Rational design of a second generation catalyst for preparation of allylsilanes using the silyl-Heck reaction

Article abstract of DOI:10.1021/ja505446y

Using rational ligand design, we have developed of a second-generation ligand, bis(3,5-di-tert-butylphenyl)(tert-butyl)phosphine, for the preparation of allylsilanes using the palladium-catalyzed silyl-Heck reaction. This new ligand provides nearly complete suppression of starting material alkene isomerization that was observed with our first-generation catalyst, providing vastly improved yields of allylsilanes from simple alkene starting materials. The studies quantifying the electronic and steric properties of the new ligand are described. Finally, we report an X-ray crystal structure of a palladium complex resulting from the oxidative addition of Me₃SiI using an analogous ligand that provides significant insight into the nature of the catalytic system.

Full text of DOI:10.1021/ja505446y

Article
pubs.acs.org/JACS
Open Access on 07/08/2015
Rational Design of a Second Generation Catalyst for Preparation
of Allylsilanes Using the Silyl-Heck Reaction
Jesse R. McAtee, Glenn P. A. Yap, and Donald A. Watson*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
ABSTRACT: Using rational ligand design, we have developed
of a second-generation ligand, bis(3,5-di-tert-butylphenyl)
(tert-butyl)phosphine, for the preparation of allylsilanes using
the palladium-catalyzed silyl-Heck reaction. This new ligand
provides nearly complete suppression of starting material
alkene isomerization that was observed with our first-
generation catalyst, providing vastly improved yields of
allylsilanes from simple alkene starting materials. The studies quantifying the electronic and steric properties of the new
ligand are described. Finally, we report an X-ray crystal structure of a palladium complex resulting from the oxidative addition of
Me3SiI using an analogous ligand that provides significant insight into the nature of the catalytic system.
1. INTRODUCTION
Allylsilanes are versatile nucleophiles in organic synthesis that
possess numerous favorable properties, including low toxicity
and high functional group compatibility.1 We recently described a
novel synthesis of allyl- (and vinyl-) silanes by palladium-catalyzed
silylation of terminal alkenes using Me3SiI.2−5 This silyl-Heck
reaction provides an expedient entry into these important
nucleophiles using widely available alkene starting materials.6
Although our initial conditions provided allylsilanes with good to
excellent selectivities for the trans isomers and were functional-
group tolerant, the yields of these reactions were moderate
(typically ∼60%) due to competitive isomerization of the alkene
starting material.2
Guided by rational ligand design, we have sought improved
catalysts that minimize competitive alkene isomerization and
increase the yield of the allylsilane. We now report a second-
generation ligand for the palladium-catalyzed silyl-Heck
reaction that converts a broad range of terminal alkenes into
allylsilanes with dramatically improved yields. In addition, these
studies have deepened our understanding of the product-
determining step that leads to allylsilane formation. Quantita-
tive studies of the steric and electronic properties of the various
ligands investigated, as well as stoichiometric experiments,
provide insight into the palladium species involved in the
oxidative addition of Me3SiI. These studies culminated in
isolation and characterization of the first monomeric palladium
complex from the oxidative addition of a silicon−halogen bond.
In total, these studies have not only greatly improved the
synthetic utility of the silyl-Heck reaction in the preparation of
allylsilanes, but also significantly increased our understanding of
the process.
Figure 1. Rational design of 2nd generation catalyst for the silyl-Heck
reaction.
reactions. This ligand was vastly superior to both all-aryl
phosphines, such as Ph3P, as well as all-alkyl phosphines, such
2
t
Cy3P and Bu3P. We rationalized that the effectiveness of
tBuPPh2 was due to it being sufficiently electron-rich to favor
oxidative addition of the Me3SiI bond (in contrast to Ph3P),
while being able to accommodate the large silyl group in the
PdSiMe3 intermediates involved in the reaction (by rotation
t
of the flat phenyl groups, in contrast to Cy3P and Bu3P).
2. RESULTS AND DISCUSSION
2.1. Ligand Design Considerations. In our previous
study, we identified tBuPPh2 as an effective ligand for silyl-Heck
Received: May 30, 2014
© XXXX American Chemical Society
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We recognized, however, that while the inclusion of phenyl
groups might be necessary for steric reasons, the inductive
electron-withdrawing nature of these aromatic substituents
dramatically lowers the electron-density of the ligand compared
Table 1. Optimization of Conditions Through Rational
Ligand Design
t
to all-alkyl congeners (e.g., Cy3P and Bu3P). In addition,
tBuPPh2 is only a moderately sterically demanding ligand.
