Stoichiometric and Catalytic Dimerization of Conjugated Dienes with (C₅R₅)Ru(diene)⁺

When Cp*Ru(η⁴-butadiene)X (Cp* = η⁵-C₅Me₅; 1a, X = Cl; 1b, X = Br) was treated with excess butadiene in the presence of silver trifluoromethanesulfonate (AgOTf) followed by carbon monoxide (1 atm), 1,5-cyclooctadiene complex [Cp*Ru(η²:η²-C₈H₁₂)(CO)]OTf (2) was isolated in 79% yield. Similar [4 + 4] cycloaddition occurred in the reaction of Cp*Ru(η⁴-isoprene)Cl (1c) with isoprene, AgOTf, and CO to give [Cp*Ru(η²:η²-cis-3,7-dimethyl-1,5-cyclooctadiene)-(CO)]OTf (4a) at ambient temperature. Similar reaction of Cp*Ru(η⁴-1,3-pentadiene)Cl (1d) with 1,3-pentadiene, however, furnished [Cp*Ru(4-methyl-(1,3-η³:6–8-η³)-nonadienediyl)]OTf (5) in the presence of AgOTf as result of regiospecific linear dimerization. This stoichiometric regioselective dimerization was extended to catalytic formation of 1,5-cyclooctadiene, dimethylcyclooctadienes, and 6-methyl-2,4,7-nonatriene from butadiene, isoprene, or 1,3-pentadiene, respectively, with a catalytic amount of 1b, 1c, or 1d and AgOTf. Alternatively, treatment of butadiene with CpRu(η⁴-butadiene)Br (Cp = η⁵-C₅H₅) and AgOTf induced linear dimerization of butadiene, and [CpRu(η⁴:η²-1,3,7-octatriene)]OTf (6) was obtained in 98% yield by way of single-step C–C bond formation.