Vanishing aromaticity: A chiral ruthenium complex catalyzes the hydrogenation of 2,6‐ or 2,7‐disubstituted naphthalenes to give chiral tetralins with up to 92 % ee. The chiral catalyst is applicable to the regio‐ and enantioselective reduction of 6‐substituted 2‐alkoxynaphthalenes and preferentially hydrogenates the alkoxy‐substituted arene rings.

The catalytic asymmetric hydrogenation of heteroarenes has been intensively studied during the last decade. [1] Nowadays, various heteroaromatics, for example, indoles, [2] pyrroles, [3] and quinolines, [4] can be reduced to the corresponding chiral heterocycles with high stereoselectivity through asymmetric catalysis. [5][6][7][8] 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. [9][10][11] However, to the best of our knowledge, the catalytic asymmetric hydrogenation of aromatics containing no heteroatoms remains unexplored. [12,13] 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 [{RuCl 2 (p-cymene)} 2 ] and the chiral bisphosphine ligand, PhTrap. [2d, 14] 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 hydro-genation 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 naphthalene-2,6-dicarboxylate (4 a) 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) [2d] catalyst. Although formation of the hydrogenation product 5 a was observed, the desired reaction was very sluggish and the enantiomeric excess of 5 a was only 22 % ee (Table 1, entry 1). Both the stereoselectivity and the reaction rate were affected by the ester substituents of the naphthalene substrate. The reaction of ethyl ester 4 b also proceeded in low yield, but the ethyl substituent brought about a remarkable improvement in the stereoselectivity (entry 2). The use of 4 c, which is a larger and more flexible ester, resulted in a moderate yield of chiral tetralin 5 c (entry 3). The enantiomeric excess of 5 c was improved to 78 % ee by conducting the hydrogenation at lower temperature (entry 4). The enantioselectivity scarcely Scheme 1. Asymmetric hydrogenation of 2-naphthylindole. Boc = tertbutoxycarbonyl. Table 1: Catalytic asymmetric hydrogenation of naphthalene-2,6-dicarboxylate 4. [a] Entry R (4) Additive Solvent Conv. [%] [b] ee [%] [c]

This enantioselective reaction is the first success in the title synthesis.