Stereoselective reduction of disubstituted aromatics on colloidal rhodium

Hydrogenation of disubstituted aromatic rings can be achieved under mild reaction conditions in a two-phase system in presence of RhCl 3 and phase transfer agents such as trioctylamine. The catalytic system was optimized for the 2-methylanisole reduction by a proper choice of the amine/Rh ratio which should be high enough to stabilize very small colloidal rhodium particles and low enough to avoid deactivation. A high chemoselectivity and a high stereoselectivity to the Z isomer are obtained. Moreover enantioselective reductions of o-cresol derivatives were performed with colloidal rhodium stabilized and modified by chiral amines. Diastereoselective reductions were also obtained using chiral auxiliaries bound to the substrate.