Synthesis of unsaturated silyl heterocycles via an intramolecular silyl-heck reaction

Article abstract of DOI:10.1021/acs.organomet.9b00498

We report the synthesis of unsaturated silacycles via an intramolecular silyl-Heck reaction. Using palladium catalysis, silicon electrophiles tethered to alkenes cyclize to form five- and six-membered silicon heterocycles. The effects of alkene substitution and tether length on the efficiency and regioselectivity of the cyclizations are described. Finally, through the use of an intramolecular tether, the first examples of disubstituted alkenes in silyl-Heck reactions are reported.

Full text of DOI:10.1021/acs.organomet.9b00498

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis of Unsaturated Silyl Heterocycles via an Intramolecular
Silyl-Heck Reaction
William B. Reid, Jesse R. McAtee, and Donald A. Watson*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
ABSTRACT: We report the synthesis of unsaturated sila-
cycles via an intramolecular silyl-Heck reaction. Using
palladium catalysis, silicon electrophiles tethered to alkenes
cyclize to form five- and six-membered silicon heterocycles.
The effects of alkene substitution and tether length on the
efficiency and regioselectivity of the cyclizations are described.
Finally, through the use of an intramolecular tether, the first
examples of disubstituted alkenes in silyl-Heck reactions are reported.
alkenes, which has not been possible in bimolecular reactions.
In addition, such a reaction would access new classes of cyclic
unsaturated silicon heterocycles.
INTRODUCTION
■
Over the past several years our laboratory has developed a
general and mild protocol for the direct synthesis of allyl and
vinyl silanes directly from unfunctionalized terminal alkenes via
the silyl-Heck reaction (Figure 1).1 Our main studies have
Silicon-containing heterocycles are important structures in
many disciplines of chemistry. Such silacycles are common
motifs in silicon derivatives of drugs, known as siladrugs.5
Silacycles are also important synthetic intermediates6 and have
been utilized in many total syntheses.7 Most commonly,
silicon-containing rings are oxidized to form complex diols.7,8
Unsaturated silacycles also serve as unique precursors for the
formation of silicon-based polymers.9 Small silacycles, such as
silacyclobutanes, have been shown to be excellent nucleophiles
for Hiyama−Denmark coupling reactions.10
Classically, silacycles have been synthesized from cyclo-
additions of reactive silenes or silylenes with dienes.11
Alternatively, intramolecular hydrosilylation is a common
strategy for the synthesis of various silacycles. Speier’s catalyst
(H2PtCl6) is commonly used and gives a strong preference for
exo cyclizations following the Chalk−Harrod mechanism,12
while more recently, Yamamoto and Trost have independently
demonstrated that endo cyclizations are possible using
aluminum trichloride or a cationic ruthenium complex.13 We
envisioned that an intramolecular silyl-Heck reaction could
provide an alternative approach for the synthesis of silicon-
containing heterocycles.
Herein, we report the first examples of intramolecular silyl-
Heck reactions (Figure 1, bottom). These studies not only
provide routes to cyclic silanes but also provide insights into
the requirements of the silyl-Heck reaction. Specifically, we
show that these transformations allow access to a variety of
five- and six-membered unsaturated silicon heterocycles with
moderate to good yields. Further, through a systematic study
of tether length and alkene substitution and their effect on
regioselectivity and efficiency, significant insight into the steric
Figure 1. Silyl-Heck reactions.
focused on palladium-catalyzed reactions, which allow for
highly regiospecific terminal silylation of alkenes with good to
excellent levels of isomeric and geometric control of the double
bond in the product. Moreover, we recently developed a
second-generation ligand for these reactions, which allows for
lower reaction temperatures, better yields, and greater product
selectivity (Figure 1, top).1e In 2014, we also reported nickel-
catalyzed conditions that allow for the preparation of vinyl
silanes.1d These methods provide a straightforward means of
preparing unsaturated silanes. Most recently, Shimada and
Nakajima have also shown similar reactions using chlorosilanes
with Lewis acid promotors.2,3
To date, however, all such studies have focused on
bimolecular cross-coupling reactions yielding linear, silicon-
containing products. We were interested in investigating an
intramolecular silyl-Heck reaction wherein a silyl halide could
cyclize onto a pendant alkene.4 We reasoned that such
reactions would allow us to study the reactivity of internal
Received: July 24, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00498
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requirements of the silyl-Heck process has been gained. Finally,
by tethering the alkene to the silicon electrophile, we have
observed the first examples of successful silyl-Heck reactions of
1,1- and 1,2-disubstituted alkenes.
