Asymmetric Synthesis of Silicon-Stereogenic Silanes by Copper-Catalyzed Desymmetrizing Protoboration of Vinylsilanes

Article abstract of DOI:10.1002/anie.202005341

The catalytic asymmetric creation of silanes with silicon stereocenters is a long-sought but underdeveloped topic, and only a handful of examples have been reported. Moreover, the construction of chiral silanes containing (more than) two stereocenters is a more arduous task and remains unexploited. We herein report an unprecedented copper-catalyzed desymmetrizing protoboration of divinyl-substituted silanes with bis(pinacolato)diboron (B₂pin₂). This method enables the facile preparation of an array of enantiomerically enriched boronate-substituted organosilanes bearing contiguous silicon and carbon stereocenters with exclusive regioselectivity and generally excellent diastereo- and enantioselectivity.

Full text of DOI:10.1002/anie.202005341

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Accepted Article
Title: Asymmetric Synthesis of Silicon-Stereogenic Silanes by Copper-
Catalyzed Desymmetrizing Protoboration of Vinylsilanes
Authors: Ge Zhang, Yanfei Li, Ying Wang, Qian Zhang, Tao Xiong, and
Qian Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202005341
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Asymmetric Synthesis of Silicon-Stereogenic Silanes by Copper-
Catalyzed Desymmetrizing Protoboration of Vinylsilanes
Ge Zhang, Yanfei Li, Ying Wang, Qian Zhang, Tao Xiong,* and Qian Zhang
[*]
Dr. G. Zhang,† Y. Li,† Y. Wang, Q. Zhang, Prof. Dr. T. Xiong, Prof. Dr. Q. Zhang
Department of Chemistry
Jilin Province Key Laboratory of Organic Functional Molecular Design & Synthesis
Northeast Normal University, Changchun 130024, China
E-mail: xiongt626@nenu.edu.cn
Prof. Dr. Q. Zhang
Department State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345Lingling Lu, Shanghai 200032, China
[†] These authors contributed equally to this work
Supporting information for this article is given via a link at the end of the document.
Abstract: The catalytic asymmetric creation of silicon-stereocenter
silanes is a long-sought but far less developed topic, and only a
handful of examples have been reported. Moreover, the construction
of (more than) two stereocenters-containing chiral silanes is a more
arduous task and remains unexploited. We herein report an
unprecedented copper-catalyzed desymmetrizing protoboration of
divinyl substituted silanes with bis(pinacolato)diboron (B2pin2). This
method allows facile preparation of an array of enantiomerically
enriched boronate-substituted organosilanes bearing contiguous
silicon- and carbon-stereocenters with exclusive regioselectivity and
generally excellent diastereo- and enantioselectivity.
Unnatural enantiomerically enriched silicon-stereogenic
organosilanes have attracted increasing attention, due to not only
their widespread applications as the vital intermediates in
asymmetric synthesis,[1] but also the potential in the field of
functional materials and pharmaceutical research.[2] Therefore,
the development of efficient and straightforward synthetic
approaches for the construction of silicon-stereogenic center has
been a long-sought but far less developed topic in the past
decades.[3] To date, two mainstream and attractive catalytic
Scheme 1. Methods for enantioselective construction of silicon-stereogenic
strategies through the desymmetrization of Si–H and Si–C bonds
activation of tetraorganosilanes have been disclosed to efficiently
synthesize chiral organosilanes with silicon-stereogenic centers.
In this regard, inspired by Corriu’s seminal work,[4] an array of
enantioselective hydrosilylation reactions of alkynes, ketones and
alkenes, as well as the carbene insertion and alcoholysis with
dihydrosilanes, have been reported with precious Pd, Rh, Ir or Pt
catalysts.[5] More recently, using low cost and environmentally
benign cobalt catalyst and chiral half-sandwich scandium catalyst,
the attractive (successive double) hydrosilylation of alkynes and
alkenes have also been developed by Huang, Hong, Lu and Hou,
respectively (Scheme 1A, i).[6,7] On the other hand, the emerging
catalytic strategy via cleavage of Si–C bonds have been elegantly
demonstrated with the contribution of Shintani, Hayashi, Tobisu,
Chatani and others by means of chiral Pd and Rh catalysts
(Scheme 1A, ii).[8] Besides, the fascinating C–H bond activation
and [2+2+2] cycloaddition protocols, wherein did not involve
silicon atom in the construction of a new bond, have also been
realized by Shintani, Nozaki, Hayashi et. al. in the presence of
chiral Pd or Rh catalysts (Scheme 1A, iii).[9] Although these
attractive approaches have been established to create silicon-
centers.
