gamma-dissociations of 3-alkylbicyclo[1.1.1]pent-1-yl radicals into [1.1.1]propellane and alkyl radicals: verification of a theoretical prediction

The gamma-dissociations of 3-substituted bicyclo[1.1.1]pent-1-yl radicals a nd cyclobutyl radicals were investigated by ab initio SCF MO (HF, MP2, MP3 and MP4/6-31G*) and density functional methods (B3LYP/6-31G*). The transiti on states were found to resemble the product alkyl radical and [1.1.1]prope llane or bicyclo[1.1.0]butane. Calculated endothermicities and energy barri ers were comparatively low for loss of the t-butyl radical from the 3-t-but ylbicyclo[1.1.1]pent-1-yl radical, which suggested that this dissociation w ould be significant under laboratory conditions. The dissociation was verif ied experimentally by means of the reaction of 1-iodo-3-t-butylbicyclo[1.1. 1]pentane with tributyltin hydride. Arrhenius parameters for this dissociat ion were determined by end product analysis. The SCF MO and density functio nal calculations resulted in much higher endothermicities and energy barrie rs for gamma-dissociations of cyclobutyl radicals, hence neither the format ion of bicyclo[1.1.0]butane nor alkyl radical addition to this bicyclic com pound was predicted to be important.