Cyclization of aromatic propargyl alcohol with a thiophene group yielding naphthothiophene aldehyde induced by a ruthenium complex

The reactions of [Cp(PPh ₃) ₂RuCl] (Cp= cyclopentadienyl) with phenyl propargylic alcohol 1 a, with a 3-thiophene group, are explored. The carbene complex 2 a, obtained exclusively from this reaction at low temperature, contains the naphthothiophene group, which is formed through a new cyclization process between the thiophene group and the inner carbon of the triple bond. Details of this process have been revealed by conducting the reaction at room temperature, affording the allenylidene complex 3 a as a side product. Complex 3 a is not converted into 2 a, indicating that the cyclization takes place while the triple bond is π coordinated to the metal center. Complex 2 a reacts with oxygen in the presence of NEt ₃ at room temperature to afford, in high yield, naphthothiophene aldehyde 4 a, ONEt ₃, OPPh ₃, and [Cp(PPh ₃) ₂RuCl]. Molecular O ₂ is likely activated by coordination to the metal center when one of the phosphane ligands dissociates. Then, NEt ₃ promotes the oxygenation process by reacting with the coordinated O ₂ to afford ONEt ₃ and possibly an unobserved oxo-carbene complex. Coupling of the oxo and carbene ligands then yields 4 a and [Cp(PPh ₃) ₂RuCl] in CHCl ₃. In a solvent system containing MeOH, the oxygenation reaction affords a mixture of 4 a and naphthothiophene ester 5 a-1. The reactions of [Cp(dppf)RuCl] (dppf=1,1′-bis(diphenylphosphino) ferrocene) with 1 a, also afford the carbene complex 2 a′, 4 a, and 5 a, which have been characterized by X-ray diffraction analyses. For the phenyl propargylic alcohol 1 b, with a 2-thiophene substituent, different naphthothiophene aldehyde and ester compounds are also obtained in high yields through a similar cyclization process followed by oxygenation under mild conditions.