Asymmetric Copper Hydride-Catalyzed Markovnikov Hydrosilylation of Vinylarenes and Vinyl Heterocycles

Article abstract of DOI:10.1021/jacs.6b13029

We report a highly enantioselective CuH-catalyzed Markovnikov hydrosilylation of vinylarenes and vinyl heterocycles. This method has a broad scope and enables both the synthesis of isolable silanes and the conversion of crude products to chiral alcohols. Density functional theory calculations support a mechanism proceeding by hydrocupration followed by σ-bond metathesis with a hydrosilane.

Full text of DOI:10.1021/jacs.6b13029

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Communication
Asymmetric Copper Hydride-Catalyzed Markovnikov
Hydrosilylation of Vinylarenes and Vinyl Heterocycles
Michael William Gribble, Michael T. Pirnot, Jeffrey S Bandar, Richard Y. Liu, and Stephen L. Buchwald
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b13029 • Publication Date (Web): 24 Jan 2017
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Asymmetric Copper Hydride-Catalyzed Markovnikov Hydrosilyla-
tion of Vinylarenes and Vinyl Heterocycles
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Michael W. Gribble Jr, Michael T. Pirnot,† Jeffrey S. Bandar,† Richard Y. Liu, and Stephen L. Buch-
wald*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
Supporting Information Placeholder
achievement in asymmetric catalysis.8 While this reac-
ABSTRACT: We report a highly enantioselective CuH-
tion is of exceptional fundamental importance, its re-
ported scope is limited; in particular, we are aware of no
applications of this methodology to the hydrosilylation
of heterocyclic or appreci-
catalyzed Markovnikov hydrosilylation of vinylarenes
and vinyl heterocycles. This method has a broad scope
and enables both the synthesis of isolable silanes and the
conversion of crude products to chiral alcohols. DFT
calculations support a mechanism proceeding by hydro-
cupration followed by ꢀ-bond metathesis with a hy-
drosilane.
Chiral silanes undergo a number of useful transfor-
mations1a,b and possess more desirable toxicological and
environmental characteristics than many related main-
group reagents.2a,b However, they can be difficult to
prepare due to the scarcity of broadly applicable meth-
ods for the construction of C(sp3)-Si bonds in functional-
ized molecules.3a,b This limitation also discourages the
pursuit of silane drug-candidates, despite the promise
organosilicon compounds hold for a variety of therapeu-
tic applications.3a-c Since functionalized alkylsilanes are
additionally important to many modern materials and
industrial processes,4a,b selective new C–Si bond-
forming reactions have the potential to be enabling tech-
nologies for researchers across a wide range of disci-
plines.
Late transition metal-catalyzed olefin hydrosilyla-
tion (Figure 1, a) is one of the core synthetic transfor-
mations of organosilicon chemistry.4b,c The archetypal
Pt-catalyzed variant4b is among the highest-volume in-
dustrial applications of homogenous catalysis,4b,5c,i and
the need to replace Pt in this role with more abundant
metals has spurred the discovery of many anti-
Markovnikov hydrosilylation catalysts based on iron,5a-f
cobalt5g-j , and nickel.5k-q In contrast, Markovnikov ole-
fin hydrosilylation catalysts are uncommon, and the syn-
thetic capabilities of the reactions are presently lim-
ited.5e,6 Hayashi’s discovery that Pd-MOP complexes
catalyze highly enantioselective Markovnikov hydrosi-
a
Figure 1. Metal-catalyzed hydrosilylation of olefins;
bPd-catalyzed asymmetric Markovnikov hydrosilylation of
lylation with trichlorosilane7a-e was
a
milestone
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vinylarenes; c,dCuH-catalyzed asymmetric hydrosilylation
and 7). These observations suggest that rapid trapping
of the alkylcopper intermediate by the silane may be a
Table 2. Hydrosilylation of Vinylarenes.
of styrenes; eProposed mechanism for the hydrosilylation.
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2
3
4
Table 1. Optimization of the CuH-catalyzed Hydrosilyla-
tion of Styrene.
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aUnless otherwise noted, reactions were conducted on
b
0.5 mmol scale; Unless otherwise noted, yields were de-
c
termined by GC with dodecane internal standard; Deter-
mined by chiral HPLC; d0.2 mmol scale; eIsolated yield.
ably Lewis-basic olefins, presumably because they are
incompatible with the sensitive trichlorosilyl group.
