Insertion reactions of silacyclopropanes: Evidence for a radical-based mechanism

Article abstract of DOI:10.1021/acs.organomet.6b00469

Silacyclopropanes reacted rapidly and selectively with p-benzoquinones to provide oxasilacyclopentanes. Ring-expansion products were observed in the absence of a catalyst, elevated temperatures, or irradiation. As substitution was increased on the silacyclopropane ring, improved stereoselectivity was observed. In some cases, the regiochemistry was controlled depending on the extent of stabilization of the reactive intermediates involved. A radical clock experiment, along with stereochemical studies, confirmed that radical intermediates were involved in the ring-expansion reaction. The scope of this radical reaction was expanded to include dienones, aryl aldehydes, and electron-deficient enones in addition to benzoquinones. In the case of aryl aldehydes and electron-deficient enones, the radical reaction can be used to generate silylenes from silacyclopropanes.

Full text of DOI:10.1021/acs.organomet.6b00469

Article
pubs.acs.org/Organometallics
Insertion Reactions of Silacyclopropanes: Evidence for a Radical-
Based Mechanism
Christina Z. Rotsides and K. A. Woerpel*
Department of Chemistry, New York University, New York, New York 10003, United States
S
* Supporting Information
ABSTRACT: Silacyclopropanes reacted rapidly and selectively
with p-benzoquinones to provide oxasilacyclopentanes. Ring-
expansion products were observed in the absence of a catalyst,
elevated temperatures, or irradiation. As substitution was increased
on the silacyclopropane ring, improved stereoselectivity was
observed. In some cases, the regiochemistry was controlled
depending on the extent of stabilization of the reactive
intermediates involved. A radical clock experiment, along with
stereochemical studies, confirmed that radical intermediates were
involved in the ring-expansion reaction. The scope of this radical reaction was expanded to include dienones, aryl aldehydes, and
electron-deficient enones in addition to benzoquinones. In the case of aryl aldehydes and electron-deficient enones, the radical
reaction can be used to generate silylenes from silacyclopropanes.
Herein, we provide evidence that strained silanes undergo
rapid, uncatalyzed reactions with carbonyl compounds at room
temperature to provide oxasilacyclopentanes. These reactions
can be highly selective, providing single diastereomers or
regioisomers of ring-expanded products. Mechanistic studies,
including determination of stereoselectivity and regioselectivity
and experiments using radical clocks,10 provide evidence that
these reactions proceed via radical intermediates.
INTRODUCTION
■
The reactions of organosilanes with carbon electrophiles are
commonly used transformations in stereoselective synthesis.1
Direct reactions of carbon−silicon bonds with carbon electro-
philes2 generally require activation, either by formation of ate
complexes or by transmetalation (for example, eqs 13 and 24).
RESULTS AND DISCUSSION
■
Stereochemical studies suggest that radical intermediates can be
formed in reactions of silacyclopropanes with carbonyl
compounds. In the presence of p-benzoquinone (11), trans-
10 and cis-10 were consumed in less than 10 minutes at 22 °C
to provide oxasilacyclopentanes 12a and 12b (eqs 4 and 5).
That these reactions were rapid contrasts with earlier
observations that elevated temperatures or catalysts were
required to achieve carbon−carbon bond formation with
other carbonyl compounds, such as aldehydes, esters, and
amides.11
The loss of stereochemical integrity in these experiments
suggests that carbon−carbon bond formation is not concerted.
Instead, short-lived ring-opened intermediates such as radicals
(e.g., A) or zwitterions (e.g., B) could intervene, leading to loss
of configuration (Figure 1). Radicals are reasonable inter-
mediates considering that benzoquinone can serve as a one-
electron oxidant in reactions with organometallic compounds.12
Furthermore, β-silyl radicals do not retain their stereochemical
configuration.13 By comparison, if intermediates such as B were
involved, they would not necessarily lose their stereochemical
By contrast, few uncatalyzed reactions5 of carbon−silicon
bonds with carbon electrophiles have been reported.6 For
example, radical intermediates were proposed in the reactions
of three-membered-ring silanes with ketones because, in some
cases, irradiation was required (eq 3)7 and because
disproportionation products were observed.8 Although those
experiments suggest the intermediacy of radical species,
processes involving siliconate intermediates could also account
for the results.9
Received: June 10, 2016
© XXXX American Chemical Society
A
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unsubstituted ketyl radical (C, X = H) would allow for some
loss of stereochemical configuration. The observation that
smaller quantities of products resulting from loss of stereo-
chemistry (i.e., 14b) were formed compared to the reactions of
monocyclic silacyclopropanes (eqs 4 and 5) could be because
the trans isomer is more strained,17 leading to little driving
force for isomerization.18
Depending upon the extent of substitution, insertions of
silacyclopropanes can be regioselective.11 Monosubstituted
silacyclopropane 17 reacted with p-benzoquinone (11) at 22
°C to give two regioisomeric products, 18 and 19 (eq 8).
Figure 1. Radical (A) and zwitterionic (B) intermediates leading to
oxasilacyclopropanes 12.
