Amine Oxidative N-Dealkylation via Cupric Hydroperoxide Cu-OOH Homolytic Cleavage Followed by Site-Specific Fenton Chemistry

Copper(II) hydroperoxide species are significant intermediates in processes such as fuel cells and (bio)chemical oxidations, all involving stepwise reduction of molecular oxygen. We previously reported a Cu-II-OOH species that performs oxidative N-dealkylation on a dibenzylamino group that is appended to the 6-position of a pyridyl donor of a tripodal tetradentate ligand. To obtain insights into the mechanism of this process, reaction kinetics and products were determined employing ligand substrates with various para-substituent dibenzyl pairs (-H,-H; -H,-Cl; -H,-OMe, and -Cl,-OMe), or with partially or fully deuterated dibenzyl N-(CH2Ph)(2) moieties. A series of ligandcopper(II) bis-perchlorate complexes were synthesized, characterized, and the X-ray structures of the -H,-OMe analogue were determined. The corresponding metastable Cu-II-OOH species were generated by addition of H2O2/base in acetone at -90 degrees C. These convert (t(1/2) approximate to 53 s) to oxidatively N-dealkylated products, producing para-substituted benzaldehydes. Based on the experimental observations and supporting DFT calculations, a reaction mechanism involving dibenzylamine H-atom abstraction or electron-transfer oxidation by the Cu-II-OOH entity could be ruled out. It is concluded that the chemistry proceeds by rate limiting Cu-O homolytic cleavage of the Cu-II-(OOH) species, followed by site-specific copper Fenton chemistry. As a process of broad interest in copper as well as iron oxidative (bio)chemistries, a detailed computational analysis was performed, indicating that a (CuOOH)-O-I species undergoes OO homolytic cleavage to yield a hydroxyl radical and (CuOH)-O-II rather than heterolytic cleavage to yield water and a Cu-II-O center dot- species.