Catalysis of Organic..

Tanielyan et al. 121We wish to report here on a new and highly efficient catalyst composition forthe aerobic oxidation of alcohols to carbonyl derivatives (Scheme 1). The catalystsystem is based on 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO), Mg(NO 3 ) 2(MNT) and N-Bromosuccinimide (NBS), utilizes ecologically friendly solvents anddoes not require any transition metal co-catalyst. It has been shown, that thedescribed process represents a highly effective catalytic oxidation protocol that caneasily and safely be scaled up and transferred to technical scale.Results and DiscussionOur initial work on the TEMPO / Mg(NO 3 ) 2 / NBS system was inspired by the workreported by Yamaguchi and Mizuno (20) on the aerobic oxidation of the alcoholsover aluminum supported ruthenium catalyst and by our own work on a highlyefficient TEMPO-[Fe(NO 3 ) 2 / bipyridine] / KBr system, reported earlier (22). On thebasis of these two systems, we reasoned that a supported ruthenium catalystcombined with either TEMPO alone or promoted by some less elaborate nitrate andbromide source would produce a more powerful and partially recyclable catalystcomposition. The initial screening was done using hexan-1-ol as a model substratewith MeO-TEMPO as a catalyst (4.1mol %) and 5%Ru/C as a co-catalyst (0.3 mol%Ru) in acetic acid solvent. As shown in Table 1, the binary composition under thestandard test conditions did not show any activity (entry 1). When either N-bromosuccinimide (NBS) or Mg(NO 3 ) 2 (MNT) was added, a moderate increase inthe rate of oxidation was seen especially with the addition of MNT (entries 2 and 3).Table 1. Oxidation of 1 to 2 using a TEMPO based catalyst composition. a# Catalyst Composition, mol% Efficiency, % Comments5%Ru/C 4 MNT NBS Rate b Cnv b Sel b1 0.3 4.1 - - 0.00 0 - Long induction (4h+)2 0.3 4.1 - 2.6 0.01 5 93 No effect3 0.3 4.1 1.9 - 0.05 39 94 No effect4 0.3 4.1 1.9 2.6 0.21 98 86 All 4 components5 0.3 4.1 1.9 2.6 0.22 100 93 Re-used Ru6 - 4.1 1.9 2.6 0.25 99 95 No need of Rua [1-Hexanol]= 2mL (16mmol), CH 3 COOH =10mL , [MeO-TEMPO, 3] =0.656mmol, [NBS]=0.416 mmol and Mg(NO 3 ) 2 = 0.30 mmol, T= 46°C, P (O 2 )15psi. Reaction time 60 minbOxidation rate, mmol O 2 /min, conversion of 1 (Cnv), selectivity to 2 (Sel).On the other hand, when all four components were used at the concentrationlevels listed in the table, a rapid oxidation took place at a high rate to give hexanal inhigh selectivity (entry 4). An even more surprising result was achieved when wetried to re-use the heterogeneous Ru/C component with a fresh portion of thehomogeneous MeO-TEMPO / Mg(NO 3 ) 2 / NBS composition (entry 5). Fullconversion of hexan-1-ol to hexanal, at much higher selectivity of 93% was observedwithin a 60 min reaction time. The data in these two last examples indicated that,most likely, the Ru/C component in the catalyst was not taking part in the overalloxidation cycle.