Activation of Acyclic and Cyclic Conjugated Dienes in Cooperation with the Three Metal Centers

Trinuclear ruthenium pentahydride complex (Cp 'Ru) 3 (μ-H) 3 (μ3-H) 2 (1 : Cp ' = η-C5Me5) reacts with a series of acyclic conjugated dienes such as butadiene, isoprene, or penta-1, 3-diene to form trinuclear μ3-1, 3-dimetalloallyl complexes 2 by way of an intermediary μ3-η2 : η2-s-cis-1, 3-diene complex. In the reactions with butadiene and isoprene, an agostic intermediate, in which three ruthenium centers concertedly activate a 1, 3-diene by taking the role of coordination sites and an activation site, is detected by 1H and 13C NMR. The intermediary 1, 3-diene complexes with agostic interaction undergo C (sp2) -H bond cleavage at room temperature to generate 1, 3-dimetalloallyl complexes. The reaction of 1 with cyclopentadiene generates a trinuclear p3-2-methylruthenacyclopentadiene complex 4 as a result of cleavage of the unactivated C (sp3) -C (sp2) bond. In the reaction of 1 with 1, 3-cyclohexadiene, cleavage of an allylic C-H bond followed by the β-H elimination affords α μ3-η2 : η2 : η2-benzene complex.