One-Pot Tandem Hiyama Alkynylation/Cyclizations by Palladium(II) Acyclic Diaminocarbene (ADC) Complexes Yielding Biologically Relevant Benzofuran Scaffolds.

A series of palladium acyclic diaminocarbene (ADC) complexes of the type cis-[(R1NH)(R2)methylidene]PdCl2(CNR1) [R1 = 2,4,6-(CH3)3C6H2: R2 = NC5H10 (2); NC4H8 (3); NC4H8O (4)] were used not only to perform the Csp2 -Csp Hiyama coupling between aryl iodide and triethoxysilylalkynes but also to subsequently carry out the one-pot tandem Hiyama alkynylation/cyclization reaction between 2-iodophenol and triethoxysilylalkynes, giving a convenient time-efficient access to the biologically relevant benzofuran compounds. The palladium ADC complexes (2-4) were conveniently synthesized by the nucleophilic addition of secondary amines, namely, piperidine, pyrrolidine, and morpholine on the cis-{(2,4,6-(CH3)3C6H2)NC}2PdCl2 in moderate yields (ca. 61-66%).

Introduction

With the success of the transition-n class="Chemical">metal N-heterocyclic carbene (NHC) complexes in homogeneous catalysis being well-recognized now,[1−15] their application in more challenging domains of catalysis such as bifunctional catalysis,[16,17] asymmetric catalysis,[18] and the tandem reactions[19] are of contemporary interest. Alongside, the quest for exploring new variants of the carbene ligands is being simultaneously pursued for meeting the demands of these intriguing catalyses.[1−3,5,6,20] In this context, a special class of heteroatom-stabilized singlet carbene ligands in the form of the acyclic diaminocarbene (ADC) is worth mentioning.[21−31] Owing to their remarkably simple preparative procedures, as opposed to the long and protracted preparation methods of the contemporary phosphine and the NHC ligands, the ADC ligands have attracted attention lately.[21−31] Furthermore, the absence of any geometric constraints that allow free orientation of the ligand substituents make these ADC ligands different from their cyclic ones, for example, the NHC ligands, thereby imparting different catalytic properties to these ligands. Because of the aforementioned reasons and also owing to our long-standing interest in the applications of transition-metal NHC complexes in biomedical applications[32−36] and in chemical catalysis,[32,33] we became interested in exploring the utility of the transition-metal ADC complexes for various catalytic applications.

The benzofuran compounds constitute important bioactive molecules such as Bn class="Chemical">NC105,[37] amiodarone,[38] cytotoxic flavonoids,[39] and the natural products, namely, daphnodorin A and B,[40] egonol,[41,42] and moracin O and P,[43,44] and hence, its synthesis by an efficient route is of sufficient interest.[45−49] In this regard, the benzofuran derivatives have been successfully prepared by different synthetic approaches[50−65] including that of the one-pot tandem C–C bond coupling/cyclization reactions,[63,66−83] and for which, we remain interested in exploring the potential of transition-metal complexes of various types of carbene ligands as catalysts for the tandem reactions.[19] As for the first component of the tandem reaction, i.e., the C–C coupling reaction, we became interested in exploring the suitability of Hiyama coupling. It is primarily for reasons that (i) the Hiyama coupling provides a much greener alternative to the Suzuki and Stille couplings with toxicity issues and (ii) that the Hiyama coupling has not been explored for the transition-metal ADC complexes thus far.[22−24,26,27]

In this article, we report a series of palladium ADC complexes {cis-[(R1NH)(R2)methylidene]PdCl2(CNR1) [R1 = 2,4,6-(CH3)3C6H2: R2 = NC5H10 (2); NC4H8 (3); NC4H8O (4)]} (Figure ), whose utility in Hiyama coupling, particularly of the Csp–Csp type coupling between aryl iodide and triethoxysilylalkyne, has been explored. Moving further, the utility of these palladium 2–4 complexes in the more challenging one-pot tandem reaction between 2-iodophenol and triethoxysilylalkynes yielding benzofuran derivatives has been studied.

Figure 1

Palladium (2–4) ADC complexes.

Results and Discussion

With the intent of exploring the application of the ADC ligands, particularly in more challenging one-pot tandem Hiyama alkynylation/cyclization reactions, for constructing the biologically relevant n class="Chemical">benzofuran derivatives, the following ADC, namely, [(R1NH)(R2)methylidene] [R1 = 2,4,6-(CH3)3C6H2: R2 = NC5H10; NC4H8; NC4H8O] was so chosen for the study. In particular, the ADC ligands were stabilized as their palladium complexes of the type cis-[(R1NH)(R2)methylidene]PdCl2(CNR1) [R1 = 2,4,6-(CH3)3C6H2: R2 = NC5H10 (2); NC4H8 (3); NC4H8O (4)] (Figure ) and were synthesized from the reactions of the secondary amines, namely, piperidene, pyrrolidene, and morpholine, with the metal precursor cis-{(2,4,6-(CH3)3C6H2)NC}2PdCl2, at room temperature in moderate yields ca. 61–66% (Scheme ) along the lines for related complexes reported by Hashmi and co-workers.[84] The metal precursor, cis-{(2,4,6-(CH3)3C6H2)NC}2PdCl2, was synthesized following a sequence of reactions starting from 2,4,6-trimethylaniline (1a) (Scheme ).[85,86]

Scheme 1

Strategy for the Synthesis of the Palladium (2–4) ADC Complexes

The 1H n class="Chemical">NMR spectrum of the palladium 2–4 complexes is worth commenting upon. In particular, the M–CcarbeneN()Mes resonance appeared highly downfield-shifted at ca. δ 8.93–9.30 ppm. Additionally, the 1H NMR spectrum indicated restricted rotation about the (Mes)(H)N–Ccarbene bond as was evident from the observation of two aromatic 2,4,6-(CH3)3C62 resonances that appeared in the region ca. δ 7.00–7.09 ppm and ca. δ 6.77–7.02 ppm, and the methyl resonances appeared as three singlets of 3 protons each at ca. δ 2.39 ppm, ca. δ 2.28 ppm, and ca. δ 2.25 ppm. The 13C{1H} spectrum of the palladium 2–4 complexes showed the characteristic carbenic M–carbeneN(H)Mes resonances at ca. δ 178.9–181.8 ppm and the Pd–≡NMes resonances at ca. δ 121.1–122.7 ppm.

Infrared spectroscopy provided valuable insight on the structures of the n class="Chemical">palladium 2–4 complexes. Particularly, the observation of ν(C≡N) stretching frequencies in the range ca. 2197–2203 cm–1 indicated the presence of a palladium bound MesN≡C moiety (Pd–C≡NMes) in the palladium 2–4 complexes as was evident by comparing with the ν(C≡N) stretching frequency of the metal precursor 1d (2214 cm–1). Furthermore, a shift of ν(C≡N) stretching frequency of ca.75–85 cm–1 in the palladium 2–4 complexes (ca. 2197–2203 cm–1) with respect to free MesN≡C (2118 cm–1) indicated the strong σ-donating nature of the MesN≡C ligand.

The molecular structures of all of the palladium 2–4 complexes have been determined by X-ray diffraction studies, and they showed that the square planar n class="Chemical">palladium center is bound to the ADC ligand moiety [(R1NH)(R2)methylidene] [R1 = 2,4,6-(CH3)3C6H2: R2 = NC5H10; NC4H8; NC4H8O] and the mesityl isonitrile moiety in a cis disposition to each other. The remaining two sites are occupied by two chloride atoms (Figure and Supporting Information Figures S1 and S2 and Table S1). The Pd–Ccarbene bond distances in 2 [2.006(3) Å], 3 [1.991(4) Å], and 4 [2.011(3) Å] are slightly shorter than the sum of the covalent radii of palladium and carbon (2.12 Å)[87] and are comparable to the other related palladium ADC complexes of similar types, namely, cis-[(R1NH)(R2)methylidene]PdCl2(CNR1) [R1 = 2,6-(CH3)2C6H3: R2 = 2,6-(CH3)2C6H3NH, Pd–Ccarbene bond distance is 2.003(7) Å;[88] R1 = C6H11: R2 = Ph2C=N–NH, Pd–Ccarbene bond distance is 1.966(3) Å;[89] R1 = 2,6-(CH3)2C6H3: R2 = 5-(NH2)C6H4NH, Pd–Ccarbene bond distance is 1.979(3) Å;[90] and R1 = t-Bu: R2 = Ph2CNH, Pd–Ccarbene bond distance is 1.994(3) Å.[91]

Figure 2

ORTEP of 4 with thermal ellipsoids drawn at the 50% probability level. Selected bond length (Å) and bond angle (deg): Pd1–C19 2.011(3), Pd1–C24 1.919(4), Pd1–Cl1 2.3770(8), Pd1–Cl2 2.3075(9), N1–C19 1.328(4), N3–C19 1.320(4), N1–H1 0.8800, Cl2–Pd1–Cl1 90.62(3), C19–Pd1–Cl2 90.22(9), C19–Pd1–Cl1 179.14(9), N3–C19–N1 120.1(3), N3–C19–Pd1 120.4(2), and N1–C19–Pd1 119.5(2).

