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 η3-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 η3-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.