Cyclization of Aromatic Propargyl Alcohol with Thiophene Group Yielding Naphthothiophene Aldehyde Catalyzed by Ruthenium Complex
Date Issued
2012
Date
2012
Author(s)
Tsai, Fu-Yuan
Abstract
Chapter 1. The chemical reactions of Cp(PPh3)2RuCl with the phenyl propargylic alcohol 1a, bearing a 3-thiophene group are explored. The carbene complex 2a, obtained exclusively from this reaction at low temperature, contains the naphthothiophene group formed via a new cyclization process between the thiophene group and the inner carbon of the triple bond. Details of this process are revealed by running the reaction at room temperature, affording the allenylidene complex 3a as a side product. Complex 3a is not converted to 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 NEt3 at room temperature to afford in high yield the naphthothiophene aldehyde 4a, ONEt3, OPPh3 and Cp(PPh3)2RuCl. Molecular O2 is likely activated by coordination to the metal center when one of the phosphine ligands dissociates. Then NEt3 promotes the oxygenation process by reacting with the coordinated O2 to afford ONEt3 and possibly an unobserved oxo-carbene complex. Coupling of the oxo and carbene ligands then yields 4a and Cp(PPh3)2RuCl in CHCl3. In a solvent system containing MeOH, the oxygenation reaction affords a mixture of 4a and the naphthothiophene ester compound 5a-1. The reactions of Cp(dppf)RuCl (dppf = 1,1''-bis(diphenyl-phosphino)ferrocene) with 1a, also afford the carbene complex 2a’ and 4a, 5a, which are characterized by X-ray diffraction analysis. For the phenyl propargylic alcohols 1b with a 2-thiophene substituent, different naphthothiophene aldehyde and ester compounds are also obtained in high yields via similar cyclization process followed by oxygenation under mild conditions.
Chapter 2.Five phenyl propargylic alcohols 1a-e, each containing either a heterocyclic group or an olefinic chain on the phenyl ring are prepared. In the presence of visible light, treatment of 1a with half equivalent of [Ru”]Cl ([Ru”] = CpRu(dppe)) and NH4PF6 under O2 at 50°C in THF for 12 hours affords aldehyde compound 4a in high yields. The other aldehydes 4b-e are similarly prepared from 1b-e, respectively. Formation of these aldehydes proceeds via a cyclization giving the carbene complex 2, which is isolated from stoichiometric reaction, followed by a facile oxygenation by O2 to give the final product. The cyclization forms a new C-C bond between the inner carbon of the triple bond and the unsaturated functional group of the heterocyclic ring. Oxygenation of 2 generating 4 is accompanied with formation of phosphine oxide of dppe. Oxygen activation possibly proceeds by coordination to the ruthenium center when one of the PPh2 of the dppe ligand dissociates. Then, the tethering dppe ligand could better react with the coordinated oxygen nearby and conceivably generates an unobserved oxo-carbene complex with partially oxidized PPh2CH2CH2P(O)Ph2 ligand. Coupling of the oxo/carbene ligands then yields 4. Presumably this partially oxidized ligand continuously promotes cyclization/oxygenation of 1 to obtain the second aldehyde 4. In a solvent system containing alcohol such as MeOH or EtOH, oxygenation reaction affords a mixture aldehyde 4 and the corresponding ester 5, and in some cases, generates acetal 8. Two carbene complexes 2a” and 2b” have been characterized by X-ray diffraction analyses. The UV-vis spectra of 2a” and 2b” consist of visible absorption bands with high extinction coefficient. From DFT theoretical calculations on 2a” and 2b”, the visible light is found to populate the LUMO anti-bonding orbital of mainly Ru=C bond, therefore, weakening the Ru=C bond in the oxygenation/demetalation reactions of 2.
Subjects
Ruthenium
cyclization
thiophene
propargylic alcohol
fused-ring systems
Cyclization
ruthenium carbene complex
oxygenation
DFT calculation
Type
thesis
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