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Letter
Enantioselective Rhodium-Catalyzed Desymmetric Hydrosilylation of Cyclopropenes Zhi-Yuan Zhao, Yi-Xue Nie, Ren-He Tang, Guan-Wu Yin, Jian Cao, Zheng Xu, Yu-Ming Cui, Zhan-Jiang Zheng, and Li-Wen Xu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02623 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019
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ACS Catalysis
Enantioselective Rhodium-Catalyzed Desymmetric Hydrosilylation of Cyclopropenes Zhi-Yuan Zhao,†, § Yi-Xue Nie,†, § Ren-He Tang,† Guan-Wu Yin,† Jian Cao,† Zheng Xu,† Yu-Ming Cui, † Zhan-Jiang Zheng,† and Li-Wen Xu*†,‡ †Key
Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, and Key Laboratory of Organosilicon Material Technology of Zhejiang Province, Hangzhou Normal University, No. 2318, Yuhangtang Road, Hangzhou 311121, P. R. China ‡State
Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute (SRI) of Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences and University of Chinese Academy of Sciences (UCAS), Lanzhou 730000, P. R. China ABSTRACT: Catalytic hydrosilylation of alkenes ranks one of the most important academic and industrial synthetic reactions in homogeneous catalysis and organosilicon chemistry. In this work, a highly enantioselective Rh-catalyzed desymmetric hydrosilylation reaction of 1,1-disubstituted cyclopropenes with the promotion of chiral DTBM-SEGPHOS is described, allowing for a straightforward access to potentially valuable chiral organosilicon compounds bearing a carbon quaternary stereocenter in high yields with good diastereo- and enantio-selectivities (up to >99:1 d.r. and >99% ee) in a 100% atom-efficient manner from readily available starting cyclopropenes and hydrosilanes. KEYWORDS: Homogeneous catalysis; hydrosilylation; cyclopropanes; asymmetric catalysis; rhodium.
Hydrosilylation of alkenes is a very important and practical methodology to access organosilicon compounds and ranks one of the most fundamental academic and industrial synthetic reactions in homogeneous catalysis and organic chemistry,1 which commonly used for the large-scale production of silane coupling agents, organosilicon polymers, and cross-linker materials.2 While transition-metal catalysts based on Pt, Rh, Ni, Cu, Fe, and Co species used in Si-C bond-forming hydrosilylation process have received steadily increased attention in the past decades,1-3 recent studies on hydrosilylation have focused intensively on enantioselective synthesis and utility of chiral organosilicon compounds that are of increasing value in organic synthesis, medicinal chemistry, and functional materials.4-6 However, the synthesis of chiral silanes is not an easy task due to the scarcity of highly efficient and broadly applicable hydrosilylation catalysts for the regio- and enantioselective construction of sp3-central chirality in Si-C bond-forming process.4 In this regard, the development of new synthetic chemistry in stereoselective hydrosilylation requires transition-metal catalysts and chiral ligands that can not only effect Si-H bond activation with good chemoselectivity but also discriminate the two enantiotopic face during enantioselective induction of silyl-functionalization on alkenes.5 In this field, a key breakthrough is Hayashi’s Pd-MOP complexes-catalyzed Markovnikov-type asymmetric hydrosilylation with trichlorosilane.7 And Chirik’s report8 on Fe-bis(imino)pyridine [such as iPrPDI, 2,6-(2,6-iPr2-C6H3N=CMe)2C5H3N] complexes
