Visible Light-Induced Divergent Deoxygenation/Hydroxymethylation of Pyridine N-Oxides

This study explores the deoxygenation of pyridine N-oxides and presents a one-step photoredox method for the direct synthesis of 2-hydroxymethylated pyridines from pyridine N-oxides. Mechanism studies elucidate the role of the catalyst and provide evidence of the possible electron transfer process and the formation of key radicals. A range of pyridine derivatives, particularly 2-hydroxymethyl-substituted pyridines, which may be difficult to obtain, can be synthesized in a single step.

Copper-Catalyzed Four-Component Domino Cyclization for the Synthesis of 2-Methylpyridines

A convenient protocol for synthesis of unsymmetrical 2-methylpyridines from acetyl ketones, ammonium salts and tertiary amines is described. The construction of two C-C bonds and two C-N bonds via [2 + 2 + 1 + 1] four-component domino cyclization reaction is achieved using Cu(OTf) as catalyst in one pot. This cyclization reaction shows good selectivity and produces multisubstituted 2-methylpyridine derivatives in good yields with various functional groups.

Synthesis of Dihydropyridine Spirocycles by Semi-Pinacol-Driven Dearomatization of Pyridines

The identification of the beneficial pharmacokinetic properties of aza-spirocycles has led to the routine incorporation of these highly rigid and three-dimensional structures in pharmaceuticals. Herein, we report an operationally simple synthesis of spirocyclic dihydropyridines via an electrophile-induced dearomative semi-pinacol rearrangement of 4-(1'-hydroxycyclobutyl)pyridines. The various points for diversification of the spirocyclization precursors, as well as the synthetic utility of the amine and ketone functionalities in the products, provide the potential to rapidly assemble medicinally relevant spirocycles.

Organoborohydride-catalyzed Chichibabin-type C4-position alkylation of pyridines with alkenes assisted by organoboranes

The first NaBEt₃H-catalyzed intermolecular Chichibabin-type alkylation of pyridine and its derivatives with alkenes as the latent nucleophiles is presented with the assistance of BEt₃, and a series of branched C4-alkylation pyridines, even highly congested all-carbon quaternary center-containing triarylmethanes can be obtained in a regiospecific manner. Therefore, the conventional reliance on high cost and low availability transition metal catalysts, prior formation of N-activated pyridines, organometallic reagents, and extra oxidation operation for the construction of a C–C bond at the C4-position of the pyridines in previous methods are not required. The corresponding mechanism and the key roles of the organoborane were elaborated by the combination of H/D scrambling experiments, ¹¹B NMR studies, intermediate trapping experiments and computational studies. This straightforward and mechanistically distinct organocatalytic technology not only opens a new door for the classical but still far less well-developed Chichibabin-type reaction, but also sets up a new platform for the development of novel C–C bond-forming methods.

The first NaBEt₃H-catalyzed intermolecular Chichibabin-type alkylation of pyridines with alkenes as the latent nucleophiles is presented in the presence of BEt₃, and a series of branched C4-alkylated pyridines were obtained in a regiospecific manner.

A convenient approach to 2,4-disubstituted quinazoline-3-oxides using active MnO2 as the oxidant

Using cheap, readily available, and economic active MnO2 as an oxidant, a series of 2,4-disubstituted quinazoline-3-oxides were synthesized in good to excellent yields by oxidation of the corresponding 1,2-dihydroquinazoline-3-oxides under mild conditions. This novel approach is simple and convenient and provides a series of 2,4-disubstituted quinazoline-3-oxides.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Photocatalyzed ortho-Alkylation of Pyridine N-Oxides through Alkene Cleavage

A photocatalyzed reaction of pyridine N-oxides with alkenes gives ortho-alkylated pyridines with cleavage of the carbon-carbon double bond. Benzyl and secondary alkyl groups are incorporated at the ortho position of pyridines in one pot.