Our group has recently developed several functional
group-tolerant Markovnikov hydrofunctionalization re-
actions that proceed through electrophilic interception of
alkylcopper intermediates generated by asymmetric hy-
drocupration of vinylarenes9a-e with an active chiral cop-
per hydride catalyst (I, Figure 1, e, step i). We reasoned
that an asymmetric Markovnikov hydrosilylation could
be achieved if the alkylcopper species II (Fig 1, e) could
undergo stereoretentive transmetallation with a hy-
drosilane (Fig 1, e, step ii).10,11a,b The hypothesized trans-
formation would constitute an attractive alternative to Pd
catalysis if it could provide isolable silanes and silane-
derivatives with high generality using readily available
catalyst precursors. However, to our knowledge, cop-
per-catalyzed olefin hydrosilylation has not been de-
scribed in the primary literature,12 despite pervasive use
of hydrosilane reductants in copper hydride cataly-
sis.9a,13a-c This observation suggested that the proposed
metathesis step could be challenging and that competi-
tive side reactions might present challenges.
With styrene as our model substrate, we observed
clean formation of the desired Markovnikov hydrosilyla-
tion product using either PhSiH3 or Ph2SiH2, but the en-
antioselectivity obtained with Ph2SiH2 was modest (Ta-
ble 1, entry 1). The selectivity with Ph2SiH2 varied with
change in the solvent, although without an obvious cor-
relation with solvent properties. Reducing the concen-
tration of silane was deleterious for enantioselectivity,
whereas conducting the reaction neat resulted in excel-
lent levels of asymmetric induction (Table 1, entries 4
aUnless otherwise noted, yields and enantiomeric excess-
es are the averages for two runs; Reaction was conducted
b
with 2.0 mol% Cu(OAc)2 and 2.2 mol% (S,S)-Ph-BPE;
cReaction mixture was stirred in a 40 °C oil bath; dReaction
e
was conducted at ambient temperature; 1.5 equiv. silane
f
were used; Enantiomeric excesses were determined for the
respective silanol derivatives;14 gExtrema are the results
from two experiments.14
key factor for obtaining high levels of enantioselectivity,
particularly since the hydrocupration step is known to be
highly enantioselective9a-e in a variety of solvents. This
notion was further supported by the observation that the
more reactive PhSiH3 underwent hydrosilylation in 96%
ee even in THF solution (Table 1, entry 8).
With the goal of generating easily isolable products,
we chose to use Ph2SiH2 in our exploration of the vi-
nylarene scope. The results of these studies are present-
ed in Table 2. The hydrosilylation occurred in high yield
and with good to excellent enantioselectivity with sub-
strates containing either electron-withdrawing or elec-
tron-donating groups and accommodated substituents at
any of the three positions on the aryl ring. The highest
levels of enantioselectivity were obtained with π- donor
substituents at either the para- or ortho-positions (e.g.,
examples 2 and 9, Table 2). Electron-withdrawing
groups were tolerated at the meta-position, and we noted
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Silanes 11 and 12 (Table 3, A) were formed with high
1
2
3
enantioselectivity, whereas a less electronically biased
heterocycle gave product with modest asymmetric in-
duction (example 13, Table 3, A). These results mir-
rored the enantioselectivity trends evident in Table 2 and
those
4
5
Table 3. Hydrosilylation of Vinyl Heterocycles.
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7
8
9
(A)a
TS-C
+21.8 (+32.6)
4.0% Cu(OAc)2
4.4% (S,S)-Ph-BPE
G
SiHPh2
Me
Me
Ph
2.27
Het
2.51
H
1.5 equiv. Ph2SiH2
neat, temp (T), time (t)
Het
P
n = 1,2
Si
1.73
Cu
n = 1,2
Ph
P
0.5 mmol
SiHPh2
Me
H 1.66
H
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Ph2HSi
Me
SiHPh2
Me
TS-C
MeO2C
N
N
MeO
N
C
PMB
LCuH
SiHPh
+
B
Boc
–0.7 (+8.2)
A
–9.1 (–3.2)
PhSiH3
11
12
61% yield
90% ee
13
77% yield
70% ee
SiH2Ph
0.0 (0.0)
LCuH +
PhSiH3
68% yieldb (79% NMR yield)c
41:1 rr (23:1 crude rr by NMR)
97% ee14
Ph
Me
H
D
CuL
CuL
+
+
T = 40 °C, t = 36 h
T = rt, t = 36 h
–11.9 (–13.7)
Ph
Ph
Me
Ph
Me
T = 40 °C, t = 48 h
SiHPh2
Me
Figure 2. DFT model for copper-catalyzed hydrosilyla-
tion of styrene (L DCyPE). M06/6-311+G(d,p)
=
N
N
SDD/SMD(THF)//B3LYP/6-31G(d)-SDD Gibbs free ener-
gy values displayed in kcal/mol. Corresponding PBE0/6-
311+G(d,p)-SDD/SMD(THF)//B3LYP/6-31G(d)-SDD val-
ues shown in brackets. Key bond distances shown in units
of Å. Carbon-bonded hydrogen atoms are omitted for
clarity.
