Phosphorus based compounds are extensively used in organic synthesis as reagents as well as ligands for metal-catalyzed reactions. This section covers some of the important applications as reagents.
5.4.1 Wittig Reaction
The reactions of alkyl halides with aldehydes or ketones in the presence of triphenylphosphine and base give alkenes in high yield (Scheme 1). The reaction is known as Wittig reaction and was awarded Nobel prize in 1979. The reaction is versatile and affords powerful tool for the construction of alkenes with excellent stereoselectivity.
Scheme 1
Mechanism
PPh3 reacts with alkyl halide to give phosphonium salt via the nucleophilic displacement (SN2) of halide by the nucleophilic phosphorus atom of triphenylphosphine (Scheme 2). The acidic hydrogen of the phosphonium salt can be removed by strong base to give phosphorus ylides (commonly known as Wittig reagent). These phosphorus ylides carry a positive and a negative charge on adjacent atoms can be represented as double bonded species, called phosphoranes. Phosphorus ylides are strong nucleophiles and add to aldehydes or ketones to form betain that collapses to a four membered ring called oxaphosphetatne, which can decompose to give the alkene and triphenylphosphine oxide.
Scheme 2