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 ^IIMe 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 ^IIIMe ₂] ⁺, followed by selective formation of ethane and the monomethyl complex (N4)Pd ^IIMe(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 ^IIMe ₂ oxidation by O ₂ to generate a Pd ^III- superoxide intermediate. In addition, reaction of (N4)Pd ^IIMe 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 ^IIMe(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.