Simplified Preparation of Trialkylvinylsilanes via the Silyl-Heck Reaction Utilizing a Second Generation Catalyst

Article abstract of DOI:10.1002/adsc.201500436

Recently we reported a second generation ligand, bis(3,5-di-tert-butylphenyl)(tert-butyl)phosphine, for the preparation of allylsilanes using the silyl-Heck reaction. We now show that this new ligand also provides superior reactivity in the preparation of vinylsilanes from styrene derivatives. For the first time, this new ligand provides exceptionally high yields of trialkylvinylsilanes using the widely available palladium pre-catalyst, tris(dibenzylideneacetone)dipalladium(0) [Pd₂(dba)₃]. Finally, we demonstrate that this new catalyst system is able to form more highly decorated all-carbon substituted vinylsilanes that have been shown to possess superior reactivity in oxidation and cross-coupling reactions.

Full text of DOI:10.1002/adsc.201500436

UPDATES
DOI: 10.1002/adsc.201500436
Simplified Preparation of Trialkylvinylsilanes via the Silyl-Heck
Reaction Utilizing a Second Generation Catalyst
Jesse R. McAtee,a Sarah B. Krause,a and Donald A. Watsona,*
a
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA
Fax: (+1)-302-831-6335; e-mail: dawatson@udel.edu
Received: May 2, 2015; Revised: May 21, 2015; Published online: July 14, 2015
Dedicated to Prof. Stephen L. Buchwald on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500436.
Abstract: Recently we reported a second genera-
tion ligand, bis(3,5-di-tert-butylphenyl)(tert-butyl)-
phosphine, for the preparation of allylsilanes using
the silyl-Heck reaction. We now show that this new
ligand also provides superior reactivity in the prepa-
ration of vinylsilanes from styrene derivatives. For
the first time, this new ligand provides exceptionally
high yields of trialkylvinylsilanes using the widely
available palladium pre-catalyst, tris(dibenzylidene-
acetone)dipalladium(0) [Pd2(dba)3]. Finally, we
demonstrate that this new catalyst system is able to
form more highly decorated all-carbon substituted
vinylsilanes that have been shown to possess superi-
or reactivity in oxidation and cross-coupling reac-
tions.
pre-catalyst.[6] Other more common and more stable
palladium(0) sources, such as Pd2(dba)3 (dba=diben-
zylideneacetone), provided significantly lower yields
(ca. 10%) under comparable conditions. We attribute
the poor performance of this latter complex to com-
petitive, non-productive ligation of dba to the metal
center.[7]
Although complex 1 is easily prepared on a gram
scale,[3a,8] is air stable, and has recently become com-
mercially available,[9] it is slightly thermally sensitive.
This liability potentially limits the adoption of the
silyl-Heck reaction in some situations. We thus have
sought alternative catalytic conditions for the trans-
formation that utilize more readily handled reagents.
In our initial study, we also reported that the palla-
dium catalyst formed using t-BuPPh2 also converts
terminal alkenes bearing allylic hydrogen atoms (a-
olefins) to the corresponding allylsilanes using the
silyl-Heck reaction, albeit in modest yields.[3a] More
recently, we have developed a second generation
ligand 2 (Figure 1) for the formation of allylsilanes in
very high yields.[3c] Although ligand 2 was designed to
À
Keywords: alkenes; palladium; silyl-Heck reaction;
vinylsilanes
Vinylsilanes are potent nucleophiles in a wide variety promote oxidative addition of the Si I bond by in-
of carbon-carbon and carbon-heteroatom bond form- creasing both the size and electron-richness of the
ing reactions.[1] Additionally, the carbon-bound silicon phosphine center,[10] we recognized that these proper-
is readily oxidized, allowing vinylsilanes to serve as ties might also inhibit deleterious binding of exogene-
masked carbonyl groups.[2] We have previously de- ous ligands.
scribed a silyl-Heck reaction, which allows for the
As such, we have reinvestigated the silylation of
preparation of unsaturated organosilanes from simple styrene derivatives by Me3SiI using our second gener-
alkenes and electrophilic silanes under palladium cat- ation ligand 2. We now report that very high yields of
alysis.[3,4,5]
the desired vinylsilanes can be obtained when this
In our initial report, we described the formation of ligand is used in combination with the widely avail-
trimethylvinylsilanes from stryene derivatives and able and bench stable Pd2(dba)3 precatalyst. We dem-
Me3SiI.[3a] The optimal ligand in this palladium-cata- onstrate that this ligand provides vastly superior
lyzed transformation was identified as t-BuPPh2, yields of vinylsilanes with this precatalyst, compared
which provided high yields (up to 98% isolated) of to the first generation ligand.
