A Silyl Substituent Can Dictate a Concerted Electrocyclic Pathway: Inward Torquoselectivity in the Ring Opening of 3-Silyl-1-cyclobutene

A preference for inward over outward rotation is found for the thermal conrotatory ring-opening reaction of 1-cyclobutene 1, to produce 1,3-butadiene 2, when a silyl substituent is located at the 3-position. This preference exists in spite of the bulkiness of the substituent.