Silver Synergistic Catalysis in Highly Enantioselective

Article pubs.acs.org/JACS

Rhodium/Silver Synergistic Catalysis in Highly Enantioselective Cycloisomerization/Cross Coupling of Keto-Vinylidenecyclopropanes with Terminal Alkynes Song Yang,† Kang-Hua Rui,† Xiang-Ying Tang,§ Qin Xu,† and Min Shi*,†,‡,§ †

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Mei-Long Road, Shanghai 200237, China ‡ State Key Laboratory and Institute of Element-Organic Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China § State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Science, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China S Supporting Information *

ABSTRACT: A rhodium/silver synergistic catalysis has been established, enabling cycloisomerization/cross coupling of keto-vinylidenecyclopropanes (VDCPs) with terminal alkynes toward the regio- and enantioselective formation of diversified tetrahydropyridin-3-ol tethered 1,4-enynes in good yields and high ee values. In this synergistic catalysis, Rh(I) and Ag(I) catalysts selectively activate keto-VDCP substrates and terminal alkynes to generate the π-allyl Rh(III) complex of oxa-rhodacyclic intermediate and Ag alkynyl intermediate, respectively. The rapid transmetalation of alkynyl groups from Ag to Rh is proposed to play a key role in realizing the regioselective cleavage of the distal bond of the three-membered ring in this transformation.

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have been reported to date.11 Very recently, Tanaka’s group and Oonishi, Sato et al., started the pioneering work by reporting Rh(I)-catalyzed enantioselective [2 + 2 + 2] cycloadditions of allenyl aldehydes with internal alkynes, respectively (Scheme 1B).11a,b The regioselectivity of the process seems to be erratic, although the enantioselectivity was good. Besides, terminal alkynes did not work in both of the two reports. Synergistic catalysis is a new synthetic strategy wherein both the nucleophile and the electrophile are simultaneously activated by two separate and distinct catalysts to afford a single chemical transformation.12−15 Recently, silver additives could be utilized as cocatalysts in transition-metal-catalyzed C− H functionalization reactions.16,17 The pioneering studies by Larrosa,17a Sanford,17b and Hartwig17c independently revealed that the silver additives could cleave the C−H bonds via a concerted metalation−deprotonation (CMD) step (Scheme 1C). The resulting nucleophilic Ag(I) intermediates would undergo rapid transmetalation to Pd or other transition metals. Although less common, silver alkynyls, formed in situ from the alkyne and the silver salt, have also been used for the same purposes.18 Combined with bimetallic synergistic catalysis, [2 + 2 + 2] cycloadditions and our ongoing investigations on the chemistry

INTRODUCTION Vinylidenecyclopropanes (VDCPs),1 which contain an allene moiety and a connected cyclopropane ring, are one of the most remarkable known organic compounds in the chemistry of highly strained small rings. Due to the much higher strained energy than simple cyclopropanes (27.5 kcal/mol), VDCPs (50.9 kcal/mol)2 are more reactive toward C−C bond cleavages upon heating and photoirradiation or mediated by Lewis acids and Brønsted acids via a cleavage of proximal bonds3 or distal bonds4 of the three-membered ring. Compared with cyclopropane, the bond angle of VDCP is bigger than that of cyclopropane (62.2° vs 60°). Therefore, the distal bond is weaker and more reactive than the proximal bond (1.531 Å vs 1.488 Å, Scheme 1A, Path I).2 However, with the coordination assistance of allenic moiety, transition metals were found to facilitate the cleavage of the proximal C−C bonds (Scheme 1A, Path II). Thus, the competing reactions via two different cleavage pathways make it challenging to regioselectively cleave the distal bond of the three-membered ring in VDCPs.5 The transition-metal-catalyzed [2 + 2 + 2] cycloadditions6,7 and cycloisomerizations8 of two C−C multiple bonds with carbonyl compounds have emerged as promising and environmentally benign methodologies for the construction of oxygencontaining heterocycles.9 However, most reports on the cycloadditions focus on 1,n-diynes or 1,n-enynes with carbonyl compounds,9d,10 only a few examples of the cycloaddition of tethered aldehydes or ketones with unsaturated compounds © 2017 American Chemical Society

