Asymmetric Hydrogenation\nof Naphthalenes with Molybdenum\nCatalysts: Ligand Design Improves Chemoselectivity

The asymmetric hydrogenation of substituted\nnaphthalenes with a\nseries of highly enantioenriched oxazoline imino(pyridine) (OIP) molybdenum\ncyclooctadiene precatalysts is described. The chemoselectivity of\nthe hydrogenation for the formation of tetralin versus decalin products\nwas systematically explored as a function of the aniline substituents\non the molybdenum precatalysts. Examples with 2,6-disubstitution with\nmethyl, ethyl, or iso-propyl substituents were the most active but\nproduced near equimolar mixtures of tetralins and decalins, both with\nhigh enantiomeric excesses. Introduction of anilines with either 2-<i>tert</i>-butyl or 2,5-di-<i>tert</i>-butyl substituents\nincreased the selectivity for decalin formation albeit with reduced\noverall hydrogenation activity. In all cases with 2,6-disubtituted\nnaphthalenes, high enantiomeric excesses are observed. Mechanistic\nstudies demonstrated that [(OIP)Mo] catalysts are inactive for tetralin\nhydrogenation and analysis of the catalytic reactions by NMR spectroscopy,\nin combination with independent synthesis, identified (OIP)Mo(η<sup>6</sup>-naphthalene) complexes as the catalyst resting states. Assaying\nproduct selectivity as a function of catalyst loading established\nthat increasing the amount of molybdenum precatalyst increased the\nselectivity for decalin formation. Substrate concentration studies\nwere also conducted and support a pathway whereby tetralin formation\narises from displacement of the arene from the coordination sphere\nof the molybdenum by incoming substrate. These studies establish general\ndesign principals for [(OIP)Mo] arene hydrogenation catalysts where\nlarger 2,6-aniline substituents increase activity, likely by promoting\nη<sup>6</sup> to η<sup>4</sup> haptotropic rearrangement\nfrom the arene resting state, while single <i>tert</i>-butyl\nsubstitution promotes more selective hydrogenation to decalin products.