The fact that significant quantities of regioisomer 19 are
formed provides strong support that the reactions proceed via
radical intermediates (D) and not zwitterionic intermediates
(E, Figure 2). Had E been an intermediate, the cationic center
configuration. Strong hyperconjugative stabilization by a
carbon−silicon bond can restrict rotation of cationic inter-
mediates,14 leading to stereospecific processes.15
In the case of a silacyclopropane that was conformationally
restricted, insertion reactions were stereoselective. The reaction
of p-benzoquinone (11) with bicyclic silacyclopropane 13
occurred within minutes at 22 °C, forming products 14a and
14b with 90% retention of configuration (eq 6). Insertion of
Figure 2. Radical (D) and zwitterionic (E) intermediates leading to
oxasilacyclopropane 18.
would experience considerably more stabilization (10 kcal·
mol−1) as a secondary β-silyl carbocation than it would as the
primary β-silyl carbocation.19 As a result, if an ionic mechanism
were operating, formation of regioisomer 19 should have been
strongly disfavored.20 By contrast, the difference in stabilities
between primary and secondary β-silyl radicals is much smaller
(≤2 kcal·mol−1), so reaction through a primary, β-silyl radical is
not as strongly disfavored as it would be for reactions
proceeding via a primary carbocation.19
In the case of a more substituted silacyclopropane ring, high
regioselectivity was observed. Geminally substituted silacyclo-
propane 20 provided a single isomer of product, although the
reaction was not clean (eq 9). The lower yield in this reaction
may be the result of developing steric interactions between the
tertiary radical and the quinone radical (intermediate F) that
decelerates this bond formation.
Vinylsilacyclopropanes also underwent regioselective car-
bon−carbon bond formation. The reaction of vinylsilacyclo-
propane 22 with p-benzoquinone (11) provided insertion
2,6-dichlorobenzoquinone (15) also occurred rapidly, provid-
ing a single diastereomer and regioisomer of oxasilacyclopen-
tane 16, in which the configuration of the starting
silacyclopropane was retained (eq 7). These stereochemical
outcomes are consistent with reactions via radical intermedi-
ates. Coupling of the nucleophilic secondary alkyl radical
component of diradical C should occur faster with the more
electron-deficient ketyl radical (C, X = Cl) because of better
matches of SOMO energies.16 The slower reaction of an
B
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assigned as 29 and 30, however, and they were formed without
opening of the cyclopropane ring. Two minor ring-opened
products, 31 and 32, were also formed.
A mechanism involving radical intermediates is consistent
with the results described in eq 10. This mechanism is
illustrated for silacyclopropane 27 (Scheme 2). Formation of
Scheme 2. Mechanism for the Insertion of Carbonyl
Compounds into the Carbon−Silicon Bond of
Silacyclopropanes
product 23 as a single regioisomer (Scheme 1). The formation
of one regioisomer suggests that the adjacent double bond and
Scheme 1. Formation of Oxasilacyclopropane 23 and
Paracyclophane 26 from Vinylsilacyclopropane 22
silyloxy group stabilize the radical intermediate.21 The
oxasilacyclopentane 23 could not be isolated in pure form,
however. Within 12 h, it rearranged to paracyclophane 26.
The hydroquinone derivative 26 could arise from a series of
rearrangements. Oxasilacyclopentane 23 could undergo a [1,2]-
alkyl shift to provide ring-expanded intermediate 24. This
intermediate is poised to undergo a subsequent rearrangement
to provide enone 25. This enone can then undergo a retro-
Claisen rearrangement to provide hydroquinone derivative
26.22
Experiments with silacyclopropanes 27 and 28 bearing
radical clocks23 provided additional evidence that radical
intermediates were involved in reactions with strained silanes.24
When these silacyclopropanes were subjected to the reaction
conditions, mixtures of products were obtained (eq 10). Due to
diradical 33 occurs,25 likely after coordination of the carbonyl
oxygen atom to the Lewis acidic silicon atom.26 Homolytic
cleavage of the activated carbon−silicon bond provides
diradical 33. Cleavage of the more substituted carbon−silicon
bond due to stabilization of the alkyl component of radical 33
also explains the trends in regioselectivity observed for other
silacyclopropanes. Radical recombination of 33 would form the
major product 29.27
Although this radical recombination28 should be rapid,
opening of the cyclopropane ring is competitive.23 Cyclo-
propylcarbinyl radical rearrangement leads to diradical 34.
Recombination of this diradical would form enone 35, which
can undergo a dienone-phenol rearrangement29 to provide
phenol 31. A single stereoisomer of 31 was formed, but, due to
the small quantities of product formed and difficulties
associated with purification, the geometry of the carbon−
carbon double bond could not be unambiguously determined.30
The formation of minor quantities of phenols 31 and 32
provides insight into the lifetime of diradical 33. The rate of
cyclopropylcarbinyl radical rearrangement of phenyl-substituted
cyclopropanes is about 1011 s−1.23 The ring closure of diradical
intermediate 33 to form the oxasilacyclopentane 29 therefore
must proceed at a competitive rate.31 In the case of a
monocyclic silacyclopropane, persistence of this radical would
lead to loss of stereochemistry (eqs 4 and 5) considering the
rapid pyramidal inversion of alkyl radicals.18
difficulties associated with purification, not all products could
be unambiguously identified. The major products could be
C
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The uncatalyzed reactions of carbon−silicon bonds were not
restricted to quinones. When dienone 36 was added to
silacyclopropane 13, ring-expanded products 37 were observed
(eq 11). The rate of this ring expansion was slow, requiring 2
diradical 43. This species (shown as its resonance form
silacarbonyl ylide 44) can undergo either electrocyclization or
dimerization, the net products of silylene transfer reactions.34
CONCLUSIONS
■
In conclusion, the reactions of carbonyl compounds with the
carbon−silicon bond of a silane can occur through a single-
electron transfer mechanism. This conclusion was supported by
experiments using substrates bearing radical clocks, which
showed that radical recombination is rapid, in addition to
stereochemical studies. Benzoquinones, electron-deficient
enones, and aryl aldehydes all react with silacyclopropanes at
room temperature in the absence of catalysts. In some cases,
the reactions of silacyclopropanes with benzoquinones were
regio- or stereoselective. The uncatalyzed reactions of aryl
aldehydes and electron-deficient enones provided the products
of net silylene transfer, indicating that the radical fragmentation
can also provide silylene intermediates.
weeks to proceed to completion compared to less than 10
minutes in the case of benzoquinone (eq 6). This difference in
relative rate is consistent with the fact that divinyl ketones are
much less prone to single-electron reduction than benzoqui-
nones, presumably because the radical intermediate is not as
delocalized.32 Furthermore, the corresponding enone, 4,4-
dimethylcyclohexenone, did not react under these conditions,
indicating that the stabilization by a single double bond is not
sufficient to stabilize a radical intermediate that would be
formed.
