Preparation of allyl and vinyl silanes by the palladium-catalyzed silylation of terminal olefins: A silyl-heck reaction

Article abstract of DOI:10.1002/anie.201200060

Installing silicon is easy! A high-yielding protocol for the palladium-catalyzed silylation of terminal alkenes is reported. This method allows facile conversion of styrenes to E-β-silyl styrenes by using iodotrimethylsilane (TMSI) or chlorotrimethylsilane/lithium iodide (see scheme). Terminal allyl silanes with good E/Z ratios are also readily accessed from α-olefins.

Full text of DOI:10.1002/anie.201200060

Angewandte
Chemie
DOI: 10.1002/anie.201200060
Synthetic Methods
Preparation of Allyl and Vinyl Silanes by the Palladium-Catalyzed
Silylation of Terminal Olefins: A Silyl-Heck Reaction**
Jesse R. McAtee, Sara E. S. Martin, Derek T. Ahneman, Keywan A. Johnson, and
Donald A. Watson*
Dedicated to Professor Robert G. Bergman on the occasion of his 70th birthday
Allyl and vinyl silanes are indispensable nucleophiles in
organic synthesis.[1] Notable applications include Hiyama
cross-coupling reactions[2] and Hosomi-Sakurai-type allyl-
ation and crotylation reactions,[3] along with many others.[1c]
Numerous methods are known for the preparation of both
allyl and vinyl silanes.[4,5] However, limitations exist with
many of the current protocols, which often makes the
synthesis of unsaturated organosilanes challenging. For
example, many current methods require highly reactive
reagents (e.g., the addition of carbon nucleophiles to electro-
philic silanes), indirect introduction of the silicon atom
Scheme 1. Putative catalytic pathway for the silyl-Heck reaction.
À
through C C bond-forming reactions (e.g., alkene metathesis,
À
C C bond-forming cross-coupling reactions, and carbonyl
olefination reactions), or reductive processes that decrease
the degree of unsaturation of the starting materials (e.g.,
alkyne hydrosilylation or metal-catalyzed allyl substitution).
Notably, methods to directly attach silyl groups to alkenes to
produce allyl or vinyl silanes have received little attention.[6–8]
Alkenes are important starting materials in organic
synthesis, as they are readily accessed and highly stable. We
would be expected to proceed under mild conditions and,
therefore, be complementary to other routes to these
unsaturated organosilanes. Hints in the literature suggest
that such a process might be possible,[11,12] but to date, no
synthetically viable silyl-Heck reactions of alkenes have been
described.
Herein, we report a high-yielding silyl-Heck reaction for
the preparation of allyl and vinyl silanes from alkenes. We
have identified tBuPPh2 as a uniquely effective ligand for this
palladium-catalyzed process. We show that the reaction
proceeds at low temperatures (RT to 508C) in good to
excellent yields and is compatible with a range of substrates.
Styrenes and related compounds lead to terminal vinyl
silanes, whereas terminal olefins that contain allylic hydrogen
atoms preferentially give terminal E-allyl silanes. This
method enables the conversion of simple alkenes into
potent nucleophiles that are capable of reacting with a wide
variety of electrophiles, thus, this process allows the facile and
recognized that oxidative addition of a low-valent transition
[9]
À
metal to a silicon halide (Si X) bond and subsequent
insertion into an alkene (analogous to the Heck reaction)
could provide a useful route to either allyl or vinyl silanes,
depending upon the site of b-hydride elimination
(Scheme 1).[10] We anticipated that this reaction manifold
would have several advantages. Not only would such a reac-
tion convert simple alkenes into more complex nucleophiles,
À
but it would also allow for the formal conversion of a C H
À
bond into a C Si bond. In so doing, this reaction would
preserve the degree of unsaturation of the alkene starting
material, thereby negating the need for more highly oxidized
substrates. Finally, as this is a palladium-catalyzed reaction, it
À
selective functionalization of the allylic or vinylic C H bonds
of terminal olefins with a wide array of functional groups.