stereogenic centers, some limitations still existed, such as: 1) only
one silicon stereocenter has been created in the existing methods,
and more challenging construction of (more than) two
stereocenters-containing
enantioenriched
organosilanes
remains much less unexploited;5g,8i 2) almost all of the
transformations rely on precious chiral Pd, Rh, Ir and Pt catalysts,
only sporadic examples by cheap and low toxicity of catalysts
have been achieved; 3) lacking an efficient method to install a
flexible functional group on the targeted organosilanes, that has
the capacity to interconvert the functional groups and hence
access to more valuable and/or complex molecules by further
elaborations.
The asymmetric hydroboration or protoboration of unsaturated
hydrocarbons has long been recognized as one of most vital
transformations in modern organic synthesis and has attracted
special interest in the past decades, owing to the organoboron
compounds having bench stability, excellent functional group
compatibility and diverse reactivity profile.[10] In this content, highly
1
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Table 1. Optimization of reaction conditions.[a]
regio- and enantioselective hydroboration or protoboration of
various alkenes including styrenes, aliphatic terminal/internal
alkenes, vinyl boronates, 1,3-dienes, conjugated enynes, strained
alkenes and others have made impressive progress in recent
years.[11] However, to the best of our knowledge, the asymmetric
protoboration of readily accessible prochiral vinyl silanes such as
1 to construct (more than) two stereocenters-containing chiral
organosilanes, remains elusive,[12] presumably due to the
formidable challenges on the well-balanced regio-, diastereo- and
stereoselectivity and surmounting the tendency of racemization of
silicon-stereogenic center, particularly in the presence of highly
reactive nucleophiles. As our continuing interest in copper
catalysis,[13] herein, we reported the first efficient and
operationally simple strategy for the construction of two adjacent
stereocenters boronate-substituted chiral organosilanes through
Entry
Catalyst
Ligand
Yield (%)[b]
ee(%)[c]
dr[d]
1
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuBr
CuOTf
Cu(OAc)2
CuCl
CuCl
CuCl
-
L1
L2
L3
L4
L5
L6
L7
L6
L6
L6
L6
L6
-
51
35
85
77
79
81
trace
65
40
84
53
49
0
95
42
94
-84
-95
97
-
＞20:1
5.8:1
5:1
2
3
4
＞25:1
＞25:1
＞25:1
-
5
6
7
8
97
96
94
94
92
-
＞25:1
＞25:1
＞25:1
＞25:1
＞25:1
-
9
a
copper-catalyzed desymmetrization of divinyl substituted
10
11[e]
12[f]
13
14
organosilanes with B2Pin2 (Scheme 1B).This strategy can be
processed with low-cost copper catalyst under very mild
conditions, and furnished an array of synthetically versatile chiral
boronate- and alkenyl-containing silanes bearing two adjacent
stereocenters with high efficiency and excellent level of regio-,
diastereo- and enantiocontrol.
