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 η ⁴-(Z)-cisoid-1,3-pentadiene and η²-(E)-transoid-1,3-pentadiene in Z-6.