Hsung et al. have reported a series of torquoselective electrocyclizations of chiral 1-azahexa-1,3,5-trienes that yield functionalized dihydropyridines.1,2 These azatriene typically feature electron-withdrawing group at the allylic stereocenter. To understand the origins of the torquoselectivities of these electrocyclic ring closures, we modeled these azaelectrocyclizations using M06-2X density functional with the 6-31+G(d,p). A new stereochemical model rationalizes the observed 1,2 stereoinduction emerges from these computations. The model likely applies to other pericyclic reactions featuring 1,2 stereoinduction and emphasizes a stabilizing hyperconjugative effect, which we have termed a transition state gauche effect. This stereoelectronic effect controls the conformational preferences at the transition states, and only in one of the allowed, disrotatory electrocyclization transition states has the ideal stereoelectronic arrangement of substituents achieved without the introduction of a steric clash. Computations confirm that this effect as a stereodeterminant as substrates with electropositive groups like a silyl substituent have different transition state conformational preferences and are predicted to undergo ring closure with the opposite stereoselectivity. This reversal of stereoselectivity for the ring closure of silyl (tert-butyldiphenylsilyl) substituted azatriene has been demonstrated experimentally. 1) Sydorenko, N.; Hsung, R. P.; Vera, E. L. Org. Lett. 2006, 8, 2611–2614. 2) Sklenicka, H. M.; Hsung, R. P.; McLaughlin, M. J.; Wei, L. I.; Gerasyuto, A. I.; Brennessel, W. B. J. Am. Chem. Soc. 2002, 124, 10435–10442.

Transition state Gauche effect controls the torquoselectivities of the clectrocyclizations of Chiral 1-azatrienes

249th National Meeting (2015)

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