Silylene transfer to allylic sulfides: Formation of substituted silacyclobutanes

Article abstract of DOI:10.1021/jo1008228

Silylene transfer to allylic sulfides results in a formal 1,2-sulfide migration. The rearrangement yields substituted silacyclobutanes, not the expected silacyclopropanes. The silacyclobutanes were elaborated by insertions of carbonyl compounds selectively into one carbon-silicon bond. A mechanism for the 1,2-sulfide migration is proposed involving an episulfonium ion intermediate.

Full text of DOI:10.1021/jo1008228

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TABLE 1. Metal Catalysts Utilized for Silylene Transfer to Allylic
Sulfide 1a
Silylene Transfer to Allylic Sulfides: Formation
of Substituted Silacyclobutanes
Bryan J. Ager, Laura E. Bourque, Kay M. Buchner,
and K. A. Woerpel*
entry
catalyst
T (°C)
yield (%)a
Department of Chemistry, University of California, Irvine,
California 92697-2025
1
2
3
4
5
6
7
8
AgO2CCF3
AgOTf
Ag3PO4
22
22
22
22
22
22
22
70
72
61
70
42
63
35
58
43
(CuOTf)2 PhMe
3
kwoerpel@uci.edu
CuBr
CuI
Received April 27, 2010
AuBr3
none
aAs determined by 1H NMR spectroscopic analysis relative to an
internal standard (PhSiMe3).
TABLE 2. Silylene Transfer to Substituted Allylic Sulfides
Silylene transfer to allylic sulfides results in a formal 1,2-
sulfide migration. The rearrangement yields substituted
silacyclobutanes, not the expected silacyclopropanes. The
silacyclobutanes were elaborated by insertions of carbo-
nyl compounds selectively into one carbon-silicon bond.
A mechanism for the 1,2-sulfide migration is proposed
involving an episulfonium ion intermediate.
entry
substrate
R1
R2
product
dra,b
yield (%)a
1
2
3
4
1b
1c
1d
1e
H
H
Me
Me
Me
i-Pr
H
3b
3c
3d
3e
92:8
100:0
86:14
66:34
41
17
55
19c
Me
aAs determined by 1H NMR spectroscopic analysis relative to an
internal standard (PhSiMe3). bRelative stereochemistry was determined
by NOE analysis of the products. Details are provided as Supporting
Information. cFive equivalents of cyclohexene silacyclopropane 2 were
required for this reaction to proceed to completion.20
Silacyclobutanes are unique strained-ring compounds1 that
have been used in a variety of chemical applications, including
ring-opening polymerization.2 These silanes are synthesized con-
ventionally by intramolecular Wurtz-type coupling3-6 or [2þ2]
cycloaddition reactions,7,8 making additional synthetic routes
desirable. Unlike three-membered-ring silanes, silacyclobutanes
are often air stable, which facilitates their handling and mani-
pulation.9 Carbonyl insertions with strained silacyclobutanes
provide a variety of synthetically useful transformations.6,10-13
In the course of exploring silylene transfer reactions, we dis-
covered that our silylene transfer conditions14-16 provided a
synthesis of four-membered-ring silanes when applied to allylic
sulfides (Table 1). In contrast to metal-catalyzed silylene inser-
tion reactions with allylic ethers,17,18 no carbon-sulfur bond
insertion products were observed. Instead, the products resulted
from formal 1,2-sulfur migration.19 A variety of metal salts
catalyzed the silylene transfer reaction at ambient temperature
in moderate yields. AgO2CCF3 was found to be the optimal
catalyst (entry 1). Silylene transfer with silacyclobutane forma-
tion occurred in the absence of catalyst, but elevated tempera-
tures were required (entry 8).
(1) Gordon, M. S.; Boatz, J. A.; Walsh, R. J. J. Phys. Chem. 1989, 93,
1584–1585.
(2) Matsumoto, K.; Shimazu, H.; Deguchi, M.; Yamaoka, H. J. Polym.
Sci., Part A: Polym. Chem. 1997, 35, 3207–3216.