Although aliphatic aldehydes did not react with the carbon−
silicon bond of silacyclopropanes, aryl aldehydes and phenyl-
substituted enones did.11a When benzaldehyde was added to
cyclohexene silacyclopropane (13), aldehyde dimer 38 was
formed within 4 h (eq 12).11b This dimer is the product of
EXPERIMENTAL SECTION
General Procedures. General experimental procedures are
provided as Supporting Information.
■
Oxasilacyclopentanes 12a and 12b (from cis-10). To a
solution of silacyclopropane cis-10 (0.055 g, 0.19 mmol of
silacyclopropane 10, contaminated with 0.070 mmol of cyclohexene
silacyclopropane 13 from the preparation of 10) in C6D6 (0.50 mL) in
a J. Young NMR tube were added 1,4-benzoquinone 11 (0.023 g, 0.21
mmol) in C6D6 (0.50 mL) and mesitylene (0.0020 mL, 0.014 mmol,
internal standard). Oxasilacyclopentanes 12a and 12b were formed in
68% yield (as a 57:43 mixture of diastereomers), and oxasilacyclo-
pentane 14 (derived from silacyclopropane 13) was formed in 43%
yield based on comparison of the standard peak (δ 2.17) and the
enone protons. Purification by flash chromatography (3:97 EtOAc/
hexanes, with silica gel that was pretreated with a Et3N solution, 1:99
Et3N/hexanes) provided oxasilacyclopentanes 12a and 12b as a
colorless oil in a 80:20 mixture with oxasilacyclopentane 14: 1H NMR
(600 MHz, C6D6) δ 6.53 (dd, J = 10.3, 3.1, 1H, HCC), 6.41−6.40
(m, 0.7H, HCC), 6.31 (dd, J = 10.0, 3.0, 0.7H, HCC), 6.25 (dd, J
= 10.1, 3.0, 1H, HCC), 6.15−6.09 (m, 3.4H, HCC), 2.05−2.00
(m, 1H, HC−CSi), 1.62−1.56 (m, 0.7H, HC−CSi), 1.16−1.14 (m,
1H, HC−Si), 1.05 (s, 9H, t-Bu), 1.00 (s, 6.5H, t-Bu), 0.96 (s, 6.5H, t-
Bu), 0.94 (appar s, 11.4H, t-Bu and CH3), 0.86 (d, J = 8.2, 3H, CH3),
0.76−0.70 (m, 0.7H, CH−Si), 0.53 (d, J = 7.3, 3H, CH3), 0.50 (d, J =
6.8, 2.1H, CH3); 13C NMR (150 MHz, C6D6) δ 185.49 (C), 185.45
(C), 152.9 (CH), 152.0 (CH), 149.1 (CH), 148.3 (CH), 81.2 (C),
79.7 (C), 48.3 (CH), 45.1 (CH), 29.5 (CH3), 29.1 (CH3), 28.9
(CH3), 28.8 (CH3), 24.7 (CH), 22.0 (C), 21.8 (C), 21.2 (C), 20.8
(C), 20.5 (CH), 14.2 (CH3), 13.2 (CH3), 13.1 (CH3), 11.6 (CH3); IR
(ATR) 1672, 1631, 1041, 831 cm−1; HRMS (TOF MS ES+) m/z
calcd for C18H31O2Si (M + H)+ 307.2089, found 307.2085.
Oxasilacyclopentanes 12a and 12b (from trans-10). To a
solution of silacyclopropane trans-10 (0.050 g, 0.14 mmol of
silacyclopropane 10, contaminated with 0.11 mmol of cyclohexene
silacyclopropane 13 from the preparation of 10) in C6D6 (0.50 mL) in
a J. Young NMR tube were added 1,4-benzoquinone 11 (0.027 g, 0.26
mmol) in C6D6 (0.50 mL) and mesitylene (0.0020 mL, 0.014 mmol,
internal standard). Oxasilacyclopentanes 12a and 12b were formed in
77% yield (as a 48:52 mixture of diastereomers), and oxasilacyclo-
pentane 14 (derived from silacyclopropane 13) was formed in 67%
yield based on comparison of the standard peak (δ 2.17) and the
enone protons. The spectroscopic data for oxasilacyclopentanes 12a
and 12b agree with those reported (from silacyclopropane cis-10,
above).
silylene transfer from silacyclopropane 13. In the absence of
other reaction partners, the silylene intermediate is transferred
to another molecule of benzaldehyde, providing dimer 38. The
reaction of enone 39 and bicyclic silacyclopropane 13 formed
both ketone dimer 40 and oxasilacyclopentene 41 (eq 13). In
the case of enone 39, dimerization could be slow considering
the steric hindrance at the carbonyl carbon atom, so 6π-
electrocyclization33 becomes competitive, providing 41 in
addition to the dimer.
