Mechanism of C-H bond activation of alkyl-substituted benzenes by cationic platinum(II) complexes

While all methyl- and ethyl-substituted benzenes react with diimine Pt(II) methyl cations to give eta(3)-benzyl products, they do not all get there by the same pathway. For toluene and p-xylene, isotopic labeling shows that initial activation occurs at aryl positions with subsequent intermolecular conversion to the benzyl product. For ethylbenzene and 1,4-diethylbenzene, initial activation takes place exclusively at aryl C-H bonds, and conversion to the eta(3)-benzyl product takes place via intramolecular isomerization. Only in the most extreme case of steric crowding-the reaction of a bulky diimine platinum methyl cation (Ar = Mes) with triethylbenzene-does direct activation of the ethyl group become preferred to aryl activation.