Mechanistic and Performance Studies on the Ligand-Promoted Ullmann Amination Reaction

Cross-coupling by a noncanonical mechanism involving the addition of aryl halide to Cu(II)

Copper complexes are widely used in the synthesis of fine chemicals and materials to catalyze couplings of heteroatom nucleophiles with aryl halides. We show that cross-couplings catalyzed by some of the most active catalysts occur by a mechanism not previously considered. Copper(II) [Cu(II)] complexes of oxalamide ligands catalyze Ullmann coupling to form the C-O bond in aryl ethers by concerted oxidative addition of an aryl halide to Cu(II) to form a high-valent species that is stabilized by radical character on the oxalamide ligand. This mechanism diverges from those involving Cu(I) and Cu(III) intermediates that have been posited for other Ullmann-type couplings. The stability of the Cu(II) state leads to high turnover numbers, >1000 for the coupling of phenoxide with aryl chloride electrophiles, as well as an ability to run the reactions in air.

Octalithium, Tetrasodium, and Decalithium Compounds Based on Pyrrolyl Ligands: Synthesis, Structures, and Activation of the C-H Bonds of Pyrrolyl Rings and C═N Bonds of a Series of Ligands by Organolithium Reagents

The organometallic compounds of lithium ions have garnered continuous interest as indispensable precursors for the syntheses of organometallic complexes of main-group metals, transition metals, lanthanide metals, and actinide metals. In this work, we present a strategy for the preparation of a series of polynuclear lithium complexes. This methodology features the utilization of organolithium reagents both as metal sources to coordinate with the ligands and as nucleophilic reagents to undergo nucleophilic addition to the C═N bonds of the ligands. Reaction of a ligand HL1 [HL1 = 2-(((1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)methylene)amino)phenol] with n-BuLi produced complex [Li8(L1a)4]·1.5Tol (1·1.5Tol) [H2L1a = 2-((1-(1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)pentyl)amino)phenol]. One prominent feature regarding the formation of 1·1.5Tol is the occurrence of nucleophilic addition of n-BuLi to the C═N bond of HL1, leading to the generation of a new [L1a]2- ligand that contains both aminophenol and 1-(2-pyrrolyl)alkylamine scaffolds. The developed protocol can be adapted to a series of organolithium reagents. Compounds [Li8(L1b)4] (2) and [Li8(L1c)4] (3) were afforded by treatment of HL1 with sec-BuLi and LiCH2SiMe3, respectively. Reaction of an analogous ligand HL2 [HL2 = 2-(((1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)methylene)amino)-4-methylphenol] with n-BuLi generated compound [Li8(L2a)4] (4). C═N bond activation was not observed in the reaction of HL1 with NaOtBu, and the complex [Na4(L1)4]·Tol (5·Tol) was obtained. A decanuclear complex [Li10(L3a)2(L3b)2] (6) was also prepared via the reaction of HL3 [HL3 = 2-(2-((((1H-pyrrol-2-yl)methylene)amino)methyl)-1H-pyrrol-1-yl)-N,N-dimethylethan-1-amine] with t-BuLi. A remarkable feature in terms of the synthesis of 6 is the simultaneous occurrence of hydrogen atom abstraction from the C-H bond of the pyrrolyl ring and nucleophilic addition to the C═N bond of the HL3 ligand by t-BuLi. A series of amines containing biologically and physiologically important moieties were achieved by hydrolysis of the crude products from the reactions of the HL1-HL3 ligands and organolithium reagents. This work provides an efficient approach to high-nuclearity lithium compounds as well as a series of amines.

Reduced-Phenalenyl-Based Molecule as a Super Electron Donor for Radical-Mediated C-N Coupling Catalysis at Room Temperature

We demonstrate that an in situ generated di-reduced phenalenyl (PLY) species accumulates sufficiently high energy and acts as a super electron donor to generate aryl radicals from aryl halides to accomplish Buchwald-Hartwig-type C-N cross-coupling reactions at room temperature. This catalytic protocol does not require any external stimuli such as heat, light, or cathodic current. This protocol shows a wide variety of substrate scope covering different genres of aryl and heteroaryl halides with various aromatic as well as aliphatic amines and late-stage functionalization of the well-known natural products. The control experiments, along with extensive density functional theory (DFT) calculations, unveil that the aryl radical is generated by a single electron transfer from the di-reduced PLY to the aryl halide substrate. The aryl radical acts as an electrophile and binds with amine, leading to the chemically driven radical-mediated C-N cross-coupling under transition-metal-free conditions.

C-N, C-O and C-S Ullmann-Type Coupling Reactions of Arenediazonium o-Benzenedisulfonimides

Arenediazonium o-benzenedisulfonimides have been used as efficient electrophilic partners in Cu(I) catalysed Ullmann-type coupling. The synthetic protocols are mild and easy, and produced either N-alkylanilines, aryl ethers, or thioethers in fairly good yields (18 positive examples, average yield 66%). o-Benzenedisulfonimide was recovered at the end of the reactions and was reused to prepare the starting salts for further reactions. It is noteworthy that diazonium salts have been used as electrophilic partners in the Ullmann-type protocol for the first time.

