beta-trimethylsilyl cyclopropylcarbenes

Thermal decomposition of the in situ generated lithium salt of the tosylhyd razone derivative of cyclopropyl trimethylsilylmethyl ketone gave 1-cyclopr opyl-1-trimethylsilylethylene, a product of exclusive silyl migration, Ther mal decomposition of the sodium salts of tosylhydrazone derivatives of 1-tr imethylsilylcyclopropyl alkyl ketones also gave methylenecyclopropane produ cts derived from trimethylsilyl migration. These reactions were interpreted in terms of rapid trimethylsilyl migration to carbene-like centers that co mpete effectively with ring expansion processes of cyclopropylcarbenes. Com putational studies (B3LYP/6-31G*) suggest that cyclopropyl stabilization of carbenes is more effective than beta -trimethylsilyl stabilization. Howeve r, beta -trimethylsilyl stabilized conformations are easily attained, and t hese conformations can lead to silyl migrations. There are two minimum ener gy conformations of methyl-1-trimethylsilylcyclopropylcarbene 27, and the r otational 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 tri methylsilyl migration in trimethylsilylmethyl cyclopropylcarbene, 11.