The Formation of Pyridines or Pyrroles by the Reaction of N-Vinyliminophosphoranes with α, β-Unsaturated Ketones or α-Bromo Ketones

Abstract

The reaction of N-(1-phenylvinyl)iminotriphenylphosphorane [1 a] and N-(1-phenylvinyl)iminotributylphosphorane [1 b] with α, β-unsaturated ketones [2 a-e] an d with α-bromo ketones [14 a-j] were investigated. The compounds [1 a, b] underwent the Michael addition with [2 a-d] followed by aza-Wittig reaction to result in the formation of phenyl-substituted pyridines [3 a-d]along with acetophenone and phosphine oxides. The isolation of diketones [5 b, c] is a good evidence for the mechanistic aspect of the reactions. On the other hand, the reaction of [l a, b] with [2 e], which has neither methyl nor phenyl group on the carbonyl carbon atom, β-carbon atom of [l a, b]underwent a charge-controlled attack to thecarbonyl group to result in the formation of the pyridine [4 e]. In the reaction of [1 a, b]with α-bromo ketones [14 a-i], phenyl-substituted pyrroles [15 a-i] was obtained via a novel C-C bond formation followed by aza-Wittig reaction. However, [1 a]with ethyl bromoacetate [14 j] underwent no aza-Wittig reaction but instead a C-C bond formation to give [17j] was observed. The ₁H-NMR and ₁₃C-NMR spectra of [l a, b] also suggested the nucleophilic property of the β-carbon atom of the vinyl group.