The Biirgi-Dunitz Angle

The same angle of attack is seen for addition reactions to an alkene. The reason for the Biirgi-Dunitz angle is that the adding nucleophile needs to attack the empty it orbital of the C=0 or C=C bond. Looking back at Sections 1.3.2 and 1.3.3, or in Appendix 3, we find that the character of this orbital on C points back and away from the bonding region. 

The molecular orbital diagrams for a C-C ir bond and a C-O ir bond on the same energy axis, along with the energy of a lone pair donor orbital. The donor orbital is closer in energy to the acceptor orbital of the C-O ir bond. 

Remember that the substitution of a heteroatom such as oxygen for carbon lowers the energy of all orbitals, both the tt and tt. In the case of the lone pair orbital on the nucleophile filling either the alkene or carbonyl tt orbital, the interaction is best with the carbonyl, because the energies of the interacting orbitals are closer. The carbonyl is a better acceptor, because its empty orbital is lower in energy and closer to the energy of the donor orbital. 

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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...

Allow the nucleophile to attack along the least hindered trajectory, taking into account the Biirgi-Dunitz angle. 

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

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