(3) Laane, J. J. Am. Chem. Soc. 1967, 89, 1538–1540.
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G. T. J. Organomet. Chem. 1972, 43, 117–120.
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Weber, W. P. J. Organomet. Chem. 1990, 391, 7–12.
(6) Okada, K.; Matsumoto, K.; Oshima, K.; Utimoto, K. Tetrahedron
Lett. 1995, 36, 8067–8070.
The reaction was general for a variety of substituted allylic
sulfides, although yieldswere lower (Table2). Silylene transfer
to R-methyl-substituted allylic sulfide 1b afforded silacy-
clobutane trans-3b with high diastereoselectivity (entry 1).
(7) Sewald, N.; Ziche, W.; Wolff, A.; Auner, N. Organometallics 1993, 12,
4123–4134.
(8) Auner, N.; Heikenwaelder, C.-R.; Herrschaft, B. Organometallics
2000, 19, 2470–2476.
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56, C7–C10.
(10) Tanaka, Y.; Yamashita, H.; Tanaka, M. Organometallics 1996, 15,
1524–1526.
(11) Takeyama, Y.; Oshima, K.; Utimoto, K. Tetrahedron Lett. 1990, 31,
6059–6062.
(12) Hirano, K.; Yorimitsu, H.; Oshima, K. Org. Lett. 2008, 10, 2199–2201.
(13) Hirano, K.; Yorimitsu, H.; Oshima, K. Org. Lett. 2006, 8, 483–485.
ꢀ
ꢀ
(14) Cirakovic, J.; Driver, T. G.; Woerpel, K. A. J. Org. Chem. 2004, 69,
4007–4012.
(15) Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2003, 125, 10659–
10663.
(16) Clark, T. B.; Woerpel, K. A. J. Am. Chem. Soc. 2004, 126, 9522–
9523.
(17) Bourque, L. E.; Cleary, P. A.; Woerpel, K. A. J. Am. Chem. Soc.
2007, 129, 12602–12603.
(18) Bourque, L. E.; Haile, P. A.; Loy, J. M. N.; Woerpel, K. A.
Tetrahedron 2009, 65, 5608–5613.
(19) Sromek, A. W.; Gevorgyan, V. Top. Curr. Chem. 2007, 274, 77–124.
DOI: 10.1021/jo1008228
r
Published on Web 07/23/2010
J. Org. Chem. 2010, 75, 5729–5732 5729
2010 American Chemical Society

Ager et al.
JOCNote
An increase in steric hindrance at the R position was observed
to increase diastereoselectivity in the reaction, but resulted in
low yield of the desired silacyclobutane (entry 2). Silylene
transfer to crotyl sulfide 1d gave silacyclobutane trans-3d with
lower diastereoselectivity (entry 3). When the reaction was
attempted with R-methyl substituted crotyl sulfide1e, product
3e was obtained in low yield and with low diastereoselectivity
(entry 4).
TABLE 3. Carbonyl Insertion Reactions with Silacyclobutane 3a
entry
R
catalyst
product
yield (%)a
dra
To assess if other functional groups could undergo 1,2-
migration,21-23 the silylene transfer conditions were applied
to allylic silane 4 and allyl bromide. Silylene transfer to allylic
silane 4 did not provide 1,2-silyl migration and afforded
silacyclopropane 5, which was subjected to the two-step,
one-flask carbonyl insertion reaction24 to afford a mixture of
products 6a-c (Scheme 1). Allylic silane 6a can result from
hydrogen atom transfer processes,25,26 and dioxasilacyclo-
pentane 6c is the product of silylene transfer to 2 equiv of
benzaldehyde.27,28 The same conditions with allyl bromide
resulted in decomposition of the starting materials.
1
2
3
4
H
H
H
Me
(CuOTf)2 PhMe
3
11a
11a
11a
11b
68
70
70
63
100:0
100:0
100:0
96:4
ZnI2
Zn(OTf)2
ZnI2
aAs determined by 1H NMR spectroscopic analysis relative to an
internal standard (PhSiMe3).
with the electrophilic silylenoid15,34,35 to give intermediate 10.