In 1991, Murai and co-workers reported a three-compo-
nent coupling of terminal alkynes, iodotrimethylsilane
(TMSI), and organostannanes catalyzed by [Pd(PPh3)4].[12]
Presumably, this reaction proceeds by a Heck-type mecha-
nism, but the use of alkenes as coupling partners was not
described. In the same year, Tanaka and co-workers reported
that styrenes give E-b-silyl styrenes upon treatment with
TMSI and a catalytic amount of [PdCl2(PEt3)2].[11] However,
the reported conditions for this reaction require a large excess
of alkene (4 equiv), are harsh (1208C for 72 h in Et3N), and
are low yielding (three examples reported: 44–54% based on
TMSI; 11–14% based on styrene). From this vantage point, it
was unclear that a synthetically viable alkene silyl-Heck
reaction was possible.
[*] J. R. McAtee, S. E. S. Martin, D. T. Ahneman, K. A. Johnson,
Prof. D. A. Watson
Department of Chemistry and Biochemistry, University of Delaware
Newark, DE 19716 (USA)
E-mail: dawatson@udel.edu
[**] We acknowledge the University of Delaware (UD) for research
support and the following sources for fellowships: NIH/NIGMS CBI
Program (5T32 GM 08550-16, SESM); David Plastino and the ACS
SURF Program (DTA); HHMI Grant 52006947 to the University of
Delaware, and the UD NUCLEUS Program (KAJ). Data were
acquired on instruments obtained through NSF and NIH funding
(NSF MIR 0421224 & CRIF MU CHE0840401, NIH P20 RR017716).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200060.
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Our effort to develop such a reaction began by examining
the silylation of 4-tert-butylstyrene using TMSI. Our initial
studies were conducted at 808C in toluene with the alkene as
the limiting reagent. To systematically evaluate the use of
various phosphine ligands, [(COD)Pd(CH2TMS)2][13] (2) was
selected as the palladium precatalyst.[14] Under these con-
ditions, neither Ph3P nor Et3P (the ligands used by Murai and
Tanaka, respectively) provided more than trace yield of the
desired silyl styrene 1 (Table 1, entries 1 and 2).[15] Bisphos-
phines were also ineffective in the reaction (Table 1, entries 3
and 4).
provided 1 in 65% yield (Table 1, entry 8). Ultimately,
tBuPPh2 (q = 1578)[17] proved to be the most effective ligand
and provided 1 in 80% yield under the assay conditions
(Table 1, entry 9). After optimization (most significantly by
lowering the temperature to 508C and decreasing the
concentration), vinyl silane 1 was obtained in 98% yield
after 24 h, as assessed by GC (Table 1, entry 10).
A wide variety of styrene substrates participate in the
silyl-Heck reaction to provide vinyl silanes in high yields
(Scheme 2). Under preparative conditions (ca. 1 mmol scale),
Table 1: Identification of the catalyst for the silyl-Heck reaction.[a]
Entry
Ligand
Yield [%][b]
Entry
Ligand
Yield [%][b]
1
2
3
4
5
Et3P
6
4
0
1
1
6
7
8
Cy3P
4
Ph3P
dppe
dcpe
tBu3P
Cy2PPh
CyPPh2
tBuPPh2
tBuPPh2
27
65
80
98
9
10[c]
[a] Conc. of substrate=2m. [b] Determined by GC. [c] 2 equiv TMSI,
2.2 equiv Et3N, tol, conc.=1m, 24 h, 508C.
From the outset, we suspected that a sterically demanding,
electron-rich ligand would be required to effect the desired
reaction in high yield. Such ligands have proven effective in
a variety of challenging palladium-catalyzed transformations,
as they allow the formation of highly reactive, low-valent
palladium intermediates, which can aid in both oxidative
addition and ligand-exchange processes.[16] Surprisingly, how-
ever, neither the use of tBu3P (Tolman Angle, q = 1828)[17] nor
Cy3P (q = 1708)[17] provided significant amounts of the desired
product (Table 1, entries 5 and 6).
