Copper-Catalyzed Asymmetric Three-Component Borylstannation: Enantioselective Formation of C-Sn Bond

Article abstract of DOI:10.1002/chem.201500060

In summary, a first copper-catalyzed synthesis of α-aryl-β-borylstannane compounds was accomplished through three-component borylstannation of aryl-substituted alkenes. In the exploration of an asymmetric variant, chiral sulfinylphosphine ligands proved a

Full text of DOI:10.1002/chem.201500060

DOI: 10.1002/chem.201500060
Communication
&
Synthetic Methods
Copper-Catalyzed Asymmetric Three-Component Borylstannation:
Enantioselective Formation of CÀSn Bond
Tao Jia,[a] Peng Cao,[a] Ding Wang,[a] Yazhou Lou,[a] and Jian Liao*[a, b]
annulation of 2-nitrosopyridine and allylstannane to furnish
Abstract: In summary, a first copper-catalyzed synthesis of
tin-substituted isoxazolidines (Scheme 1a,b). However, to our
a-aryl-b-borylstannane compounds was accomplished
knowledge,
a general and versatile catalytic asymmetric
through three-component borylstannation of aryl-substi-
tuted alkenes. In the exploration of an asymmetric variant,
chiral sulfinylphosphine ligands proved advantageous in
controlling stereochemistry of BÀCu addition and in pro-
moting transmetalation of enantioenriched alkylÀCu spe-
cies. The stereochemical outcome supported a sequential
syn-borylcupration and configuration-retentive transmeta-
lation mechanism. Moreover, a-chiral b-borylstannanes
were easily transformed into a diverse array of secondary
alkylstannanes and triarylethane with high enantiomeric
purity. The applications of chiral sulfinylphosphine ligands
to other tandem CuÀB addition reactions are currently
under investigation in our group.
method for the construction of acyclic alkylstannanes remains
unreported.[10] Herein we present a new Cu-catalyzed asym-
metric dimetalation (three-component reaction) strategy to
access to enantiomerically enriched organostannanes
(Scheme 1c).
Bis-metalated alkene moieties are of increasing importance
in the preparation of medical reagents and complex molecules
through cascade transformations.[2g,11a] Asymmetric catalytic
approaches to dimetalation have relied primarily on noble
metal (Pt, Rh, and Pd) catalysis.[11] Very recently, Hoveyda and
co-workers disclosed a copper-catalyzed double borylcupration
to alkynes that afforded 1,2-diboronated alkanes in high enan-
tioselectivities.[12] Borylcupration has also been widely em-
ployed as an elegant strategy for enantioselective hydro/di-
functionalization of alkenes,[13] including hydroboration (Hovey-
da),[14] aminoboration (Miura),[15] and borylcarbonation (Lin).[16]
Furthermore, this difunctionalization strategy was recently
demonstrated in the non-asymmetric borylstannation of al-
kynes and activated alkenes by Yoshida and Takaki, to afford
achiral or racemic vicinal borylstannanyl difunctionality.[17] In
this context, we envisioned that the chiral ligand-directed bor-
Configurationally stable nonracemic organometallic com-
pounds (organoboron, organosilane, and organotin)[1] are
emerging as a class of important chiral reagents. Their carbon–
metal bonds can be readily transformed to stereogenic
carbon–carbon[2] and carbon–heteroatom bonds,[3] providing
novel and concise routes to syntheses of natural products and
chiral drugs. Among these organometallic compounds, secon-
dary alkylstannanes are configurationally robust and easily
stored, and have been utilized in many asymmetric transforma-
tions, such as Stille-type cross-coupling reactions,[4] allylstanna-
tions,[5] and transmetalations to other organometallic species.[6]
Conventional methods to reach optically active organostan-
nanes include chiral resolution,[4f] substrate control,[4c,7] and re-
agent induction,[8] whereas asymmetric catalysis has been
rather less thoroughly investigated. To date, only two cases re-
lating to chiral secondary cyclic alkylstannane synthesis have
been reported. In 2004, Gevorgyan and co-workers developed
a RhI-catalyzed enantioselective hydrostannation of cyclopro-
pene to generate cyclopropylstannanes.[9a] More recently,
Studer and co-workers[9b] reported a CuI-catalyzed asymmetric
[a] Dr. T. Jia, Prof. Dr. P. Cao, D. Wang, Y. Lou, Prof. Dr. J. Liao
Natural Products Research Center, Chengdu Institute of Biology
Chinese Academy of Sciences, Chengdu, 610041 (China)
E-mail: jliao@cib.ac.cn
[b] Prof. Dr. J. Liao
College of Chemical Engineering
Sichuan University, Chengdu, 610052 (China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500060.
