Simple phosphoranes are very reactive and are unstable in the presence of air or moisture. They are therefore prepared in a scrupulously dry solvent under nitrogen and the carbonyl compound is added as soon as the phosphorane has been formed.

More stable phosphoranes are obtained when a –M substituent is adjacent to the anionic carbon (Scheme 4). However, although they react with aldehydes, they do not do so effectively with ketones and for this purpose modification has been introduced in which triphenylphosphine is replaced by triethyl phosphate (Wadsworth-Emmons Reaction).

Scheme 4

Examples:

10.1.2 Wittig Indole Synthesis

Indole is an important structural unit present in many natural products and biologically active compounds. The reaction of (2-amidobenzyl)triphenylphosphonium salt with base allows the synthesis of 2-substituted indoles (Scheme 5-6).

Scheme 5

Mechanism

Scheme 6