MECHANISTIC STUDY ON CP-MEDIATED AND CP-ASTERISK-RU+-MEDIATED CYCLOADDITION BETWEEN CONJUGATED DIENES AND ACETYLENE

When Cp*RuCl(eta4-diene), where diene = butadiene (1a) or 1,3-pentadiene (1b), was treated with excess acetylene in the presence of silver triflate, [Cp*Ru{eta2,eta4-C6H7(CH=CH2)}]+ (3a) and [Cp*Ru{eta2,eta4-C6H6(CH3)(CH=CH2)}]+ (3b) were formed as predominant products in 84 and 71% yields, respectively. On the basis of the distribution patterns of the deuteration experiments with C2D2, the new hydrocarbon ligand was found to be generated by the following sequence of steps; (1) [4 + 2] cycloaddition of acetylene and the coordinated diene ligand, (2) C-H activation to form a Ru(IV) cyclohexadienyl-hydride intermediate, (3) insertion of the second acetylene molecule into the formed Ru(IV)-H bond, and (4) the resultant vinyl ligand coupling with the cyclohexadienyl ligand. Dehydrogenation or methane elimination from the [4 + 21 cycloadducts, 1,4-cyclohexadiene intermediate, as well as cyclic trimerization of acetylene, resulted in the formation of stable [Cp*Ru(eta6-arene)]OTf (2a and 2b) as minor products (10-22%). Alternatively, the formation of the cationic arene complexes (2a and 2b, 2d or 2e) became predominant, and that of 1:2 adducts was completely suppressed, when Cp*RuCl(eta4-isoprene) (1c) or CpRuCl(eta4-diene) complexes, diene = butadiene (1d) and isoprene (1e), were employed.