Selective Alcohol Oxidation with Molecular Oxygen Catalyzed by Os−Cr and Ru−Cr Complexes

Abstract

The heterobimetallic complexes [Y][M(N)R 2 (µ-O) 2 CrO 2 ] (where Y is either N(n-Bu) 4 + or PPh 4 + ; M is either Ru or Os; and R is an alkyl or aryl ligand) catalyze the selective oxidation of alcohols with molecular oxygen. The rate of the reaction is higher with a ruthenium-containing complex than with an analogous osmiumcontaining catalyst. The rate decreases with steric bulk in either the catalyst or substrate. Single-crystal X-ray diffraction studies of [N(n-Bu) show the chromate group coordinated to each nitrido(dialkyl)metal center through two oxo ligands. The oxidation of benzyl alcohol by [N(n-Bu) 4 ][Os(N)(CH 2 SiMe 3 ) 2 (µ-O) 2 CrO 2 ] was examinined in detail. It is first order in alcohol and substrate, and, when oxygen partial pressure is low, the rate depends directly on the O 2 partial pressure. A mechanism in which alcohol coordinates to the osmium center and is oxidized by β-hydrogen elimination is consistent with the data. [N(n-Bu) 4 ][Os(N)(CH 2 SiMe 3 ) 2 (µ-O) 2 CrO 2 ] catalyzes the oxidation of dppe with O 2 through a different pathway.

Ru-catalyzed oxidation of primary alcohols

“…Among those "green" oxidants, molecular oxygen is especially attractive. Suitable active metals for both homogeneously and heterogeneously catalyzed oxidations with molecular oxygen include ruthenium and other group VIII metals like palladium [9][10][11][12][13][14][15], cobalt [16][17][18], iron [19], nickel and osmium [20][21][22] and also the groups I, V, VI, and VII metals such as copper [23][24][25][26][27][28][29], vanadium [30,31], molybdenum [32], and manganese [33,34]. Ruthenium catalysts are known to oxidize alcohols and other substrates according to different reaction mechanisms such as hydridometal or oxometal pathways and even radical mechanism, depending on the oxidation states of ruthenium, oxidant and cocatalysts or additives [23][24][25][26][27][28][29][30][31][32][33][34][35].…”

“…Ruthenium catalysts are known to oxidize alcohols and other substrates according to different reaction mechanisms such as hydridometal or oxometal pathways and even radical mechanism, depending on the oxidation states of ruthenium, oxidant and cocatalysts or additives [23][24][25][26][27][28][29][30][31][32][33][34][35]. Various homogeneous Ru catalysts, like RuCl 3 [36], perruthenate [37,38] or Ru complexes [20,39,40] preferentially with salen [41], amine [42][43][44]50] or phosphine [45][46][47][48][49][50][51] ligands, also in combination with bases and TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) as cocatalyst were successfully employed in the aerobic oxidation. Bäckvall and co-workers [52] developed methods for the oxidation of alcohols via biomimetic multi-stage electron transfers.…”

Oxidations catalyzed by osmium compounds. Part 1: Efficient alkane oxidation with peroxides catalyzed by an olefin carbonyl osmium(0) complex

“…Surprisingly, although osmium-catalyzed oxidation reactions of olefins [63][64][65][66][67][68][69][70][71][72][73] and alcohols [74][75][76][77][78] have been reported and are used in organic synthesis, much less is known about catalysis of alkane transformations by soluble osmium compounds [79][80][81][82]. Oxide of high-valent osmium, OsO 4 [79], and osmium chlorides [80] have been used in alkane oxidations with hydrogen peroxide in organic solvents.…”

Catalytic selective oxidation of benzyl alcohols to aldehydes with rhenium complexes

“…Mechanistic studies demonstrate that metal mediated alcohol oxidation can proceed via abstraction of a hydrogen atom by a metal oxo-or peroxo-unit, or by means of a concerted ␤-hydrogen elimination pathway [9].…”