Rh-Catalyzed Highly Enantioselective Synthesis of Aliphatic Sulfonyl Fluorides.

Rh-catalyzed, highly enantioselective (up to 99.8% ee) synthesis of aliphatic sulfonyl fluorides was accomplished. This protocol provides a portal to a class of novel 2-aryl substituted chiral sulfonyl fluorides, which are otherwise extremely difficult to access. This asymmetric synthesis has the feature of mild conditions, excellent functional group compatibility, and wide substrate scope (51 examples) generating a wide array of structurally unique chiral β-arylated sulfonyl fluorides for sulfur(VI) fluoride exchange (SuFEx) click reaction and drug discovery.

Introduction

Since the seminal work reported by K. B. Sharpless group in 2014 (Dong et al., 2014a, Dong et al., 2014b), sulfur(VI) fluoride exchange (SuFEx) click reaction has grown into a powerful synthetic tool, attracting increasing interest with wide applications in various disciplines such as polymer chemistry (Dong et al., 2014a, Dong et al., 2014b, Yatvin et al., 2015, Oakdale et al., 2016, Brendel et al., 2017, Gao et al., 2017, Wang et al., 2017, Zhang et al., 2019), surface chemistry (Brooks et al., 2016), bioconjugation (Zelli et al., 2016, Li et al., 2016), protein target identification (Jones, 2018a, Jones, 2018b, Mortenson et al., 2018, Wang et al., 2018a, Wang et al., 2018b, Wang et al., 2018c, Wang et al., 2018d, Zhao et al., 2017), and covalent protein inhibition (Alvarez et al., 2017, Chen et al., 2016a, Chen et al., 2016b, Fadeyi et al., 2017, Gehringer and Laufer, 2019, Hett et al., 2015, Liu et al., 2018, Narayanan and Jones, 2015, Shishido et al., 2017). Sulfonyl fluoride moiety as the sulfur(VI)-containing functional group at the heart of SuFEx methodology is imbued with a stability and chemoselectivity profile that is highly desirable for click chemistry applications (Chinthakindi and Arvidsson, 2018, Mukherjee et al., 2018, Chinthakindi et al., 2016, Kwon and Kim, 2019, Smedley et al., 2018, Leng and Qin, 2018, Thomas and Fokin, 2018). For instance, sulfonyl fluoride headed molecules have gained a renewed interest for both organic and medicinal chemists as privileged warheads in chemical biology and drug discovery (Figure 1) (Akçay et al., 2016, Brouwer et al., 2012, Dalton et al., 2018, Dubiella et al., 2014, Jones, 2018a, Jones, 2018b, Tschan et al., 2013). Moreover, the synthesis of 2-substituted ethenesulfonyl fluorides has recently attracted significant attention because of their unique properties as both “perfect” Michael acceptors and electrophiles for SuFEx manipulation (Allgäuer et al., 2017, Chen et al., 2016a, Chen et al., 2016b, Chen et al., 2017, Chen et al., 2018, Chen et al., 2019, Chinthakindi et al., 2017, Li et al., 2018, Ncube and Huestis, 2019, Qin et al., 2016, Ungureanu et al., 2015, Wang et al., 2018a, Wang et al., 2018b, Zha et al., 2017a, Zha et al., 2017b), because the pioneering work by Truce and Hoerger in 1954 (Truce and Hoerger, 1954). However, β-arylethenesulfonyl fluorides have rarely been explored as latent precursors for the constructions of chiral sulfonyl fluoride molecules (Barrow et al., 2019).

Figure 1

Representative Molecules Bearing Sulfonyl Fluoride Moiety with Biological Significance

Di(hetero)arylalkanes are ubiquitous and important structures as building blocks in drug discovery (Figure 2) (Zhou et al., 2013, He et al., 2018, Graffner-Nordberg et al., 2001, Boyd et al., 2001, Hsin et al., 2002, Hills et al., 1998, Silva et al., 1999, Malhotra et al., 2009, Hu et al., 2010, Pathak et al., 2010, Ameen and Snape, 2013). To accelerate the discovery of new covalent drug candidates, we plan to build diversified compound libraries bearing both di(hetero)arylalkane and sulfonyl fluoride functionalities (Schreiber, 2000, O’Connor et al., 2012, Nadin et al., 2012).

