Enantioenriched Molybdenum Dearomatization: Dissociative Substitution with Configurational Stability

The preparation and properties of the complex (RMo,R)-MoTp­(NO)­(DMAP)­(η2-α-pinene) are described (~10 g scale; DMAP = 4-(dimethylamino)­pyridine; Tp = hydridotris­(pyrazolyl)­borate). This complex undergoes exchange of the pinene with a wide range of other π ligands including acetone, ethyl acetate, N,N-dimethylformamide, acetonitrile, and naphthalene. Treatment of the α-pinene complex with iodine results in the complex (S)-MoTp­(NO)­(DMAP)­(I), which is recovered in enantioenriched form (er = 99:1; yield >90%; scale 4.6 g). Reduction of this molybdenum­(I) precursor results in enantioenriched molybdenum(0) complexes, including (R)-MoTp­(NO)­(DMAP)­(η2-trifluorotoluene). Sequential treatment of this arene complex with acid, a masked enolate, and iodine regenerates MoTp­(NO)­(DMAP)­(I) along with an alkylated 1-(trifluoromethyl)­cyclohexa-1,3-diene with an er value as high as 99:1. This process demonstrates the efficient transfer of asymmetry from α-pinene to the diene product. Accompanying studies with (1R)-myrtenal reveal a redox-catalyzed pinene/myrtenal ligand exchange occurring through Mo­(I) intermediates.