Quite interestingly, the stronger trans-effect of the n class="Chemical">ADC ligand [(R1NH)(R2)methylidene] [R1 = 2,4,6-(CH3)3C6H2: R2 = NC5H10; NC4H8; NC4H8O] as compared to that of the mesityl isonitrile ligand was evident from the observation of a significantly longer Pd–Cl bond by ca. 0.0571–0.0762 Å located trans to the ADC ligand in the complexes 2 [2.3825(9) Å], 3 [2.3695(12) Å], and 4 [2.3770(8) Å] than the adjacent Pd–Cl bond located trans to the mesityl isonitrile ligand in the complexes 2 [2.3063(8) Å], 3 [2.3124(13) Å], and 4 [2.3075(9) Å]. The same has been observed in the other related complexes, namely, cis-[(R1NH)(R2)methylidene]PdCl2(CNR1) [R1 = 2,6-(CH3)2C6H3: R2 = 2,6-(CH3)2C6H3NH, 2.3838(18), 2.316(2) Å;[88] R1 = C6H11: R2 = Ph2C=N–NH, 2.3671(17), 2.3232(7) Å;[89] R1 = 2,6-(CH3)2C6H3: R2 = 5-(NH2)C6H4NH, 2.3843(7), 2.3289(8) Å;[90] and R1 = t-Bu: R2 = Ph2CNH, 2.3698(8), 2.3241(8) Å.[91]

Furthermore, extensive pπ–pπ delocalization in the n class="Chemical">ADC ligand [(R1NH)(R2)methylidene] [R1 = 2,4,6-(CH3)3C6H2: R2 = NC5H10; NC4H8; NC4H8O] was evident from the fact that the two C–N bond lengths in the complexes 2 [N2–C11 1.328(4) Å, N3–C11 1.330(4) Å], 3 [N1A–C10A 1.329(5) Å, N2A–C10A 1.329(5) Å], and 4 [N1–C19 1.328(4) Å, N3–C19 1.320(4) Å] fall between the C–N single bond length of 1.469 Å in amine[92] and the C=N double bond length of 1.279 Å in imines.[92]

Csp–Csp Type Hiyama Alkynylation

With the intent of exploring the one-pot tandem Hiyama alkynylation/cyclization reaction yielding biologically relevant benzofuran compounds, we decided to explore the first component of this tandem reaction that tested the ability of our n class="Chemical">palladium ADC complexes, (2–4), in carrying out the Hiyama alkynylation reaction. In this context, it is worth noting that there exists no report of the use of ADC-based catalysts for any type of Hiyama coupling reactions. In this backdrop, our efforts in exploring the utility of the palladium ADC complexes (2–4) in the Hiyama alkynylation coupling reaction between an triethoxysilylalkyne containing the Csp center and aryl iodide, a substrate containing the Csp center assumes importance (Chart ).

Chart 1

Hiyama Coupling Reaction of Iodobenzene (5) with Triethoxysilylalkynes (6–13) as Catalyzed by Pd–ADC Complex (4)

Significantly enough, a representative n class="Chemical">ADC complex 4 successfully carried out the Csp–Csp Hiyama alkynylation coupling between aryl iodide with the triethoxysilylalkyne substrate. In particular, the Hiyama alkynylation between various triethoxysilylalkyne reagents covering aryl alkyne moieties in triethoxy(phenylethynyl)silane (6),[93] triethoxy(p-tolylethynyl)silane (7), triethoxy(4-fluorophenylethynyl)silane (10), triethoxy(4-chlorophenylethynyl)silane (11), triethoxy(4-bromophenylethynyl)silane (12), and triethoxy(naphthalen-1-ylethynyl)silane (13) to the aliphatic ones in triethoxy(hex-1-yn-1-yl)silane (8),[93] triethoxy(hept-1-yn-1-yl)silane (9) with iodobenzene was catalyzed by the palladium ADC complex 4 at 2 mol % of the catalyst loading at 80 °C in 4 h of reaction time (Chart ). Interestingly enough, higher yields were obtained for the aryl triethoxysilylalkyne (ca. 40–76%) as compared to the aliphatic alkyne silyl ether (ca. 35–41%) reagents (Table ). The significant amplification of the product yield for the Hiyama alkynylation coupling reaction between triethoxy(phenylethynyl)silane (6) and iodobenzene (5) by palladium ADC complex 4 was observed when the yield of 76% was compared to the blank experiments that showed 10% of the yield and with control experiments with PdCl2 that exhibited 39% of the yield and with Cl2Pd(MeCN)2 exhibiting 42% of the yield. The homogeneous nature of the catalysis were further established by performing the Hg-drop experiment that showed a near-equal yield of (ca. 74%) the product formed in the presence and in the absence of Hg (ca. 76%) (Supporting Information Table S2).

Table 1

Selected Results for Hiyama Cross-Coupling Reaction of Iodobenzene with Triethoxysilylalkynes as Catalyzed by Pd–ADC Complex 4a

Reaction conditions: iodobenzene (1.00 mmol), triethoxysilylalkyne (1.20 mmol), NaOH (3.00 mmol), in the presence of catalyst 4 (2 mol %) and 6 mL of a mixed medium of 1,4-dioxane/H2O (4:2 v/v ratio) at 80 °C for 4 h.

Isolated yields.

Important is the comparison of the Hiyama coupling products obtained using our catalyst to those obtained by other reactions. In particular, n class="Chemical">diphenyl acetylene was obtained in a 76% yield at 2 mol % of catalyst 4 loading under our Hiyama alkynylation condition, which is slightly lower than the near-quantitative yield of 99% obtained using CuI (10 mol %),[94] of 94% obtained using a palladium hydroxysalen complex (3 mol %),[95] and of 85% obtained using a [1,3-bis(2,6-Pr2C6H3)imidazol-2-yilidene]PdCl2[3-ClC6H4N] complex (5 mol %)[96] using decarboxylative coupling between alkynyl carboxylic acid with aryl iodide/boronic acid/diazonium salts. Apart from this, diphenyl acetylene in high yield has been reported by reaction of aryl halides with alkynyl magnesium bromide in a 94% yield in the presence of a Cu(I) PNP pincer complex as the catalyst (5 mol %)[97] and also by Sonogashira coupling.[98,99] Likewise, for other substrates too, the decarboxylative cross-coupling reaction produced better yields than our Hiyama alkynalation reaction.

The Hiyama alkynylation coupling was also performed under environment-friendly and much desired n class="Chemical">fluoride-free conditions.[100−104] More importantly, the Hiyama coupling by the representative palladium ADC complex 4 represents the first ever report of a Csp–Csp coupling between an aryl halide substrate containing a Csp center and a triethoxysilylalkyne reagent containing a Csp center. We have earlier reported the more common Csp–Csp variant of the Hiyama coupling with the palladium NHC complexes[105] and abnormal NHC complexes.[106]

A proposed mechanism involves the generation of palladium(0) species (A) which undergoes oxidative addition with n class="Chemical">phenyl iodide to give the palladium aryl iodide species (B), which upon reacting with alkynyl nucleophile of the triethoxysilylalkyne reagent gives the corresponding palladium aryl[Ph(C≡CPh)]alkynyl derivative (C). Subsequently, reductive elimination from the species C gives the Hiyama product along with generation of palladium(0) intermediate A (Scheme ).

Scheme 2

Proposed Mechanism for the Hiyama Coupling Reaction between Two Representative Iodobenzene and Triethoxy(phenylethynyl)silane Substrates as Catalyzed by Pd–ADC Complex (4)

Csp–Csp-Type Hiyama Alkynylation/Cyclization

Having successfully performed the Csp–n class="Gene">Csp alkynylation coupling between aryl iodide and triethoxysilylalkyne by a representative palladium ADC complex 4, we proceeded to attempt the more challenging one-pot tandem Hiyama alkynylation/cyclization reaction for constructing the much desired biologically relevant benzofuran scaffold. Indeed, the palladium ADC complex successfully carried out the one-pot tandem Hiyama alkynylation/cyclization reaction of 2-iodophenol and triethoxysilylalkyne, giving a benzofuran derivative (Chart ).

Chart 2

Hiyama Coupling Reaction Followed by Cyclization of Iodophenol (22) with Triethoxysilylalkynes (6–13) as Catalyzed by Pd–ADC Complex (4)

In particular, the solvent optimization study carried out on the representative substrate 2-iodophenol (22) and n class="Chemical">triethoxy(phenylethynyl)silane (6) as catalyzed by representative palladium ADC complex 4 at 2 mol % catalyst loading in the presence of NaOH as the base showed a maximum yield (57%) of the 2-phenylbenzofuran (23) product in the mixed media of 1,4-dioxane/H2O (2:1 v/v) at 80 °C and at 4 h of the reaction time (Supporting Information Table S3). Similarly, the base variation study on the same two representative substrates performed using the same representative palladium ADC complex 4 produced a maximum yield of 57% in the presence of NaOH as the base (Supporting Information Table S4). The time-dependence study performed on the same substrates using the representative palladium ADC complex 4 showed saturation of the reaction yield after 4 h of reaction time (57%) and showed no significant increment in the yield even after 12 h of reaction time (59%), and hence, the 4 h of reaction time was chosen for the subsequent runs (Supporting Information Table S6). The significant amplification of the product yield for the one-pot tandem Hiyama alkynylation/cyclization reaction for the same substrates by palladium ADC complex 4 was observed when compared to the yield of 57% of the blank experiments that showed no product formation and with control experiments with PdCl2 that exhibited 25% of yield and with Cl2Pd(MeCN)2 exhibiting 24% of yield. The homogeneous nature of the catalysis was confirmed from the Hg-drop experiment that showed a near-equal yield of the product formed in the presence (yield of 54%) and in the absence of Hg (yield of 57%) (Supporting Information Table S7). Furthermore, a catalyst optimization study, performed on the representative substrate, namely, 2-iodophenol (22) and triethoxy(phenylethynyl)silane (6), displayed the higher yield for the morpholine-derived palladium ADC complex 4, and which are used for the subsequent substrate scope study (Supporting Information Table S5).