These results are consistent with the proposal that radical
intermediates are involved. β-Silyl radical intermediates 42
could undergo fragmentation reactions, leading to silylene
transfer products (eq 14). Coordination of the silicon atom of
Oxasilacyclopentanes 14a and 14b. To a solution of 1,4-
benzoquinone 11 (0.051 g, 0.47 mmol) in C6H6 (2 mL) was added
cyclohexene silacyclopropane 13 (0.15 g, 0.67 mmol) as a solution in
C6H6 (0.5 mL). After 10 min, the reaction mixture was concentrated in
silacyclopropane 13 followed by homolytic bond cleavage
provides diradical 42. Elimination of cyclohexene affords new
D
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vacuo. Purification by flash chromatography (5:95 EtOAc/hexanes)
provided oxasilacyclopentanes 14a and 14b as a colorless oil (90:10
mixture of diastereomers, 0.144 g, 92%): 1H NMR (500 MHz, CDCl3)
δ 7.01 (dd, J = 10.3, 3.0, 1H, HCC), 6.76 (dd, J = 10.1, 2.9, 0.1H,
HCC), 6.72 (dd, J = 10.0, 3.0, 0.1H, HCC), 6.62 (dd, J = 10.0,
3.1, 1H, HCC), 6.15 (dd, J = 10.0, 2.0, 1.1H, HCC), 6.11 (dd, J =
10.3, 1.9, 1.1H, HCC), 2.34−2.32 (m, 1H, HC−CSi), 2.06−2.02
(m, 0.1H, HC−CSi), 1.92−1.82 (m, 1.1H, CH2), 1.72−1.70 (m, 1.1H,
CH2), 1.59−1.55 (m, 2.2H, CH2), 1.49−1.40 (m, 2.2H, CH2), 1.30−
1.26 (m, 1.1H, CH2), 1.22−1.17 (m, 1.1H, CH2), 1.13 (appar s, 10H,
t-Bu and HC−Si), 1.08 (s, 1.0H, t-Bu and HC-Si), 1.07 (s, 0.9 H, t-
Bu), 1.03 (s, 9H, t-Bu); 13C NMR (125 MHz, CDCl3) δ 186.1 (C),
186.0 (C), 153.4 (CH), 152.1 (CH), 150.9 (CH), 148.7 (CH), 128.2
(CH), 128.1 (CH), 128.0 (CH), 127.1 (CH), 81.0 (C), 78.4 (C), 51.3
(CH), 44.4 (CH), 29.9 (CH), 29.3 (CH3), 28.7 (CH3), 28.6 (CH2),
28.4 (CH3), 28.0 (CH3), 27.9 (CH2), 27.7 (CH2), 26.4 (CH2), 26.3
(CH2), 25.9 (CH2), 24.3 (CH), 23.4 (CH2), 22.8 (CH2), 21.7 (C),
21.5 (C), 21.0 (C), 20.4 (C); 29Si (99 MHz, CDCl3) δ 34.3, 28.2; IR
(ATR) 1673, 1631, 1039, 822 cm−1; HRMS (TOF MS ES+) m/z
calcd for C20H33O2Si (M + H)+ 333.2244, found 333.2243.35
(M + H)+ 499.3058, found 499.3070. Anal. Calcd for C29H46O3Si2: C,
69.82; H, 9.29. Found: C, 70.00; H, 9.55.
Oxasilacyclopentane 21. To a solution of 1,1-dimethyl-di-tert-
butylsilacyclopropane 20 (0.030 g, 0.15 mmol) in C6D6 (0.50 mL) in a
J. Young NMR tube were added a solution of 1,4-benzoquinone 11
(0.50 mL, 0.32 M in C6D6, 0.16 mmol) and mesitylene (0.0020 mL,
0.014 mmol, internal standard), and the unpurified reaction mixture
was analyzed by NMR spectroscopy after 10 min. Oxasilacyclopentane
21 was formed in 37% yield based on comparison of the standard peak
1
(δ 6.71) and the enone protons: H NMR (500 MHz, C6D6) δ 6.60
(d, J = 10.2, 2H, HCC), 6.05 (d, J = 10.1, 2H, HCC), 1.00 (s,
18H, 2t-Bu), 0.84 (s, 6H, 2CH3), 0.74 (s, 2H, CH2); 13C NMR (125
MHz, C6D6) δ 185.2 (C), 151.4 (CH), 126.7 (CH), 82.1 (C), 45.6
(C), 31.4 (CH3), 29.5 (CH3), 24.8 (CH2), 20.7 (C).36
Oxasilacyclopentane 23 and Hydroquinone 26. To a solution
of vinylsilacyclopropane 22 (0.064 g, 0.17 mmol, from silylene transfer
step) in C6D6 (0.54 mL) in a J. Young NMR tube were added a
solution of 1,4-benzoquinone 11 (0.48 mL, 0.54 mM in C6D6, 0.26
mmol) and mesitylene (0.0020 mL, 0.014 mmol, internal standard),
and the unpurified reaction mixture was analyzed by 1H NMR
spectroscopy after 10 min. Oxasilacyclopentane 23 was formed in 70%
yield based on comparison of the standard peak (δ 2.17) and the
enone protons. Hydroquinone 26 was also formed in 16% yield after
10 min. The reaction mixture was allowed to sit at room temperature
overnight (12 h), and hydroquinone 26 was formed in 78% yield from
vinylsilacyclopropane 22 based on comparison of the standard peak (δ
2.17) and the aryl protons.37
Oxasilacyclopentane 16. To a solution of 2,6-dichlorobenzoqui-
none 15 (0.087 g, 0.49 mmol) in C6H6 (2 mL) was added cyclohexene
silacyclopropane 13 (0.149 g, 0.66 mmol) as a solution in C6H6 (0.50
mL). After 10 min, the reaction mixture was concentrated in vacuo.
Purification by flash chromatography (3:97 EtOAc/hexanes, with silica
gel that was pretreated with a Et3N solution, 1:99 Et3N/hexanes)
provided oxasilacyclopentane 16 as a white solid (single isomer, 0.135
Oxasilacyclopentane 23. 1H NMR (500 MHz, C6D6) δ 6.54 (dd,
J = 10.3, 3.1, 1H, HCC), 6.38 (dd, J = 10.1, 3.1, 1H, HCC), 6.26
(d, J = 11.7, 1H, HCC), 6.14 (dd, J = 10.1, 1.9, 1H, HCC), 6.10
(dd, J = 10.3, 1.9, 1H, HCC), 4.76 (dd, J = 11.7, 9.3, 1H, HCC),
2.58 (ddd, J = 13.1, 9.1, 7.1, 1H, CH), 1.06 (br s, 21H, Si(i-Pr3), 0.98
(appar s, 10H, t-Bu and 1 H of H2C−Si), 0.96 (s, 9H, t-Bu), 0.73 (dd,
J = 14.9, 13.2, 1H, 1 H of H2C−Si); 13C NMR (125 MHz, C6D6) δ
185.2 (C), 151.4 (CH), 147.6 (CH), 142.6 (CH), 129.2 (CH), 129.0
(CH), 111.0 (CH), 80.4 (C), 47.8 (CH), 28.8 (CH3), 28.5 (CH3),
20.9 (C), 20.3 (C), 18.3 (CH3), 15.2 (CH2), 12.6 (CH).
