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

Results are reported of photolyses of rhodium dicarbonyl complexes of C 3 -symmetric Tp Menth and Tp Mementh 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 Tp Menth Rh(CO) 2 was found to contain an η 2 -Tp Menth ligand in the solid state by X-ray crystallography but was shown to be an interconverting mixture of η 2 and η 3 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 Tp Menth Rh(CO) 2 under a N 2 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) allowed assignment of all 1 H and 13 C resonances arising from the functionalized pyrazolyl group and identification of the R -C J, R -Rh absolute configurations of these new stereocenters in the major product. Various evidence, including spin-saturation transfer NMR and kinetics data, is reported that suggests (i) the minor isomer differs only in the configuration at C J and (ii) that the major and minor isomers thermally equilibrate via a mechanism involving rate-limiting alkyl−hydride reductive elimination and subsequent oxidative addition to either methyl group of the same pyrazolyl unit without inversion of configuration at the metal. The net result is cyclometalation with complete enantiocontrol at the rhodium center.