The substrate-scope study showed that the palladium ADC complex 4 was successfully used to carry out the one-pot tandem Hiyama alkynylation/cyclization between 2-iodophenol and a variety of triethoxysilylalkyne substrates with moderate to good yields (ca. 14–57%). It is noteworthy that a relatively higher yield was observed in the case of the aryl triethoxysilylalkyne substrates as compared to aliphatic triethoxysilylalkynes (Table ).

Table 2

Selected Results for the Tandem Hiyama/Cyclization Reaction Iodophenol and Various Triethoxysilylalkynes as Catalyzed by Pd–ADC Complex 4a

Reaction conditions: 2-iodophenol (1.00 mmol), triethoxysilylalkyne (1.20 mmol), NaOH (3.00 mmol), in the presence of catalyst 4 (2 mol %) and 6 mL of a mixed medium of 1,4-dioxane/H2O (4:2 v/v ratio) at 80 °C for 4 h.

Isolated yields.

In this regard, it is worth noting that the 2-aryl-substituted benzofuran framework is prepared by many synthetic methodologies including directly attaching the phenyl moiety on the 2-pon class="Chemical">sition of the benzofuran ring by various C–C bond-forming reactions such as the Suzuki–Miyaura cross-coupling,[50−55] the Heck/Heck–Matsuda-type coupling,[56−59] the Hiyama–Denmark cross-coupling,[60,61] and by cyclization reactions of 1-halo-2-(phenylethynyl)benzene substrates,[62−65] and also by tandem C–C coupling/cyclization reactions of 2-iodophenol and phenylacetylene.[63,66−83] Of the various approaches employed for preparing the benzofuran scaffold, the one-pot tandem reaction involving C–C coupling/cyclization starting from 2-iodophenol and phenylacetylene substrates has mainly been reported for the Sonogashira alkynylation/cyclization reactions[66,67,69,71,74,76−79] and the Heck alkynylation/cyclization.[70] Hence, against this backdrop, the 2–4 catalyzed one-pot tandem reaction for the synthesis of benzofuran compounds represents the first report of a Hiyama alkynylation/cyclization procedure giving convenient time-efficient access to the class of biologically relevant benzofuran motifs.

Important is the comparison of the yields of the aryl benzofuran products obtained in our Hiyama alkynylation/cyclization reaction to those obtained by other methods. In particular, n class="Chemical">2-phenylbenzofuran was obtained in 57% at 2 mol % of catalyst 4 loading under our Hiyama alkynylation/cyclization condition which is lower than the near-quantitative yield of 93% obtained using Et2Zn (5 mol %),[68] of 90% obtained using a CuI2(pip)2 complex (1%),[69] and of 88% obtained using Pd(OAc)2-NCB (5 mol %),[107] all obtained using tandem Sonogashira/cyclization reaction between 2-iodophenol and phenylacetylene substrates. The 2-phenylbezofuran synthesis has also been reported in 88% of yield using [1,3-bis(2,6-Pr2C6H3)imidazol-2-yilidene]AuCl (5 mol %) from 1-(alkynyl)-7-oxabicyclo[4.1.0]heptan-2-ones.[108] Furthermore, 2-phenylbenzofuran has been reported in high yield (72%) by reaction of arylation of 2-(gem-dibromovinyl)phenols with sodium arylsulfinates using PdCl2 (5 mol %)/Cu(OAc)2 (10 mol %) as the catalyst.[109] Using one-pot Suzuki coupling of 1,2-dichlorovinyl ethers and organoborane reagents, 2-phenylbenzofuran has been obtained in 75% yield using Pd2(dba)3 (2.5 mol %).[54] Furthermore, Heck alkynylation/cyclization reaction using a palladium-triazole-based NHC complex (1 mol %) produced phenylbenzofuran in high yields (81%).[70] Likewise, for other aryl benzofuran substrates too, one-pot Suzuki coupling and the Sonogashira coupling/cyclization reactions produced better yields than the current Hiyama alkynylation/cyclization.

The proposed mechanism for the one-pot tandem Hiyama alkynylation/cyclization is initiated with the formation of the palladium(0) intermediate (A) from the precatalysts (2–4) in the presence of n class="Chemical">NaOH as the base. The oxidative addition of the aryl iodide bond leads to the formation of square planar intermediate (B) which reacts with the alkynyl nucleophile of the triethoxysilylalkyne to give the intermediate (C) that upon reductive elimination gives back starting intermediate (A) and 2-(phenylethynyl)phenol. 2-(phenylethynyl)phenol subsequently enters the second catalytic cycle by coordinating with the palladium center in the intermediate (A). The cyclization of alkyne-coordinated species (D) leads to the formation of the benzofuran-bound palladium complex (E). Lastly, the reductive elimination of the benzofuran derivative 2-phenylbenzofuran (23) from intermediate (E) simultaneously gives the desired benzofuran product along with the generation of the intermediate (A) (Scheme ). It is noteworthy that the conversion of 2-(phenylethynyl)phenol to 2-phenylbenzofuran by the catalyst (4) under the same catalytic conditions in good yield of ca. 81% further proves that the proposed mechanism having two consecutive cycles (see the Experimental Section from page 50, line 18 to page 51, line 13 and Supporting Information Figures S233–S238).

Scheme 3

Proposed Mechanism for the Hiyama Alkynylation/Cyclization Reaction between Two Representative 2-Iodophenol and Triethoxy(phenylethynyl)silane Substrates as Catalyzed by Pd–ADC Complex (4)

Conclusions

In summary, a series of palladium ADC complexes, namely, n class="Chemical">cis-[(R1NH)(R2)methylidene]PdCl2(CNR1) [R1 = 2,4,6-(CH3)3C6H2: R2 = NC5H10 (2); NC4H8 (3); NC4H8O (4)] have been synthesized and structurally characterized. These palladium 2–4 complexes were used not only to carry out the Csp–Csp Hiyama coupling of an aryl iodide containing the Csp center with a triethoxysilylalkyne containing the Csp center but also to successfully perform the one-pot tandem Hiyama alkynylation/cyclization reaction of a variety of 2-iodophenol and triethoxysilylalkyne reagents yielding the biologically relevant benzofuran compounds. The work represents the first report of a convenient time-efficient one-pot tandem Hiyama alkynylation/cyclization reaction for the preparations of the benzofuran compounds.

Experimental Section

General Procedures

All manipulations were carried out using a glovebox and standard Schlenk techniques. n class="Chemical">PdCl2 was purchased from SD Fine Chemicals (India), and pyrrolidine, piperidine, morpholine, phenylacetylene, and 1-ethynyl-4-fluorobenzene were purchased from Spectrochem Pvt. Ltd. (India) and used without any further purification. 1-Ethynyl-4-methylbenzene, hex-1-yne, and hept-1-yne were purchased from Sigma-Aldrich and used without any further purification. 1-Chloro-4-ethynylbenzene, 1-bromo-4-ethynylbenzene, and 1-ethynylnaphthalene were prepared by a modified literature procedure.[110]N-Formyl-2,4,6-trimethylaniline (1b),[85] 2,4,6-trimethylphenyl isonitrile (1c),[86] and cis-[PdCl2(2,4,6-trimethylphenyl isonitrile)2] (1d)[86] were synthesized by a modified literature procedure as described below. 2-(phenylethynyl)phenol was synthesized by a known literature procedure.[111]1H and 13C{1H} NMR spectra were recorded on Bruker 400 and 500 MHz NMR spectrometers. 1H NMR peaks are labeled as singlet (s), doublet (d), triplet (t), quartet of doublet (qd), and septet (sept). Mass spectrometry measurements were done on a Micromass Q-Tof spectrometer and Bruker Maxis impact. Elemental analysis was carried out on a Thermo Quest Flash 1112 Series (CHNS) elemental analyzer. For the catalysis runs, the gas chromatography–mass spectrometry (GCMS) analysis was done on using Agilent Technologies 7890A GC systems with a 5975C inert XL EI/CI MSD Triple-Axis detector. The X-ray diffraction data were collected on a Rigaku Hg 724+ diffractometer and refined by full-matrix least-square procedures on F2 with SHELXTL (version 6.10).[112−114] CCDC 951308 (2), 918896 (3), and 1524127 (4) contain the supplementary crystallographic data for this paper (Supporting Information). These data can be obtained free of charge from the Cambridge Crystallographic Data center via www.ccdc.cam.ac.uk/data_request/cif.

Synthesis of N-Formyl-2,4,6-trimethylaniline (1b)[85]

A mixture of HCOOH (5.31 g, 115 mmol) and n class="Chemical">(CH3CO)2O (4.68 g, 45.8 mmol) was stirred at room temperature for 1 h under a nitrogen atmosphere, after which a solution of 2,4,6-trimethylaniline (5.00 g, 37.0 mmol) in dry CH2Cl2 (ca. 30 mL) was added. The temperature of the reaction mixture was maintained between (5–10) °C during the course of the addition. The reaction mixture was further stirred at room temperature for 16 h and then refluxed for another 4 h, after which the solvent was removed under reduced pressure. The residue was dissolved in CHCl3 (ca. 100 mL), washed with a saturated solution of aqueous NaHCO3 (3 × 100 mL) and subsequently with H2O (ca. 100 mL), then dried over Na2SO4, filtered, and finally, the solvent was removed under reduced pressure to obtain as a yellow solid. The yellow solid was further washed with hot Et2O to give the product as a white solid (3.77 g, 62%). Isomer (major). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.32–8.30 (m, 1H, CO), 7.17–7.08 (m, 1H, N), 6.93 (s, 2H, 2,4,6-(CH3)3C62), 2.28 (s, 3H, 2,4,6-(C3)3C6H2), 2.18–2.17 (m, 6H, 2,4,6-(C3)3C6H2). Isomer (minor). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.04 (d, 1H, 3JHH = 12 Hz, CO), 7.37–7.31 (m, 1H, N), 6.87 (s, 2H, 2,4,6-(CH3)3C62), 2.25 (s, 9H, 2,4,6-(C3)3C6H2).

