The four stereoisomeric cyclobutanes from thermal cycloaddition of 1,1-dichloro-2,2-difluoroethylene (“1122”) to the stereoisomeric 2,4-hexadienes1 at 80° have been isolated by preparative vapor phase chromatography and their configurations as previously assigned have been confirmed by nuclear magnetic resonance. Tetrafluoroethylene at 120˚ adds to trans,trans-2,4-hexadiene to yield only the two stereoisomeric trans-propenylcyclo-butanes in a trans/cis ratio in the ring of 4.2, and with no isomerization of the recovered diene. At this same temperature the addition of 1122 to trans,trans,trans-2,4-hexadiene is attended by isomerization of the diene and formation of minor amounts of cis-propenylcyclobutanes along with major amounts of the trans-propenylcyclobutanes. The isomerization is interpreted as due to reversal of the initial biradical formation, which may occur after rotation in the biradical. The similar amounts of loss of configuration with tetrafluoroethylene and 1122 lead to the suggestion that the rate of ring closure is controlled, not by the rate constant of radical-radical combination, but by the rate of rotations about the three single bonds of the biradical to bring it into a conformation suitable for ring closure from an original distribution of non-cis conformations. © 1969, American Chemical Society. All rights reserved.
