Not a Diels-Alderreaction 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 /Bu4 NBH4 (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.
The absolute control of the regiochemistry of a cobalt-catalyzed 1,4-hydrovinylation reaction is achieved by alternation of the ligands applied. While the dppe/dppp ligands led to the formation of the branched product, the herein described application of the SchmalzPhos ligand generates the corresponding linear product in both excellent yields and regioselectivities. The catalyst system exhibits a high tolerance toward functional groups, and the very mild reaction conditions allow the synthesis of 1,4-dienes without isomerization into conjugated systems.