The phosphorus reagents have three characteristics: the ease with which phosphorus (III) is converted into phosphorus (V); the relatively strong bonds formed between phosphorus and oxygen; and the availability of vacant 3d orbitals for bonding.

10.1.1 Wittig Olefination

The Wittig reaction for the synthesis of alkenes stems from two properties: First, it is specific for the conversion of aldehydes or ketone to alkenes. Second, the carbonyl compounds can contain a variety of other functional groups (Scheme 1).

Mechanism

The phosphorus ylides are prepared by quaternizing a tervalent phosphorus compounds with an alkyl halide and treating the salt with base. The phosphorane adds as a carbon nucleophile to the carbonyl group and the resulting intermediate reacts via a cyclic intermediate to form the alkene (Scheme 2-3).

Scheme 2

Notes:

Other phosphines may be used for this reaction, but the choice should not contain a proton that could be abstracted by base, because a mixture of desired and undesired ylides would be formed.

Usually strong bases such as BuLi, NaH and NaNH₂ are used.

Scheme 3