opened a new access to the earth-abundant transition metalcatalyzed alkene hydrosilylation and its asymmetric version subsequently reported by Lu, Huang, and Buchwald, et al. (Scheme 1, equation a).9-12 To date, enantioselective hydrosilylation of terminal alkenes,9 conjugated dienes, 10 allenes, 11 and alkynes12 have been well established for the synthesis of chiral alkylsilanes and silicon-stereogenic silanes.13 Although these catalytic hydrosilylation processes have generally developed to make functionalized linear alkylsilanes with good regio- and enantioselectivities, the limitation that deficiency of highly enantioselective hydrosilylation of internal alkenes and carbocycles motivates us to aim at developing a highly efficient and enantioselective construction of structurally diverse and chiral silyl carbocycle compounds, providing a straightforward and efficient approach to a wide range of optically pure and silyl-functionalized polysubstituted carbocycles and organosilicon compounds. Meanwhile, among carbocycle chemistry, the cyclopropane ring is unique and extremely important building blocks in organic synthesis due to its inherent ring strain and unusual electronic properties.14 Over the past 20 years, asymmetric preparation and utility of substituted cyclopropanes or its derivatives based on direct functionalization of prochiral or achiral three-membered carbocycles has consistently been a very interesting and hot research topic in synthetic chemistry and catalysis.15 In asymmetric transition-metal catalyzed direct hydrofunctionalization of cyclopropenes,14-15 the key
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cyclopropenylmetal intermediates having a carbon-stereogenic center generally formed by transmetalation can smoothly undergo enantioselective cross-coupling process in the presence of organostannanes, 16 boranes,17 aromatic aldehydes, 18 aryl boric acids, 19 alkynes20, oximes21, diorganozinc reagents,22 amines,23 and others (Scheme 1, equation b).24 However, largely different from that of hydroboration or hydrostannation reaction16-17, we are aware of no report on the desymmetric hydrosilylation of cyclopropenes, presumably because of the difficult in the Si-H bond activation on hydrosilane and in-situ formation of chiral cyclopropenylmetal intermediate on the three-membered carbocycles.
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diastereo- and enantioselectivity of the Rh-catalyzed hydrosilylation of the 1,1-disubstituted cyclopropene 1a by varying the reaction Table 1. Optimization of the Rh-catalyzed Asymmetric Hydrosilylation of the 1, 1-Disubstituted Cyclopropene 1a O O
+ [Si]-H
1a
[Rh(cod)2]BF4 (3.0 mol%) (R)-DTBM-SEGPHOS (3.6 mol%) [B(Ar)4]Na (3.0 mol%) Hexane, 30 oC, 20 h
O O
[Si]
H2 Si
[Si]
Pd, Fe, Co, Cu, Sc
R
[Si]
R = alkyl, aromatic
+ R
R
[Si]-H =
[B(Ar)4]Na (B1):
Ar =
(b) Transition-metal-catalyzed hydrofunctionalizations of cyclopentenes R1
catalyst R2
R1
R2
X reagent
E
Yield (%)b
d.r. (%)b
ee (%)c
None
84
>99:1
98
2e
Without additive
30
87/13
75
3e
RhCl3 instead [Rh(cod)2]BF4
of
n.r
-
-
4e
[Rh(OAc)2]2 [Rh(cod)2]BF4
of
n.r
-
-
5e
[Rh(PPh3)3]Cl [Rh(cod)2]BF4
instead
of
n.r
-
-
6e
[Rh(nbd)2]Cl2 [Rh(cod)2]BF4
instead
of
11
82/18
54
7e
[PdCl(C3H5)]2 [Rh(cod)2]BF4
instead
of
19
47/53
0
8e
L2 instead of L1
44
95/5
53
9e
L3 instead of L1
34
35/65
31
10e
L4 instead of L1
54
85/15
20
11e
L5 instead of L1
Traced
-
-
12e,f
CoCl2/L6 complex instead of Rh/L1
Traced
-
-
13
[B(Ph)4]Na instead of B1
58
97/3
92
14
AgSbF6 instead of B1
34
94/6
80
15
THF instead of Hexane
30
80/20
79
16
DCM instead of Hexane
52
96/4
96
17
(S)-L1 instead of (R)-L1
65
>99:1
96
Entrya
Variation conditions
1
E = alkyl, aromatic , amine, borane, or metal