Received: February 27, 2017 Published: April 7, 2017 5957

DOI: 10.1021/jacs.7b02027 J. Am. Chem. Soc. 2017, 139, 5957−5964

Journal of the American Chemical Society

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Scheme 1. Previous Work and This Work

Article

RESULTS AND DISCUSSION

To test the feasibility of our hypothesis, we initially investigated the reaction of keto-VDCP 1a with phenylacetylene 2a in the presence of the cationic Rh(I) complexes in situ generated from silver salts and chiral bisphosphine ligands (Table 1). To our delight, when [Rh(cod)Cl]2 and AgNTf2 were used as catalysts with (R)-Binap as ligand, product 3aa could be successfully furnished in 69% yield with −81% ee value (entry 1). Encouraged by the result, various chiral bisphosphine ligands were next investigated. A better enantioselectivity was realized Table 1. Optimization of the Reaction Conditions for Asymmetric Cycloisomerization/Cross Coupling of KetoVDCP 1a with Phenylacetylene 2aa

of VDCPs, we envisioned that the cationic Rh(I) complexes prefer to insert into the weaker distal bond of the threemembered ring to give the corresponding π-allylic Rh(III) complex of oxa-rhodacyclic intermediate B (Scheme 1D). Simultaneously, a Ag alkynyl intermediate C may be generated in the presence of silver salt. Then, the oxa-rhodacyclic intermediate B derived from distal bond cleavage may be captured rapidly by nucleophilic Ag alkynyl intermediate C via transmetalation of alkynyl group from Ag to Rh. This rapid transmetalation could avoid the side reaction via a cleavage of the proximal C−C bonds. Thus, the regioselective cleavage of the distal bond rather than the proximal C−C bonds would be achieved. After reductive elimination and protonolysis from intermediate D, a novel cross coupling between terminal alkynes and keto-VDCPs could be realized. Herein, we wish to disclose such an unprecedented example of a synergistic Rh(I)/ Ag(I) dual-catalysis that promotes the regio- and stereoselective cycloisomerization/cross coupling of keto-VDCPs with terminal alkynes toward the formation of diversified tetrahydropyridin-3-ol tethered 1,4-enynes. This transformation can not be accomplished using either Rh(I) complex or Ag(I) catalyst alone. As far as we know, piperidines and aliphatic sixmembered nitrogen-containing heterocycles are among the most promising therapeutic agents for a wide variety of diseases, including Alzheimer’s disease and Parkinson’s disease.19 In addition, 1,4-enynes are important structural motifs in many natural products20 and have been also extensively applied in organic synthesis.21 Thus, the Nheterocyclic 1,4-enynes will be the key intermediates in the preparation of more drug-like substances or biologically active compounds. To this regard, this new synthetic method is of great significance in organic synthesis or medicinal chemistry, as other existing methods used to construct such scaffolds usually suffer from multiple-step operations.

entrya

Ag salt

ligand

solvent

yield (%)b

ee (%)c

1 2 3 4 5 6 7 8 9 10 11 12 13 14d 15e 16f

AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgOTf AgSbF6 AgNTf2 AgNTf2 AgNTf 2 AgNTf2 AgNTf2

L1 L2 L3 L4 L5 L6 L7 L8 L9 L7 L7 L7 L7 L7 L7 L7

DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE dioxane PhCl dioxane dioxane dioxane

69 67 63 27 82 82 73 trace 35 69 70 85 78 85 68 35

−81 −92 86 −87 92 −93 99 − 65 99 99 99 99 99 99 99

a

Reaction conditions: 1a (0.10 mmol), 2a (0.15 mmol), [Rh(cod)Cl]2 (5 mol %), Ag salt (12 mol %), ligand (12 mol %), and solvent (1.5 mL) were used; 4−12 h. bIsolated yield. cDetermined by HPLC on a chiral stationary phase. d[Rh(cod)Cl]2 (2.5 mol %), AgNTf2 (6.0 mol %), and (R)-SDP (6.0 mol %) were employed. e0.5 mL Dioxane was employed. fThe reaction was conducted at 60 °C. cod = cyclo-1,5octadiene, Ts = 4-toluenesulfonyl.

