| We have investigated the one electron oxidation of diarylcarbenes in an attempt to generate and observe diarylcarbene cation radicals in solution and describe their reactivity. Chemical oxidation of di-(4-methylphenyl)-diazomethane in the presence of excess 2,3-dimethyl-2-butene results in formation of 1,1-di-(4-methylphenyl)-2,2,3,3-tetramethylcyclopropane in 75% yield. Using stop-flow kinetics and a double nanosecond laser experiment, the product forming intermediate was determined to be di-(4-methylphenyl)-carbene cation radical. Reaction of di-(4-methylphenyl)-carbene cation radical with stereoisomeric alkenes yields completely stereospecific cyclopropanes. Investigations of the mechanism for reaction of di-(4-methylphenyl)-carbene cation radical with alkenes using an induced kinetic isotope study and radical clock kinetics suggest that the stereospecific reaction proceeds via a stepwise mechanism with the initial formation of a 1,3 cation radical intermediate with subsequent ring closure prior to bond rotation. The reactivity of diarylcarbene cation radicals was investigated using absolute bimolecular rate constants and product studies. Diarylcarbene cation radicals are exceptionally reactive species reacting with alkenes, alcohols, arenes, oxygen, and hydrogen atom donors with bimolecular rate constants ranging from 1 e+06 to 5 e+09 M-1 sec-1. The reactivity can be described as stemming from a dichotomy of radical- and cation-like character. The sp2 structure proposed by Bally et.al. of a planar aromatic stabilized pi cation and a sigma radical is supported by the observed solution reactivity. Aromatic substitution, both electronic and steric, results in modification of the reactivity consistent with stabilization of this proposed structure. |
