Highly enantioselective Michael addition of 3-arylthio ...

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Cite this: Chem. Commun., 2014, 50, 15179 Received 15th August 2014, Accepted 7th October 2014 DOI: 10.1039/c4cc06417h

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Highly enantioselective Michael addition of 3-arylthio- and 3-alkylthiooxindoles to nitroolefins catalyzed by a simple cinchona alkaloid derived phosphoramide† Wei-Ming Gao, Jin-Sheng Yu, Yu-Lei Zhao, Yun-Lin Liu, Feng Zhou, Hai-Hong Wu* and Jian Zhou*

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A new cinchona alkaloid derived bifunctional tertiary aminephosphoramide C1e is identified as a highly enantioselective catalyst for Michael addition of both unprotected 3-arylthio- and 3-alkylthiooxindoles to nitroolefins. The phosphoramide moiety of C1e plays an indispensable role in this reaction.

The integration of H-bond donors with other functionalities has been established as a fruitful strategy to develop efficient multifunctional catalysts to realize reactivity and stereoselectivities unattainable by monocatalysis.1 In this context, the merger of a certain H-bond donor with tertiary amines to design new catalysts is in particular attractive, as tertiary amines can serve as Lewis bases2a–c or Brønsted bases2d to activate many types of nucleophiles. To date, (thio)urea,3 sulfamide4 and squaramide5 derived bifunctional tertiary amines have been well established and have found ever-increasing application in new reaction development.6 However, less attention has been paid to explore the potential of phosphoramide,7 although such three-dimensional H-bond donors possess two unique characteristics: (1) two amide groups can serve as shielding groups for two directions, useful for the control of stereoselectivities; (2) the pKa value of the N–H bond can be readily tuned by varying the amide group from an aryl to an alkoxy group, which in turn effectively modifies the steric and electronic properties of the catalysts. Attracted by these advantages, along with our efforts in asymmetric syntheses of fully substituted carbon stereocenters by bifunctional tertiary amine catalysis,8 we started to exploit tertiary amine–phosphoramide catalysis. We have developed a cinchona alkaloid derived phosphoramide C1a as a powerful catalyst for the Michael addition of 3-aryl or 3-alkyl oxindoles,9a and an analogous phosphinamide for the Strecker reaction.9b These promising results prompted us to apply these easily available catalysts to develop new reactions, and to investigate the role of the amide N–H bond. Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, 3663N, Zhongshan Road, Shanghai 200062, China. E-mail: [email protected], [email protected]; Fax: +86-21-6223-4560 † Electronic supplementary information (ESI) available: Experimental details and spectra data. See DOI: 10.1039/c4cc06417h

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On the other hand, the demand for privileged scaffolds in chemistry, biology and medicinal research10 has stimulated the diverse synthesis of 3,3-disubstituted oxindoles, widely present in natural products, drugs and pharmaceutically active compounds.11 Despite achievements,12 the highly enantioselective introduction of a sulfur atom at the C3 position of oxindole is still very limited, although sulfide moieties are present in many pharmaceuticals.13a Recently, several oxindole derivatives with interesting biological activities have been reported, possessing a C3 S-containing tetrasubstituted carbon,13 the absolute configuration of which can significantly affect the bioactivity.13b Therefore, it is highly desirable to develop asymmetric syntheses of optically active quaternary 3-thiooxindoles for the structure–activity relationship studies. In 2012, Feng et al. pioneered the asymmetric electrophilic sulfenylation of 3-prochiral oxindoles, using their own powerful chiral C2-symmetric N,N0 -dioxide–Sc(III) complex.14a The corresponding organocatalytic versions were also independently reported by Enders,14b Jiang14c and Li and Cheng,14d respectively.

