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Relevant academic research and scientific papers

Modular synthesis of functionalisable alkoxy-tethered N-heterocyclic carbene ligands and an active catalyst for buchwald-hartwig aminations

Modular syntheses of functionalised, alkoxy-tethered 1,3-bis(2,4,6- trimethylphenyl)imidazolium (IMes*H+) and 1,3-bis(2,6- diisopropylphen-l)imidazolium (IPr*H+) derivatives 1,3-bis(4-alkyl- -2,4,6-trimethylphenyl)imidazolium (IXyOR*H+) and 1,3-bis(4-alkyl-oxy-2,6-diisopropylphenyl)imidazolium (IPrOR*H +) are reported. A reliable synthesis of the key starting material 4-amino-3,5-diisopropylphenol is also described. Etherification of hydroxydecorated ligand intermediates before formation of the imidazolium core and subsequent modification, or direct etherification of the versatile synthon IPrOH·HCl, allowed access to various linker types including triethoxysilyl, primary amino and norbornenyl, which are not accessible by other methods. An IPrOR-palladium(II) complex was prepared, and its catalytic activity was evaluated in challenging Buchwald- Hartwig aminations of aryl chlorides. This precatalyst displayed excellent activity and selectivity under mild reaction conditions, achieving in some cases a 10-fold improvement in TOF relative to the IPr-based version. An unexpected activity profile was observed wherein sterically demanding anilines were coupled more easily than those lacking orthosubstitution.

Palladium-catalysed amination of aryl- and heteroaryl halides using tert-butyl tetraisopropylphosphorodiamidite as an easily accessible and air-stable ligand

The phosphorus compound tert-butyl tetraisopropylphosphorodiamidite, prepared from bis(diisopropylamino)chlorophosphine, is an excellent ligand for palladium-catalysed Buchwald-Hartwig amination of aryl- and heteroaryl chlorides and bromides. Based on its ready accessibility and air-stability, this amination protocol is a practical approach to the synthesis of industrially important aryl- and heteroarylamines. Copyright

(IPr)Pd(pydc) (pydc = pyridine-2,6-dicarboxylate) - A highly active precatalyst for the sterically hindered C-N coupling reactions

A new class of well-defined NHC-Pd complexes incorporating a pyridine-2-carboxylate or pyridine-2,6-dicarboxylate ligand has been synthesized. These novel complexes exhibited prominent catalytic activity in the sterically hindered C-N coupling reactions at elevated temperature, but relatively inferior reactivity at low temperature. The distinctly different reactivity of these NHC-Pd complexes was presumed to be associated with their unique structures of ancillary ligands.

Characterization, reactivity, and potential catalytic intermediacy of a cyclometalated tri-tert -butylphosphine palladium acetate complex

Palladium acetate and tri-tert-butylphosphine react at room temperature via C-H activation of a tert-butyl group to form the novel palladium(II) complex [(PtBu3)Pd(CH2C(CH3) 2PtBu2)(OAc). This cyclometalated complex can be reduced to Pd(PtBu3)2 by either heat or hydrogen, but is resistant to reduction by alkoxide bases and amines. As a result of the existence of this cyclometalated complex, the production of catalytically active palladium(0) species generated in situ from Pd(OAc) 2 and tri-tert-butylphosphine is diminished, as evident by the catalytic inactivity of this complex. Additionally, datively bound tri-tert-butylphosphine is easily displaced by amines and less basic phosphines to form a series of novel cyclometalated palladium(II) complexes.

Transition metal-free amination of aryl halides-A simple and reliable method for the efficient and high-yielding synthesis of N-arylated amines

