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

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) Å 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‐l. In CD2C12 at ‐90°C, δ31P is shifted to +3.5 ppm to low field, suggesting virtually complete ring closure under these conditions.