Oxidations by the system “hydrogen peroxide–dinuclear manganese(IV) complex–carboxylic acid”Part 5. Epoxidation of olefins including natural terpenes

Abstract

Dinuclear manganese(IV) complex [LMn(O)3MnL](PF6)2 (1, L = 1,4,7-trimethyl-1,4,7-triazacyclononane) efficiently catalyzes epoxidation of sterically accessible olefins, including natural compounds by hydrogen peroxide in acetonitrile at room temperature if a small amount of a carboxylic acid is present in the solution. The kinetics of dec-1-ene epoxidation and accompanying dioxygen evolution (catalase activity) under the action of this system in the presence of acetic acid has been studied. The initial rates of both epoxidation and O 2 evolution are proportional to the catalyst initial concentration. First order has been found for catalyst 1 for both processes, whereas the rate dependences of the dec-1-ene epoxidation is first order and the O 2 evolution is second order for H2O2. The epoxidation rate increases and the O2 evolution rate decreases with growing of acetic acid concentration. Zero order has been found for dec-1-ene in its epoxidation. The reaction proceeds with an induction period for a few minutes during which changes in the electronic spectra of the reaction solution are observed. It has been proposed that the processes of the alkane oxidation and dioxygen evolution on the one hand and of the olefin epoxidation on the other hand are induced by different intermediate species. An assumption has been made that the epoxidation occurs with participation of oxo-hydroxy Mn(V) derivative [LMn V ( O)(O)2(HO)Mn IV L] 2+ , whereas di(hydroperoxy) complex [LMn III (OOH)(O)2(HOO)Mn IV L] + is responsible for the alkane oxidation with simultaneous dioxygen evolution. The following equations for the initial rates were proposed for [CH 3CO2H] = 0.25 mol dm −3 : d[epoxide]/dt = keff (epoxide)[1][H2O2] with keff (epoxide) = 2.8–3.7 mol −1 dm 3 s −1 ; d[CyOOH]/dt = keff (CyOOH)[1][H2O2] 2 [CyH] with keff (CyOOH) = 4.1–6.2 mol −3 dm 9 s −1 ; d[O2]/dt = keff (O2)[1][H2O2] 2 with keff (O2) = 2.8–7.0 mol −2 dm 6 s −1 . Many different carboxylic acids were checked as cocatalysts and it has been found that oxalic acid acts with the highest efficiency in the epoxidation whereas the accompanying catalase activity of the system is very low in this case. It has been also demonstrated that complex 1 is unique catalyst because similar compounds containing only one Mn(IV) center (2) or dinuclear complex with bridging phenylboronic acid (3) are very poor catalysts in the olefin epoxidation. No epoxidation has been found when hydrogen peroxide was replaced by tert-butyl hydroperoxide. The system based on 1 ,H 2O2 and acetic and/or oxalic acid was employed for the efficient epoxidation of terpenes limonene, citral, carvone and linalool, while other terpenes containing sterically hindered double bonds (citronellal, - and -isomers of pinene) were epoxidized only with <15% yield. Using limonene as example, it has been demonstrated that regioselectivity of the epoxidation (predominant formation of product with addition of the O atom either to internal ring or external double bond) can be controlled by replacing acetic acid by oxalic acid. © 2004 Elsevier B.V. All rights reserved.