Dunitz angle

Now that we have decided which are the important conformations, how do we know which gives the product We need to decide which is the most reactive. All we need to do is to remember that any nucleophile attacking the carbonyl group will do so from the Biirgi-Dunitz angle—about 107° from the C=0 bond. The attack can be from either side of C=0, and the following diagrams show the possible trajectories superimposed on the two conformations we have selected, which are in equilibrium with one another. 

It is a common misunderstanding to think that the Biirgi-Dunitz angle implies that the angles are acute. They can be sometimes, but they are not usually—the... 

Fig. 5.2 The Salem-Klopman equation applied to the Biirgi-Dunitz angle...

Attack by a nucleophile breaks the re-bond, and the electrons reside on the oxygen atom. This is energetically favourable, as a strong o-bond is formed at the expense of a weaker re-bond. Examination of crystal structures has shown that the nucleophile approaches the carbonyl carbon at an angle of around 107°. This is known as the Burgi-Dunitz angle (see Section 4.10). 

Figure 4.6. (a) Orientation of the superimposed carbonyl and nitrogen atoms, (b) Superimposed plot of the N, C, and O atoms of structures A-F, and the variance of the RRC plane from the RRO plane, a is the Burgi-Dunitz angle, 107°. Reprinted with permission from ref. [21], copyright 1973, American Chemical Society. 

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