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    Home > ACS catalyst: Series C − H / C − o coupling reaction of heteroaromatics catalyzed by nickel

    ACS catalyst: Series C − H / C − o coupling reaction of heteroaromatics catalyzed by nickel

    • Last Update: 2018-11-27
    • Source: Internet
    • Author: User
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    Metal mediated cross coupling reaction has become an important and versatile synthesis strategy, which can be used to synthesize various biaryl skeletons In general, aryl halides are often used as electrophilic reagents, but they are not very environmentally friendly Therefore, the electrophilic reagent derived from oxygen atom has become a potential substitute In 1979, wenkert reported the pioneering work of the cross coupling of aryl ether and Grignard reagent via C − O bond cleavage catalyzed by nickel, but it was not until the end of the 21st century that this reaction was re emphasized In the past, carbamates, carbonates, ethers and some phenols and alcohols have been successfully coupled, and nickel seems to be the best catalyst for the activation of inert C (aryl) - O bonds Recently, the coupling reaction of benzimidazole or heterocyclic aromatic derivatives with inert aryl methyl ether was established by using Ni as catalyst and Ni mediated C-O bond activation and C-H cleavage of Grignard reagent Relevant articles were published in ACS catalyst under the title of "nickel catalyzed heteroarenes cross coupling via tandem C − H / C − o activation" Firstly, we optimized the reaction conditions with 2-methoxynaphthalene (1a) and 1-methylbenzimidazole (2a) The catalyst, solvent, additive, ligand and Grignard reagent were screened The results showed that the target product 3AA with 92% yield was obtained by using Ni (COD) 2 as catalyst, m-xylene as solvent, LiCl as additive, IPR as ligand and p-methyl-phenylmagnesium bromide as Grignard reagent (source: ACS catalyst.) then, under the optimized reaction conditions, the author investigated the reaction range of a series of methoxynaphthalene derivatives 1 and 2A (Table 1) The author first explored the change of C6 in methoxynaphthalene The yield of 3ba is 50% by the reaction of 1B containing silicon group Pyridine (1c) and Diphenylamino (1D) derivatives can also be successfully converted into coupling products 3CA (79%) and 3DA (82%) 2-methoxy-6-phenylnaphthalene (1E) can also be activated by C − O bond to obtain the corresponding coupling product 3EA with a yield of 60% In addition, several kinds of 2-methoxy-6-arylnaphthalene (1f-1j) were investigated, and the products were obtained in medium to good yields It is worth mentioning that naphthalene with C6 substituted by aromatic ring or heteroaromatic ring will be used as substrate to construct organic photosensitizer materials, such as (3KA - 3mA) In addition, the substituted naphthalene (1n - 1R, 1t) of C7 or C1 is also an effective substrate (source: ACS catalyst.), the author screened the heterocyclic aromatic hydrocarbon substrate range (Table 3) Under the same reaction conditions, electron rich 1,5,6-trimethylbenzimidazole (2b) was coupled with 1a to obtain 6BA, with a yield of 81% Similarly, the yield of coupling 1A with 1-ethylbenzimidazole (2C) is 75%, while the yield of coupling 1-benzylbenzimidazole (2D) is only 30%, which may be due to the large volume of benzyl group in 2D The yield of coupling products of 1-phenylbenzimidazole (2e), 1-m-methylbenzimidazole (2G), 1-p-methylbenzimidazole (2H) and 1-tert-butylbenzimidazole (2I) was 57-86%, while 1-o-methylbenzimidazole (2f) could not get the target product due to the existence of steric hindrance It is worth noting that 6ja can be obtained from 1 - (p-methoxyphenyl) benzimidazole (2J) and 1a in 43% yield, which makes methoxy intact in the reaction process In addition to benzimidazole compounds, imidazole derivatives and other heterocyclic aromatic hydrocarbons (2k-2qn) are also suitable for the reaction conditions, and the target products are obtained in low to medium yields In addition, benzimidazole derivatives can react with anisole 4a to obtain corresponding products The results show that Grignard reagent and nickel compounds not only participate in the C-H bond activation of heterocyclic aromatic hydrocarbons, but also play a key role in controlling the by-products in the coupling reaction Therefore, the author proposed a reasonable mechanism (scheme 9) In the presence of excessive rmgbr, 1-methylbenzimidazole (2a) was deprotinated under the action of nickel catalyst to obtain intermediate II, which was activated by C − H bond to form complex III Adding 2-methoxynaphthalene (1a) to Ⅲ will produce nickel (0) complex IV, which can be further converted into intermediate V by oxidative addition Then, V completes the catalytic cycle by reducing and eliminating, and the target product 3AA is obtained (source: ACS catalyst.) Summary: Based on the strategy of simultaneous activation of C − O / C − H bond, the author has realized the cross coupling reaction of inert aryl methyl ether with benzimidazole or heterocyclic aromatic derivatives, which is suitable for naphthyl methyl ether, anisole and other heterocyclic aromatic derivatives.
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