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entries 8 and 9). Then, inductively coupled plasma optical emission (ICP-OES) was used to confirm the quantity of Ag in the catalyst [Rh(cod)2]BF4 purchased from 9-Ding Chemistry (see Figure S1 in the Supporting Information). The analysis reports showed that the mass fraction of Ag in the [Rh(cod)2]BF4 is 608.70 ppm. Moreover, no Ag was detected after the [Rh(cod)2]BF4 catalyst was preprocessed by a short flash column chromatography. Besides, 46% yield of 3aa could be also afforded in the presence of [Rh(cod)Cl]2 (5 mol %) and CuI (10 mol %) (Table 2, entry 10). Decreasing the catalyst loading of AgNTf2 has no obvious influence on the formation of product 3aa (Table 2, entry 11). Furthermore, silver phenylacetylide18 could indeed catalyze the reaction with [Rh(cod)2]BF4, affording the desired product 3aa in 40% along with the formation of its homocoupling product in 20% yield (Table 2, entry 12). These results implied that the silver salts were used not only to generate the cationic Rh(I) complexes but also as cocatalysts, which probably behave in a similar way as well-known Sonogashira coupling reactions.23 The terminal alkyne may react with silver salt, yielding a Ag alkynyl intermediate, which could then undergo transmetalation with Rh complex. The scope of this asymmetric cycloisomerization/cross coupling process was then assessed through variation of the keto-VDCPs and terminal alkynes under the optimized conditions. We first examined the ketone moiety of ketoVDCPs 1. As shown in Table 3, the substrate scope of this protocol was broad. As for the substituents at the benzene ring, whether they were electron-rich or electron-poor, the reactions proceeded smoothly to deliver the desired products 3ba−3ka in 70−84% yields with 97−99% ee values. Besides, no obvious erosion of yields and ee values was observed when different halogen atoms such as F, Cl, or Br were introduced. Changing R1 to 1-naphthyl, 2-thienyl or 3-indolyl, the reactions also worked very well and furnished the desired products 3la−3na in 83−90% yields with 95−99% ee values. In the case of 1o (R1 = Me), the desired product 3oa could be obtained in 79% yield with 98% ee value. Satisfactorily, 3pa (R2 = Me) could be afforded as a single diastereoisomer in 64% yield with −97% ee when (R)-H8-Binap was employed as ligand. The relative configuration of 3pa was determined by nuclear Overhauser effect spectroscopy (see Supporting Information, p S107). Next, we examined the VDCP moiety and the links between ketone and VDCP (Table 4). As for substrates 1q−1u, (R3 = primary, secondary, even tertiary alkyl groups), the desired products 3qa−3ua were obtained in good to excellent yields and outstanding ee values. Although this transformation proceeds efficiently for alkyl-substituted VDCPs, yield is slightly lower for phenyl-substituted VDCP (R3 = Ph, 1v). Moreover, as for substrates 1w and 1y, in which ketone and VDCP moieties are connected by BsN and carbon anchors, the corresponding products 3wa and 3ya could be obtained in 85% and 34% yields, respectively, along with −93% ee values if using (R)-H8-Binap as ligand. However, changing the link as an oxygen atom or extending the carbon chain as a (CH2)2 tether, only traces of expected products (3xa and 3za) could be detected by thin-layer chromatography (TLC) monitoring. With respect to terminal alkynes, various substituents at the benzene ring were first examined. As we can see in Table 5, both of the electron-withdrawing and electron-donating substituents have no obvious impact on the reaction activity, even for these strongly electron-withdrawing substituents such as nitryl and cyano groups. Interestingly, when 1,4-dieth-

