Metal-catalyzed silacyclopropanation of mono- and disubstituted alkenes

Article abstract of DOI:10.1021/ja020566i

As an alternative to the strongly reducing conditions necessary for the formation of silacyclopropanes, silylene transfer was developed as a mild, functional group tolerant method of silacyclopropanation. Complex silacyclopropanes were formed from functio

Full text of DOI:10.1021/ja020566i

Published on Web 07/19/2002
Metal-Catalyzed Silacyclopropanation of Mono- and Disubstituted Alkenes
Jelena CÄ irakovic´, Tom G. Driver, and K. A. Woerpel*
Department of Chemistry, UniVersity of California, IrVine, California 92697-2025
Received April 22, 2002
Table 1. Silacyclopropanation of Monosubstituted Alkenes (eq 1)
Silacyclopropanes are highly reactive compounds that undergo
a number of stereoselective carbon-carbon bond-forming reac-
tions.1 The most general method for forming silacyclopropanes
involves the reaction of an alkene with a free silylene or a silylenoid
intermediate. The substrate compatibility of alkene silacyclopro-
panation, however, is limited because the generation of a silylene
species requires the use of elevated temperatures, strongly reducing
conditions, or photolysis.2-4 To develop milder conditions for alkene
silacyclopropanation, we hoped to employ a catalyst to generate
and transfer silylenes.5 While metal-catalyzed reactions are com-
monly employed for the synthesis of three-membered rings from
alkenes,6,7 the catalytic silacyclopropanation of alkenes is un-
known. In this communication we report a metal-catalyzed silacy-
clopropanation of mono- and disubstituted alkenes that proceeds
at -27 °C. This transformation allows for the one-step conversion
of alkenes to oxasilacyclopentanes.
Since t-Bu2Si can be generated thermally from silacyclopropane
1,2 we sought to promote this reaction at reduced temperatures
through the use of a catalyst (eq 1). A survey of metal salts as
possible promoters of silylene transfer was undertaken. Several salts
reduced the temperature of silylene transfer to room temperature
or below, including those of CeCl3, Cu(OTf)2, and (CuOTf)2-
benzene. The transfer could be promoted at temperatures as low
as -27 °C with silver triflate as catalyst.8
a As determined by 1H NMR spectroscopic analysis of the product
mixture relative to an internal PhSi(CH3)3 standard. b An excess (ca. 5 equiv)
of alkene was used.
Table 2. Silacyclopropanation of Disubstituted Olefins
Once silver triflate was identified as the most active catalyst,
the effects of solvent, concentration, temperature, and catalyst
loading were examined. A screen of solvents revealed that toluene
and methylene chloride afforded the best yields and allowed the
lowest temperatures for transfer. The yield of the transfer did not
vary with concentrations between 0.1 and 0.5 M. As little as 1
mol % AgOTf effected complete silylene transfer in 2 h at -27
°C. Lower catalyst loadings required higher temperatures (room
temperature) and increased reaction times.
a As determined by 1H NMR spectroscopic analysis of product mixture
relative to an internal standard of PhSi(CH3)3 or 1,3 dimethoxybenzene.
b An excess of alkene was used. c One diastereomer visible by 1H and 29Si
NMR spectroscopy (dr g95:5).
A variety of silacyclopropanes were formed from monosubsti-
tuted alkenes using the silver triflate-catalyzed conditions (eq 1,
Table 1).8 Because of their increased sensitivity under the reaction
conditions, the silacyclopropanes were not isolated.9 The transfer
of the silylenoid species was not affected by the size of the alkene
substituent (entries 1-3). The functional group tolerance of the
silver triflate-catalyzed silacyclopropanation is illustrated in entries
5-10. Aryl, benzyl, and silyl ethers are compatible with the reaction
conditions, and even primary and aryl pivaloate esters (entries 7
and 10) afforded high yields of the corresponding silacyclopropanes.
