Phosphoranes alkylidenephosphoranes

A variety of 1-azirines are available (40-90%) from the thermally induced extrusion (>100 °C) of triphenylphosphine oxide from oxazaphospholines (388) (or their acyclic betaine equivalents), which are accessible through 1,3-dipolar cycloaddition of nitrile oxides (389) to alkylidenephosphoranes (390) (66AG(E)1039). Frequently, the isomeric ketenimines (391) are isolated as by-products. The presence of electron withdrawing functionality in either or both of the addition components can influence the course of the reaction. For example, addition of benzonitrile oxide to the phosphorane ester (390 = C02Et) at... 

Quinquecovalent phosphoranes readily undergo ligand exchange on treatment with organolithium compounds. With alkyl-lithiums the resulting alkylphosphoranes give alkylidenephosphoranes, e.g. (2), by loss of benzene. 

The P=C bond of ), a -phosphoranes is also accessible to a 1,3-dipolar cycloaddition reaction. The formation of 97 (Scheme 8.23), of 3,5-dihydro-1,2,4-diazaphosphole sulfide 132 from an amino(methylene)thioxophosphorane (190) and of phosphirane imine 133 from an amino(imino)alkylidenephosphorane (191) (Scheme 8.29) illustrate the chemical behavior of phosphaalkenes containing a A, a -phosphoms atom. 

The reaction of the phosphonium salts 28, generated from the -halogencarboxylic esters 26, with sodium ethoxide in DMF or sodium hydride in DMSO yields the phosphoranes 29 which, on treatment with aldehydes 30, are converted into (Z)-alkenoic esters 31 49,51 53). To synthesize As-unsaturated acids and higher homologous acids of type 31 (m 11) Bergelson et al. 49,54) olefinated ethyl co-formyl-alkanoates 34, obtained from co-iodoalkanoic esters 32, with alkylidenephosphoranes 35 to the ethyl esters 31 (Scheme 7). 

A full account has appeared of the reactions of acyl azides with jS-keto-alkylidenephosphoranes. These phosphoranes with nitrile oxides gave isoxazoles (54) and the stable ylides (56) or (57), formed as shown via the quinquecovalent 1 1 adducts (55). ... 

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