when (R)-Tol-Binap was employed (entry 2). Among biaryl bisphosphine ligands examined, both of (S)-Segphos and sterically more-demanding (R)-DTBM-Segphos gave inferior yields and ee values. Gratifyingly, after further screening of ligands, the best enantioselectivity was achieved by employing (R)-SDP as ligand (entries 5−7). Nonbiaryl bisphosphine ligands L8 and L9 have been also examined in this transformation, but both of them gave inferior results (entries 8 and 9). A subsequent survey on other silver salts and solvents indicated that AgNTf2 and 1,4-dioxane were the best choice, promoting an increased yield to 85% (entries 10−13). As the catalytic activity of these Rh(I) and Ag(I) catalysts were very high, the reaction could even be carried out in the presence of 2.5 mol % of the Rh catalyst without erosion of the product’s yield and ee value (entry 14). In addition, increasing the concentration and lowering the reaction temperature did not give better results (entries 15 and 16). The absolute configuration of 3aa has been assigned as S by X-ray diffraction. The ORTEP drawing and the CIF data are summarized in the Supporting Information.22 To elucidate our hypothesis for this asymmetric cycloisomerization/cross coupling proceeding through a synergistic Rh(I)/Ag(I) dual-catalysis, a series of control experiments were conducted. When commercially available [Rh(cod)2]BF4 or [Rh(cod)2]OTf (purchased from 9-Ding Chemistry) was used as catalyst with (R)-Binap as ligand, product 3aa could be successfully furnished in moderate yield (Table 2, entries 1 and Table 2. Mechanistic Study about the Role of Silver Salts in This Reaction

entry

Rh complex

Ag salt

yield (%)a

b

[Rh(cod)2]BF4 [Rh(cod)2]OTf [Rh(cod)2]BF4 [Rh(cod)2]OTf [Rh(cod)2]BF4 [Rh(cod)2]OTf [Rh(cod)Cl]2 [Rh(cod)Cl]2 − [Rh(cod)Cl]2 [Rh(cod)Cl]2 [Rh(cod)2]BF4

− − − − AgNTf2 AgNTf2 AgNTf2 − AgNTf2 Cul AgNTf2 PhCC−Ag

50 45 N.D. N.D. 58 46 85 N.R. N.R. 46 83 40

1 2b 3c 4c 5c 6c 7 8 9 10 11d 12c a

Determined by 1H NMR spectroscopy. bThe Rh catalyst was purchased from 9-Ding Chemistry and was used without further purification. cThe Rh catalyst was preprocessed by a short flash column chromatography. d5 mol % AgNTf2 was employed.

2). Notably, no desired product could be detected by TLC monitoring when these commercially available cationic Rh(I) catalysts were preprocessed by a short flash column chromatography (Table 2, entries 3 and 4). Intriguingly, 46− 58% yields of 3aa could be obtained by using these preprocessed cationic Rh(I) catalysts with extra AgNTf2 added again (Table 2, entries 5 and 6). The combination of [Rh(cod)Cl]2 with AgNTf2 also gave 3aa in 85% yield in the presence of rac-BINAP (Table 2, entry 7). No reaction occurred in the absence of [Rh(cod)Cl]2 or AgNTf2 (Table 2, 5959


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Journal of the American Chemical Society Table 3. Substrate Scope of the Asymmetric Cycloisomerization/Cross Coupling of Keto-VDCPs 1 and Phenylacetylene 2aa−c

Table 5. Substrate Scope of the Asymmetric Cycloisomerization/Cross Coupling of Keto-VDCP 1a and Terminal Alkynes 2a−c

a

Reaction conditions: keto-VDCPs 1 (0.10 mmol), phenylethyne 2a (0.15 mmol), [Rh(cod)Cl]2 (2.5 mol %), AgNTf2 (6.0 mol %), (R)SDP (6.0 mol %), and dioxane (1.0 mL) were used, 4−12 h. bIsolated yield. cDetermined by HPLC on a chiral stationary phase. d(R)-H8Binap was employed in place of (R)-SDP.

