1-acyl-4-benzylpyridinium tetrafluoroborates: Stability, structural properties, and utilization for the synthesis of acyl fluorides

1-Acyl-4-benzylpyridinium salts 4 containing nonnucleophilic anions X- such as CF3SO3-, FSO3-, and BF4- can be generated quantitatively and in situ fr om 1-acyl-4-alkylidene-1,4-dihydropyridines 1a-f and the corresponding acid , HX. The BF4- salts reveal an interesting and unexpected thermal instabili ty which allows the convenient synthesis of carboxylic acid fluorides 5b-f. This procedure offers advantages over known methods: All operations can be performed in a standard glass apparatus and do not require high pressures. The formation of RCOF 5 is assisted by the pyridine moiety of 4, which spl its off and functions as a Lewis base to intercept the BF3 acid. The struct ural and electronic relationships as well as dominating differences between the very reactive cations of 4 and their almost "inert" uncharged precurso rs, the dihydropyridines 1, are discussed both on the fundament of experime ntal evidence (X-ray structures of 1f and the extremely reactive and very l abile 4f) and theoretical investigations tab initio and DFT MO calculations ).