In addition to silacyclopropanes 2, a range of silacyclopropanes
can be formed from disubstituted alkenes under the silver triflate
conditions (Table 2). This reaction is stereospecific, since trans-
and cis-2-butene (entries 1 and 2) afford solely the corresponding
* To whom correspondence should be addressed. E-mail: kwoerpel@uci.edu.
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J. AM. CHEM. SOC. 2002, 124, 9370-9371
10.1021/ja020566i CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
as compared to previous methods used to form silacyclopropanes.2-4,11
The product oxasilacyclopentanes have been previously shown to
undergo stereoselective carbon-carbon bond-forming reactions
through formation of an oxocarbenium ion and trapping with the
appropriate nucleophile.1,15 The catalytic silver triflate conditions
greatly enhance the synthetic utility of silacyclopropanation by
increasing the availability of functionalized silacyclopropanes.
Table 3. One-Flask Silylene Transfer, Methyl Formate Insertion
(eq 2)
Acknowledgment. This research was supported by the National
Institute of General Medical Sciences of the National Institutes of
Health (GM54909). K.A.W. thanks the Camille and Henry Dreyfus
Foundation, Merck Research Laboratories, Johnson & Johnson, and
the Sloan Foundation for awards to support research. T.G.D. thanks
Eli Lilly and Company for a predoctoral fellowship. We thank Dr.
Phil Dennison for assistance with NMR spectrometry, and Dr. John
Greaves and Dr. John Mudd for mass spectrometry data.
Supporting Information Available: Experimental procedures,
spectroscopic and analytical data for the products (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
References
a As determined by 1H NMR spectroscopic analysis of crude product
mixture. b Isolated yield from alkene after purification by flash chroma-
tography.
(1) Franz, A. K.; Woerpel, K. A. Acc. Chem. Res. 2000, 33, 813-820.
(2) Boudjouk, P.; Black, E.; Kumarathasan, R. Organometallics 1991, 10,
2095-2096.
(3) Boudjouk, P.; Samaraweera, U.; Sooriyakumaran, R.; Chrusciel, J.;
Anderson, K. R. Angew. Chem., Int. Ed. Engl. 1988, 27, 1355-1356.
(4) Schafer, A.; Weidenbruch, M.; Peters, K.; Schnering, H. Angew. Chem.,
Int. Ed. Engl. 1984, 23, 302-303.
trans- and cis-dimethyl silacyclopropanes (3 and 4, respectively).10
The silver-catalyzed silacyclopropanation was also found to be
diastereoselective (entries 3-5). One product diastereomer was
observed in good yield for norbornylene (entry 3). The silylenoid
approaches from the more accessible exo face to provide 5.11
â-Pinene and the substituted cyclopentene 7 also reacted in a
stereoselective manner to afford single diastereomers of 6 and 8
by 1H and 29Si NMR spectroscopy (entries 5 and 6). These
selectivities represent an improvement in the diastereomeric ratios
relative to those obtained under conditions for the thermal transfer
of silylene.11,12
The catalyzed silacyclopropanation reaction could be used as
part of a one-step procedure to convert alkenes into oxasilacyclo-
pentanes without isolation of air-sensitive silacyclopropane inter-
mediates. Silver triflate-catalyzed silacyclopropanation of mono-
substituted alkenes was followed by zinc bromide-catalyzed
insertion of methyl formate into the carbon-silicon bond of the
silacyclopropane intermediate to afford the products 9 (eq 2, Table
3).13,14 High yields were observed over the two-step sequence with
the butyl-, isopropyl-, and benzyl-substituted alkenes (entries 1, 2,
and 4). While the silylene-transfer reaction was not affected by
the steric bulk of alkene, the regioselectivity of the formate insertion
decreased with the large tert-butyl substituent on the silacyclopro-
pane (entry 3).13 The second insertion step of the reaction sequence
was less functional group tolerant than the silylene transfer, but
good yields were observed with primary benzyl and silyl ethers
(entries 5 and 6). Aryl silyl ethers (entry 7) were also tolerated in
this reaction.
