(Z)-stereospecific Olefination (Wittig reaction analogue)

Utilising Still-Gernnari reagent

Why is this reaction stereospecific?

The 'erythro' adduct is more favourable than the 'threo' adduct as the transition state is the less hindered, therefore thermodynamically stable, thus in step 2, the (Z)-alkene is formed:

Refer to www.case.edu/artsci/chem/courses/chem435/Wittig_Reaction.pdf for full account.

Journal: Still, W.C.; Gennari, C., TETRAHEDRON LETTERS, 1983, 24, 4405