The continuum of carbon–hydrogen (C–H) activation mechanisms and terminology

Abstract

As a rapidly growing field across all areas of chemistry, C-H activation/functionalisation is being used to access a wide range of important molecular targets. Of particular interest is the development of a sustainable methodology for alkane functionalisation as a means for reducing hydrocarbon emissions. This Perspective aims to give an outline to the community with respect to commonly used terminology in C-H activation, as well as the mechanisms that are currently understood to operate for (cyclo)alkane activation/functionalisation.

Synthesis of imidazolocoumarins by the amide-directed oxidative cyclisation of enol-Ugi derivatives

“…Prepared by method A from enol-Ugi adduct 9c (0.5 mmol) and obtained as a yellow solid. Mixture of two rotamers A : B in ratio 0.85 : 0.15; 66%; IR (cm −1 ) 3441, 3283, 2927, 2853, 1722, 1636, 1597, 1498, 1459, 1325, 1169, 1128, 1068, 744; 1 H NMR (500 MHz, CDCl 3 ) δ 7.73-7.43 (m, 1H), 7.40 (d, J = 7.9 Hz, 2H), 7.31-7.24 (m, 2H), 7.22 (t, J = 7.9 Hz, 2H), 7.11-7.04 (m, 2H), 6.99 (dt, J = 6.5, 2.8 Hz, 1H), 6.94 (d, J = 7.9 Hz, 1H), 6.84 (t, J = 7. 3 [1,3]dioxol-5-yl)-N-cyclohexylacetamide (10f ).…”

“…4,5 The two major challenges of C-H activation are the low reactivity of C-H bonds and the poor selectivity when several or many similar C-H bonds are present in a molecule. 6 In the last years, many activation methods have been developed. Although their success generally relies on transition metal catalysts, 7 more sustainable oxidizing agents, such as hypervalent iodine reagents, have recently been proposed as an advantageous alternative to avoid the environmental drawbacks of toxic metals.…”

A noble-metal-free metal-organic framework for alkane borylation

“…Functionalization of unactivated alkanes is of great interest but presents a signi cant challenge [1][2][3][4][5][6] .…”

Interplay between the Directing Group and Multifunctional Acetate Ligand in Pd-Catalyzed anti-Acetoxylation of Unsymmetrical Dialkyl-Substituted Alkynes

“… 2 For example, acetate ligands have enabled mechanisms for C–H activation alternative to direct oxidative addition by acting as an inner-sphere base to assist deprotonation of the C–H bond concomitant with M–C bond formation, which is referred to as concerted metalation–deprotonation (CMD) or amphiphilic metal–ligand activation (AMLA). 3 Another general class of related mechanism refers to proton transfer between two different ligands without involving any metal hydride known as ligand-to-ligand hydrogen transfer (LLHT). 4 However, in this type of reaction, most often the protonated ligand subsequently leaves the metal to afford the product.…”