1
g, 69%): mp = 46−47 °C; H NMR (600 MHz, C6D6) δ 7.11 (d, J =
2.8, 1H, HCC), 6.63 (d, J = 2.8, 1H, HCC), 1.99 (t, J = 7.3, 1H,
HC−CSi), 1.47−1.41 (m, 2H, CH2), 1.25−1.14 (m, 4H, HC−Si and
CH2), 1.05−1.02 (m, 1H, CH2), 0.99 (s, 9H, t-Bu), 0.94−0.93 (m, 1H,
CH2), 0.83 (s, 9H, t-Bu), 0.80−0.76 (m, 1H, CH2); 13C NMR (150
MHz, C6D6) δ 173.0 (C), 149.4 (CH), 147.2 (CH), 131.7 (C), 130.5
(C), 83.7 (C), 45.8 (CH), 29.5 (CH3), 28.6 (CH3), 26.6 (CH2), 26.2
(CH2), 24.2 (CH), 23.9 (CH2), 23.2 (CH2), 21.7 (C), 21.1 (C); IR
(ATR) 2933, 2859, 1691 cm−1; HRMS (TOF MS ES+) m/z calcd for
C20H31Cl2O2Si (M + H)+ 401.1465, found 401.1462. Anal. Calcd for
C20H30Cl2O2Si: C, 59.84; H, 7.53. Found: C, 59.57; H, 7.67.
Hydroquinone 26. 1H NMR (400 MHz, C6D6) δ 7.03 (d, J = 8.8,
2H, Ar−H), 6.84 (d, J = 8.9, 2H, Ar−H), 6.18−6.10 (m, 1H, HCC),
5.97−5.96 (m, 1H, HC), 5.75 (dd, J = 15.2, 5.4, 1H, HCC), 1.84 (d,
J = 8.0, 2H, CH2), 1.18 (br s, 21H, Si(i-Pr)3), 1.10 (s, 18H, 2 t-Bu);
13C NMR (125 MHz, C6D6) δ 151.4 (C), 151.2 (C), 130.9 (CH),
130.7 (CH), 121.3 (CH), 119.9 (CH), 98.3 (CH), 28.8 (CH3), 22.32
(C), 22.28 (C), 18.7 (CH3), 18.6 (CH3), 18.4 (CH2), 13.2 (CH).37
Radical Clock Experiment with Silacyclopropanes 27. To a
solution of silacyclopropanes 27 (0.027 g, 0.074 mmol, from silylene
transfer step) in C6D6 (0.50 mL) in a J. Young NMR tube were added
1,4-benzoquinone 11 (0.15 mL, 0.5 M in C6D6, 0.079 mmol) and
mesitylene (0.0020 mL, 0.014 mmol, internal standard), and the
Oxasilacyclopentanes 18 and 19. To a solution of silacyclo-
propane 17 (0.073 g, 0.19 mmol, from silylene transfer step) in C6D6
(0.8 mL) in a J. Young NMR tube were added a solution of 1,4-
benzoquinone 11 (0.40 mL, 0.54 mM in C6D6, 0.22 mmol) and
mesitylene (0.0020 mL, 0.014 mmol, internal standard). After 10 min,
the reaction mixture was analyzed by 1H NMR spectroscopy, and all of
the starting material had been consumed. The reaction mixture was
concentrated in vacuo. Purification by flash chromatography (3:97
EtOAc/hexanes) provided oxasilacyclopentanes 18 and 19 as a
colorless oil (62:38 mixture of regioisomers, 0.087 g, 87%): 1H
NMR (500 MHz, CDCl3) δ 7.13−7.11 (m, 0.6H, Ar−H), 7.09−7.05
(m, 1.6H, Ar−H), 7.03−7.01 (m, 1H, Ar−H), 6.95−6.84 (m, 3.8H,
Ar−H and HCC), 6.81−6.76 (m, 2.6H, HCC), 6.23 (dd, J =
10.3, 1.8, 1H, HCC), 6.20 (dd, J = 10.0, 1.7, 1H, HCC), 6.12 (dd,
J = 10.1, 1.8, 0.6H, HCC), 6.02 (dd, J = 10.3, 1.9, 0.6H, HCC),
3.01−3.00 (m, 1.2H, CH2), 2.62−2.55 (m, 1H, CH), 2.51 (dd, J =
13.2, 3.9, 1H, 1 H of CH2), 2.31 (dd, J = 12.9, 11.5, 1H, 1 H of CH2),
2.03−2.01 (m, 1.2H, CH2), 1.89−1.88 (m, 0.6H, HC−Si), 1.20 (s,
5.4H, t-Bu), 1.10 (s, 5.4H, t-Bu), 1.09 (s, 9H, t-Bu), 1.02 (s, 9H, t-Bu),
1.00 (s, 5.4H, t-Bu), 0.99 (s, 9H, t-Bu), 0.95−0.92 (m, 1H, 1 H of
H2C−Si), 0.73 (dd, J = 14.8, 13.7, 1H, 1 H of H2C−Si), 0.26 (s, 1.8H,
H3C−Si), 0.23 (s, 1.8 H, H3C−Si), 0.22 (s, 3H, H3C−Si), 0.18 (s, 3H,
H3C−Si); 13C NMR (150 MHz, CDCl3) δ 186.0 (C), 185.6 (C),
153.8 (C), 153.6 (C), 152.4 (CH), 152.1 (CH), 150.6 (CH), 148.5
(CH), 132.4 (C), 130.7 (CH), 130.4 (CH), 129.8 (C), 128.6 (CH),
128.2 (CH), 127.3 (2 CH), 127.2 (CH), 126.2 (CH), 121.3 (CH),
121.2 (CH), 118.9 (CH), 118.8 (CH), 79.3 (C), 75.3 (C), 47.2 (CH),
42.8 (CH2), 33.3 (CH2), 30.0 (CH2), 29.4 (CH), 28.7 (CH3), 25.3
(CH3), 28.1 (CH3), 26.2 (CH3), 26.1 (CH3), 25.2 (CH3), 21.9 (C),
20.63 (C), 20.60 (C), 20.1 (C), 18.6 (C), 18.5 (C), 12.5 (CH2), −3.6
(CH3), −3.7 (CH3), −3.8 (CH3), −4.0 (CH3); IR (ATR) 1673, 1632,
1041, 830 cm−1; HRMS (TOF MS ES+) m/z calcd for C29H47O3Si2
1
unpurified reaction mixture was analyzed by H NMR spectroscopy
after 10 min. Phenol 31 was formed in 13% yield based on comparison
of the standard peak (δ 2.17) and the alkene protons along with a
mixture of at least three distinct products. Spectroscopic signatures
(listed below) suggest that oxasilacyclopentanes 29 were formed (46%
yield based on comparison of the standard peak (δ 2.17) and the
quinone protons), but their structures could not be unambiguously
assigned. The products (29) could not be separated from each other.
