Beta-trimethylsilyl cyclopropylcarbenes.

Thermal decomposition of the in situ generated lithium salt of the tosylhydrazone derivative of cyclopropyl trimethylsilylmethyl ketone gave 1-cyclopropyl-1-trimethylsilylethylene, a product of exclusive silyl migration. Thermal decomposition of the sodium salts of tosylhydrazone derivatives of 1-trimethylsilylcyclopropyl alkyl ketones also gave methylenecyclopropane products derived from trimethylsilyl migration. These reactions were interpreted in terms of rapid trimethylsilyl migration to carbene-like centers that compete effectively with ring expansion processes of cyclopropylcarbenes. Computational studies (B3LYP/6-31G) suggest that cyclopropyl stabilization of carbenes is more effective than beta-trimethylsilyl stabilization. However, beta-trimethylsilyl stabilized conformations are easily attained, and these conformations can lead to silyl migrations. There are two minimum energy conformations of methyl-1-trimethylsilylcyclopropylcarbene, 27, and the rotational barrier to interconversion of these conformations (5.4 kcal/mol) is substantially lower than in the parent cyclopropylcarbene (15 kcal/mol). The onset of a stabilizing interaction in the transition state between the carbene vacant orbital with the adjacent Si-C sigma-orbital is proposed. Computational studies also show a very small (2.0 kcal/mol) barrier for trimethylsilyl migration in trimethylsilylmethyl cyclopropylcarbene, 11.