X = Et2Zn, RMgX, heterocycle, Ar-B(OH)2 , pinBH, alkyne, HSnMe3, R2NH, etc. This work: (c) Rh-catalyzed Hydrosilylation synergized with desymmetrization R1
R2
R2
R1
SiHArR Desymmetrization
[Rh] Chiral ligand
R1 R2
Ar2SiH2
R1
Hydrosilylation SiHAr2
CF3
from
standard
[Si]
[Si] = HSiCl3, ArSiH2 , ArRSiH
High diastereoselectivity High enantioselectivity
3a
CF3
2a
Previous work: (a) Transition-metal-catalyzed hydrosilylation of olefins
instead
R2
SiAr2 HO
Scheme 1. From Catalytic Hydrosilylation of Terminal Alkenes to Hydrofunctionalizations of Cyclopropenes (previous work): Enantioselective Hydrosilylation of Cyclopropenes Featured with Internal Alkenes (this work) In linked with our program to develop new catalytic strategy on desymmetric construction of quaternary carbon seterocenters, 25 we herein report our exploration on the establishment of highly diastereo- and enantioselective rhodium-catalyzed desymmetric hydrosilylation of 1,1disubstituted cyclopropene derivatives (Scheme 1, equation c), in which a straightforward access to chiral organosilicon compounds and corresponding double carbon stereocenters were completed by catalytic asymmetric hydrosilylation in high yields with excellent diastereo- and enantio-selectivities. For our initial studies, we want to know the different reactivity of hydrosilanes in the rhodium-catalyzed hydrosilylation of cyclopropene 1a. Interestingly, it was found the diphenylsilane 2a was a suitable reagent that better than that of dimethyl(phenyl)silane, triethylsilane, and triethoxysilane in this reaction, and only diphenylsilane exhibited high reactivity under Rh-based reaction conditions. Then we choose diphenylsilane 2a as a model hydrosilane to explore the
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a
Unless otherwise noted, the standard reaction conditions were as follows: 1 (0.2 mmol), 2 (0.26 mmol), and solvent (2.0 mL, DCM was used solvent in entries 2-14). b Determined by crude 1H NMR analysis. c The ee value of 3a was determined by chiral HPLC analysis. d Determined by GC-MS analysis and trace is 99%ee
R1
[Rh(cod)2]BF4 (3.0 mol%) (R)-DTBM-Segphos (3.6 mol%) [B(Ar)4]Na (3.0 mol%) Hexane, 30 oC, 20 h
O O
SiH O O 3a 80%, 94% ee, 94:6 dr
Scheme 2. Gram-scale Hydrosilylation Reaction conditions, including rhodium salts, chiral ligands, additives, and solvents as summarized in Table 1 (for more details for the optimization of reaction conditions, see the Supporting Information). After screening different ligands and reaction parameters, we were pleased to observe that the desired chiral organosilicon product 3a could be obtained with excellent diastereo- and enantioselectivity (>99:1 d.r. and 98% ee) by the use of [Rh(cod)2]BF4 as the Rh catalyst source in hexane with (R)-DTBM-SEGPHOS as chiral ligand (Table 1, entry 1) in the presence of sodium tetrakis[3,5bis(trifluoromethyl)phenyl]borate (B1). No additive or the use of other additives, such as [B(Ph)4]Na or AgSbF6, led to inferior yield, diastereoselectivty and enantioselectivity (Entries 1, 2, 13 and 14). The importance of additive B1 was also confirmed by 31P NMR (See Figure S7 of Supporting Information). These experimental results suggested that the electron-deficient anion would enhanced the coordinated ability between Rh and chiral P-ligand, which is beneficial to the stereoselectivity enhancement. And in Table 1, the experimental results showed the crucial role of the rhodium salts and chiral ligands in the stereoselectivity of the hydrosilylation reaction (entries 2–6). The low conversion and poor stereoselectivity was observed when RhCl3, [Rh(OAc)2]2, [Rh(PPh3)]Cl, or [Rh(nbd)2]Cl2 was used in DCM (entries 36). Notably, generally reported palladium and cobalt catalysts (for example, (iPrIPOiPr)FeCl2)7,9 that well-established for asymmetric hydrosilylation of terminal alkenes were not suitable for this hydrosilylation of 1,1-disubstituted cyclopropene 1a (entries 7 and 12). Notably, after assessing chiral ligands (entries 8–11 for representative examples; see the Supporting Information for a full list of chiral P-ligands or other ligands tested), no additional ligands can provide good
