Rearrangement Reactions of the Transient Lewis Acids (CF3) 3B and (CF3)3BCF2: An Experimental and Theoretical Study

Article abstract of DOI:10.1021/ic0350640

Short-lived (CF₃)₃B and (CF₃) ₃BCF₂ are generated as intermediates by thermal dissociation of (CF₃)₃BCO and F-abstraction from the weak coordinating anion [B(CF₃)₄]^-, respectively. Both Lewis acids cannot be detected because of their instability with respect to rearrangement reactions at the B-C-F moiety. A cascade of 1,2-fluorine shifts to boron followed by perfluoroalkyl group migrations and also difluorocarbene transfer reactions occur. In the gas phase, (CF ₃)₃B rearranges to a mixture of linear perfluoroalkyldifluoroboranes C_nF_2n+1BF₂ (n = 2-7), while the respective reactions of (CF₃)₃BCF ₂ result in a mixture of linear (n = 2-4) and branched monoperfluoroalkyldifluoroboranes, e.g., (C₂F₅)(CF ₃)FCBF₂. For comparison, the reactions of [CF ₃BF₃]^- and [C₂F₅BF ₃]^- with AsF₅ are studied, and the products in the case of [CF₃BF₃]^- are BF₃ and C₂F₅BF₂ whereas in the case of [C ₂F₅BF₃]^-, C₂F ₅BF₂ is the sole product. In contrast to reports in the literature, it is found that CF₃BF₂ is too unstable at room temperature to be detected. The decomposition of (CF₃) ₃BCO in anhydrous HF leads to a mixture of the new conjugate Bronsted-Lewis acids [H₂F][(CF₃)₃BF] and [H₂F][C₂F₅BF₃]. All reactions are modeled by density functional calculations. The energy barriers of the transition states are low in agreement with the experimental results that (CF₃)₃B and (CF₃)₃BCF₂ are short-lived intermediates. Since CF₂ complexes are key intermediates in the rearrangement reactions of (CF₃)₃B and (CF₃)₃BCF₂, CF₂ affinities of some perfluoroalkylfluoroboranes are presented. CF₂ affinities are compared to CO and F^- affinities of selected boranes showing a trend in Lewis acidity, and its influence on the stability of the complexes is discussed. Fluoride ion affinities are calculated for a variety of different fluoroboranes, including perfluorocarboranes, and compared to those of the title compounds.