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Catalytically Asymmetric Synthesis of 1,3-Bis(silyl)propenes via Copper-Catalyzed Double Proto-Silylations of Polar Enynes Fei-Fan Meng, Jia-Hao Xie, Yun-He Xu, and Teck-Peng Loh ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00999 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018
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ACS Catalysis
Catalytically Asymmetric Synthesis of 1,3-Bis(silyl)propenes Copper-Catalyzed Double Proto-Silylations of Polar Enynes
via
Fei-Fan Meng,a Jia-Hao Xie,a Yun-He Xu*a and Teck-Peng Loha,b a
Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, P. R. China 230026
b
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 Supporting Information Placeholder
ABSTRACT: A copper catalyzed double proto-silylations of electron-deficient conjugated enynes to synthesize functional bis(silyl)propenes was developed. Under mild reaction conditions, the silylboronate PhMe2Si-Bpin reacts with the enynes via cascade proto-silylations and yields the corresponding Z-specific bis(silyl)propene products in up to 98% yield and 95% ee. In addition, the enantioselective synthesis of these compounds was also achieved by using enantiopure pyridine-oxazoline-based ligand.
KEYWORDS: copper, silylborane, conjugate addition, proto-silylation, bis(silyl)propene
INTRODUCTION 1
Among the functional allylic silanes, the bis(silyl)propenes are one of the most important and useful building blocks in organic synthesis due to their wide range of selective 2 transformations. Therefore, some efforts have been devoted to preparing these bis(silyl)propene compounds. Among them, most of the cases are featured by pre-installation of a silyl group into the starting materials. Subsequently, another silyl group can be introduced into the silyl-substituted 3 substrates. So far, only few examples have been developed to simultaneously fix two silyl groups on the substrates for direct synthesis of bis(silyl)propenes. For example, the organolithium agents generated in situ were found efficiently react with chlorosilanes and yield the bis(silyl)propenes 4 (Scheme 1, eqs 1. and 2). In 1990, Luh and co-workers reported a nickel-catalyzed alkylative alkenation of orthothioesters to synthesize the bis(silyl)propenes (Scheme 5 1, eq 3). Recently, Nakamura group developed an iron-catalyzed carbosilylation reaction of alkynes, in which one example for synthesis of 1,3-bis(silyl)propene product mainly with an E-configuration was demonstrated by using 6 symmetric oct-4-yne. Despite these advances, the limitations of reaction efficiency, substrate scope and functionality tolerance still spur us to discover new and more effective methods for the synthesis of these compounds. Additionally, to construct more complex molecules, the preparation of region- and stereodefined functional 2g, 7 bis(silyl)propenes is also highly desirable. Unfortunately, so far the detailed studies to access these multi-functional bis(silyl)propenes especially with high Z/E ratio and 3g, 3h, 6 enantioselectivities are still rare. Inspired by the 8 pioneering works on copper-catalyzed proto-silylation and combined with our continuous interest in C-Si bond 9 formation , herein we would like to communicate the results of copper-catalyzed double proto-silylations of
electron-deficient conjugated enynes in one-pot manner. The desired racemic and enantioenriched 1,3-bis(silyl)propene products could be synthesized in high efficiency, respectively.
Scheme 1. Copper-catalyzed Double Proto-Silylations of Electron-deficient Enynes.
Table 1. CuCl Catalyzed Double Proto-Silylations of 1a.a
entry 1
2 (equiv) 3.0
2
3.0
2
12
82
14
--
3
3.5
2
12
95c
--
---
4d
3.5
2
12
--
3
97
5
3.5
--
12
3
71
--
6
3.5
2
--
10
40
--
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DIPEA (equiv) 0.1
L1 (mol%) 12
3a (%)b 34
4a (%) 63
1a (%) --
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Reaction was run under the following reaction conditions: 0.1 mmol 1a, 0.35 mmol 2 (3.5 equiv), 10 mol% CuCl, 12 mol% L1 and 0.2 mmol DIPEA (2.0 equiv) were stirred in 0.5 mL b MeOH at 30 °C for 12 h under argon atmosphere. Yields was 1 c d determined by H NMR. Yield is isolated yield. Without CuCl. DIPEA: N,N-Diisopropylethylamine.