Given our interests in the oxindole chemistry,8,15 we considered an alternative strategy, the asymmetric functionalization of nucleophilic 3-prochiral thiooxindoles 1, and reported that thiooxindoles 1 could be easily deprotonatively activated for a highly enantioselective amination reaction.16 To expand the synthetic utility of compound 1, we further tried its Michael addition using nitroolefins.17 During our studies, Lu nicely

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utilized 1–5 mol% of the multifunctional catalyst C2 to realize highly stereoselective addition of 3-alkylthiooxindoles to nitroolefins.18 However, only one example of 3-arylthiooxindole was tried, with only 79% ee obtained. Herein, we wish to report our result that a new phosphoramide catalyst C1e, readily available in 2 steps, can enable a general and highly enantioselective Michael addition of both unprotected 3-arylthio and 3-alkylthiooxindoles 1 to nitroolefins to give the desired products in excellent ee values. The reaction development began with evaluation of our previously reported cinchonidine derived phosphoramide C1a in the reaction of 3-arylthio oxindole 1a and nitroolefin 2a, in dichloromethane at 10 1C. However, the desired product 3a was obtained in only 2 : 1 dr and 76% ee (entry 1, Table 1). Accordingly, we tried varying the R1 group of the catalyst to improve the stereoselectivities. Indeed, the newly prepared analogues C1b and c, bearing the bulkier isopropoxy or n-butoxy group, gave product 3a with a higher ee value (entries 2 and 3). We also tried using a 2,2,2-trifluoroethoxy group to enhance the acidity of the amide N–H bond, but corresponding catalyst C1d proved to be inferior in terms of reactivity and ee value (entry 4). Further modification revealed that catalyst C1e, with a 3-pentyloxy group, could achieve 2.7 : 1 dr with 90% ee for product 3a (entry 5), but catalysts C1f and g with a cyclohexyloxy or phenoxy group were less effective (entries 6 and 7). The corresponding phosphinamide C1h gave poor results as well. These results are very interesting, which showed for the first time that varying the size of the alkoxy group of phosphoramide could effectively improve Table 1

the enantiofacial control,7 useful for the further development of such types of chiral catalysts. The following study of the effect of solvent was based on catalyst C1e (entries 9–14). Et2O proved to be the best, improving the dr and the ee value of product 3a to 5.4 : 1 and 94%, respectively (entry 14), which were further enhanced to 7 : 1 and 96% when the reaction was run at 20 1C with the addition of anhydrous powered MS 5 Å (entry 16). For details of additive efforts, see the ESI.† Noticeably, even the use of only 5 mol% or 1 mol% C1e could still achieve uneroded yield and stereoselectivities for 3a (entries 17 and 18). In the following, the scope of the reaction was examined by using 5 mol% of catalyst C1e in Et2O, in the presence of MS 5 Å. Our protocol had a broad scope with respect to substituted 3-arylthio- or 3-alkylthio oxindoles. No matter whether the R1 substituent was an aryl or an alkyl group, the desired products 3a–g were obtained in excellent ee values (entries 1–7, Table 2). But the R1 group obviously influenced the reactivity. For example, the p-chlorophenyl group accelerated the reaction that was run at 30 1C (entry 3), whilst the p-methoxyphenyl one slowed down the reaction which should be run at 0 1C (entry 4). The reaction of 3-benzylthio- or 3-allylthiooxindoles must be run at 0 1C using 10 mol% C1e, to realize reasonable yield (entries 6 and 7). In addition, substituents on the oxindole had no obvious effect on the reaction outcome, and products 3h–k were all obtained in 494% ee (entries 8–11). Both b-aryl and alkyl nitroolefins are viable substrates to give the desired products 3l–u in excellent ee values (entries 12–21). To show the practicability of our protocol, we further tried a gram-scale synthesis. It turned out

Reaction optimization Table 2

Substrate scope

1

Entry Cat. Solvent 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15d 16d 17d 18d

C1a C1b C1c C1d C1e C1f C1g C1h C1e C1e C1e C1e C1e C1e C1e C1e C1e C1e

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 Toluene CH3CN MeOAc Acetone THF Et2O Et2O Et2O Et2O Et2O

X

T (1C) Time (h) Yielda (%) drb

eec (%)

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 5 1

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 20 20 20

76 80 86 51 90 82 76 46 92 60 90 89 93 94 96 96 96 96

8 15 15 50 8 8 15 15 8 5 11 20 20 17 21 36 40 144

95 93 94 72 96 97 93 95 96 98 94 97 97 95 91 96 91 95

2.0 : 1 2.0 : 1 2.3 : 1 1.5 : 1 2.7 : 1 2.0 : 1 2.0 : 1 1.2 : 1 3.4 : 1 2.0 : 1 3.1 : 1 3.4 : 1 4.8 : 1 5.4 : 1 6.5 : 1 7.0 : 1 7.1 : 1 7.1 : 1

a Isolated yield. b Determined by 1H NMR analysis of the crude reaction mixture. c Determined by chiral HPLC analysis, for the major diastereomer. d 50 mg of powdered MS 5 Å was added.

