Recent Advances in Functionalization of Pyrroles and their Translational Potential

Among the known aromatic nitrogen heterocycles, pyrrole represents a privileged aromatic heterocycle ranging its occurrence in the key component of "pigments of life" to biologically active natural products to active pharmaceuticals. Pyrrole being an electron-rich heteroaromatic compound, its predominant functionalization is legendary to aromatic electrophilic substitution reactions. Although a few excellent reviews on the functionalization of pyrroles including the reports by Baltazzi in 1963, Casiraghi and Rassu in 1995, and Banwell in 2006 are available, they are fragmentary and over fifteen years old, and do not cover the modern aspects of catalysis. A review covering a comprehensive package of direct functionalization on pyrroles via catalytic and non-catalytic methods including their translational potential is described. Subsequent to statutory yet concise introduction, the classical functionalization on pyrroles using Lewis acids largely following an ionic mechanism is discussed. The subsequent discussion follows the various metal-catalyzed C-H functionalization on pyrroles, which are otherwise difficult to implement by Lewis acids. A major emphasize is given on the radical based pyrrole functionalization under metal-free oxidative conditions, which is otherwise poorly highlighted in the literature. Towards the end, the current development of pyrrole functionalization under photocatalyzed and electrochemical conditions is appended. Only a selected examples of substrates and important mechanisms are discussed for different methods highlighting their scopes and limitations. The aromatic nucleophillic substitution on pyrroles (being an electron-rich heterocycle) happened to be the subject of recent investigations, which has also been covered accentuating their underlying conceptual development. Despite great achievements over the past several years in these areas, many challenges and problems are yet to be solved, which are all discussed in summary and outlook.

Well-Defined Palladium N-Heterocyclic Carbene Complexes: Direct C-H Bond Arylation of Heteroarenes

A series of palladium N-heterocyclic carbene (NHC) complexes of type trans-{(NHC)PdCl₂L} (L = C₅H₅N, 3-ClC₅H₄N, and PPh₃) (3-5) have been developed as efficient precatalysts for direct C-H bond arylation of various heteroarenes. In particular, an in situ generated new NHC ligand derived from {1,3-di-(2,6-diethylphenyl)acenaphtho[1,2-d] imidazolium} chloride (2) is used for the stabilization of the palladium metal center. Among the screened palladium precatalysts (3-5), the most active PEPPSI themed complex (3) was successfully employed toward direct C-H bond arylation of various heteroarenes and aryl bromides. A range of functional groups on aryl bromides as well as on heteroarenes sustained throughout the standard reaction conditions for easy access of various arylated heterocyclic compounds. Significantly, the utility of the protocol was demonstrated by the effective synthesis of a precursor of raloxifene, a selective estrogen receptor modulator.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.