Disproportionation of hydrophobic dihydroarenes by recyclable rhodium and palladium catalysts in aqueous microemulsions

In the course of our attempts to replace the traditional, but harmful, organic solvents in synthetic processes by environmentally favored benign media, we studied the disproportionation of hydrophobic vicinal dihydroarenes in water. This reaction takes place efficiently by either sol–gel-entrapped [(C 8 H 17 ) 3 NCH 3 ][RhCl 4 ] ion pair or by immobilized Pd(OAc) 2 , together with a suitable surfactant to give tetrahydroarenes and fully aromatic products. Since a slow dehydrogenation process accompanies this process the latter products are usually formed in excess. The reactions are also accompanied by transformation of the heterogenized catalysts into sol–gel-stabilized metallic particles, which can be recycled without loss of activity for at least five runs. The rhodium (but not the palladium) promoted disproportionations that follow the pseudo first order rate law. The reactions are influenced by both the electronic and steric structures of the dihydroarenes, as well as by the nature of the surfactant and by the hydrophobicity of the ceramic sol–gel support. Deuterio-labeling indicates slight involvement of the water in the disproportionation process.