This effect is much more stabilizing when the

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Unformatted text preview: Dunbar, K and Petrik, I UIUC Chemistry 436 IV. Molecular Orbital Theory Applied to Carbonyl Addition Reactions: A B A standard nucleophilic attack at a carbonyl is shown above. The lone pairs on the nucleophile attack the π* orbital of the carbonyl carbon. The molecular orbital diagram for such an attack is shown below. It should be noted that the angle of attack is close to 104o to allow for good overlap with the π* orbital of the carbonyl. A: B: The stereoselectivity of attacks at carbonyls can be explained by extending our analysis of the carbonyl compound to consider the orbitals present at the α ­carbon. A σ* orbital present at this position that is anti ­ periplanar to the incoming nucleophile can help stabilize the nucleophile π* interaction, which lowers the energy of the electrons as they begin to form a bond with the carbonyl carbon. This interaction (shown below) is built into the Felkin ­Ahn model. Dunbar, K and Petrik, I UIUC Chemistry 436 The case above shows the interaction when the anti ­periplanar group is an alkyl group. This effect is much more stabilizing when the anti ­periplanar is an electronegative heteroatom such as oxygen, fluorine or chlorine. In these instances the σ* orbital is significantly lower in energy and a more favorable interaction can take place between the incipient bond and this anti ­bonding orbital....
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