Abstract
According to our earlier observations, the products of photocatalytic oxygenations of furan and thiophene derivatives of benzobicyclo[3.2.1]octadiene with anionic and cationic manganese(III) porphyrin at pH = 7 strongly depended on the type and position of the heteroatom in the aromatic ring, as well as the charge of the photocatalyst. Hence, a significant pH increase (to 10) in these systems offered a reasonable tool to affect the diversity and yields of the oxygenation products. They were quantitatively separated by TLC and identified with NMR analyses. The results clearly indicated that the increase of HO− concentration, in most cases, considerably changed the product yield, e.g., enhanced it to 70% for the hydroxy-furyl derivative. Accordingly, the selectivity of the oxygenation of the furan compound could be improved in this way. In the case of one thienyl compound, however, even an additional product appeared, while the yields of the products of the other thiophene derivative (with cationic catalyst) decreased to zero, suggesting the application of lower pH for preparative purposes. The pH effects indicate that oxygenation reactions in these systems involve more photochemically generated oxidative agents, e.g., •OH and (P)Mn(V)=O), the role of which is affected by the pH increase in various ways.
Highlights
Several types of porphyrins have been used for photocatalytic oxygenations of various organic compounds
Both free-base ligands and metalloporphyrins proved to be efficient for such reactions
The presentation of theintroductory experimental results along with with theirthe interpretation is organically to the corresponding part. It begins photocatalytic organically connected the corresponding introductory part. It the begins with ofthe1 oxygenation of another,to3-furyl derivative (2) for an easy comparison with behavior photocatalytic oxygenation of another, 3-furyl derivative for an easy comparison the, a constitutional isomer of 2
Summary
Several types of porphyrins have been used for photocatalytic oxygenations of various organic compounds. Both free-base ligands and metalloporphyrins proved to be efficient for such reactions. Some aromatic (and aliphatic) alcohols and aldehydes were oxidized to carboxylic acids by solar irradiation of a Sb(V)-porphyrin [7]. Photocatalytic oxygenation of cyclohexene was carried out in the system sensitized by Sn(IV)-porphyrin adsorbed on Pt loaded TiO2 nano-particles [8]. In air- or oxygen-saturated solutions, free-base porphyrins generate only singlet oxygen in these photocatalytic reactions, while metalloporphyrins can be involved in other light-induced processes producing species with higher oxidation potential than that of 1 O2
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