Burgi-Dunitz trajectory

Any other portions of the molecule that get in the way of (or, in other words, that cause steric hindrance to) the Burgi-Dunitz trajectory will greatly reduce the rate of addition and this is another reason why aldehydes are more reactive than ketones. The importance of the Burgi-Dunitz trajectory will become more evident later—particularly in Chapter 34. 

Figure 4.5. Compounds whose X-ray structures provided the basis for the Burgi-Dunitz trajectory.

Fig. 6.40. Eschenmoser s interpretation of glycolaldehyde phosphate aldomerisation reactions [37] It is generally appreciated... that the BUrgi-Dunitz trajectory... for nucleophilic addition to C = 0 groups must be taken into account as steric interactions between reaction center substituents are evaluated. The drawings in (this figure) remind the reader why. While it can be difficult to weigh the contributions of the four relevant interactions for an aldehyde/ketone-enolate pair, the problem for the case of an aldehyde/aldehyde-enolate pair turns out to have a unique solution the one indicated in (this figure), where none of the interacting substituents is juxtaposed with a non-H-atom partner ...

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 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.

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