PNNL

Abstract

CuH-catalyzed olefin hydrogenation is rare compared to those of carbonyl-derived substrates. Olefin insertion into Cu-H to form Cu-alkyl is ubiquitous; however, subsequent H2 activation remains unknown to our knowledge. Herein, we investigated the transformations of ß-H elimination, H2 cleavage, and catalytic olefin hydrogenation in a series of linear and trigonal planar Cu(I)-alkyl complexes supported by monodentate N-heterocyclic carbene and bidentate naphthyridine-bis(carbene) ligands, respectively. Contrary to unreactive linear species, trigonal planar variants promote ß-H elimination, hydrogenolysis, and catalytic hydrogenation of unactivated alkenes at mild temperature and H2 pressure. The rare isolation of a naphthyridine-bis(carbene)CuH monomer further affirms two predominant competing pathways for H2 cleavage of metal-ligand cooperativity at Cu(I)-alkyl or internal electrophilic substitution at Cu(I)-OH. Employing either isolated or in situ generated Cu(I)-OH complex, via protonolysis of alkyl precatalyst by adventitious water, significantly accelerated catalysis compared to that operating primarily by the metal-ligand cooperativity pathway. DFT calculations and energy decomposition analysis on the disparate ß-H elimination reactivity between linear and trigonal planar tert-butyl complexes and the mechanism of H2 activation at a hydroxide complex, indicate that coordination geometry at Cu(I) and properties of the naphthyridine-bis(carbene) ligand are integral to the transformations reported here.