Sterically Congested Phosphite Ligands: Synthesis, crystallographic characterization, and observation of unprecedented eight‐Bond 31P,31P coupling in the 31P‐NMR spectra

Abstract

AbstractThe synthesis and characterization of 2‐{1‐{3,5‐bis(1,1‐dimethylethyl)‐2‐{[2,4,8,10‐tetrakis(1,1‐dimethylethyl)dibenzo[d,f][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 Å is found, which is within 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 optimizations (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* = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo[d,f][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.