RESULTS AND DISCUSSION To test the feasibility of copper-catalyzed double proto-silylations of electron-deficient enynes, we embarked on the study with use of enynoate 1a and the silylboronate 8j, 10 PhMe2Si-Bpin as model reaction. Pleasingly, when 12 mol% 4,4'-di-tert-butyl-2,2'-bipyridine (L1) as ligand, 10 mol% DIPEA (N,N-diisopropylethylamine) as base, and 10 mol% CuCl as catalyst (Table 1, entry 1) were applied in this reaction. The target bis(silyl)propene product 3a with an exclusive E-configuration could be isolated in 34% yield and along with formation of the mono-protosilylation product 4a in 63% yield. Therefore, the loadings of silylboronate 2 and DIPEA were improved, and the yield of product 3a was up to 95% (Table 1, entries 2-3). Further study showed that the pure mono-protosilylation product 4a could also be fully converted into the bis(silyl)propene 3a under the optimized reaction conditions. Thus, the double proto-silylations of conjugated enynes in one pot was established. Finally, the control experiments showed that the copper catalyst, DIPEA base and 4,4'-di-tert-butyl-2,2'-bipyridine ligand were all essential to guarantee a high yield of the product (Table 1, entries 4-6).
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After determining the optimized reaction conditions, different electron-deficient conjugated enynes were subjected to current copper-catalyzed double proto-silylations reaction (Table 2). Firstly, the substrates with various aryl substituents at 4-position were tested, all the desired products were obtained in high to excellent yields (Table 2, 3a-3g). Next, different enynoates were also examined to react with silylboronate 2 under the optimized reaction conditions, the isopropyl-substituted enynoate was found to be less efficient to afford the desired product (Table 2, 3i-3l). On the other hand, when other electron-deficient conjugated enynes such as (E)-5-phenylpent-4-en-2-ynenitrile and (E)-1,7-diphenylhept-6-en-4-yn-3-one were used for this reaction, the corresponding bis(silyl)propene products 3m and 3n bearing a nitrile or ketone functionality were produced in good yield, respectively. It is worth mentioning that diverse aliphatic-substituted enynoates tethering with other functionalities such as halides, hydroxyl, ether, and phthalimidyl were also well tolerated in this reaction to give the corresponding products in high to excellent yields (Table 2, 3q-u). Only the product 3o was obtained in a low yield which was possibly due to the steric hindrance caused by the cyclohexyl group.
Table 3. Optimization of Copper-Catalyzed Asymmetric Double Proto-Silylations of 1a with PhMe2Si-Bpina
Table 2. Copper Catalyzed Double Proto-Silylations of Various Substrates with PhMe2Si-Bpina
entry
[Cu]
ligand
1
CuCl
L2
temp. (℃) 30
yield (%)b 36
ee (%)d 53
2
CuCl2
L2
30
25
53
3
CuI
L2
30
19
46
4
Cu(OAc)2
L2
30
24
64
5
CuSO4
L2
30
89
56
6
Cu(OTf)2
L2
30
90
58
7
Cu(OTf)2
L3
30
34
63
8
Cu(OTf)2
L5
30
23
10
9
Cu(OTf)2
L8
30
99
75
10
Cu(OTf)2
L8
0
99
86
11
Cu(OTf)2
L8
-10
99
87
12
Cu(OTf)2
L8
-20
99
89
13
Cu(OTf)2
L6
-20
--
--
14
Cu(OTf)2
L9
-20
99
88
15
Cu(OTf)2
L4
-20
99
61
16
Cu(OTf)2
L7
-20
99
89
17
Cu(OTf)2
L10
-20
99
91
18e
Cu(OTf)2
L10
-20
99 (90)c
91
a
a
Condition A: Reactions were carried out in MeOH (0.2 M), in the presence of 1a-u (0.20 mmol), 10 mol% of CuCl, 12 mol% of L1, 2.0 equiv of DIPEA, 3.5 equiv of PhMe2Si-Bpin at o b 30 C under Ar for 12~19h (see SI). EtOH as solvent.