Gevorgyan laboratory involving a silicon-tethered substrate.20
In that case, 6-endo cyclization was also observed, which
presumably is a result of the stereoelectronic effects imparted
by the long Si−C bonds contained within the product. To
better understand if the observed regioselectivity in the
cyclization of 1 was a manifestation of the preference for the
silicon center to react at the alkene terminus or is inherent in
Heck-type cyclizations involving small silacycles, we undertook
a more systemic study.
RESULTS
■
For the initial exploration of an intramolecular silyl-Heck
reaction, we sought to design a substrate that closely resembled
those of our prior bimolecular reactions, which involved silyl
iodides and terminal monosubstituted alkenes. To this end, 4-
pentenyldimethyliodosilane (1) was identified as a suitable
substrate due to minimal steric and electronic bias. However,
because of the sensitivity of the molecule toward moisture, and
the propensity of trace hydroiodic acid to result in alkene
isomerization, 1 could only be prepared in limited purity (ca.
To facilitate that investigation, substrates that would be
easier to prepare and purify and products that would be easier
to isolate and analyze were desired. Toward this end, we
elected to investigate chlorosilane substrates. We have
previously shown that chlorosilanes can participate in
palladium-catalyzed silyl-Heck reactions activated in situ by
the addition of iodide salts.21 Moreover, chlorosilanes can be
synthesized under mild reaction conditions, potentially
allowing for higher yield and purity in substrate synthesis.
Initially, we focused on preparing chlorodiphenylsilane
substrates. We reasoned that the added molecular mass of
the two phenyl groups would lower the volatility of the
products and allow easier isolation. As predicted, we were able
14,15
1
80%, according H NMR analysis).
Despite those limitations, we decided to explore the
cyclization of compound 1 nonetheless. In principle, 1 can
undergo either a 6-endo (2 and/or 3) or 5-exo (4) cyclization
(Scheme 1). On the basis of the known selectivities of Heck
Scheme 1. Intramolecular Cyclization of Iodosilane 1
1
to prepare 7 in higher purity (>95% according to H NMR
analysis, eq 1). Unfortunately, however, this chlorodiphenylsi-
lane substrate proved sluggish in the silyl-Heck reaction. Even
with use of lithium iodide as an additive, only a 41% combined
yield of products 8 and 9 was obtained.22 Interestingly,
however, like the earlier cyclization, only products arising from
a 6-endo cyclization were observed; no 5-exo product (10)
could be detected.
We suspected that the poor reactivity of 7 was due to the
steric demands of the two phenyl groups attached to silicon. As
a means of modulating the size of the silicon center, we then
turned to chloromethylphenylsilyl substates. Encouragingly,
using this electrophilic silicon center, we were able to prepare
substrate 11 in an analytically pure fashion. Moreover, using
the combination of catalytic Pd2dba3 and JessePhos, with LiI
additive, 11 underwent smooth cyclization to lead to a 1:1
mixture of 12 and 13 in 81% combined yield (Table 1, entry
1). As before, product 14 (which would arise from 5-exo
cyclization) was not observed.
cyclization involving carbon electrophiles,4 the 5-exo product
(4) was expected to strongly predominate. However, when 1
was subjected to reaction conditions similar to those employed
in our earlier work (Pd2dba3/JessePhos),1f a surprising result
was observed. Instead of the expected 5-exo pathway, 1
underwent cyclization to exclusively provide a mixture of
products 2 and 3 in a 1:1 ratio in 61% yield (as determined by
1H NMR against an internal standard). Apparently, these
products arise from 6-endo cyclization, followed by non-
selective β-hydride elimination from intermediate 6. Unfortu-
nately, due to their volatility, 2 and 3 proved challenging to
isolate and separate. However, samples of sufficient analytical
purity for structural characterization were obtained using
preparative gas chromatography.16 In addition, in nearly all
cases in prior studies, a strong preference for the allylic isomer
has been observed in silyl-Heck transformations where its
formation is possible. Here a mixture of allyl and vinyl isomers
is obtained. It is unclear in this case if the observed alkene
mixture results from kinetic or thermodynamic selectivity.