The silver-bound silyl species could open the episulfonium ion
concurrent to four-membered ring closure to afford silacyclo-
butane 3 (Scheme 2). The trace crossover products could arise
from loss and recombination of the sulfide group on inter-
mediate 10.36
SCHEME 1. Silylene Transfer to an Allylic Silane
A crossover experiment was performed with allylic sul-
fides 1b and 1f to provide insight into the mechanism for the
allylic sulfide rearrangement (eq 1). Only trace amounts of
crossover products were observed, indicating that the 1,2-
sulfide migration occurs through a stepwise mechanism in
which fragments largely combine intramolecularly. The
likely mechanism involves episulfonium ion formation29-33
SCHEME 2. Episulfonium Ion Formation in the 1,2-Sulfide
Migration Mechanism
(20) Silacyclobutanes trans-3e and cis-3e were the only identifiable pro-
ducts of this reaction. Details are provided as Supporting Information.
(21) Suginome, M.; Takama, A.; Ho, Y. J. Am. Chem. Soc. 1998, 120,
1930–1931. and references cited therein.
(22) Ishikawa, M.; Nakagawa, K.-I.; Kumada, M. J. Organomet. Chem.
1981, 214, 277–288.
(23) Miyake, H.; Yamamura, K. Bull. Chem. Soc. Jpn. 1988, 61, 3752–
3754.
(24) Buchner, K. M.; Clark, T. B.; Loy, J. M. N.; Nguyen, T. X.; Woerpel,
K. A. Org. Lett. 2009, 11, 2173–2175.
(25) Seyferth, D.; Duncan, D. P.; Shannon, M. L. Organometallics 1984,
3, 579–583.
(26) Bodnar, P. M.; Palmer, W. S.; Ridgway, B. H.; Shaw, J. T.; Smi-
trovich, J. H.; Woerpel, K. A. J. Org. Chem. 1997, 62, 4737–4745.
(27) Bourque, L. E.; Woerpel, K. A. Org. Lett. 2008, 10, 5257–5260.
(28) Lim, Y. M.; Park, C. H.; Yoon, S. J.; Cho, H. M.; Lee, M. E.; Baeck,
K. K. Organometallics 2010, 29, 1355–1361.
(29) Bland, J. M.; Stammer, C. H. J. Org. Chem. 1983, 48, 4393–4394.
(30) Auvray, P.; Knochel, P.; Normant, J. F. Tetrahedron 1988, 44, 4495–
4508.
(31) Hirabayashi, K.; Sato, H.; Kuriyama, Y.; Matsuo, J.-i.; Sato, S.;
Shimizu, T.; Kamigata, N. Chem. Lett. 2007, 36, 826–827.
(32) Kim, J. T.; Kel’in, A. V.; Gevorgyan, V. Angew. Chem., Int. Ed. 2003,
42, 98–101.
Carbonyl insertion reactions were performed with sila-
cyclobutane 3a to afford oxasilacyclohexanes as single
diastereomers (Table 3).6,11-13 Various metal catalysts
were employed in the insertion reaction with silacyclobu-
tane 3a, and zinc catalysts were observed to be optimal
(entry 2). No reaction was observed in the absence of
catalyst. Benzaldehyde and acetophenone were competent
in the insertion reaction, but both linear and branched
aliphatic aldehydes and ketones were not.25 The observed
diastereoselectivity could occur to minimize the unfavor-
able 1,3-diaxial interaction that would arise between the
phenyl group of the carbonyl and a tert-butyl group on
silicon.
(33) Peng, L.; Zhang, X.; Zhang, S.; Wang, J. J. Org. Chem. 2007, 4, 1192–
1197.
(34) Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2004, 126, 9993–
10002.
(35) Mayoral, J. A.; Rodrı
Chem. 2010, 1231–1234.
(36) Schmid, G. H.; Fitzgerald, P. H. J. Am. Chem. Soc. 1971, 93, 2547–
2548.
guez-Rodrıguez, S.; Salvatella, L. Eur. J. Org.
´ ´
5730 J. Org. Chem. Vol. 75, No. 16, 2010

Ager et al.
