Catalytic asymmetric hydrogenation of naphthalenes.

The catalytic asymmetric hydrogenation of heteroarenes has been intensively studied during the last decade. Nowadays, various heteroaromatics, for example, indoles, pyrroles, and quinolines, can be reduced to the corresponding chiral heterocycles with high stereoselectivity through asymmetric catalysis. Glorius and co-workers recently found that a chiral N-heterocyclic carbene–ruthenium catalyst allowed the site-selective hydrogenation of the carbocyclic rings of some quinoxalines, producing chiral 5,6,7,8-tetrahydroquinoxalines with up to 88% ee. However, to the best of our knowledge, the catalytic asymmetric hydrogenation of aromatics containing no heteroatoms remains unexplored. Herein, we report the first successful enantioselective hydrogenation of carbocyclic arenes, naphthalenes, through asymmetric catalysis. Previously, we had developed the highly enantioselective hydrogenation of N-Boc indoles with a ruthenium catalyst generated from [{RuCl2(p-cymene)}2] and the chiral bisphosphine ligand, PhTrap. During the course of this study, we attempted the reduction of 2-naphthylindole 1 with the PhTrap–ruthenium catalyst (Scheme 1). To our surprise, none of the expected product 2 was obtained. The hydrogenation was accompanied by the partial reduction of the naphthalene ring, yielding tetrahydronaphthylindoline 3 with 90% ee. This observation suggested that the ruthenium complex was capable of reducing carbocyclic aromatic rings. In this context, we began to study the catalytic asymmetric hydrogenation of naphthalenes. In our initial attempt, a solution of dimethyl naphthalene2,6-dicarboxylate (4a) in 1,4-dioxane was stirred at 60 8C under hydrogen gas (50 atm) in the presence of [RuCl(p-cymene){(S,S)-(R,R)-PhTrap}]Cl (6) catalyst. Although formation of the hydrogenation product 5a was observed, the desired reaction was very sluggish and the enantiomeric excess of 5a was only 22% ee (Table 1, entry 1). Both the