From cyclic (alkyl)(amino)carbene (CAAC) precursors to fluorinating reagents. Experimental and theoretical study

Abstract

Addition of anhydrous HF to the hydrochloride [^MeCAACH][Cl(HCl)_0.5] resulted in the formation of salts with high HF content. By stepwise removal of HF in vacuo, we selectively prepared [^MeCAACH][F(HF)₂] (3) and [^MeCAACH][F(HF)₃] (4). We also characterised a salt with [F(HF)₄]⁻ anions within the structure of [^MeCAACH][F(HF)_3.5] (5). Compounds with a lower content of HF were not accessible under vacuum conditions. ^MeCAAC(H)F (1) was selectively prepared by abstraction of HF from 3 with CsF or KF, while [^MeCAACH][F(HF)] (2) was prepared by mixing 3 and 1 in a 1 : 1 ratio. Compound 2 proved to be quite unstable as it tends to disproportionate into 1 and 3. This observation triggered our computational study, in which the structural relationships between CAAC-based fluoropyrrolidines and dihydropyrrolium fluorides were investigated using different DFT methods. The study showed that the results were very sensitive to the computational method used. For a correct description, the quality of the triple-ζ basis set was crucial. Surprisingly, the isodesmic reaction of [^MeCAACH][F] + [^MeCAACH][F(HF)₂] → [^MeCAACH][F(HF)] + [^MeCAACH][F(HF)] did not confirm the low thermodynamic stability of 2. Furthermore, the use of 3 as a nucleophilic fluorinating reagent was tested on a range of organic substrates, as it is the most stable compound in this series. It was found to have the potential to fluorinate benzyl bromides, 1- and 2-alkyl bromides, silanes and sulfonyls with good to excellent yields of the target fluorides.