Spectroscopic data for phenol 31 and diagnostic peaks for the
remaining products (29) are listed below.
Mixture of Oxasilacyclopentanes 29. 1H NMR (600 MHz,
C6D6, diagnostic peaks) δ 6.65 (dd, J = 10.7, 3.2, 1H, HCC), 6.63
(dd, J = 10.3, 3.1, 1H, HCC), 6.53 (dd, J = 10.1, 3.1, 1H, HCC),
6.26 (m, 3H, HCC), 6.17 (m, 2H, HCC), 6.13 (dd, J = 10.1, 2.0,
1H, HCC), 6.09 (dd, J = 10.0, 2.0, 1H, HCC), 5.88 (dd, J = 10.1,
3.1, 1H, HCC), 5.85 (dd, J = 10.3, 2.0, 1H, HCC), 1.93 (dd, J =
13.3, 8.0, 1H, CH2), 1.85 (m, 2H, CH2 and HC), 1.62 (m, 1H, CH2),
1.46 (ddd, J = 9.4, 7.7, 6.2, 1H, HC), 1.30 (m, 1H, CH2), 1.26 (m, 1H,
CH2), 1.22 (m, 2H, CH2), 1.11 (m, 1H, CH2), 1.10 (s, 9H), 1.07 (m,
1H, H2C−Si), 1.01 (s, 9H), 0.97 (s, 9H and m, 1H, t-Bu and H2C−Si),
0.93 (s, 9H, t-Bu), 0.84 (s, 9H, t-Bu), 0.81 (m, 1H, H2C−Si), 0.71 (s,
E
DOI: 10.1021/acs.organomet.6b00469
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
9H, t-Bu), 0.61 (m, 1H, H2C−Si), 0.50 (dd, J = 14.8, 6.7, 1H, H2C−
Si); 13C NMR (150 MHz, C6D6) δ 186.0 (C), 185.8 (C), 185.2 (C),
152.0 (CH), 148.1 (CH), 129.3 (CH), 129.0 (CH), 128.7 (CH),
127.9 (CH), 126.6 (CH), 116.5 (CH), 79.14 (C), 78.99 (C), 75.3
(C), 48.2 (CH), 46.1 (CH), 43.9 (CH2), 30.0 (CH), 28.9 (CH3), 28.8
(3 CH3), 28.4 (CH3), 28.2 (CH3), 27.90 (CH), 27.86 (C), 27.3 (CH),
25.8 (CH2), 20.3 (CH2), 20.2 (CH2), 18.02 (CH2), 17.99 (CH2), 13.7
(CH2), 13.4 (CH2).
(100 MHz, CDCl3) δ 137.5 (CH), 137.3 (CH), 136.8 (CH), 136.2
(CH), 131.8 (CH), 130.7 (CH), 129.8 (CH), 127.6 (CH), 80.2 (C),
77.8 (C), 53.2 (CH), 46.2 (CH), 34.3 (C), 33.8 (C), 30.7 (CH3), 30.6
(CH3), 30.3 (CH2), 30.0 (CH3), 29.6 (CH3), 29.5 (CH3), 29.34
(CH3), 29.29 (CH), 29.2, (CH3), 28.72 (CH2), 28.69 (CH2), 28.5
(CH3), 27.5 (CH2), 27.3 (CH2), 26.9 (CH2), 25.3 (CH2), 25.0 (CH),
23.1 (CH2), 22.2 (C), 21.9 (C), 21.4 (C), 20.7 (C); IR (ATR) 2928,
1012, 820 cm−1; HRMS (TOF MS ES+) m/z calcd for C22H39OSi (M
+ H)+ 347.2765, found 347.2772. Anal. Calcd for C22H38OSi: C,
76.23; H, 11.05. Found: C, 75.97; H, 11.01.
Dioxasilacyclopentane 38. To a solution of benzaldehyde 2
(0.10 mL, 0.10 mmol) in C6D6 (0.3 mL) in a J. Young NMR tube were
added a solution of cyclohexene silacyclopropane 13 (0.10 mL, 1.3 M
in C6D6, 0.13 mmol) and mesitylene (0.0020 mL, 0.014 mmol,
internal standard). The reaction was monitored by 1H NMR
spectroscopy. After 4 h, the reaction mixture was concentrated in
vacuo. Purification by flash chromatography (3:97 EtOAc/hexanes)
provided dioxasilacyclopentane 38 as a colorless oil (0.023 g, 100%).
