Palladium-Catalyzed Regioselective Alkoxylation via C-H Bond Activation in the Dihydrobenzo[c]acridine Series

Ruthenium Catalyzed Intramolecular C-X (X=C, N, O, S) Bond Formation via C-H Functionalization: An Overview

Ruthenium catalyzed C-H activation is well known for its high tolerance towards the functional group and broad applicability in organic synthesis and molecular sciences, with significant applications in pharmaceutical industries, material sciences, and polymer industry. In the last few decades, enormous progress has been observed with ruthenium-catalyzed C-H activation chemistry. Notably, the vast majority of the C-H functionalization known in the literature are intermolecular, although the intramolecular variant provides fascinating new structural facet starting from the simple molecular scaffolds. Intramolecular C-H functionalization is atom economical and step efficient, results in less formation of undesired products which is easy to purify. This has created a lot of interest in organic chemistry in developing new synthetic strategies for such functionalization. The focus of this review is to present the relatively unexplored intramolecular functionalization of C-H bonds into C-X (X=C, N, O, S) bonds utilizing versatile ruthenium catalysts, their scope, and brief mechanistic discussion.

The role of hypervalent iodine(iii) reagents in promoting alkoxylation of unactivated C(sp3)-H bonds catalyzed by palladium(ii) complexes

Although Pd(OAc)2-catalysed alkoxylation of the C(sp3)-H bonds mediated by hypervalent iodine(iii) reagents (ArIX2) has been developed by several prominent researchers, there is no clear mechanism yet for such crucial transformations. In this study, we shed light on this important issue with the aid of the density functional theory (DFT) calculations for alkoxylation of butyramide derivatives. We found that the previously proposed mechanism in the literature is not consistent with the experimental observations and thus cannot be operating. The calculations allowed us to discover an unprecedented mechanism composed of four main steps as follows: (i) activation of the C(sp3)-H bond, (ii) oxidative addition, (iii) reductive elimination and (iv) regeneration of the active catalyst. After completion of step (i) via the CMD mechanism, the oxidative addition commences with an X ligand transfer from the iodine(iii) reagent (ArIX2) to Pd(ii) to form a square pyramidal complex in which an iodonium occupies the apical position. Interestingly, a simple isomerization of the resultant five-coordinate complex triggers the Pd(ii) oxidation. Accordingly, the movement of the ligand trans to the Pd-C(sp3) bond to the apical position promotes the electron transfer from Pd(ii) to iodine(iii), resulting in the reduction of iodine(iii) concomitant with the ejection of the second X ligand as a free anion. The ensuing Pd(iv) complex then undergoes the C-O reductive elimination by nucleophilic attack of the solvent (alcohol) on the sp3 carbon via an outer-sphere SN2 mechanism assisted by the X- anion. Noteworthy, starting from the five coordinate complex, the oxidative addition and reductive elimination processes occur with a very low activation barrier (ΔG ‡ 0-6 kcal mol-1). The strong coordination of the alkoxylated product to the Pd(ii) centre causes the regeneration of the active catalyst, i.e. step (iv), to be considerably endergonic, leading to subsequent catalytic cycles to proceed with a much higher activation barrier than the first cycle. We also found that although, in most cases, the alkoxylation reactions proceed via a Pd(ii)-Pd(iv)-Pd(ii) catalytic cycle, the other alternative in which the oxidation state of the Pd(ii) centre remains unchanged during the catalysis could be operative, depending on the nature of the organic substrate.

Catalyzed Mizoroki-Heck Reaction or C-H Activation

In the last few decade, research conducted on the development by catalytic processes of C-C bonds formation on the one hand and on the other hand on the activation of C-H bonds has grown considerably [...]

Recent advances in intramolecular C–O/C–N/C–S bond formationviaC–H functionalization

This review presents the construction of C–X bonds (X = O/N/S) by using intramolecular C–H functionalization for the synthesis of heterocyclic compounds.