Stoichiometric Regio- and Stereoselective Oxidative Coupling Reactions of Conjugated Dienes with Ruthenium(0). A Mechanistic Insight into the Origin of Selectivity

Treatment of [Ru(η6-C10H8)(η4-1,5-COD)] (1) with excess butadiene at room temperature produces supine,prone-[Ru(η3:η3-2,6-octadiene-1,8-diyl)(η4-1,5-COD)] (supine,prone-2). Similar treatment of 1 with isoprene and (E)- and (Z)-1,3-pentadiene also gives the corresponding analogues of supine,prone-2, while 2,3-dimethylbutadiene does not react. A low-temperature NMR study of the reaction of 1 with butadiene shows initial formation of [Ru(η4-cisoid-butadiene)(η2-transoid-butadiene)(η4-1,5-COD)] (5) as an intermediate, which is spontaneously converted into supine,prone-2 upon warming to room temperature. Similarly, treatment of 1 with (E)-1,3-pentadiene gives two intermediates, [Ru{η4-(E)-cisoid-1,3-pentadiene}{η2-(E)-transoid-1,3-pentadiene}(η4-1,5-COD)] (E-6) and [Ru{η4-(Z)-cisoid-1,3-pentadiene}{η2-(E)-transoid-1,3-pentadiene}(η4-1,5-COD)] (Z-6), in 44/56 ratio at −50 °C. These intermediates are converted into anti-supine,syn-prone-[Ru(η3:η3-3,7-decadiene-2,9-diyl)(η4-1,5-COD)] (anti-supine,syn-prone-4). The observed stereochemistries can be explained by a mechanism that involves oxidative coupling between η4-(Z)-cisoid-1,3-pentadiene and η2-(E)-transoid-1,3-pentadiene in Z-6.