The Wittig Reaction

Question 1

What is the TD driving force?

Answer 1

Formation of P=O

Question 2

Describe the chemoselectivity, regioselectivity and stereoselectivity of Witting reaction.

Answer 2

Chemoselectivity - ylid reactive towards all types of aldehydes and ketones (aldehydes much more reactive than ketones). Generally NO reaction wit nitriles, esters, amides, nitro groups etc.

Regioselectivity - a,B unsaturated aldehydes/ketones react exclusively in 1,2 fashion

Stereoselectivity - dependent on variety of factors, mostly importantly type of ylid

Question 3

How are phosphonium ylids formed?

Answer 3

1 - Formation of phosphonium salts. 2 - deprotonation of phosphonium salts.

Question 4

Describe ylid classification

Answer 4

Non-stabilised ylids give Z alkenes.
Stabilised ylids give E alkenes.
Semi-stabilised are less stereoselective.

Question 5

Describe the mechanism under salt-free conditions.

Answer 5

Under KINETIC control. Stereoselective formation of cis or trans oxaphosphetane. Stereospecific breakdown of oxaphosphetane to alkene. Salt-free conditions means no betaine formation.

Question 6

What is the effect of the ligands on P?

Answer 6

Triphenylphosphonium ylids give better Z selectivity than trialkylphosphonium ylids.

Question 7

What is the effect of the presence of Li salts?

Answer 7

Reduces cis/trans selectivity of initial cycloaddition. Facilitates retrocycloaddition back to aldehyde and ylid, introducing degree of equilibration into rxn which will increasingly favour TD product. Other metal halides soluble in organic solvents can also erode Z stereoselectivity.