Origins of inward torquoselectivity by silyl groups and other sigma-acceptors in electrocyclic reactions of cyclobutenes.

Recent reports of inward torquoselectivities in thermal electrocyclic ring-opening reactions of 3-silylcyclobutenes have revealed that saturated silyl substituents, just like the extensively studied pi-acceptors, can exert contrasteric effects. The origins of torquoselectivity for substituents lacking pi orbitals have been explained using B3LYP density functional calculations. Orbital interactions involving the substituent vacant orbitals and the occupied orbitals associated with the breaking bonds are found to control these contrasteric torquoselectivities, with minor contributions from electrostatic effects. Reaction energetics and transition states for electrocyclic ring-opening reactions of 3-silyl, fluorosilyl, difluorosilyl, trifluorosilyl, methylsilyl, methyl, fluoromethyl, difluoromethyl, trifluoromethyl, ammonio, phosphonio, formyl, and borohydrido cyclobutenes are reported to complement previous extensive studies of unsaturated substituents. Inward stereoselectivities are predicted for various silyl and phosphonium substituents, along with potent pi-acceptors studied earlier. Cope and Diels-Alder reactions involving silyl substituents are also computed.