Structural Effects Affecting the Thermal Electrocyclic Ring Closure of Vinylallenes to Alkylidenecyclobutenes

The thermal electrocyclic ring closure of (2 E,7 E )-3,4,7-trialkylnona-2,4,5,7-tetraenes (divinyl-4,5-allenes) is regioselective, occurring at the most sterically congested vinylallene subunit to afford the trisubstituted alkylidenecyclobutene. Ring closure of both divinylallenes and vinylallenes displays high torquoselectivity (exclusive formation of the ( E )-alkylidenecyclobutenes) when the substituent at C 4 is a sterically demanding alkyl group and the substituent at C 2 is a formyl group. Ab initio calculations clearly demonstrate the dominant steric influence of the bulky C 4 substituent in these selectivities. However, torquoselectivity appears to be enhanced if cyclization involves loss of conjugation by a π system encompassing the C 2 substituent.