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

• Published in: Journal of the American Chemical Society 2012-02, Vol.134 (4), p.2414-2422
• Main Authors: Khusnutdinova, Julia R, Rath, Nigam P, Mirica, Liviu M
• Format: Article
• Language: English
• Subjects: Crystallography, X-Ray; Models, Molecular; Molecular Conformation; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Oxidation-Reduction; Palladium - chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja210841f

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.