Synthesis of 2,4,6-Trimethylphenyl Isonitrile (1c)[86]

POCl3 (2.738 g, 18.3 mmol) was added dropwise to a solution of n class="Chemical">N-formyl-2,4,6-trimethylaniline (1b) (1.00 g, 6.13 mmol) in CH2Cl2 (ca. 50 mL) at −60 °C over a period of 5 min. The reaction mixture was stirred for another 20 min when NEt3 (5.57 g, 55.0 mmol) was added to the suspension over a period of 10 min and then the resulting suspension was let to stir overnight at room temperature. CH2Cl2 (ca. 100 mL) was added to the reaction mixture, and the organic layer was separated, washed with saturated aqueous NaHCO3 (3 × 30 mL), dried over anhydrous Na2SO4, and finally, the solvent was removed under reduced pressure. The residue thus obtained was purified by column chromatography using silica gel as a stationary phase and by eluting with a CH2Cl2/CH3OH mixture (99:1 v/v) to obtain the compound as a colorless solid (0.402 g, 45%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 6.78 (s, 2H, 2,4,6-(CH3)3C62), 2.30 (s, 6H, 2,4,6-(C3)3C6H2), 2.21 (s, 3H, 2,4,6-(C3)3C6H2). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 166.9 (2,4,6-(CH3)36H2), 138.7 (2,4,6-(CH3)36H2), 134.5 (2,4,6-(CH3)36H2), 128.4 {2[(2,4,6-(CH3)36H2)]}, 124.3 (2,4,6-(CH3)36H2N), 21.1 (2,4,6-(H3)3C6H2), 18.7 {2(2,4,6-(H3)3C6H2)}. IR data (KBr pellet): 2976 (s), 2922 (w), 2118 (s), 1606 (s), 1479 (s), 1376 (w), 1039 (w), 851 (w) cm–1.

Synthesis of cis-[PdCl2(2,4,6-Trimethylphenyl isonitrile)2] (1d)[86]

A mixture of 2,4,6-trimethylphenyl isonitrile (n class="Chemical">1c) (0.221 g, 1.54 mmol) and [(CH3CN)2PdCl2] (0.200 g, 0.771 mmol) in toluene (ca. 10 mL) was stirred at room temperature for 12 h, after which the solvent was removed under vacuum. The residue was washed with n-pentane (3 × 30 mL) and dried under reduced pressure. Then, the residue was purified by column chromatography using silica gel as a stationary phase and by eluting with a CH3Cl/CH3OH mixture (99/1 v/v) to give a pure product as a yellow solid (0.253 g, 70%). Isomer (major). 1H NMR (DMSO-d6, 400 MHz, 25 °C): δ 7.11–7.04 (m, 2H, 2,4,6-(CH3)3C62), 2.36 (s, 6H, 2,4,6-(C3)3C6H2), 2.30 (br, 3H, 2,4,6-(C3)3C6H2). Isomer (minor). 1H NMR (DMSO-d6, 400 MHz, 25 °C): δ 6.97–6.81 (m, 2H, 2,4,6-(CH3)3C62), 2.25–2.14 (m, 9H, 2,4,6-(C3)3C6H2). IR data (KBr pellet): 2214 (s), 1603 (w), 1470 (w), 1381 (w), 1307 (w), 1039 (w), 854 (w) cm–1. HRMS calcd for [C20H22Cl2 N2Pd + Na]+, 491.0080; found m/z, 491.0089. Anal. Calcd for C20H22Cl2N2Pd: C, 51.36; H, 4.74; N, 5.99%. Found: C, 50.72; H, 4.32; N, 5.20%.

Synthesis of cis-[((2,4,6-Trimethylphenylamino)(piperidin-1-yl)methylidene)]PdCl2(2,4,6-trimethylphenylisonitrile) (2)

To a solution of cis-{(2,4,6-(CH3)3C6H2)NC}2PdCl2 (1d) (0.176 g, 0.376 mmol) in n class="Chemical">tetrahydrofuran (THF, ca. 10 mL) at 0 °C, piperidine (0.032 g, 0.376 mmol) was added, and the reaction mixture was stirred overnight at room temperature. The solvent was then removed under reduced pressure, and the residue so obtained was purified by column chromatography using silica gel as a stationary phase and by eluting with a CHCl3/CH3OH mixture (95:5 v/v) to give the product (2) as an yellow solid (0.138 g, 66%). 1H NMR, (DMSO-d6, 400 MHz, 25 °C): δ 9.07 (s, 1H, N), 7.09 (s, 2H, 2,4,6-(CH3)3C62), 7.00 (s, 1H, 2,4,6-(CH3)3C62), 6.77 (s, 1H, 2,4,6-(CH3)3C62), 4.57 (br, 1H, NC510), 4.28 (br, 1H, NC510), 3.86 (br, 2H, NC510), 2.39 (s, 3H, 2,4,6-(C3)3C6H2), 2.28 (s, 3H, 2,4,6-(C3)3C6H2), 2.25 (s, 3H, 2,4,6-(C3)3C6H2), 2.21 (s, 6H, 2,4,6-(C3)3C6H2), 1.93 (s, 3H, 2,4,6-(C3)3C6H2), 1.72–1.54 (m, 6H, NC510). 13C NMR (DMSO-d6, 125 MHz, 25 °C): δ 179.9 (NHN), 140.8 (2,4,6-(CH3)36H2), 137.4 (2,4,6-(CH3)36H2), 137.3 (2,4,6-(CH3)36H2), 135.4 (2,4,6-(CH3)36H2), 135.2 (2,4,6-(CH3)36H2), 135.0 (2,4,6-(CH3)36H2), 134.9 (2,4,6-(CH3)36H2), 134.3 (2,4,6-(CH3)36H2), 129.7 (2,4,6-(CH3)36H2), 129.1 (2,4,6-(CH3)36H2), 129.1 (2,4,6-(CH3)36H2), 128.5 (2,4,6-(CH3)36H2), 122.7 (N–2,4,6-(CH3)3C6H2), 56.6 (N5H10), 47.6 (N5H10), 26.2 (N5H10), 26.1 (N5H10), 23.6 (N5H10), 21.0 (2,4,6-(H3)3C6H2), 20.7 (2,4,6-(H3)3C6H2), 19.6 (2,4,6-(H3)3C6H2), 18.2 (2,4,6-(H3)3C6H2), 17.9 {2(2,4,6-(H3)3C6H2)}. IR data (KBr pellet): 3249 (s), 2924 (s), 2198 (s), 1608 (w), 1560 (s), 1443 (w), 1342 (w), 1247 (w), 1023 (w), 855 (w), 655 (w) cm–1. HRMS calcd for [C25H33N3Cl2Pd – Cl]+, 518.1389; found m/z, 518.1389. Anal. Calcd for C25H33Cl2N3Pd: C, 54.31; H, 6.02; N, 7.60%. Found: C, 54.47; H, 5.67; N, 7.11%

Synthesis of cis-[((2,4,6-Trimethylphenylamino)(pyrrolidin-1-yl)methylidene)]PdCl2(2,4,6-trimethylphenylisonitrile) (3)

To a solution of cis-{(2,4,6-(CH3)3C6H2)NC}2PdCl2 (1d) (0.192 g, 0.411 mmol) in n class="Chemical">THF (ca. 10 mL) at 0 °C, pyrrolidine (0.0292 g, 0.411 mmol) was added, and the reaction mixture was stirred overnight at room temperature. The solvent was removed under reduced pressure, and the residue so obtained was purified by column chromatography using silica gel as an stationary phase and by eluting with a CHCl3/CH3OH mixture (95:5 v/v) to give product (3) as a yellow solid (0.146 g, 64%). 1H NMR, (DMSO-d6, 400 MHz, 25 °C): δ 8.93 (s, 1H, N), 7.09 (s, 2H, 2,4,6-(CH3)3C62), 7.01 (s, 1H, 2,4,6-(CH3)3C62), 6.79 (s, 1H, 2,4,6-(CH3)3C62), 4.30–4.28 (m, 2H, NC48), 4.15–4.11 (m, 2H, NC48), 2.39 (s, 3H, 2,4,6-(C3)3C6H2), 2.28 (s, 3H, 2,4,6-(C3)3C6H2), 2.25 (s, 3H, 2,4,6-(C3)3C6H2), 2.20 (s, 6H, 2,4,6-(C3)3C6H2), 2.10–1.95 (m, 4H, NC48), 1.93 (s, 3H, 2,4,6-(C3)3C6H2). 13C NMR (DMSO-d6, 100 MHz, 25 °C): δ 178.9 (NHN), 140.8 (2,4,6-(CH3)36H2), 137.3 (2,4,6-(CH3)36H2), 137.2 (2,4,6-(CH3)36H2), 137.0 (2,4,6-(CH3)36H2), 135.4 (2,4,6-(CH3)36H2), 135.3 (2,4,6-(CH3)36H2), 135.2 (2,4,6-(CH3)36H2), 135.0 (2,4,6-(CH3)36H2), 134.1 (2,4,6-(CH3)36H2), 129.4 (2,4,6-(CH3)36H2), 129.1 (2,4,6-(CH3)36H2), 128.4 (2,4,6-(CH3)36H2), 122.1 (N–2,4,6-(CH3)3C6H2), 55.7 (N4H8), 49.1 (N4H8), 25.1 (N4H8), 24.6 (N4H8), 20.9 (2,4,6-(H3)3C6H2), 20.7 (2,4,6-(H3)3C6H2), 19.6 (2,4,6-(H3)3C6H2), 18.1 (2,4,6-(H3)3C6H2), 17.8 {2(2,4,6-(H3)3C6H2)}. IR data (KBr pellet): 3181 (s), 2919 (s), 2197 (s), 1556 (s), 1453 (w), 1034 (w), 856 (w) cm–1. HRMS calcd for [C24H31N3Cl2Pd – Cl]+, 502.1241; found m/z, 502.1245. Anal. Calcd for C24H31Cl2N3Pd: C, 53.50; H, 5.80; N, 7.80%. Found: C, 53.29; H, 5.60; N, 7.62%.