Scheme 1. Catalytic enantioselective synthesis of organostannanes. pin=pi-
nacolato.
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ylstannation of simple olefins would generate enantioenriched
dimetalated alkanes with a primary CÀB bond and a secondary
CÀSn bond. Subsequent selective carbon–metal bond conver-
sion would then allow easy diversification of organoboron and
organostannane reagents, as well as alkene difunctionalities.[2,3]
The proposed method essentially concerns the enantiospecific
copper-to-tin transmetalation process, which remains a chal-
lenge in organometallic chemistry.[5b]
oxide–phosphine ligands developed by our group[18] efficiently
promoted the borylstannation of styrene with high enantiose-
lectivity. In the presence of L3, optically active 4a was ob-
tained in 90% NMR yield and 92% ee (Table 1, entry 7). This is
the first example of Cu-catalyzed enantioselective hetero-dime-
talation reaction. To investigate the ligands’ effects on asym-
metric borylstannation, sulfinyphosphines L4–L7, bearing
a benzodioxole skeleton, were tested (Table 1, entries 8–11). L5
proved to be the optimal ligand and provided 4a in 92% yield
(isolated product) with 95% ee.
Our investigation began with the racemic borylstannation of
styrene (1a) with (Bpin)2 (2; pin=pinacolato) and Bu3SnOMe
(3; Table 1).We found that an in situ-prepared complex of
copper salt and Cy3P, in a 1:1 ratio, efficiently catalyzed the
title reaction at room temperature and CuI salts showed signifi-
cantly higher reactivity than CuII salts (Table 1, entries 1 and 2).
The desired a-phenyl-b-borylstannane (4a) was afforded in ex-
cellent yield (92%) when using CuCl and Cy3P with THF as the
solvent (Table 1, entry 4).We then turned our attention to an
asymmetric variant. Initially, the commercially available chiral
mono- and biphosphine ligands that we examined didn’t show
promising yields or enantiomeric excesses.(Table 1, entries 5
and 6; see also the Supporting Information). Gratifyingly, sulf-
Table 2 outlines the application of the optimized reaction
conditions on a variety of styrene derivatives. Interestingly, the
stereoelectronic properties of aryl groups had minimal effect
on the enantioselectivity (87–96% ee). Electron-withdrawing
aryl groups facilitated the borylstannation process with full
conversion of olefins. Halogen-substituted styrenes with o-Br
(1b), o-Cl (1c), m-Br (1 f), m-Cl (1g), p-Br (1j), p-Cl (1k), and p-F
(1l) substituents provided good to excellent yields of 4 (75–
99%) and excellent selectivities without competitive cross-cou-
pling reactions.[19] Electron-rich styrenes were slightly less reac-
tive, although reactions proceeded smoothly by increasing
temperature without loss in enantioselectivity. At 458C, p-Me-
(1n) and p-tBu-substituted (1m) vinylbenzenes smoothly pro-
vided adducts 4n and 4m, respectively, in good yields (86%
and 82%) and excellent enantiomeric excesses (91% ee and
96% ee). Interestingly, o-Me- (1d) and o-OMe-substituted (1e)
vinylbenzenes were quite reactive at 208C and their products
