Ylids, phosphorus from phosphonium salts

In the Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also called a phosphorane) to give an alkene. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from the phosphine and an alkyl halide (10-44) ... 

If we view the Wittig reaction from an alkyl halide starting material (alkyl halide phosphonium salt phosphorus ylid alkene), the halogen-bearing carbon of an alkyl halide must contain at least one hydrogen as in 59 (for deprotonation at the phosphonium salt stage). 

Phosphorus ylids are prepared by treatment of phosphonium salts with bases such as phenyllithium, butyllithium. dimsyl anion, or potassium f-butoxide. They are not isolated but used directly after preparation because they are sensitive to oxygen and moisture. The requisite phosphonium salts are available in great variety from the reaction of triphenylphosphine and primary or secondary alkyl halides, usually bromides [24]. 

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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