Dioxasilacyclopentane 38 is known, but the data were previously
collected in CDCl3.38 1H NMR (600 MHz, C6D6) δ 7.27−7.26 (m,
4H, Ar−H), 7.11−7.09 (6H, Ar−H), 4.96 (s, 2H, 2 CH), 1.24 (s, 18H,
2 t-Bu); 13C NMR (150 MHz, C6D6) δ 140.4, 128.8, 128.5, 127.6,
85.4, 27.9, 14.7; HRMS (TOF MS ES+) m/z calcd for C22H31O2Si (M
+ H)+ 355.2088, found 355.2086.
Dioxasilacyclopentane 40 and Oxasilacyclopentene 41. To a
solution of trans-4-phenyl-3-buten-2-one (0.030 g, 0.21 mmol) in
C6D6 (0.30 mL) in a J. Young NMR tube were added a solution of
cyclohexene silacyclopropane 13 (0.21 mL, 1.3 M in C6D6, 0.27
mmol) and mesitylene (0.0020 mL, 0.014 mmol, internal standard),
and the unpurified reaction mixture was analyzed by 1H NMR
spectroscopy after 20 min. Dimer 40 was formed in 54% yield, and
oxasilacyclopentene 41 was formed in 30% yield based on comparison
of the standard peak (δ 2.17) and the alkene protons. The products
were inseparable.
Hydroquinone 31. 1H NMR (600 MHz, C6D6) δ 7.46 (d, J = 8.0,
2H, Ar−H), 7.11−7.08 (m, 2H, Ar−H), 7.04−7.01 (m, 4H, Ar−H),
6.88−6.79 (m, 4H, Ar−H and hydroquinone Ar−H), 6.72 (br s, 1H,
hydroquinone Ar−H), 5.28−5.23 (m, 1H, HCC), 4.51−4.46 (m,
1H, HCC), 3.64 (dd, J = 12.1, 2.9, 1H, H2C−CC), 2.33−2.29
(m, 1H, H2C−CC), 1.84−1.80 (m, 1H, H2C−Si), 1.66−1.62 (m,
1H, H2C−Si), 1.05 (s, 9H), 0.96 (s, 9H); 13C NMR (150 MHz, C6D6)
δ 151.4 (C), 146.9 (C), 146.7 (C), 146.1 (C), 136.1 (CH), 133.0
(CH), 132.4 (C), 130.2 (CH), 129.1 (CH), 128.9 (CH), 128.7 (CH),
127.8 (CH), 126.5 (CH), 124.7 (CH), 120.4 (CH), 118.5 (CH), 61.4
(C), 42.1 (CH2), 28.2 (CH3), 27.8 (CH3), 22.7 (C), 21.5 (C), 18.5
(CH2); IR (ATR) 3527, 3058, 1011, 822 cm−1; HRMS (TOF MS ES
+) m/z calcd for C31H39O2Si (M + H)+ 471.2714, found 471.2714.
Radical Clock Experiment with Silacyclopropanes 28. To a
solution of silacyclopropanes 28 (0.049 g, 0.17 mmol, from silylene
transfer step) in C6D6 (0.50 mL) in a J. Young NMR tube were added
1,4-benzoquinone 11 (0.19 mL, 0.98 M in C6D6, 0.19 mmol) and
mesitylene (0.0020 mL, 0.014 mmol, internal standard), and the
1
unpurified reaction mixture was analyzed by H NMR spectroscopy
after 10 min. Hydroquinone 32 was formed in 13% yield based on
comparison of the standard peak (δ 2.17) and the methylene protons
along with a mixture of at least three distinct products. Spectroscopic
signatures (listed below) suggest that oxasilacyclopentanes 30 were
formed (44% based on comparison of the standard peak (δ 2.17) and
the quinone protons), but their structures could not be unambiguously
assigned. Upon silica gel chromatography, the reaction mixture
completely decomposed, and the products were not fully charac-
terized. Spectroscopic signatures (in situ data) are listed below.
Mixture of Oxasilacyclopentanes 30. 1H NMR (600 MHz,
C6D6, diagnostic peaks) δ 2.24 (m, 1H, CH), 1.98 (m, 2H, CH2), 1.75
(m, 4H, H2C and HC), 1.42 (m, 4H, H2C and HC), 0.79 (m, 2H,
CH2), 0.73 (m, 2H, CH2), 0.56 (m, 5H, H2C and HC), 0.51 (m 1H,
H2C−Si), 0.46 (m, 1H, H2C−Si); 13C NMR (150 MHz, C6D6,
diagnostic peaks) δ 185.7 (C), 185.4 (C), 185.2 (C), 184.9 (C), 79.3
(C), 77.2 (C), 75.4 (C), 75.3 (C), 53.7 (CH), 53.4 (CH), 44.0 (CH),
43.6 (CH), 14.6 (CH2), 14.0 (CH2), 13.7 (CH2), 12.9 (CH2).
Dioxasilacyclopentane 40. 1H NMR (500 MHz, C6D6,
diagnostic peaks) δ 7.07−7.05 (m, 1H, HCC), 6.81 (d, J = 16.1,
1H, HCC), 6.49 (d, J = 16.1, 1H, HCC), 6.21 (d, J = 15.6, 1H,
HCC), 1.48 (s, 3H, CH3), 1.42 (s, 3H, CH3), 1.19 (s, 9H, t-Bu),
1.01 (s, 9H, t-Bu); 13C NMR (125 MHz, C6D6, diagnostic peaks) δ
138.12 (CH), 138.09 (CH), 137.0 (CH), 85.3 (C), 84.5 (C), 23.6
(CH3).
