Combinations of Alkyllithiums and Lithium Aminoalkoxides for Generation of Functional Pyridine Organometallics and Derivatives

Expanding the Scope of C-Glycoside Synthesis from Unstable Organolithium Reagents Using Flow Microreactors

C-glycosides are versatile scaffolds for drugs and bioactive compounds. The common organolithium-based synthesis of C-glycosides is limited by low reaction temperatures and a restricted substrate scope. To address these issues, a flow microreactor (FMR) was utilized for rapid mixing and precise temperature control, enabling C-glycoside synthesis at temperatures up to 40 °C and expanding the substrate scope. Continuous C-glycoside synthesis was achieved with a throughput of 21.9 g h-1, demonstrating the industrial potential of FMRs.

meta-Selective C-H Functionalization of Pyridines

The pyridine moiety is an important core structure for a variety of drugs, agrochemicals, catalysts, and functional materials. Direct functionalization of C-H bonds in pyridines is a straightforward approach to access valuable substituted pyridines. Compared to the direct ortho- and para-functionalization, meta-selective pyridine C-H functionalization is far more challenging due to the inherent electronic properties of the pyridine entity. This review summarizes currently available methods for pyridine meta-C-H functionalization using a directing group, non-directed metalation, and temporary dearomatization strategies. Recent advances in ligand control and temporary dearomatization are highlighted. We analyze the advantages as well as limitations of current techniques and hope to inspire further developments in this important area.

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.

Isolation and characterization of a lithiated pyridine - aggregation in the solid state and in solution

Pyridyllithiums are ubiquitous intermediates used for the electrophilic functionalization of pyridines. In this work, the isolation of a pyridyllithium is reported. Single crystal XRD revealed a dimer in the solid state. In THF solution it was identified as a monomer using DOSY NMR spectroscopy. Calculations showed that the pyridyllithium is a very electron rich pyridine superbase, which may account for its high reactivity.

Catalyst Control in Switching the Site Selectivity of C-H Olefinations of 1,2-Dihydroquinolines: An Approach to Positional-Selective Functionalization of Quinolines

A unique approach to achieve site-selective C-H olefinations exclusively at the C-3- or C-8-positions in the quinoline framework has been developed by catalyst control. Distal C(3)-H functionalization is achieved by using palladium catalysis, whereas proximal C(8)-H functionalization is obtained by employing ruthenium catalysis. Switching the site selectivity within a single substrate directly indicates two diverse pathways, which are operating under the palladium- and ruthenium-catalyzed reaction conditions.

Deprotonative Coupling of Pyridines with Aldehydes Catalyzed by an HMDS-Amide Base Generated in Situ

Herein, the deprotonative functionalization of pyridine derivatives with aldehydes under ambient conditions has been demonstrated using an amide base generated in situ from a catalytic amount of CsF and a stoichiometric amount of tris(trimethylsilyl)amine (N(TMS)₃). Pyridine substrates bearing two electron-withdrawing substituents (i.e., fluoro, chloro, bromo, and trifluoromethyl moieties) at the 3- and 5-positions efficiently react at the 4-position with various aldehydes including arylaldehydes, pivalaldehyde, and cyclohexanecarboxaldehyde.

Deprotonative Metalation of Methoxy-Substituted Arenes Using Lithium 2,2,6,6-Tetramethylpiperidide: Experimental and Computational Study

The reaction pathways of lithium 2,2,6,6-tetramethylpiperidide (LiTMP)-mediated deprotonative metalation of methoxy-substituted arenes were investigated. Importantly, it was experimentally observed that, whereas TMEDA has no effect on the course of the reactions, the presence of more than the stoichiometric amount of LiCl is deleterious, in particular without an in situ trap. These effects were corroborated by the DFT calculations. The reaction mechanisms, such as the structure of the active species in the deprotonation event, the reaction pathways by each postulated LiTMP complex, the stabilization effects by in situ trapping using zinc species, and some kinetic interpretation, are discussed herein.

Case for Lithium Tetramethylpiperidide-Mediated Ortholithiations: Reactivity and Mechanisms

Rate and mechanistic studies of ortholithiations by lithium 2,2,6,6-tetramethylpiperidide focus on four arenes: 1,4-bis(trifluoromethyl)benzene, 1,3-bis(trifluoromethyl)benzene, 1,3-dimethoxybenzene, and 4,4-dimethyl-2-phenyl-2-oxazoline. Metalations occur via substrate-dependent combinations of monosolvated monomer, disolvated monomer, and tetrasolvated dimer (triple ions). Density functional theory computational studies augment the experimental data. We discuss the challenges presented by shifting dimer-monomer proportions in determining the observable reaction orders and our mathematical treatment of such shifting in reactant structure.

Sodium Diisopropylamide in Tetrahydrofuran: Selectivities, Rates, and Mechanisms of Arene Metalations

Sodium diisopropylamide (NaDA)-mediated metalations of arenes in tetrahydrofuran (THF)/hexane or THF/Me₂NEt solutions are described. A survey of >40 benzenoid- and pyridine-based arenes with a range of substituents demonstrates the efficacy and regioselectivity of metalation. Metalations of activated disubstituted arenes and selected monosubstituted arenes are rapid at -78 °C. Rate studies of 1,3-dimethoxybenzene and related methoxylated arenes show exclusively monomer-based orthometalations with two or three coordinated THF ligands. Rate studies of the isotopic exchange of benzene and monosubstituted arenes with weakly activating groups reveal analogous di- and trisolvated monomer-based metalations. Cooperative inductive, mesomeric, steric, and chelate effects are discussed.

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.