Large, highly electron-rich ligands have been shown to
promote oxidative addition.7 As it was likely that competition
between oxidative addition to the SiI bond (leading to
productive silyl-Heck products) and background isomerization
of the alkene starting material (likely by trace metal hydride
species) was responsible for the limited yields of allylsilanes in
our original report, we hypothesized that catalysts employing
larger and more electron-rich phosphine ligands would further
improve the reaction. Specifically, we hoped that by increasing
the electron-donating character of the arene groups on the
ligand backbone, a more efficient catalyst might arise. At the
same time, additional substituents on the aromatic groups
would increase the size of the ligand, potentially leading to
further improvements in the reaction.
2.2. Ligand Synthesis. To investigate the possibility that
larger, more electron-rich ligands would lead to more efficient
allylsilane synthesis, we prepared a series of diaryl-tert-butyl
phosphine ligands with varied electronic and steric properties of
the aryl groups. These ligands were prepared by addition of the
aromatic organolithium reagent to dichloro-tert-butylphosphine
(eq 1). Yields ranged from 47 to 83% after purification by
a
Yield determined by NMR using internal standard (1,3,5-C6H3(OMe)3).
Yield reflects total silylated products (allylsilane + trace vinyl silane); in all
cases, allylsilane/vinylsilane ≥ 93:7, E-allyl/Z-allyl ≈ 85:15.
While highly encouraging, this latter result was also
perplexing. Examination of molecular models revealed that
the flanking isopropyl groups force the dimethylamino group
into a conformation where the nonbonding electron pair of the
nitrogen is perpendicular to the π-system of the aromatic ring
(Figure 2). In such a conformation, little electronic benefit from
distillation or recrystallization. Further details regarding ligand
synthesis are provided in the Supporting Information (SI).
2.3. Ligand Optimization Studies. To assess the effect of
ligand design on the silyl-Heck reaction, we selected the coupling
of 1-decene and Me3SiI as our model system (Table 1). Our
t
t
initial investigation compared the use of BuPPh2 (3) and BuP-
(4-C6H4OMe)2 (6) as ligands under palladium-catalyzed con-
ditions. Consistent with our hypothesis, the reaction involving the
more electron-rich ligand 6 provided markedly higher yield of
desired product 2 compared to that using tBuPPh2 (entries 1 vs 4).
Further, when ligands bearing meta electron-withdrawing sub-
stituents were employed (4 and 5, entries 2 and 3), lower yields
were observed.
To further increase the electron-donor ability of the ligand,
as well as its size, para-anisole derivatives bearing meta alkyl
groups were prepared (ligands 7 and 8, entries 5 and 6). The
yield of desired product 2 improved with both methyl and tert-
butyl substituents, and increased with the size of the alkyl
group. With 3,5-di-tert-butyl-4-methoxyphenyl-derived ligand 8,
the product was produced in 92% yield.
Figure 2. Conformational analysis of dimethylamino group in ligand 11.
the dimethyl amino group is expected. This analysis led us to
question the need for a para-heteroatom substituent.
Accordingly, we prepared one final series of ligands bearing only
meta-alkyl substituents on the aromatic rings (ligands 12−14).
Pleasantly, and surprisingly, this series of ligands was also highly
effective in the model transformation. Ligands bearing either
methyl or isopropyl groups showed significant improvement over
tBuPPh2 (entries 10 and 11). Most significantly, however, the
ligand bearing meta,meta-di-tert-butylphenyl groups (14) provided
product 1 in 99% assay yield (entry 12). As ligand 14 can be easily
prepared on multigram scale in only a single step from
commercially available materials and is air stable (no detectable
oxidation was observed by 31P NMR after one year of storage on
the benchtop under air), we selected this ligand to explore as a
second generation ligand for the preparation of allylsilanes.
2.4. Substrate Scope of Allylsilane Synthesis Using
Ligand 14. Using ligand 14, vastly improved yields of allylsilanes
are obtained with a broad range of α-olefin substrates (Figure 3).
