Valence-bond isomer chemistry. Part II. Diels–Alder reactions of hexafluorobicyclo[2,2,0]hexa-2,5-diene : pyrrole as a diene

Hexafluorobicyclo[2,2,0]hexa-2,5-diene is an active dienophile in the Diels–Alder reaction, reacting at room temperature with 1,3-diphenylisobenzofuran, cyclopentadiene, furan, pyrrole, αα-dimethylfulvene, cyclohexa-1,3-diene, 2,3-dimethylbuta-1,3-diene, 2-methylbuta-1,3-diene, and buta-1,3-diene to give 1 : 1 adducts, and to give 1 : 2 adducts, except with the least reactive dienes, butadiene and cyclohexadiene. Reaction times vary from a few hours with the most reactive dienes, 1,3-diphenylisobenzofuran and cyclopentadiene, to several months with the least reactive ones, butadiene and cyclohexadiene. It is argued, on the basis of spectroscopic data, that reaction occurs by exo-addition to the bicyclo[2,2,0]hexa-2,5-diene, and by exo-addition to the cyclic dienes. This is rationalized in terms of steric factors operating in the transition state.