15180 | Chem. Commun., 2014, 50, 15179--15182

Entry R

R1

R2

3

Yielda (%) drb

eec (%)

1 2d 3d 4e 5 6e,g 7e,g 8d 9d 10d 11 12d 13 14 15 16 17 18e,g 19e,g 20f,h 21f,h

2-Naphthyl Ph p-ClC6H4 p-MeOC6H4 p-MeOC6H4 Bn Allyl 2-Naphthyl 2-Naphthyl 2-Naphthyl 2-Naphthyl 2-Naphthyl 2-Naphthyl 2-Naphthyl 2-Naphthyl 2-Naphthyl 2-Naphthyl 2-Naphthyl p-ClC6H4 p-ClC6H4 p-ClC6H4

p-ClC6H4 p-ClC6H4 p-ClC6H4 p-ClC6H4 p-CF3C6H4 p-ClC6H4 p-ClC6H4 p-ClC6H4 p-ClC6H4 p-ClC6H4 p-ClC6H4 p-CF3C6H4 p-FC6H4 p-BrC6H4 Ph p-MeC6H4 2-Thienyl n-Pr n-Pr i-Pr BnCH2

3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k 3l 3m 3n 3o 3p 3q 3r 3s 3t 3u

95 93 92 90 91 98 82 90 91 99 97 95 98 99 94 99 95 96 96 80 97

96 96 97 93 96 92 91 96 95 94 95 97 96 96 96 95 94 97 98 92 97

H H H H H H H F Cl Et MeO H H H H H H H H H H

8:1 7:1 7:1 5:1 6:1 7:1 4:1 7:1 6:1 6:1 5:1 7:1 7:1 8:1 6:1 5:1 7:1 6:1 6:1 5:1 4:1

a Isolated yield. b Determined by 1H NMR or by HPLC analysis of the crude mixture. c For the major diastereomer, by HPLC analysis. d 30 1C. e 0 1C. f 25 1C. g 10 mol% catalyst. h 20 mol% catalyst.


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that 1.0 mol% C1e catalyzed the reaction well at 20 1C to give 3c in 97% yield (1.34 g), 6 : 1 dr and 97% ee, comparable to the result by using 5.0 mol% C1e (entry 3 of Table 2).

The broad scope and excellent enantioselectivity achieved by the easily available catalyst C1e was impressive, which encouraged us to compare it with other well-known bifunctional catalysts C3–69c,d in the reaction of 1a and 2a. Under the standard conditions, only sulfamide C3 achieved 91% ee and 5 : 1 dr values for 3a, slightly lower than those obtained by C1e (96% ee and 7 : 1 dr). Other bifunctional catalysts (amide C4, thiourea C5 and squaramide C6) all gave 3a in a much lower ee value. These results justified the further exploration of bifunctional tertiary amine–phosphoramide catalysis.

To probe whether the amide N–H bond played a role in the reaction, the corresponding N-methyl catalyst C7 was prepared. Not unexpectedly, it proved to be impotent in terms of reactivity and stereoselectivity, giving 3a in only 24% yield, 1.4 : 1 dr and 2% ee. This result unambiguously confirmed the indispensable role of the amide N–H bond, which was proposed as shown in working model I: the H-bonding activation of nitroolefins by the amide N–H bond facilitated the attack from Re-face by thiooxindole 2 that was simultaneously deprotonatively activated by the quinuclidine moiety of C1e, but the detailed reaction mechanism is still under investigation.

In conclusion, we have developed a new cinchonidine derived bifunctional phosphoramide catalyst C1e, which enables a general and highly enantioselective Michael addition of 3-prochiral thiooxindoles to nitroolefins, with catalyst loading down to 1.0 mol%.


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Importantly, this research confirms the role of phosphoramide as an effective H-bond donor to cooperate with tertiary amines to improve the reactivity, and the possibility of varying the amide alkoxy group to improve the stereoselectivities. The development of new bifunctional phosphoramide catalysts to realize unprecedented asymmetric reactions is ongoing in our laboratory. The financial support from the 973 program (2011CB808600), NSFC (21172075 and 21222204), the Ministry of Education (NCET11-0147 and PCSIRT) and the Program of SSCS (13XD1401600) is appreciated.

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