O
14d
83% yield
88% ee
T = rt, t = 16 h
ORTEP diagram of 14
(B)a
KF, H2O2
KHCO3
K2EDTA
4.0% Cu(OAc)2
OH
Me
SiH2Ph
4.4% (S,S)-Ph-BPE
Het
Me
5.0 equiv. PhSiH3
neat, rt, 12 h
THF:MeOH
rt, 20 h
Het
Het
n = 1,2
our group has observed in the CuH-catalyzed hydroami-
nation of vinylarenes.9e
n = 1,2
n = 1,2
(not isolated)
OH
1 mmol
A crystal structure of 14 (Table 3, A) showed the ab-
solute stereochemistry of the major enantiomer to be (S),
which is consistent with the mechanism shown in Figure
1, e.9c,d However, we noted that several entries in Tables
1 and 2 exhibited lower ee’s than were obtained in pre-
viously reported reactions thought to proceed by the
same hydrocupration step.9c,d We considered two mech-
anistic hypotheses that might account for these discrep-
ancies: one posits a racemization step occurring after
hydrocupration (i.e., affecting intermediate II of Figure
1, e); the second invokes gradual formation of an unde-
sired catalytically active species that undergoes hydro-
cupration with poor selectivity in the presence of certain
substrates. In either case, one would expect that accel-
erating the transmetallation (Figure 1, e, step ii), e.g., by
employing the more reactive PhSiH3, would result in
higher enantioselectivity. PhSiH3 would also be ideal
for derivatization attempts using crude products, since it
could be easily evaporated beforehand. Hydrosilylation
with PhSiH3 occurred in <12 h at room temperature in
most examples, and we were able to perform Tamao
oxidations15 on the crude products by incorporating
EDTA into the reaction mixtures (Table 3, B), which
suppresses copper-catalyzed disproportionation of hy-
drogen peroxide. The enantioinduction obtained with
PhSiH3 was indeed broadly superior to that observed
with Ph2SiH2 and further appeared to be less sensitive to
OH
OH
Me
N
Me
Me
N
N
N
N
MeO
N
O
15
85% yield
98% ee
17
16
64% yield
96% ee
80% yield
96% ee
HO
OH
Me
HO
HO
Me
Me
Me
N
N
N
O
S
F
Boc
40
18e,f
75% yield
96% ee
19e,
20
74% yield
97% ee
21
41
42
43
44
f
66% yield
67% yield
90% ee
57-66% ee14
aYields and enantiomeric excess values are averages
b
from two runs; Isolated yield pertains to the regioisomer
c
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mixture; NMR yield pertains to the major regioisomer;
d2.0 equiv. silane used; MTBE used as the solvent; Reac-
e
f
tion run for 36 h.
that conducting the reaction at room temperature bene-
fited the selectivity in those cases. Certain functional
groups, such as the nitro group and halogens other than
fluorine, were incompatible with the reaction, as were
sterically demanding ortho-substituents. Underscoring
the utility of the protocol, we conducted a reaction on 10
mmol scale without significant loss of yield or enanti-
oselectivity (eq 1).
Table 3 illustrates that a variety of vinyl heterocy-
cles proved to be viable substrates for the reaction.
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substrate electronic bias (compare 16, 17 [Table 3, B]
Page 4 of 5
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To sharpen our mechanistic hypothesis,16 we per-
formed DFT calculations on the CuH-catalyzed hydrosi-
lylation
of
styrene
with
PhSiH3
using
bis(dicyclohexylphosphino)ethane (DCyPE) as the mod-
el ligand (Figure 2). After hydrocupration, we located a
ꢀ-complex
C
upon interaction of copper with
phenylsilane. From here, ꢀ-bond metathesis may pro-
ceed irreversibly through a thermally accessible four-
membered transition state TS-C (+30.9 or +35.8
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favorable (A vs. D).
