Catalytic enantioselective hydrogenolysis of [Cr(CO)3(5,8-dibromonaphthalene)].

Highly enantiomerically enriched (h-arene)tricarbonylchromium(0) complexes whose chirality originates from the 1,2disubstitution pattern of the arene and the coordination of the metal to one enantiotopic face of the arene are powerful chirons in asymmetric synthesis. Robust, planar chiral arene complexes also increasingly find application as chiral ligands in asymmetric catalysis. Enantiomerically enriched arene complexes are accessible by both resolution and asymmetric synthesis. The latter method includes diastereoselective complexation, diastereoselective or enantioselective nucleophilic addition/hydride abstraction, and diastereoselective and enantioselective lithiation/electrophile addition. While these approaches are potent methods that often give the target complexes in high enantiomeric purity, they rely on the use of stoichiometric quantities of chiral information and the diastereoselective methods often require additional steps for the introduction and the removal of chiral auxiliaries. A potentially very attractive catalytic route is the desymmetrization of mesocomplexes by a chiral catalyst. In pioneering studies, Uemura, Mishimura, and Hayashi reported palladium-catalyzed asymmetric cross-coupling reactions of alkenyl metal and aryl metal compounds with [Cr(CO)3(1,2-dichlorobenzene)]. [3] A palladium-catalyzed Miyaura–Suzuki coupling reaction using a chiral bidentate ferrocene ligand ((S,R)-1-[1-(dimethylamino)ethyl]-2-(diphenylphosphino)ferrocene ((S,R)-PPFA) afforded the biaryl complex in 55% yield with 69% ee. More recently, a very similar level of induction and yield was reported for a methoxycarbonylation of the same substrate (47% yield, 63% ee). In this particular reaction, kinetic resolution in the second step allowed the isolation of the highly enantiomerically enriched product (95% ee) in 31% yield. Bidentate ferrocenyl ligands again were the best performers in this as well as in analogous reactions of [Cr(CO)3(2,6-dichlorotoluene)]. [4b] To complete the short list of precedents of asymmetric desymmetrizations, a report by Kamikawa et al. appeared during the preparation of this manuscript. It describes an asymmetric intramolecular Heck reaction of [Cr(CO)3(2,6-bisbutenyl chlorobenzene)] with the best results reaching 78% yield and 73% ee. We conclude that despite considerable efforts, a highly asymmetric, highyield desymmetrization of a meso arene complex has not yet been realized. Herein, we present the results of our studies to carry out such a transformation and show that this goal can be realized by using a new bulky phosphoramidite ligand. We selected [Cr(CO)3(h -5,8-dibromonaphthalene)] (2 ; see Scheme 1) as the substrate and asymmetric hydrogenolysis as the reaction. A highly enantiomerically enriched planar chiral bromonaphthalene complex could be a valuable precursor for asymmetric synthesis, for planar chiral ligands, and as a chiral {Cr(CO)3}-transfer agent. [7] To our knowledge, desymmetrization of meso dihalides by asymmetric hydrogenolysis has not been reported. We were aware that by choosing a naphthalene complex we compounded the problem because of the lability of the metal– arene bond in this class of compounds. Dibromonaphthalene complex 2 was obtained as a single regioisomer in 76% yield by treating [Cr(CO)3(NH3)3] with BF3·OEt2 in the presence of the arene 1 (Scheme 1). This mild and high-yielding procedure avoids insertion of a zerovalent {Cr(CO)n} fragment into the aryl–bromine bond, a process that results in the decomposition of the starting materials.