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Relevant academic research and scientific papers

Flexible Steric Bulky Bis(Imino)acenaphthene (BIAN)-Supported N-Heterocyclic Carbene Palladium Precatalysts: Catalytic Application in Buchwald-Hartwig Amination in Air

To achieve efficient palladium-catalyzed cross-coupling reaction under mild reaction conditions with the flexible steric bulk strategy, a series of Pd-PEPPSI (PEPPSI: pyridine-enhanced precatalyst preparation, stabilization, and initiation) complexes C1-C6 were synthesized and characterized, in which unsymmetric flexible steric bulk was introduced on the N-aryl of ancenaphthyl skeleton. These well-defined palladium complexes were found to be excellent precatalysts for Buchwald-Hartwig amination of aryl chlorides with amines in air. The electronic effect of the Pd-PEPPSI complexes and the effect of ancillary pyridine ligands were evaluated, among which complex C3 exhibited the most efficiency. It was demonstrated that the cross-coupling products were obtained in excellent yields in the presence of 0.5-0.1 mol % palladium loading. A wide range of aryl- and heteroaryl chlorides as well as various amines were compatible. The oxidative addition of aryl chlorides is revealed to be the rate-determining step in the catalytic cycle. The catalytic activity can be enhanced by introducing electron-donating groups to the Pd-PEPPSI complexes. This type of Pd-PEPPSI precatalyst showed the most efficiency reported to date for the challenging C-N cross-coupling reactions requiring no anhydrous and inert atmosphere protections, suggesting flexible steric bulk as a promising catalyst design strategy.

Strong Short-Range Cooperativity in Hydrogen-Bond Chains

Chains of hydrogen bonds such as those found in water and proteins are often presumed to be more stable than the sum of the individual H bonds. However, the energetics of cooperativity are complicated by solvent effects and the dynamics of intermolecular interactions, meaning that information on cooperativity typically is derived from theory or indirect structural data. Herein, we present direct measurements of energetic cooperativity in an experimental system in which the geometry and the number of H bonds in a chain were systematically controlled. Strikingly, we found that adding a second H-bond donor to form a chain can almost double the strength of the terminal H bond, while further extensions have little effect. The experimental observations add weight to computations which have suggested that strong, but short-range cooperative effects may occur in H-bond chains.

Arylation of Amines in Alkane Solvents by using Well-Defined Palladium-N-Heterocyclic Carbene Complexes

The use of the ITent-based series of Pd-N-heterocyclic carbenes precatalysts (Tent= Tentacular) enables the Buchwald-Hartwig amine arylation reaction in apolar alkane solvents. The use of such solvents is rare because of their poor solvation properties. Nonetheless, they could be of interest for industrial applications, as they can be disposed of in an energetically favorable manner and may potentially simplify product isolation. The excellent solubility of the precatalysts permitted the desired products to be obtained in yields ranging from 75 to 96 %, even with deactivated and functionalized coupling partners. Fueling cross-couplings: The use of ITent-based Pd precatalysts (Tent=tentacular) enables the Buchwald-Hartwig amine arylation reaction in alkane solvents. Such solvents could be of interest for industrial applications, as they can be disposed of in an energetically favorable manner and may potentially simplify product isolation. Yields ranging from 75 to 96 % are obtained, even with challenging substrates.

SYNTHESES OF N-HETEROCYCLIC CARBENES AND INTERMEDIATES THEREFOR

A method of preparing a 2,6 disubstituted anilines includes, reacting a 2-amino isophtha!ic acid diester with sufficient Grignard reagent R2CH2MgX to form the corresponding diol product, dehydrating the diol product to the corresponding dialkene; and hydrogenating the diol product to form the corresponding aniline. The 2,6 disubstituted anilines can be used to produce N-Heterocyciic Carbenes (NHCs). The NHCs can find application in various fields such as organic synthesis, catalysis and macromolecular chemistry. Palladium catalysts containing the NHCs are also described.

Large yet flexible N-heterocyclic carbene ligands for palladium catalysis

A straightforward and scalable eight-step synthesis of new N-heterocyclic carbenes (NHCs) has been developed from inexpensive and readily available 2-nitro-m-xylene. This process allows for the preparation of a novel class of NHCs coined ITent ("Tent" for "tentacular") of which the well-known IMes (N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IPr (N,N′-bis(2,6-di(2-propyl)phenyl)imidazol-2-ylidene) and IPent (N,N′-bis(2,6-di(3-pentyl)phenyl)imidazol-2-ylidene) NHCs are the simplest and already known congeners. The synthetic route was successfully used for the preparation of three members of the ITent family: IPent (N,N′-bis(2,6- di(3-pentyl)phenyl)imidazol-2-ylidene), IHept (N,N′-bis(2,6-di(4-heptyl) phenyl)imidazol-2-ylidene) and INon (N,N′-bis(2,6-di(5-nonyl)phenyl) imidazol-2-ylidene). The electronic and steric properties of each NHC were studied through the preparation of both nickel and palladium complexes. Finally the effect of these new ITent ligands in Pd-catalyzed Suzuki-Miyaura and Buchwald-Hartwig cross-couplings was investigated. Tentacular NHCs: A straightforward, scalable eight-step synthesis of N-heterocyclic carbenes (NHCs) has been developed using inexpensive and readily available 2-nitro-m-xylene, allowing the preparation of a class of NHCs designated ITent ("Tent" for "tentacular"). The electronic and steric properties of each NHC were studied through the preparation of both nickel and palladium complexes, and the effect of these new ITent ligands in Pd-catalyzed Suzuki-Miyaura and Buchwald-Hartwig cross-couplings was investigated.

Effect of electronic enrichment of NHCs on the catalytic activity of [Pd(NHC)(acac)Cl] in Buchwald-Hartwig coupling

A series of [Pd(NHC)(acac)Cl] (NHC = N-heterocyclic carbene) complexes have been synthesized and characterized to investigate the electronic and steric effects of NHC ligands in catalysis. Their reactivity in Buchwald-Hartwig coupling has been explored an