2-Chloro-4-nitropyridine N-oxide

In the title compound, C5H3ClN2O3 (systematic name: 2-chloro-4-nitropyridin-1-ium-1-olate), the nitro group is essentially coplanar with the aromatic ring, with a twist angle of 6.48 (8)°. The molecular packing exhibits a herringbone pattern with the zigzag running along the b axis; here, there are no short contacts, hydrogen bonds, or π–π interactions.

C-H Functionalization of Azines

Azines, which are six-membered aromatic compounds containing one or more nitrogen atoms, serve as ubiquitous structural cores of aromatic species with important applications in biological and materials sciences. Among a variety of synthetic approaches toward azines, C-H functionalization represents the most rapid and atom-economical transformation, and it is advantageous for the late-stage functionalization of azine-containing functional molecules. Since azines have several C-H bonds with different reactivities, the development of new reactions that allow for the functionalization of azines in a regioselective fashion has comprised a central issue. This review describes recent advances in the C-H functionalization of azines categorized as follows: (1) S_NAr reactions, (2) radical reactions, (3) deprotonation/functionalization, and (4) metal-catalyzed reactions.

Metal-Free Synthesis of C-4 Substituted Pyridine Derivatives Using Pyridine-boryl Radicals via a Radical Addition/Coupling Mechanism: A Combined Computational and Experimental Study

Density functional theory investigations revealed that the pyridine-boryl radical generated in situ using 4-cyanopyridine and bis(pinacolato)diboron could be used as a bifunctional "reagent", which serves as not only a pyridine precursor but also a boryl radical. With the unique reactivity of such radicals, 4-substituted pyridine derivatives could be synthesized using α,β-unsaturated ketones and 4-cyanopyridine via a novel radical addition/C-C coupling mechanism. Several controlled experiments were conducted to provide supportive evidence for the proposed mechanism. In addition to enones, the scope could be extended to a wide range of boryl radical acceptors, including various aldehydes and ketones, aryl imines and alkynones. Lastly, this transformation was applied in the late-stage modification of a complicated pharmaceutical molecule.

Synthesis and Utility of Dihydropyridine Boronic Esters

When activated by an acylating agent, pyridine boronic esters react with organometallic reagents to form a dihydropyridine boronic ester. This intermediate allows access to a number of valuable substituted pyridine, dihydropyridine, and piperidine products.

A three-component synthesis of aryl(heteroaryl)acylamides

α-Heteroarylamides of diverse structures are assembled in a three-component reaction of azole or azineN-oxides, 1,1-difluorostyrenes and amines.

Direct, catalytic, and regioselective synthesis of 2-alkyl-, aryl-, and alkenyl-substituted N-heterocycles from N-oxides

A one-step transformation of heterocyclic N-oxides to 2-alkyl-, aryl-, and alkenyl-substituted N-heterocycles is described. The success of this broad-scope methodology hinges on the combination of copper catalysis and activation by lithium fluoride or magnesium chloride. The utility of this method for the late-stage modification of complex N-heterocycles is exemplified by facile syntheses of new structural analogues of several antimalarial, antimicrobial, and fungicidal agents.

Metal-free (Boc)2O-mediated C4-selective direct indolation of pyridines using TEMPO

Metal-free C-4-selective indolation of pyridines using (Boc)₂O as an activating agent.

Copper-catalyzed direct C–H arylation of pyridine N-oxides with arylboronic esters: one-pot synthesis of 2-arylpyridines

Copper-catalyzed direct C–H arylation of pyridine N-oxides with arylboronic esters for the one-pot synthesis of 2-arylpyridines without an additional reductant.

Brønsted acid catalyzed intramolecular benzylic cyclizations of alkylpyridines

Aldehyde and ketone electrophiles incorporated into the side chains of 2- and 4-alkylpyridines participate in intramolecular aldol-like condensations with pyridine benzylic carbons in the presence of Brønsted acid catalysts.