Dunitz-Burgi attack

The n co orbital is has a larger coefficient at C than at O. The 7t cc orbital has equal coefficients on Q and C2. Therefore, the negative overlap between the nucleophile Nu and the distal extremity of the double bond (O or C2), which is the cause of the Dunitz-Burgi obtuse angle of attack, is larger with n cc than with ji co ... 

Scheme 9.42. A representation of the base-catalyzed aldol condensation reaction (using the hydroxide ion [OH ] as a typical base) between two equivalents of ethanal (acetaldehyde [CH3CHO]). The steps, both deprotonation-protonation and carbon-carbon bond forming are reversible (the retroaldol reaction). The carbon-carbon bond forming step is shown as occurring on the re face of the aldehyde and with an approximate Burgi-Dunitz trajectory. Attack at the si face is equally likely and the product is racemic.

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). 

The same angles, the Burgi-Dunitz and the Flippin-Lodge, will have their counterparts for nucleophilic attack on a C=C bond, but the former at least ought to be muted, because all three factors 5.50-5.52 will be reduced when the oxygen atom of the carbonyl group is replaced by a carbon atom. 

Although we now know precisely from which direction the nucleophile attacks the C=0 group, this is not always easy to represent when we draw curly arrows. As long as you bear the Burgi-Dunitz trajectory in mind, you are quite at liberty to write any of the variants shown here, among others. 

Figure 9.8. A pictorial representation of Crain s rule (the Felkin-Anh model), which suggests that attack at the carbonyl group (along the Burgi-Dunitz trajectory ) is strongly influenced by the steric effects introduced by neighboring substituents.

Scheme 10.28. A repeat of Scheme 9.35. Here, Nu represents an amino function and the attack of the incoming group is drawn to be greater than 90° as suggested by Biirgi et al. (BUrgi, H. B. Dunitz, J. D. Shefter, E. J. Am. Chem. Soc., 1973,95,5065). Affirming the data in Table 10.4, the carbonylamines shown here generally lose water to produce compounds with carbon-nitrogen double bonds. See Scheme 10.29.

The attack of carbonyl by nucleophile is a simple example of an outsie stereochemical preference Burgi HB, Dunitz JD (1983) Acc. Chem. Res. 16 153... 

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