Rhodium-catalyzed linear codimerization and cycloaddition of ketenes with alkynes.

A novel rhodium-catalyzed linear codimerization of alkyl phenyl ketenes with internal alkynes to dienones and a novel synthesis of furans by an unusual cycloaddition of diaryl ketenes with internal alkynes have been developed. These reactions proceed smoothly with the same rhodium catalyst, RhCl(PPh(3))(3), and are highly dependent on the structure and reactivity of the starting ketenes.

1. Introduction

Ketenes are very important intermediates in the field of organic synthesis [1,2,3], and much attention has been focused on the ketene-metal complexes [4]. In general, ketenes coordinate to transition-metal complexes in two ways: 1) coordination through a C=C bond in ketenes [5], and 2) coordination through a C=O bond in ketenes [6,7,8,9,10]. If these coordination modes can be controlled through the selection of ketenes in transition-metal catalysis, completely different methods for the construction of novel organic molecules could be developed according to the structure and reactivity of ketenes using the same transition-metal catalyst.

We have previously developed a ruthenium-catalyzed synthesis of pyranopyrandiones by the ring-opening carbonylation of cyclopropenones [11] and a novel synthesis of 2-pyranones by the ruthenium- or rhodium catalyzed ring-opening dimerization of cyclobutenones [12], as well as a rhodium-catalyzed synthesis of 2-substituted phenols from cyclobutenones and alkenes via cleavage of a carbon-carbon bond [13]. We propose that (η4-bisketene)- and (η4-vinylketene)metal complexes are important key intermediates in these reactions; however, there are still few examples of transition-metal complex-catalyzed transformations of ketenes themselves [14,15,16,17,18,19,20]. Thus, we focused our attention on the development of novel reactions of ketenes with unsaturated compounds in the presence of ruthenium or rhodium catalysts, and recently developed rhodium-catalyzed decarbonylative coupling reactions of diphenyl ketene with 2-norbornenes and electron-deficient alkenes [21]. Then, the reactions of ketenes with alkynes were investigated in the presence of several transition-metal catalysts. After many trials, we developed the novel RhCl(PPh3)3-catalyzed linear codimerization of alkyl phenyl ketenes with internal alkynes and a novel synthesis of furans by the unusual RhCl(PPh3)3-catalyzed cycloaddition of diaryl ketenes with internal alkynes. In these reactions, the catalyst is the same but the products are completely different, depending on the structure and reactivity of the starting ketenes.

2. Results and Discussion

Treatment of alkyl phenyl ketenes 1a-c with internal alkynes 2 in the presence of 5 mol % RhCl(PPh3)3 in mesitylene at 120 ºC for 12 h under an argon atmosphere gave the corresponding dienones 3 in high yield with high stereoselectivity (Scheme 1). For example, RhCl(PPh3)3-catalyzed reaction of ethyl phenyl ketene (1a) with 3-hexyne (2a) gave only (2Z, 5E)-5-ethyl-3-phenylocta-2,5-dien-4-one (3a) in 92% yield, and no stereoisomers were obtained at all.

Scheme 1

Rhodium-catalyzed linear codimerization of ketenes with alkynes to dienones.

First, the catalytic activities of several transition-metal complexes were examined in the reaction of 1a with 2a, and the results are summarized in Table 1. Among the catalysts examined, RhCl(PPh3)3 (3a, 92%) showed the highest catalytic activity. RhCl(CO)(PPh3)2 (3a, 46%) and RhCl3.3H2O (3a, 26%) also showed moderate catalytic activity; however, other rhodium complexes, such as RhH(PPh3)4 and RhH(CO)(PPh3)3, as well as ruthenium complexes, such as RuCl2(PPh3)3, [RuCl2(CO)3]2, and RuH2(PPh3)4, and an iridium complex, IrCl(CO)(PPh3)2, were totally ineffective, whereas Pd(PPh3)4 showed slight catalytic activity (3a, 29%). An attempt to reduce the amount of RhCl(PPh3)3 catalyst from 5.0 mol % to 2.0 mol % resulted in vain (Entry 2).

Table 1

Catalytic activity of several transition-metal complexes in the reaction of 1a with 2a to 3a.

