CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

If a strongly electronegative group, e. g., a halogen atom is present α to the C=O
group, a dipolar model is suggested to predict the stereochemical outcome of the reaction.
The dipoles of the carbonyl bond and the C-X bond oppose each other and so they are
placed anti as in the model 26 so as to minimize the dipolar repulsion. The nucleophile
adds from the side of S giving the major product as shown in Scheme I.3.9.
Nu
X
S
R
M
O
X
R
Nu
S OH
M
26 27
Scheme I.3.9
Desptite correctly predicting the stereochemical course of the reactions, the Cram's
models often fails to give a quantitative assessment of the asymmetric induction in terms
of steric interactions. A few alternative models have been proposed to predict the
stereochemical outcome of the designated reactions in more quantitative terms. Of these
the Felkin-Anh model, discussed below has gained consensus.
In Felkin-Anh model, 24 In this model, two reactive conformations 28 and 29
(Scheme I.3.10) have been considered in which either the largest (L) or the most electron
withdrawing group (which provides the greatest σ*-π* overlap with the carbonyl π*
orbital, allowing delocalization of the electron density by hyperconjugation from the
reaction centre towards L) at C α is placed at right angle to the C=O double bond. Between
the two, the first with M opposing C=O and S gauche to R is usually preferred. The nonbonded
interactions which involve R and S (rather than R and M as in 29) are thus
minimized. The model predicts the same stereochemistry as Cram’s, but provides a more
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