We wished to understand the origins of the seemingly
unusual regioselectivity. On one hand, in all prior examples of
silyl-Heck reactions,17 silylation occurs exclusively at the
terminal position of the alkene and the observed selectivity
might be due to the same effect. On the other hand, 6-endo
intramolecular Heck reactions using carbon-based electrophiles
are not unprecedented but typically require electronically
biased alkenes18 or are thought to undergo rearrangements and
other nontraditional reaction mechanisms.19 One notable
exception is a recently reported Heck cyclization from the
Before continuing to a more exhaustive study, we also
wanted to examine the nature of the catalyst. We have
previously reported the use of a single-component JessePhos
palladium complex as an effective precatalyst for the silyl-Heck
Table 1. Cyclizations of Chloromethylphenyl Silanes
entry
catalyst (mol %)
combined yield (%) 12:13:14
1
2
Pd2dba3 (5), JessePhos (10)
(JessePhos)2PdCl2 (5)
81
88
1:1:0
1:1:0
B
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reaction.23 The use of these precatalysts simplifies reaction
setup and often leads to higher and more consistent yields.
Using the single-component catalyst (JessePhos)2PdCl2 (15),
methyl(phenyl)silane 11 cyclized to yield a 1:1 ratio of 12 and
13 in 88% combined yield (entry 2). This catalyst system was
selected for further study.
With an isolable and sufficiently reactive class of silicon
electrophiles and a proper catalyst identified, we set out to
study the effects of both tether length and alkene substitution
on the intramolecular silyl-Heck reaction. A series of
homologous substrates were prepared16 and then subjected
to the silyl-Heck reaction conditions. The results of those
cyclization studies are presented below.
silyl-Heck reaction (22) was not detected, indicating that the
7-endo pathway is less favorable than internal silylation.
However, the overall low yield in this reaction indicates the
difficulties associated with both pathways.
Unfortunately, attempting to drive larger ring cyclization
with the five-atom linker of alkene 25 failed to provide a
product (eq 5); neither seven- nor eight-membered silacycles
were observed.
Substrate 16 bearing a four-carbon tether was prepared and
subjected to the reaction conditions using catalyst 15 (eq 2).
After determining the range of reactive chain lengths, we
next turned our attention to studying the effects of alkene
substitution. To date, only monosubstituted alkenes have been
found to be suitable substrates in bimolecular silyl-Heck
reactions. Tolerance of higher alkene substitution is presum-
ably disfavored due to unfavorable steric interactions. A similar
limitation is observed in bimolecular Heck reactions employing
carbon electrophiles, wherein the rates of reactivity decrease
dramatically with increased olefin substitution under most
reaction conditions.30,31 Related intramolecular Heck cycliza-
tions, however, have been shown to tolerate tri- and
tetrasubstituted alkenes.32 Considering this background, we
sought to define the tolerance of alkene substitution in the
intramolecular silyl-Heck reaction.
With this butenyl substrate, cyclization occurred to provide a
1:1 mixture of 5-endo products 17 and 18 in 80% combined
yield. The observed result is notable, as it again proved
dissimilar to Heck cyclizations of carbon electrophiles; 5-endo-
trig cyclizations are typically disfavored due to the distortion
required for orbital overlap.24 Although 5-endo products have
been observed in Heck reactions25 to form indoles and related
compounds, they are thought react through six-membered
palladacycle intermediates.25i,26 In this case, however, the
substrate lacks suitable electronic bias to favor such a pathway.
Additionally, although four-membered-ring formation has been
observed in Heck cyclizations of carbon electrophiles,27 in this
case silacyclobutane 19 resulting from 4-exo cyclization was
not observed.
We began with a substrate bearing a tethered 1,1-
disubstituted terminal alkene (26, Scheme 2). Like other
Scheme 2. 6-Endo Cyclization of 26
We also prepared a substrate bearing a tether one atom
shorter yet (20, eq 3). However, when this allyl substrate was
subjected to the optimized reaction conditions, no cyclized
product was observed. Even at elevated temperatures, only
unreacted or isomerized starting material remained.
Next, we next sought to examine longer alkene tether
lengths. When substrate 21 (eq 4), bearing a four-carbon
disubstituted alkenes, gem-disubstituted olefins have proven to
be poor substrates in bimolecular silyl-Heck reactions.