Pyridine C3-arylation of nicotinic acids accessible via a multicomponent reaction: an entry to all-substituted-3,4-diarylated pyridines

An efficient route for the synthesis of all-substituted/functionalized pyridines has been developed. Several of the synthesized compounds exhibited significant anti-proliferative properties.

Sodium Diisopropylamide in N,N-Dimethylethylamine: Reactivity, Selectivity, and Synthetic Utility

The reactivities and chemoselectivities of sodium diisopropylamide (NaDA) in N,N-dimethylethylamine (DMEA) are compared with those of lithium diisopropylamide (LDA) in tetrahydrofuran (THF). Metalations of arenes, epoxides, ketones, hydrazones, dienes, and alkyl and vinyl halides are represented. The positive attributes of NaDA-DMEA include high solubility, stability, resistance to solvent decomposition, and ease of preparation. The high reactivities and chemoselectivities often complement those of LDA-THF.

Functionalizations of Mixtures of Regioisomeric Aryllithium Compounds by Selective Trapping with Dichlorozirconocene

The reaction of mixtures of aryllithium regioisomers obtained either by directed lithiation or by Br/Li exchange with substoichiometric amounts of Cp₂ZrCl₂ proceeds with high regioselectivity. The least sterically hindered regioisomeric aryllithium is selectively transmetalated to the corresponding arylzirconium species leaving the more hindered aryllithium ready for various reactions with electrophiles. As an application, these regioselective transmetalations from Li to Zr were used to prepare all three lithiated regioisomers of 1,3‐bis(trifluoromethyl)benzene.

The microwave-assisted ortho-alkylation of azine N-oxides with N-tosylhydrazones catalyzed by copper(i) iodide

A regioselective direct ortho-alkylation of azine N-oxides has been achieved under ligand-free conditions using copper(i) iodide and microwave conditions.

Deprotometalation-iodolysis and computed CH acidity of 1,2,3- and 1,2,4-triazoles. Application to the synthesis of resveratrol analogues

1-Aryl- and 2-aryl-1,2,3-triazoles were synthesized by N-arylation of the corresponding azoles using aryl iodides. The deprotometalations of 1-phenyl-1,2,3-triazole and -1,2,4-triazole were performed using a 2,2,6,6-tetramethylpiperidino-based mixed lithium-zinc combination and occurred at the most acidic site, affording by iodolysis the 5-substituted derivatives. Dideprotonation was noted from 1-(2-thienyl)-1,2,4-triazole by increasing the amount of base. From 2-phenyl-1,2,3-triazoles, and in particular from 2-(4-trifluoromethoxy)phenyl-1,2,3-triazole, reactions at the 4 position of the triazolyl, but also ortho to the triazolyl on the phenyl group, were observed. The results were analyzed with the help of the CH acidities of the substrates, determined in THF solution using the DFT B3LYP method. 4-Iodo-2-phenyl-1,2,3-triazole and 4-iodo-2-(2-iodophenyl)-1,2,3-triazole were next involved in Suzuki coupling reactions to furnish the corresponding 4-arylated and 4,2'-diarylated derivatives. When evaluated for biological activities, the latter (which are resveratrol analogues) showed moderate antibacterial activity and promising antiproliferative effect against MDA-MB-231 cell line.

A novel one-pot method for the synthesis of substituted furopyridines: iodine-mediated oxidation of enaminones by tandem metal-free cyclization

A novel iodine-mediated oxidative tandem cyclization reaction of simple enaminones has been developed for the synthesis of substituted furopyridines through C–C/C–N/C–O bond formation in a one-pot procedure.

50 years of superbases made from organolithium compounds and heavier alkali metal alkoxides

A review of reactions of organolithium compounds (RLi) with alkali metal alkoxides is presented. On the one hand, simple lithium alkoxides form adducts with RLi the reactivity of which differs only slightly from that of RLi. On the other hand, after mixing heavier alkali metal alkoxides (R’OM, M = Na, K, Rb, Cs) with RLi, a new system is formed, which has reactivity that dramatically exceeds that of the parent RLi. A metal interchange, according to the equation RLi + R’OM = RM + R’OLi, occurs in this system, giving rise to a superbase. This reaction is frequently used for the preparation of heavier alkali metal organometallic compounds. Similar metal interchange takes place between R’OM and compounds such as lithium amides and lithium enolates of ketones or esters, thus demonstrating the general nature of this procedure. Superbases react easily with many types of organic compounds (substrates), resulting in the formation of a heavier alkali metal derivative of the substrate (metalation). The metalated substrate can react in situ with an electrophile to yield the substituted substrate, a procedure that is frequently used in synthetic and polymer chemistry. An improved mechanism of metal interchange and reaction of superbases with substrates is proposed.

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.

New avenues in the directed deprotometallation of aromatics: recent advances in directed cupration

Advances in directed aromatic deprotometallation are reported in the context of recent developments in our understanding of lithium cuprates.

Radical-based regioselective C-H functionalization of electron-deficient heteroarenes: scope, tunability, and predictability

Radical addition processes can be ideally suited for the direct functionalization of heteroaromatic bases, yet these processes are only sparsely used due to the perception of poor or unreliable control of regiochemistry. A systematic investigation of factors affecting the regiochemistry of radical functionalization of heterocycles using alkylsulfinate salts revealed that certain types of substituents exert consistent and additive effects on the regioselectivity of substitution. This allowed us to establish guidelines for predicting regioselectivity on complex π-deficient heteroarenes, including pyridines, pyrimidines, pyridazines, and pyrazines. Since the relative contribution from opposing directing factors was dependent on solvent and pH, it was sometimes possible to tune the regiochemistry to a desired result by modifying reaction conditions. This methodology was applied to the direct, regioselective introduction of isopropyl groups into complex, biologically active molecules, such as diflufenican (44) and nevirapine (45).