A new catalytic system using the combination
RuCl2(DMSO)4/CH3CH2COONa/DMSO/carboxylic
acid
which enables selective dimerization of acrylonitrile to
linear dimers without formation of propionitrile as a
byproduct has been developed on the basis of a new
working hypothesis of the desirable reaction mechanism
involving protonolysis of a C−Ru bond in the intermediate.
For the catalyst system of ruthenium and carboxylic acid, which is useful for the efficient tail-to-tail dimerization of acrylonitrile, the TON increases as the ruthenium catalyst concentration is decreased. Furthermore, the addition of aromatic solvents of equal volume to that of acrylonitrile improves the catalyst activity. Especially, the use of diphenyl ether leads to a 1.7 time improvement of the TON.
A new type of catalyst system using ruthenium and carboxylic acid is useful for the tail-to-tail dimerization of acrylonitrile, proceeding without the formation of undesired by-product propionitrile. Carboxylic acids having pKa 3.5–5 are suitable as co-catalysts for the dimerization of acrylonitrile. The relationship between the logarithm of the relative rate in the dimer formation and the pKa of m- and p-substituted benzoic acids (Brønsted plot) was linear (R2 = 0.946) with a slope of −0.199. The role of the carboxylic acids can be considered to be effective protonation in the protonolysis of the carbon–ruthenium bond of an intermediate Ru complex.
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