In general, high yields, good E/Z selectivities, and outstanding
allyl/vinyl selectivities were observed.8 For example, the model
substrate gave rise to allylsilane 2 in 98% isolated yield as a 84:16
mixture of E/Z isomers, with only trace amounts (≤7%) of the
For comparison, we also prepared the ligand series bearing
para-dimethylamino substitution (ligands 9−11). In this series,
the results were less straightforward. The reaction involving
ligand 9, which lacked meta substitution, provided similar yields
t
of 2 compared to BuPPh2 (entry 7). In contrast, the ligand
bearing meta-methyl groups (10) provided increased yields
(entry 8), similar to dimethylanisole ligand 7. Surprisingly,
however, the 3,5-di-iso-propyl-4-dimethylaminophenyl derived
11 provided vastly superior results, giving the desired product
in nearly quantitative yield (entry 9).
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amount of alkene isomers are observed, even in the these highly
thermodynamically favorable cases, suggests that the observed
allylsilanes are the kinetic products of the silyl-Heck reaction and
are not the result of postreaction alkene isomerization.
Complex, natural product derived substrates can also be
silylated using this protocol. For example, silylation of an
allylated cholesterol derivative gave rise to allylsilane 25 in
excellent yield. Importantly, this example also demonstrates
that terminal alkenes are silylated selectively over internal
alkenes, and that stereogenic centers at the homoallylic position
of the starting material can be maintained with full stereo-
chemical fidelity during the reaction.
The effect of steric bulk proximal to the site of β-hydride-
elimination was studied by the preparation of allylsilanes 26−
28. In particular, we desired to understand the effects of steric
crowding on allyl- vs vinylsilane selectivity. With modest levels of
steric encumbrance (tertiary carbon center) at either the δ- or
ε-position relative to the terminal alkene starting material, no
erosion of selectivity for allylsilanes was observed (26 and 27). In
fact, even branching at the ∂-position resulted in good selectivity for
allylsilanes as long as there was a hydrogen atom available for
β-hydride-elimination (28). Notably, however, in these more sterically
encumbered cases, a slight increase in catalyst loading was
required to maintain high yield (see Figure 3). We attribute this
requirement to increased difficulty in the migratory insertion step.
In contrast, more severe steric interactions did decrease selectivity
for allylsilanes. For example, the presence of a fully substituted
carbon at the γ-position of the starting material noticeably decreased
selectivity, leading to a 72:28 mixture of allyl and vinyl isomers (29,
see Discussion below). Interestingly, however, when trimethylallyl-
silane was used, the product symmetry allows for the production of
unsaturated disilane 30 as a single isomer.
With our previous catalyst system, we observed poor allyl vs
vinyl selectivity in the formation of silyl ether 31. As we
believed this to be an informative reaction in understanding the
factors controlling allyl vs vinyl selectivity, we reinvestigated
this reaction using the second-generation catalyst (Figure 3).
While the yield of 31 is greatly improved using the new catalyst,
a similar mixture of isomeric products is observed. The reasons
for this erosion of selectivity are discussed below.
Figure 3. Scope of silyl-Heck reaction to form allylsilanes with second-
generation ligand.
vinylsilane isomer. Similar results were obtained for other n-alkyl
α-olefins (e.g., 15 and 16). The allyl vs vinyl and E-allyl vs Z-allyl
selectivity are very similar to the first-generation catalyst system.2
Importantly, however, and in contrast to the earlier system using
tBuPPh2, almost no alkene isomerization of the starting material
was observed. This not only significantly improves the yield of the
allylsilane, but also greatly simplifies purification of the product.
The silyl-Heck reaction also demonstrates a broad range of
functional group compatibility, although in some cases slightly
greater catalyst loadings where required to achieve high yields
(see Figure 3). Protected alcohols, including pivaloyl esters and
silyl ethers can be used without incident (17−19). Alkyl halides
are also well tolerated. With alkyl chlorides, high yields of the
desired allylsilane are achieved (20). Alkyl bromides can also be
used, but higher catalyst loading and a modified metal to ligand
ratio (1:1) are required to obtain desired product 21 in modest
yield. Alkyl boronic esters participate smoothly in this reaction
giving a potentially versatile dimetalloid (22).