Entry	Catalyst	Yield of 3a (%) a	Entry	Catalyst	Yield of 3a (%) a
1	RhCl(PPh3)3­	92	7	RuCl2(PPh3)3	1
2 b	RhCl(PPh3)3	5	8 c	[RuCl2(CO)3]2	0
3	RhCl(CO)(PPh3)2	46	9	RuH2(PPh3)4	0
4	RhCl3·3H2O	26	10	Pd(PPh3)4	29
5	RhH(PPh3)4	2	11	IrCl(CO)(PPh3)2	0
6	RhH(CO)(PPh3)3	0
a GLC yield; b RhCl(PPh3)3 (2.0 mol %, 0.020 mmol) for 40 h; c [RuCl2(CO)3]2 (0.025 mmol).

The results obtained in the reaction of several alkyl phenyl ketenes 1a-c with alkynes 2a and b under the optimized reaction conditions are summarized in Table 2. Ethyl phenyl ketene (1a) reacted with 6-dodecyne (2b) to give the corresponding dienone 3b in an isolated yield of 68% (Entry 2). As for ketenes, cycloalkyl phenyl ketenes, such as 1b and 1c, also reacted with 2a to give the corresponding dienones, 3c and 3d, in isolated yields of 52% and 40%, respectively (Entries 3 and 4). Unfortunately, when terminal alkynes, such as phenylacetylene and 1-hexyne, were used in RhCl(PPh3)3-catalyzed reaction with ethyl phenyl ketene (1a), the corresponding dienones were obtained in low yield (below 10%) together with various byproducts, probably due to the formation of a (vinylidene)Rh species.

Table 2

RhCl(PPh3)3-catalyzed linear codimerization of alkyl phenylketenes 1a-c with internal alkynes 2a,b to dienones 3a-d.a

Entry	Ketene		Alkyne	Product		Isolated Yield (%)
1		1a			3a	74 (92) b
			2a
2	1a				3b	68
			2b
3		1b	2a		3c	52
4		1c	2a		3d	40
a Ketene (1.0 mmol), alkyne (3.0 mmol), RhCl(PPh3)3 (0.050 mmol), and mesitylene (1.0 mL) at 120 ºC for 12 h under an argon atmosphere; b GLC yield.

In sharp contrast, treatment of diaryl ketenes 1d and e instead of alkyl phenyl ketenes 1a-c with internal alkynes 2 in the presence of the same RhCl(PPh3)3 catalyst (5 mol %) in mesitylene at 120 ºC for 12 h under an argon atmosphere gave unusual cycloadducts, the furans 4, instead of dienones 3, in good to high yields (Scheme 2). The structure of furan 4a was confirmed by 13C Inadequate NMR measurement (see Experimental, Figure 2).

Scheme 2

Rhodium-catalyzed cycloaddition of ketenes with alkynes to furans.

Figure 2

13C Inadequate NMR spectrum of 4a.

The catalytic activities of several transition-metal complexes were also examined in the reaction of diphenyl ketene (1d) with 3-hexyne (2a), and the results are summarized in Table 3. Among the catalysts examined, only RhCl(PPh3)3 showed catalytic activity (4a, 74%). Other rhodium, ruthenium, iridium and palladium complexes were totally ineffective in the present reaction.

Table 3

Catalytic activity of several trasition-metal complexes in the reaction of 1d with 2a to 4a.

Entry	Catalyst	Yield of 4a (%) a	Entry	Catalyst	Yield of 4a (%) a
1	RhCl(PPh3)3­	74	7	[Cp*RuCl2]2	0
2	RhCl(CO)(PPh3)2	2	8	RuCl2(PPh3)3	0
3	RhCl3·3H2O	0	9	[RuCl2(CO)3]2	0
4	[RhCl(cod)]2	0	10	IrCl(CO)(PPh3)2	0
5	[RhCl(C2H4)2]2	0	11	Pd(PPh3)4	0
6	RhH(PPh3)4	0
a GLC yield.

6-Dodecyne (2b), as well as 4-octyne (2c) and 5-decyne (2d), reacted with 1d to give the corresponding furans 4b-d in moderate yields (Entries 2-4 in Table 4). As for ketenes, di(4-chloro-phenyl) ketene (1e) also reacted with 2a to give the corresponding furan 4e in an isolated yield of 51% (Entry 5).

