Remote C-H Bond Functionalizations. Группа авторов
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Source: (a) Modified from Tang et al. [31]; (b) Modified from Yang et al. [32].
2.2.3.2 Benzylamine Derivatives
In 2014, Yu and coworkers reported the first meta‐selective C–H acetoxylation of benzylamine derivatives using the template that previously worked for anilines (Scheme 2.24) [31]. The versatility of the reaction was demonstrated with both acyclic and cyclic benzylamines, although only a few examples were investigated. Notably, since 2‐phenylpyrrolidine and 2‐phenylpiperidine motifs are often found in medicinally important heterocycles, this meta‐selective C–H acetoxylation method is a potentially powerful methodology for accessing diverse structures of medicinally importance.
Scheme 2.24 meta‐C–H acetoxylation of benzylamine derivatives.
Source: Modified from Tang et al. [31].
Besides meta‐C–H acetoxylation of benzylamines, meta‐C–H olefination was also reported by Xu, Xu, Jin, and coworkers through a quaternary ammonium salt assembly of the template for tertiary benzylamines [33]. Remarkably, this method was demonstrated to be applicable to several distal arene‐tethered tertiary amine derivatives, although only a few examples of benzylamine derivatives were tested (Scheme 2.25).
Scheme 2.25 meta‐C–H olefination of tertiary benzylamines and distal arene‐tethered tertiary amine derivatives.
2.2.3.3 Phenylethylamine Derivatives
In 2015, the group of Li achieved the first meta‐C–H olefination of N‐methyl‐phenylethylamine derivatives using the 2‐cyanobenzoyl group as the CF (Scheme 2.26) [34], followed by the isolated examples of meta‐C–H olefination of tertiary phenylethylamine derivatives mentioned in Section 2.2.3.2 [33]. In presence of Pd(OAc)2 and Ac‐Gly‐OH under nitrogen atmosphere, the reaction proceeded smoothly with a broad scope of substrate, and the directing template is structurally very simple and commercially available. Moreover, to increase the potential of application of direct C–H transformations in organic synthesis, using 2‐cyanobenzoyl group as the common original directing functionality to access regiodivergent C–H activation was also demonstrated in a sequential remote‐selective C–H olefination of 2‐fluorophenylethylamine (Scheme 2.27). Notably, the first remote‐selective C–H olefination occurs with the secondary amide 67 leaving the proximal aromatic ortho‐C
Scheme 2.26 meta‐C–H olefination of phenylethylamine derivatives.
Source: Modified from Li et al. [34].
Scheme 2.27 Sequential remote‐selective regiodivergent C–H olefination of 2‐fluorophenylethylamine.
2.2.3.4 N‐Heterocyclic Arene Derivatives
Due to the prevalence of N‐heterocycles in biologically important molecules, selective activation of a C
Scheme 2.28 (a) Conformation promoted meta‐selective activation. (b) meta‐C–H olefination of tetrahydroquinoline derivatives.
Source: (a) Modified from Tang et al. [31].
Subsequently, Movassaghi, Yu, and coworkers developed a novel nitrile‐based sulfonamide directing template for indolines, since previous directing template for tetrahydroquinolines was not viable presumably due to that the aryl group of indolines is more electron rich and the new skeleton requires a different template to accommodate (Scheme 2.29a) [35]. The new electronically withdrawing sulfonamide linkage is crucial for the meta‐selective C–H functionalization of electron‐rich indolines that are otherwise highly reactive toward electrophilic palladation at the electron‐rich C5‐positions. With this new template and the established reaction conditions,