Regioselective Functionalization of Quinolines through C-H Activation: A Comprehensive Review

Quinoline is a versatile heterocycle that is part of numerous natural products and countless drugs. During the last decades, this scaffold also became widely used as ligand in organometallic catalysis. Therefore, access to functionalized quinolines is of great importance and continuous efforts have been made to develop efficient and regioselective synthetic methods. In this regard, C-H functionalization through transition metal catalysis, which is nowadays the Graal of organic green chemistry, represents the most attractive strategy. We aim herein at providing a comprehensive review of methods that allow site-selective metal-catalyzed C-H functionalization of quinolines, or their quinoline N-oxides counterparts, with a specific focus on their scope and limitations, as well as mechanistic aspects if that accounts for the selectivity.

The 2-Pyridyl Problem: Challenging Nucleophiles in Cross-Coupling Arylations

Azine-containing biaryls are ubiquitous scaffolds in many areas of chemistry, and efficient methods for their synthesis are continually desired. Pyridine rings are prominent amongst these motifs. Transition-metal-catalysed cross-coupling reactions have been widely used for their synthesis and functionalisation as they often provide a swift and tuneable route to related biaryl scaffolds. However, 2-pyridine organometallics are capricious coupling partners and 2-pyridyl boron reagents in particular are notorious for their instability and poor reactivity in Suzuki-Miyaura cross-coupling reactions. The synthesis of pyridine-containing biaryls is therefore limited, and methods for the formation of unsymmetrical 2,2'-bis-pyridines are scarce. This Review focuses on the methods developed for the challenging coupling of 2-pyridine nucleophiles with (hetero)aryl electrophiles, and ranges from traditional cross-coupling processes to alternative nucleophilic reagents and novel main group approaches.

Relative Strength of Common Directing Groups in Palladium-Catalyzed Aromatic C-H Activation

Efficient functionalization of C-H bonds can be achieved using transition metal catalysts, such as Pd(OAc)2. To better control the regioselectivity in these reactions, some functional groups on the substrate may be used as directing groups, guiding the reactivity to an ortho position. Herein, we describe a methodology to score the relative strength of such directing groups in palladium-catalyzed aromatic C-H activation. The results have been collected into a scale that serves to predict the regioselectivity on molecules with multiple competing directing groups. We demonstrate that this scale yields accurate predictions on over a hundred examples, taken from the literature. In addition to the regioselectivity prediction on complex molecules, the knowledge of the relative strengths of directing groups can also be used to work with new combinations of functionalities, exploring uncharted chemical space.

Pyridinium salts: from synthesis to reactivity and applications

This review highlights the pyridinium salts in terms of their natural occurrence, synthesis, reactivity, biological properties, and diverse applications.

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.

Sequential decarboxylative azide-alkyne cycloaddition and dehydrogenative coupling reactions: one-pot synthesis of polycyclic fused triazoles

Herein, we describe a one-pot protocol for the synthesis of a novel series of polycyclic triazole derivatives. Transition metal-catalyzed decarboxylative CuAAC and dehydrogenative cross coupling reactions are combined in a single flask and achieved good yields of the respective triazoles (up to 97% yield). This methodology is more convenient to produce the complex polycyclic molecules in a simple way.

Copper-mediated formally dehydrative biaryl coupling of azine N-oxides and oxazoles

A copper-mediated formally dehydrative biaryl coupling of azine N-oxides and oxazoles has been developed. The C-C bond-forming process proceeds, accompanied by the removal of the oxygen atom from the azine core, to directly afford the azine-oxazole biaryl linkage. Moreover, this system requires no noble transition metals such as palladium and rhodium, which are common promotors in the related dehydrogenative couplings with the azine N-oxide. Thus, the present protocol can provide a unique and less expensive approach to the azine-containing biheteroaryls of substantial interest in pharmaceutical and medicinal chemistry.

Palladium-Catalyzed C8-Selective C-H Arylation of Quinoline N-Oxides: Insights into the Electronic, Steric and Solvation Effects on the Site-Selectivity by Mechanistic and DFT Computational Studies

We report herein a palladium-catalyzed C–H arylation of quinoline N-oxides that proceeds with high selectivity in favor of the C8 isomer. This site selectivity is unusual for palladium, since all of the hitherto described methods of palladium-catalyzed C–H functionalization of quinoline N-oxides are highly C2 selective. The reaction exhibits a broad synthetic scope with respect to quinoline N-oxides and iodoarenes and can be significantly accelerated to subhour reaction times under microwave irradiation. The C8-arylation method can be carried out on a gram scale and has excellent functional group tolerance. Mechanistic and density functional theory (DFT) computational studies provide evidence for the cyclopalladation pathway and describe key parameters influencing the site selectivity.