Anions Antarafacial process

By contrast, in the [1,2] anionic H shift, the dominant FMO interaction is between the Is orbital of the one-atom component and t/q of the two-atom component. For there to be positive overlap between orbitals where bond-making and bond-breaking take place, the two-atom component must react antarafacially. Thus, for this shift to be a thermally allowed process, the H atom must have partial bonds to the top and bottom faces of the C2 unit simultaneously. Because this arrangement is geometrically impossible, [1,2] anionic H shifts are thermally disallowed reactions. 

We may further extend the analysis of pericyclic reactions by considering that a single p orbital, denoted by the symbol m, can be a participant in a pericyclic reaction. In this analysis, one lobe of the p orbital makes up the top face of a one-atom n system, while the other lobe makes up the bottom face. The participation of a single p orbital is suprafacial if both cycloaddition processes involve only one of the two lobes of the p orbital, and it is antarafacial if the cycloaddition involves both. We may thus predict that the conrotatory opening of the cyclopropyl anion to an allyl anion (Figure 11.72) should take place via an -F 2 ] pathway. Conversely, the opening of the cation would be a -F 2 ] process, giving the opposite stereochemistry in the product." ... 

The [1,6] shifts in pentadienyl anions have rarely been observed. Thermal [1,6] sigmatropic hydrogen shifts occur in the acyclic systems, but not in the analogous cyclic ions. This indicates that an antarafacial interaction on the TT-system is necessary to the migration, as is predicted by the theory. Further confirmation for these results comes from the finding that the cyclic pentadienyl anion is photochemically isomerized by a [1,6] hydrogen shift the favourable [ 2s +, 6J interaction is available to the excited state process. 

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