Table 4. Substrate Scope of the Asymmetric Cycloisomerization/Cross Coupling of Keto-VDCPs 1 and Phenylacetylene 2aa−c a

Reaction conditions: keto-VDCP 1a (0.10 mmol), terminal alkynes 2a (0.15 mmol), [Rh(cod)Cl]2 (2.5 mol %), AgNTf2 (6.0 mol %), (R)-SDP (6.0 mol %), and dioxane (1.0 mL) were used, 4−12 h. b isolated yield. cDetermined by HPLC on a chiral stationary phase. d (R)-H8-Binap was employed in place of (R)-SDP.

ynylbenzene was employed, only 3am was isolated in moderate yield with excellent ee value. A possible product with two molecules of 1a involved was not observed. As for substrate 2n (R4 = 2-thienyl), the reaction also proceeded very well. On the other hand, the use of terminal alkynes with alkyl or trimethylsilyl (TMS) group could also afford the desired cycloisomerization/cross coupling products (3ao, 3ap, 3aq, and 3ar) in moderate yields with outstanding ee values. As long as 3as could be obtained with propargyl alcohol without the use of protecting group, the known bioactive molecule ethynyl estradiol24 was employed to demonstrate this synthetic methodology’s potential application and extension. As shown in Scheme 2, the expected product 3at could be afforded in moderate yield with high ee value when (R)-H8-Binap was used as ligand. Transformations of the present asymmetric cycloisomerization/cross coupling reaction products 3 were also examined briefly (Scheme 3). Hydroarylation of 3aa and 3ba with

a

Reaction conditions: keto-VDCPs 1 (0.10 mmol), phenylethyne 2a (0.15 mmol), [Rh(cod)Cl]2 (2.5 mol %), AgNTf2 (6.0 mol %), (R)SDP (6.0 mol %), and dioxane (1.0 mL) were used, 4−12 h. bIsolated yield. cDetermined by HPLC on a chiral stationary phase. d(R)-H8Binap was employed in place of (R)-SDP.

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Journal of the American Chemical Society Scheme 2. Rh-Catalyzed Asymmetric Cycloisomerization/ Cross Coupling of Keto-VDCP 1a with Ethynyl Estradiol 2t

Scheme 4. Control Experiments

Scheme 3. Derivatizations of Products 3

hydroxy group of 2b. The incomplete deuterium transfer may be due to exchange with protic impurities (e.g., H2O). The reaction was then conducted under CO atmosphere (Scheme 4, eq 2). The possible reaction intermediate 8aa was isolated in 20% yield. Furthermore, another reaction intermediate 9aa was obtained in 26% yield when the reaction was performed in the absence of 2a (Scheme 4, eq 2). The structures of 8aa and 9aa were confirmed by NMR spectroscopic and HRMS analysis. It is noteworthy that 8aa was not stable and easily decomposed (for more details, see Table S1 in the Supporting Information). No reaction occurred when diphenylacetylene was employed to take the place of terminal alkyne (Scheme 4, eq 3). These results were in accordance with the transmetalation process, rather than routine migratory insertion. On the basis of above observations and the previous literatures, a plausible mechanism is outlined in Scheme 5 Scheme 5. Proposed Reaction Mechanisms phenylboronic acid proceeded smoothly with high syn-stereoselectivity in the presence of [Rh(OH)(cod)2] and racBINAP.25 Moreover, hydroalkoxylation and olefin isomerization of 3aa could be achieved upon treating with a different base, respectively.26 The hydroalkoxylation product 5aa was obtained in 80% yield upon treatment of 3aa with 1,5diaza(5,4,0)undec-5-ene (DBU) under reflux in THF. In addition, the olefin isomerization product 6aa could be afforded in the presence of NaH at 5−10 °C. The structures of 4ba and 6aa were unambiguously assigned by X-ray diffraction (see Figure S3 and Figure S4 in the Supporting Information).27,28 The relative configuration of 5aa was determined by nuclear Overhauser effect spectroscopy (see Supporting Information, pp S171−172). To show further synthetic utility of the present catalytic process, the N-tosyl group was removed with magnesium dust and NH4Cl under reflux in MeOH, giving the desired product 7aa in 62% yield and 99% ee value along with the recovery of 3aa in 34% yield (Scheme 3, eq (iii)). To investigate the mechanistic insights, some control experiments were conducted. The deuterium-labeling experiment was performed by subjecting deuterated substrate [D]-2b in DCE (Scheme 4, eq 1). The cycloisomerization/cross coupling of 1a and [D]-2b provided [D]-3ab in 75% yield along with 30% D incorporation, in which the deuterium atom was transferred from the terminal position of alkyne to the

using 1a and 2a as model substrates. First, a rhoda-cyclobutene intermediate A was generated from oxidative addition of the weaker distal C−C bond along with isomerization.29 A subsequent ketone carbometalation led to an oxa-rhodacyclic intermediate B.30 Simultaneously, the terminal alkyne would react with AgX (X = NTf2), yielding a Ag alkynyl intermediate 5961