Table 4

RhCl(PPh3)3-catalyzed unusual cycloaddition of diaryl ketenes 1d, e with internal alkynes 2a-d to furans 4a-e.a

Entry	Ketene			Alkyne	Product		Isolated Yield (%)
1		1d				4a	64 (74) b
				2a
2	1d					4b	43
				2b
3	1d					4c	52
				2c
4	1d					4d	43
				2d
5			1e	2a		4e	51 (70)b
a Ketene (1.0 mmol), alkyne (3.0 mmol), RhCl(PPh3)3 (0.050 mmol), and mesitylene (1.0 mL) at 120 ºC for 12 h under an argon atmosphere; b GLC yield.

While the reaction mechanism is not yet clear, the possible mechanisms are illustrated in Scheme 3 and Scheme 4. Scheme 3 shows a possible mechanism of the reaction of alkyl phenyl ketenes 1a-c with internal alkynes 2 to give dienones 3. We now believe that the initial step is the coordination of alkyl phenyl ketenes 1 to an active rhodium center through a C=C bond in ketenes. Oxidative cyclization of alkyl phenyl ketenes 1a-c with alkynes 2 would give rhodacyclopentenone intermediates I [5]. Stereoselective β-hydrogen elimination, followed by reductive elimination, would give the corresponding dienones 3 stereoselectively. In addition, we now consider that a catalytically active species is a Rh(I) bearing a chloro ligand, and RhCl3.3H2O would be reduced to a Rh(I)-Cl species by crystal water to show some catalytic activity (Entry 4 in Table 1).

Scheme 3

A possible mechanism of linear codimerization of alkyl phenyl ketenes 1a-c with internal alkynes 2 to give dienones 3.

Scheme 4

A possible mechanism of the synthesis of furans 4 by unusual cycloaddition of diaryl ketenes 1d and e with internal alkynes 2.

On the other hand, a possible mechanism for the reaction of diaryl ketenes 1d and e with internal alkynes 2 to furans 4 is shown in Scheme 4. In the synthesis of furans 4, the reaction starts from the coordination of diaryl ketenes to an active rhodium center through a C=O bond in ketenes (not through a C=C bond in ketenes). Oxidative cyclization of diaryl ketenes 1d, and e with alkynes 2 gives an oxametallacycle intermediate II [22,23,24,25]. β-Hydrogen elimination and insertion of an allenyl group in an intermediate III into a Rh-H bond, followed by reductive elimination/isomerization, would give the desired furans 4.

3. Experimental

3.1. General

GLC analyses were carried out on a Shimadzu GC-18A gas chromatograph equipped with a glass column (2.8 mm i.d. × 3 m) packed with Silicone OV-17 (2% on Chromosorb W(AW-DMCS), 60–80 mesh). Recycling preparative HPLC was performed with an LC-918 (Japan Analytical Industry Co. Ltd.) equipped with JAIGEL-1H and 2H columns (GPC) using CHCl3 as an eluent. 1H-NMR spectra were recorded at 300 or 400 MHz, and 13C-NMR spectra were recorded at 75 or 100 MHz. Samples were analyzed in CDCl3, and the chemical shift values are expressed relative to Me4Si as an internal standard. IR spectra were obtained on a Nicolet Impact 410 spectrometer. Elemental analyses were performed at the Microanalytical Center of Kyoto University.

3.2. Materials

Ketenes were synthesized as described in the literature [26,27]. Alkynes were obtained commercially and purified before use by standard procedures. RhCl3.·3H2O, [RhCl(cod)]2, [Cp*RhCl2]2, [RuCl2(CO)3]2, IrCl(CO)(PPh3)2, and Pd(PPh3)4 were obtained commercially and used without further purification. RhCl(PPh3)3 [28], RhCl(CO)(PPh3)2 [29], RhH(PPh3)4 [30], RhH(CO)(PPh3)3 [31], [RhCl(C2H4)2]2 [32], RuCl2(PPh3)3 [33], and RuH2(PPh3)4[34] were prepared as described in the literature.

3.3. General procedure for the rhodium-catalyzed reaction of ketenes with alkynes to give dienones and furans

A mixture of ketene 1 (1.0 mmol), alkyne 2 (3.0 mmol), RhCl(PPh3)3 (0.050 mmol), and mesitylene (1.0 mL) was placed in a two-necked 20-mL Pyrex flask equipped with a magnetic stirring bar and a reflux condenser under a flow of argon. The reaction was carried out at 120 ºC for 12 h with stirring. After the reaction mixture was cooled, the products were analyzed by GLC and isolated by Kugelrohr distillation followed by recycling preparative HPLC.

