Regioselective allene hydrosilylation catalyzed by N-heterocyclic carbene complexes of nickel and palladium

Article abstract of DOI:10.1021/ja407749w

Regioselective methods for allene hydrosilylation have been developed, with regioselectivity being governed primarily by the choice of metal. Alkenylsilanes are produced via nickel catalysis with larger N-heterocyclic carbene (NHC) ligands, and allylsilan

Full text of DOI:10.1021/ja407749w

Communication
pubs.acs.org/JACS
Regioselective Allene Hydrosilylation Catalyzed by N‑Heterocyclic
Carbene Complexes of Nickel and Palladium
Zachary D. Miller,‡ Wei Li,‡ Tomas R. Belderrain,† and John Montgomery*,‡
́
‡Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055, United States
†Laboratorio de Catal
́ ́ ́
isis Homogenea, Unidad Asociada al CSIC, CIQSO-Centro de Investigacion en Química Sostenible and
Departamento de Química y Ciencias de los Materiales, Campus de El Carmen s/n, Universidad de Huelva, 21007-Huelva, Spain
S
* Supporting Information
Scheme 2
ABSTRACT: Regioselective methods for allene hydro-
silylation have been developed, with regioselectivity being
governed primarily by the choice of metal. Alkenylsilanes
are produced via nickel catalysis with larger N-heterocyclic
carbene (NHC) ligands, and allylsilanes are produced via
palladium catalysis with smaller NHC ligands. These
complementary methods allow either regioisomeric
product to be obtained with exceptional regiocontrol.
lkenylsilanes and allylsilanes are useful reagent classes that
1
Aare employed in numerous synthetic transformations. For
example, the Hiyama cross-coupling2 and Sakurai-type
Table 1
allylation and crotylation reactions3 are widely used methods
for C−C bond formation. Currently, the most direct and atom-
economical routes to vinyl- and allylsilanes proceed via metal-
catalyzed hydrosilylations of π-components (Scheme 1). The
hydrosilylation of alkynes to afford alkenylsilanes4 and the
hydrosilylation of 1,3-dienes to afford allylsilanes5 are versatile
entry
precatalyst
L·HX
% yield
regiosel. (3:4)
methods that have been widely employed. In both instances,
control of regiochemistry is required with unsymmetrical
substrates, and even the most efficient hydrosilylation
procedures are often plagued by the lack of complete
regioselectivity for a variety of substrates without directing
group effects. Compounding these challenges is the difficulty
typically seen in the separation of regioisomeric allyl- or
vinylsilanes produced by these methods.
In contrast to the significant developments that have been
reported in hydrosilylations of alkynes and 1,3-dienes, the
hydrosilylation of allenes has rarely been examined.6 The
hydrometalation of allenes presents considerable challenges in
1
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
5a
5b
6a
6b
7
22
15
58
47
84
trace
56
75
74
80
33:67
2
40:60
3
85:15
4
81:19
5
>98:<2
not determined
14:86
6
7
7
6b
6a
5b
5a
8
12:88
9
2:98
10
<2:>98
Scheme 1
selectivity since 1,2- or 2,1-addition to either of the two
contiguous π-systems can occur, potentially providing two
regioisomeric allylsilanes and two regioisomeric vinylsilanes, in
Received: July 27, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja407749w | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 3
Table 2
some cases as mixtures of alkene stereoisomers. The hydro-
boration of allenes has been widely developed, but these
processes are typically uncatalyzed, thus providing a single
allylborane product governed by substrate steric and electronic
characteristics.7,8 Significant advances have appeared in the
regioselective bis-metalation of allenes, through catalytic
reactions such as diboration9 and silaboration,10 but regiodi-
vergent processes in simpler hydrometalations of allenes are
lacking. Given the synthetic utility of allyl- and vinylsilanes and
the potential to provide regiocontrolled access to both structure
classes from a common allene substrate, we report herein our
efforts to develop regioselective, catalyst-controlled methods for
the hydrosilylation of allenes.
As part of our laboratory’s broader interest in the
development of regioselective catalytic processes,11 we initially
hoped to develop ligand-controlled regioselectivity reversal in
the nickel-catalyzed hydrosilylation of allenes. However, upon
screening a broad range of nickel catalysts modified with N-
heterocyclic carbene or phosphine ligands, combinations that
allowed high-yielding and highly regioselective access to both
alkenylsilane and allylsilane products were not identified.