(4d and 4e) showed highly optical purities (95% ee and
86% ee, respectively). Acid/base-sensitive functional groups,
such as OTf (1h), allyloxy (1i), and OBz (1o; Bz=benzoic),
were robust on exposure to the borylstannation process, with-
out any interference in the reactivity or enantioselectivity. This
method also enabled the synthesis of a-naphthyl- (4p and 4q)
and a-indolyl-b-borylstannane (4r) in good yields (57–69%)
with excellent enantiomeric excess (92% ee). In the examina-
tion of 1,2-substituted olefins, electron-deficient and strained
olefins showed excellent reactivities and regioselectivities. For
instance, the borylstannation of methyl crotonate (1s) provid-
ed b-boryl-a-stannyl ester (4s) with good enantioselectivity
(89% ee for the major isomer) and moderate diastereoselectiv-
ity (d.r.=70:30). Distinctively, chemo- and stereoselective dime-
talation of norbornadiene proceeded smoothly to furnish opti-
cally active 4t in excellent yield with a single endo- and syn-
configuration, which might represent a common stereochemi-
cal outcome in CuI-catalyzed alkene-borylstannation.[17b] Nota-
bly, most of the borylstanation products (4) were readily con-
verted to b-hydroxystannanes (5) without any interference at
the chiral CÀSn bond (see the Supporting Information). To de-
termine absolute configurationd, Ph3SnOMe was used instead
of Bu3SnOMe in the enantioselective borylstannation of 2-vinyl-
naphthalene. The formation of (R)-4u,[20] which was confirmed
by X-ray crystal structure analysis, was induced by sulfinylphos-
phine (R)-L5.
Table 1. Optimization of CuI-catalyzed enantioselective borylstannation
of styrene 1a.[a]
Entry
Cu salt
Ligand
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
Cu(OAc)2
CuOAc
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
Cy3P
Cy3P
Cy3P
Cy3P
L1
L2
L3
L4
L5
toluene
toluene
toluene
THF
THF
THF
THF
THF
THF
THF
65
83
84
96(92)[d]
27
10
90
88
95(92)[d]
82
–
–
–
–
À38
66
92
92
95
89
93
9
10
11
CuCl
CuCl
L6
L7
THF
87
[a] Reactions conditions: 1a (0.2 mmol), 2 (0.3 mmol), 3 (0.3 mmol), CuCl
(10 mol%, 0.02 mmol, 2.0 mg), L (10 mol%, 0.02 mmol) in THF (2.0 mL),
208C, 12 h. [b] NMR yield of 4a. [c] Enantiomeric excess determined by
chiral HPLC analysis of b-hydroxystannane (5a) obtained by oxidation
(NaOH, H2O2) of enantiomerically enriched 4a. [d] Value in parentheses is
yield of isolated 4a.
As reported by Ito, Sawamura and co-workers[21] and further
reinforced by the syn-configuration of 4t, CuI-catalyzed alkene
borylstannation proceeded by syn addition of CuÀBpin species
to the C=C bond, followed by stereoretentive transmetala-
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Table 2. The synthesis of enantioenriched b-boryl alkylstannanes[a]
Scheme 2. The proposed stereochemical model of enantioselective boryl-
stannation.
the re-face of the styrene is favored (Scheme 2, path a) as it
avoids the steric hindrance between phenyl group of styrene
and the tert-butylsulfinyl group of the ligand (Scheme 2,
path b). The absolute configuration of (R)-4u[20] also rationaliz-
es the stereochemical model brought about in path a.
