Triplet [2 + 2] cycloadditions. Spin-inversion control of stereoselectivity

Spin-inversion mechanisms in triplet [π,2_S + π2_S] complexes fall into three classes. These differ in their stereochemical results: (a) bis pyramidalization (BP) which leads to 2s + 2a product; (b) intramolecular disrotation (ID) which leads to 2s + 2a product; (c) bis intramolecular disrotation (BID) which leads to 2a + 2a product. The polarity (i.e., the donor-acceptor relationship) and the triplet excitation energies of the olefins will determine the relative efficiency of these mechanisms; when the olefin pair is nonpolar (i.e., both olefins are electron donors or electron acceptors) 2s + 2s is the main product. As polarity increases, 2s + 2a becomes the major product. Further increase of polarity results again in 2s + 2s cycloaddition. On the other hand, decreasing the triplet excitation energies of the olefins at constant polarity will increase 2s + 2s production. The efficiency of ID and BP is larger in short intermolecular distances and hence anti regiochemistry is expected. Photochemical data (e.g., cycloaddition of olefins to cyclohexenones, type Arearrangement, and the di-x-methane rearrangement) are discussed in the light of these predictions.