the desired products across a range of styrene deriva-
These new conditions allow the preparation of vi-
tives. These transformations, however, required the nylsilanes from styrene derivatives using the silyl-
use of (COD)Pd(CH2SiMe3)2 (1) as the palladium
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Jesse R. McAtee et al.
from the model reaction (3) was isolated in 99% yield
(entry 1). Simple styrene gives rise to trimethylsilane
4 in 96% isolated yield (entry 2). Like the earlier gen-
eration silyl-Heck reactions, this catalyst system was
tolerant of a wide range of functional groups, includ-
ing aryl chlorides, aryl fluorides, esters, ethers, tertiary
amines, silyl ethers, dioxalanes, strained rings and bor-
onic esters (entries 3–11). Both electron-poor and
electron-rich aromatic styrenes were well tolerated
(entries 5–7), as were sterically hindered substrates
(entry 12). Finally, even complex molecules such as
natural product derivatives could be functionalized
(entry 13). In all cases, uniformly high yields of the vi-
nylsilane were observed, and in all cases, only the
trans-isomer of the product was detected. In some
cases, as noted in Table 2, 2 equivalents of Me3SiI
Figure 1. Improved synthesis of vinylsilanes using our
second generation catalyst.
Heck reaction with completely bench-stable and com- were required to achieve the observed yields.
mercially available catalytic precursors.[11]
Consistent with the results shown in Table 1, across
As a model system to examine the interplay of the the substrates examined, the second generation ligand
ligand and palladium precatalyst, we investigated the provide far superior results compared to t-BuPPh2.[13]
conversion of 4-tert-butylstyrene to vinylsilane 3 using
We also briefly examined the use of the Pd2(dba)3/
Me3SiI in the presence of Et3N. As we have previous- ligand 2 catalyst system for the silylation of a-olefins.
ly reported, with use of 5 mol% 1 and 10.5 mol% t- Using 1-decene as a model system, this catalyst
BuPPh2 in toluene at 508C, this transformation can be system provided the desired allylsilane 16 in 87%
carried out in 98% isolated yield (Table 1, entry 1).[3a] yield, with the mass balance being decene isomers
With 2.5 mol% Pd2(dba)3 (5 mol% Pd), only 12% of 3 (Table 3, entry 1). The E/Z and allyl/vinyl selectivities
is observed under otherwise identical conditions in the reaction were nearly identical to those which
(entry 2). In contrast, with the use of second-genera- we have previously reported.[3a,c] At room tempera-
tion ligand 2 (7.5 mol%) and 2.5 mol% Pd2(dba)3 ture, however, only trace reactivity was observed
(DCE at 408C, conditions similar to those used in our (entry 2). Although this catalyst system is much more
second generation allylsilane synthesis),[3c] 91% of the efficient than that employing t-BuPPh2 (entry 3), it re-
desired product is observed along with trace amounts mains significantly less effective than with the use of
of starting material (entry 3).[12] With lower ligand precatalyst 1 and ligand 2 under our previously re-
loading (5 mol%), a quantitative yield of silane 3 and ported conditions (entry 4).[3c]
no remaining alkene is observed (entry 4). Demon-
Given the greater reactivity of the Pd2(dba)3/ligand
strating that it is ligand 2, and not just the altered 2 catalyst system, we also wished to examine if vinyl-
conditions, that account for the improved reactivity silanes containing groups other than trimethylsilane
using Pd2(dba)3, the use of t-BuPPh2 under the same could be prepared using palladium catalysis. Previous-
conditions provides only 75% of the desired product ly, Me3SiI has proven to be the only mono-electro-
(entry 5).
philic silane with sufficient reactivity to participate in
Using the catalyst derived from Pd2(dba)3 and palladium-catalyzed silyl-Heck reactions. In particu-
ligand 2, excellent yields of vinylsilanes were obtained lar, we wished to prepare vinylsilanes bearing benzyl
with a broad range of styrenes (Table 2). The product groups (which have been shown to be highly effective
Table 1. Interplay of palladium precatalyst and ligand on yield.
Entry
Me3SiI
Solvent
Temperature
Ligand
[Pd]
Yield of 3
1
2
3
4
5
2.0 equiv.
2.0 equiv.
