Cyclization of Aromatic Propargyl Alcohol with a Thiophene Group Yielding Naphthothiophene Aldehyde Induced by a Ruthenium Complex

Abstract

The reactions of [Cp(PPh(3))(2) RuCl] (Cp=cyclopentadienyl) with phenyl propargylic alcohol 1a, with a 3-thiophene group, are explored. The carbene complex 2a, 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 3a as a side product. Complex 3a is not converted into 2a, indicating that the cyclization takes place while the triple bond is π coordinated to the metal center. Complex 2a reacts with oxygen in the presence of NEt(3) at room temperature to afford, in high yield, naphthothiophene aldehyde 4a, ONEt(3), OPPh(3), and [Cp(PPh(3))(2)RuCl]. 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 4a and [Cp(PPh(3))(2) RuCl] in CHCl(3). In a solvent system containing MeOH, the oxygenation reaction affords a mixture of 4a and naphthothiophene ester 5a-1. The reactions of [Cp(dppf)RuCl] (dppf=1,1'-bis(diphenylphosphino)ferrocene) with 1a, also afford the carbene complex 2a', 4a, and 5a, which have been characterized by X-ray diffraction analyses. For the phenyl propargylic alcohol 1b, 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.

Oxygenation of Ruthenium Carbene Complexes Containing Naphthothiophene or Naphthofuran: Spectroscopic and DFT Studies

“…In EtOH, the reaction of 1a generates a mixture of 4a and 5a' (R=Et; see Table 2) in a ratio of 1:1.1. In a similar reaction using [Cp(PPh 3 ) 2 RuCl] in EtOH, the much lower yield of 5a' relative to 5a was previously attributed to the bulkier OEt group 8. For reactions that use the dppe complex, yields of 5a and 5a' are somewhat comparable.…”

“…The highly conjugated naphthothiophene ring is nearly a plane. The bite angle P1‐Ru1‐P2 (82.7(3)°) of 2a is significantly smaller than that of the analogous ruthenium complex (97.93(3)°) with a bidentate 1,1′‐bis(diphenylphosphino)ferrocene (dppf) ligand, isolated previously only as a minor product 8. This smaller bite angle of dppe, which makes more space for the approach of the thiophenyl group to the triple bond in the reaction of 1a with [Ru]Cl, might favor the formation of 2a as the major product.…”

“…The metal‐induced cyclization reactions of enyne with a functionality of propargylic alcohol are known to proceed first through π coordination of the alkynyl ligand 2. This could be followed by a nucleophilic attack of the tethering unsaturated olefinic group to Cβ of the alkynyl group to give a carbene complex with the cyclized moiety,3, 8 or alternatively an allenylidene4 species formed from the π‐alkynyl ligand might undergo cyclization to yield an acetylide complex. For the pathway that leads to carbene, a few stable metal–carbene complexes after the cyclization reaction could be isolated 5.…”

“…For the oxidation of metal–carbenoid intermediates to yield organic carbonyl compounds from alkyne precursors in catalytic reactions, only a few examples have been reported 7. In our previous investigation, cyclization of aryl propargylic alcohol that contains a thiophenyl group on the aromatic ring by [Cp(PPh 3 ) 2 RuCl] (Cp=cyclopentadienyl ligand) was found to proceed through such a metal–carbene intermediate, which then underwent oxygenation to afford aldehyde and/or ester compounds in good yields in the presence of promoter NEt 3 8. However, two steps are required for this reaction.…”

Ruthenium‐Induced Cyclization of Heteroatom‐Functionalized Alkynes: Progress, Challenges and Perspectives

“…More vigorous validations of the “non‐vinylidene‐involving” pathways came from the independent works of Lin and Wong on isolating the Ru–alkyne reaction intermediates. In 2012, Lin and co‐workers reported the isolation of a series of Ru–carbene complexes 1‐car from the reactions between thiophenylphenyl‐substituted propargylic alcohols S1 and Ru II precursors [CpRu(PPh 3 ) 2 Cl], [CpRu(dppf)Cl] and [CpRu(dppe)Cl] (Scheme ; dppf=1,1’‐bis(diphenylphosphino)ferrocene) . It is noteworthy that performing these reactions at room temperature gave the Ru–carbene and Ru–allenylidene ( 1‐all ) complexes as major and minor products, respectively, whereas performing these reactions at −10 °C gave exclusively the carbene complexes.…”