Synthesis of N-alkoxycarbonyl Pyrroles from O-Substituted Carbamates: A Synthetically Enabling Pyrrole Protection Strategy

The condensation of readily available O-substituted carbamates with 2,5-dimethoxytetrahydrofuran gives N-alkoxycarbonyl pyrroles in a single step and in good yield. By this method, several common amine protecting groups can be introduced on the pyrrole nitrogen. With the exception of N-Boc, N-alkoxycarbonyl groups have seen only minimal use for protection of the pyrrole nitrogen to date. Here, we show that N-alkoxycarbonyl protection can endow pyrrole with distinct reactivity in comparison with N-sulfonyl protection, for example, in a pyrrole acylation protocol employing carboxylic acids with a sulfonic acid anhydride activator.

One-Pot Synthesis of Unprotected 2-Acylpyrroles from 1,2,3-Triazoles and 2-Hydroxymethylallyl Carbonates

An efficient, tandem one-pot approach to synthesize multisubstituted 2-acylpyrroles from readily prepared N-tosyl triazoles and 2-hydroxymethylallyl carbonates is reported. The reaction proceeds via Rh(II)-catalyzed O-H insertion, [3,3]-sigmatropic rearrangement, Pd(0)-catalyzed oxidative addition, intramolecular cyclization, DBU-promoted E1cB elimination, double bond isomerization, and aromatization, enabling the disconnection and formation of multiple bonds in one reactor. The approach represents a highly regioselective way to access di-, tri-, and tetra-substituted NH pyrroles with high efficiency.

Recent Advances in Transition-Metal-Catalyzed C–H Addition to Nitriles

Transition-metal-catalyzed C–H bond addition to nitriles has emerged as a powerful synthetic approach for the construction of C–C bonds in organic synthesis. Due to the merits of atom- and step-economy, as well the easy availability of the starting materials, these transformations not only deliver acyclic aryl ketone products with nitriles­ as C-building blocks, but can also be utilized for the highly efficient­ assembly of azaheterocyclic skeletons using nitriles as C–N building blocks. This short review summarizes recent progress on transition-metal-catalyzed C–C bond-forming reactions based on C(sp2)–H and C(sp3)–H additions to nitriles.

1 Introduction

2 Palladium-Catalyzed C–H Addition to Nitriles

2.1 Palladium-Catalyzed C–H Addition to Nitriles for the Preparation of Ketone (Imine) Products

2.2 Palladium-Catalyzed C–H Addition to Nitriles for the Preparation of Azaheterocycles

2.3 Palladium-Catalyzed C–H Addition to Nitriles/1,2-Rearangement

3 Other Transition-Metal-Catalyzed C–H Additions to Nitriles

4 Summary and Outlook

[3 + 2 + 1] Pyridine Skeleton Synthesis Using Acetonitrile as C4N1 Units and Solvent

The first [3 + 2 + 1] methodology for pyridine skeleton synthesis via cascade carbopalladation/cyclization of acetonitrile, arylboronic acids, and aldehydes was developed. This reaction proceeds via six step tandem reaction sequences involving the carbopalladation reaction of acetonitrile, a nucleophilic addition, a condensation, an intramolecular Michael addition, cyclization, and aromatization. Delightfully, both palladium acetate and supported palladium nanoparticles catalyzed this reaction with similar catalytic performance. The characterization results of the fresh and used supported palladium nanoparticle catalysts indicated that the reaction might be performed via a Pd(0)/Pd(II) catalytic cycle that began with Pd(0). Furthermore, the products showed good fluorescence characteristics. The green homogeneous/heterogenous catalytic methodologies pave a new way for constructing the pyridine skeleton.

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.

N-Acyl pyrroles: chemoselective pyrrole dance vs. C–H functionalization/aroylation of toluenes

Chemoselectivity is one of the most challenging issues facing the chemical sciences.

The recent advances in K2S2O8-mediated cyclization/coupling reactions via an oxidative transformation

Recently the K₂S₂O₈ mediated cyclization/coupling reactions to construct carbon–carbon/carbon–heteroatom bond via oxidative transformation is became much interesting in organic synthesis.

The synthesis of fluorescent benzofuro[2,3-c]pyridines via palladium-catalyzed heteroaromatic C–H addition and sequential tandem cyclization

A Pd-catalyzed tandem reaction of 2-(cyanomethoxy)chalcones with heteroarenes (such as thiophenes, furans, pyrroles and indoles) for the synthesis of fluorescent benzofuro[2,3-c]pyridines.

Regioselective C5 alkenylation of 2-acylpyrroles via Pd(ii)-catalyzed C–H bond activation

A Pd(ii)-catalyzed regioselective C5 alkenylation of 2-acylpyrroles to synthesize various C5-alkenylated 2-acylpyrroles using a readily removable N-protecting group is developed.

Copper-catalyzed C–S direct cross-coupling of thiols with 5-arylpenta-2,4-dienoic acid ethyl ester

A selective copper (Cu)-catalyzed C–S bond direct cross-coupling of thiols with 5-arylpenta-2,4-dienoic acid ethyl ester was developed. Notably, various biologically active 5-phenyl-3-phenylsulfanylpenta-2,4-dienoic acid ethyl ester derivatives were efficiently synthesized under moderate conditions. Finally, a plausible Cu(i)/Cu(iii) reaction mechanism was proposed.

A selective copper (Cu)-catalyzed C–S bond direct cross-coupling of thiols with 5-arylpenta-2,4-dienoic acid ethyl ester was developed.