(2Z,5E)-5-Ethyl-3-phenylocta-2,5-dien-4-one (3a). Yellow liquid; b.p. 130 ºC (3.0 mmHg, Kugelrohr); IR (cm-1) 1652 (CO); 1H-NMR (CDCl3, 300 MHz): δ 0.98 (t, 3H, J = 7.52 Hz, C8-H), 1.03 (t, 3H, J = 7.52 Hz, 5-CH2CH3), 1.72 (d, 3H, J = 7.16 Hz, C1-H), 2.24 (dq, 2H, J = 7.52 Hz, C7-H), 2.42 (q, 2H, J =7.52 Hz, 5-CH2CH3), 6.15 (q, 1H, J = 7.16 Hz, C2-H), 6.64 (t, 1H, J = 7.52 Hz, C6-H), 7.19–7.37 (m, 5H, 3-phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.09 (5-CH2CH3), 13.79 (C8), 15.33 (C1), 18.12 (5-CH2CH3), 22.22 (C7), 125.06 (C2), 125.55 (3-phenyl), 127.25 (3-phenyl), 128.48 (3-phenyl), 129.19 (C3 or C5), 142.85 (3-phenyl-C1), 145.41 (C3 or C5), 150.28 (C6), 200.61 (C4); MS (EI) m/z 228 (M+). Anal. Calcd. for C16H20O: C 84.36, H 9.00. Found: C 84.14, H 8.83. A nuclear Overhauser enhancement (NOE) study was undertaken to determine the stereochemistry of dienone 3a. Irradiation of olefinic CH at δ 6.15 ppm gave a 6.7% NOE of the phenyl group at δ 7.26–7.28 ppm, while irradiation of CH2 in ethyl group at δ 2.24 ppm showed 5.2% NOE with CH3 in the other ethyl group at δ 1.03 ppm. The stereochemistry of 3a was therefore assigned to 2Z and 5E (Figure 1). The same method was used to determine the stereochemistry of 3b-d.

Figure 1

The NOE study of 3a.

(2Z,5E)-5-Pentyl-3-phenylundeca-2,5-dien-4-one (3b). Colorless liquid; b.p. 140 ºC (1.0 mmHg, Kugelrohr); IR (cm-1): 1652 (CO) ; 1H-NMR (CDCl3, 400 MHz): δ = 0.84 (t, 3H, C11-H), 0.89 (t, 3H, 5-CH2(CH2)3CH3), 1.17–1.35(m, 12H, 5-CH2(CH2)3CH3/C8-H/C9-H/C10-H), 1.72 (d, 3H, J = 7.16 Hz, C1-H), 2.21 (dt, 2H, C7-H), 2.37 (t, 2H, 5-CH2(CH2)3CH3), 6.14 (q, 1H, J = 7.16 Hz, C2-H), 6.66 (t, 1H, C6-H), 7.18-7.31 (m, 5H, 3-phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.93, 14.04, 15.38 (C1), 22.40, 22.51, 24.97, 28.09, 28.61, 29.03, 31.89, 44.87, 125.09 (C2), 125.63 (3-phenyl), 127.14 (3-phenyl), 128.61(3-phenyl), 134.58 (C3 or C5), 141.94 (3-phenyl-1), 144.80 (C3 or C5), 149.70 (C6), 200.62 (C4); MS (EI) m/z 312 (M+).

(3E)-1-Cyclopentylidene-3-ethyl-1-phenylhex-3-en-2-one ( Yellow liquid; b.p. 140 oC (2.0 mmHg, Kugelrohr); IR (cm-1) 1650 (CO); 1H-NMR (CDCl3, 400 MHz): δ 0.96 (t, 3H, J =7.52 Hz, C6-H), 0.98 (t, 3H, J = 7.52 Hz, 3-CH2CH3), 1.61-1.71 (m, 4H, 1-cyclopentylidene-H), 2.21 (q, 2H, J = 7.52 Hz, 1-cyclopentylidene-H), 2.30-2.46 (m, 6H, 1-cyclopentylidene-H/C5-H/3-CH2CH3), 6.63 (t, 1H, J = 7.52 Hz, C4-H), 7.17-7.33 (m, 5H, 1-phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.31 (3-CH2CH3), 13.77 (C6), 18.51 (3-CH2CH3), 22.19 (C5), 26.34 (1-cyclopentylidene), 26.38 (1-cyclopentylidene), 32.18 (1-cyclopentylidene), 32.39 (1-cyclopentylidene), 126.50 (1-phenyl), 127.82 (1-phenyl), 128.06 (1-phenyl), 132.91 (1-cyclopentylidene), 138.53 (C1 or C3), 142.08 (1-phenyl-1), 146.26 (C1 or C3), 148.48 (C4), 200.16(C2); MS (EI) m/z 268 (M+).

