Abstract
On the basis of earlier results with furan and thiophene derivatives of benzobicyclo[3.2.1]octadiene, photocatalytic oxygenation of novel furo- and thieno heterostilbenes with water-soluble manganese(III) porphyrins offered suitable possibilities to study their reactivities and reaction pathways depending on the heteroatom and the catalyst charge. The experiments were carried out in two reactors types (batch and microflow) to investigate the geometric effects. NMR spectroscopy, GC, and UPLC/MS analyses were applied for identification and quantification of the products. As our results indicated, the 2-thienyl and the common p-tolyl groups in the starting compounds remained intact due to their stronger aromaticity. Hence, the thieno derivative underwent oxygenation only at the open-chain part of the molecule, and the rates of its reactions were much lower than those of the furyl analogue. The less stable furan ring was easily oxygenated, its products with highest ratios were 2-furanon derivatives. Epoxide formation occurred at the open-chain parts of both substrates preferably by the anionic catalyst. Nevertheless, the conversion rates of the substrates were higher with the cationic porphyrin, according to electrophilic attacks by photogenerated Mn(V)=O species. Additionally, the reactions were significantly faster in microflow reactors due to the more favorable circumstances of mass transfer, diffusion, and light penetration.
Highlights
In recent years, numerous research groups in the field of organic synthetic chemistry put their focus on photochemistry as a relatively young science
Very promising results are obtained by reactions in which porphyrin complexes coordinated with certain metals are used as catalysts; scientists have studied a whole range of metals within porphyrin structures as well as their advantages in various catalytic oxidations [2]
There are examples of porphyrin complexes coordinated with ruthenium [3]
Summary
Numerous research groups in the field of organic synthetic chemistry put their focus on photochemistry as a relatively young science. In the 1H NMR spectrum of photoproduct 8, the integral of four protons at about 2 ppm can be found beside the signal of the methyl group All these pieces of information together with the masses of six products [3,4,5,6,7,8], confirmed by UPLC/MS analysis, clearly show the tendency of the starting furostilbene 1 under the conditions of photooxygenation using Mn(III) porphyrins and the reactivity of the furan ring. The higher reactivity of the furyl derivative in these oxygenation processes is demonstrated by the incorporation of two oxygen atoms in most of its products, while those originated from 2 contained only one O
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