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

Oxidation of the Pd II  complex (N4)Pd II Me 2  (N4 =  N , N ′-di- tert -butyl-2,11-diaza[3.3](2,6)pyridinophane) with O 2  or ROOH (R = H,  tert -butyl, cumyl) produces the Pd III  species [(N4)Pd III Me 2 ] + , followed by selective formation of ethane and the monomethyl complex (N4)Pd II Me(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)Pd II Me 2  oxidation by O 2  to generate a Pd III -superoxide intermediate. In addition, reaction of (N4)Pd II Me 2  with cumene hydroperoxide involves a heterolytic O–O bond cleavage, implying a two-electron oxidation of the Pd II  precursor and formation of a transient Pd IV  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 Pd IV  species. Moreover, the (N4)Pd II Me(OH) complex formed upon ethane elimination reacts with weakly acidic C–H bonds of acetone and terminal alkynes, leading to formation of a new Pd II –C bond. Overall, this study represents the first example of C–C bond formation upon aerobic oxidation of a Pd II  dimethyl complex, with implications in the development of Pd catalysts for aerobic oxidative coupling of C–H bonds.