Subjecting alkene 26 to the reaction conditions gave rise to
products 27 and 28 in 88% yield as a 5:1 ratio of isomers,
making this the first successful example of the use of a
disubstituted alkene in a silyl-Heck reaction. Consistent with
the previous intramolecular cases, the reaction proceeds with
exclusive 6-endo selectivity, placing the silicon atom at the
terminus of the alkene. However, in contrast to the previous
examples the vinyl isomer is largely favored over the allylic
isomers.
Next, we investigated the reactivity of several internal
alkenes. When the methyl-substituted (Z)-alkene 31 was
subjected to the optimal reaction conditions, 5-exo product 32
was formed in 18% yield (Scheme 3). Only the (Z)-alkene
isomer was observed, the configuration of which was
established using one- and two-dimensional NMR methods.
The geometry of the product is consistent with a Heck-like
mechanism involving syn-facial migratory insertion, C−C σ-
bond rotation, and syn-periplanar β-hydride elimination (via
tether, was subjected to the reaction conditions, products 23
and 24 were obtained in 31% combined yield as a 3:1 mixture.
Interestingly, both of these products are the result of 6-exo
cyclization. These are the first examples of silyl-Heck
cyclizations proceeding via an exo pathway. More importantly,
however, they are also the first examples of the internal
silylation of an alkene using this method and demonstrate that
addition of the silicon atom to the internal carbon is possible.28
Notably, although 7-endo Heck cyclizations have been
reported,20,29 the product from 7-endo cyclization in the
C
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order to proceed. Finally, unlike bimolecular silyl-Heck
reactions where the position of the double bond in the
unsaturated organosilane product is readily predicted by the
nature of the starting material, intramolecular silyl-Heck
reactions provide less predictable and less selective mixtures
of allyl and vinyl silane products.
Scheme 3. 5-Exo Cyclizations of 31 and 33
CONCLUSION
■
In conclusion, for the first time we have explored the feasibility
of intramolecular silyl-Heck reactions. We have found that this
method is effective for the preparation of both five- and six-
membered unsaturated silicon heterocycles, provided that the
reaction can proceed to place the silicon atom at the
unsubstituted terminus of the tethered alkene. Both endo-
and exo-selective reactions are possible. The selectivity
between endo and exo modes is best understood by a
consideration of both ring size and the steric requirements of
the alkene.
These studies have also demonstrated the first examples of
more highly substituted alkenes participating in silyl-Heck
reactions. Both 1,1- and 1,2-disubstituted alkenes can undergo
cyclization, but the success of such reactions is dependent on
the geometric and steric considerations of the reaction.
Overall, these studies have provided further insights into the
steric requirements of the silyl-Heck reaction and will provide
insights into how to further develop silyl-Heck reactions of
highly substituted alkenes.
intermediates 35−37). Notably, cyclization of the closely
related (Z)-styrenyl substrate 33 led to a very similar result in
terms of product selectivity (only 34 was observed), but the
reaction was considerably more efficient (49% yield). This
indicated that the electronic nature of the alkene is also
important in the outcome of the silyl-Heck cyclization.
The successful cyclizations of 31 and 33 are significant, as
these are the first internal alkenes that have been observed to
participate in silyl-Heck reactions. Moreover, in contrast to
substrates 11 and 26, which bear identical tether lengths, these
substrates cyclize with complete preferential 5-exo selectivity.
This indicates that, in the absence of a steric preference,
stereoelectronic effects similar to those observed in Heck
cyclizations of carbon-based electrophiles predominate.4a,33
Successful cyclization of internal alkenes, however, appears
to be very substrate dependent. For example, simply switching
the alkene geometry of the substrate from cis to trans resulted
in no observed cyclization products from substrate 38 (eq 6).
EXPERIMENTAL SECTION
■
General Procedure for Silyl-Heck Reactions. In a nitrogen-
filled glovebox, a 1 dram vial equipped with a magnetic stir bar was
charged with (JessePhos)2PdCl2 (15; 13.9 mg, 0.125 mmol, 5 mol %),
LiI (47 mg, 0.35 mmol, 1.4 equiv), Et3N (175 μL, 1.25 mmol, 5.0
equiv), PhCF3 (500 μL, 0.5M), and silyl chloride (0.25 mmol, 1.0
equiv). The vial was sealed, and the contents were stirred at 45 °C for
24 h. The reaction mixture was removed from heat, cooled to room
temperature, and opened to air, and 1,3,5-trimethoxybenzene (28 mg,
2/3 equiv) was added. A small aliquot was taken for NMR analysis
without concentration; the sample was returned to the crude mixture
and then filtered through Celite with Et2O and concentrated in vacuo.
