Remote C-H Bond Functionalizations. Группа авторов
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Название: Remote C-H Bond Functionalizations
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9783527824144
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Scheme 2.29 (a) meta‐C–H olefination of indoline derivatives. (b) Removal of directing template.
Source: (a) Modified from Yang et al. [35].
Based on the success of meta‐C–H olefination of electron‐rich indolines, the meta‐C–H arylation of indolines under previously reported meta‐C–H cross‐coupling reaction conditions for hydrocinnamic acid derivatives was also achieved, demonstrating the versatility of this new template for diverse meta‐C–H functionalizations of indolines (Scheme 2.30) [35]. Several indoline substrates reacted with arylboronic acid pinacol esters to afford meta‐arylated indoline derivatives in synthetically useful yields.
Scheme 2.30 meta‐C–H arylation of indoline derivatives.
Source: Modified from Yang et al. [35].
Since the meta‐hydroxylated indolines are biologically important, meta‐C–H acetoxylation of indolines using the well‐established oxidation conditions was also investigated (Scheme 2.31) [35]. It was found that although the meta‐acetoxylated indolines were obtained as the major products, the meta‐selectivity was much lower than olefination and para‐acetoxylated indolines (∼10%) were also formed probably due to the electrophilic palladation at the electron‐rich C5 position under the reaction conditions. It should be mentioned that the nonsubstituted indoline substrate was not compatible with these oxidation conditions, since the indoline was readily oxidized to give a mixture of unidentified compounds instead. Further investigation of this reaction to find better reaction conditions is required.
Scheme 2.31 meta‐C–H acetoxylation of indoline derivatives.
Source: Modified from Yang et al. [35].
In 2018, Yu and coworkers developed a novel simple ortho‐sulfonyl benzonitrile template to achieve remote ortho or meta‐C–H olefination of six different classes of N‐heterocycles, including indoline, indole, and tetrahydroquinoline (Scheme 2.32) [37]. It was demonstrated that the site‐selectivity could be predicted based on distance and geometry, and template‐directed meta‐C–H activation was possible through macrocyclopalladation processes with smaller ring size (10‐membered organometallic ring for indoline and indole).
Scheme 2.32 meta‐C–H olefination of N‐heterocycles.
Source: Modified from Dutta et al. [36].
Stoichiometric installation of the template is not step‐economic, or it would not be attached to the substrate while there is no appropriate functional group to form the covalent linkage. Thus, the design of a catalytic template that binds the substrate via a reversible coordination instead of a covalent linkage is very desirable. In 2017, Yu and coworkers reported a remarkable breakthrough for meta‐C–H olefination of 3‐phenylpyridines by using a catalytic pyridine‐based bifunctional template (Scheme 2.33) [12]. In this reaction, the novel template coordinated two metal centers, one of which one reversibly anchored substrates near the catalyst. The other metal cleaved the meta‐C–H selectively. Notably, unlike previous reaction conditions, the addition of the new copper acetate additive was crucial, since both the yield and the meta‐selectivity would decrease greatly in the absence of copper acetate.
Scheme 2.33 meta‐C–H olefination of 3‐phenylpyridines.
Source: Modified from Zhang et al. [12].
Subsequently, Maiti and coworkers reported a novel nitrile‐based bifunctional template for meta‐C–H olefination of 3‐phenylpyridines with Pd(acac)2 as the catalyst under similar reaction conditions (Scheme 2.34) [13]. Notably, this nitrile‐based bis‐amide template was easily prepared, which was beneficial for its application in the synthesis of complex molecular structures.
Scheme 2.34 meta‐C–H olefination of 3‐phenylpyridines using nitrile‐based bifunctional template.
Source: Modified from Achar et al. [13].
2.2.4 Sulfonic Acid Derivatives
In 2015, Maiti and coworkers developed the first meta‐C–H activation of benzylsulfonic acid derivatives using aforementioned commercially available the 2‐hydroxybenzonitrile directing template with high and controllable mono‐selectivity (Scheme 2.35a) [38]. Thus, meta‐selective homo‐diolefination and sequential hetero‐diolefination of benzylsulfonyl ester derivatives were made possibly with this protocol, providing a novel method for the synthesis of hetero‐dialkenylated products that are difficult to access using conventional methods. Notably, the directing template could be converted to an alkenyl group using modified Julia olefination conditions (Scheme 2.35b).
Scheme 2.35 (a) meta‐C–H olefination of benzylsulfonic acid derivatives. (b) Elaboration of product via olefination.
Source: (a) Modified from Bera et al. [38].
Subsequently, Maiti and coworkers also achieved the meta‐C–H olefination of 2‐phenethylsulfonic acid СКАЧАТЬ