Hydrogenation and dehalogenation of aryl chlorides and fluorides by the sol-gel entrapped RhCl₃-aliquat 336 ion pair catalyst

The SiO₂ sol-gel entrapped ion pair [(C₈H₁₇)₃NMe]⁺[RhCl₄nH₂O]^-, generated from RCl₃H₂O, Aliquat 336® and Si(OMe)₄ catalyzes at 80°C and 16 atm. H₂, the hydrogenation of aryl fluorides and chlorides to give halogen-free hydroaromatics. Fluorobenzenes initially yield fluorocyclohexanes which eliminate HF, in a non-catalytic process, followed by hydrogenation of the cyclohexenes. In contrast, chlorobenzenes are first dehalogenated to the corresponding benzene derivatives, that in a second step are hydrogenated to cyclohexanes. Aryl bromides undergo dehalogenation and hydrogenation as well, but only in the presence of a free radical scavenger. The catalyst in these reactions is leach-proof and recyclabe with little or no loss in activity.