Thiolate-assisted copper(i) catalyzed C–S cross coupling of thiols with aryl iodides: scope, kinetics and mechanism

The scope and mechanism of the C–S cross coupling of thiophenols with aryl iodides using a Cu(i) catalyst in a ligand-free environment is disclosed.

Fe-MIL-101 modified by isatin-Schiff-base-Co: a heterogeneous catalyst for C–C, C–O, C–N, and C–P cross coupling reactions

Fe-MIL-101-isatin-Schiff-base-Co was synthesized and applied as a catalyst for Ullmann-type, Buchwald–Hartwig, Hirao, Hiyama and Mizoroki–Heck cross-coupling reactions of aryl halides.

Chan-Evans-Lam N1-(het)arylation and N1-alkеnylation of 4-fluoroalkylpyrimidin-2(1H)-ones

The Chan–Evans–Lam reaction of 1-unsubstituted 4-fluoroalkylpyrimidin-2(1Н)-ones with arylboronic acids is reported as a facile synthetic route to hitherto unavailable N1-(het)aryl and N1-alkenyl derivatives of the corresponding pyrimidines. An efficient C–N bond-forming process is also observed by using boronic acid pinacol esters as coupling partners in the presence of Cu(II) acetate and boric acid. The 4-fluoroalkyl group on the pyrimidine ring significantly assists in the formation of the target N1-substituted products, in contrast to the 4-methyl and 4-unsubstituted substrates which do not undergo N1-arylation under similar reaction conditions.

The Quest for the Ideal Base: Rational Design of a Nickel Precatalyst Enables Mild, Homogeneous C-N Cross-Coupling

Palladium-catalyzed amination reactions using soluble organic bases have provided a solution to the many issues associated with heterogeneous reaction conditions. Still, homogeneous C-N cross-coupling approaches cannot yet employ bases as weak and economical as trialkylamines. Furthermore, organic base-mediated methods have not been developed for Ni(0/II) catalysis, despite some advantages of such systems over those employing Pd-based catalysts. We designed a new air-stable and easily prepared Ni(II) precatalyst bearing an electron-deficient bidentate phosphine ligand that enables the cross-coupling of aryl triflates with aryl amines using triethylamine (TEA) as base. The method is tolerant of sterically congested coupling partners, as well as those bearing base- and nucleophile-sensitive functional groups. With the aid of density functional theory (DFT) calculations, we determined that the electron-deficient auxiliary ligands decrease both the pK_a of the Ni-bound amine and the barrier to reductive elimination from the resultant Ni(II)-amido complex. Moreover, we determined that the preclusion of Lewis acid-base complexation between the Ni catalyst and the base, due to steric factors, is important for avoiding catalyst inhibition.

Brønsted acid/visible-light-promoted Markovnikov hydroamination of vinylarenes with arylamines

A Brønsted acid/visible-light-promoted Markovnikov hydroamination of vinylarenes with arylamines in the presence of TPT and CF₃CO₂H has been developed. This transformation provides a green approach to alpha-amino-substituted arylalkanes under metal-free conditions.

Synthetic and computational studies on CuI/ligand pair promoted activation of C(Aryl)-Cl bond in C-N coupling reactions

Cu/ligand-mediated coupling reactions have been widely investigated in the recent past. However, activation of cheaper aryl chlorides is still a great limitation of these reactions. During the course of present investigations efforts have been made to develop a normal and facile CuI/ligand pair protocol for arylation of phthalimide using aryl chlorides. The protocol has also been extended for arylation of amines. On the basis of experimental and theoretical results, a catalytic cycle has also been proposed and it has been established that these reactions follow oxidative addition-reductive elimination (OA-RE) pathway. These studies have indicated that tetracoordinated [Cu(L1)(L2)]⁺ complex is active catalytic species in these reactions.

Direct Synthesis of Diphenylamines from Phenols and Ammonium Formate Catalyzed by Palladium

Arylamines are commercially and synthetically useful compounds with a wide variety of applications. Their preparation has been traditionally achieved using metal-catalyzed C-N coupling reactions with aryl halides. In this work, 17 different diarylamines are prepared from phenols by using ammonium formate as the aminating reagent. Phenolic compounds are more desirable feedstocks, owing to their availability from lignin, making them valuable biorenewable alternatives to aryl halides. Ammonium formate is found to be a convenient surrogate for ammonia and a useful aminating reagent for phenols. Diarylamine products are obtained in good to excellent yields while only water and CO₂ are generated as byproducts of the transformation.

Visual kinetic analysis

Naked-eye comparison of reaction profiles is a powerful tool to extract mechanistic information – FAQ, video tutorial and templated examples included.

Visual kinetic analyses extract meaningful mechanistic information from experimental data using the naked-eye comparison of appropriately modified progress reaction profiles. Basic kinetic information is obtained easily and quickly from just a few experiments. Therefore, these methods are valuable tools for all chemists working in process chemistry, synthesis or catalysis with an interest in mechanistic studies. This minireview describes the visual kinetic analyses developed in the last fifteen years and provides answers to the most common queries of new users. Furthermore, a video tutorial is attached detailing the implementation of both VTNA and RPKA.