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(cod)]2 (5 mol % Rh), rac-BINAP (12 mol % Rh), and 1,4-dioxane (1.0 mL). The reaction mixture was stirred at 90 °C for 4−10 h. The solvent was removed under reduced pressure, and the residue was purified by a flash column chromatography on silica gel to give the corresponding products 4. Compound 4aa. A light yellow oil, 85% yield (48 mg). 1H NMR (CDCl3, 400 MHz, TMS) δ 1.64 (s, 3H), 2.41 (s, 3H), 2.76 (s, 1H), 2.86 (d, J = 11.2 Hz, 1H), 3.13 (d, J = 18.0 Hz, 1H), 3.32 (d, J = 18.0 Hz, 1H), 3.36 (d, J = 16.0 Hz, 1H), 3.43 (d, J = 11.2 Hz, 1H), 3.91 (d, J = 16.0 Hz, 1H), 4.66 (d, J = 1.2 Hz, 1H), 5.01 (d, J = 1.6 Hz, 1H), 6.89 (s, 1H), 7.03−7.05 (m, 2H), 7.13−7.17 (m, 5H), 7.21−7.25 (m, 3H), 7.29−7.33 (m, 5H), 7.40−7.42 (m, 2H), 7.61 (d, J = 8.0 Hz, 2H). 13C NMR (CDCl3, 100 MHz, TMS) δ 17.9, 21.5, 37.8, 49.7, 58.0, 73.0, 118.6, 126.1, 126.6, 126.8, 127.0, 127.3, 127.8, 127.9, 128.15, 128.18, 128.20, 128.6, 129.8, 130.3, 132.5, 137.7, 138.3, 138.4, 141.7, 142.3, 142.5, 143.9. IR (CH2Cl2) ν 3688, 3675, 3658, 2987, 2970, 2921, 2853, 2360, 2346, 1730, 1597, 1493, 1449, 1405, 1394, 1381, 1349, 1250, 1154, 1077, 1066, 1056, 1028, 987, 895, 864, 806, 762, 698, 663 cm−1. HRMS (ESI) calcd for C36H39N2O3S (M + NH4)+: 579.2676, found: 579.2674. [α]20 D = 43.8 (c 1.00, CH2Cl2)]. Typical Procedure for the Preparation of Compound 5aa. To a flame-dried Schlenk tube were added compounds 3aa (0.1 mmol), DBU (2.0 equiv), and THF (1.0 mL). The reaction mixture was heated to reflux for 10 h. The solvent was removed under reduced pressure, and the residue was purified by a flash column chromatography on silica gel to give the corresponding product 5aa in 80% yield. Compound 5aa. A light yellow oil. 80% yield (39 mg). 1H NMR (CDCl3, 400 MHz, TMS) δ 1.99 (s, 3H), 2.39 (s, 3H), 2.97 (d, J = 11.6 Hz, 1H), 3.29 (d, J = 16.4 Hz, 1H), 3.35 (d, J = 15.6 Hz, 1H), 3.40 (d, J = 15.6 Hz, 1H), 3.67 (dd, J1 = 0.8 Hz, J2 = 11.6 Hz, 1H), 4.00 (d, J = 16.4 Hz, 1H), 4.80 (s, 1H), 4.89 (s, 1H), 5.04 (s, 1H), 7.05 (d, J = 8.4 Hz, 2H), 7.11−7.18 (m, 4H), 7.22−7.29 (m, 6H), 7.50 (d, J = 8.4 Hz, 2H). 