Mechanistic Studies of Fujiwara Hydroarylation. C−H Activation versus Electrophilic Aromatic Substitution

The addition of the C−H bonds of arenes across alkynes is catalyzed by Pd(OAc) 2 in CF 3 CO 2 H (Fujiwara hydroarylation). Previously it has been suggested that this transformation proceeds by C−H activation of the arene followed by trans -addition of a palladium−arene bond across an alkyne. We have investigated the kinetic isotope effects of intramolecular hydroarylation with deuterated arene substrates and found that catalytic hydroarylation exhibits an inverse KIE. The observed inverse isotope effect is not consistent with known KIEs for C−H activation by electrophilic palladium; thus it suggests that the reaction proceeds by electrophilic aromatic substitution.