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.