13C NMR (CDCl3, 100 MHz, TMS) δ 19.3, 21.5, 40.4, 50.3, 54.9, 79.0, 104.1, 111.6, 126.2, 126.5, 126.9, 127.65, 127.70, 127.81, 128.3, 129.5, 129.6, 130.2, 133.1, 134.7, 136.9, 141.6, 143.6, 152.6. IR (CH2Cl2) ν 3688, 3675, 3658, 3515, 2989, 2972, 2901, 2360, 1697, 1596, 1489, 1450, 1394, 1383, 1350, 1305, 1253, 1195, 1173, 1051, 1090, 1066, 1047, 1028, 988, 941, 908, 869, 852, 807, 793, 757, 732, 704, 692, 662 cm−1. HRMS (ESI) calcd for C30H33N2O3S (M + NH4)+: 501.2206, found: 501.2201. [α]20 D = 26.8 (c 1.00, CH2Cl2)]. Typical Procedure for the Preparation of Compound 6aa. To a flame-dried Schlenk tube were added compounds 3aa (0.1 mmol), NaH (60% dispersion in mineral oil, 2.0 equiv), and THF (1.0 mL). The reaction mixture was heated to reflux for 10 h. The solvent was removed under reduced pressure, and the residue was purified by a flash column chromatography on silica gel to give the corresponding product 6aa in 77% yield. Compound 6aa. A white solid. 77% yield (37 mg). Mp 133−135 °C. 1H NMR (CDCl3, 400 MHz, TMS) δ 1.74 (d, J = 1.2 Hz, 3H), 1.77 (s, 3H), 2.43 (s, 3H), 2.82 (s, 1H), 2.92 (d, J = 11.6 Hz, 1H), 3.40 (d, J = 16.0 Hz, 1H), 3.48 (d, J = 11.6 Hz, 1H), 3.85 (d, J = 16.0 Hz, 1H), 5.34 (d, J = 1.2 Hz, 1H), 7.25−7.27 (m, 4H), 7.28−7.35 (m, 6H), 7.39 (dd, J1 = 1.6 Hz, J2 = 8.8 Hz, 2H), 7.63 (d, J = 8.0 Hz, 2H). 13 C NMR (CDCl3, 100 MHz, TMS) δ 18.3, 21.1, 21.5, 49.6, 57.5, 73.2, 86.7, 94.4, 111.4, 123.5, 126.2, 127.38, 127.82, 127.87, 127.94, 128.2, 128.6, 129.8, 131.2, 132.5, 138.6, 141.4, 144.0, 147.1. IR (CH2Cl2) ν 3515, 3059, 2956, 2922, 2854, 2811, 2254, 2196, 2018, 1700, 1596, 1490, 1449, 1348, 1305, 1247, 1152, 1091, 1045, 1021, 987, 941, 908, 807, 756, 731, 704, 691, 662 cm−1. HRMS (ESI) calcd for C30H33N2O3S (M + NH4)+: 501.2206, found: 501.2204. [α]20 D = 18.6 (c 1.00, CH2Cl2)].

