Spin Inversion in Triplet Diels-Alder Reactions

Spin inversion in triplet [_π4_s + _n2_s] complexes can be induced by stereochemically different mechanisms. Consequently, triplet Diels-Alder reactions can be 4s + 2s stereospecific, 4s + 2a (4a + 2s) stereospecific, or stereorandom. The selection of one mechanism depends on the donor-acceptor relationship and the triplet energies of the reactants. It is predicted that enhancing the donor-acceptor relationship of the reactants, i.e. increasing the reaction polarity, will increase 4s + 2s product formation relative to all other products. On the other hand, decreasing the triplet energies, at constant polarity, will result in predominant 4s + 2a product formation. Solvent polarity will have a more pronounced effect on the yield of 4s + 2s adducts. The competition of [4 + 2] and [2 + 2] cycloaddition is discussed. It is predicted that the [4 + 2]/[2 + 2] product ratio will increase as polarity increases and it will decrease as the triplet energy of one or both reaction partners decreases.