Disproportionation of dihydroarenes by sol-gel entrapped RhCl₃-quaternary ammonium ion pair catalysts

The ion pairs generated from RhCl₃·3H₂O and the quaternary ammonium salts [(C₈H₁₇)₃NMe]Cl (Aliquat 336) and [Me₃N(CH₂)₃Si(OMe)₃]Cl were physically and chemically entrapped, respectively, in SiO₂ sol-gel matrices under mild conditions. The resulting immobilized ion pairs proved to be stable, leach-proof, and recyclable catalysts for disproportionation of 1,3-cyclohexadiene and several other vic-dihydroarenes. In these reactions, equimolar quantities of the respective tetrahydro- and fully aromatic compounds were obtained. The entrapped catalysts, in most cases, proved to be more efficient and more selective than their homogeneous analogues. The reaction rates and conversions were shown to depend strongly on steric effects of substituents and on the bulkiness of the substrate skeleton. The recorded first order kinetics in the substrates (organic reactants) suggests that the mechanism involves stepwise addition of two molecules of the dihydroarenes to the rhodium nucleus, and that the addition of the first substrate molecule is rate limiting.