Question

In a stereospecific wittig reaction: Discuss the betaine intermediate of the Wittig reaction and why the stereochemistry of the intermediate determines the stereochemistry of the alkene product.

Answer 1

Solution:

The Wittig reaction has been described as a stepwise ionic process.The hypothetical betaine intermediates were never observed, but lithium halide adducts could be isolated in some of the early Wittig experiments. The newest hypothesis attributes stereoselectivity to a combination of steric effects and varying degrees of rehybridization at
phosphorous in the formation of the covalent oxaphosphetane.

The experimental and theoritical insights of witting reaction is summarized as,

• Under salt-free, aprotic conditions, ylides Ph3P=CHR (R=alkyl, alkenyl, phenyl) react with aldehydes to produce the oxaphosphetane directly via four-centered transition states.
• The Z:E ratio of alkenes corresponds to the cis-trans ratio of oxaphosphetanes in typical reactions (kinetic control). This is true of nonstabilized ylides as well as carbonyl-stabilized ylides, although there are exceptions.
• The oxaphosphetane decomposes by a syn-cycloreversion process to the alkene.
• There are no zwitterionic or diradical intermediates having significant lifetimes.
• Betaines are energetically uphill compared to reactants as well as to oxaphosphetanes.