The crude oil was purified via flash silica gel chromatography with the
indicated eluent in parentheses.
This remained true even with the use of elevated temperatures
and longer reaction times. We attribute the lack of reactivity to
steric congestion during the migratory insertion between the
groups on silicon and the methyl group of the (E)-alkene,
which indicates the degree of difficulty associated with silyl-
palladation of internal alkenes using these methods.
1,1-Dimethyl-1,2,3,4-tetrahydrosiline (2) and 1,1-Dimethyl-
DISCUSSION
■
1,2,3,6-tetrahydrosiline (3). In a nitrogen-filled glovebox, a 1 dram
Overall, the results presented above paint an initial picture of
the steric and stereoelectronic requirements for intramolecular
silyl-Heck reactions. The predominant factor in evaluating the
facility of a silyl-Heck cyclization appears to depend upon the
steric nature of the alkene. With tethered, terminal alkenes,
both 5- and 6-endo cyclizations appear to occur readily and are
dictated by the ability of the cyclization to place the large
silicon group at the nonsterically encumbered terminus of the
alkene. Endo-selective cyclizations can also appear to tolerate
additional substitution at the internal carbon, so long as the
terminal position remains unsubstituted. 3-exo, and 4- and 7-
endo cyclizations appear to be much more challenging. In the
first two cases, no cyclization products are formed. In the last
case, 6-exo cyclization is preferred but is not efficient
presumably due to the challenges of placing the silicon group
at the internal carbon of the alkene. Finally, in some cases,
through use of an intramolecular tether, 1,2-disubstituted
alkenes can participate in silyl-Heck reactions to some extent.
However, yields in these cyclizations are low and appear to
require the less sterically demanding cis-alkene geometry in
vial with
a magnetic stir bar was charged with tris-
(dibenzylideneacetone)dipalladium (Pd2dba3, 11 mg), JessePhos
(12 mg), Et3N (175 μL), and PhCF3 (500 μL). The vial was capped
and heated at 45 °C, and the contents were stirred for 5 min. The vial
was removed from heat, and silyl iodide 1 (64 mg) was added in one
portion without cooling. The vial was then resealed and stirred at 45
°C for 24 h. The reaction was removed from heat and cooled to room
temperature. Mesitylene (35 μL) was added, and a small aliquot was
taken for NMR analysis without concentration. The volatile organic
compounds (including products) of the crude mixture were vacuum-
transferred, to separate them from the catalyst and ligand, and
analytical amounts of the two isomeric products were purified to
≥70% purity by preparatory gas chromatography. Data for 2
1
(vinylsilane): H NMR (400 MHz, C6D6) δ 6.63 (dt, J = 14.1, 3.9
Hz, 1H), 5.79 (dt, J = 14.1, 2.1 Hz, 1H), 2.04−1.92 (m, 2H), 1.78−
1.64 (m, 2H), 0.68−0.57 (m, 2H), 0.08 (s, 6H); 13C NMR (101
MHz, C6D6) δ 149.1, 127.1, 31.2, 21.5, 12.3, −1.6; HRMS (LIFDI)
calcd for [C7H14Si] 126.0865, found 126.0847. Data for 3
1
(allylsilane): H NMR (400 MHz, C6D6) δ 5.86 (dtt, J = 10.1, 5.0,
1.8 Hz, 1H), 5.74 (dtt, J = 10.5, 4.4, 1.8 Hz, 1H), 2.21 (tdt, J = 6.4,
3.8, 1.9 Hz, 2H), 1.17 (dq, J = 4.0, 1.9 Hz, 2H), 0.63 (t, J = 6.9 Hz,
2H), 0.00 (s, 6H); 13C NMR (101 MHz, C6D6) δ 130.5, 126.2, 23.2,
D
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1
13.3, 10.3, −2.5; HRMS (LIFDI) calcd for [C7H14Si] 126.0865,
found 126.0835.
