New Functionalized Polycycles Obtained by Photocatalytic Oxygenation Using Mn(III) Porphyrins in Basic Media

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.

Photocatalytic Oxygenation of Heterostilbenes—Batch versus Microflow Reactor

“…Moreover, only one product kept the aromatic furan ring (as a hydroxylated one, 6), the transformation ceased the aromaticity of this heterocycle in the other cases. These results confirm that the aromatic character of furan is much weaker than that of thiophene, in accordance with our earlier observations in similar systems [16,18].…”

“…Concerning the results obtained for the furyl derivative 1, all the photoproducts, 3-8 (Scheme 1), were isolated by repeated column and thin-layer chromatography and characterized by spectroscopic methods. From the 1 H NMR spectra and UPLC/MS analyses, taking our previous results [14,17,18] as reference, the structures of all the new compounds were determined. Molecular ions m/z 216 (3-5, 7, 8) and m/z 198 (6) indicated that one or two oxygen atoms were incorporated in the starting substrate 1, having masses 32 or 14 higher than the mass for 1 with m/z 184 (Scheme 1).…”

“…In our laboratory, a simple and efficient protocol is utilized for the synthesis of functionalized benzobicyclo[3.2.1]octenes/octadienes ( Figure 1E,F) from organic substrates (A-C) having no characteristic functional groups other than reactive double bonds [14][15][16][17][18]. Utilization of anionic and cationic manganese(III) porphyrins ( Figure 2) as photocatalysts resulted in the formation of epoxy or hydroxy derivatives as main products on the reactive double bonds of the furan or thiophene rings or on the free double bond, as well as other minor photooxygenation products.…”

A simple and easy to perform synthetic route to functionalized thienyl bicyclo[3.2.1]octadienes

“…As previously emphasized, the prepared products 3-7 and 12, due to the presence of a double bond in their structure, could serve as potential starting precursors for further functionalization. These functionalizations, beside the addition reaction, could involve photooxygenation reactions (Scheme 5) [1,[22][23][24][25], previously studied in our laboratory. These reactions could result in a completely new spectrum of products, with preserved bicyclo[3.2.1.…”

“…The bicyclo [3.2.1]octane skeleton has become the subject of intensive research in recent years [1][2][3]. Its presence in numerous biologically active natural compounds ( Figure 1) [4][5][6][7], their strenuous isolation procedures from plants, as well as their complicated multistage synthesis due to the complexity of their structure, encouraged us to develop a simple one-step synthetic procedure based on a photochemical methodology [8][9][10][11][12][13][14][15][16][17][18][19][20][21].…”