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<\/a>Spring is nearly here in Massachusetts. The snow has almost completely melted, and the days are getting longer. Soon the first flowers will bloom and some of those flowers are catalysts for photoredox cross-coupling reactions. Wait, what? File this work<\/a> that appeared recently in the RSC Green Chemistry (open access) under \u00ab\u00a0Things we would never think to try\u00a0\u00bb and \u00ab\u00a0Why we love science\u00a0\u00bb. It’s a fascinating example of photoredox chemistry with organic dyes.<\/p>\nThink about the last reaction that you ran. Perhaps you dried a solvent, distilled a reagent or sparged to remove oxygen. If it is a photoredox reaction hopefully you selected the appropriate wavelength and thought about your light intensity<\/a>. If you ever considered adding dried flower petals, take a bow. But that’s exactly what Prof. Jan J. Weigand and coworkers<\/a> at the Technische Universit\u00e4t Dresden did for several photoredox cross-coupling reactions (Ref 1). And to take a step back, it is a brilliant, incredibly fun well-characterized paper. One of our recent favorites.<\/p>\n As we all know by now, numerous photoredox synthetic methodologies have been developed for more than a decade (Ref 2). To many, light has the promise to be an efficient and clean energy source for synthetic chemistry. We certainly think so<\/a>. But if you stop to think about your catalysts, you can quickly see the problems with basing so much of our chemistry on rare metals such as iridium or rhodium (or the synthetic requirements of the complex ligand systems). Metal-free synthetic dyes offer a solution but themselves require multistep syntheses and are often insufficient for the energy requirements of a reaction (more on this in a bit). So what are the opportunities for photoredox chemistry with organic dyes? Let’s jump in…<\/p>\n To find a sustainable catalyst, the authors looked to the plant genus Hypericum<\/em> L. (hypericaceae) which includes 460 herbal species including Hypericum perforatum<\/em> (St. John’s Wort). Hypericum perforatum <\/em>is one of the world’s oldest medicinal herbs which some believe to be a cure everything from burns to depression or for use as an anti-viral compound (Ref 3). Certainly, someone out there has tested it against COVID-19 by now (Googling…oh, look they have and sadly it’s not the only hit<\/a>?). Hypercin, one of the active ingredients in hypericum,<\/em> is known to generate reactive oxygen species and believed to be responsible for hypericin’s phototoxicity towards bacteria and fungi. Additional, hypercin has been demonstrated to localize in cancer cells and has been studied for use in photodynamic therapy to kill cancer cells (Ref 4).<\/p>\n Figure 1:<\/strong> Hypercin analogues found in Hypericum<\/em><\/p>\n Relevant to this work, hypercin and pseudohypericin are found in fresh plant material with the highest levels of hypericin analogues found in the flowers. The unstable forms protohypericin and protopseudophypericin can be converted to hypericin and pseudohypericin with light (Figure 1). The authors selected ten species of hypericum<\/em> (collected from Germany, Austria and Greece) for their initial screen. To prepare their catalysts, the plant material was dried for 24 h at 40 \u00b0C in the dark. The flowers were ground into a fine powder and added directly to the reaction mixture for the debromination cross coupling of 2-bromobenzonitrile and N-methyl pyrrole. (Figure 2)<\/p>\n Figure 2: <\/strong>Flower catalyzed chemistry To everyone’s surprise (at least ours?), the reactions worked well with yields ranging from 31-68% varying by species of flower. The authors characterized the total concentration of the hypericin analogues found in each species by HPLC which varied greatly ranging from 0.016 wt% to 2.00 % (generally corresponding to conversion) as well as the concentration of individual analogues. For their controls, no reaction occurred in the absence of plant material or with plant material in the dark. For reactions performed with pure isolated hypericin analogues, each analogue demonstrated some activity with isolated hypericin having a similar conversion (73%) as the best reaction from the initial screen. With optimal conditions in hand, the authors expanded both the reaction scope and additional chemistry. Both hypericum vesiculosum <\/em>flowers and isolated hypericin successfully catalyzed 30+ additional examples of \u00ab\u00a0potpourri catalysis\u00a0\u00bb (our term, not theirs). Importantly, the authors point out that the plant material can be removed with simple column chromatography. Check out the full paper for additional discussion on the reactivity of individual analogues, their prevalence in different species and an in-depth in situ<\/em> UV-vis spectroelectrochemical study on the catalytic hypericin species and a complete mechanistic picture. Just a very nice work. Now, will we all be running our reactions tomorrow with flower blossoms? Probably not right away. But it is not unreasonable to think of a scenario where bulk plant matter could be useful on scale at some point.<\/p>\n Reusable Dyes<\/strong><\/p>\n In the same light, with a focus on sustainability Pieber and coworkers from Max Planck recently reported a dye-based self-assembly system for nickel-catalyzed photoredox reactions<\/a> (Ref 5). Numerous carbon-carbon and carbon-hetero bond formations using photoredox nickel catalysis have been reported with most utilizing iridium or ruthenium photocatalysts to initiate the cycle (Ref 6). Iridium and ruthenium bipyridine based photocatalysts are particularly useful due to their high excited state triplet energies and long-lived triplet state which can facilitate the bimolecular reaction with the Ni-catalyst. Many organic dyes have excited state redox potentials that theoretically are suitable to initiate the nickel catalysts (often with lower energy light); however, the short-lived singlet excited states of most dyes do not enable the reaction in homogenous solution. To overcome this problem, the authors report a series of dye-sensitized metallophotoredox catalysts (DSMP) containing an immobilized dye and nickel catalyst on a TiO2 <\/sub>bead (Figure 3).<\/p>\n Figure 3<\/strong>: Development of Dye-sensitized metallophotoredox catalysts (DSMP)<\/p>\n Happy to announce that our most recent paper came out in @ACSCatalysis<\/a> today! Congratulation to first author @SusanneReisch<\/a> for the excellent work @BioMolSys<\/a> @MpiciPotsdam<\/a> — PieberLab (@PieberLab) November 2, 2020<\/a><\/p><\/blockquote>\n![\"Hypercin](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2021\/03\/photochemistry-organic-dyes-figure1.gif\")
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