2
1
SiH O
R
R
3a, R = H, 84% yield, 98% ee, d.r. = >99:1 3b, R = F, 80% yield, 98% ee, d.r. = >99:1 3c, R = Cl, 76% yield, 93% ee, d.r. = 93:7 3d, R = Br, 80% yield, 96% ee, d.r. = 94:6 3e, R = OMe, 39% yield, 88% ee, d.r. = 93:7 3f, R = Me, 94% yield, 96% ee, d.r. = >99:1 3g, R = t-Bu, 94% yield, 96% ee, d.r. = 93:7
3h, R = F, 80% yield, >99% ee, d.r. = 98:2 3i, R = Cl, 90% yield, >99% ee, d.r. = 95:5 3j, R = Me, 94% yield, >99% ee, d.r. = 98:2 F
O
SiH
O O (CH2)2Ph 3v, 89% yield, 98% ee, d.r. = 95:5
O O
O
O
O
3w, R = i-Pr, 80% yield, 96% ee, d.r. = >99:1 3x, R = n-Bu, 87% yield, 98% ee, d.r. = 98:2 3y, R = Et, 95% yield, 98% ee, d.r. = >99:1 3z, R = -cyclo-(CH2)6, 70% yield, 93% ee, d.r. = 98:2 3aa, R = -cyclo-(CH2)5 , 70% yield, 96% ee, d.r. > 99:1
O
O
3t, 80% yield, >99% ee, d.r. = 93:7
O R
O
SiH
O
O B O
SiH
3u, 81% yield, 96% ee, d.r. = >99:1
Me
SiH
SiH
O O
3ab, 67% yield, 90% ee, d.r. = >99:1
O Bn
X-ray strcuture fof 3t (CCDC 1915910)
O
3ab, 70% yield, 97% ee, d.r. = 90:10
SiH O
SiH
O SiH
Cl
F
SiH
SiH
O
3q, 94% yield, >99% ee, d.r. = 95:5
O
3s, 70% yield, 92% ee, d.r. = 95:5
SiH O
3p, 85% yield, >99% ee, d.r. = 96:4
O
O
O R2
Cl
O F 3k, R = F, 73% yield, 99% ee, d.r. = >99:1 3l, R = Cl, 65% yield, 94% ee, d.r. = 95:5 3m, R = Br, 80% yield, 99% ee, d.r. = 94:6 3n, R = OMe, 82% yield, 97% ee, d.r. = 96:4 3o, R = CF3, 82% yield, 95% ee, d.r. = 95:5 Cl
SiH
Cl
3
O
3r, 89% yield, 92% ee, d.r. >99:1
Cl
O
R
O
O
R
SiH
SiH
O
SiH
1
3ac, 30% yield, 7% ee, d.r. = 80:20
Scheme 3. Enantioselective Desymmetric Hydrosilylation of Cyclopropenes
O
O
3ad, 40% yield, 86% ee
Rhodium-catalyzed 1, 1-Disubstituted
Then with the optimized conditions showed in Table 1, we explored the substrate scope of the Rh-catalyzed desymmetric hydrosilylation reaction with respect to the cyclopropene substrates for one-step construction of two carbon stereocenters. A series of 1, 1-didubstituted cyclopropenes was used in the Rh-catalyzed desymmetric hydrosilylation with
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diphenylsilane. Representative results showed with a broad scope are summarized in Scheme 3. Similarly to that of methyl 1-phenylcycloprop-2-enecarboxylate (1a), Most of ortho-, para-, or meta-substituted aryl cyclopropenes undergo smooth hydrosilylation and desymmetrization, providing the corresponding chiral silanes 3 in good to excellent yields with excellent enantioselectivities (up to >99% ee). It should be noted that most of the silyl disubstituted cyclopropane 3 were obtained with excellent diastereoisomers (up to >99:1 d.r.), confirming that the desymmetric hydrosilylation proceeds with the aid of the direction of ester group. For the hydrosilylation reactions leading to silyl cyclopropanes 3, we found that the introduction of electron-deficient or -neutral groups, such as F, Cl, Br, provides better enantiomeric ratio and diastereoselectivity in comparison to that of electron-donating methoxyl-substituted aryl cyclopropane (3a-3g). Interestingly, the steric effect on the reactivity and enantioselectivity was also studied under the optimized condition. It was found that all the desired product 3h-3j were obtained with >99% ee and high diastereoselectivity (95:5 to 98:2 d.r.).
