C−S Bond Cleavage of Benzo[b]thiophene at Ruthenium

Abstract

The ruthenium(II) (tetrahydroborate)hydride complex [(triphos)RuH(BH4)] (1) reacts with benzo[b]thiophene (BT) in THF at 40 °C yielding, after 3 h, a mixture of four different compounds: [(triphos)RuH{BH3(o-S(C6H4)CH2CH3)}] (2), [(triphos)Ru{η4-S(C6H4)CH(CH3)}] (3), [(triphos)RuH(μ-S(C6H4)CH2CH3)2HRu(triphos)] (4), and [(triphos)RuH(μ-BH4)HRu(triphos)]+ (5+) (triphos = MeC(CH2PPh2)3). After two further hours of reaction, 3 disappears and is completely converted to 4. Further heating at 40 °C does not change the product composition (2, 4, and 5+ in a 4:1.3:3 ratio). A variety of independent reactions with isolated compounds have been performed with the aim of elucidating the mechanism of the C−S insertion/hydrogenation of BT to the 2-ethylthiophenolate ligand. The μ-thiolate complex 4 reacts in THF with dihydrogen (≥160 °C, 30 bar H2) or with HBF4·OEt2 (20 °C) yielding ethylbenzene and 2-ethylthiophenol, respectively. No reaction occurs with LiHBEt3. All of the reactions and new complexes reported have been studied by multinuclear NMR spectroscopy. An X-ray analysis has been carried out on a single crystal of 2.