The following investigation focused on transformations of
vic-borylstannyl alkanes (Scheme 3). Several new enantiomeri-
cally enriched a-arylstannanes (6, 7 and 8) were derived from
4a (94% ee). The insertion of methylene into the CÀB bond of
4a provided 6,[22] the subsequent oxidation of which furnished
g-hydroxy-a-phenylstannane (7). The b-boryl alkylstannane 4a
could also be readily converted into 8 through Suzuki cross-
coupling.[3] The stereospecific cleavage of the non-functional-
ized C(sp3)ÀSn bond has seldom been reported.[4f] To our de-
light, the asymmetric cross-coupling of 8 with C(sp2)ÀX bond
successfully generated chiral triarylethane 9, analogue to
CDP840, a phosphodiesterase 4 inhibitor, with high enantio-
meric purity.[23]
[a] Reactions conditions (unless otherwise stated):
1 (0.2 mmol), 2
(0.3 mmol), (0.3 mmol), CuCl (10 mol%, 0.02 mmol, 2.0 mg), L5
3
(10 mol%, 0.02 mmol) in THF (2.0 mL), 208C, 12 h. [b] Yields of isolated 4.
[c] Enantiomeric excess determined by chiral HPLC analysis of b-hydroxy-
stannane obtained by oxidation (NaOH, H2O2) of enantiomerically en-
riched 4. [d] Enantiomeric excess determined by chiral HPLC analysis of 4.
[e] Reaction performed at 458C. [f] Diastereoselectivity of 4. [g] The rela-
tive configuration is confirmed in ref. [16b]. [h] Ph3SnOMe (0.3 mmol) was
used instead of 3 and the absolute configuration was confirmed by the
X-ray structure of 4u[20] (see the Supporting Information).
Scheme 3. Conversions of 4a through the cleavage of CÀB and CÀSn bonds.
tion.[17b] The stereochemical determination involved the enan-
tiodiscriminative borylcupration of styrene. A catalytic cycle
and stereochemical model are proposed in Scheme 2. The cat-
alyst precursor [{(R)-L7ÀCuCl}2][20] was isolated and character-
ized by X-ray crystal structure analysis. In the proposed catalyt-
ic cycle, the dimer dissociates first to give the monomer and
then transforms into the active intermediate [(R)-LÀCu(Bpin)],
which approaches the styrene with tetrahedral coordination
geometry at the copper atom. Therefore, CuÀB addition from
Experimental Section
In a glove box, to a flame-dried tube was added CuCl (2.0 mg,
0.02 mmol), L5 (9.3 mg 0.02 mmol), and dry THF (1.0 mL). The re-
sultant solution was stirred for 30 min, followed by the addition of
(Bpin)2 (76 mg, 0.3 mmol) and THF (1.0 mL). The tube was taken
out of the glove box and styrene (0.2 mmol) and Bu3SnOMe (87 mL,
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Acknowledgements
We thank the NSFC(No. 21272226, 21202160 and 21072186),
Chengdu Institute of Biology of CAS (Y0B1051100) and the
Major State Basic Research Development Program (973 pro-
gram, 2010CB833300).
Keywords: asymmetric synthesis · borylation · stannation ·
copper · homogeneous catalysis
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[23] a) R. P. Alexander, G. J. Warrellow, M. A. W. Eaton, E. C. Boyd, J. C. Head,
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Received: January 7, 2015
Published online on && &&, 0000
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Communication
COMMUNICATION
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Synthetic Methods
T. Jia, P. Cao, D. Wang, Y. Lou, J. Liao*
&& – &&
Copper-Catalyzed Asymmetric Three-
Component Borylstannation:
Enantioselective Formation of CÀSn
Bond
CÀtin arrangement: The copper-cata-
lyzed asymmetric three-component bor-
ylstannation of aryl-substituted alkenes
is presented. In the presence of a chiral
sulfinylphosphine ligand, a-aryl-b-boryl-
stannanes are obtained in highly enan-
tiomeric purity and site-selective cleav-
age of CÀB and CÀSn bonds is realized
with complete retention of the initial
enantioselectivity.
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Chem. Eur. J. 2015, 21, 1 – 6
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!