C, which could then undergo transmetalation to Rh to afford intermediate D rapidly. The reductive elimination from intermediate D gave the corresponding alkoxy Rh intermediate E, protonolysis of which afforded the final cycloisomerization/ cross coupling product 3aa. Carbonylation of intermediate B, followed by reductive elimination along with double-bond isomerization via intermediate F, yielded the corresponding lactone derivative 9aa (also see Scheme S1 in the Supporting Information). The isolation of 9aa would strongly rationalize the intermediacy of oxa-rhodacyclic species B. This synergistic rhodium/silver catalysis is in accordance with the control experiments and explains why internal alkynes did not work under identical conditions. As the path of alkynyl migratory insertion could not be excluded thoroughly, an alternative reaction mechanism is also proposed in Scheme S1 in the Supporting Information. Besides, a proposed mechanism for the formation of 8aa has been also shown in Scheme S2 in the Supporting Information.

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CONCLUSION In conclusion, an unprecedented highly regio- and enantioselective cycloisomerization/cross coupling of keto-VDCPs with terminal alkynes has been achieved by using a synergistic rhodium/silver dual catalysis. Control experiments and relevant previous reports showed that the silver salt was used not only to generate cationic Rh(I) catalyst but also as a cocatalyst. The substrate scope is broad allowing the generation of a range of six-membered ring systems containing multiple functional groups such as tertiary hydroxyl group and alkenyl as well as alkynyl moieties in good yields along with high ee values. These findings described here enhance significantly the scope of the catalysis platform outlined in Scheme 5 (B−D), opening up numerous avenues for further development. The utilization of this synthetic methodology for the synthesis of biologically active molecules is currently under investigation.

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EXPERIMENTAL SECTION

General Procedure for the Synthesis of Product 3. To a flame-dried Schlenk tube were added keto-VDCP 1 (0.1 mmol, 1.0 equiv), [Rh(cod)Cl]2 (2.5 mol %), (R)-SDP (6.0 mol %), and AgNTf2 (6.0 mol %). The tube was evacuated and backfilled with argon (repeated three times). Then, terminal monoyne 2 (1.2 equiv) and dioxane (1.0 mL) were added into the tube. The reaction mixture was stirred at 80 °C for 4−10 h. The solvent was removed under reduced pressure, and the residue was purified by a flash column chromatography on silica gel to give the corresponding product 3. Compound 3aa. A white solid, 85% yield (41 mg). Mp: 122−124 °C. 1H NMR (CDCl3, 400 MHz, TMS) δ 1.81 (s, 3H), 2.43 (s, 3H), 2.88 (s, 2H), 2.91 (brs, 1H), 2.98 (d, J = 11.2 Hz, 1H), 3.43 (d, J = 15.6 Hz, 1H), 3.47 (d, J = 11.2 Hz, 1H), 3.84 (d, J = 15.6 Hz, 1H), 4.71 (d, J = 1.2 Hz, 1H), 5.37 (d, J = 1.6 Hz, 1H), 7.23−7.27 (m, 5H), 7.28−7.35 (m, 5H), 7.41 (d, J = 7.2 Hz, 2H), 7.62 (d, J = 8.0 Hz, 2H). 13 C NMR (CDCl3, 100 MHz, TMS) δ 18.3, 21.5, 27.8, 49.5, 57.5, 73.0, 83.4, 86.6, 118.5, 123.6, 126.2, 127.4, 127.7, 127.81, 127.85, 128.2, 129.7, 129.8, 131.4, 132.6, 136.7, 139.9, 141.5, 143.9. IR (CH2Cl2) ν 3052, 2984, 2970, 2918, 2360, 2342, 1597, 1490, 1448, 1380, 1346, 1248, 1153, 1091, 1041, 987, 911,861, 812, 790, 757, 731, 702, 792, 662 cm−1. HRMS (ESI) calcd for C30H33N2O3S (M + NH4)+: 501.2206, found: 501.2206. Enantiomeric excess was determined by HPLC with a Chiralcel IC-H column [λ = 254 nm; eluent: hexane/ isopropanol = 80/20; flow rate: 0.50 mL/min; tminor = 28.77 min, tmajor = 37.71 min; ee% > 99%; [α]20 D = +10.2 (c 1.00, CH2Cl2)]. Typical Procedure for the Preparation of Compound 4. Under argon atmosphere, to a flame-dried Schlenk tube were added compound 3 (0.1 mmol), phenylboronic acid (3.0 equiv), [Rh(OH)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02027. Experimental procedures, characterization data, NMR spectra (PDF) 5962


Article


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crystallographic data for 3aa (CIF) crystallographic data for (CIF) crystallographic data for 3aa (CIF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Min Shi: 0000-0003-0016-5211 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Basic Research Program of China (973)-2015CB856603, the Strategic Priority Research Program of the Chinese Academy of Sciences, grant no. XDB20000000, the National Natural Science Foundation of China (20472096, 21372241, 21572052, 20672127, 21421091, 21372250, 21121062, 21302203, and 20732008), and the Fundamental Research Funds for the Central Universities 222201717003.

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Silver Synergistic Catalysis in Highly Enantioselective

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