Plausible PEPPSI catalysts for direct C-H functionalization of five-membered heterocyclic bioactive motifs: synthesis, spectral, X-ray crystallographic characterizations and catalytic activity

In this study, a series of benzimidazolium salts were synthesized as asymmetric N-heterocyclic carbene (NHC) precursors. Nine novel palladium complexes with the general formula [PdX2(NHC)(pyridine)] were synthesized using benzimidazolium salts in the PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) theme. All synthesized Pd(ii) complexes are stable. The synthesized compounds were thoroughly characterized by respective spectroscopic techniques, such as 1HNMR, 13C NMR, FTIR spectroscopy, X-ray crystallography and elemental analysis. The geometric structure of the palladium N-heterocyclic carbene has been optimized in the framework of density functional theory (DFT) using the B3LYP-D3 dispersion functional with LANL2DZ as a basis set. The on/off mechanism of pyridine assisted Pd-NHC complexes made them the best C-H functionalized catalysts for regioselective C-5 arylated products. Five membered heterocyclic compounds such as 2-acetyl furan, furfuryl acetate 2-acetylthiophene and N-methylpyrrole-2-carboxaldehyde were treated with numerous aryl bromides and arylchlorides under optimal catalytic reaction conditions. Interestingly, all the prepared catalysts possessed essential structural features that facilitated the formation of desired coupling products in quantitative yield with excellent selectivity. The arylation reaction of bromoacetophenone was highly catalytically active with only 1 mol% catalyst loading at 150 °C for 2 hours. To check the efficiency of the synthesized complexes, three different five member heterocyclic substrates (2-acetylfuran, 2-acetylthiophen, 2-propylthaizole) were tested with a number of aryl bromides bearing both electron-donating and electron-withdrawing groups on para position. The data in Tables 2-4. Indicated that electron-donating groups on the para position of aryl halide decreased the catalytic conversion while electron-withdrawing groups increased the catalytic conversion this was due to the high nucleophilicity of the electron-donating substituents.

Synthesis, structures, DFT calculations, and catalytic application in the direct arylation of five-membered heteroarenes with aryl bromides of novel palladium-N-heterocyclic carbene PEPPSI-type complexes

A new series of Pd-catalysts was synthesized and fully characterized via spectroscopic methods and the Pd–NHC complexes were evaluated via the direct arylation process.

Organo-selenium mediated regio- and stereoselective iodoselenylation of alkynes in an aqueous medium: simple access to (E)-β-iodoalkenyl selenides

A method for the simple, efficient, and atom economical iodoselenylation of simple alkynes under mild conditions using molecular iodine (I₂) and 1,2-diselanes as starting materials was developed.

Convergent Synthesis of PI3K Inhibitor GDC-0908 Featuring Palladium-Catalyzed Direct C-H Arylation toward Dihydrobenzothienooxepines

A practical convergent synthesis of PI3K inhibitor GDC-0908 (1) is described. The process features a dihydrobenzothienooxepine formation via palladium-catalyzed intramolecular direct C-H arylation and a Negishi coupling to construct the key C-C bonds. We further developed a general synthesis of dihydrobenzothienooxepines in good to excellent yields via palladium-catalyzed intramolecular direct C-H arylation, which tolerates both electronically and sterically diverse substituents on the phenyl ring.

An Iron-Based Long-Lived Catalyst for Direct C-H Arylation of Arenes and Heteroarenes

Direct C-H arylation of arenes and heteroarenes to biaryls at ambient temperature has been accomplished using a phenalenyl-supported iron(III) catalyst. The present catalyst requires a chemical reductant such as potassium and functions without any light stimulation. C-H arylation of various heteroarenes including pyridine as well as unactivated arene such as benzene delivered good to excellent yield (28 examples, up to 92 %) at room temperature. A combined effort based on experiments and theoretical calculations established that a phenalenyl-based radical species (generated by chemical reduction of the iron(III) coordinated phenalenyl complex) plays key role during the catalysis. Furthermore, this catalyst displayed remarkable stability during the catalysis, as evident from the fact that it was still usable over ten consecutive catalytic runs without losing its catalytic efficiency.

