The reactivities and facial stereoselectivities of Diels-Alder reactions of C5-X cyclopentadienes were studied using density functional theory. Burnell and coworkers have shown that the facial selectivity results from the energy required to distort the reactants in the transition state geometries. The differences in the distortion energies are associated with the out-of-plane bending required of the cyclopentadiene C5-X bond. C5-X σ-donors contribute to the aromatic sextet as pseudo π-donors by Schleyer’s hyperconjugative aromaticity. The effect of hyperconjugative aromaticity is maximized when the C5-X bond distorts as to align with the cyclopentadiene π-orbitals. This predistortion favors addition anti to the cyclopentadiene C5-X substituent. C5-X σ-acceptors cause an opposite distortion to minimize the effect of negative hyperconjugation, and syn selectivity is observed.

Distortion and hyperconjugative aromaticity/antiaromaticity determine the reactivity and facial stereoselectivity of 5-substituted cyclopentadienes

251st National Meeting (2016)

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