N-Vinylic Phosphazenes. A Useful Tool for the Synthesis of Acyclic and Heterocyclic Compounds

The synthesis and reactivity of the N-vinylic phosphazene derivatives is the subject of this review. The first part gives a survey of different methodologies for the preparation of N-vinylic phosphazenes. These N-vinylic phosphazenes can be prepared either, by Staudinger reaction of N-vinylic azides with phosphines, or by reaction of phosphorus ylides and nitriles. The second part covers the high synthetic potential of these N-vinylic phosphazenes as valuable building blocks in preparative organic synthesis. N-Vinylic phosphazenes can be used as starting materials for the preparation of enamines by hydrolysis under basic or acidic conditions, and can be transformed into aminophosphonium salts by treatment with hydrogen halides or acyl halides. However, the most important synthetic application of N-vinylic phosphazenes is their use for the construction of carbon-nitrogen double bonds by means of aza-Wittig reaction with carbonyl compounds. This aza-Wittig reaction of N-vinylic phosphazenes has been widely applied for the preparation of simple and functionalized 2-aza-1,3-butadienes. These 2-azadienes are excellent starting materials for the construction of acyclic and heterocyclic compounds, and more elaborated polycyclic derivatives. Also the reactivity of N-vinylic phosphazenes with cumulene double bonds as carbon dioxide, carbon disulfide, isocyanates, isothiocyanates, ketenes and triple bonds, and the use of this process for the synthesis of acyclic and cyclic derivatives has been reviewed.