(vinylsilane): H NMR (400 MHz, CDCl3) δ 7.61−7.50 (m, 2H),
7.41−7.31 (m, 3H), 5.51 (s, 1H), 2.11 (t, 2H), 1.89 (d, 3H), 1.88−
1.79 (m, 2H), 0.94−0.69 (m, 2H), 0.31 (s, 3H). Data for 28
1-Methyl-1-phenyl-1,2,3,4-tetrahydrosiline (12) and 1-Methyl-1-
phenyl-1,2,3,6-tetrahydrosiline (13). According to the general
procedure, silyl chloride 11 (56 mg, 0.25 mmol), (JessePhos)2PdCl2
(15; 13.9 mg, 0.125 mmol, 5 mol %), LiI (47 mg, 0.35 mmol, 1.4
equiv), Et3N (175 μL, 1.25 mmol, 5.0 equiv), and PhCF3 (500 μL, 0.5
M) were combined and stirred at 45 °C for 24 h. Analysis of the crude
reaction mixture via 1H NMR revealed an 88% yield. The crude
material was purified via silica gel chromatography (hexanes) to afford
a mixture of 12 and 13 as a colorless oil (38 mg, 81%). Data for 12
1
(allylsilane): H NMR (400 MHz, CDCl3) δ 7.60−7.49 (m, 2H),
7.41−7.31 (m, 3H), 5.51 (s, 1H), 2.29−2.21 (m, 2H), 1.79 (d, J = 1.8
Hz, 3H), 1.53−1.29 (m, 2H), 0.97−0.68 (m, 2H), 0.31 (s, 3H). Data
for 27 and 28 (mixture): 13C NMR (101 MHz, CDCl3) δ 159.0,
139.6, 134.2, 133.8, 129.1, 128.9, 127.9, 127.8, 124.3, 118.4, 100.1,
35.3, 29.5, 28.5, 22.8, 21.6, 17.4, 10.7, 9.0, −2.8; FTIR (cm−1) 2924,
1608, 1427, 1250, 1111, 815, 731, 698; HRMS (CI) m/z calcd for
[C13H18Si] 202.1178, found 202.1174.
1
(vinylsilane): H NMR (600 MHz, CDCl3) δ 7.59−7.51 (m, 2H),
(Z)-2-Ethylidene-1-methyl-1-phenylsilolane (32). According to
the general procedure, silyl chloride 31 (240 mg, 1.0 mmol),
(JessePhos)2PdCl2 (15; 56 mg, 0.5 mmol, 5 mol %), LiI (188 mg, 1.4
mmol, 1.4 equiv), Et3N (700 μL, 5.0 mmol, 5.0 equiv) and PhCF3 (2
mL, 0.5 M) were combined and stirred at 45 °C for 24 h. Analysis of
7.39−7.32 (m, 3H), 6.91 (dt, J = 14.1, 4.0 Hz, 1H), 5.91−5.79 (m,
1H), 2.25−2.18 (m, 2H), 1.90−1.80 (m, 2H), 1.03−0.80 (m, 2H),
0.35 (s, 3H). Data for 13 (allylsilane): 1H NMR (600 MHz, CDCl3) δ
7.61−7.51 (m, 2H), 7.40−7.32 (m, 3H), 5.91−5.80 (m, 1H), 5.78−
5.66 (m, 1H), 2.36−2.27 (m, 2H), 1.63−1.36 (m, 2H), 1.04−0.80
(m, 2H), 0.33 (s, 3H). Data for 12 and 13 (mixture): 13C NMR (151
MHz, CDCl3) δ 150.8, 139.0, 138.9, 134.2, 133.8, 130.7, 129.2, 129.1,
128.0, 127.9, 125.9, 124.7, 31.1, 23.0, 21.2, 12.1, 11.6, 9.4, −3.0, −3.8;
FTIR (cm−1) 2907, 1590, 1427, 1251, 1111, 809, 699; HRMS (CI)
m/z calcd for [C12H16Si] 188.1021, found 188.1012.
1
the crude reaction mixture via H NMR revealed a 18% yield. The
crude material was purified via silica gel chromatography (pentane) to
afford 32 as a colorless oil (36 mg, 17%): 1H NMR (600 MHz,
CDCl3) δ 7.61−7.52 (m, 2H), 7.40−7.30 (m, 3H), 6.29 (qt, J = 6.6,
2.0 Hz, 1H), 2.39−2.33 (m, 2H), 1.85−1.69 (m, 2H), 1.64 (dt, J =
6.7, 1.9 Hz, 3H), 0.98−0.81 (m, 2H), 0.50 (s, 3H); 13C NMR (151
MHz, CDCl3) δ 142.5, 138.1, 134.3, 133.8, 129.1, 127.9, 39.2, 25.5,
19.8, 15.0, −3.9; FTIR (cm−1) 2916, 1428, 1250, 1112, 732, 698;
HRMS (CI) m/z calcd for [C13H18Si] 202.1178, found 202.1177.
