The Aerobic Oxidation of a Pd(II) Dimethyl Complex Leads to Selective Ethane Elimination from a Pd(III) Intermediate

Oxidation of the PdII complex (N4)­PdIIMe2 (N4 = N,N′-di-tert-butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane) with O2 or ROOH (R = H, tert-butyl, cumyl) produces
the PdIII species [(N4)­PdIIIMe2]+, followed by selective formation of ethane and the monomethyl
complex (N4)­PdIIMe­(OH). Cyclic voltammetry studies and
use of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as
a spin trap suggest an inner-sphere mechanism for (N4)­PdIIMe2 oxidation by O2 to generate a PdIII-superoxide intermediate. In addition, reaction of (N4)­PdIIMe2 with cumene hydroperoxide involves a heterolytic O–O
bond cleavage, implying a two-electron oxidation of the PdII precursor and formation of a transient PdIV intermediate.
Mechanistic studies of the C–C bond formation steps and crossover
experiments are consistent with a nonradical mechanism that involves
methyl group transfer and transient formation of a PdIV species. Moreover, the (N4)­PdIIMe­(OH) complex formed
upon ethane elimination reacts with weakly acidic C–H bonds
of acetone and terminal alkynes, leading to formation of a new PdII–C bond. Overall, this study represents the first
example of C–C bond formation upon aerobic oxidation of a PdII dimethyl complex, with implications in the development of
Pd catalysts for aerobic oxidative coupling of C–H bonds.