The reaction profiles of the (hetero) Diels–Alder reactions acrolein + ethene [Eq. (a)] and butadiene + ethene are surprisingly different. Extensive AM1 /CI calculations revealed that, for reaction (a), the uncomplicated RHF method gave satisfactory results. RHF, UHF, and CI calculations gave only slightly differing results for the properties of the second transition state, which is generally rate-determining. The existence of a polar-radicaloid intermediate only slightly stabilized relative to the two transition states cannot be ruled out.
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Tietze et al. (1989) studied this question.
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