53. Sterically Congested Phosphite Ligands: Synthesis, Crystallographic Characterization, and Observation of Unprecedented Eight-Bond 31P,31P Coupling in the 31P-NMR Spectra

Article abstract of DOI:10.1002/hlca.19930760214

The synthesis and characterization of 2-<1-<3,5-bis(1,1-dimethylethyl)-2-<<2,4,8,10-tetrakis(1,1-dimethyl)dibenzo<1,3,2>dioxaphosphepin-6-yl>oxy>phenyl>ethyl>-4,6-bis(1,1-dimethylethyl)phenyl diphenyl phosphite (6) is described.In the 31P-NMR spectrum (1H-decoupled) of 6, an unprecedented eight-bond P,P coupling of J=72.8 Hz is observed.In the X-ray crystal structure of 6, an intramolecular P-P distance of 3.67 Angstroem is found, which is in the sum of the van-der-Waals radii of the P-atoms.The observed intramolecular P-P distance suggests that a through-space coupling mechanism is operative.The solid-state conformation of 6 is compared to the conformation obtained by semi-empirical MO geometry optimization (PM3 method).The calculated geometry suggests that the solid-state structure is near a true energy minimum, but that crystal-packing forces decrease the intramolecular P-P distance in the solid state.In the absence of crystal-packing forces, however, the collisional and vibrational energy available in solution may lead to the population of states with a shortened intramolecular P-P distance in 6.The proximity of the P-atoms in 6 is due to restricted conformational freedom resulting from steric congestion within the molecule.The free energy of activation (ΔG(excit.) = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo<1,3,2>dioxaphosphepin ring in 6 is determined by variable-temperature 31P-NMR spectroscopy.Semi-empirical MO calculation on model compounds suggest that the structure of the transition state for ring inversion has the two aryl rings and O-atoms in a common plane, with the P-atom lying above this plane.