(Z)-1-Methyl-2-(4-methylbenzylidene)-1-phenylsilolane (34). Ac-
cording to the general procedure, silyl chloride 33 (158 mg, 0.5
mmol), (JessePhos)2PdCl2 (15; 28 mg, 0.25 mmol, 5 mol %), LiI (94
mg, 0.7 mmol, 1.4 equiv), Et3N (350 μL, 2.5 mmol, 5.0 equiv), and
PhCF3 (1.0 mL, 0.5M) were combined and stirred at 45 °C for 24 h.
1-Methyl-1-phenyl-2,3-dihydro-1H-silole (17) and 1-Methyl-1-
phenyl-2,5-dihydro-1H-silole (18). According to the general
procedure, silyl chloride 16 (53 mg, 0.25 mmol), (JessePhos)2PdCl2
(15; 13.9 mg, 0.125 mmol, 5 mol %), LiI (47 mg, 0.35 mmol, 1.4
equiv), Et3N (175 μL, 1.25 mmol, 5.0 equiv), and PhCF3 (500 μL, 0.5
M) were combined and stirred at 45 °C for 24 h. Analysis of the crude
reaction mixture via 1H NMR revealed an 80% yield. The crude
material was purified via silica gel chromatography (pentane) to afford
a mixture of 17 and 18 as a colorless oil (31 mg, 71%). Data for 17
1
Analysis of the crude reaction mixture via H NMR revealed a 49%
1
yield. The crude material was purified via silica gel chromatography
(hexanes) to afford 34 as a colorless oil (57 mg, 41%): 1H NMR (400
MHz, CDCl3) δ 7.58−7.52 (m, 2H), 7.37−7.31 (m, 3H), 7.26 (s,
1H), 7.08 (d, J = 8.1 Hz, 2H), 6.94 (d, J = 7.8 Hz, 2H), 2.66−2.58
(m, 2H), 2.25 (s, 3H), 1.98−1.85 (m, 1H), 1.76−1.63 (m, 1H), 0.97
(dd, J = 7.8, 6.6 Hz, 2H), 0.38 (s, 3H); 1H NMR (400 MHz, C6D6) δ
7.62−7.53 (m, 2H), 7.35 (s, 1H), 7.26−7.21 (m, 2H), 7.22−7.17 (m,
3H), 6.79 (d, J = 7.8 Hz, 2H), 2.66−2.52 (m, 2H), 1.98 (s, 3H),
1.94−1.79 (m, 1H), 1.71−1.57 (m, 1H), 0.99−0.84 (m, 2H), 0.41 (s,
3H); 13C NMR (101 MHz, CDCl3) δ 144.0, 139.1, 138.4, 136.6,
136.6, 134.2, 129.2, 128.8, 128.0, 127.9, 42.8, 24.8, 21.3, 15.8, −5.0
(15_C); 13C NMR (101 MHz, C6D6) δ 143.6, 140.0, 138.6, 137.1,
136.8, 134.5, 129.5, 129.1, 128.3, 43.0, 25.1, 21.1, 16.1, −4.9; FTIR
(cm−1) 2920, 1510, 1428, 1110, 809, 699; HRMS (CI) m/z calcd for
[C19H22Si] 278.1491, found 278.1493.
(vinylsilane): H NMR (600 MHz, CDCl3) δ 7.55−7.48 (m, 2H),
7.42−7.32 (m, 3H), 7.00 (dt, J = 10.1, 2.7 Hz, 1H), 6.07 (dt, J = 10.1,
2.3 Hz, 1H), 2.70−2.51 (m, 2H), 1.06−0.82 (m, 2H), 0.48 (s, 3H).
1
Data for 18 (allylsilane): H NMR (600 MHz, CDCl3) δ 7.61−7.55
(m, 2H), 7.43−7.31 (m, 3H), 5.97 (s, 2H), 1.69−1.43 (m, 4H), 0.48
(s, 3H). Data for 17 and 18 (mixture): 13C NMR (101 MHz, CDCl3)
δ 155.2, 138.9, 138.3, 134.0, 133.8, 131.2, 129.4, 129.2, 128.7, 128.0,
127.9, 32.4, 17.7, 8.8, −3.0, −3.7; FTIR (cm−1) 3019, 2905, 1114;
HRMS (CI) m/z calcd for [C11H14Si] 174.0865, found 174.0858.