JOCNote
TABLE 4. Carbonyl Insertion Reactions with Silacyclobutane 3b
did not undergo carbonyl insertion when subjected to the
zinc-catalyzed conditions.46
In conclusion, metal-catalyzed silylene transfer conditions
were utilized in a rearrangement reaction with allylic sulfides
to afford silacyclobutanes. The resulting four-membered-
ring compounds were subjected to carbonyl insertion reac-
tion conditions to afford substituted oxasilacyclohexanes.
A mechanism for the 1,2-sulfide migration was proposed
utilizing a sulfonium ion intermediate.
entry
catalyst
yield 12a (%)a
1
2
3
(CuOTf)2 PhMe
3
25
35
29
ZnI2
Zn(OTf)2
aAs determined by 1H NMR spectroscopic analysis relative to an
internal standard (PhSiMe3).
The increased ring substitution on silacyclobutane 3b
decreased the efficiency of the carbonyl insertion reaction
(Table 4). The most competent catalyst in the benzaldehyde
insertion reaction was ZnI2. The yield was moderate, how-
ever, and isolation of the product proved difficult due to the
significant number of unidentifiable decomposition pro-
ducts. Carbonyl insertion occurred into the more substituted
carbon-silicon bond in moderate yield to afford oxasilacy-
clohexane 12a as a single diastereomer. Allyl silane 12b was
observed as a minor product resulting from metal-mediated
rearrangement of the starting material.37 The regiochemistry
is in contrast to that observed for analogous zinc-catalyzed
carbonyl insertions into silacyclopropanes.38 The formation
of allylic silane 14 from silacyclobutane 3a in the absence of
aldehyde supports the existence of transmetalation inter-
mediate 13 (eq 2). The regioselectivity of this reaction is
analogous to the copper-mediated transmetalation observed
for silacyclopropanes.39,40
Experimental Section
Silylene Transfer to Allylic Sulfides (Procedure A). Silacy-
clobutane 3a. To a solution of allylic sulfide 1a (5.00 g, 33.3
mmol) in toluene (140 mL) was added cyclohexene silacyclo-
propane 215,47 (8.96 g, 39.9 mmol) and AgO2CCF3 (0.073 g, 0.33
mmol). The reaction mixture was stirred overnight, filtered
through a pad of SiO2 with hexanes to remove residual catalyst,
and concentrated in vacuo. Purification by column chromatog-
raphy (hexanes) afforded silacyclobutane 3a as a white solid (6.2
g, 64%): mp 36-38 °C; 1H NMR (400 MHz, C6D6) δ 7.40 (d,
J = 7.2 Hz, 2H), 7.10 (t, J = 7.6 Hz, 2H), 6.98 (t, J = 7.3 Hz,
1H), 3.88 (quint, J = 9.2 Hz, 1H), 1.57-1.50 (m, 2H), 1.32-1.25
(m, 2H), 0.93 (s, 9H), 0.92 (s, 9H); 13C NMR (125 MHz, CDCl3)
δ 137.5, 130.0, 128.7, 125.8, 37.8, 28.0, 27.8, 19.3, 18.9, 18.7; 29Si
NMR (99.3 MHz, CDCl3) δ 18.9; IR (thin film) 3082, 3059,
2935, 2854, 1583, 1464 cm-1; HRMS (GC-MS) m/z calcd for
C17H29SSi (M þ H)þ 293.1759, found 293.1759. Anal. Calcd for
C17H28SSi: C, 69.79; H, 9.65. Found: C, 69.89; H, 9.65.
Silacyclobutane 3b. Procedure A was employed using allylic
sulfide 1b (4.01 g, 24.4 mmol), AgO2CCF3 (0.53 g, 2.4 mmol),
and cyclohexene silacyclopropane 215,47 (16.4 g, 73.1 mmol).
