Studies on the thermal ring-opening reactions of cis-3,4-bis(organosilyl)cyclobutenes

[GRAPHICS] Three cis-3,4-bis(organosilyl) cyclobutenes were synthesized, and their thermal ring-opening reactions were studied. The ring-opening reaction of cis-3,4-bis( trimethylsilyl) cyclobutene proceeded remarkably faster than that of cis-3,4-dimethylcyclobutene. The significant rate acceleration was explained by assuming (i) stabilization of the transition state by electron delocalization from the cyclobutene HOMO to the Si-CH3 sigma* orbital, (ii) destabilization of the ground state by intramolecular interaction between the C-Si sigma orbitals and the pi orbital of cyclobutene, and (iii) through-space steric repulsion of the two bulky trimethylsilyl groups in a cis arrangement. The ring-opening reaction of unsymmetrical cis-3,4-bis( arylsilyl) cyclobutenes having electronically different arylsilyl groups was also examined. The inward preference increased in the order, p-CH3OC6H4-Si, C6H5-Si, p-CF3C6H4-Si, supporting the interpretation of the origin of the inward preference of silyl substituents on the basis of a stabilizing interaction between the cyclobutene HOMO and the Si-C sigma* orbital at the transition state.