O
+ R 1a
Si H2 2
substituents on the hydrosilane showed significant influences on the activity and enantioselectivity. Notably, to confirm the absolute configuration of the silyl cyclopropanes 3, X-ray crystallographic analysis of 3t unambiguously established its structure and the absolute stereochemistry of Si- and ester -linked carbon center was determined as (S, R)-configuration respectively. As a supplement, we compared the calculated electronic circular dichroism (ECD) and experimental spectrum of the compound 3a (see Supporting Information, Figure S1 and S2) to confirm the (S, R)-configuration of the products 3 because ECD has been proved to be another reliable option to determine the stereochemistry of chiral molecules.26 R1 O
R2
H2 Si
O +
SiH R
Hexane, 30 oC, 20 h
O
O
3
O
SiH
3af, 80% yield, d.r. = 52:48 99% ee (major), 99% ee (minor)
O
SiH Me
O
3ag, 78% yield, d.r. = 53:47 99% ee (major), 99% ee (minor)
O
O
trace (99% ee). In addition, we have also checked the reaction with cyclopropenes containing Ph/alkyl groups and alkyl/ester groups, and found these new experimental results could support the importance of both ester and aromatic rings on the cyclopropenes 1 due to the strong coordination of the ester group to the Rh catalyst. At the same time, we found other silanes, such as PhMeSiH2 and Et2SiH2, exhibited poor reactivity in this reaction. In addition, we found the construction of silicon-setereogenic center is really difficult because the diastereoselectivity is poor. Some representative results are summarized in Scheme 4. We found that the
O
Si OH
O
O
4b, 90% yield, 96% ee, d.r. = >99:1
O
Si O
O
4c, 75% yield, 96% ee, d.r. = >99:1
Si OH
OH
O R 4f, R = -cyclo-(CH2)6 , 68% yield, 93% ee, d.r. = >99:1 4g, R = -cyclo-(CH2)5, 58% yield, 96% ee, d.r. = >99:1 O
O
4d, 64% yield, 98% ee, d.r. = >99:1
O R2
O
Si OH
O
Si OH
O
2
4
O
4a, 80% yield, 96% ee, d.r. = >99:1
Si OH
30 oC, 40 h
2a
1
3
SiH
O
(a) [Rh(cod)2 ]BF4 (3.0 mol%) (R)-DTBM-SEGPHOS (3.6 mol%) [B(Ar)4]Na (3.0 mol%) Hexane, 30 oC, 20 h R1 (b) CH CN:H O (1:1),
Si OH
[Rh(cod)2]BF4 (3.0 mol%) (R)-DTBM-Segphos (3.6 mol%) [B(Ar)4]Na (3.0 mol%)
O
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4e, 84% yield, 98% ee, d.r. = >99:1
Scheme 5. Enantioselective Rhodium-catalyzed Tandem Desymmetric Hydrosilylation/oxidation of 1,1Disubstituted Cyclopropenes To further examine the substrate scope of hydrosilylation and desymmetrization process for one-step construction of two carbon stereocenters, we became greatly interested in the enantioselective and one-pot synthesis of silanol product by subsequent oxidation of cyclopropanyl hydrosilanes 3, because the corresponding silanol moiety is an important type of Si-based molecules in organosilicon chemistry.27 However, attempts to obtain a chiral silanol were not an easy task, which always gave an inseparable mixture of the siloxanes or other side-products as a result of Si-O coupling reaction under the reported reaction conditions. 28 Herein, we established a simple and in-situ rhodium-promoted oxidation of chiral cyclopropanyl hydrosilane 3 to corresponding silanols 4 with excellent yields (Scheme 5). As shown in Scheme 5, when water (CH3CN/H2O = 1:1) was added to reaction mixture after the desymmetric hydrosilylation reaction of cyclopropenes with the diphenylsilane that carried out in the presence of Rh/(R)-DTBM-SEGPHOS catalyst system, the desired silanol 4a was exclusively formed in high yields (80% yield) without loss of enantioselectivity (96% ee, >99:1 d.r.). In order to check this practical and simple protocol, we then investigated the substrate scope with representative examples under the standard reaction conditions (Scheme 5). Fortunately in theory, all these cyclopropenes are suitable for the tandem desymmetric hydrosilylation/oxidation reaction, leading to the
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chiral silanol products 4 with excellent yields with high level of stereoselectivities (>99:1 d.r. and 93-98% ees). Hence, cyclopropanyl-functionalized silanols bearing two Cstereogenic centers or a quaternary carbon stereocenter can be accessed easily from the catalytic desymmetric hydrosilylation and one-pot oxidation by Rh catalysis.