Figure 2

Representative Drugs and Natural Products Possessing di(hetero)arylmethane Functionality

Carbon-carbon (C-C) bond formation represents one of the most straightforward and atom-efficient strategy for the construction of new organic molecules because the framework of most organic molecules is a carbon chain (Gruttadauria and Giacalone, 2011, Jacobsen et al., 1999, Jumde et al., 2016, Mu et al., 2017, Schmidt et al., 2016, Schwarzwalder et al., 2019, Wang et al., 2018a, Wang et al., 2018b, Wang et al., 2018c, Wang et al., 2018d; Liang and Fu, 2015). Particularly, in recent years, organoboron reagents participated in rhodium-catalyzed asymmetric 1,4- conjugate additions to activated alkenes for the synthesis of C-C bonds have emerged as robust, reliable, and versatile methods to construct chiral gem-diaryl alkanes, whereas diverse aryl and alkenyl groups are incorporated with high enantioselectivity (Sidera and Fletcher, 2015, Tian et al., 2012, Edwards et al., 2010, Hayashi and Yamasaki, 2003, Fagnou and Lautens, 2003, Müller and Alexakis, 2012). The Rh/binap catalyzed asymmetric addition of arylboronic acids to conjugated enones was firstly reported by Hayashi and Miyaura in 1998 (Takaya et al., 1998). This pioneering method has been rapidly developed in addition to various functional groups attached alkenes such as α,β-unsaturated esters (Duchemin and Cramer, 2019, Paquin et al., 2005a, Paquin et al., 2005b, Sakuma et al., 2000), amides (Yuan and Sigman, 2018, Wang et al., 2014, Hargrave et al., 2006, Sakuma and Miyaura, 2001, Senda et al., 2001), carbonyl (Bocknack et al., 2004, Kadam et al., 2017, Khiar et al., 2013, Moragues et al., 2015, Paquin et al., 2005a, Paquin et al., 2005b, Shintani et al., 2006, Yasukawa et al., 2015), phosphonates (Hayashi et al., 1999), imines (Cui et al., 2011, Jagt et al., 2006, Lee and Kim, 2015, Nishimura et al., 2012a, Nishimura et al., 2012b, Shintani et al., 2010, Trincado and Ellman, 2008, Wu et al., 2018), sulfonyl (Lim and Hayashi, 2015, Liu et al., 2019, Mauleon and Carretero, 2005, Nishimura et al., 2012a, Nishimura et al., 2012b, Takechi and Nishimura, 2015, Yan et al., 2019), nitro compounds (Wang et al., 2010, Hayashi et al., 2000, He et al., 2015, Miyamura et al., 2017), borylalkenes (Sasaki and Hayashi, 2010), and other electron-deficient alkenylarenes (Pattison et al., 2010, Saxena and Lam, 2011). We envision that through using Rh(I) catalyst and appropriate chiral ligand, the reaction of 2-arylethenesulfonyl fluorides with arylboronic acids would furnish a class of novel chiral molecules bearing both chiral gem-diarylmethane moiety and sulfonyl fluoride functionality (Scheme 1). However, to the best of our knowledge, the asymmetric addition of organometallic reagents to α,β-unsaturated sulfonyl fluorides for producing chiral β,β-diarylethanesulfonyl fluorides has not been divulged because there are two major challenges: first, the sulfonyl fluoride moiety (R-SO2F) is fragile in the presence of bases such as Et3N, NaHCO3, and DBU to undergo nucleophilic reactions (Chen et al., 2017, Chen et al., 2018, Dong et al., 2014a, Dong et al., 2014b, Ungureanu et al., 2015), whereas for the Rh(I)-catalyzed 1,4-addition system, strong bases such as NaOH, KOH, CsOH, and K2CO3 are typically required to drive the desired transformation to occur (Sidera and Fletcher, 2015, Tian et al., 2012, Edwards et al., 2010, Hayashi and Yamasaki, 2003, Fagnou and Lautens, 2003, Müller and Alexakis, 2012), which could partially or even completely destroy the S(VI)-F functionality; second, the -SO2F motif is much more electron withdrawing comparing with other sulfonyl groups, carbonyl groups, phosphonates, and nitro counterparties, which makes the olefins conjugated with -SO2F a lot more (more than 100 times) reactive than alkenes conjugated with other electron-withdrawing groups; therefore, ethenesulfonyl fluoride performs as “perfect” Michael acceptor to proceed the addition in very short time (Allgäuer et al., 2017, Chen et al., 2016a, Chen et al., 2016b), which further makes the control of enantioselectivity a lot more challenging.

Scheme 1

Proposed Enantioselective Addition of Arylboronic Acids to 2-Arylethenesulfonyl Fluorides

In the course of our research program on SuFEx chemistry, we have developed an efficient entry into diverse α,β-ethenesulfonyl fluorides (Qin et al., 2016, Zha et al., 2017a, Zha et al., 2017b); herein, we report the first example (to the best of our knowledge) of Rh-catalyzed highly enantioselective conjugate addition of aryl boronic acids to this category of vinyl sulfonyl fluorides to generate a class of novel chiral sulfonyl fluoride compounds with potential pharmaceutical significance for drug discovery (Scheme 1) (Herrán et al., 2005, Hayashi et al., 2005, Nishimura et al., 2006).

