The Aza-Wittig Reaction

The aza-Wittig reaction [112-117] is the nitrogen analog of the Wittig olefi-nation process and involves the reaction of an iminophosphorane [118-121] with a carbonyl group. The reaction provides an excellent method for the construction of carbon-nitrogen double bonds via inter and intramolecular aza-Wittig reaction.

The reaction is useful in the synthesis of acyclic imines [122-124] and heterocumulenes [112-117] and in the intramolecular formation of carbon-nitrogen double bonds in heterocyclic synthesis [112-117]. On the other hand aza-Wittig type reactions of iminophosphoranes with carbon dioxides, carbon disulphides, isocyanates, isothiocyanates and ketenes render access to functionalized heterocumulenes as highly reactive intermediates able to undergo a plethora ofheterocyclization reactions [112-117].

Stability, basicity, and nucleophilicity of iminophosphoranes are mainly determined by the substituents at the nitrogen atom. General reactivity trends of N- and P-substituted series (R' and R", respectively) are summarized as follows [108-110,112,118-121]:

R'N = PPh3 » ArN = PPh3 » R'CON = PPh3 - R'SO2N = PPh3 R'N = P(R/;)3 > R'N = P(Ar)3 > R'N = P(OR")3 > R'N = P(OPh)3

Carbonyl groups of aldehydes, ketones, acid halides, and heterocumulenes are generally reactive [108-110,112-117]. In the intramolecular version, amide, imide, and ester carbonyl groups are also reactive giving rise to imino-cyclization (see Sect. 2.1.3) [112-117].

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