Oxasilacyclopentene 41. Oxasilacyclopentene 41 is a known
compound, but data were collected in CDCl3. The data below were
collected in situ due to difficulty separating oxasilacyclopentene 41
from dioxasilacyclopentane 40:33 1H NMR (500 MHz, C6D6,
diagnostic peaks) δ 4.89−4.87 (m, 1H, HCC), 3.54−3.52 (m,
1H, HC−Ar), 1.87−1.86 (m, 3H, CH3), 1.12 (s, 9H, t-Bu), 0.85 (s,
9H, t-Bu); 13C NMR (125 MHz, C6D6, diagnostic peaks) δ 158.3 (C),
103.4 (CH), 28.2 (CH3), 27.8 (CH3), 18.6 (CH3).
1
Hydroquinone 32. H NMR (600 MHz, C6D6, diagnostic peaks)
δ 6.45 (d, J = 2.9, 1H, HCC), 5.44−5.40 (m, 1H, HCC), 5.33−
5.32 (m, 1H, HCC), 4.55−4.50 (m, 1H, HCC), 3.21−3.17 (m,
1H, H2C−CC), 2.06−2.05 (m, 1H, H2C−CC), 1.91 (m, 1H,
overlaps with cyclohexene from silylene transfer step, H2C−Si), 1.51
(m, 1H, overlaps with cyclohexene from silylene transfer step, H2C−
Si); 13C NMR (150 MHz, C6D6, diagnostic peaks) δ 149.6 (C), 143.2
(C), 135.8 (CH), 131.8 (CH), 125.7 (CH), 45.7 (CH), 38.0 (CH2),
28.8 (CH2).
ASSOCIATED CONTENT
■
S
Oxasilacyclopentanes 37. To a solution of dienone 36 (0.016 g,
0.13 mmol) in C6D6 (0.30 mL) in a J. Young NMR tube were added a
solution of cyclohexene silacyclopropane 13 (0.11 mL, 1.3 M in C6D6,
0.14 mmol) and mesitylene (0.0020 mL, 0.014 mmol, internal
standard). The reaction mixture was monitored by 1H NMR
spectroscopy. After 2 weeks, all of silacyclopropane 13 was consumed
and the reaction mixture was concentrated in vacuo. Purification by
flash chromatography (3:97 EtOAc/hexanes) provided oxasilacyclo-
pentanes 37 as a colorless oil (80:20 mixture of diastereomers, 0.030 g,
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00469.
Additional experimental procedures with analytical and
spectroscopic data, NMR spectra, and stereochemical
proof for oxasilacyclopentane 16 (PDF)
1
66%): H NMR (400 MHz, CDCl3) δ 6.06 (d, J = 10.4, 0.2H, HC
C), 5.95 (dd, J = 2.3, 10.3, 1H, HCC), 5.65−5.52 (m, 4H, HCC),
2.13−2.09 (m, 1H, HC−C), 1.80−1.67 (m, 3.6H, HC−C minor and 3
CH2 minor, 3 CH2 major), 1.64−1.60 (m, 2H, CH2), 1.54−1.44 (m,
3.6H, HC−Si minor and 3 CH2 minor, 3 CH2 major), 1.40−1.34 (m,
1.4H, HC−Si major and CH2 minor), 1.19 (s, 9H, t-Bu), 1.14 (s, 1.8H,
t-Bu), 1.11 (s, 1.8H, t-Bu), 1.08 (s, 9H, t-Bu), 1.00 (s, 3H, CH3), 0.993
(s, 0.6H, CH3), 0.990 (s, 0.6H, CH3), 0.98 (s, 3H, CH3); 13C NMR
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kwoerpel@nyu.edu
Notes
The authors declare no competing financial interest.
F
DOI: 10.1021/acs.organomet.6b00469
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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ACKNOWLEDGMENTS
■
This research was supported by the National Science
Foundation (CHE-1362709). NMR spectra collected using
the TCI CryoProbe at NYU were supported by an S10 grant
from the National Institutes of Health (OD016343). We thank
Dr. Chin Lin (NYU) for his assistance with NMR spectroscopy
and mass spectrometry. We thank Jillian Sanzone (NYU) for
insightful discussions.
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1
(30) The H NMR spectra suggest that hydroquinone 31 exists in a
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compared to other substrates. The origin of this preference may lie in
stabilization of the resulting silacarbonyl ylide 44. This ylide would
develop positive charge on the carbon atom bearing R and R1.
Considering benzaldehyde (2), it is likely that developing positive
charge at a benzylic center would be favored over generating positive
charge at an oxygen atom of the benzoquinone. This analysis is
supported by proton affinities, which show that benzaldehyde (2) is
more basic than benzoquinone (11) in the gas phase: Hunter, E. P. L.;
Lias, S. G. J. Phys. Chem. Ref. Data 1998, 27, 413−656.
(35) Two attempts at combustion analysis did not provide
satisfactory results.
(36) All attempts to purify 21 resulted in decomposition.
(37) All attempts to purify 26 resulted in decomposition.
(38) Ager, B. J.; Bourque, L. E.; Buchner, K. M.; Woerpel, K. A. J.
Org. Chem. 2010, 75, 5729−5732.
(13) (a) Masterson, D. S.; Porter, N. A. Org. Lett. 2002, 4, 4253−
4256. (b) Bourque, L. E.; Haile, P. A.; Loy, J. M. N.; Woerpel, K. A.
Tetrahedron 2009, 65, 5608−5613.
(14) (a) Wierschke, S. G.; Chandrasekhar, J.; Jorgensen, W. L. J. Am.
Chem. Soc. 1985, 107, 1496−1500. (b) Lambert, J. B. Tetrahedron
1990, 46, 2677−2689.
G
DOI: 10.1021/acs.organomet.6b00469
Organometallics XXXX, XXX, XXX−XXX