1-Boryl-3,4-dimethylphosphole trimer: Synthesis, crystal structure and quantum chemical calculations

A novel cycloadduct of 1-boryl-3,4-dimethylphosphole was prepared by reaction of 3,4-dimethylphospholyl anion with monobromoborane–methylsulfide complex (CH3)2S · BH2Br at −60 °C. It was characterized as a six-membered trimer by spectroscopic means, and its structure confirmed by an X-ray crystal analysis and quantum chemical calculations. Density functional theory calculations (B3LYP) showed that the cyclic trimer is by far more stable than the monomer, dimers or open-chain forms. Various molecular and spectroscopic properties of the borylphosphole monomer and trimer were evaluated. In particular, the changes of the 31P NMR chemical shifts upon oligomerization were examined. The six-membered ring was demonstrated to exist preferentially in a chair-like conformation. Computed NMR chemical shifts (1H, 13C and a lesser extent 31P) appear to be a highly sensitive analytical tool for distinguishing ring conformations having only small energy differences.