Malonates are tolerated in the reaction, as shown by 23. In
this particular case, a small amount of alkene isomerization (11%)
was observed, wherein the alkene moved into conjugation with
the diester group, giving the homoallylsilane. A similar result was
obtained with the use of homoallylbenzene. In this case, product
24 was contaminated with approximately 8% of the E-
homoallylsilane, wherein the alkene moved into conjugation
with the phenyl group. We believe that the fact that such a low
Finally, in our previous study, aryl-substituted allyl compounds
were a particularly challenging class of substrates due to the
propensity of the alkene of the starting material to migrate into
conjugation with the aromatic group. Second-generation ligand
14 provides vastly improved silyl-Heck reactions across a range
of such substrates (32−37, Figure 4). With electron-neutral and
heteroaromatic substrates, very little alkene isomerization was
observed (33 and 37). With highly electron-rich allyl aromatic
substrates, alkene isomerization could not be fully suppressed,
resulting in slightly diminished yields (32, 34−36). However,
in all cases, reactions involving ligand 14 were dramatically
t
improved over those using BuPPh2. In these cases, only the
E-allylsilanes are observed. In no case, were Z-allyl- or vinylsilane
was detected (NMR and GC/MS).
2.5. Steric and Electronic Effects in Allylsilane vs
Vinylsilane Selectivity. As mentioned above, the limited
amount of alkene isomerization observed in the formation of
compounds 23 and 24 suggests that the observed allyl/vinyl
ratio is likely kinetic in nature. To further determine if the
allylsilane products might arise from isomerization of vinyl-
silanes under the reaction conditions, we independently
prepared vinylsilane 38 and added it to the silyl-Heck reaction
of 1-hexene and Me3SiI (Scheme 1). After the reaction, allylsilane
C
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(A-value = 4.9 kcal/mol) is larger than the trimethylsilyl group
(A-value = 2.5 kcal/mol), strictly steric selectivity would favor
formation of vinylsilane product vinyl-29.9 In contrast, due to
the α silicon effect and the fact that the hydrogen atoms α to
the silicon center have increased acidity and decreased
hydricity,10 electronic control would favor β-elimination toward
the tert-butyl group. The fact that allylsilane allyl-29 is the
dominant product suggests a significant electronic contribution
in product determination in the silyl-Heck reaction. The importance
of electronic factors in β-hydride elimination have been
documented in other palladium-catalyzed Heck-type reactions.11,12
Importantly, in substrates lacking such bulk substituents,
such as those leading to allylsilanes 2 and 15−28, the large
trimethylsilyl group is the most sterically demanding in the
proximity of the palladium center in the metal alkyl inter-
mediate. In such cases, steric and electronic factors reinforce each
other, helping to explain the high allylsilane selectivity observed.
Finally, we believe electronic factors also explain the low
allyl/vinyl ratio observed in the formation of product 31
(Figure 3). Product 27 provides a comparable steric environ-
ment to this product. However, unlike 31, the formation of
compound 27 vastly favors formation of the allyl isomer, ruling
out steric factors as an explanation for the observed selectivity
in 31. Instead, we believe that the poor selectivity observed in
the formation of 31 is primarily electronic in nature and is due
to the lowered hydricity of the hydrogen atoms β to the
electron-withdrawing ether and aromatic group.
Figure 4. Silyl-Heck reactions with aryl-substituted allyl substrates.
Scheme 1. Kinetic Stability of Vinylsilanes Under Silyl-Heck
Reaction Conditions
This deeper understanding of the importance of electronic,
as well as steric, factors in product selection will undoubtedly
be important in planning synthetic applications of the silyl-
Heck reaction in complex allylsilane synthesis.
2.6. Understanding the Effects of Steric and
Electronic Ligand Properties on the Silyl-Heck Reaction.
Having demonstrated that ligand 14 is vastly superior to the
earlier system in the formation of allylsilanes, we wanted to
better understand how the electronic and steric properties of
the various ligands contributed to their performance in the silyl-
Heck reaction.
To gauge the steric properties of the ligands, we obtained
X-ray crystal structures of the ligands that could be crystallized
(in all, 8 of the 12 ligands studied). Using these structures, we
computed the Equivalent Cone Angle (ECA), employing the
method of Guzei and assuming a metal-phosphine bond
distance of 2.28 Å (Table 2).13,14
From these data, and in comparison to the data presented in
Table 1, the importance of size becomes obvious. In general,
ligands with larger steric size outperform smaller ligands.
