azaWittig

Aza-Wittig, or the aza-Wittig reaction, is a chemical transformation in organic synthesis in which an iminophosphorane species reacts with a carbonyl compound (aldehyde or ketone) to form an imine (Schiff base) with concomitant loss of a phosphine oxide. It is conceptually analogous to the Wittig reaction, which forms alkenes, but replaces the carbonyl-derived step with nitrogen transfer to yield C=N bonds.

The iminophosphorane, often represented as R3P=NR', can be generated in situ by a Staudinger reaction between an azide and triphenylphosphine, or used as a preformed reagent. On reaction with an aldehyde or ketone, the carbonyl carbon is activated toward attack by the iminophosphorane nitrogen, forming a betaine-like intermediate that collapses to the corresponding imine and triphenylphosphine oxide. The process is typically conducted under mild, neutral conditions and is compatible with a range of functional groups.

Scope and applications of the aza-Wittig reaction include converting carbonyl groups to imines under relatively mild conditions, enabling the preparation of N-substituted imines that serve as valuable intermediates in organic synthesis. It has been employed in the synthesis of heterocycles, natural product scaffolds, and in protective-group strategies. Limitations can include sensitivity to steric hindrance and competing side reactions; solvent choice and additives can influence reaction efficiency and, in some cases, the stereochemical outcome of imine formation.

See also: Wittig reaction, Staudinger reaction, imine, iminium.