The aerobic reactivity of N-heterocyclic-carbene-coordinated reduced palladium complexes

Palladium-catalyzed reactions are among the most versatile methods for the selective aerobic oxidation of organic molecules. Little can be ascertained about the mechanism of catalyst reoxidation from examination of these systems: the catalyst reoxidation steps are kinetically invisible under catalytic conditions. As facile catalyst reoxidation is critical to attain efficient catalytic turnover, the reactivity of molecular oxygen with N-heterocyclic-carbene-coordinated reduced palladium complexes has been investigated. NHCs have been shown to be useful ligands in palladium oxidase catalysis.;This report examines the reaction of (IMes)2Pd0 with O2, which forms an eta2-peroxopalladium(II) complex. Subsequent addition of HOAc to (IMes)2Pd(O2) yielded the first example of a hydroperoxopalladium species derived from molecular oxygen.;Additionally, this report describes the synthesis of trans-(IMes) 2Pd(H)(OBz) and a mechanistic study of its reaction with molecular oxygen, which yields trans-(IMes)2Pd(OOH)(OBz). The reaction was monitored by 1H NMR spectroscopy in benzene-d 6, and kinetic studies revealed a two-term rate law: kobs = k1 + k2[BzOH]. The data support a stepwise mechanism for this conversion consisting of rate-limiting reductive elimination of BzOH from (IMes)2PdII(H)(OBz) followed by oxidation of Pd0 by O2 as described above. The second term of the rate law reflects the ability of protic additives (such as benzoic acid) to catalyze the reductive elimination of BzOH from (IMes)2Pd II(H)(OBz). Introduction of an electron rich carboxylate ligand trans to the hydride (-O2CC6H4OMe) allowed the observation of a second independent mechanism for the reaction of palladium(II)-hydride with O2 involving an H-atom abstraction pathway.;Finally, this report details that in acetonitrile solvent, (IMes) 2PdII(H)(OBz) rapidly dissociates benzoate anion to form the solvent-coordinated cationic palladium hydride complex [trans -(IMes)2Pd(H)(NCCD3)][OBz]. Exposure of this compound to molecular oxygen yields the solvent-associated PdII-cationic hydroperoxide [trans-(IMes)2Pd(OOH)(NCCD 3)][OBz]. Kinetic studies of this reaction revealed a complex rate law, rate = k1[Pd-H][-OBz]/( k-1[CD3CN] + k2[ -OBz]) + k3[Pd-H][-OBz]. The data support a stepwise mechanism for the conversion consisting of two parallel pathways, both of which feature rate limiting deprotonation, one of a solvent-coordinated and the other of a solvent-dissociated palladium(II)-hydride complex to form (IMes)2Pd0. The latter species reacts with O2 as described above.