Chemical Behavior of Cyclopropylmethyl Radicals: Relative Unimportance of Cyclopropylmethyl to 3-Butenyl Rearrangement in the Cycloaddition Reactions Proceeding via Allylically Stabilized Diradicals

Article abstract of DOI:10.1021/jo00177a022

The reaction of cis-1-cyclopropyl-1,3-butadiene (1c) with fluorenylidene (Fl:), which was thermally generated from 9-diazofluorene (2), produced 3c in addition to fluorenone azine (4) and 9,9'-bifluorenylidene (5).The reaction of 1t with 2 gave a mixture of two adducts, 3t and 6, in which 6 was proved to be the secondary product derived from 3t.The addition of Fl: was thus regiospecific and gave 3, and the stereochemistry of the C1-C2 double bond in 1 was retained throughout the addition.The reaction of 1,1-dicyclopropyl-1,2-propadiene (7) with Fl: took place exclusively at the C1-C2 double bond to give 13.On being heated at 140 deg C, 13 underwent the anticipated skeletal isomerization to give 14.At 180 deg C, more extensive rearrangements took place to produce 15, 16Z, and 16E in a 2.9:1.0:3.7 ratio. 1,1-Dibromo-2,2-dicyclopropylcyclopropane (19), on the other hand, produced a mixture of 20Z and 20E on being heated at 140 deg C.In contrast to the results obtained in the Fl: addition reactions, the radical addition of bromotrichloromethane (22) to 1 yielded cyclopropane-cleaved 23 as the exclusive product.It may thus be concluded that the cyclopropylmethyl to 3-butenyl rearrangement in the diradicals, in which the cyclopropyl-substituted site is stabilized with allylic resonance, is unimportant relative to the intramolecular coupling of the two radicals sites at least at 140 deg C or below, whereas the intermolecular atom abstraction of the cyclopropyl-substituted allyl radical occurs relatively slowly and hence is accompanied by the cyclopropane cleavage.