L6
0
-
-
Our investigation was initiated by employing prochiral
bis(vinyl)silane 1a, which could be easily obtained from the
trichlorophenylsilane, alcohol and vinyl Grignard reagent in one
operation, and B2pin2 as model reactants for the Cu-catalyzed
desymmetrizing protoboration reaction (Table 1). Pleasingly,
when the reaction was conducted at room temperature in the
presence of CuCl (2 mol%), NaOtBu (1.5 equiv), MeOH (2.0
equiv), and (S,S)-QuinoxP* L1 (3 mol%) in THF, the desired
consecutive silicon- and carbon-stereogenic containing
organosilane 2a was furnished in moderate yield with excellent
level of enantioselectivity and more than 20 : 1 diastereomeric
ratio (dr) (Table 1, entry 1). Encouraged by this promising result,
we further evaluate the efficiency of other chiral ligands, including
L2−L7 (Table 1, entries 2−7). To our delight, good yield with
excellent enantio- and diastereoselectivity (81% yield, 97% ee, dr
> 25 : 1) was obtained by employing (R,R)-ipr-Duphos L6 as the
chiral ligand (Table 1, entry 6). Among this type of ligand skeleton,
the enantioselectivity improved with the increasing the steric
hindrance (ranging from < -84% to 97% ee) and the similar
reactivity profiles were observed (Table 1, entries 4-6). In addition,
compared to these chiral bisphosphine ligands, chiral
monophosphine ligand, namely (R)-Me-Rajphos L7, showed
almost no reactivity (Table 1, entry 7). We next proceeded to
screen various copper catalysts and the nature of the proton
sources. Experimental results revealed that both copper (I) and
copper (II) catalysts were valid catalysts, albeit showing some
difference on reactivity (Table 1, entries 8-10). Compared with the
effect of steric hindrance of the ligands L4-L6, a reversed
relationship between the reactivity and the steric hindrance of
alcohols was observed, and MeOH exhibited the best reactivity
than that of EtOH and iPrOH (Table 1, entries 11-12). Additionally,
no reaction occurred either in the absence of copper catalyst or
ligand (Table 1, entries 13-14). Further investigations of the bases,
temperature, and solvents were also carried out (see the
Supporting Information for details). In addition, the absolute
configuration of the product 2a was also unequivocally
determined by single crystal X-ray diffraction.
[a] Conditions: 0.2 mmol vinylsilane 1a (1.0 equiv), B2pin2 (1.5 equiv), copper
catalyst (2 mol%), Ligand (3 mol%), NaOtBu (1.5 equiv), alcohol (2.0 equiv), in
dry THF (2.0 mL) at 30 oC. [b] Yields were determined by 1H NMR analysis of
the crude reaction mixture with dibromomethane as an internal standard. [c] ee
values were determined by chiral HPLC analysis. [d] d.r. were determined by 1H
NMR analysisof the crude reaction mixture. [e]Use EtOH instead of MeOH. [f]
Use iPrOH instead of MeOH.
protoboration. First, the effect of alkoxyl groups in bis(vinyl)
silanes was examined. As illustrated in Table 2, a wide variety of
substrates 1 bearing various alkoxyl substituents attached to the
silicon atom processed smoothly, furnishing the corresponding
chiral boron-substituted silanes 2b–2j in good yields with over
90% ee values and 20 :1 diastereoselectivities in most of the
cases. These experimental results also clearly demonstrated that
the steric hinderance of the alkoxyl groups in substrates 1 has
marginal effect on the enantiocontrol, while small size of methoxyl
substituent in 1d leads diastereoselectivity drop dramatically to 4 :
1. In addition, tert-butyldimethylsilyl (TBS) protected ether and
alkyl substituted vinyl functional groups are both tolerated well,
delivering chiral products 2k and 2l in good yields and
enantioselectivities. In addition to examination of the scope of
alkoxyl substituents, we also found that electron-donating or
electron-withdrawing functional groups at either ortho-, meta- or
para-position on the aromatic ring, which directly attached to
silicon atom in 1, had negligible effect on diastereo- and
enantiocontrol. For instance, chiral organosilanes 2m-2q were
obtained in moderate to good yields with more than 95% ee and
15 : 1 diastereoselectivity, despite relatively low enantioselectivity
for 2p. The 4-Me and 4-OMe styryl substituted silanes 1r and 1s
were also examined, providing corresponding chiral silanes 2r
and 2s in moderate yields with excellent diastereo- and
enantioselectivies. The divinyl substituted silane 1t could also be
transformed smoothly to anti-Markovnikov product 2t in good
yield and enantioselectivity. This reversed regioselectivity
compared to that of 1a-1s might be ascribed to the stability of the
organocopper intermediates. Furthermore, in order to investigate
With the optimized reaction conditions in hand, we then
investigated the substrate scope of this desymmetrizing
2
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the possibility of the alkoxyl functional groups in substrates 1 as
the directing groups in this transformation, a range of linear,
branched and cyclic aliphatic alkyl-containing prochiral
organosilanes 1u-1ab were synthesized (Table 3). To our delight,
these substrates underwent smoothly under the current
Table 3. Substrate Scope of aryl, alkyl substituted divinyl organosilanes.[a][b]
Table 2. Substrate Scope of aryl, alkoxyl substituted divinyl organosilanes.[a][b]
[a] Reaction conditions: 0.2 mmol vinylsilane 1 (1.0 equiv), B2pin2 (1.5 equiv),
CuCl (0.02 equiv), L6 (0.03 equiv), NaOtBu (1.5 equiv), MeOH (2.0 equiv), in
THF (2.0 mL) at rt under N2 atmosphere for 36 h unless otherwise stated. [b]
Yields were determined by 1H NMR analysis of the crude reaction mixture with
dibromomethane as an internal standard, ee were determined by chiral HPLC
analysis and dr were determined by 1H NMR analysis. [c] Use Bpin-Bdan
instead of B2pin2.