Further, the best preforming ligands (8, 11, and 14) have the
largest computed cone angles.15
16 (arising from 1-hexene) was observed in quantitative yield
(>95%, GC/MS). In addition, vinylsilane 38 remained
unchanged, being returned from the reaction in >95% yield
(GC/MS). Importantly, allylsilane 15, which is the product of
alkene isomerization of 38, was not detected. These results
support the proposal that the observed allyl/vinyl ratios from the
silyl-Heck reaction are kinetic in nature, and implicates β-hydride
elimination from the alkyl-palladium intermediate as the product-
determining step.
Allylsilanes are typically the vastly major product in this class
of silyl-Heck reactions. These products result from β-hydride
elimination away from the silicon center, and in principle either
steric or electronic factors could contribute to the observed
selectivity. The observed ratio of allyl- and vinylsilanes in the
formation of tert-butyl substituted product 29 (Figure 3) sheds
considerable light on the nature of this selectivity.
Considering allyl-palladium intermediate 38 that forms en
route to 29, β-hydride elimination can occur either toward the
tert-butyl group or the trimethylsilyl group in an otherwise
symmetrical molecule (Figure 5). As the tert-butyl group
Steric data, however, do not provide a complete explanation
for the variance in ligand performance. In particular, these data
do not explain the superiority of ligands 11 and 14 (ECA 149°
for both) over ligand 8 (ECA 153°). In particular, ligands 8 and
14 differ only by the presence of a para-methoxy group on the
aromatic rings. The difference in the ECAs appears to be too
small (and is in opposite trend) to explain the difference in
reactivity.
To gauge the σ-donor ability of the ligands, each was con-
verted to the phosphine selenide by treatment with elemental
(black) selenium in CDCl3. Previous studies have shown that
the σ-donor ability of phosphine ligands is directly proportional
77
to the inverse of the 31P Se coupling constant in the NMR
Figure 5. Steric vs Electronic Control in β-Hydride Elimination
Leading to Product 29.
spectra (1JPSe).16,17 Measurement of this coupling constant for
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In the context of the silyl-Heck reaction, the ligand data
outlined above correlate very strongly with the observed trends
in yield. The two most effective ligands (11 and 14) are the two
with the best combination of electron-richness and large steric
size. These factors are expected to favor oxidative addition, and
are consistent with our hypothesis that led to the rational
design of these second-generation ligands.
Table 2. Computed Equivalent Cone Angles (ECA) of
Ligands
A more subtle result that emerges from the electronic
analysis of the ligands is the comparative lack of electron-
richness of ligand 8, particularly compared to ligand 14, which
lacks the presumably electron-donating para-methoxy group.
One possible explanation for this surprising result is that the
large flanking alkyl groups of ligand 8 restrict the conformation
of the methoxy group to be out of plane with the aromatic ring,
so that the beneficial overlap between the nonbonding
electrons on the oxygen and the π-system of aromatic ring is
poor and offset by the inductive electron-withdrawing effect of
the oxygen atom.
We believe that this is the first time that the electronic
properties of these large phosphine substituents have been
quantified in a systematic way. Beyond the scope of the silyl-
Heck reaction, these data might inform on ligand design in
other classes of novel sterically demanding, electron-rich
phosphine ligands.
2.7. Insights into Catalyst Structure. The silyl-Heck
reactions with this second-generation ligand (as well as with the
other high-yielding ligands presented herein) are particularly
sensitive to the ligand/metal (L/M) ratio employed. Empiri-
cally (as reflected in Figure 2), L/M ratios between 1:1 and
1.5:1 provide the highest yields of allylsilanes. With L/M ratios
at or above 2:1, a pronounced drop in reactivity is observed.
These results are suggestive of the importance of low-valent
metal centers in the catalytic reaction.19
a
ECA = Equivalent Cone Angle, MP bond =2.28 Å.
