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Recently, nature has reported the most reductive organic photocatalyst, mesacr. Its excited oxidation potential can reach e * ox = -3.36 EV, which is stronger than that of lithium (E * ox = -3.29 EV). It can reduce those chemical bonds that are difficult to reduce. Article link: https://doi.org/10.1038/s41586-020-2131-1 jump reading → chemical plus search: more than 20000 enterprises, more than 3 million products, 120 million compound data The C-X bond of aryl halides has always been a research hotspot of synthetic chemists. Due to the influence of halogen and aryl conjugation, the c-halogen bond is often inert chemical bond, especially the C-Cl and C-F bond in aryl. The cutting off of aryl carbon halide bond often depends on high energy free radicals, noble metal catalysis and strong reducing agent. In the early days, chemists could use tin or silicon radicals to cut off the free radicals of aryl carbon halide bonds; they could also use simple metals with strong reducibility to reduce aryl carbon halide bonds, but the overall functional compatibility of these methods was poor. With the development of organometallic chemistry, synthetic chemists can use PD and other metals to oxidize and insert activated aryl halides. However, due to the inertia of aryl chlorides, such substrates are often limited by application. With the in-depth study of photochemistry in recent years, the reduction of aryl halides by photocatalytic oxidation-reduction method has attracted more attention because of its mild conditions, good substrate compatibility and convenience for subsequent transformation. However, there are also great challenges: the reduction potential of aryl halides, especially aryl chlorides (some bromines) and aryl fluorides is too high (usually they are In addition, when there is an electron donating substituent (EDG) on the aryl group, the reduction potential of very few photocatalysts can reach - 2.5V, which results in very little application of this type of substrate in photocatalytic oxidation and reduction. How to solve the application of this type of substrate in photoreaction is a big problem for Chemists (Fig. 1). Figure 1: Recently, Nature reported the most reductive organic photocatalyst mesacr. The oxidation potential of its excited state can reach e * ox = -3.36 EV, which is stronger than that of lithium (E * ox = -3.29 EV). It can reduce those chemical bonds that are difficult to be reduced, such as C-Cl bond and n-ts bond in chlorobenzene. The compound was discovered by David A. niewicz team of Chapel Hill University of North Carolina. The highly oxidized mesacr + and cobalt dichloride were reduced to mesacr by single electron after illumination, which showed very strong reducibility and could survive stably in an oxygen free environment (Fig. 2). Fig. 2: mesacr with strong reducibility. Photo source: naturedavid a. niewicz research group has been committed to the development of chemical reaction methodology using visible light oxidation-reduction organic catalyst, especially the use of 9-furyl acridine salt mesacr for organic synthesis reaction. Mesacr + BF4? Is a kind of photocatalyst with strong oxidation in the excited state, which can directly oxidize the C (SP2) - H bond of electron rich aryl group. In 2015, the research group reported on the use of such catalysts to realize the directional amination of aryl groups in Science (Science, 2015, 349, 1326). At the same time, the research group also reported that the catalysts can also oxidize the hydrocarbon bonds of olefins and other electron rich aryls (Fig. 3). The reduction of mesacr + BF4? With strong oxidization has certain reducibility, but the reducibility of converting mesacr. To excited state under light has not been reported. In this paper, the reducibility of the excited state mesacr. Has been studied in detail, which has been applied to the reduction of inert aryl halides and the deprotection of n-ts. Figure 3: David A. niewicz group realizes the directional amination of aryl group with mesacr +. Photo source: Nature
By analyzing the dynamics of mesacr. Excited state, it is found that there are two excited state modes with different energy: in the low energy dual state (D1) and in the high energy twisted intramolecular charge transfer state (TICT). By DFT calculation and experimental data, it is shown that mesacr. In low energy dual state (D1) has a reduction potential of - 2.91 V, while TICT has a reduction potential of - 3.36 v. Through calculation, the author also studies the distribution of molecular orbital of mesacr: the single electron occupied molecular orbital density (SOMO) is mainly distributed in the acridine ring, while the lowest unoccupied molecular orbital (LUMO) is distributed in the n-benzene ring. The author speculates that mesacr may contain phenylanion radical through single