Temperature-dependent Ring-Opening and -Closure of a Cyclic Phosphane-Borane

Article abstract of DOI:10.1002/cber.19971300722

A phosphane-borane (3) with the P-B bond integrated into a seven-membered ring was prepared from 4-pentenyl-diphenylphosphane (4) by hydroboration using 9-borabicyclononane (9-BBN). The product was confirmed to have a ring structure in the solid state by single crystal X-ray diffraction. The P-B distance of 2.057(2) A is indicative of a standard donor-acceptor bond similar to the type found in homologous five- and six-membered rings. The room-temperature ³¹P-NMR signal of the compound in various solvents (δ = - 15) is not compatible with the cyclic structure and suggests that ring opening occurs as the compound is dissolved. Variable-temperature NMR work corroborated this assumption, and the enthalpy of ring closure was determined to be ΔH = -30.5(4) kJmol^-1. In CD₂Cl₂ at -90°C, δ³¹P is shifted to +3.5 ppm to low field, suggesting virtually complete ring closure under these conditions. VCH Verlagsgcsellschaft mbH,.