1.4 equiv.
1.4 equiv.
1.4 equiv.
PhMe
PhMe
DCE
DCE
DCE
508C
508C
408C
408C
408C
10.5 mol% t-BuPPh2
10.5 mol% t-BuPPh2
7.5 mol% 2
5 mol% 2
5 mol% t-BuPPh2
5 mol% 1
98%
12%
91%
99%
75%
2.5 mol% Pd2(dba)3
2.5 mol% Pd2(dba)3
2.5 mol% Pd2(dba)3
2.5 mol% Pd2(dba)3
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Simplified Preparation of Trialkylvinylsilanes via the Silyl-Heck Reaction
Table 2. Scope of the second generation silyl-Heck reaction
Table 3. Reactions of a-olefins.
to form vinylsilanes.
Entry
1
Temperature
Catalyst
Yield of 16
408C
2.5 mol% Pd2(dba)3
5 mol% 2
2.5 mol% Pd2(dba)3
87%[a]
2
3
4
r.t.
trace[b]
20%[b]
98%[a,c]
5 mol% 2
408C
r.t.
2.5 mol% Pd2(dba)3
5 mol% t-BuPPh2
2 mol% 1
3 mol% 2
[a]
Isolated yield.
[b]
[c]
Yield determined using NMR against internal standard.
Ref.[3c]
To this end, we were pleased to find that
Bn(Me)2SiI, Ph(Me)2SiI and (5-methyl-2-furyl)-
(Me)2SiI could all be used in the reaction with 4-tert-
butylstyrene, leading to the corresponding trans-vinyl-
silanes (17–19) in high yield (Figure 2). Unfortunately,
the new ligand does not completely solve steric limita-
tions with respect to the silyl electrophile. With use of
Et3SiI only a trace of product 20 was observed. None-
theless, the ability to prepare functionalized vinylsi-
lanes using the palladium-catalyzed silyl-Heck reac-
tion marks a significant advance in the utility of this
methodology.
In conclusion, we have now shown that our second
generation ligand effectively promotes the prepara-
tion of vinylsilanes from styrene derivatives via the
silyl-Heck reaction using the bench stable Pd2(dba)3
precatalyst. Thus, silylation of styrene derivatives
[a]
Yield determined using NMR against internal standard.
Isolated yield.
Using 2.0 equiv. Me3SiI.
[b]
[c]
in fluoride-promoted Hiyama cross-coupling reac-
tions),[14] phenyl groups (which are excellent sub-
strates for both oxidation and acylation),[1o,p,2] and 5-
methyl-2-furyl groups (which can be oxidized under
extremely mild conditions).[15]
Figure 2. Second generation silyl-Heck reaction for the syn-
thesis of functionalized vinylsilanes.
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Experimental Section
General Procedure
In
a
nitrogen-filled glovebox Pd2(dba)3 (25.0 mmol,
2.5 mol%), phosphine 2 (50.0 mmol, 5.0 mol%), dichloro-
ethane (1.0 mL), triethylamine (1.0 mL), trimethylsilyl
iodide (0.2 mL, 1.4 mmol, 1.4 equiv.) and alkene (1.0 mmol,
1 equiv.) were added to a 4.0-mL vial containing a magnetic
stir bar. The vial was sealed with a Teflon lined cap and re-
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a 408C oil bath and the mixture was allowed to stir for 24 h.
The vial was then cooled to room temperature, and opened
to air. The reaction was quenched with ~0.5 mL of brine
and diluted with ~1 mL diethyl ether. The layers were sepa-
rated and the aqueous layer was extracted with a second
~1 mL of diethyl ether. The combined organic layers were
dried over anhydrous magnesium sulfate, filtered, and con-
centrated under vacuum. The crude residue was purified by
flash column chromatography, vacuum distillation, or subli-
mation to yield the pure products.
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[4] For a nickel-catalyzed silyl-Heck reaction see: J. R.
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Acknowledgements
The University of Delaware (UD), the Research Corporation
(Cottrell Scholars Program), and the NSF (CAREER
CHE1254360) are gratefully acknowledged for support.
NMR and MS data were acquired at UD on instruments ob-
tained with the assistance of NSF (NSF CHE0421224,
CHE1229234 and CHE0840401) and NIH (NIH
P20M103541, P20M104316, P30M110758, S10RR02692, and
S10OD016267) funding.
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Simplified Preparation of Trialkylvinylsilanes via the Silyl-Heck Reaction
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