1-Methyl-2-methylene-1-phenylsilinane (23) and 1,6-Dimethyl-
1-phenyl-1,2,3,4-tetrahydrosiline (24). According to the general
procedure, silyl chloride 21 (240 mg, 1.0 mmol), (JessePhos)2PdCl2
(15; 56 mg, 0.5 mmol, 5 mol %), LiI (188 mg, 1.4 mmol, 1.4 equiv),
Et3N (700 μL, 5.0 mmol, 5.0 equiv) and PhCF3 (2 mL, 0.5 M) were
combined and stirred at 45 °C for 24 h. Analysis of the crude reaction
1
mixture via H NMR revealed a 31% yield. The crude material was
purified via silica gel chromatography (hexanes) to afford a mixture of
23 and 24 as a colorless oil (23 mg, 12%). Data for 23 (exo): H
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00498.
■
1
S
NMR (400 MHz, CDCl3) δ 7.59−7.51 (m, 3H), 7.40−7.33 (m, 3H),
5.60 (dd, J = 3.3, 1.6 Hz, 1H), 5.19 (dt, J = 3.6, 1.2 Hz, 1H), 2.49−
2.27 (m, 2H), 1.96−1.83 (m, 1H), 1.70−1.60 (m, 1H), 1.54−1.39
(m, 1H), 1.20−1.09 (m, 1H), 0.86−0.72 (m, 1H), 0.34 (s, 3H). Data
1
for 24 (endo): H NMR (400 MHz, CDCl3) δ 7.61−7.49 (m, 2H),
NMR spectra (PDF)
7.43−7.30 (m, 3H), 6.49 (dq, J = 4.2, 1.8 Hz, 1H), 2.23−2.12 (m,
2H), 1.84−1.76 (m, 2H), 1.71 (q, J = 2.0 Hz, 3H), 1.00−0.89 (m,
1H), 0.86−0.71 (m, 1H), 0.37 (s, 3H). Data for 23 and 24 (mixture):
13C NMR (101 MHz, CDCl3) δ 150.8, 143.9, 138.2, 137.0, 134.4,
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.A.W.: dawatson@udel.edu.
ORCID
Donald A. Watson: 0000-0003-4864-297X
Notes
The authors declare no competing financial interest.
■
134.3, 131.9, 129.2, 129.1, 127.9, 127.9, 123.5, 40.0, 31.0, 30.6, 24.5,
21.9, 21.4, 13.7, 11.9, −4.3, −4.9; FTIR (cm−1) 2921, 2852, 1653;
HRMS (CI) m/z calcd for [C13H18Si] 202.1178, found 202.1176.
1,5-Dimethyl-1-phenyl-1,2,3,4-tetrahydrosiline (27) and 1,5-
Dimethyl-1-phenyl-1,2,3,6-tetrahydrosiline (28). According to the
general procedure, silyl chloride 26 (60 mg, 0.25 mmol),
(JessePhos)2PdCl2 (15; 13.9 mg, 0.125 mmol, 5 mol %), LiI (47
mg, 0.35 mmol, 1.4 equiv), Et3N (175 μL, 1.25 mmol, 5.0 equiv), and
PhCF3 (500 μL, 0.5 M) were stirred at 45 °C for 24 h. Analysis of the
crude reaction mixture via 1H NMR revealed an 88% yield. The crude
material was purified via silica gel chromatography (pentane) to afford
a mixture of 27 and 28 as a colorless oil (42 mg, 82%). Data for 27
ACKNOWLEDGMENTS
■
The University of Delaware and the National Science
Foundation (CAREER CHE-1254360 and CHE-1800011)
are gratefully acknowledged for support. Gelest, Inc. (Topper
E
DOI: 10.1021/acs.organomet.9b00498
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Article
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Grant Program), is acknowledged for gifts of chemical supplies.
Data were acquired at UD on instruments obtained with the
assistance of NSF and NIH funding (NSF CHE-0421224,
CHE-0840401, CHE-1229234; NIH S10 OD016267, S10
RR026962, P20 GM104316, P30 GM110758).
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H
DOI: 10.1021/acs.organomet.9b00498
Organometallics XXXX, XXX, XXX−XXX