Synthesis of cis-[((2,4,6-Trimethylphenylamino)(morpholino)methylidene)]PdCl2(2,4,6-trimethylphenylisonitrile) (4)

To a solution of cis-{(2,4,6-(CH3)3C6H2)NC}2PdCl2 (1d) (0.349 g, 0.746 mmol) in n class="Chemical">THF (ca. 10 mL) at 0 °C, morpholine (0.065 g, 0.747 mmol) was added, and the reaction mixture was stirred overnight at room temperature. The solvent was then removed under reduced pressure, and the residue so obtained was purified by column chromatography using silica gel as a stationary phase and by eluting with a CHCl3/CH3OH mixture (95:5 v/v) to give the product (4) as a yellow solid (0.253 g, 61%). 1H NMR, (DMSO-d6, 400 MHz, 25 °C): δ 9.30 (s, 1H, N), 7.09 (s, 2H, 2,4,6-(CH3)3C62), 7.02 (s, 1H, 2,4,6-(CH3)3C62), 6.77 (s, 1H, 2,4,6-(CH3)3C62), 4.46–4.42 (m, 2H, NC48O), 3.93–3.86 (m, 2H, NC48O), 3.80–3.74 (m, 4H, NC48O), 2.39 (s, 3H, 2,4,6-(C3)3C6H2), 2.28 (s, 3H, 2,4,6-(C3)3C6H2), 2.25 (s, 3H, 2,4,6-(C3)3C6H2), 2.21 (s, 6H, 2,4,6-(C3)3C6H2), 1.92 (s, 3H, 2,4,6-(C3)3C6H2). 13C{1H} NMR (DMSO-d6, 100 MHz, 25 °C): δ 181.8 (NHN), 140.6 (2,4,6-(CH3)36H2), 137.2 (2,4,6-(CH3)36H2), 137.1 (2,4,6-(CH3)36H2), 135.2 (2,4,6-(CH3)36H2), 135.1 (2,4,6-(CH3)36H2), 134.8 {2(2,4,6-(CH3)36H2)}, 134.6 (2,4,6-(CH3)36H2), 129.4 (2,4,6-(CH3)36H2), 128.9 {2(2,4,6-(CH3)36H2)}, 128.4 (2,4,6-(CH3)36H2), 122.5 (N–2,4,6-(CH3)3C6H2), 66.6 (N4H8O), 65.7 (N4H8O), 55.3 (N4H8O), 47.2 (N4H8O), 20.8 (2,4,6-(H3)3C6H2), 20.6 (2,4,6-(H3)3C6H2), 19.4 (2,4,6-(H3)3C6H2), 18.0 (2,4,6-(H3)3C6H2), 17.8 {2(2,4,6-(H3)3C6H2)}. IR data (KBr pellet): 3196 (s), 2921 (s), 2203 (s), 1605 (w), 1555 (s), 1439 (w), 1275 (w), 1237 (w), 1115 (w), 1027 (w), 853 (w) cm–1. HRMS calcd for [C24H31Cl2N3OPd – Cl]+, 520.1184; found m/z, 520.1184. Anal. Calcd for C24H31Cl2N3OPd: C, 51.95; H, 5.63; N, 7.57%. Found: C, 52.34; H, 5.59; N, 7.82%.

General Procedures for Triethoxysilylalkyne Preparation

A mixture of terminal alkyne and EtMgBr (2.0 M in THF) in Et2O (ca. 30 mL) was added in a 1.2:1 ratio at room temperature and refluxed for 2 h. The reaction mixture was cooled to room temperature, and Si(OEt)4 (1.8 times of EtMgBr) was added. The reaction mixture was again refluxed further for 12 h. The resulting mixture was filtered, and the volatiles were removed under reduced pressure. The crude product was then purified by fractional distillation under reduced pressure at ambient temperature by a Kugelrohr short path distillation apparatus at the temperature range of 40–60 °C.

Triethoxy(phenylethynyl)silane (6)[93]

Phenyl acetylene (2.38 g, 23.3 mmol), EtMgBr (2.0 M in THF, 9.7 mL, 19.4 mmol), Si(OEt)4 (7.8 mL, 34.9 mmol). Yellow liquid; yield (1.073 g, 21%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.52–7.48 (m, 2H, C65), 7.36–7.30 (m, 3H, C65), 3.85 (q, 6H, 3JHH = 7 Hz, Si(OC2CH3)3), 1.24 (t, 9H, 3JHH = 7 Hz, Si(OCH2C3)3). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 132.3 (6H5), 129.3 (6H5), 128.3 (6H5), 121.9 (6H5), 104.1 (Si(OCH2CH3)3), 85.1 (C6H5), 59.1 (Si(OH2CH3)3), 18.1 (Si(OCH2H3)3). GC–MS (ESI): 264 [M]+.

Triethoxy(p-tolylethynyl)silane (7)

1-Ethynyl-4-methylbenzene (1.022 g, 8.79 mmol), EtMgBr (2.0 M in THF, 3.7 mL, 7.33 mmol), Si(OEt)4 (2.9 mL, 13.2 mmol). Yellow liquid; yield (0.409 g, 20%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.41 (d, 2H, 3JHH = 8 Hz, 4-CH3C64), 7.12 (d, 2H, 3JHH = 8 Hz, 4-CH3C64), 3.94 (q, 6H, 3JHH = 7 Hz, Si(OC2CH3)3), 2.35 (s, 3H, 4-C3C6H4), 1.29 (t, 9H, 3JHH = 7 Hz, Si(OCH2C3)3). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 139.7 (4-CH36H4), 132.4 (4-CH36H4), 129.2 (4-CH36H4), 119.0 (4-CH36H4), 104.6 (Si(OCH2CH3)3), 84.4 (C6H5), 59.2 (Si(OH2CH3)3), 21.7 (4-H3C6H4), 18.2 (Si(OCH2H3)3). GC–MS (ESI): 278 [M]+.

Triethoxy(hex-1-yn-1-yl)silane (8)[93]

Hex-1-yne (2.25 g, 27.3 mmol), EtMgBr (2.0 M in THF, 11.4 mL, 22.8 mmol), Si(OEt)4 (9.2 mL, 41.01 mmol). Yellow liquid; yield (1.83 g, 33%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 3.84 (q, 6H, 3JHH = 7 Hz, Si(OC2CH3)3), 2.23 (t, 2H, 3JHH = 7 Hz, CH3CH2CH2C2), 1.51 (quint, 2H, 3JHH = 7 Hz, CH3CH2C2CH2), 1.41 (quint, 2H, 3JHH = 7 Hz, CH3C2CH2CH2), 1.22 (t, 9H, 3JHH = 7 Hz, Si(OCH2C3)3), 0.88 (t, 3H, 3JHH = 7 Hz, C3CH2CH2CH2). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 107.5 (Si(OCH2CH3)3), 75.9 (C6H5), 58.9 (Si(OH2CH3)3), 30.3 (CH3CH2CH2H2), 21.9 (CH3CH2H2CH2), 19.3 (CH3H2CH2CH2), 18.1 (Si(OCH2H3)3), 13.6 (H3CH2CH2CH2). GC–MS (ESI): 244 [M]+.

Triethoxy(hept-1-yn-1-yl)silane (9)

Hept-1-yne (1.00 g, 10.4 mmol), EtMgBr (2.0 M in THF, 4.3 mL, 8.66 mmol), Si(OEt)4 (3.5 mL, 15.6 mmol). Yellow liquid; yield (0.624 g, 28%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 3.88 (q, 6H, 3JHH = 7 Hz, Si(OC2CH3)3), 2.24 (t, 2H, 3JHH = 7 Hz, CH3CH2CH2CH2C2), 1.53 (quint, 2H, 3JHH = 7 Hz, CH3CH2CH2C2CH2), 1.38–1.30 (m, 4H, CH3C2C2CH2CH2), 1.27–1.23 (m, 9H, Si(OCH2C3)3), 0.89 (t, 3H, 3JHH = 7 Hz, C3CH2CH2CH2CH2). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 107.5 (Si(OCH2CH3)3), 75.9 (C6H5), 58.9 (Si(OH2CH3)3), 31.0 (CH3CH2CH2CH2H2), 27.9 (CH3CH2CH2H2CH2), 22.2 (CH3CH2H2CH2CH2), 19.6 (CH3H2CH2CH2CH2), 18.1 (Si(OCH2H3)3), 14.0 (H3CH2CH2CH2CH2). GC–MS (ESI): 243 [M – CH3]+.