Purification by column chromatography (hexanes) afforded a
mixture of silacyclobutanes trans-3b and cis-3b (83:17 dr) as a
pale yellow oil (1.0 g, 15%). Silacyclobutane trans-3b: 1H NMR
(400 MHz, C6D6) δ 7.53 (d, J = 7.1 Hz, 2H), 7.08 (t, J = 7.4 Hz,
2H), 7.00 (t, J = 7.3 Hz, 1H), 3.51 (dt, J = 10.6, 9.8 Hz, 1H),
1.66 (dq, J = 11.3, 7.3 Hz, 1H), 1.54 (dd, J = 14.6, 8.9 Hz, 2H),
1.26 (d, J = 7.4 Hz, 3H), 0.98 (s, 9H), 0.85 (s, 9H); 13C NMR
(125 MHz, C6D6) δ 136.4, 132.57, 128.6, 126.4, 48.4, 29.3, 28.4,
28.1, 19.8, 19.5, 18.2, 14.5; 29Si NMR (119.2 MHz, C6D6) 17.1;
IR (neat) 3059, 2931, 2858, 1583, 1471, 1265 cm-1; HRMS (GC-
MS) m/z calcd for C18H31SSi (M þ H)þ 307.1916, found
To increase the general utility of the 1,1-di-tert-butyl-3-
thiophenyl-1-silacyclobutanes, lithiation reactions were per-
formed to see if the sulfide moiety could be functionalized
selectively. Silacyclobutane 3a underwent lithium-sulfide
exchange41-45 followed by trapping with chlorotrimethylsi-
lane to afford silacyclobutane 15 (eq 3). Silacyclobutane 15
1
307.1912. Silacyclobutane cis-3b: H NMR (400 MHz, C6D6,
(37) Allyl silane 12b was isolated in 6% yield when the reaction was
performed on preparative scale, although it was present in such small
amounts that it was not observed by 1H NMR spectroscopic analysis of
the unpurified reaction mixture.
(38) Franz, A. K.; Woerpel, K. A. Angew. Chem., Int. Ed. 2000, 39, 4295–
4299.
(39) Franz, A. K.; Woerpel, K. A. J. Am. Chem. Soc. 1999, 121, 949–957.
(40) The thiophenyl silane analogue of compound 14 was observed as a
minor product due to rearrangement of the starting material . Details are
provided as Supporting Information.
(41) Screttas, C. G.; Micha-Screttas, M. J. Org. Chem. 1979, 44, 713–719.
(42) Zhu, S.; Cohen, T. Tetrahedron 1997, 53, 17607–17624.
(43) Yus, M.; Herrera, R. P.; Guijarro, A. Chem.;Eur. J. 2002, 8, 2574–
2584.
distinctive peaks) δ 7.34 (d, J = 7.3 Hz, 2H), 4.19 (q, J = 9.7 Hz,
1H), 2.14-2.05 (m, 1H), 1.37 (d, J = 8.2 Hz, 3H), 1.01 (s, 9H),
0.96 (s, 9H); 13C NMR (125 MHz, C6D6, distinctive peaks) δ
135.3, 132.56, 128.7, 124.7, 42.9, 28.7, 28.0, 17.2, 12.9.
Silacyclobutane 3c. Procedure A was employed using allylic
sulfide 1c (0.55 mL, 0.24 M in C6D6, 0.13 mmol), AgO2CCF3
(0.001 g, 0.005 mmol), and cyclohexene silacyclopropane 215,47
(0.085 g, 0.38 mmol). After 24 h, the reaction had afforded
silacyclobutane 3c as a single diastereomer in 17% yield as
1
determined by H NMR spectroscopy relative to an internal
standard (PhSiMe3) using a single scan: H NMR (400 MHz,
1
C6D6, distinctive peaks) δ 3.57 (q, J = 9.5 Hz, 1H), 0.94 (s, 9H),
0.88 (s, 9H); LRMS (GC-MS) Rt = 19.3 min; m/z calcd for
C20H34SSi (M)þ 334.22, found 334.17; m/z calcd for C16H25SSi
(M - C4H9)þ 277.14, found 277.19.
(44) Screttas, C. G.; Heropoulos, G. A.; Micha-Screttas, M.; Steele, B. R.;
Catsoulacos, D. P. Tetrahedron Lett. 2003, 44, 5633–5635.
