Conformational Behavior of Tris(pentafluorophenyl)borane−Benzotriazole Adducts

The stereomutations in a class of tris(pentafluorophenyl)borane−benzotriazole adducts are investigated by means of variable-temperature 19F NMR and X-ray crystallography. At low temperature the presence of a pair of conformational enantiomers is confirmed by NMR spectra obtained in a chiral medium. Two different energy barriers, corresponding to an enantiomerization process and a B−N rotation, were observed and their values measured (ΔG⧧ = 10.7 and 12.8 kcal mol-1 in the case of 1). In the case of the bis-adduct 3, two conformational diastereoisomers with different populations were detected at low temperature, while only one diastereoisomer is present in the crystalline state.