Question

1. Who received the Noble Prize for the discovery of the Wittig reaction?
2. The Wittig reaction usually gives a mixture of cis and trans isomers, yet in this
reaction, only the trans isomer is formed. The stereochemistry of product is set
in the first step of the reaction of the ylid with the aldehyde. Draw the
intermediate oxaphosphetane that leads to the trans-alkene geometry, and the
one that would lead to the cis - alkene geometry, showing the stereochemistry at
all of the chiral centers. Can you suggest a reason for the fact that only the trans
alkene is formed?
3. It is often possible to make an alkene from two different combinations of a
carbonyl group and a phosphonium salt. What is the other combination of
carbonyl compound and phosphonium ylide that could be used to make the
product in this reaction?

Answer 1

1. Georg Wittig was awarded the Nobel Prize in Chemistry in 1979 for the discovery of the Wittig reaction.

2. Both cis and trans alkenes can be formed from the Wittig reaction, but it is a stereoselective reaction which gives the cis alkene as the major product.

The question that why this reaction leads to mainly the cis alkenes is not solved yet. The probable explanation is :

Due to the rules of orbital symmetry the carbonyl compound and the ylide approach perpendicular to each other. In this approach two intermediates are possible. The first intermediate shown in the picture is less stable because the large substituents are on the same side. In the second intermediate the large substituents are far apart from each other; this intermediate leads to the cis product.

3. The combinations of carbonyl compound and phosphonium ylide are shown in the second picture.