Mechanochemical and Conformational Study of N-heterocyclic Carbonyl-Oxime Transformations

New mechanochemical pathways for the transformation of six N-heterocyclic carbonyl compounds into oximes using hydroxylamine hydrochloride were explored. Reactions were performed first without any base since the heterocyclic moieties (imidazole, benzimidazole, pyridine and quinuclidine) have an intrinsic basic nitrogen atom. This green, solvent free method was suitable for all compounds (up to quantitative yields) except for N-benzyl substituted imidazole and benzimidazole-2-carbaldehyde. For the slower reacting aldehydes, reactions with liquid assisted grinding and addition of sodium hydroxide were performed as well. Conformational analysis and quantum-chemical calculations revealed steric and electronic reasons for the lower reactivity of N-benzyl substituted derivatives.