electron reduction (Fig. 4). Fig. 4: mesacr in TICT excited state. Potential calculation and orbital distribution. Source: nature tested mesacr transient absorption spectrum in order to verify such conjecture. The transient absorption wavelengths in ground state and excited state are 550 nm and ~ 650 nm respectively. In fact, 550 nm is the absorption wavelength of acridine ring, while the absorption of aromatic free radical anion is generally 600-800 Nm, the instantaneous absorption wavelength of the excited state ~ 650 nm is between 600-800 nm, which proves that the excited state mesacr. Has the existence of phenyl anion radical (Fig. 5). Fig. 5: mesacr in the excited state. Photo source: mesacr in the excited state of nature. The key to the high reducibility of the compound is the presence of phenyl anion radicals. Its property is similar to naphthalene anion radical produced by sodium naphthalene solution, and it has strong reducibility. Similarly, mesacr with phenyl anion radical shows similar reducibility. Stimulated by light, the benzene anion radical can easily reduce the carbon bromine bond in the molecule. Similarly, the benzene anion radical can also reduce the carbon bromine bond through single electron transfer Reduction of halogenated aryl group by carbon halide bond. In order to stabilize the newly generated phenyl anion radicals, the benzene ring will be twisted to obtain greater conjugation with the acridine ring of mesacr. Based on the TD-DFT calculation of the instantaneous density functional theory, the author finds that the n-benzene ring radical anion tends to be planarized with the acridine ring structure by twisting, and its angle with the acridine surface is 36 ° (compared with the angle of mesacr * in the ground state & gt; 80 °). The author thinks that this plays a greater role in the excited state of TICT (Fig. 6). Fig. 6: mesacr in TICT excited state can reduce the halogen in the molecule. Photo source: nature has such high reducibility that the author has tested many chemical bonds that are difficult to be cut off by reduction, such as p-methoxychlorobenzene (EP / 2 & lt; - 2.9 EV vs SCE) and p-methoxybromobenzene (EP / 2 = - 2.9 EV vs SCE). Among the tested aryl bromides, the substrates 6-10 are all methoxy ome with strong electron donor. The carbon bromine bond of this kind of substrate type is usually difficult to cut through reduction. In this strategy, the conversion can be completed in high yield (yield & gt; 86%). At the same time, the method has good compatibility, and the substrates 14 and 15 contain carboxyl and trifluoromethyl groups, which are not affected by mesacr. Reduction (Fig. 7). Figure 7: mesacr. Reduced aryl bromide in TICT excited state Photo source: nature is the same. For the C-Cl bond with more inert aryl chloride, the method is also very efficient. For the substrate containing fluorine atom instead of 18, the method selectively reduces the carbon chloride bond. At the same time, the method shows the functional group compatibility of the substrate: cyano 22, trifluoromethyl 23, carboxylic acid 24, 25 and other groups in the substrate are not reduced In particular, when olefin 29, fatty ketone 30 and fatty carboxylic acid 31 are present in the substrate, the strategy does not reduce the easily reducible substrate, and the product yield is high. If there are two bromine atoms in the molecule, mesacr. Can reduce two bromine atoms at the same time; for the substrate 34 containing two chlorine atoms, a mixture of full reduction and semi reduction will be generated (Fig. 8). Figure 8: mesacr. Reduced aryl chloride in TICT excited state Photo source: the author of nature also tested the cutting and reduction of n-ts. in traditional methods, strong reducing agents (such as mg or Na) are usually needed for the removal of TS. in this method, high selective reduction of n-ts can be achieved by using high reducing activity mesacr *. For keto carbonyl structure 48, olefin 57, ester 43, carboxylic acid 44 existing in the molecule They are not affected, and most of these groups will be reduced by metals (such as Na / naphthalene solution). At the same time, the method can also realize the gram scale reaction of substrate 64, which can be applied to the deprotection in the middle and later stages of drug molecular synthesis (Fig. 9). Figure 9: mesacr in TICT excited state. Photo source: nature summary: the author found that mesacr in TICT excited state has strong reducibility through spectrum and theoretical calculation. The spectrum data shows that the excited state process is mainly the formation of phenyl anion radicals, and plays an important role through n-benzene ring twisting effect. This property has been applied to deprotection of halogenated aromatic hydrocarbons and n-ts which are difficult to be reduced. Writer: Caesar
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