However, in the course of these exploratory studies, we were
attracted to the recent disclosures of well-defined N-
heterocyclic carbene (NHC) complexes of palladium and
nickel.12 To our surprise, simple modification of the metal,
while leaving the ligand and reaction conditions unchanged,
provided an encouraging lead toward identifying routes to both
product regioisomers. As illustrated in the hydrosilylation of
cyclohexylallene with triethylsilane, the use of nickel complex 1
provided the vinylsilane 3 with 85:15 regioselectivity, whereas
exactly the same procedure and same ligand, but employing the
palladium complex 2 instead, provided the allylsilane 4 with
88:12 regioselectivity (Scheme 2). Reactions involving the
presynthesized nickel complex 1 required an elevated temper-
ature, likely due to the need to dissociate styrene to provide the
a
Unless otherwise noted, reactions were conducted on a 0.3 mmol
b
c
scale. Conducted on 6.0 mmol scale. Conducted with benchtop
assembly of the catalyst and ligand followed by three vacuum/purge
cycles before allene and silane introduction. Conducted at 2.5 mol %
d
loading of Pd2(dba)3.
active catalyst form, whereas palladium complex 2 was
catalytically active at rt or elevated temperature. This unusual
example of regiochemistry reversal with a common ligand
scaffold and within a single metal triad presented an unexpected
and effective strategy for accessing both allylsilanes and
alkenylsilanes from a common allene precursor.
To further boost the regioselectivity of these complementary
processes, we examined ligand variation for both nickel- and
palladium-catalyzed hydrosilylations. In comparing catalytic
Scheme 4
B
dx.doi.org/10.1021/ja407749w | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
catalyst loading (2.5 mol % Pd2(dba)3) proceeded in 72%
isolated yield (Table 2, entry 15), whereas attempts to lower
the catalyst loading further resulted in more significant drops in
chemical yield.
Table 3
In order to evaluate the effects of introducing allene 1,1-
disubstitution, we subjected vinylidene cyclohexane to both
nickel- and palladium-catalyzed reaction conditions (Scheme
3). The nickel-catalyzed reaction resulted in an outcome
consistent with the previous observations for monosubstituted
allenes, where the silyl group is installed at the central allenic
carbon to afford vinylsilane 10. Reactions with palladium,
however, afforded a new observed allylsilane isomer 11, with
the C−Si bond forming at the least substituted allenic carbon.
This observation offers a new entry to a useful allylsilane
traditionally accessed via the hydrosilylation of 1,3-dienes.
The palladium-based procedure tolerates variation of the
silane structure with minimal change in regioselectivity and
yield. For example, additions to cyclohexyl allene cleanly
proceed with a range of synthetically versatile silane structures
(Table 3).13 Alternatively, the nickel-based procedure is
currently more limited with silane variation leading to a drop
in yield and erosion of regioselectivy.
While the origin of regiocontrol is at present uncertain, we
propose that a likely mechanism involves a change from a
silylmetalation pathway with nickel to a hydrometalation
pathway with palladium (Scheme 4). Migratory insertions to
allenes typically favor addition to the central allene carbon with
the formation of a metal π-allyl complex.9g,10g Through this
common mechanistic feature, silylmetalation of nickel allene
complex 12 to generate π-allyl intermediate 13, followed by C−
H reductive elimination, would afford vinylsilane product 8.
Alternatively, hydrometalation of palladium allene complex 14
to generate π-allyl intermediate 15, followed by C−Si reductive
elimination, would afford allylsilane product 9. By utilizing a
large ligand with nickel, disposition of the silyl group trans to
the ligand in intermediate 12 reinforces the preference for
silylmetalation. However, by utilizing a small ligand with
palladium, disposition of the silyl group cis to the ligand in 14
reinforces the preference for hydrometalation. Detailed studies
from Morken and Suginome in diborations and silaborations of
allenes provide significant insight into the factors that control
regiochemistry in these related processes, and the mechanistic
model described in Scheme 4 draws from the insights of those
studies.9g,10g Future work will be devoted to evaluating the
intriguing mechanistic basis of these regiodivergent hydro-
silylation processes.
entry
silane
product (% yield)
regiosel.