Triethoxy((4-fluorophenyl)ethynyl)silane (10)

1-Ethynyl-4-fluorobenzene (1.00 g, 8.32 mmol), EtMgBr (2.0 M in THF, 3.5 mL, 6.93 mmol), Si(OEt)4 (2.8 mL, 12.5 mmol). Yellow liquid; yield (0.507 g, 26%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.51–7.46 (m, 2H, 4-FC64), 7.02–6.97 (m, 2H, 4-FC64), 3.92 (q, 6H, 3JHH = 7 Hz, Si(OC2CH3)3), 1.31 (t, 9H, 3JHH = 7 Hz, Si(OCH2C3)3). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 162.9 (d, 1JCF = 248 Hz, 4-F6H4), 134.2 (4-F6H4), 134.1 (4-F6H4), 118.4 (C6H5), 115.8 (d, 2JCF = 22 Hz, 4-F6H4), 82.7 (Si(OCH2CH3)3), 59.2 (Si(OH2CH3)3), 18.2 (Si(OCH2H3)3). GC–MS (ESI): 282 [M]+.

((4-Chlorophenyl)ethynyl)triethoxysilane (11)

1-Chloro-4-ethynylbenzene (1.00 g, 7.32 mmol), EtMgBr (2.0 M in THF, 3.1 mL, 6.10 mmol), Si(OEt)4 (2.5 mL, 10.98 mmol). Yellow liquid; yield (0.602 g, 33%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.51–7.42 (m, 2H, 4-ClC64), 7.33–7.28 (m, 2H, 4-ClC64), 3.87 (q, 6H, 3JHH = 8 Hz, Si(OC2CH3)3), 1.26 (t, 9H, 3JHH = 8 Hz, Si(OCH2C3)3). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 135.1 (4-Cl6H4), 133.5 (4-Cl6H4), 128.8 (4-Cl6H4), 121.3 (4-Cl6H4), 105.6 (Si(OCH2CH3)3), 90.0 (C6H5), 59.3 (Si(OH2CH3)3), 18.2 (Si(OCH2H3)3). GC–MS (ESI): 298 [M]+.

((4-Bromophenyl)ethynyl)triethoxysilane (12)

1-Bromo-4-ethynylbenzene (1.00 g, 5.52 mmol), EtMgBr (2.0 M in THF, 2.3 mL, 4.60 mmol), Si(OEt)4 (1.9 mL, 8.28 mmol). Yellow liquid; yield (0.436 g, 27%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.45 (d, 2H, 3JHH = 8 Hz 4-BrC64), 7.34 (d, 2H, 3JHH = 8 Hz, 4-BrC64), 3.85 (q, 6H, 3JHH = 7 Hz, Si(OC2CH3)3), 1.24 (t, 9H, 3JHH = 7 Hz, Si(OCH2C3)3). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 133.7 (4-Br6H4), 131.7 (4-Br6H4), 128.5 (4-Br6H4), 123.3 (4-Br6H4), 105.9 (Si(OCH2CH3)3), 90.7 (C6H5), 59.3 (Si(OH2CH3)3), 18.2 (Si(OCH2H3)3). GC–MS (ESI): 343 [M]+.

Triethoxy(naphthalen-1-ylethynyl)silane (13)

A mixture of 1-ethynylnaphthalene (1.00 g, 6.57 mmol) and EtMgBr (2.0 M in THF, 16.4 mL, 32.9 mmol) in Et2O (ca. 30 mL) was added at room temperature and refluxed for 2 h. The reaction mixture was cooled to room temperature, and Si(OEt)4 (2.6 mL, 11.8 mmol) was added. The reaction mixture again refluxed further for 12 h. The resulting mixture was filtered, and the volatiles were removed under reduced pressure. The crude product was then purified by fractional distillation under reduced pressure at ambient temperature by a Kugelrohr short path distillation apparatus at the temperature range of 40–60 °C.

Yellow liquid; yield (0.283 g, 14%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.36 (d, 1H, 3JHH = 8 Hz, C107), 7.85 (d, 2H, 3JHH = 8 Hz, C107), 7.74 (d, 1H, 3JHH = 7 Hz, C107), 7.59 (t, 1H, 3JHH = 7 Hz, C107), 7.52 (t, 1H, 3JHH = 7 Hz, C107), 7.42 (t, 1H, 3JHH = 7 Hz, C107), 3.85 (q, 6H, 3JHH = 7 Hz, Si(OC2CH3)3), 1.24 (t, 9H, 3JHH = 7 Hz, Si(OCH2C3)3). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 133.7 (10H7), 133.2 (10H7), 131.4 (10H7), 129.5 (10H7), 128.4 (10H7), 127.1 (10H7), 126.6 (10H7), 126.4 (10H7), 125.2 (10H7), 120.6 (10H7), 104.9 (Si(OCH2CH3)3), 94.4 (C6H5), 59.3 (Si(OH2CH3)3), 18.3 (Si(OCH2H3)3). GC–MS (ESI): 314 [M]+.

General Procedure for the Csp–Csp Hiyama Alkynylation Reaction of Iodobenzene with Triethoxysilylalkyne

In a typical catalysis run, performed in air, a 25 mL round-bottom flask was charged with a mixture of n class="Chemical">iodobenzene, triethoxysilylalkyne, and NaOH, in the molar ratio of 1:1.2:3. Palladium complex 4 (2 mol %) was added to the mixture, followed by 6 mL solvent (dioxane/H2O, 4:2 v/v), and the reaction mixture was heated at 80 °C for 4 h. The reaction mixture was cooled to room temperature, and water (ca. 12 mL) was added. The resultant mixture was extracted with EtOAc (ca. 50 mL). The aqueous layer was further extracted with EtOAc (ca. 3 × 20 mL). The organic layers were combined and vacuum-dried to obtain a crude product that was subsequently purified by column chromatography using silica gel as a stationary phase and eluting it with mixed medium of petroleum ether/EtOAc to give the desired product.

Procedure for the Mercury (Hg) Drop Test

A 25 mL round-bottom flask was charged with a mixture of the iodobenzene, n class="Chemical">triethoxysilylalkyne, and NaOH in the molar ratio of 1:1.2:3. Palladium complex 4 (2 mol %) and excess Hg were added to the mixture followed by 6 mL solvent (dioxane/H2O, 4:2 v/v), and the reaction mixture was heated at 80 °C for 4 h. The reaction mixture was cooled to room temperature, and water (ca. 12 mL) was added. The resultant mixture was extracted with EtOAc (ca. 50 mL). The aqueous layer was further extracted with EtOAc (ca. 3 × 20 mL). The organic layers were combined and vacuum-dried to obtain a crude product that was subsequently purified by column chromatography using silica gel as a stationary phase and eluting it with mixed medium of petroleum ether/EtOAc to give the desired product.

1,2-Diphenylethyne (14)

Triethoxy(phenylethynyl)silane (6) (0.317 g, 1.20 mmol), iodobenzene (0.204 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield (0.136 g, 76%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.56–7.54 (m, 2H, C65), 7.38–7.34 (m, 3H, C65). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 131.7 (6H5), 128.5 (6H5), 128.4 (6H5), 123.4 (6H5), 89.5 (C6H5). Anal. Calcd for C14H10: C, 94.34; H, 5.66%. Found: C, 94.57; H, 5.43%. GC–MS (ESI): 178 [M]+.

1-Methyl-4-(phenylethynyl)benzene (15)

Triethoxy(p-tolylethynyl)silane (7) (0.333 g, 1.20 mmol), iodobenzene (0.204 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield (0.131 g, 68%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.53 (d, 2H, 3JHH = 8 Hz, C65), 7.44 (d, 2H, 3JHH = 8 Hz, 4-CH3C64), 7.37–7.33 (m, 3H, C65), 7.17 (d, 2H, 3JHH = 8 Hz, 4-CH3C64), 2.38 (s, 3H, 4-H3C6H4). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 138.5 (4-CH36H4), 131.7 (4-CH36H4), 131.6 (6H5), 129.3 (6H5), 128.5 (6H5), 128.2 (6H5), 123.6 (4-CH36H4), 120.3 (4-CH36H4), 89.7 (4-CH3C6H4), 88.8 (C6H5), 21.6 (4-H3C6H4). Anal. Calcd for C15H12: C, 93.71; H, 6.29%. Found: C, 93.61; H, 6.33%. GC–MS (ESI): 192 [M]+.

Hex-1-yn-1-ylbenzene (16)

Triethoxy(n class="Gene">hex-1-yn-1-yl)silane (8) (0.293 g, 1.20 mmol), iodobenzene (0.204 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). Colorless oil; yield (0.064 g, 41%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.40–7.38 (m, 2H, C65), 7.28–7.26 (m, 3H, C65), 2.41 (t, 2H, 3JHH = 7 Hz, CH3CH2CH2C2), 1.60 (quint, 2H, 3JHH = 7 Hz, CH3CH2C2CH2), 1.47 (quint, 2H, 3JHH = 7 Hz, CH3C2CH2CH2), 0.95 (t, 3H, 3JHH = 7 Hz, C3CH2CH2CH2). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 131.7 (6H5), 128.3 (6H5), 127.6 (6H5), 124.2 (6H5), 90.5 (C6H5), 80.6 (CH3CH2CH2CH2), 31.0 (CH3CH2CH2H2), 22.2 (CH3CH2H2CH2), 19.2 (CH3H2CH2CH2), 13.8 (H3CH2CH2CH2). Anal. Calcd for C12H14: C, 91.08; H, 8.92%. Found: C, 91.15; H, 8.60%. GC–MS (ESI): 158 [M]+.