Table 3. 31P−77Se Coupling Constants of Phosphine
Selenides
To further probe the relevance of low-valent palladium
complexes in silyl-Heck reactions using our second-generation
ligands, stoichiometric experiments were undertaken. These
studies shed considerable light onto the catalytic process. For
example, combining ligand 7 with precatalyst 1 and excess
Me3SiI in a solvent mixture of toluene, benzene, and pentane at
−35 °C results in the formation of light green crystals, which
were determined to be oxidative addition complex 39 using
X-ray crystallography (Figure 6). This thermally and air-sensitive
a
1JPSe measured in CDCl3 from in situ generated phosphine selenide.
each of the in situ generated phosphine selenides reveled three
trends (Table 3).18 First, as expected, the substituted ligands
bearing electron-donor groups are in fact more electron-rich
t
than the parent BuPPh2. Second, while electron-rich ligands
give generally improved catalytic performance, only those
ligands that are also sterically bulky provide highly improved
results compared to tBuPPh2. For example, ligand 9 bearing the
N,N-dimethylaniline group is measured to be the most
electron-rich, but its small cone angle and exposed basic
t
amine make it only negligibly better than BuPPh2. Third, 11
and 14 are two of the most electron-rich ligands and appear to
have greater σ-donor ability than ligand 8.
Figure 6. Preparation and structure of complex 39.
E
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T-shaped monophosphine complex has an open coordination site
opposite the silicon ligand (no intramolecular interactions,
including agostic interactions with the tert-butyl group, nor
intermolecular interactions are observed in this open coordination
site). This compound is the first monomeric palladium complex
resulting from oxidative addition of a silicon−halide bond to be
characterized, and is also the first such complex to be supported by
a phosphine ancillary ligand.20 This complex was fully characterized
in solution (1H, 13C and 31P NMR in CD2Cl2 at 0 °C), and the
data are consistent with the structure shown in Figure 6.
AUTHOR INFORMATION
Corresponding Author
dawatson@udel.edu
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Mr. Gabriel Andrade (UD) is thanked for assistance with
crystallography. The University of Delaware (UD), the Research
Corporation (Cottrell Scholars Program), and the NSF (CAREER
CHE1254360) are gratefully acknowledged for support. NMR and
other data were acquired at UD on instruments obtained with
the assistance of NSF and NIH funding (NSF CHE0421224,
CHE1229234, CHE0840401, and CHE1048367; NIH P20
GM103541 and S10 RR02692).
Complex 39 is a competent precatalyst in the silyl-Heck
reaction. The use of 2 mol % of this complex with an additional
1 mol % of 7 as catalyst (mimicking the optimized metal/ligand
ratio) provided allylsilane 2 in 72% yield, which is comparable
to the in situ generated catalyst (Table 1). Further, complex 39
is the major product observed (31P NMR) in stoichiometric
experiments performed in DCE (1 equiv each of 1 and 7, 15
equiv each of Me3SiI and Et3N), which is the solvent of choice
for the catalytic reaction. Combined, these experiments establish
the relevance of complex 39 under catalytically relevant conditions.
Finally and significantly, a highly similar signal is observed by
31P NMR when ligand 14 is used in the stoichiometric reaction
(in DCE, as above), suggesting that an analogous complex is
formed with the optimal ligand.21 To date, however, we have been
unable to crystallize the complex using the more soluble ligand 14.
These results, along with the steric and electronic data outlined
above and the optimized reaction conditions, are highly suggestive
that the active catalyst is a palladium−monophosphine (L1M)
complex. Such a catalytic intermediate parallels those involved in
oxidative additions of carbon−halogen bonds using large, electron-
rich phosphines.7 Moreover, the structure of 39, particularly the
disposition of the aromatic groups on the phosphine, clearly
illustrates how this class of diaryl-tert-butylphosphine ligands
accommodates the steric bulk of the large Me3Si group. The
rotation of these groups away from the silicon center is consistent
with our previous hypothesis for why these ligands are superior in
the silyl-Heck reaction.2
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3. CONCLUSIONS
Using rational ligand design, we have developed an easily
prepared, bench-stable second-generation ligand for the
preparation of allylsilanes using the silyl-Heck reaction. Unlike
the previously reported system, the new ligand provides
extremely high yields of allylsilane products using simple
terminal α-olefin substrates, with good E/Z selectivity, very
little formation of vinylsilanes, and only trace isomerization of
the alkene starting materials. Quantitative studies demonstrate
that this new ligand is much larger and more electron-rich than
the first-generation ligand, and the isolation of an oxidative
addition complex of Me3SiI sheds significant light onto the
effectiveness of this ligand class. Overall, we believe that this
new ligand provides a simple, direct, and highly useful means to
access high-value allylsilanes from readily available starting
materials using the silyl-Heck reaction.