Reaction was run under the following reaction conditions: 0.1 mmol 1a, 0.35 mmol 2 (3.5 equiv), 10 mol% Cu(OTf)2, 22 mol% L10 and 0.2 mmol DIPEA (2.0 equiv) in 0.5 mL MeOH b 1 for 12 h under Ar atmosphere. Yield was determined by H c d NMR. Isolated yield. The values of ee were determined by e HPLC. 5 mol% Cu(OTf)2 and 11 mol% L10.
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ACS Catalysis It is well known that the enantiomerically pure allylic silanes are feasible and facile building blocks for the 11 synthesis of chiral natural products and biologically active 12 molecules . Thus the enantioselective synthesis of bis(silyl)propenes was investigated. The enantiopure pyridine-oxazoline-based compounds were applied as ligands to coordinate with copper catalysts. Pleasingly, the product 3a* could be obtained in 36% yield and with 53% enantiomeric excess (ee) when CuCl and (S)-i-PrPyrOx (L2) were applied (Table 3, entry 1). Accordingly, other copper catalysts and ligands were screened, the combination of Cu(OTf)2/L8 displayed relatively higher efficiency (Table 3, entry 2-9). Further reducing the temperature to -20°C would significantly improve the ee value of the product 3a* to 89% without diminishing the product yield (Table 3, entries 10-12). Then more efforts to tune the electronic and steric effects of the ligands were pursued to improve the product’s enantioselectivity (Table 3, entry 13-17). Finally, it was found that L10 was more effective to give the desired product in 90% isolated yield and up to 91% ee value. Importantly, decreasing the loadings of Cu(OTf)2 catalyst and ligand to half had no negative effect on the result (Table 3, entry 18).
Table 4. Copper-Catalyzed Asymmetric Double Proto-Silylations of Various Enynaoates with PhMe2Si-Bpina EWG + PhMe 2Si-Bpin R
Cu(OTf)2 (5 mol%) L10 (11 mol%) DIPEA (2 equiv) MeOH, -20 °C, 12 h, Ar
PhMe 2Si 3*
Me
CO 2Me PhMe 2Si
SiMe2Ph
3a*, 90% 91% ee
Furthermore, the enynoate (Z)-1a was also prepared and subjected to this reaction under the optimized reaction conditions. Unfortunately, a complex mixture including 1 about 12% yield of 3a (determined by crude H NMR) was obtained (Scheme 2, eq 1). It is noteworthy that current copper-catalyzed double proto-silylations of enynoate could be scaled up for gram-scale synthesis of 3a in a 95% yield and with 90 % ee (Scheme 2, eq 2). Interestingly, different silyl groups could be introduced sequentially into 1a in a one-pot manner with different silylboranes (Scheme 2, eq 3).
SiMe2Ph
2
1
EWG
R
or 2-thienyl group at δ-position both furnished the corresponding bis(silyl)propenes in excellent yields and with high ee values (Table 4, 3g* and 3h*). Besides the methyl ester 3a*, the ethyl, isopropyl, and benzyl analogues were also obtained in high yields and with slightly decreased ee values (Table 4, 3i*-3k*). In the reactions of 1i and 1j, to avoid the formation of methanol transesterification products, the ethanol with less nucleophilic ability replaced the methanol as the solvent without significant diminishing the product’s yield and ee value. When the nitrile 1m and ketone 1n were examined under the standard reaction conditions, the ee values of the products were lower comparing with the methyl esters. (Table 4, 3m* and 3n*) Unfortunately, the cyclohexyl substituted enynoate was unfavorable for this enantioselective double proto-silylations. And the desired product 3o* was only obtained in 55% yield and with 56% ee value even after prolonging the reaction time to 56h.
