Remote C-H Bond Functionalizations - Группа авторов

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amino acid significantly improved the yield of the reaction and improved the selectivity as well with the optimal HFIP solvent that was crucial for the full conversion of the substrate. It was found in subsequent reports that this set of novel reaction conditions was also highly effective for many of the directing templated assisted meta‐C–H transformations. In this transformation, not only the intrinsic electronic biases of the substrate were overridden (9, 10), but also challenging steric hindrance was overcome by the template (11). Intriguingly, biaryl acid substrate that has the same length between the chelating nitrile group and the target meta‐C
H bond as hydrocinnamic acid could also undergo meta‐selective C–H olefination of the remote aryl ring (12). Finally, the directing template could be easily removed by hydrolysis under basic conditions at room temperature, leading to the meta‐olefinated hydrocinnamic acids and the recycled directing template.

Chemical reaction depicts the meta-C–H olefination of hydrocinnamic acid derivatives. Chemical reaction depicts the (a) 2-hydroxybenzonitrile template assisted mono meta-selective olefination of hydrocinnamic acids; (b) meta-selective hetero-di-olefination of hydrocinnamic acids.

      Source: (a) Modified from Modak et al. [14].

H bond ortho to the carboxy group on the same aryl intact, although carboxy group is well‐known to be a good ortho‐directing group (Scheme 2.6b). Moreover, tuning the electronic and steric properties of the carboxy group by switching the hydrogen atom ortho to the carboxy group with a fluorine atom, the yield as well as the site selectivity could be improved to some extent (Scheme 2.6c). The possible presence of κ2 coordination in assisting remote‐selective C–H activation may inspire the exploration of novel site‐selectivity of the carboxyl assisted C–H activation reactions.

Chemical reaction depicts the (a) Proposed remote-selective C-H activation via κ2 coordination of the carboxyl. (b) Remote-selective meta-C–H olefination of hydrocinnamic acids. (c) Improved site-selectivity and reactivity with a modified carboxyl-containing template.

      Source: (a) Modified from Li et al. [15].

Chemical reaction depicts the (a) Rh(III)-catalyzed directing template assisted remote meta-C–H olefination of hydrocinnamic acids. (b) Rh(III)-catalyzed meta-C–H alkenylation of hydrocinnamic acids using alkynes.

      Source: (a) Modified from Xu et al. [16]; (b) Modified from Xu et al. [17].

H bonds with arylboronic esters to afford meta‐arylated hydrocinnamic acids derivatives (Scheme 2.8) [18]. A more electron‐rich nitrile‐based directing template with methoxy substitutions was found to better assist this reaction. Moreover, the privileged solvent HFIP and the MPAA ligand (Ac‐Gly‐OH) previously used for meta‐C–H olefination of hydrocinnamic acid was still crucial for the reaction. Notably, tetrabutylammonium (TBA) salt tetrabutylammonium hexafluorophosphate (TBAPF6), which might prevent undesired agglomeration of Pd(0) species to give unreactive palladium black, dramatically improved the reaction yield. It is worth mentioning that biaryl compounds are import structural motifs that are frequently found in numerous pharmaceuticals and agrochemicals.

Chemical reaction depicts the meta-C–H arylation of hydrocinnamic acids with arylboronic esters.