(6) For a related transformation, see: Nakamura, S.; Yonehara, M.;
Uchiyama, M. Chem.Eur. J. 2008, 14, 1068.
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(f) Surry, D. S.; Buchwald, S. L. Chem. Sci. 2011, 2, 27.
(8) See SI for exact allyl/vinyl selectivity.
(9) (a) Winstein, S.; Holness, N. J. J. Am. Chem. Soc. 1955, 77, 5562.
(b) Kitching, W.; Olszowy, H. A.; Drew, G. M.; Adcock, W. J. Org.
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(10) (a) Whitmore, F. C.; Sommer, L. H. J. Am. Chem. Soc. 1946, 68,
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, crystallographic, and spectral data.
This material is available free of charge via the Internet at
http://pubs.acs.org
(11) Werner, E. W.; Sigman, M. S. J. Am. Chem. Soc. 2011, 133, 9692.
(12) Recently, iridium-catalyzed dehydrogenative silylation of alkenes
to form vinylsilanes have been reported. β-hydride elimination from a
F
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Journal of the American Chemical Society
Article
β-silyl alkyl metal hydride complex has been proposed as a key step in
those processes. However, it appears to occur towards the silicon
center in those cases. See: (a) Cheng, C.; Simmons, E. M.; Hartwig, J.
F. Angew. Chem., Int. Ed. 2013, 52, 8984. (b) Lu, B.; Falck, J. J. Org.
Chem. 2010, 75, 1701.
(13) Guzei, I. A.; Wendt, M. Dalton Trans. 2006, 3991.
(14) Tolman, C. A. Chem. Rev. 1977, 77, 313.
(15) The larger than expected cone angle of ligand 11, which
contains meta isopropyl substituents on the aromatic rings, is
explained by the gearing of those groups off of the adjacent para-
dimethylamino groups. This conformation, which is observed in the X-
ray crystal structure, makes the isopropyl groups project steric bulk
similar to tert-butyl groups. See CIF in SI for further details.
(16) (a) Pinnell, R. P.; Megerle, C. A.; Manatt, S. L.; Kroon, P. A. J.
Am. Chem. Soc. 1973, 95, 977. (b) Allen, D. W.; Taylor, B. F. J. Chem.
Soc., Dalton Trans. 1982, 51. (c) Socol, S. M.; Verkade, J. G. Inorg.
Chem. 1984, 23, 3487. (d) Socol, S. M.; Verkade, J. G. Inorg. Chem.
1986, 25, 2658. (e) Beckmann, U.; Susluyan, D.; Kunz, P. C.
̈
̈
Phosphorus, Sulfur Silicon Relat. Elem. 2011, 186, 2061.
(17) For recent applications of this method, see: (a) Sergeev, A. G.;
Spannenberg, A.; Beller, M. J. Am. Chem. Soc. 2008, 130, 15549.
(b) Berhal, F.; Esseiva, O.; Martin, C.-H.; Tone, H.; Genet, J.-P.; Ayad,
T.; Ratovelomanana-Vidal, V. Org. Lett. 2011, 13, 2806. (c) Kawaguchi,
S.-i.; Minamida, Y.; Ohe, T.; Nomoto, A.; Sonoda, M.; Ogawa, A.
Angew. Chem., Int. Ed. 2013, 52, 1748.
(18) It is known that 1JP‑Se is sensitive to the pyramidalization of the
phosphine atom, which can be affected by steric bulk of the
phosphorus substituents. See ref 16e. However, analysis of the X-ray
crystal structures reveals that the pyramidalization is nearly identical in
all ligands studied (see SI for details). Thus, we believe the differences
in coupling constants for the phosphine selenides reflects electronic
differences at the phosphorus center.
(19) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122,
4020.
(20) Esposito, O.; Roberts, D. E.; Cloke, F. G. N.; Caddick, S.;
Green, J. C.; Hazari, N.; Hitchcock, P. B. Eur. J. Inorg. Chem. 2009,
2009, 1844.
(21) See SI for details.
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