Building an Alkene Spacer by the Wittig Reaction: Synthesis of 4-[2-(4-Nitrophenyl)ethenyl]benzonitrile

Abstract

The Wittig reaction is one of the most common and valuable methods for the formation of alkenes, starting from a carbonyl compound and a primary or secondary alkyl halide. The reaction involves the formation of an alkene from an aldehyde or a ketone using a phosphorous ylide derivative, R 2 C − -P + (C 6 H 5 ) 3 . This latter species is obtained from a nucleophilic substitution reaction of an alkyl halide by triphenylphosphine, followed by deprotonation of the correspond- ing phosphonium salt by a strong base. This reaction has wide applicability, using different aldehydes and ketones and alkyl halides, despite its relatively slow rate reaction with ketones. Knowledge of the exact position of the double bond is also an advantage of the method. The aim of this work is the synthesis of the 4-[2-(4-nitrophenyl)ethenyl]benzonitrile and the isolation of the (E)-isomer. This example illustrates the use of the Wittig reaction to join two reacting species by the formation of a new carbon–carbon double bond, with the consequent increase in size and complexity of the molecule. The new double bond links two phenyl rings, extending the carbon chain and the electronic delocalization through the molecule, allowing electronic transfer processes between the substituents of each ring. The presence of a nitrile group allows the coordination to transition metal moieties or the transformation into other functional groups. Isolation of the (E)-isomer is possible by selective precipitation.