Cycloaddition Reaction of Some Representative 1-Cyclopropyl-1,3-butadienes with Tetracyanoethylene and Reaction of the Resultant Vinylcyclobutanes. An Easy Vinylcyclobutane-Cyclohexene Rearrangement

Article abstract of DOI:10.1021/ja00522a045

In the reaction with TCNE, (Z)-1-cyclopropyl-1,3-butadiene (1b) as well as several 1,1-disubstituted 1,3-butadienes (1c-f) yielded vinylcyclobutane 2 as the major product particularly in a polar solvent, whereas the E isomer 1a gave the cyclohexene 3 exclusively.The resultant vinylcyclobutanes, except for 2f, isomerized easily to 3.The most reactive of all was 2c while 2b was the least reactive.The isomerization of 2c in acetonitrile in the presence of 1a yielded virtually no cross product 3a, supporting the intramolecular nature of the transformation.Since the intermediate was trapped by p-toluenethiol, the rearrangement will most probably be stepwise.The solvent effects and the substituent effects on the rate of the reaction indicate that the ionic mechanism is operating.In contrast, 2f did not rearrange at all, but, especially at elevated temperatures, it split into the two fragments, i.e., methylenemalononitrile and 4f.The lack of effect of solvent polarity on the rate suggests that the fragmentation would be a diradical stepwise process.In the reaction of other vinylcyclobutanes, also, the fragmentation became appreciable at elevated temperatures in solvents of low polarity.The extent of the fragmentation depends upon the substituent(s) at the terminal carbon of the vinyl group.