Catalytic C-H bond functionalisation chemistry: the case for quasi-heterogeneous catalysis

This feature article examines the potential of heterogeneous Pd species to mediate catalytic C-H bond functionalisation processes employing suitable substrates (e.g. aromatic/heteroaromatic compounds). A focus is placed on the reactivity of supported and non-supported Pd nanoparticle (PdNPs) catalysts, in addition to the re-appropriation of well-established heterogeneous Pd catalysts such as Pd/C. Where possible, reasonable comparisons are made between PdNPs and traditional 'homogeneous' Pd precatalyst sources (which form PdNPs). The involvement of higher order Pd species in traditional cross-coupling processes, such as Mizoroki-Heck, Sonogashira and Suzuki-Miyaura reactions, allows the exemplification of potential future topics for study in the area of catalytic C-H bond functionalisation processes.

Synthesis of Biheterocycles Based on Quinolinone, Chromone, and Coumarin Scaffolds by Palladium-Catalyzed Decarboxylative Couplings

An efficient Pd-catalyzed decarboxylative coupling of heterocyclic carboxylic acids with heterocyclic halides to achieve the synthesis of biheterocycles of biological interest has been reported. In all cases, the cross-coupling reactions take place rapidly in DMSO in good yields and efficiently proceed in the presence of a PdBr2/DPEphos catalytic system, furnishing the novel biheterocycles based on quinolin-4-one, quinolin-2-one, chromone, and coumarin scaffolds.

Direct arylation and heterogeneous catalysis; ever the twain shall meet

The formation of aryl-aryl bonds and heteroaryl analogues is one of the most important C-C bond forming processes in organic chemistry. Recently, a methodology termed Direct Arylation (DA) has emerged as an attractive alternative to traditional cross-coupling reactions (Suzuki-Miyaura, Stille, Negishi, etc.). A parallel focus of the pharmaceutical and other chemical industries has been on the use heterogeneous catalysis as a favourable substitute for its homogeneous counterpart in cross-coupling reactions. Only very recently has heterogeneous catalysis been proposed and applied, to DA reactions. In this perspective, we consider the terms 'heterogeneous' and 'homogeneous' and the problems associated with their delineation in transition-metal catalysed reactions. We highlight the reports at the interface of DA and heterogeneous catalysis and we comment briefly on the methods used which attempt to classify reaction types as homo- or heterogeneous. In future work we recommend an emphasis be placed on kinetic methods which provide an excellent platform for analysis. In addition two analytical techniques are described which if developed to run in situ with DA reactions would illuminate our understanding of the catalysis. Overall, we provide an entry point, and bring together the mature, yet poorly-understood, subject of heterogeneous catalysis with the rapidly expanding area of DA, with a view towards the acceleration of catalyst design and the understanding of catalyst behaviour.

Modification of purine and pyrimidine nucleosides by direct C-H bond activation

Transition metal-catalyzed modifications of the activated heterocyclic bases of nucleosides as well as DNA or RNA fragments employing traditional cross-coupling methods have been well-established in nucleic acid chemistry. This review covers advances in the area of cross-coupling reactions in which nucleosides are functionalized via direct activation of the C8-H bond in purine and the C5-H or C6-H bond in uracil bases. The review focuses on Pd/Cu-catalyzed couplings between unactivated nucleoside bases with aryl halides. It also discusses cross-dehydrogenative arylations and alkenylations as well as other reactions used for modification of nucleoside bases that avoid the use of organometallic precursors and involve direct C-H bond activation in at least one substrate. The scope and efficiency of these coupling reactions along with some mechanistic considerations are discussed.

Synthesis and cytostatic activity of 7-arylsulfanyl-7-deazapurine bases and ribonucleosides

A series of novel 6-substituted 7-arylsulfanyl-7-deazapurine bases and nucleosides has been prepared for screening of anticancer activity. 7-Thienylsulfanyl-7-deazapurine bases exerted micromolar cytostatic affects.

A general Pd/Cu-catalyzed C–H heteroarylation of 3-bromoquinolin-2(1H)-ones

The Pd(OAc)₂/CuI system effectively catalyzes the coupling of 3-bromoquinolin-2(1H)-ones with a series of azoles to give 3-(heteroaryl)quinolin-2(1H)-ones in good yields.

Purinyl N1-directed aromatic C-H oxidation in 6-arylpurines and 6-arylpurine nucleosides

Palladium-catalyzed C-H bond activation and oxidation of C6 arylpurines as well as C6 arylpurine nucleosides can be accomplished using Pd(OAc)2/PhI(OAc)2 in CH3CN. Despite the presence of four nitrogen atoms in the purine moiety as well as the polyoxygenated saccharide and a labile glycosidic bond in the nucleosides, these reactions can be effectively conducted. Notably, the generally more labile 2'-deoxyribonucleosides also undergo reaction. The reaction conditions can be tuned to yield either monoacetoxylated or diacetoxylated products predominantly. In the course of these investigations, a dimeric Pd(II)-containing cyclopalladated C6 naphthylpurine derivative has been obtained and crystallographically characterized. This compound is competent in catalyzing the oxidization with PhI(OAc)2, indicating its plausible intermediacy in the chemistry. The X-ray structure of a monoacetoxylated product from this reaction has also been obtained.