Esters phosphonate, ylids from

More general methods depend on Wittig reactions with functionalised ylids. The ylid24 from 116 and the lithium derivatives of 117-120 all react with aldehydes and ketones to give enol ethers that can be hydrolysed to chain-extended aldehydes. Yields with the ylid from 116 are not always wonderful and the phosphonate ester 120a with a chelating substituent generally does better.25... 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

As the ylid 51 is stabilised by the nitrile as well as the benzene ring, the phosphonate ester 54 is preferred in the manufacture and the reaction is strongly trans selective.13 The by-product is the anion of dimethyl phosphate 55 which is water-soluble and very easy to separate from the product 50. By contrast, triphenylphosphine oxide is insoluble in water and can be difficult to separate from the alkene. 

A.iu. Phosphine Oxides and Phosphonate Esters. Many extensions of the Wittig reaction have been introduced that improve or modify the reactivity and/or stereoselectivity of the ylid. Horner et al. showed that a-lithiophosphine oxides such as that derived from 552 react with aldehydes or ketones to give a p-hydroxy phosphine oxide (553) as an isolable species. Subsequent treatment with base liberates the alkene, (554). Wadsworth and Emmons modified the Horner reaction to use phosphonate ester derivatives such as... 

---