A simple and reliable reaction protocol for the clean, fast, and high-yielding synthesis of various N-arylated amines derived from reactions of aryl halides with various (also sterically hindered) amines under transition metal-free reaction conditions is presented. Dioxane and KN(Si(CH3)3)2 were found to be the ideal solvent and base for this transformation. The conversion rates and yields observed are excellent and in the majority of the reactions performed significantly higher than that obtained in their catalyzed versions. Furthermore, the selective synthesis of 6-halopyridin-2-amines and asymmetric pyridine-2,6-diamines (derived from consecutive reactions of 2,6-dibromopyridine and 2,6-dichloropyridine, respectively, with different amines) is possible in almost quantitative yields (relative to 2,6-dihalopyridine) within very short reaction times. Purification of the 6-halopyridin-2-amine intermediates is not necessary, allowing the synthesis of pyridine-2,6-diamines in 'one-pot'. However, catalysts are in many cases not required to efficiently and selectively couple aryl halides with amines, making transition metal-free versions of the Buchwald-Hartwig reaction extremely attractive for the synthesis of N-arylated amines with substrates containing substituents on the aryl halide, which either promote regioselectivity and/or do not require regioselective aminations.

Development of a practical Buchwald-Hartwig amine arylation protocol using a conveniently prepared (NHC)Pd(R-allyl)Cl catalyst

Continuing efforts to establish a more general "user-friendly" protocol for the palladium-catalysed arylation of amines (Buchwald-Hartwig reaction) are described herein. Significant advances have been made through the use of the versatile (SIPr)Pd(methallyl)Cl complex in conjunction with the reliable base lithium hexamethyldisilazide (LHMDS). The Royal Society of Chemistry.

Modified (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes for room-temperature Suzuki-Miyaura and Buchwald-Hartwig reactions

A series of (NHC)Pd(R-allyl)Cl complexes [NHC: IPr = N,N′-bis(2,6- diisopropylphenyl)imidazol-2-ylidene, SIPr = N,N′-bis(2,6- diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene; R = H, Me, gem-Me2, Ph] have been synthesized and fully characterized. When compared to (NHC)Pd(allyl)Cl, substitution at the terminal position of the allyl scaffold favors a more facile activation step. This translates into higher catalytic activity in the Suzuki-Miyaura and Buchwald-Hartwig reactions, allowing for the coupling of unactivated aryl chlorides at room temperature in minutes. In the Suzuki-Miyaura reaction, aryl triflates, bromides, and chlorides react with boronic acids using very low catalyst loading. In the N-aryl amination reaction, a wide range of substrates has been coupled efficiently; primary-, secondary-, alkyl-, or aryl-amines react in high yields with unactivated, neutral, and activated aryl chlorides and bromides. In both reactions, extremely hindered substrates such as tri-ortho-substituted biaryls and tetra-ortho-substituted diarylamines can be produced without loss of activity. Finally, the present catalytic system has proven to be efficient with as low as 10 parts-per-million (ppm) of precatalyst in the Buchwald-Hartwig reaction and 50 ppm in the Suzuki-Miyaura reaction.

(IPr)Pd(acac)Cl: An easily synthesized, efficient, and versatile precatalyst for C-N and C-C bond formation

A very straightforward synthesis of (IPr)Pd(acac)Cl from two commercially available starting materials, Pd(acac)2 and IPr·HCl [acac = acetylacetonate; IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], has been developed. The resulting complex, (IPr)Pd(acac)Cl (1), has proven to be a highly active PdII precatalyst in the Buchwald-Hartwig and the α-ketone arylation reactions. A wide range of substrates has been screened, including unactivated, sterically hindered, and heterocyclic aryl chlorides.

NUCLEOPHILIC HETEROCYCLIC CARBENE DERIVATIVES OF PD(ACAC)2 FOR CROSS-COUPLING REACTIONS

Embodiments in accordance with the present invention provide for a palladium complex characterized by the general Formula (I): where A is a bidentate monoanionic ligand, NHC is a nucleophilic heterocyclic carbene, and Z is an anionic ligand. Such palladium complexes are useful in initiating cross-coupling reactions.

Catalyst for aromatic C—O, C—N, and C—C bond formation

The present invention is directed to a transition metal catalyst, comprising a Group 8 metal and a ligand having the structure wherein R, R′ and R″ are organic groups having 1-15 carbon atoms, n=1-5, and m=0-4. The present invention is also directed to a method of forming a compound having an aromatic or vinylic carbon-oxygen, carbon-nitrogen, or carbon-carbon bond using the above catalyst. The catalyst and the method of using the catalyst are advantageous in preparation of compounds under mild conditions of approximately room temperature and pressure.