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

Abstract

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.

Synthesis of conjugated diene complexes of ruthenium(0) derived from Ru(η6-naphthalene)(η4-1,5-COD): Z to E isomerisation of coordinated 1,3-pentadiene

“…In the isomerisation of Z-5l to E-5l, the rate-determining loss of PPh 3 must be followed by other, more rapid steps and no insight into the mechanism of the C]C bond rotation can be gained. Recently, we established the existence of reversible E/Z-isomerisation in the complex [Ru(h 4 -cisoid-1,3-pentadiene)(h 2 -transoid-1,3-pentadiene)(h 4 -1,5- COD)] [12]. EXSY measurements [34] ruled out the "envelope shift" mechanism that appears to be widely accepted to account for the E/Z isomerisation in other 1,3-diene complexes [30e32] but were consistent with an initial h 4À -diene to h 3 -allylic transformation induced by metal-carbon bond rupture (either heterolytic or homolytic) in the ene-diyl form of the h 4 -diene; rotation about the CeC bond in the resulting carbanion or radical would then lead to Z/ E isomerisation of the diene, as illustrated in Scheme 3.…”

“…These are of interest because there are several examples of the interconversion of the E and Z-isomers coordinated both to ruthenium [12,28,29] and to other transition elements [26,27,30e32], although the mechanism of interconversion remains a matter for discussion. (2) 1.425(4) C(2)eC (3) 1.417(4) C(3)eC (4) 1.410(4) C(1)eC (5) 1.478(4) C(4)eC (11) 1.491(4) (17) 174.2 (3) Ru (1)eC (1)eC (2) 69.08(16) Ru(1)eC(4)eC (3) 69.77 (16) C(1)eRu(1)eC (4) 78.20(10) C(1)eC (2) i.e., whether the complex is E-5a or Z-5a, but the 1 H NMR spectrum of the complex formed by displacement of acetonitrile by PPh 3 at room temperature showed the former alternative to be correct.…”

“…An excess of butadiene in the absence of acetonitrile completely displaces naphthalene from complex 1 to give supine, prone-[Ru(h 3 :h 3 -2,6-octadiene-1,8-diyl)(h 4 -1,5-COD)] as a result of oxidative coupling of the butadiene units in an intermediate [Ru(h 4 -cisoid-butadiene)(h 2 -transoid-butadiene)(h 4 -1,5-COD)] that can be detected at low temperature [12]. Similar mechanisms probably apply also to the other cross dimerisations mentioned above.…”

Mechanistic insights into catalytic linear cross-dimerization between conjugated dienes and styrenes by a ruthenium(0) complex

“…Because the NMR experiments for the stoichiometric reaction using 2,3-dimethylbuta-1,3-diene and styrene gave a complex mixture, we described the mimic reaction of [Ru(η 4 -buta-1,3-diene)(η 4 -1,5-COD)(NCMe)] We have tentatively assigned this species as [Ru(η 4 -buta-1,3-diene)(η 2 -methyl acrylate)(η 4 -1,5-COD)] (7) by 1 H NMR and 1 H-1 H COSY (Scheme 3). ˚C by 1 H NMR, where the η 2 -transoid-buta-1,3-diene is labile and undergoes rapid exchange with free buta-1,3-diene while the η 4 -cisoid-buta-1,3-diene binds tightly to the Ru center [12].…”

Oxidative coupling reactions at ruthenium(0) and their applications to catalytic homo- and cross-dimerizations¶

“…Stoichiometric oxidative coupling reactionsOn exposure of a benzene solution of 1a to excess butadiene for 30 min at room temperature, a bis( 3 -allylic)ruthenium(II) complex, supine,prone-[Ru( 3 : 3 -C 8 H 12 )( 4 -1,5-COD)](10)was formed as the exclusive product; its molecular structure was revealed by X-ray structuralanalysis (Scheme 7 andFig.5)[71,72]. pentadiene must rotate during the reaction.…”