(3E)-1-Cyclohexylidene-3-ethyl-1-phenylhex-3-en-2-one (3d). Yellow liquid; b.p. 160 oC (8.0 mmHg, Kugelrohr); IR (cm-1) 1641 (CO); 1H-NMR (CDCl3, 400 MHz): δ 0.91 (t, 3H, C6-H), 1.04 (t, 3H, 3-CH2CH3), 1.60 (br, 6H, 1-cyclohexylidene-H), 2.13 (br, 4H, 1-cyclohexylidene-H), 2.23 (br, 2H, C5-H), 2.31 (q, 2H, J = 7.32 Hz, 3-CH2CH3), 6.78 (t, 1H, J = 7.32 Hz, C4-H), 7.21–7.31 (m, 5H, 1-phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.62 (3-CH2CH3), 13.79 (C6), 18.46 (3-CH2CH3), 22.35 (C5), 26.54 (1-cyclohexylidene), 27.96 (1-cyclohexylidene), 28.36 (1-cyclohexylidene), 30.90 (1-cyclohexylidene), 32.88 (1-cyclohexylidene), 126.61 (1-phenyl), 128.77 (1-phenyl), 129.03 (1-phenyl), 133.32 (1-cyclohexylidene), 137.20 (C1 or C3), 140.50 (C1 or C3), 142.81 (1-phenyl-1), 148.96 (C4), 200.32(C2); MS (EI) m/z 282 (M+).

2-(Diphenylmethyl)-3-ethyl-5-methylfuran (4a). Yellow liquid; b.p. 135–145 ºC (0.1 mmHg, Kugelrohr); 1H-NMR (CDCl3, 400 MHz): δ 1.05 (t, 3H, J = 7.57 Hz, 3-CH2CH3), 2.20 (s, 3H, 5-CH3), 2.29 (q, 2H, J = 7.49 Hz, 3-CH2CH3), 5.40 (s, 1H, 2-CHPh2), 5.85 (s, 1H, C4-H), 7.18–7.29 (m, 10H, phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.85 (5-CH3), 15.31 (3-CH2CH3), 18.24 (3-CH2CH3), 48.43 (2-CHPh2), 107.14 (C4), 123.09 (C3), 126.16 (phenyl), 128.24 (phenyl), 128.73 (phenyl), 142.30 (phenyl-1), 147.46 (C2), 150.22 (C5); MS (EI) m/z 276 (M+). Anal. Calcd for C20H20O: C 86.92, H 7.29. Found: C 87.02, H 7.29. The relationship of the substituted group is confirmed by 13C Inadequate NMR measurement (Figure 2).

2-(Diphenylmethyl)-5-n-butyl-3-n-pentylfuran (4b). Yellow liquid; b.p. 170 ºC (0.1 mmHg, Kugelrohr); 1H-NMR (CDCl3, 400 MHz): δ 0.82 (t, 3H, J = 6.84 Hz), 0.89 (t, 3H, J = 7.32 Hz), 1.22 (m, 6H, 3-(CH2)2(CH2)2CH3/5-(CH2)2CH2CH3), 1.43 (m, 2H, 3-CH2CH2(CH2)2CH3), 1.56 (m, 2H, 5-CH2CH2CH2CH3), 2.27 (dt, 2H, J = 2.12 Hz, 7.69 Hz, 3-CH2(CH2)3CH3), 2.53 (t, 2H, J = 7.33 Hz, 5-CH2(CH2)2CH3), 5.37 (s, 1H, 2-CHPh2), 5.82 (s, 1H, C4-H), 7.18–7.29 (m, 10H, phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.99, 14.16, 22.36, 22.60, 24.97 (3-CH2(CH2)3CH3), 27.86 (5-CH2(CH2)2CH3), 30.20 (5-CH2CH2CH2CH3), 30.34 (3-CH2CH2(CH2)2CH3), 31.04, 48.38 (2-CHPh2), 106.54 (C4), 121.41 (C3), 126.09 (phenyl), 128.01 (phenyl), 128.73 (phenyl), 142.48 (phenyl-1), 147.54 (C2), 154.63 (C5); MS (EI) m/z 360 (M+).