Scheme 2. Scope of the silyl-Heck reaction for substrates that lack
allylic hydrogen atoms. Conc. of substrate=1m. For 8, 11, and 12:
[(COD)Pd(CH2TMS)2] (10 mol%), tBuPPh2 (21 mol%), sol-
vent=PhCF3. For 9: [(COD)Pd(CH2TMS)2] (10 mol%), tBuPPh2
(21 mol%), solvent=PhCF3, TMSI (4 equiv), Et3N (4.4 equiv),
conc.=0.5m, after cleavage of the silyl enol ether, 48 h. For 10 and 13:
[(COD)Pd(CH2TMS)2] (10 mol%), tBuPPh2 (21 mol%),TMSI (3 equiv)
Et3N (3.5 equiv), 48 h.
Consideration of the intermediates in the proposed
catalytic cycle (Scheme 1) suggested that the reason for the
failure of these all-alkyl ligands might be related to the large
steric demand of the TMS group. We reasoned that large,
electron-rich, all-alkyl ligands, which provide an electronic
vinyl silane 1 was isolated in 97% yield.[18] Unsubstituted
styrene was also an excellent substrate and gave a 95% yield
of isolated 3. Both electron-rich and electron-poor substrates
proved viable in the reaction; compounds 4 and 8 were
prepared in 96% and 81% yield, respectively. The reaction
also tolerates functional groups well, with substrates that
contain aryl ethers, esters, chlorides, fluorides, and ketones all
providing products in high yields.[19] Steric hindrance on the
arene of the styrene was also tolerated; compound 10, which
contains a methyl group ortho to the alkene, was isolated in
87% yield. However, substrates that have increased substi-
tution on the alkene, such as in a- or b-methyl styrene, failed
to react in the silylation. Although the formation of highly
Lewis-basic products, such as pyridine derivative 11, was
possible, the yields in these reactions were diminished.
Increased steric demand around the site of the Lewis base,
such as in picoline derivative 12, greatly improved the
reaction, which suggests that the low yield for 11 may be
a result of ligation of the pyridyl group to the palladium
benefit to the reaction, might sterically disfavor oxidative
0
À
addition of Pd to the TMS I bond. To address the apparent
dichotomy between steric and electronic factors, we examined
ligands that contain both phenyl groups and large alkyl
groups. Although these ligands are less commonly used in
catalysis, we reasoned that they might be sufficiently electron-
donating and large enough to support low-valent palladium
complexes, and also provide sufficient space around the
resulting PdII center to accommodate the sterically demand-
ing TMS group. In accordance with this hypothesis, the use of
Cy2PPh (q = 1618)[17] significantly increased the yield of the
desired product (27%, Table 1, entry 7). This result is
particularly remarkable given the failure of both Ph3P and
Cy3P, which contain only a single type of phosphine substitu-
ent. The silyl-Heck reaction was further improved by switch-
ing to the slightly smaller CyPPh2 (q = 1538),[17] which
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Table 2: Identification of conditions for a-olefin substrates.
catalyst. Finally, other electron-rich alkenes that lack allylic
hydrogen atoms are also viable substrates in the reaction. For
example, the use of vinyl carbazole gave rise to silane 13 in
86% yield. In all cases, only the E isomer of the vinyl silane
was detected (by 1H NMR spectroscopy and GC). It is worth
noting that the reactions are operationally simple to conduct;
all preparative reactions were set up by using standard bench-
top techniques, and both 2 and tBuPPh2 were stored and
handled in air.[20]
Entry
T [8C]
Solvent
Conversion [%][a]
Yield [%][a]
1
2
3
4
5
6
7
8
50
50
50
50
50
50
RT
RT
toluene
dioxane
hexanes
MeCN
DCE
PhF
PhF
PhCF3
72
34
46
2
63
73
80
93
33
18
20
0
29
41
55
68
We were cognizant that TMSI is more expensive than
chlorotrimethylsilane (TMSCl) and that a procedure that uses
TMSCl would be highly attractive. However, the oxidative
À
addition of transition-metal complexes to Si Cl bonds is
À
known to be challenging because of the Si Cl bond strength
(ca. 97 kcalmolÀ1 versus ca. 57 kcalmolÀ1 for Si I). It was
[21]
À
[a] Determined by GC.
