Journal of the American Chemical Society, Vol.118, No.8, 1881-1891, 1996
Structural Effects Affecting the Thermal Electrocyclic Ring-Closure of Vinylallenes to Alkylidenecyclobutenes
The thermal electrocyclic ring closure of (2E,7E)-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 pi system encompassing the C-2 substituent.