organosilanes 2a was obtained in moderate yield without any
erosion in enantioselectivity with 1 mol% CuCl and 1.5 mol%
chiral ligand L6 (Scheme 2A). We also briefly examined the
further applications of these novel chiral boron-containing
organosilanes. For example, organosilane 2a could undergo a
highly chemoselective epoxidation of the C–C double bond to
furnish highly functionalized product 3 under very common
oxidation reaction conditions, although the diastereocontrol need
to further enhanced (Scheme 2B, top). The enantioenriched 2a
could also be smoothly transformed into synthetically versatile but
quite challenge chiral allylic organosilicon 4 in almost prefect
enantiospecificity (es).[14] During this procedure, the C=C bond
and C–Si bond in 2a were inert (Scheme 2B, middle). In addition,
silane 2u can be oxidized efficiently to form secondary alcohol 5
in nearly quantitative yield without loss of diastereoselectivity
(Scheme 2B, bottom).[15]
[a] Reaction conditions: 0.2 mmol vinylsilane 1 (1.0 equiv), B2pin2 (1.5 equiv),
CuCl (0.02 equiv), L6 (0.03 equiv), NaOtBu (1.5 equiv), MeOH (2.0 equiv), in
THF (2.0 mL) at rt under N2 atmosphere for 36 h unless otherwise stated. [b]
Yields were determined by 1H NMR analysis of the crude reaction mixture with
dibromomethane as an internal standard, ee were determined by chiral HPLC
analysis and dr were determined by 1H NMR analysis.
conditions, delivering highly enantiomerically enriched
organosilanes 2u-2ab in generally high yields with high to
excellent level of diastereo- and stereoselectivity with the
exception of 2x, probably due to the big steric hindrance. These
results reveal that the alkoxyl functional groups in prochiral
organosilanes 1 are merely substituents rather than directing
groups in the present transformations. Moreover, similar tendency
was observed again, namely smaller size of substituents showing
low level of diastereocontrol (e.g. 2u, 2v versus 2w, 2y-2ab). In
addition to the successful installation of Bpin group into the
prochiral organosilane 1, the Bdan (dan, naphthalene-1,8-
diaminato) group could also be introduced into substrate with the
similar reactivity and enantioselectivity compared with Bpin such
as 2u’.
Scheme 2. Gram-scale synthesis and applications of chiral boron-containing
organosilanes.
In conclusion, we have reported an efficient desymmetrizing
method for synthesis of chiral organosilanes via Cu-catalyzed
highly enantioselective protoboration reactions of bis(vinyl)
silanes. A variety of chiral borated-silanes containing consecutive
silicon- and carbon-stereogenic centers could be synthesized in
A gram-scale synthesis was conducted to demonstrate the
practicability of the present approach, and the target chiral
3
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Acknowledgements
We thank the NSFC (Grants 21801039, 21672033, 21831002),
Jilin Educational Committee
( JJKH20190269KJ),
the
Fundamental Research Funds for the Central Universities
(2412018QD007), and the Ten Thousand Talents Program for
generous financial support.
Keywords: copper catalysis
desymmetrization • protoboration
•
chiral organosilane
•
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[15] We found that 2a can’t be transformed into alcohol under the same
oxidation conditions, might owing to the steric hindrance.
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10.1002/anie.202005341
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
A mild and efficient desymmetrizing protoboration of bis(vinyl)silanes with B2pin2 or Bpin-Bdan by (R,R)-iPr-Duphos-ligated copper
catalyst was disclosed for the first time. Consequently, an array of synthetic versatile but challenging enantioenriched borated
organosilanes bearing contiguous silicon- and carbon-stereocenters were obtained with generally excellent level of diastereo- and
enantioselectivities.
5
This article is protected by copyright. All rights reserved.