Hept-1-yn-1-ylbenzene (17)

Triethoxy(n class="Gene">hept-1-yn-1-yl)silane (9) (0.3101 g, 1.20 mmol), iodobenzene (0.204 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). Colorless oil; yield (0.061 g, 35%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.40–7.39 (m, 2H, C65), 7.28–7.26 (m, 3H, C65), 2.41 (t, 2H, 3JHH = 7 Hz, CH3CH2CH2CH2C2), 1.62 (quint, 2H, 3JHH = 7 Hz, CH3CH2CH2C2CH2), 1.44 (quint, 2H, 3JHH = 7 Hz, CH3CH2C2CH2CH2), 1.37 (quint, 2H, 3JHH = 7 Hz, CH3C2CH2CH2CH2), 0.93 (t, 3H, 3JHH = 7 Hz, C3CH2CH2CH2CH2). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 131.7 (6H5), 128.3 (6H5), 127.6 (6H5), 124.3 (6H5), 90.6 (C6H5), 80.7 (CH3CH2CH2CH2CH2), 31.3 (CH3CH2CH2CH2H2), 28.6 (CH3CH2CH2H2CH2), 22.4 (CH3CH2H2CH2CH2), 19.5 (CH3H2CH2CH2CH2), 14.1 (H3CH2CH2CH2CH2). Anal. Calcd for C13H16: C, 90.64; H, 9.36%. Found: C, 90.59; H, 9.48%. GC–MS (ESI): 172 [M]+.

1-Fluoro-4-(phenylethynyl)benzene (18)

Triethoxy((4-fluorophenyl)ethynyl)silane (10) (0.338 g, 1.20 mmol), iodobenzene (0.204 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield (0.133 g, 68%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.53–7.50 (m, 4H, 4-FC64), 7.37–7.33 (m, 3H, C65), 7.05 (t, 2H, 3JHH = 8 Hz, C65). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 162.6 (d, 1JCF = 247 Hz, 4-F6H4), 133.6 (d, 3JCF = 8.75 Hz, 4-F6H4), 131.7 (6H5), 128.5 (6H5), 128.4 (6H5), 123.2 (6H5), 119.5 (4-F6H4), 115.9 (d, 2JCF = 22 Hz, 4-F6H4), 89.2 (4-FC6H4), 88.4 (C6H5). Anal. Calcd for C14H9F: C, 85.69; H, 4.62%. Found: C, 85.52; H, 4.32%. GC–MS (ESI): 196 [M]+.

1-Chloro-4-(phenylethynyl)benzene (19)

((4-Chlorophenyl)ethynyl)triethoxysilane (11) (0.358 g, 1.20 mmol), iodobenzene (0.204 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield (0.103 g, 48%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.54–7.52 (m, 2H, 4-ClC64), 7.46 (d, 2H, 3JHH = 8 Hz, 4-ClC64), 7.36–7.34 (m, 4H, C65), 7.32 (br, 1H, C65). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 134.4 (4-Cl6H4), 132.9 (6H5), 131.7 (6H5), 128.8 (6H5), 128.6 (4-Cl6H4), 128.5 (6H5), 123.0 (4-Cl6H4), 121.9 (4-Cl6H4), 90.4 (4-ClC6H4), 88.4 (C6H5). Anal. Calcd for C14H9Cl: C, 79.07; H, 4.27%. Found: C, 78.80; H, 4.56%. GC–MS (ESI): 212 [M]+.

1-Bromo-4-(phenylethynyl)benzene (20)

((4-Bromophenyl)ethynyl)triethoxysilane (12) (0.412 g, 1.20 mmol), iodobenzene (0.204 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield (0.149 g, 58%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.54–7.52 (m, 2H, 4-BrC64), 7.48 (d, 2H, 3JHH = 8 Hz, 4-BrC64), 7.39 (d, 2H, 3JHH = 8 Hz, C65), 7.36–7.34 (m, 3H, C65). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 133.1 (4-Br6H4), 131.7 (4-Br6H4), 131.7 (4-Br6H4), 128.6 (6H5), 128.5 (4-Br6H4), 123.0 (6H5), 122.6 (6H5), 122.3 (6H5), 90.6 (4-BrC6H4), 88.4 (C6H5). Anal. Calcd for C14H9Br: C, 65.40; H, 3.53%. Found: C, 65.18; H, 3.56%. GC–MS (ESI): 257 [M]+.

1-(Phenylethynyl)naphthalene (21)

Triethoxy(naphthalen-1-ylethynyl)silane (13) (0.377 g, 1.20 mmol), iodobenzene (0.204 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). Colorless oil; yield (0.092 g, 40%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 8.47 (d, 1H, 3JHH = 8 Hz, C107), 7.88 (d, 1H, 3JHH = 8 Hz, C107), 7.86 (d, 1H, 3JHH = 8 Hz, C107), 7.79 (d, 1H, 3JHH = 7 Hz, C107), 7.67 (d, 2H, 3JHH = 8 Hz, C65), 7.62 (t, 1H, 3JHH = 8 Hz, C107), 7.56 (t, 1H, 3JHH = 8 Hz, C107), 7.49 (t, 1H, 3JHH = 8 Hz, C107), 7.47–7.43 (m, 3H, C65). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 133.4 (6H5), 133.3 (6H5), 131.8 {2(10H7)}, 130.5 (10H7), 128.9 (10H7), 128.6 {2(10H7)}, 128.5 (6H5), 128.4 (10H7), 126.9 (10H7), 126.6 (10H7), 126.4 (10H7), 125.4 (6H5), 123.5 (6H5), 121.0 (6H5), 94.4 (C10H7), 86.7 (C6H5). Anal. Calcd for C18H12: C, 94.70; H, 5.30%. Found: C, 94.92; H, 4.96%. GC–MS (ESI): 228 [M]+.

General Procedure for the Csp–Csp Hiyama Alkynylation/Cyclization Reaction of 2-Iodophenol and Triethoxysilylalkyne

In a typical catalysis run, performed in air, a 25 mL round-bottom flask was charged with a mixture of n class="Chemical">2-iodophenol, a triethoxysilylalkyne, and NaOH in the molar ratio of 1:1.2:3. Palladium complex 4 (2 mol %) was added to the mixture, followed by 6 mL solvent (dioxane/H2O, 4:2 v/v), and the reaction mixture was heated at 80 °C for 4 h. The reaction mixture was cooled to room temperature, and water (ca. 12 mL) was added. The resultant mixture was extracted with EtOAc (ca. 50 mL). The aqueous layer was further extracted with EtOAc (ca. 3 × 20 mL). The organic layers were combined and vacuum-dried to obtain a crude product that was subsequently purified by column chromatography using silica gel as a stationary phase and eluting it with mixed medium of petroleum ether/EtOAc to give the desired product.

A 25 mL round-bottom flask was charged with a mixture of 2-iodophenol, n class="Chemical">triethoxysilylalkyne, and NaOH in the molar ratio of 1:1.2:3. Palladium complex 4 (2 mol %) and excess Hg were added to the mixture followed by 6 mL solvent (dioxane/H2O, 4:2 v/v), and the reaction mixture was heated at 80 °C for 4 h. The reaction mixture was cooled to room temperature, and water (ca. 12 mL) was added. The resultant mixture was extracted with EtOAc (ca. 50 mL). The aqueous layer was further extracted with EtOAc (ca. 3 × 20 mL). The organic layers were combined and vacuum-dried to obtain a crude product that was subsequently purified by column chromatography using silica gel as a stationary phase and eluting it with mixed medium of petroleum ether/EtOAc to give the desired product.

2-Phenylbenzofuran (23)

Triethoxy(phenylethynyl)silane (6) (0.317 g, 1.20 mmol), iodophenol (0.220 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield (0.110 g, 57%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.88 (d, 2H, 3JHH = 8 Hz, C65), 7.59 (d, 1H, 3JHH = 8 Hz, C85O), 7.54 (d, 1H, 3JHH = 8 Hz, C85O), 7.46 (t, 2H, 3JHH = 8 Hz, C85O), 7.37 (t, 1H, 3JHH = 8 Hz, C65), 7.29 (t, 1H, 3JHH = 8 Hz, C65), 7.24 (t, 1H, 3JHH = 7 Hz, C65), 7.04 (s, 1H, C85O). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 156.1 (8H5O), 155.0 (8H5O), 130.6 (8H5O), 129.4 (6H5), 128.9 (6H5), 128.7 (8H5O), 125.1 (6H5), 124.4 (8H5O), 123.1 (8H5O), 121.0 (6H5), 111.3 (8H5O), 101.4 (8H5O). GC–MS (ESI): 194 [M]+. Anal. Calcd for C14H10O: C, 86.57; H, 5.19%. Found: C, 86.71; H, 4.91%. GC–MS (ESI): 194 [M]+.

2-(p-Tolyl)benzofuran (24)

Triethoxy(p-tolylethynyl)silane (7) (0.333 g, 1.20 mmol), iodophenol (0.220 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield: (0.079 g, 38%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.76 (d, 2H, 3JHH = 8 Hz, 4-CH3C64), 7.58 (d, 1H, 3JHH = 8 Hz, C85O), 7.51 (d, 1H, 3JHH = 8 Hz, C85O), 7.27 (d, 2H, 3JHH = 8 Hz, 4-CH3C64), 7.21 (t, 2H, 3JHH = 8 Hz, C85O), 6.97 (s, 1H, C85O), 2.40 (s, 3H, 4-C3C6H4). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 156.3 (8H5O), 154.9 (8H5O), 138.7 (8H5O), 129.6 (4-CH36H4), 129.4 (8H5O), 127.9 (4-CH36H4), 125.0 (8H5O), 124.1 (4-CH36H4), 123.0 (4-CH36H4), 120.9 (8H5O), 111.2 (8H5O), 100.7 (8H5O), 21.5 (4-H3C6H4). Anal. Calcd for C15H12O: C, 86.51; H, 5.81%. Found: C, 86.68; H, 5.95%. GC–MS (ESI): 208 [M]+.

