Unusual Regioselectivity in Pd-Catalyzed Heterogeneous Aromatic Hydrogenation of P^V-Functionalized Naphthalene...

Regioselective hydrogenation of a single aromatic ring in naphthalene derivatives represents a persistent challenge in synthetic chemistry, particularly when aiming for selective reduction of the functionalized ring. Herein, we report the first heterogeneous Pd-catalyzed hydrogenation of P^V-substituted naphthalene derivatives using a commercially available and recyclable Pd/Al₂O₃ catalyst. Unusual regioselectivity was observed during the hydrogenation of naphthalen-1-yl phosphine oxides, leading to 1,2,3,4-tetrahydronaphthalen-1-yl phosphine oxides with high regioselectivity (up to >95:5 rr) and yield (up to 99%). In contrast, shifting the P^V-group to the C2-position resulted in a reversal of regioselectivity, affording 5,6,7,8-tetrahydronaphthalen-2-yl phosphine oxides with similarly high selectivity and efficiency. This practical method was successfully applied to the synthesis of a novel chiral P,N-ligand and an H₈-Cy-BINAP-type phosphine ligand, underscoring its significant synthetic utility. Mechanistic investigations suggest that the reaction proceeds via a tandem process including 3,4-hydride and subsequent 1,2-hydride additions, with unusual regioselectivity likely arising from a combination of electronic and steric effects. This strategy is anticipated to provide a general approach for the regioselective hydrogenation of functionalized naphthalene derivatives.