Not a Diels–Alder reaction but a 1,4-hydrovinylation takes place on treatment of 1,3-dienes with functionalized alkenes in the presence of the catalyst system [CoBr2(dppe)]/ZnI2/Bu4NBH4 (dppe = 1,2-bis(diphenylphosphanyl)ethane). With this reaction the 1,4-dienes with different substituents R1–R4 can be obtained in high selectivity and good to excellent yields (see scheme).
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Hilt et al. (2001) studied this question.