Geminal bond participation is essential for the contradictory torquoselectivities in retro-Nazarov reactions

Recently we have shown that retro-Nazarov reactions are under the control of geminal bond participation. In retro-Nazarov reactions, electrocyclic ring-opening must occur in a conrotatory manner, so that a geminally 4,4-disubstituted substrate and cis -4,5-disubstituted substrate may exhibit the same torquoselectivity with a substituent effect. For amino-hydroxy substituted substrates, however, cis -5-amino-4-hydroxy substituted 9 was calculated preferentially to give E -amino- Z -hydroxy product Z - 12 , while geminally disubstituted 4-amino-4-hydroxy 11 was expected to predominantly afford Z -amino- E -hydroxy product E - 12 . We demonstrate here that this discrepancy in torquoselectivity can be explained in terms of geminal bond participation: in 9 , the most electron-donating σ-bond is σ CH geminal to the amino group, which should rotate inwardly. Thus, the amino group should rotate outward. On the other hand, the σ CN bond is more electron-donating than the σ CO bond, which results in inward rotation of the amino group in 11 .