Homogeneous oxidation of 1-octene by dioxygen catalyzed by rhodium species: comparison of the ‘rhodium-copper-lithium’ and ‘rhodium-bismuth-lithium’ systems

Abstract

Complexes of the type [Rh III(H 2O) 6](A −) 3 (A − = ClO 4 −, BF 4 −,…) have been tested with other rhodium compounds as catalyst precursors in the oxidation of terminal olefins to methyl ketones by dioxygen. The effect of a cocatalyst (Cu(II) or Bi(III) salt) is reported and new systems are devised. It is possible to obtain good activities as well as good selectivities with substrate/catalyst ratios of 120 ( P(O 2) = 1 atm, 40 ⩽ T(°C) ⩽ 70). Novel homogeneous systems based on rhodium(III), copper(II) complexes and lithium chloride are more effective in the oxidation of 1-olefins dissolved in ethanol than the ‘RhCl 3·3H 2O’/Cu(ClO 4) 2·4HMPA system, (the latter gives a precipitate of cuprous chloride). Together with rhodium-copper based systems, rhodium complexes in association with bismuth chloro-complexes and/or LiCl were found to be equally efficient in the oxidation of 1-alkenes in alcoholic media, but there is less solvent cooxidation in the ‘Rh III/Cu II/Li I’ systems. The yields and the rate of reaction are not sensitive to phosphine concentration ( e.g. P(C 6F 5) 3 and P(n-Bu) 3, within the limits Rh:P =1:1 to 1:2). Cycloocta-1,5-diene is mono-oxygenated by the ‘Rh III/ Cu II/Li’ system at 60 °C, in a reaction leading to cyclooct-4-en-1-one (yield 82%). Our experimental results and those of other groups suggest that the key intermediates are a hydrido-rhodium complex: ‘[HRh IIICl xL y ]’ and a hydroperoxy complex ‘[HOORhCl xL y]’. The reaction of dioxygen with HRhCl 2[PPh 2(n-Bu)] 3 in dry ethanol-oct-1-ene mixtures affords octane-one (yield 45% based on rhodium, T = 60 °C).