Reduction of Polycyclic Arenes by ‐Boranes, II. Borane Catalyzed Hydrogenation of Naphthalenes to Tetralins

Abstract

Abstract Tetrapropyldiborane(6) (TPDB) and triethylborane (TEB) catalyze the regioselective and partial hydrogenation of naphthalene (N) and a number of substituted naphthalenes at 170–200°C and hydrogen pressures of 25–100 bar. Tetralin (T) is formed quantitatively. Naphthalene derivatives are mainly hydrogenated in the least substituted ring. In the case of alkyl substituents, Lewis acid catalyzed migration and, to a lesser extent CC bond rupture, lower the yield of the main tetralin derivative. Chlorinated naphthalenes and at the O‐atom derivatized naphthols undergo also partial loss of the chloro or oxygen functional groups. The initially added borane acts only as a precatalyst and is slowly converted to catalytically active polyboranes of as yet unknown structures.