(45) Streiff, S.; Ribiero, N.; Desaubry, L. J. Org. Chem. 2004, 69, 7592–
7598.
(46) The lithiation reaction with silacyclobutane 3a suffered from irre-
producibility. Attempts to trap the alkyllithium intermediate with a carbon
electrophile were unsuccessful, as were attempts to lithiate the sulfide
functionality on oxasilacyclohexane 11a.
(47) Boudjouk, P.; Samaraweera, U.; Sooriyakumaran, R.; Chrusciel, J.;
Anderson, K. R. Angew. Chem., Int. Ed. Engl. 1988, 27, 1355–1356.
J. Org. Chem. Vol. 75, No. 16, 2010 5731

Ager et al.
JOCNote
Silacyclobutane 3d (same as compound 3b). Procedure A was
employed using crotyl sulfide 1d (0.62 g, 3.7 mmol), cyclohexene sila-
cyclopropane 215,47 (2.53 g, 11.3 mmol), and AgO2CCF3 (0.038 mg,
0.19 mmol). Purification by column chromatography (hexanes;
1:99 EtOAc/hexanes) afforded silacyclobutanes trans-3d and cis-3d
(80:20 dr) as a colorless oil (0.28 g, 24%). Full characterization data
were reported for silacyclobutane 3b, vide supra.
Oxasilacyclohexane 12a and Allyl Silane 12b. Procedure B was
employed using silacyclobutane 3b (0.319 g, 1.04 mmol, 83:17 dr),
benzaldehyde (0.32 mL, 3.1 mmol), and ZnI2 (0.032 g, 0.10 mmol).
The reaction mixture was heated to 70 °C. Benzaldehyde was
removed in vacuo at 100 °C (0.4 mmHg). Purification by column
chromatography (1:199 EtOAc/hexanes) afforded oxasilacyclo-
hexane 12a as a white solid (0.072 g, 16%) and allyl silane 12b as a
colorless oil (0.019 g, 6%). Oxasilacyclohexane 12a: mp 80-82 °C;
1H NMR (500 MHz, CDCl3) δ 7.49 (d, J=7.7 Hz, 2H), 7.40-7.34
(m, 8H), 4.61 (d, J=9.5 Hz, 1H), 3.58 (ddd, J=11.9, 10.5, 4.4 Hz,
1H), 1.88 (tq, J=10.3, 6.7 Hz, 1H), 1.40 (dd, J=14.9, 4.5 Hz, 1H),
1.15 (dd, J=15.1, 11.9 Hz, 1H), 1.08 (s, 9H), 1.04 (s, 9H), 0.90 (d,
J=6.7 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 144.6, 135.2,
132.8, 128.9, 128.2, 127.6, 127.2, 127.0, 84.4, 53.0, 45.7, 28.4, 27.6,
22.2, 20.1, 17.6, 16.2; IR (thin film) 3070, 2929, 2856, 1581, 1471,
1387 cm-1; HRMS (GC-MS) m/z calcd for C25H37OSSi (M þ H)þ
413.2334, found 413.2320. Anal. Calcd for C25H36OSSi: C, 72.76;
H, 8.79. Found: C, 72.48; H, 8.91. Allyl Silane 12b: 1H NMR (500
MHz, CDCl3) δ 7.60-7.28 (m, 5H), 5.48 (dt, J = 15.1, 7.5 Hz,
1H), 5.37 (dq, J = 15.0, 6.3 Hz, 1H), 1.80 (d, J = 7.6 Hz, 2H), 1.67
(d, J = 6.3 Hz, 3H), 1.13 (s, 18H); 13C NMR (125 MHz, CDCl3) δ
136.2, 136.1, 128.7, 126.93, 126.87, 125.1, 28.9, 18.3, 18.1; IR (neat)
3072, 2933, 1768, 1583, 1473, 1389 cm-1; HRMS (GC-MS) m/z
calcd for C18H34NSSi (M þ NH4)þ 324.2181, found 324.2171.