CuCl2 (2.0 eq.), CuI (3 mol%) KF (1.2 eq.)
Si R O
O
Si F
THF, r.t., 12 h (R = H)
O
O
3a/4a, 94% ee, d.r. = > 99:1
(1)
NEt3 (3 eq.) TMSCl (3 eq.) DCM, r.t., 4 h
Si
§(Z.-Y. Zhao, Y.-X. Nie) These authors contributed equally.
Funding Sources We thank National Natural Science Foundation of China (NSFC 21702211, 21703051, and 21773051), and Zhejiang Provincial Natural Science Foundation of China (LZ18B020001, LY17B030005, and LY18B020013) for financial support of this work. The authors declare no competing financial interests.
Si OSiPh2 H
O
(2)
DIBAL-H (2.0 eq.) ether, r.t., 5 h
Experimental procedures, characterization of products and spectroscopic data (PDF); Crystallographic data for 3t (CIF).
Si OTMS
(3)
O
7, 75% yield, 96% ee, d.r. = >99:1
OH 8, 75% yield, 94% ee, d.r. = >99:1
Scheme 6. Transformation of Chiral Silylcyclopropanes Finally, elaborations of chiral silylcyclopropanes 3a and 4a were also examined to demonstrate the stability of chiral organosilicon compounds and potentially synthetic utility of chiral silanes or silanols obtained in this work (Scheme 6). These experimental results suggested that the chiral Si-C bond is quite stable during silylation and reduction reaction, and the silylcyclopropane 3a could be used for the synthesis of fluorine-containing chiral organosilicon compound (Equation 1). In summary, we have presented an intermolecular Rhcatalyzed desymmetric hydrosilylation reaction of 1, 1disubstituted cyclopropenes with the promotion of chiral DTBM-SEGPHOS, allowing for a straightforward and highly enantioselective access to potentially valuable chiral organosilicon compounds in high yields with excellent diastereo- and enantio- selectivities (up to >99:1 d.r. and >99% ee) in a 100% atom-efficient manner from readily available starting cyclopropenes. The rhodium-catalyzed process demonstrated a broad scope of desymmetric construction of chiral three-membered carbocycles bearing a carbon quaternary stereocenter and remarkable ligand-directed highly enantioselective hydrosilylation process of cyclopropenes, which also provides a mild and convenient approach to a variety of chiral silyl cyclopropanes and silanols with various substituents through Si-C bond-forming reactions. Further investigation regarding the mechanism of syn-selective hydrosilylation reaction is ongoing and will be reported in due course.
ASSOCIATED CONTENT AUTHOR INFORMATION
The Supporting Information is available free of charge on the ACS Publications website. At DOI: 10.1021/acscatal.xxxx.
O
6, 80% yield, 96% ee, d.r. = >99:1
O
Author Contributions
Supporting Information H
Et3N (1.5 eq.) DCM, r.t., 4 h
Si OTMS
*
[email protected] Notes
5, 70% yield 94% ee, d.r. = 96:4
R = OH Cl
Corresponding Author
Acknowledgements The authors thank Dr. K.Z. Jiang, Dr. K.F. Yang, and Dr. X. Q. Xiao (all at HZNU) for their technical and analytical support.
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(28) With nanoAg catalyst for oxidation, see: Dong, X. Y.; Gao, L. X.; Zhang, W. Q.; Cui, Y. M.; Yang, K. F.; Gao, Z. W.; Xu, L. W. Catalytic Construction of Si-H -Containing Semi-Penetrating Networks: Evolution of Cobalt Catalysis and Its Application in Nanosilver-Catalyzed Alkynylation of Paraformaldehyde ChemistrySelect 2016, 1, 4034-4043.
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