Antarafacial sigma bonds

Hoffman rules (9) such a pathway would be thermally allowed if it involved an antarafacial sigma bond opening. 

Anomeric effect, 82, 310-311, 305 Antarafacial, 163 examples, 164 sigma bonds, 167 Anti-Bredt olefin, 102 Approximations of MO theory Born-Oppenheimer, 22 Hartree-Fock, 222 Huckel, 35, 86 independent electron, 35 LCAO, 229 nonrelativistic, 22 SHMO, 87... 

The problem may be restated in terms of the symmetry of all of the orbitals involved. In the transition state the pair of electrons forming the sigma bond to hydrogen may now be considered part of the n system the hydrogen will move suprafacially or antarafacially depending on whether the HOMO is a or b. For (4n + 2)77- electrons a thermal change is allowed, and for (471)77- electrons a first-excited-state process is allowed. 

For conrotatory ring opening of cyclobutene there is retention at C3 and inversion at C4. Sigma bond thus participates in antarafacial manner. Addition across rt-bond is then suprafacial and this pathway is classified as o s+Ji s t e... 

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