Wittig Reaction
The Wittig Reaction allows the preparation of an alkene by the reaction of an aldehyde or ketone with the ylide generated from a phosphonium salt.
The geometry of the resulting alkene depends on the reactivity of the ylide. If R'' is Ph or R is an electron withdrawing group, then the ylide is stabilized and is not as reactive as when R'' and R are alkyl. Stabilized ylides give (E)-alkenes whereas non-stabilized ylides lead to (Z)-alkenes (see also Wittig-Horner Reaction).
Mechanism of the Wittig Reaction
(2+2) Cycloaddition of the ylide to the carbonyl forms a four-membered cyclic intermediate, an oxaphosphetane. Preliminary posultated mechanisms lead first to a betaine as a zwitterionic intermediate, which would then close to the oxaphosphetane. The intermediacy of such betaines plays an important role in the Schlosser Modification. Betaines may be stabilized by lithium salts leading to side products; therefore, suitable bases in the Wittig Reaction are for example: NaH, NaOMe, NEt3.
The driving force is the formation of a very stable phosphine oxide:
Reactive ylides give rapid reaction and subsequent rapid ring opening to give the (Z)-alkene:
FOLLOW AND VISIT OUR BLOG (ELIMU YETU BLOG )DAILY