1
2
3
4
HSiMe2Ph
12a (91)
12b (93)
12c (82)
12d (75)
>98:2
>98:2
>98:2
>98:2
HSiMe2Bn
HSiMe(OSiMe3)2
H2Si(t-Bu)2
reactions of 1 and 2 with the corresponding complexes
prepared in situ from Ni(COD)2 or Pd2(dba)3, the chemical
yields are similar and regioselectivities are identical in
comparing the catalysts presynthesized or prepared in situ
(comparing Scheme 2 to Table 1, entries 3 and 8). The use of
the presynthesized complexes comes with the advantage of a
higher catalyst turnover and ease of accurate weighing on small
scale that becomes difficult for the in situ protocol. However, to
facilitate more rapid screening of a variety of ligands, catalysts
for the optimization studies were prepared in situ by treatment
of Ni(COD)2 or Pd2(dba)3 with the HCl or HBF4 salt of the
desired NHC ligands.
From the ligand variation studies, an interesting trend
emerged that regioselectivities for alkenylsilane products using
nickel were enhanced with larger ligands, whereas regioselec-
tivities for allylsilanes using palladium were enhanced with
smaller ligands. To illustrate this trend using Ni(COD)2 as the
precatalyst, IMes and SIMes slightly favored allylsilane
production (Table 1, entries 1 and 2), IPr and SIPr favored
alkenylsilane production with good selectivity (Table 1, entries
3 and 4), and the very bulky ligand DP-IPr afforded the
alkenylsilane in 84% isolated yield with >98:2 regioselectivity
(Table 1, entry 5). In comparison, using Pd2(dba)3 as the
precatalyst, the use of DP-IPr afforded a low-yielding mixture of
regioisomers (Table 1, entry 6), IPr and SIPr favored allylsilane
production with good selectivity (Table 1, entries 7 and 8), and
IMes and SIMes provided exceptional regioselectivities (Table
1, entries 9 and 10), with IMes providing an 80% isolated yield
of the allylsilane with >98:2 regioselectivity.
The above guidelines for the in situ generated Ni(0) and
Pd(0) hydrosilylation catalysts were utilized in the reactions for
a variety of allene substrate classes (Table 2). In addition to the
use of cyclohexyl allene (entries 1 and 2), allenes containing
less hindered aliphatic groups (Table 2, entries 3 and 4),
aromatic groups (Table 2, entries 5 and 6), unprotected
hydroxyls (Table 2, entries 7 and 8), and silyloxy groups (Table
2, entries 9 and 10) were successfully hydrosilylated using
either Ni(COD)2 with ligand 7 (DP-IPr; conditions A) or
Pd2(dba)3 with ligand 5a (IMes; conditions B) with
consistently excellent regioselectivities (>98:2) and in high
chemical yields. In addition, an allene containing a phthalimido
moiety underwent smooth hydrosilylation to afford either
regioisomeric product (Table 2, entries 11 and 12). To further
explore the scaleup and utility in reactions not using glovebox
manipulations, the palladium-catalyzed procedure with cyclo-
hexyl allene was optimized on a 6 mmol scale, which proceeded
in 98% isolated yield in >98:2 regioselectivity (Table 2, entry
13). Additionally, a procedure conducted with a standard
benchtop assembly purged with a nitrogen line proceeds in
74% isolated yield with slightly eroded (96:4) regioselectivity
(Table 2, entry 14). Finally, an experiment with a reduced
In summary, the complementary use of metals (Ni vs Pd)
results in regiochemical reversals in allene hydrosilylations.
Alterations in NHC ligand structure can further improve the
regioselectivities to provide regiodivergent access to a wide
range of alkenylsilanes or allylsilanes in high yields and with
exceptional regiocontrol. The above study demonstrates the
special role that metal identity can play in governing
regioselective catalytic reactions. Future studies will explore
these effects in other classes of catalytic reactions.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and analytical data. This material is
available free of charge via the Internet at http://pubs.acs.org.
C
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Journal of the American Chemical Society
Communication
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AUTHOR INFORMATION
Corresponding Author
jmontg@umich.edu
■
Funding
No competing financial interests have been declared.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-1265491),
the National Institutes of Health (GM57014) and MINECO
(CTQ2011−24502) for support of this research. NSF supports
a program including a comparative analysis of base metal and
precious metal catalysis. NIH supports a program in the design
of NHC ligands for regioselective transformations. Professor
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Soc. 2010, 132, 6304. (b) Liu, P.; Montgomery, J.; Houk, K. N. J. Am.
Chem. Soc. 2011, 133, 6956. (c) Li, W.; Chen, N.; Montgomery, J.
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́
Pedro Perez (Universidad de Huelva) is thanked for the
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