2-(Diphenylmethyl)-5-ethyl-3-n-propylfuran (4c). Yellow liquid; b.p. 140–150 ºC (0.1 mmHg, Kugelrohr); 1H-NMR (CDCl3, 400 MHz): δ 0.85 (t, 3H, J = 7.33 Hz, 3-CH2CH2CH3), 1.17 (t, 3H, J = 7.33 Hz, 5-CH2CH3), 1.47 (sixtet, 2H, J = 7.42 Hz, 3-CH2CH2CH3), 2.26 (t, 2H, J = 7.57 Hz, 3-CH2CH2CH3), 2.56 (q, 2H, J = 7.53 Hz, 5-CH2CH3), 5.38 (s, 1H, 2-CHPh2), 5.83 (s, 1H, C4-H), 7.10–7.29 (m, 10H, phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 12.14 (5-CH2CH3), 14.06 (3-CH2CH2CH3), 21.46 (5-CH2CH3), 23.79 (3-CH2CH2CH3), 27.08 (3-CH2CH2CH3), 48.36 (2-CHPh2), 105.86 (C4), 121.32 (C3), 126.18 (phenyl), 128.11 (phenyl), 128.82 (phenyl), 142.56 (phenyl-1), 147.81 (C2), 156.00 (C5); MS (EI) m/z 304 (M+).

2-(Diphenylmethyl)-3-n-butyl-5-n-propylfuran (4d). Yellow liquid; b.p. 150–160 ºC (0.1 mmHg, Kugelrohr); 1H-NMR (CDCl3, 300 MHz): δ 0.84 (t, 3H, J = 7.25 Hz, 3-CH2(CH2)2CH3), 0.91 (t, 3H, J = 7.52 Hz, 5-(CH2)2CH3), 1.24 (m, 4H, 3-CH2(CH2)2CH3), 1.59 (sixtet, 2H, J = 7.38 Hz, 5-CH2CH2CH3), 2.27 (t, 2H, J = 7.52 Hz, 3-CH2(CH2)2CH3), 2.50 (t, 2H, J = 7.43 Hz, 5-CH2CH2CH3), 5.37 (s, 1H, 2-CHPh2), 5.83 (s, 1H, C4-H), 7.17-7.27 (m, 10H, phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.71, 13.89, 21.39 (5-CH2CH2CH3), 22.41, 24.58 (3-CH2(CH2)2CH3), 30.10 (5-CH2CH2CH3), 32.72, 48.36 (2-CHPh2), 106.79 (C4), 121.52 (C3), 126.23 (phenyl), 128.16 (phenyl), 128.88 (phenyl), 142.67 (phenyl-1), 147.77 (C2), 154.67 (C5); MS (EI) m/z 332 (M+). Anal. Calcd for C24H26O: C 86.70, H 8.43. Found: C 86.43, H 8.67.

2-[Bis(4-chloropheny)methyl]-3-ethyl-5-methylfuran (4e). Yellow liquid; b.p. 150 ºC (3.0 mmHg, Kugelrohr); 1H-NMR (CDCl3, 300 MHz): δ 1.05 (t, 3H, J = 7.57 Hz, 3-CH2CH3), 2.20 (s, 3H, 5-CH3), 2.28 (q, 2H, J = 7.49 Hz, 3-CH2CH3), 5.31 (s, 1H, 2-CH(p-ClPh)2), 5.86 (s, 1H, C4-H), 7.06–7.31 (m, 8H, phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.63 (5-CH3), 15.10 (3-CH2CH3), 18.06 (3-CH2CH3), 47.08 (2-CH(p-ClPh)2), 107.35 (C4), 128.47 (C3), 130.13 (phenyl), 132.03 (phenyl), 132.38 (phenyl), 140.56 (phenyl-1), 146.54 (C2), 150.87 (C5); MS (EI) m/z 344 (M+).

4. Conclusions

In conclusion, we have developed a novel rhodium-catalyzed cross-coupling reaction of ketenes with alkynes. The different coordination modes of ketenes to rhodium, which highly depend on the structure and reactivity of the starting ketenes, realized the selective formation of totally different products, dienones and furans in the presence of the same rhodium catalyst, RhCl(PPh3)3. Both reactions proceed via characteristic rhodacyclic intermediates.