Substituent effect of group 14 elements on the ring-opening reaction of cyclobutene

A series of cyclobutenes bearing group 14 elements at the 3-position were synthesized, and their thermal ring-opening reactions were studied. Carbon-substituted cyclobutene underwent the ring-opening reaction through an outward pathway to afford the E-diene exclusively. On the other hand, the ring-opening reaction of the silyl, germyl, and stannyl substituted cyclobutenes occurred in both outward and inward directions giving a mixture of E and Z isomers. The structural features of the calculated transition state and population analysis suggested that the formation of the Z-isomer could be ascribed to the donor/acceptor interaction between the HOMO and the sigma* orbital of group 14 elements. Interestingly, the order of inward preference was Si > Sn > Ge. These rotational behaviors of silyl, germyl, and stannyl substituents were explained by taking into account the energy gap and the magnitude of overlap between the sigma* orbital and HOMO.