Oxygenation of ruthenium carbene complexes containing naphthothiophene or naphthofuran: Spectroscopic and DFT studies

The aryl propargylic alcohol 1-[2-(thiophen-3-yl)phenyl]prop-2-yn-1-ol (1a) is readily prepared from 2-(thiophen-3-yl)benzaldehyde. In the presence of visible light, treatment of 1a with one-half mole equivalent of [Ru]Cl ([Ru]-Cp(dppe)Ru) (dppe=1,2-bis(diphenylphosphino)ethane) and NH ₄PF₆ in O₂ affords the naphtha[2,1-b]thiophene- 4-carbaldehyde (4a) in high yields. The cyclization reaction of 1a proceeds through the formation of the carbene complex 2a that contains the naphtha[2,1-b]thiophene ring, which is isolated in a 1:1 stoichiometric reaction. The C-C bond formation between the inner carbon of the terminal triple bond and the heterocyclic ring is confirmed by structure determination of 2a using single-crystal X-ray diffraction analysis. Facile oxygenation of 2a by O₂ yields the aldehyde product 4a accompanied by the formation of phosphine oxide of dppe. Oxygen is most likely activated by coordination to the ruthenium center when one PPh₂ unit of the dppe ligand dissociates. This dissociated PPh₂ unit then reacts with the coordinated oxygen nearby to generate half-oxidized dppe ligand and an unobserved oxo-carbene intermediate. Coupling of the oxo/carbene ligands followed by demetalation then yields 4a. Presumably the resulting complex with the half-oxidized dppe ligand continuously promotes cyclization/oxygenation of 1a to yield the second aldehyde molecule. In alcohol such as MeOH or EtOH, the oxygenation reaction affords a mixture of 4a and the corresponding esters 5a or 5a'. Four other aryl propargylic alcohols 1b, 1c, 1d, 1e, which contain thiophen-2-yl, isopropenyl, fur-3-yl, and fur-2-yl, respectively, on the aryl ring are also prepared. Analogous aldehydes 4b, 4c, 4d, 4e are similarly prepared from 1b, 1c, 1d, 1e, respectively. For oxygenations of 1b, 1d, and 1e in alcohol, mixtures of aldehyde 4, ester 5, and acetal 8 are obtained. The carbene complex 2b obtained from 1b was also characterized by single-crystal X-ray diffraction analysis. The UV/Vis spectra of 2a and 2b consist of absorption bands with a high extinction coefficient. From DFT calculations on 2a and 2b, the visible light is found to populate the LUMO antibonding orbital of mainly Ru-C bonds, thereby weakening the Ru-C bond and promoting the oxygenation/demetalation reactions of 2. Making light of things: In a one-batch process, cyclization of phenyl propargylic alcohols 1 a with an olefinic group on the ruthenium-1,2-bis(diphenylphosphino) ethane (dppe) complex first afforded the carbene complexes 2 a, then oxygenation of 2 a is promoted by visible light to cause cleavage of the Ru-C bond to generate aldehyde products 4 a (see scheme).