not surprising, therefore, that no reaction occurred when
TMSCl was used in place of TMSI. However, TMSI can be
prepared in situ from TMSCl by using iodide salts.[22] After
optimization, we identified LiI as an additive that allows the
direct use of TMSCl in the silyl-Heck reaction of styrenes. In
most cases, similar yields are obtained by using this procedure
relative to the TMSI conditions [Eq. (1)].[23,24]
product was obtained (Table 2, entry 6). Lowering the
reaction temperature from 508C to RT further improved
the yield (Table 2, entry 7). Ultimately, we found that a,a,a-
trifluorotoluene was most effective (Table 2, entry 8). Using
this solvent in the reaction, allyl silane 14 was obtained in
68% yield after 24 h at RT, with nearly complete consump-
tion of the starting alkene (by GC).[27] Isomerized starting
material accounted for the mass balance.
Under preparative conditions, 14 was isolated in 60%
yield as an 83:17 mixture of E/Z allyl silanes (Scheme 3). The
product was readily separable from the isomerized starting
material by simple vacuum distillation. These reaction con-
ditions proved applicable to the preparation of other terminal
allyl silanes. On a preparative scale (9–15 mmol, ca. 3 g) the
catalyst loading could often be lowered to 2.5 mol%
palladium without a loss of yield. In general, the selectivity
for allyl over vinyl silane was greater than 93:7, and often
better than 95:5.[28] For example, allyl silane 15 was isolated in
57% yield from the reaction with allyl benzene. In this case,
high (greater than 95:5) E/Z selectivity was obtained.
Having established high-yielding protocols for the silyla-
tion of styrenes and related substrates, we turned to terminal
alkenes that contain allylic hydrogen atoms. Silyl-Heck
reactions to prepare allyl silanes from terminal alkenes have
not been reported previously.[25,26]
Our initial investigations focused on 1-decene as a model
substrate. Under the optimized reaction conditions for the
silylation of styrenes, modest amounts (33% yield by GC) of
silylated products were obtained. These
products were almost exclusively allyl
silane products 14 in an approximate
85:15
E/Z ratio (Table 2, entry 1). However,
significant amounts of alkene isomers of
decene were also obtained (as shown by
GC/MS), which indicates nonproductive
consumption of the starting material. In
an attempt to favor the formation of allyl
silanes, we undertook a survey of reaction
solvents. Although most solvents, such as
dioxane, hexanes, and acetonitrile, gave
inferior results relative to toluene
(Table 2, entries 2–4), the use of 1,2-
dichloroethane (DCE) led to slightly less
isomerization of the starting material, but
provided a similar yield of the desired
product (Table 2, entry 5). Based upon
these results, we examined the use of
Scheme 3. Substrate scope for formation of allyl silanes. For 14 and 22: [(COD)Pd(CH2TMS)2]
(5 mol%), tBuPPh2 (10.5 mol%). For 15: 358C, 48 h. For 20a, b, and c: [(COD)Pd(CH2TMS)2]
other halogenated solvents. With fluoro-
benzene, an increased yield of the desired (10 mol%), tBuPPh2 (21 mol%). For 21: 508C, 48 h, solvent=PhH.
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Although the silylation was equally effective with 4-phenyl-
butene, a small amount (ca. 3%) of the homoallyl silane was
also produced in this reaction, likely as a result of isomer-
ization of the alkene of 16 into conjugation with the arene.
Ethers were well tolerated in this reaction; silanes 17 and 18
were isolated in 67% and 66% yield, respectively. In the
aliphatic series, silyl ethers were compatible with the reaction
conditions (for example, 19). Substrates that contain pivalate-
protected alcohols can also be used. Interestingly, the
proximity of the pivalate to the alkene had a remarkable
effect on the reaction. Whereas 20a and 20b were produced
in good yields (58–59%), the product derived from 3-buten-1-
ol (20c) was not obtained (the crude reaction mixture only
contained starting material by GC). The origins of this
phenomenon are not clear at this point. Notably, allyltrime-
thylsilane was also an effective substrate for the silylation
reaction and gave disilane 21 in 49% yield.[29] Finally, we note
that the ratio of allyl to vinyl silane products appears to be
dependent upon the degree of substitution at the homoallyl
position in the starting material, with increased substitution at
this position partially eroding selectivity for the allyl silane;
for example, 22 was isolated as an approximate 2:1 mixture of
E-allyl to E-vinyl silanes. Presumably, increased substitution
at the homoallylic position disfavors the b-hydride elimina-
tion that leads to the allyl product, thereby making the
pathway to the vinyl silane more competitive.
