Activity of homobimetallic ruthenium alkylidene complexes on intermolecular [2+2+2] cyclotrimerisation reactions of terminal alkynes

The activity of homobimetallic ruthenium alkylidene complexes, [(p-cymene)Ru(Cl)(μ-Cl)2Ru(Cl)(CHPh)(PCy3)] [Ru-I] and [(p-cymene)Ru(Cl)(μ-Cl)2Ru(Cl)(CHPh)(IPr)] [Ru-II], on intermolecular [2+2+2] cyclotrimerisation reactions of monoynes has been investigated for the first time. It was found that these complexes can catalyse the chemo and regioselective cyclotrimerisation reactions of alkynes at both 25 and 50 °C in polar, aprotic solvents. The catalytic activity of [Ru-I] and [Ru-II] was compared to other well-known ruthenium catalysts such as Grubbs first generation catalyst [RuCl2(CHPh)(PCy3)2] [Ru-III], [RuCl(μ-Cl)(p-cymene)]2 [Ru-IV] and [RuCl2(p-cymene)PCy3] [Ru-V] complexes. To examine the effect of the steric hinderance of substrates on the regioselectivity of the reaction, a series of sterically hindered silicon containing alkynes (1a, 1b, 1c) were used. It was shown that the isomeric product distribution of the reaction shifts from 1,2,4-trisubstituted arenes to 1,3,5-trisubstituted arenes as the steric hinderance on the substrates increases. These homobimetallic ruthenium alkylidene complexes also catalysed regio- and chemo-selective cross-cyclotrimerisation reactions between silicon-containing alkynes (1a, 1b, 1c) and aliphatic alkynes (1d–g).

Graphical abstractIntermolecular [2+2+2] cyclotrimerisation reactions of two different monoynes catalysed by various homobimetallic ruthenium alkylidene complexes proceed smoothly in a chemoselective manner to give substituted benzene derivatives in good yields by slow addition of one monoyne. The products obtained by these reactions were both characterised by 1H, 13C NMR and GC–MS.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Silicon containing benzene derivatives were synthesized by cyclotrimerisation reactions of monoynes. ► Homobimetallic ruthenium alkylidene complexes were employed as efficient cyclotrimerisation catalysts. ► A chemo and regio selective process was developed. ► The steric effect of monoynes on product distribution was investigated.