Phosphonium ylid acylation

The physical and chemical properties of the X -phosphorins 118 and 120 are comparable to those of phosphonium ylids which are resonance-stabilized by such electron-pulling groups as carbonyl or nitrile substituents Thus they can be viewed as cyclic resonance-stabilized phosphonium ylids 118 b, c, d). As expected, they do not react with carbonyl compounds giving the Wittig olefin products. However, they do react with dilute aqueous acids to form the protonated salts. Similarly, they are attacked at the C-2 or C-4 positions by alkyl-, acyl- or diazo-nium-ions Heating with water results in hydrolytic P—C cleavage, phosphine oxide and the hydrocarbon being formed. 

Treatment of the salt, [PhsBiCFhCOR1] BF4 with base generates triphenylbis-muthonium 2-oxoalkylide (84 R1 = Buf, Ph). This reacts with 1,2-dicarbonyls to give 2,3-diacyloxiranes (85 from acyclic reactants, MeCOCOR2, R2 = Me, OEt) or 2-acyl-3-hydroxytropones [e.g. (86), from the tetrachloro-o-quinone].123 Both reaction types are of considerable synthetic utility, and both are in marked contrast to the routes followed by the corresponding phosphonium ylids. 

Several aromatic acyl silanes and the fascinating but unstable pink carbonyl bis(trimethylsilane) have been prepared in reasonably good yields by the oxidation of phosphonium ylids (Scheme 17)13. 

This chapter will discuss carbanion-like reactions that utilize enolate anions. The acid-base reactions used to form enolate anions will be discussed. Formation of enolate anions from aldehyde, ketones, and esters will lead to substitution reactions, acyl addition reactions, and acyl substitution reactions. Several classical named reactions that arise from these three fundamental reactions of enolate anions are presented. In addition, phosphonium salts wiU be prepared from alkyl halides and converted to ylids, which react with aldehydes or ketones to form alkenes. These ylids are treated as phosphorus-stabilized car-banions in terms of their reactivity. 

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