High-Yield Thermolytic Conversion of Imidazolium Salts into Arduengo Carbene Adducts with BF3 and PF5

Thermolysis of 1-ethyl-3-methyl-1H-imidazolium tetrafluoroborate (1) and 1,3-dimethyl-1H-imidazolium hexafluorophosphate (3) under reduced pressure eliminates HF to furnish the BF3 and PF5 adducts of the corresponding Arduengo carbenes (2 and 4) in high yields. The intuitively anticipated imidazole N adducts with BF3 and/or PF5 arising from elimination of alkyl fluorides are not detected at all. These observations represent the first examples of a direct bond rearrangement of the type [C–H] + E–F → [C–E] + H–F (E = B, P). DFT computational studies suggest a single-step mechanism for the reaction 1 → 2 + HF. Lower yield thermolysis of a 1,2,3-trimethyl-1H-imidazolium hexafluorophosphate (11) into [(1,3-dimethyl-1H-imidazolium-2-yl)­methyl]­pentafluorophosphate (12) supports the generality of this transformation.