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In summary, we have developed a broadly applica-
ble base-metal-catalyzed asymmetric hydrosilylation
that provides access to bench-stable silanes and chiral
alcohol derivatives. The method uses mild conditions,
employs commerically available catalyst-precursors, and
enables the functionalization of a variety of medicinally
relevant heterocyclic olefins. While our mechanistic
hypotheses remain speculative at this time, we believe
they provide a useful framework for rationalizing the
observed selectivity trends. We also believe that a thor-
ough mechanistic investiation will be of value to our
group’s ongoing efforts to develop new CuH-catalyzed
transformations.
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is
available free of charge on the ACS Publications website.
Experimental procedures and characterization data for all
compounds (PDF, CIF)
AUTHOR INFORMATION
Corresponding Author
*Email: sbuchwal@mit.edu
Author Contribution
†M.T.P. and J.S.B. made equal contributions to this work.
Notes
(8) The Nishiyama group has described a rhodium-catalyzed asym-
metric synthesis of bench-stable silanes, but the regioselectivity of
this reaction is modest in many cases: Naito, T.; Yoneda, T.; Ito, J-I.;
Nishiyama, H. Synlett 2012, 23, 2957-2960.
The authors declare no competing financial interests.
(9) See, e.g.: (a) Pirnot, M. T.; Wang, Y-M.; Buchwald, S. L. An-
gew. Chem. Int. Ed. 2016, 55, 48-57. (b) Bandar, J. S.; Ascic, E.;
Buchwald, S. L. J. Am. Chem. Soc. 2016, 138, 5821-5824. (c) Wang,
Y-M.; Buchwald, S. L. J. Am. Chem. Soc. 2016, 5024-5027. (d) Yang,
Y.; Perry, I. B.; Buchwald, S. L. J. Am. Chem. Soc. 2016, 138, 9787-
9790. (e) Bandar, J. S.; Pirnot, M. T.; Buchwald, S. L. J. Am. Chem.
Soc. 2015, 137, 14812-14818.
(10) Marks has proposed this mechanism for a lanthanide-catalyzed
hydrosilylation of styrenes. Fu, P-F; Brard, L.; Li, Y.; Marks, T. J. J.
Am. Chem. Soc. 1995, 117, 7157-7168. Also see ref. 5(k).
ACKNOWLEDGMENT
The National Institutes of Health under award number
GM46059 supported research reported in this publica-
tion. M.T.P. and J.S.B. thank the National Institutes of
Health for postdoctoral fellowships (1F32GM113311
[M.T.P], GM112197 [J.S.B.]). We thank Dr. Yi-Ming
Wang (MIT) for advice on the preparation of this manu-
script, Jonathan Becker (MIT) for X-ray crystallographic
analysis, and the National Institutes of Health for a sup-
plemental grant for the purchase of supercritical fluid
chromatography (SFC) equipment (GM058160-17S1).
(11) For a mechanistically analogous Cu-catalyzed hydroboration,
see: (a) Noh, D.; Chea, H.; Ju, J.; Yun, J. Angew. Chem. Int. Ed. 2009,
48, 6062-6064. (b) Noh, D.; Yoon, S. K.; Won, J.; Lee, J. Y.; Yun, J.
Chem. Asian J. 2011, 6, 1967-1969.
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(12) Dow Corning holds a patent claiming certain copper-based
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Krause, N.; Lipshutz, B. H. Chem. Rev. 2008, 108, 2916-2927.
(14) See supporting information.
(15) Tamao, K.; Ishida, N.; Tanaka, T.; Kumada, M. Organometal-
lics, 1983, 2, 1694-1696.
catalysts for the hydrosilylation of olefins. Brandstadt, K.; Cook, S.;
Nguyen, B. T.; Surgenor, A.; Taylor, R.; Tzou, M-S. Copper Contain-
ing Hydrosilylation Catalysts and Compositions Containing the Cata-
lysts. WO 2013043792 A, March 28, 2013.
1
2
3
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(13) See, e.g., (a) Deschamp, J.; Chuzel, O.; Hannedouche, J.; Riant,
O. Angew. Chem. Int. Ed. 2006, 45, 1292-1297. (b) Rendler, S. Oes-
(16) For a discussion of metal-catalyed hydrosilylation mechansims,
see refs. 4(b) and 4(c).
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7
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44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
ACS Paragon Plus Environment