Epoxidation of strained alkenes catalysed by (1,2-dimethyl-4(1H)pyridinone-3-olate)2MnIIICl

Abstract

The mild epoxidation of strained alkenes using (DMPO) 2 MnCl catalyst (DMPO = 1,2-dimethyl-4(1H)-pyridinone-3-olate) in the presence of various oxidants was studied. Hydrogen peroxide and monopersulfate were found to be the best oxidants when used with imidazole in acetonitrile at 4 o C, with up to 94% conversion. Dismutation of hydrogen peroxide was also observed when used as an oxidant. The epoxidation using hydrogen peroxide or monoperoxysulfate appears to be mild and very selective for strained alkenes. A mechanism is proposed where imidazole is required for activation of the oxidant and where a detected Mn V =O species is proposed as the active species. Competitive reaction between H 2 O 2 and the substrate for the active species is proposed and homolytic vs heterolytic scissions of the O-O bond of the oxidant are discussed.

Reaction landscape of a mononuclear MnIII–hydroxo complex with hydrogen peroxide

“…[24][25][26][27][28] These catalysts have been used for a variety of oxidation reactions, including sulfoxidation 29 and C-H hydroxylation; [30][31][32][33][34] however, these catalysts are most widely used in olefin epoxidation reactions due to their high efficiencies and remarkable selectivity in asymmetric epoxidations. 24,30,[35][36][37][38][39][40][41][42][43] Mechanistic studies indicate that the active catalytic species in these reactions is a Mn V -oxo species produced via heterolytic O-O cleavage of a Mn III -hydroperoxo intermediate. [43][44][45][46][47][48] This cleavage is often facilitated by a carboxylic acid, such as acetic acid, that hydrogen bonds with the hydroperoxo ligand.…”