Regioselective CH or NH bond cleavage reactions of heterocyclic compounds by [Ru(1,5-COD)(1,3,5-COT)]/monodentate phosphine

Reactions of [Ru(1,5-COD)(1,3,5-COT)] (1) (COD=cyclooctadiene, COT=cyclooctatriene) with benzo[b]thiophene, thiophenes, benzo[b]furan, and furans in the presence of PEt3 result in the regioselective CH bond cleavage of these heterocyclic compounds giving [Ru(1-5-η5-C8H11)(R)(PEt3)2] [R=2-benzo[b]thienyl (4a), 2-thienyl (4b), 5-(2-ethoxylcarbonyl)thienyl (4c), 5-(2-acetyl)thienyl (4d), 5-(3-acetyl)thienyl (4e), 2-benzo[b]furyl (5a), 2-furyl (5b), 5-(2-acetyl)furyl (5c)]. Similar treatments of 1 with indoline and pyrrole lead to cleavage of the NH bond giving [Ru(1-5-η5-C8H11)(R)(PEt3)2] [R=1-indolyl (6a), 1-pyrrolyl (6b)]. The time-course study for the reaction of 1/PEt3 with benzo[b]thiophene monitored by use of NMR suggests prior formation of a zero-valent complex [Ru(1-4-η4-1,3,5-COT)(PEt3)3] (2a) followed by production of 4a. Kinetic study reveals that the rate of the reaction of 2a with benzo[b]thiophene is the first-order dependence on [2a], [benzo[b]thiophene], and [PEt3]−1, respectively. This fact suggests prerequisite dissociation of PEt3 from a coordinatively saturated complex 2a giving a vacant site for interaction of Ru center with benzo[b]thiophene.