Iodine-mediated regio- and stereoselective iodothiocyanation of alkynes in aqueous ethanol

Iodothiocyanation of alkynes with ammonium thiocyanate (NH₄SCN) and molecular iodine (I₂) has been described under transition-metal-free conditions in green aqueous ethanol. A series of β-iodo vinylthiocyanates were obtained from functional alkynes in good to excellent isolated yields. The synthetic protocol has the advantages of operational simplicity, broad substrate scope, good stereo-selectivity, the use of easily available starting materials.

High performance p-type molecular electron donors for OPV applications via alkylthiophene catenation chromophore extension

The synthesis of key 4-alkyl-substituted 5-(trimethylsilyl)thiophene-2-boronic acid pinacol esters 3 allowed a simplified alkylthiophene catenation process to access bis-, ter-, quater-, and quinquethiophene π-bridges for the synthesis of acceptor–π-bridge-donor– π-bridge-acceptor (A–π-D–π-A) electron donor molecules. Based on the known benzodithiophene-terthiophene-rhodanine (BTR) material, the BXR series of materials, BMR (X = M, monothiophene), BBR (X = B, bithiophene), known BTR (X = T, terthiophene), BQR (X = Q, quaterthiophene), and BPR (X = P(penta), quinquethiophene) were synthesised to examine the influence of chromophore extension on the device performance and stability for OPV applications. The BT^xR (x = 4, butyl, and x = 8, octyl) series of materials were synthesised by varying the oligothiophene π-bridge alkyl substituent to examine structure–property relationships in OPV device performance. The devices assembled using electron donors with an extended chromophore (BQR and BPR) are shown to be more thermally stable than the BTR containing devices, with un-optimized efficiencies up to 9.0% PCE. BQR has been incorporated as a secondary donor in ternary blend devices with PTB7-Th resulting in high-performance OPV devices with up to 10.7% PCE.

Pd-Catalyzed one-pot sequential cross-coupling reactions of tetrabromothiophene

Unsymmetrical one-pot sequential cross-coupling reactions of sterically hindered tetrabromothiophene with arylboronic acid and an alkyne/alkene to afford selective bi-, tri-, and tetrasubstituted aryl/alkynyl-thiophenes with the aid of a palladium catalyst were described.

Syntheses via a direct arylation method of push-pull molecules based on triphenylamine and 3-cyano-4-hexyloxythiophene moieties

Synthetic access to new push-pull molecules based on 3-cyano-4-hexyloxythiophene and triphenylamine moieties is presented herein using a clean methodology. The key step involves a direct heteroarylation coupling reaction in the presence of a homogeneous or heterogeneous [Pd] catalyst followed by Knoevenagel condensation performed in ethanol as a solvent. Structure-electronic property relationships of the new molecular materials are discussed and then their use as donors in bilayer planar heterojunction solar cells is investigated.

Direct C-H arylation of thiophenes at low catalyst loading of a phosphine-free bis(alkoxo)palladium complex

An efficient phosphine-free direct C-H arylation of thiophenes at the α-position has been developed at low catalyst loading of bis(alkoxo)palladium complex (Cat.I, 0.1-0.2 mol %). The developed synthetic method can be applied to the synthesis of α-aryl/heteroaryl thiophenes from aryl or heteroaryl bromides in good to excellent yields and is compatible with the substrates bearing electron-donating or electron-withdrawing groups. The reactivities of the 2- and 5-positions of thiophenes are equivalent and not dependent on steric hindrance under optimal conditions. This condition can also be applied to other heterocyclic moieties such as benzothiophene, benzofuran, and pyrrole with high conversion yields.

Reactivity of bromofluorenes in palladium-catalysed direct arylation of heteroaromatics

The palladium-catalysed direct arylation using 2-bromofluorene or 2,7-dibromofluorene and heteroaromatics as the coupling partners proceeded using only 0.1–0.5 mol% catalyst.

Synthesis of heteroarylated polyfluorobiphenyls via palladium-catalyzed sequential sp2 C-H bonds functionalizations

The higher reactivity of C5-H bonds of heteroarenes as compared to C-H bonds of bromopolyfluorobenzenes for palladium-catalyzed direct arylation allows the selective synthesis of the polyfluoroaryl-heteroarenes in moderate to high yields, without C-H bond functionalization of the polyfluorobenzene ring. In most cases, low loading of Pd(OAc)2 catalyst (0.5-1 mol %) was employed. Then, from these heteroarylated polyfluorobenzenes, the palladium-catalyzed C-H bond functionalization of the polyfluorobenzene ring allows the synthesis of heteroarylated polyfluorobiphenyls.