(5) For recent examples refer to: (a) Amoroso, D.; Haaf, M.; Yap, G. P. A.;
West, R.; Fogg, D. E. Organometallics 2002, 21, 534-540. (b) Mork, B.
V.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 9702-9703. (c) Sharma,
H. K.; Pannell, K. H. Organometallics 2001, 20, 7-9. (d) Tobita, H.;
Sato, T.; Okazaki, M.; Ogino, H. J. Organomet. Chem. 2000, 611, 314-
322. (e) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Maciejewski, H.
Organometallics 1998, 17, 5599-5601.
(6) For recent examples refer to: (a) Boche, G.; Lohrenz, J. C. W. Chem.
ReV. 2001, 101, 697-756. (b) (aziridination) Dauban, P.; Sanie`re, L.;
Tarrade, A.; Dodd, R. H. J. Am. Chem. Soc. 2001, 123, 7707-7708. (c)
(epoxidation) Daly, A. M.; Renehan, M. F.; Gilheany, D. G. Org. Lett.
2001, 3, 663-666. (d) (cyclopropanation) Davies, H. M. L.; Panaro, S.
A. Tetrahedron 2000, 56, 4871-4880.
(7) Palmer, W. S.; Woerpel, K. A. Organometallics 1997, 16, 4824-4827.
(8) To screen lower reaction temperatures, TMEDA was added to sequester
the metal and prevent further reaction so analysis by 1H NMR spectroscopy
of the reaction mixtures could be performed at ambient temperature.
(9) The silver triflate-catalyzed silacyclopropanation reaction was monitored
by 1H and 29Si NMR spectroscopy with an internal standard of PhSi-
(CH3)3. Refer to Supporting Information for more details.
(10) The identification of the products was determined by correlation with
previously prepared cis- and trans-dimethyl silacyclopropane.
(11) Driver, T. G.; Franz, A. K.; Woerpel, K. A. J. Am. Chem. Soc. 2002,
124, 6524-6525.
(12) The thermolysis of silacyclopropane 1 in the presence of â-pinene and
cyclopentene 7 provided silacyclopropanes 6 and 8 as 70:30 and 94:6
mixtures of diastereomers.
(13) Franz, A. K.; Woerpel, K. A. Angew. Chem., Int. Ed. 2000, 39, 4295-
4299.
(14) Representative Method for the One-Flask Construction of Oxasila-
cyclopentanes from Monosubstituted Alkenes: To a cooled, hetero-
geneous solution (-27 °C) of AgOTf (0.010 g, 0.039 mmol) in 2 mL of
toluene was added 1-hexene (0.125 mL, 0.996 mmol) followed by the
dropwise addition of a solution of 1 (0.210 g, 0.936 mmol) in 1 mL of
toluene. After 2 h the black solution was cooled to -78 °C, and methyl
formate (0.175 mL, 2.83 mmol) and ZnBr2 (0.034 g, 0.15 mmol) were
added. The heterogeneous mixture was allowed to warm to 25 °C over
16 h. After addition of 10 mL of H2O, the mixture was extracted with 3
× 5 mL of Et2O. The combined organic phases were washed with 10 mL
of NaHCO3 (saturated aqueous) and 10 mL of brine. The resulting organic
phase was dried (MgSO4), filtered, and concentrated in vacuo to afford
the oxasilacyclopentane 9 as a 76:24 mixture of diastereomers by 1H NMR
spectroscopy. Purification by flash chromatography (3:97 Et2O:hexanes)
provided the product as a clear oil (0.232 g, 87%).
Silver triflate-catalyzed di-tert-butyl silylene transfer from sila-
cyclopropane 1 to functionalized alkenes followed by in situ ring
expansion with methyl formate provides an efficient one-flask
synthesis of functionalized oxasilacyclopentanes from alkenes. The
low temperature and short reaction time for silylene transfer result
in an improvement in diastereoselectivity and substrate compatibility
(15) Shaw, J. T.; Woerpel, K. A. Tetrahedron 1997, 53, 16597-16606.
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