Lithiation of the Sulfide Functionality. Silacyclobutane 15. A
procedure reported by Cohen42 was adapted to prepare silacy-
clobutane 15. A cooled (-78 °C) solution of Li pellets (0.030 g,
4.3 mmol) in THF (1.0 mL) was prepared under an atmosphere
of argon. A solution of naphthalene (0.014 g, 0.11 mmol) in
THF (1.0 mL) was added, followed by a solution of silacy-
clobutane 3a (0.24 g, 0.80 mmol) in THF (1.5 mL). After 2 h at
-78 °C, Me3SiCl (0.15 mL, 1.2 mmol) was added to the reaction
mixture. After 2 h, the reaction mixture was warmed to 22 °C
and diluted with pentane (10 mL). An aqueous solution of
saturated NH4Cl was added and the layers were separated.
The organic layer was washed with H2O and brine, dried over
Na2SO4, and concentrated in vacuo. Purification by column
chromatography (hexanes) afforded silacyclobutane 15 as a pale
Silacyclobutane 3e. Procedure A was employed using crotyl
sulfide 1e (0.372 g, 2.09 mmol), cyclohexene silacyclopropane 215,47
(2.36 g, 10.5 mmol), and AgO2CCF3 (0.023 g, 0.10 mmol). The
crude mixture contained a mixture of silacyclobutanes trans-3e/
cis-3e (66:34 dr). Purification by column chromatography
(hexanes;1:99 EtOAc/hexanes) afforded an impure yellow oil
containing silacyclobutane cis-3e (0.06 g, 10%) in addition to an
impure mixture containing silacyclobutanes trans-3e/cis-3e
(55:45 dr) as a yellow oil (0.22 g, 32%). Silacyclobutane trans-
3e: 1H NMR (400 MHz, C6D6, distinctive peaks) δ 7.58 (dd, J =
8.0, 2.0 Hz, 2H), 3.07 (t, J = 11.5 Hz, 1H). Silacyclobutane cis-
3e: 1H NMR (400 MHz, C6D6, distinctive peaks) δ 7.37 (d, J =
7.3 Hz, 2H), 7.07 (t, J = 7.7 Hz, 2H), 3.96 (dd, J = 12.6, 9.3 Hz,
1H), 2.06 (quint, J = 8.8 Hz, 1H), 1.87-1.78 (m, 1H), 1.36 (d,
J = 8.2 Hz, 3H), 1.29 (d, J = 5.7 Hz, 3H),48 1.03 (s, 9H), 0.98 (s,
9H); 29Si NMR (119.2 MHz, C6D6) δ 14.1; IR (neat) 3059, 2935,
2860, 1581, 1473, 1363 cm-1; HRMS (GC-MS) m/z calcd for
C19H31SSi (M - H)þ 319.1916, found 319.1920.
Carbonyl Insertion into Silacyclobutanes (Procedure B). Oxa-
silacyclohexane 11a. To a solution of silacyclobutane 3a (1.4 g, 4.8
mmol) in toluene (22 mL) was added benzaldehyde (1.5 mL, 15
mmol) and ZnI2 (0.16 g, 0.50 mmol). The reaction mixture was
heated to 100 °C. After 30 h, the reaction mixture was filtered
through a pad of SiO2/Celite (1:9) with hexanes to remove residual
catalyst and concentrated in vacuo. Benzaldehyde was removed in
vacuo at 100 °C (0.4 mmHg). Purification by column chromatog-
raphy (1:99 EtOAc/hexanes) afforded oxasilacyclohexane 11a as a
1
white solid (0.69 g, 36%): mp 56-58 °C; H NMR (500 MHz,
CDCl3) δ 7.42 (d, J = 8.1 Hz, 2H), 7.35-7.32 (m, 6H), 7.28-7.24
(m, 2H), 4.99 (appar d, J=11.2 Hz, 1H), 3.73 (tdd, J=12.7, 4.2, 2.3
Hz, 1H), 2.17 (appar dd, J=13.9, 1.7 Hz, 1H), 1.60 (dt, J=13.7,
11.7 Hz, 1H), 1.34 (ddd, J=14.4, 4.5, 2.1 Hz, 1H), 1.09 (s, 9H), 1.07
(s, 9H), 0.89 (appar t, J= 13.8 Hz, 1H); 13C NMR (125 MHz,
CDCl3) δ 145.0, 134.6, 132.4, 129.0, 128.3, 127.2, 127.1, 125.2, 77.3,
45.0, 44.7, 28.2, 27.6, 22.2, 20.2, 13.8; IR (thin film) 3061, 2931,
1601, 1583, 1471, 1092 cm-1; HRMS (GC-MS) m/z calcd for
C24H35OSSi (M þ H)þ 399.2178, found 399.2180. Anal. Calcd
for C24H34OSSi: C, 72.30; H, 8.60. Found: C, 72.04; H, 8.59.
