P‐Heterosubstituted Phosphorus Ylides

Abstract

This chapter is conccrned with the synthesis and properties of phosphorus ylides substituted on the phosphorus atom of the P=C group with various heteroatoms. In reccnt years the chemistry of P-heteroatoni-substituted (or, simply, P-heterosubstituted) phosphorus ylides has been extensively d~velopcd'--~. Ylides containing heteroatoms attached to the phosphorus atom have especially interesting and frequently unusual properties, bccause of diffcrcnt effects of the substituent heteroatoms on the distribution of the electron density in the molecule of these compounds. X = RO, R2N, RS, Hlg, R3Si, R2P, etc. P-Heterosubstituted phosphorus ylides open a route to the preparation of many new types of organophosphorus compound to afford the possibility of interesting basic research into structure, reactivity, tautomeric transformations, and isorneri~ation~. P-0 YlidesAlkoxyphosphonium ylides are the type of P a phosphorus ylide studled in most detail. Ylides bearing ArO groups on the phosphorus are less studled. The characteristic feature of alkoxyphosphonium ylides is their tendency to dealkylate during both storage and chemical reactions. P-OAlk ylides are, therefore, less accessible than triphenylphosphonium ylides. P-0 ylides cannot be prepared by traditional synthetic methods. They cannot be synthesized by the salt method because of the instability of the starting alkoxyphosphonium salts3". SynthesisSeveral convenient routes to P-alkoxylides have been developed3. These compounds are usually obtained by ((oxidative ylidatiom of phosphorus(II1) compounds. The term 274 5 P-Heterosubstituted Ylides ((oxidative ylidatiom implies reactions as a result of whch the coordmtion number of the tervalent phosphorus atom increases and a P=C bond with a tetracoordinate phosphorus atom is f~r m e d~.~.The ((oxidative ylidation)) of tervalent phosphorus compounds, in particular of tertiary phosphmes and phosphonites containing a mobile hydrogen atom on the a carbon atom and behaving as CH acids of tervalent phosphorus, must be regarded as a general approach to the synthesis of P-heterosubstituted ylides. Compounds 1 containing electron-acceptor substituents R2 on the a carbon atolm, thus increasing the mobility of the hydrogen atoms and stabilizing the ylide function, are ylidated very Often, however, successful ylidation of weak CH acids, for example, trialkylphosphines, is possible (Fi q.5.1):Phosphorus(II1) CH acids are carbon analogs of phosphorus(I1I)OH acids, whch can be converted into ylides in the same way as the OH acids of tervalent pho~phorus~-~. In particular, phosphorus(II1)CH acids undergo reactions resembling the Atherton-Todd, Michaelis-Backer, and Pudovik reactions characteristic of dialkyl phosphites. Certain alkylphosphines of type 1 isomerize to ylides with a P-H bond, analogous to the prototropic P(0)H forms of the phosphorus(I1I)OH acids; this emphasizes still further the similarity of these types of organophosphorus compound. Depending on the structure of the initial tervalent phosphorus compounds, their ylidatio...