DFT studies of reductive elimination, C–H activation and β-hydride elimination in alkyl and aryl palladium amine complexes

Abstract

The factors affecting C–N bond formation via reductive elimination from Pd(I t Bu)(neopentyl)(morpholide) (I t Bu = 1,3-Di-tert-butyl-imidazol-2-ylidene) are studied computationally. DFT calculations indicate that choosing an alkyl group without β hydrides, such as neopentyl, has a detrimental effect on the possibility of C–N reductive elimination. In the absence of β hydride elimination, a pathway of lower energy than reductive elimination is found, namely morpholide-promoted C–H activation of neopentyl t Bu has a significantly lower activation energy than reductive elimination. Changing the ancillary ligand from I t Bu to tricyclopentylphosphine (P(Cyp)3) has little impact. By contrast, replacing neopentyl by phenyl leads to a c. 50% reduction in activation energy. Study of Pd(I t Bu)(2-methylpropyl)(morpholide) permits comparison of the potential energy surfaces for three possible processes; (1) reductive elimination (2) morpholide-promoted C–H activation and (3) β-hydride elimination and reveals that the activation energies for these processes increase in the order of (3) < (2) < (1). Reductive elimination, C–H activation and β-hydride elimination processes in alkyl and aryl palladium amine complexes are studied computationally, in order to establish why reductive elimination in the alkyl complexes is so elusive.