ENANTIOSELECTIVE INVERSION OF A CHIRAL PHOSPHORYL RADICAL - A SINGLE-CRYSTAL ESR ANALYSIS OF X-IRRADIATED BIS(2,4,6-TRI-TERT-BUTYLPHENYL)PHOSPHINIC CHLORIDE

ESR experiments on an X-irradiated single crystal of enantiomerically pure bis(2,4,6-tri-tert-butylphenyl)phosphinic chloride (Ar2P(O)Cl) reveal the formation of the corresponding phosphoryl radical (1a,Ar2PO) via a dissociative electron-capture reaction of the PCl bond. At 120 K, this radical exhibits an exclusive and enantioselective stereoinversion in the single-crystal matrix leading to the phosphoryl radical structure 1b. This process was monitored in detail using anisotropic ESR spectroscopy combined with X-ray crystallographic data. An analysis of the nonbonded steric interactions demonstrates that the stereoinversion is controlled intramolecularly and occurs in a synchronous reorientation of the PO bond and two methyl groups of the ortho tert-butyl substituents. Besides the stereoinversion, other radical reactions are observed in the crystal matrix. Ab initio quantum chemical calculations are employed to assess the electronic structure of the phosphoryl radical in more detail. An ROHF/6-31G* geometry optimization within C(s) symmetry constraints yields two equilibrium structures which basically differ by their PO bond length (1.478 and 1.616 angstrom). The two geometries correspond to the canonical valence bond structures: H2P+-O- and H2P-O, respectively. Calculated P-31 hyperfine interactions of the H2P+-O- equilibrium structure are in good agreement with experiment.