2-Butylbenzofuran (25)

Triethoxy(hex-1-yn-1-yl)silane (8) (0.293 g, 1.20 mmol), iodophenol (0.220 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). Colorless oil; yield: (0.041 g, 24%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.48 (d, 1H, 3JHH = 7 Hz, C85O), 7.42 (d, 1H, 3JHH = 9 Hz, C85O), 7.22–7.16 (m, 2H, C85O), 6.34 (s, 1H, C85O), 2.77 (t, 2H, 3JHH = 7 Hz, CH3CH2CH2C2), 1.75 (quint, 2H, 3JHH = 7 Hz, CH3CH2C2CH2), 1.44 (quint, 2H, 3JHH = 7 Hz, CH3C2CH2CH2), 0.96 (t, 3H, 3JHH = 7 Hz, C3CH2CH2CH2). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 159.9 (8H5O), 154.7 (8H5O), 129.2 (8H5O), 123.1 (8H5O), 122.5 (8H5O), 120.3 (8H5O), 110.8 (8H5O), 101.9 (8H5O), 29.9 (CH3CH2CH2H2), 28.3 (CH3CH2H2CH2), 22.4 (CH3H2CH2CH2), 13.9 (H3CH2CH2CH2). Anal. Calcd for C12H14O: C, 82.72; H, 8.10%. Found: C, 82.90; H, 8.44%. GC–MS (ESI): 174 [M]+.

2-Pentylbenzofuran (26)

Triethoxy(n class="Gene">hept-1-yn-1-yl)silane (9) (0.310 g, 1.20 mmol), iodophenol (0.220 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). Colorless oil; yield: (0.026 g, 14%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.48 (d, 1H, 3JHH = 7 Hz, C85O), 7.41 (d, 1H, 3JHH = 7 Hz, C85O), 7.21–7.14 (m, 2H, C85O), 6.37 (s, 1H, C85O), 2.76 (t, 2H, 3JHH = 7 Hz, CH3CH2CH2CH2C2), 1.75 (quint, 2H, 3JHH = 7 Hz, CH3CH2C2CH2CH2), 1.39–1.34 (m, 4H, CH3C2CH2CH2CH2), 0.91 (t, 3H, 3JHH = 7 Hz, C3CH2CH2CH2CH2). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 159.9 (8H5O), 154.7 (8H5O), 129.2 (8H5O), 123.1 (8H5O), 122.5 (8H5O), 120.3 (8H5O), 110.8 (8H5O), 101.9 (8H5O), 31.5 (CH3CH2CH2CH2H2), 28.6 (CH3CH2CH2H2CH2), 27.5 (CH3CH2H2CH2CH2), 22.6 (CH3H2CH2CH2CH2), 14.1 (H3CH2CH2CH2CH2). Anal. Calcd for C13H16O: C, 82.94; H, 8.57%. Found: C, 82.64; H, 8.36%. GC–MS (ESI): 188 [M]+.

2-(4-Fluorophenyl)benzofuran (27)

Triethoxy((4-fluorophenyl)ethynyl)silane (10) (0.338 g, 1.20 mmol), iodophenol (0.220 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield: (0.083 g, 39%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.86–7.83 (m, 2H, 4-FC64), 7.58 (d, 1H, 3JHH = 8 Hz, C85O), 7.52 (d, 1H, 3JHH = 8 Hz, C85O), 7.28 (t, 1H, 3JHH = 8 Hz, C85O), 7.23 (t, 1H, 3JHH = 8 Hz, C85O), 7.14 (t, 2H, 3JHH = 8 Hz, 4-FC64), 6.96 (s, 1H, C85O). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 163.0 (d, 1JCF = 247 Hz, 4-F6H4), 155.1 (8H5O), 155.0 (8H5O), 134.7 (d, 3JCF = 9 Hz, 4-F6H4), 126.9 (4-F6H4), 126.8 (8H5O), 124.4 (8H5O), 123.2 (8H5O), 121.0 (8H5O), 116.0 (d, 2JCF = 22 Hz, 4-F6H4), 111.3 (8H5O), 101.2 (8H5O). Anal. Calcd for C14H9FO: C, 79.23; H, 4.27%. Found: C, 78.96; H, 3.90%. GC–MS (ESI): 212 [M]+.

2-(4-Chlorophenyl)benzofuran (28)

((4-Chlorophenyl)ethynyl)triethoxysilane (11) (0.358 g, 1.20 mmol), iodophenol (0.220 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield: (0.094 g, 41%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.79 (d, 2H, 3JHH = 9 Hz, 4-ClC64), 7.59 (d, 1H, 3JHH = 8 Hz, C85O), 7.51 (d, 1H, 3JHH = 8 Hz, C85O), 7.42 (d, 2H, 3JHH = 9 Hz, 4-ClC64), 7.30 (t, 1H, 3JHH = 7 Hz, C85O), 7.23 (t, 1H, 3JHH = 7 Hz, C85O), 7.01 (s, 1H, C85O). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 155.0 (8H5O), 154.9 (8H5O), 134.4 (4-Cl6H4), 129.2 (8H5O), 129.2 (4-Cl6H4), 126.3 {2(4-Cl6H4)}, 124.7 (8H5O), 123.2 (8H5O), 121.1 (8H5O), 111.3 (8H5O), 101.9 (8H5O). Anal. Calcd for C14H9ClO: C, 73.53; H, 3.97%. Found: C, 73.76; H, 3.63%. GC–MS (ESI): 228 [M]+.

2-(4-Bromophenyl)benzofuran (29)

((4-Bromophenyl)ethynyl)triethoxysilane (12) (0.412 g, 1.20 mmol), iodophenol (0.220 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield: (0.117 g, 43%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.73 (d, 2H, 3JHH = 7 Hz, 4-BrC64), 7.59–7.56 (m, 3H, C85O, 4-BrC64), 7.51 (d, 1H, 3JHH = 8 Hz, C85O), 7.30 (t, 1H, 3JHH = 8 Hz, C85O), 7.23 (t, 1H, 3JHH = 8 Hz, C85O), 7.03 (s, 1H, C85O). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 155.1 (8H5O), 154.9 (8H5O), 132.1 (4-Br6H4), 129.6 (8H5O), 129.2 (4-Br6H4), 126.5 (4-Br6H4), 124.7 (8H5O), 123.2 (8H5O), 122.6 (4-Br6H4), 121.2 (8H5O), 111.3 (8H5O), 102.0 (8H5O). Anal. Calcd for C14H9BrO: C, 61.57; H, 3.32%. Found: C, 61.77; H, 3.64%. GC–MS (ESI): 273 [M]+.

2-(Naphthalen-1-yl)benzofuran (30)

Triethoxy(naphthalen-1-ylethynyl)silane (13) (0.377 g, 1.20 mmol), iodophenol (0.220 g, 1.00 mmol), NaOH (0.120 g, 3.00 mmol), catalyst (4) (1.11 × 10–3 g, 0.02 mmol). White solid; yield: (0.066 g, 27%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.51 (d, 1H, 3JHH = 8 Hz, C85O), 7.96–7.91 (m, 3H, C107), 7.70 (d, 1H, 3JHH = 8 Hz, C85O), 7.64–7.56 (m, 4H, C107), 7.39–7.30 (m, 2H, C85O), 7.11 (s, 1H, C85O). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 155.7 (8H5O), 155.1 (8H5O), 134.1 (8H5O), 130.8 (8H5O), 129.7 (10H7), 129.2 (8H5O), 128.7 (10H7), 128.4 (8H5O), 127.4 (10H7), 127.0 (10H7), 126.3 (10H7), 125.6 (10H7), 125.4 (10H7), 124.5 (10H7), 123.1 (10H7), 121.1 (10H7), 111.4 (8H5O), 106.1 (8H5O). Anal. Calcd for C18H12O: C, 88.50; H, 4.95%. Found: C, 88.58; H, 4.54%. GC–MS (ESI): 244 [M]+.

Synthesis of 2-Phenylbenzofuran (23) from 2-(Phenylethynyl)phenol

A 25 mL round-bottom flask was charged with a mixture of 2-(phenylethynyl)phenol (0.194 g, 1.00 mmol), n class="Chemical">NaOH (0.120 g, 3.00 mmol), and catalyst 4 (1.11 × 10–3 g, 0.02 mmol) in the mixed medium of dioxane/H2O as the solvent (ca. 6 mL, 4:2 v/v). The reaction mixture was heated at 80 °C for 4 h. The reaction mixture was cooled to room temperature, and water (ca. 12 mL) was added. The resultant mixture was extracted with EtOAc (ca. 50 mL). The aqueous layer was further extracted with EtOAc (ca. 3 × 20 mL). The organic layers were combined and vacuum-dried to obtain a crude product that was subsequently purified by column chromatography using silica gel as a stationary phase and eluting it with mixed medium of petroleum ether/EtOAc to give the desired product as a white solid; yield (0.157 g, 81%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 7.88 (d, 2H, 3JHH = 8 Hz, C65), 7.59 (d, 1H, 3JHH = 8 Hz, C85O), 7.54 (d, 1H, 3JHH = 8 Hz, C85O), 7.46 (t, 2H, 3JHH = 8 Hz, C85O), 7.36 (t, 1H, 3JHH = 7 Hz, C65), 7.29 (t, 1H, 3JHH = 7 Hz, C65), 7.24 (t, 1H, 3JHH = 8 Hz, C65), 7.04 (s, 1H, C85O). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): δ 156.1 (8H5O), 155.0 (8H5O), 130.6 (8H5O), 129.3 (6H5), 128.9 (6H5), 128.7 (8H5O), 125.1 (6H5), 124.4 (8H5O), 123.1 (8H5O), 121.0 (6H5), 111.3 (8H5O), 101.4 (8H5O). GC–MS (ESI): 194 [M]+.