Superjacent orbital control

Since the stablest conformation of compounds of this type (as well as anions) is determined by a stabilizing interaction between a nonbonding HOMO level and a superjacent (LUMO) level, David et al. (175), to describe such interaction, proposed the term superjacent orbital control. 

David S, Eisenstein O, Hehre WJ, Salem L, Hoffman R (1973) Superjacent orbital control. Interpretation of the anomeric effect. J Am Chem Soc 95 3806-3807... 

After it was recognized that the interaction of the LUMO with such a subjacent orbital, as it was named by Berson and Salem [46], can be strong enough to allow an otherwise forbidden pathway, and thus to control the stereochemistry of a reaction, the frontier electrons lost much of their uniqueness. Instances were then cited in which the superjacent orbital, the one lying immediately above the LUMO, seems to have a dominant influence on the course of the reaction. [47]... 

The FMO analysis of the cycloaddition behaviour of 7,7-dimethylfulvene toward cyclopentadiene has been critieally examined. There are three [4 2]addition modes (the one in whieh cyclopentadiene acts as the diene and a ring C=C unit of the fulvene plays the part of the dienophile leads to the experimentally observed product) and one [6 4- 4] mode. The [6 -I- 4] mode has previously been predicted to be the preferred combination on the basis of FMO results. However, superjacent orbital effects i.e. effects arising from contributions of MO s lying above the frontier M O manifold) are now shown to be decisive, with a marked preference for the observed [4 + 2] addition. It is recommended that in the application of FMO theory an inspection of the coefficients of those MO s which lie outside the FMO manifold be carried out. It should nevertheless be noted that the identity if the kinetically controlled product of the above cycloaddition has yet to be verified although, apparently, such studies are now in progress. 

The size of the reagent is not the only controlling factor since a change in the hardness of the nucleophile implies a modification of its valence orbitals and also of its HOMO energy level (Scheme 29) (160). When the level is high (i.e., in a hard nucleophile), frontier-orbital interaction is predominant. When the level is low (i.e., in a soft nucleophile), the relative importance of the nucleophile HOMO-substrate superjacent MO (161) interaction is increased. The structure of the latter is shown below (Scheme 30) for the Si-F bond. The big lobe of the Si hybrid orbital points to the rear in the superjacent MO, which therefore favors inversion of configuration. 

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