OMe
CO 2Me PhMe 2Si
SiMe2Ph
CO 2Me PhMe 2Si
O
SiMe2Ph
3c*, 85% 94% ee
3b*, 92% 92% ee
Cl
F
O CO 2Me PhMe 2Si
SiMe2Ph
3d*, 93% 92% ee
PhMe 2Si
SiMe2Ph
PhMe 2Si
SiMe2Ph
3g*, 92% 93% ee
S PhMe 2Si
CO 2R'
CO 2Me SiMe2Ph
O
PhMe 2Si
SiMe2Ph
3m*, 65% 85% ee
SiMe2Ph
R' = Et, 3i*, 94% (89% ee)b R' = iPr, 3j*, 85% (88% ee) R' = Bn, 3k*, 97% (79% ee)b
3h*, 88% 95% ee
Scheme 2. (1) Reactivity of (Z)-1a under Condition B; (2) Large Scale Experiment; (3) Introducing Two Silyl Groups in One-Pot Process.
Ph
CN PhMe 2Si
SiMe2Ph
3f*, 87% 90% ee
3e*, 92% 91% ee
CO 2Me PhMe 2Si
CO 2Me
CO 2Me
CO 2Me PhMe 2Si
SiMe2Ph
3n*, 90% 74% ee
PhMe 2Si
SiMe2Ph
3o*, 55%c 56% ee
a
Condition B: Reactions were carried out in MeOH (0.2 M), in the presence of 1a-k, 1o (0.20 mmol), 5 mol% of Cu(OTf)2, 11 mol% of L10, 2.0 equiv of DIPEA, 3.5 equiv of o b PhMe2Si-Bpin at -20 C under Ar for 12h. EtOH as solvent. c 56 h. Having established the optimized reaction conditions for the enantioselective synthesis of functional bis(silyl)propenes, then different enynoates were tested by employing 5 mol% Cu(OTf)2 and 11 mol% chiral pyridine-oxazoline-based ligand L10 (Table 4). Notably, all the methyl enynoates with different substituted on the phenyl ring were found to be effective for this conversion. Moreover, the substrates with a 2-naphthyl
Scheme 3. Determination of Configuration of Compound 3a*.
the
Absolute
To determine the absolute configuration of enantiomerically pure bis(silyl)propene 3a*, its derivation reactions were performed (Scheme 3). Firstly, the compound 3a* was treated with DIBAL-H to give the aldehyde 5a*, 3
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which was sequentially oxidized into acid 6a*. Following that, condensation of 6a* with 3-bromocarbazole gave the amide product 7a* as solid with 99% ee value after 14 recrystallization. Finally, single-crystal X-ray diffraction analysis produced the molecular structure of amide 7a* with (S)-configuration (CCDC Nos: 1816910).
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indicated their potential applications in the synthesis of useful dicarbonyl compounds. Further experiments on exploring diverse utility of these functional bis(silyl)propene compounds are in progress in our laboratory.
AUTHOR INFORMATION Corresponding Author
[email protected] ORCID Yun-He Xu: 0000-0001-8817-0626
Notes i) TBAT, ethyl glyoxalate, THF, r. t. ii) 1) TBAT, CO2 balloon, DMSO, 30 ; 2) CH3I. TBAT: tetra-n-butylammonium difluorotriphenylsilicate. The synthetic utilities of the bis(silyl)propene 3a were also demonstrated by its transformations (Scheme 4). When the chiral compound 3a* and ethyl glyoxylate were treated with TBAT in THF solution, the 1,7-diester 8a could be generated in a moderate yield via desilylation/nucleophilic addition 15 processes. On the other hand, a 1,6-diester compound (9a) was obtained in 52% yield by using CO2 as electrophile. Unfortunately, no chirality transformation of the products was observed in both cases.
Scheme 4. Synthetic Applications of Compound 3a
The authors declare no competing financial interests.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Supporting spectra and characterization data (PDF) Crystallographic data for compound 7a* (CIF)
ACKNOWLEDGMENT We gratefully acknowledge the funding support of the National Natural Science Foundation of China (21672198), the State Key Program of National Natural Science Foundation of China (21432009), the Fundamental Research Funds for the Central Universities (WK2060190082), the Open Project of Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University (2017110) and the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChEM) for financial support.
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Conjugate
Addition
Silylcupration
to
of
α,β-Unsaturated
Terminal
Alkynes
to
Crotylation
of
Aldimines
with
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