Oxasilacyclohexane 11b. Procedure B was employed using
silacyclobutane 3a (0.29 g, 1.0 mmol), acetophenone (0.35 mL, 3.0
mmol), and ZnI2 (0.035 g, 0.11 mmol). Acetophenone was removed
in vacuo at 100 °C (0.4 mmHg). Purification by column chroma-
tography (1:99 EtOAc/hexanes) afforded oxasilacyclohexane 11b
as a colorless oil (0.22 g, 54%): 1H NMR (500 MHz, CDCl3) δ 7.44
(d, J=7.2 Hz, 2H), 7.41 (d, J=7.3 Hz, 2H), 7.36 (t, J=7.6 Hz, 2H),
7.31-7.26 (m, 3H), 7.20 (t, J=7.3 Hz, 1H), 3.78 (appar td, J=
12.7, 3.5 Hz, 1H), 2.45 (appar d, J=13.7 Hz, 1H), 1.85 (appar t, J=
12.9 Hz, 1H), 1.46 (s, 3H), 1.30 (ddd, J=14.2, 4.2, 2.2 Hz, 1H), 1.10
(s, 9H), 0.87 (s, 9H), 0.78 (appar t, J=13.8 Hz, 1H); 13C NMR (125
MHz, CDCl3) δ 150.4, 135.2, 131.5, 129.0, 127.9, 126.9, 126.3,
124.2, 76.4, 45.2, 40.6, 32.0, 27.6, 27.4, 20.9, 20.6, 14.6; IR (neat)
3058, 2967, 1601, 1583, 1471, 1011 cm-1; HRMS (GC-MS) m/z
calcd for C25H37OSSi (M þ H)þ 413.2334, found 413.2328. Anal.
Calcd for C25H36OSSi: C, 72.76; H, 8.79. Found: C, 72.55; H, 8.85.
1
yellow oil (0.15 g, 70%): H NMR (500 MHz, CDCl3) δ 1.44
(quint, J=10.8 Hz, 1H), 1.10 (s, 9H), 0.99 (s, 9H), 0.89 (d, J=
10.7 Hz, 4H), -0.04 (s, 9H); 13C NMR (125 MHz, CDCl3) δ
28.0, 27.7, 20.3, 19.2, 17.3, 8.5, -3.6; IR (neat) 2951, 2858, 1470,
1248, 1115, 829 cm-1; HRMS (GC-MS) m/z calcd for
C14H36NSi2 (M þ NH4)þ 274.2386, found 274.2388.
Acknowledgment. This research was supported by the
National Institute of General Medical Sciences of the Na-
tional Institutes of Health (GM-54909). K.M.B. and L.E.B.
thank the Department of Education (GAANN) for predoc-
toral fellowships. B.J.A. thanks the National Science Foun-
dation (Chem-SURF) for undergraduate research support.
K.A.W. thanks Amgen and Lilly for awards to support
research. We thank Dr. P. Dennison (UCI) for assistance
with NMR spectroscopy and Dr. J. Greaves and Ms. S.
Sorooshian (UCI) for mass spectroscopy.
Supporting Information Available: General experimental
information and additional experimental procedures and
spectroscopic and analytical data for the products. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(48) An NOE enhancement was applied to resolve the splitting of this peak.
5732 J. Org. Chem. Vol. 75, No. 16, 2010