Pd0/PR3-Catalyzed Arylation of Nicotinic and Isonicotinic Acid Derivatives

Directing Group-Free Regioselective meta-C-H Functionalization of Pyridines

The pyridine core is among the most common motifs found in pharmaceuticals and agrochemicals. Consequently, the C-H functionalization of pyridine is a prized reaction, as it can help access a broad spectrum of valuable chemicals. However, the intrinsic electronic properties of pyridines hinder their meta-C-H functionalization, requiring drastic conditions affecting functional group compatibility. A synthetic manoeuvre to overcome this challenge involves the temporary conversion of pyridines into electron-rich intermediates and subsequent regioselective electrophilic functionalization. This was recently accomplished by a ring-opening ring-closing sequence via Zincke imine intermediates by McNally and co-workers, and a dearomatization-rearomatization sequence via oxazino-pyridine intermediates by the Studer group. The mildness and simplicity of these protocols enable them to work with complex molecular setups for synthesizing natural products and bioactive molecules.

Palladium-Catalyzed Electrophilic Functionalization of Pyridine Derivatives through Phosphonium Salts

Herein, we report a highly efficient and practical method for pyridine-derived heterobiaryl synthesis through palladium-catalyzed electrophilic functionalization of easily available pyridine-derived quaternary phosphonium salts. The nice generality of this reaction was goes beyond arylation, enabling facile incorporation of diverse carbon-based fragments, including alkenyl, alkynyl, and also allyl fragments, onto the pyridine core. Notably, the silver salt additive is revealed to be of vital importance for the success of this transformation and its pivotal role as transmetallation mediator, which guarantees a smooth transfer of pyridyl group to palladium intermediate, is also described.

Highly Selective Palladium-Catalyzed Arene C-H Acyloxylation with Benzothiadiazole as a Modifiable Directing Group

An efficient protocol for the palladium-catalyzed direct arene C-H acyloxylation of benzothiadiazole-arene derivatives is reported for the first time. The key strategy is the employment of benzothiadiazole as a modifiable directing group. The highly selective mono- or bis-acyloxylation can be achieved by tuning the reaction conditions, affording various acyloxylated benzothiadiazole derivatives, which could offer a rational tailoring of their electronic properties and be applied in organic electronic and optoelectronic materials. Finally, by diversity-oriented modification of the benzothiadiazole directing group, the acyloxylated products can be readily converted into various valuable functionalized (hetero)biaryls.

Copper-Mediated Late-Stage Functionalization of Heterocycle-Containing Molecules

One long-standing issue in directed C-H functionalization is that either nitrogen or sulfur atoms present in heterocyclic substrates may bind preferentially to a transition-metal catalyst rather than to the desired directing group. This competitive binding has largely hindered the application of C-H functionalization in late-stage heterocycle drug discovery. Reported here is the use of an oxazoline-based directing group capable of overriding the poisoning effect of a wide range of heterocycle substrates. The potential use of this directing group in pharmaceutical drug discovery is illustrated by diversification of Telmisartan (an antagonist for the angiotensin II receptor) through copper-mediated C-H amination, hydroxylation, thiolation, arylation, and trifluoromethylation.

C-H and C-N Activation at Redox-Active Pyridine Complexes of Iron

Pyridine activation by inexpensive iron catalysts has great utility, but the steps through which iron species can break the strong (105-111 kcal mol-1 ) C-H bonds of pyridine substrates are unknown. In this work, we report the rapid room-temperature cleavage of C-H bonds in pyridine, 4-tert-butylpyridine, and 2-phenylpyridine by an iron(I) species, to give well-characterized iron(II) products. In addition, 4-dimethylaminopyridine (DMAP) undergoes room-temperature C-N bond cleavage, which forms a dimethylamidoiron(II) complex and a pyridyl-bridged tetrairon(II) square. These facile bond-cleaving reactions are proposed to occur through intermediates having a two-electron reduced pyridine that bridges two iron centers. Thus, the redox non-innocence of the pyridine can play a key role in enabling high regioselectivity for difficult reactions.

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.

Recent Advances in the C-H-Functionalization of the Distal Positions in Pyridines and Quinolines

This review summarizes recent developments in the C-H-functionalization of the distal positions of pyridines, quinolines and related azaheterocycles. While the functionalization of the C2 position has been known for a long time and is facilitated by the proximity to N1, regioselective reactions in the distal positions are more difficult to achieve and have only emerged in the last decade. Recent advances in the transition metal-catalyzed distal C-H-functionalization of these synthetically-important azaheterocycles are discussed in detail, with the focus on the scope, site-selectivity and mechanistic aspects of the reactions.

Direct oxidative arylation of aryl C - H bonds with aryl boronic acids via pd catalysis directed by the N,N-dimethylaminomethyl group

Biaryl skeletons were directly constructed via palladium-catalyzed ortho-arylation of N,N-dimethyl benzylamine with aryl boronic acids with high efficiency and high regioselectivity under open-flask conditions. The N,N-dimethylaminomethyl group was first applied as a directing group in such an oxidative coupling. Various substrates proved to be efficient coupling partners, furnishing the corresponding ortho-monoarylated or -diarylated arenes in moderate to good yields under mild conditions.

Direct arylation of N-heteroarenes with aryldiazonium salts by photoredox catalysis in water

A highly effective visible light-promoted "radical-type" coupling of N-heteroarenes with aryldiazonium salts in water has been developed. The reaction proceeds at room temperature with [Ru(bpy)3 ]Cl2 ⋅6 H2 O as a photosensitizer and a commercial household light bulb as a light source. Pyridine and a variety of substituted pyridines are effective substrates under these reaction conditions, and only monosubstituted products are formed with different regioselectivities. Using aqueous formic acid as solvent, an array of xanthenes, thiazole, pyrazine, and pyridazine are compatible with this new arylation approach. The broad substrate scope, mild reaction conditions, and use of water as reaction solvent make this procedure a practical and environmentally friendly method for the synthesis of compounds containing aryl-heteroaryl motifs.