Diastereoselective Intramolecular C−H bond Activation by Optically Active Tris(pyrazolyl)hydroborate Complexes of Rhodium

Abstract

Results are reported of photolyses of rhodium dicarbonyl complexes of C3-symmetric TpMenth and TpMementh ligands that illustrate the capability of these scorpionates to exhibit a high degree of regio- and stereocontrol in reactions involving intramolecular attack of ligand substituent C−H bonds. The starting material TpMenthRh(CO)2 was found to contain an η2-TpMenth ligand in the solid state by X-ray crystallography but was shown to be an interconverting mixture of η2 and η3 forms in toluene solution by IR and NMR spectroscopy. A mechanism for the fluxionality based on one previously suggested for achiral systems was supported by line shape analysis of VT-NMR data. Irradiation of TpMenthRh(CO)2 under a N2 purge in a variety of solvents resulted in the generation of an 85:15 mixture of diastereomeric alkyl hydrides resulting from intramolecular cyclometalation reactions involving the methyl substituents on the ligand isopropyl group. Analysis of two-dimensional NMR data (DQ-COSY, NOESY, HMQC, TOCSY spectra...