We expect the direct silylation of alkenes to find
applications in the synthesis of complex organic molecules.
To highlight the utility of this process, we have examined the
reactivity of the silylated products in a number of previously
reported types of reactions. For example, allyl silanes are well
known to participate in allyl transfer reactions and proceed
with good scope.[1] Treatment of 19 with benzaldehyde
dimethylacetal and BF3·Et2O gave a 70% yield of isolated
methyl ether 23 as a 84:16 mixture of diastereomers
(Scheme 4a).[3a] As the reactivity of allyl silanes is orthogonal
to nonsilylated alkenes, crotylation reactions can often be
carried out without purification of the intermediate allyl
silane. For example, silylation of 4-phenylbutene and subse-
quent treatment of the crude product with butyraldehyde and
TiCl4 provided a 59% yield of isolated homoallyl alcohol 24
as a 95:5 mixture of diastereomers, based on the starting
alkene (Scheme 4b).[3a,30,31] Finally, branched allylic alcohols
can be prepared by the oxidation of allyl silanes without
purification of the intermediate allyl silane. Silylation of
decene and subsequent treatment of the crude product with
m-chloroperbenzoic acid and then tetrabutylammonium
fluoride produced allyl alcohol 25 in 54% overall yield
(Scheme 4c).[32] Overall, this method provides a highly flex-
ible route for allylic functionalization, which complements the
existing direct allylic functionalization reactions.[33]
In summary, we have demonstrated a high-yielding
protocol for the silyl-Heck reaction of alkenes. Key to this
discovery is the identification of tBuPPh2 as a uniquely
effective ligand, which we believe is simultaneously large and
electron-rich enough to provide a highly active palladium
catalyst, yet small enough to accommodate the bulky TMS
group. This method allows the direct silylation of monosub-
stituted alkenes. In the case of substrates that lack allylic
hydrogen atoms, such as styrenes, high yields of E-vinyl
silanes result. Conditions that use TMSI, as well as the less
expensive TMSCl, have been developed. Substrates that
contain allylic hydrogen atoms are transformed into terminal
allyl silanes in good yields and with good levels of E/Z
stereocontrol. When combined with existing methods, the
conversion of a-olefins into allyl silanes allows the facile and
selective functionalization of the allylic position, which results
in a rapid increase of molecular complexity from simple
starting materials.
Received: January 4, 2012
Revised: February 6, 2012
Published online: March 1, 2012
Keywords: alkenes · allyl silanes · Heck reaction · palladium ·
.
vinyl silanes
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(CH2TMS)2] (5 mol%), tBuPPh2 (10.5 mol%), TMSI (2 equiv), Et3N
(2.2 equiv), solvent=PhCF3, RT, 24 h.
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than 99% yield.
[19] In the case of ketone 9, the corresponding silyl enol ether was
initially obtained and was subsequently converted to the ketone
by using mild acid.
[20] Other palladium precatalysts, such as Pd(OAc)2 and [Pd2(dba)3],
were not effective in this transformation.
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gave only trace yields of product.
[25] Allyl silane was observed in the silylation of allylbenzene in the
zirconium catalyzed chemistry described in Ref. [6a]. However,
this is not a general process; other aliphatic a-olefins gave vinyl
silanes.
[26] For a related nucleophilic process that involves silylzincation of
alkenes see ref. [5e].
[27] The mechanistic underpinnings for the positive effect of the
halogenated solvents are currently being investigated.
[28] See the Supporting Information for the exact quantities of vinyl
silanes formed.
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