A variety of carbonyl compounds participated in the
cycloaddition employing alkyne 10 as the dipolarophile
(Table 1). The cycloaddition reaction tolerated R,ꢀ-unsatur-
Scheme 2. Silylene Transfer to Aldehydes
Table 1. Silver-Catalyzed Silylcarbonyl Ylide Cycloaddition
to formation of oxasilacyclopropanes. When n-butyraldehyde
(8) was subjected to the same catalyst screen, copper(II)
bromide-catalyzed silylene transfer afforded the intermo-
lecular 1,3-dipolar cycloaddition product, dioxasilacyclo-
pentane acetal 9, in 89% yield (Scheme 2). A competition
experiment between aldehydes 5 and 8 yielded dioxasilacy-
clopentane 7 as the sole product, suggesting that the
difference in regioselectivity of the corresponding products
7 and 9 is caused by a divergence in the mechanistic
pathways.
Silylene transfer to benzaldehyde in the presence of
electron-deficient alkyne 10 led to a different three-
component coupling reaction. When 1 equiv of alkyne 10
and 1 equiv of benzaldehyde (5) were treated under metal-
catalyzed silylene transfer conditions, oxasilacyclopentene
11 was formed in 85% yield (Scheme 3). This reaction is
a At 50 °C for 12 h. b For 3 h. c 4 equiv of 6 and 4 equiv of 12e.
Scheme 3. Formation of Oxasilacyclopentene
ated aldehydes and formate esters (entries 1 and 2). Forma-
mides also reacted to form N,O-acetal oxasilacyclopentene
13c, although the reaction was slower. Ketones underwent
cyclization to form oxasilacyclopentenes containing tetra-
substituted carbon atoms (entry 4). R,ꢀ-Unsaturated ester 12e
reacted with alkyne 10 to form oxasilacyclopentene 13e,
although silylene transfer to the R,ꢀ-unsaturated ester was
also observed (Scheme 4).10 Excess ester and silacyclopro-
pane were employed to optimize the formation of oxasila-
cyclopentene 13e (entry 5).
The highly electron-deficient alkyne 10 was the only
alkyne that participated in these reactions. Treatment of
silacyclopropane 6 and carbonyl compounds with other
dipolarophiles led to either silylene transfer to the alkyne to
form silacyclopropenes or dimerization with the aldehyde
to form dioxasilacyclopentane 7 (Scheme 5).
complementary to our earlier synthesis of oxasilacyclopentenes.8
That method involved a two-step process whereupon silacy-
clopropenes, prepared from alkynes, underwent insertion reac-
tions with carbonyl compounds. Electron-deficient alkynes,
however, did not participate in that reaction because the
silylenoid intermediate is electrophilic, not nucleophilic.9
(6) (a) Jutzi, P.; Eikenberg, D.; Bunte, E.-A.; Mo¨hrke, A.; Neumann,
B.; Stammler, H.-G. Organometallics 1996, 15, 1930–1934. (b) Belzner,
J.; Ihmels, H.; Pauletto, L.; Noltemeyer, M. J. Org. Chem. 1996, 61, 3315–
3319. (c) Franz, A. K.; Woerpel, K. A. J. Am. Chem. Soc. 1999, 121, 949–
957.
Control experiments indicated that the formation of the
oxasilacyclopentene did not involve a silacyclopropene
intermediate. When electron-deficient alkyne 10 was treated
´
(7) Cirakovic´, J.; Driver, T. G.; Woerpel, K. A. J. Org. Chem. 2004,
69, 4007–4012.
(9) Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2004, 126, 9993–
(8) (a) Seyferth, D.; Vick, S. C.; Shannon, M. L. Organometallics 1984,
3, 1897–1905. (b) Clark, T. B.; Woerpel, K. A. J. Am. Chem. Soc. 2004,
126, 9522–9523.
10002.
(10) Calad, S. A.; Woerpel, K. A. J. Am. Chem. Soc. 2005, 127, 2046–
2047.
5258
Org. Lett., Vol. 10, No. 22, 2008