Results and Discussion

We commenced our investigation by testing the feasibility of asymmetric 1,4-conjugate addition of (4-(methylthio)phenyl)boronic acid (2a) to (E)-2-phenylethenesulfonyl fluoride (1a). To attain the desired 1,4-addition product with high ee, different chiral phosphene ((R)- or (S) binap) and diene ligands (L1-L6) (Table 1) were evaluated subsequently. In reaction with the use of only rhodium catalyst (no ligand), no conversion was observed (entry 1). The use of the most widely applied rhodium-bisphosphine complex, [RhCl((R)-binap)]2 or RhCl((S)-binap)]2 complex, afforded desired addition product in only a trace amount (entries 2 and 3). To our delight, chiral diene ligands L1–L3 with ester functional groups, from a readily available natural product (R)-phellandrene, exhibited excellent catalytic activity for achieving enantioselectivity (entries 4–6). Surprisingly, the use of ligand L3 bearing a less bulky group ((2,6-dimethyl)phenyl ester) afforded the addition product in even higher yield of 85% and better enantiopurity, 92% ee, than the using of ligand L2 bearing a more bulky group ((2,6-diisopropyl)phenyl ester) (81% yield with 80% ee) (entry 5 vs entry 6). The chiral diene ligands with amide moieties (L4 and L5) displayed less catalytic activities than those chiral diene ligands with ester moieties (entries 7 and 8). And, the chiral diene ligand bearing ketone moiety (L6) also showed less catalytic activity and poor enantioselectivity providing the product 3a in 65% yield with 49% ee (entry 9). Therefore, the condition of entry 6 with L3 was utilized for further substrate scope exploration and functional group compatibility examination.

Table 1

Screening Ligands for Rhodium-Catalyzed Asymmetric Addition of (4-(Methylthio)phenyl)Boronic Acid (2a) to (E)-2-Phenylethenesulfonyl Fluoride (1a)

Entry	Ligand	Yield (%)a, b	ee (%)c
1	–	0	ND
2	R-BINAP	trace	ND
3	S-BINAP	trace	ND
4	L1	84	70
5	L2	81	80
6	L3	85	92
7	L4	80	41
8	L5	84	51
9	L6	65	49

Reaction conditions: a mixture of 1a (0.25 mmol), 2a (0.5 mmol), [RhCl(L*)]2 (10 mol%), and CsF (0.5 mmol) was dissolved in EA + H2O (2.5 + 0.25 mL) and reacted at 50°C for 12 h under argon atmosphere.

Isolated yield.

Determined by chiral HPLC analysis.

Substrate Scope Study

The obtained promising results persuaded us to explore the scope of [RhCl(L3)]2 catalyzed asymmetric 1,4-addition of various arylboronic acids 2 to phenylethenesulfonyl fluoride 1a, as summarized in Scheme 2. The aryl boronic acids 2 containing either electron donating or electron withdrawing groups at the para-positions of aromatic rings reacted with phenylethenesulfonyl fluoride 1a smoothly to afford desired chiral β-phenyl β-arylethanesulfonyl fluoride products in good to excellent yields (75–99%) with excellent enantioselectivities (61%–>99% ee) (3a-3k). However, boronic acid bearing 2,4-difluoro electron withdrawing group (2l) reacted with the vinyl sulfonyl fluoride 1a sluggishly to furnish desired product 3l in 77% yield with slightly lower enantioselectivity (89% ee). Notably, no conversion was observed when the reaction was performed with ortho-substituted phenylboronic acids such as 2-Cl, 2-Br, 2-I, 2-Me, and 2-iPr. Arylboronic acids (2m-2p) possessing electron withdrawing groups at meta-positions afforded the desired products in high yields (84%–99%) and high enantioselectivities (94%–97% ee) (3m-3p). Interestingly, the boronic acid 2q containing strong electron-donating group at the meta-position provided the corresponding product 3q in 87% yield; however, the enantioselectivity was significantly low (74% ee). Sterically hindered arylboronic acids (2r-2t) also proceeded the addition to the vinyl sulfonyl fluoride 1a successfully furnishing their corresponding products (3r-3t) in good to high yields (75%–89%) with excellent enantioselectivity (97%–98% ee). Remarkably, the heteroaryl boronic acids (2u-2w) containing N-, O-, S- hetero atoms also underwent the addition smoothly providing their corresponding 1,4-addition products (3u-3w) in high yields (82–99%) with excellent enantioselectivities (>99% ee). The reactions of benzofuran-3-yl boronic acid (2x) and benzo[b]thiophen-3-yl boronic acid (2y) with vinyl sulfonyl fluoride 1a were much slower than using other arylboronic acids, providing the corresponding 1,4-addition products (3x, 3y) in moderate yield 63% (3x, 89% based on recovery of starting material 1a) and 50% (3y, 92% based on recovery of starting material 1a) respectively due to the incomplete conversion of 1a, whereas their enantioselectivities were excellent (3x, >99% ee and 3y, 97% ee).

