1-Acyl-4-benzylpyridinium Tetrafluoroborates: Stability, Structural Properties, and Utilization for the Synthesis of Acyl Fluorides

Abstract

1-Acyl-4-benzylpyridinium salts 4 containing nonnucleophilic anions X– such as CF3SO3–, FSO3–, and BF4– can be generated quantitatively and in situ from 1-acyl-4-alkylidene-1,4-dihydropyridines 1a–f and the corresponding acid, HX. The BF4– salts reveal an interesting and unexpected thermal instability 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 splits off and functions as a Lewis base to intercept the BF3 acid. The structural and electronic relationships as well as dominating differences between the very reactive cations of 4 and their almost “inert” uncharged precursors, the dihydropyridines 1, are discussed both on the fundament of experimental evidence (X-ray structures of 1f and the extremely reactive and very labile 4f) and theoretical investigations (ab initio and DFT MO calculations).