Scheme 2

Rhodium-Catalyzed Asymmetric Addition of Arylboronic Acids (2) to (E)-2-Phenylethenesulfonyl Fluoride (1a)

a Reaction conditions: a mixture of 1a (0.5 mmol), 2 (1.0 mmol), [RhCl(L3)]2 (10 mol%), and CsF (1.0 mmol) was dissolved in EA + H2O (5.0 + 0.5 mL) and reacted at 50°C for 12 h under argon atmosphere.

b Determined by chiral HPLC analysis.

c Based on recovery of 1a.

Next, the scope of the addition of phenylboronic acid 2z to a variety of α,β-unsaturated ethenesulfonyl fluorides 1 was also evaluated as summarized in Scheme 3. The 2-aryletheneulfonyl fluorides 1 bearing electron donating or withdrawing group at para-position of the aromatic rings underwent the asymmetric addition efficiently to bestow corresponding 1,4-addition products (4a-4j) in good to excellent yields (62–96%) with moderate to excellent enantioselectivities (51%–98% ee). The absolute configuration of 4d was confirmed using X-ray crystallography analysis (see Supplemental Information). Furthermore, meta-substituted aromatic rings with either electron rich or deficient groups of the α,β-unsaturated 2-arylethenesulfonyl fluorides 1k-1p also proceeded the corresponding asymmetric addition to produce desired addition products (4k-4p) in high yield (88%–96%) with lower enantioselectivities (65%–80% ee). Gratifyingly, 2-arylethenesulfonyl fluoride 1q with ortho-substitution on the aromatic ring also smoothly participated in the asymmetric addition to afford the desired addition product 4q in 90% yield and 75% ee in contradiction to the unsuccessful additions of ortho-substituted arylboronic acid to 2-arylethenesulfonyl fluoride 1a (Scheme 2). The β-1-naphthyl-substituted ethenesulfonyl fluoride 1r was transformed to the corresponding addition product 4r in 84% yield with 86% ee. Notably, the additions of phenylboronic acid 2z to 2-heteroarylethenesulfonyl fluorides containing S-, O-, and N- heteroatoms (1s-1y) exhibited excellent enantioselectivities (88%–95% ee). Interestingly and remarkably, through the asymmetric additions of different boronic acids to 2-arylethenesulfonyl fluoride 1a (Scheme 2), and additions of the same arylboronic to different arylethenesulfonyl fluorides (Scheme 3), both enantiomers of each of the addition products can be obtained, for example, 3b of Scheme 2 vs 4c of Scheme 3, 3d of Scheme 2 vs 4b of Scheme 3.

Scheme 3

Rhodium-Catalyzed Asymmetric Addition of Phenylboronic Acid (2z) to α,β-Unsaturated Sulfonyl Fluorides (1)

a Reaction conditions: a mixture of 1 (0.5 mmol), 2z (1.0 mmol), [RhCl(L3)]2 (10 mol%), and CsF (1.0 mmol) was dissolved in EA + H2O (5.0 + 0.5 mL) and reacted at 50°C for 12 h under argon atmosphere.

b Determined by chiral HPLC analysis.

Afterward, diversifications of the 1,4-addition products were examined to demonstrate the further utility of these chiral sulfonyl fluorides (Scheme 4). Reaction of compound 3w with amine 5 in the presence of triethyl amine produced the corresponding sulfonamide 6w in 98% yield and greater than 99% ee. The SuFEx click reaction of compound 3w with phenol 7 afforded the corresponding sulfonate 8w in 99% yield and higher than 99.9% ee. Compound 9 obtained from corresponding boronic acid and α,β-unsaturated sulfonyl fluoride was also successfully transformed into the corresponding sulfonyl amide 11 in 88% yield and 98% ee through a SuFEx click process with benzylamine 10. And the sulfonyl amide 11 proceeded an intramolecular C-H amination (Martínez et al., 2016) to generate a cyclic amine 12 in 80% yield and 92% ee.

Scheme 4

Diversification of the Chiral Sulfonyl Fluorides

Conclusion

In summary, Rh-catalyzed, highly enantioselective, conjugate additions of arylboronic acids to α,β-ethenesulfonyl fluorides was achieved providing a portal to a class of novel 2-aryl substituted chiral sulfonyl fluorides, which are extremely difficult to access otherwise. This method has feature of mild conditions, excellent functional group compatibility, and wide scope generating a wide array of structurally diverse β-arylated sulfonyl fluorides. Further developments and synthetic applications of these molecules in chemical biology and drug discovery are in progress.

Limitations of the Study

The results of examination of substrate scope showed that the present method was not suitable for the conjugate addition of ortho-substituted arylboronic acids to 2-arylethenesulfonyl fluorides.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.