Alexey G. Sergeev

Selective, Nickel-Catalyzed Hydrogenolysis of Aryl Ethers

A catalyst that cleaves aryl-oxygen bonds but not carbon-carbon bonds may help improve lignin processing. Selective hydrogenolysis of the aromatic carbon-oxygen (C-O) bonds in aryl ethers is an unsolved synthetic problem important for the generation of fuels and chemical feedstocks from biomass and for the liquefaction of coal. Currently, the hydrogenolysis of aromatic C-O bonds requires heterogeneous catalysts that operate at high temperature and pressure and lead to a mixture of products from competing hydrogenolysis of aliphatic C-O bonds and hydrogenation of the arene. Here, we report hydrogenolyses of aromatic C-O bonds in alkyl aryl and diaryl ethers that form exclusively arenes and alcohols. This process is catalyzed by a soluble nickel carbene complex under just 1 bar of hydrogen at temperatures of 80 to 120°C; the relative reactivity of ether substrates scale as Ar-OAr>>Ar-OMe>ArCH2-OMe (Ar, Aryl; Me, Methyl). Hydrogenolysis of lignin model compounds highlights the potential of this approach for the conversion of refractory aryl ether biopolymers to hydrocarbons.

A Heterogeneous Nickel Catalyst for the Hydrogenolysis of Aryl Ethers without Arene Hydrogenation

A heterogeneous nickel catalyst for the selective hydrogenolysis of aryl ethers to arenes and alcohols generated without an added dative ligand is described. The catalyst is formed in situ from the well-defined soluble nickel precursor Ni(COD)(2) or Ni(CH(2)TMS)(2)(TMEDA) in the presence of a base additive, such as (t)BuONa. The catalyst selectively cleaves C(Ar)-O bonds in aryl ether models of lignin without hydrogenation of aromatic rings, and it operates at loadings down to 0.25 mol % at 1 bar of H(2) pressure. The selectivity of this catalyst for electronically varied aryl ethers differs from that of the homogeneous catalyst reported previously, implying that the two catalysts are distinct from each other.

A General and Efficient Catalyst for Palladium-Catalyzed C−O Coupling Reactions of Aryl Halides with Primary Alcohols

An efficient procedure for palladium-catalyzed coupling reactions of (hetero)aryl bromides and chlorides with primary aliphatic alcohols has been developed. Key to the success is the synthesis and exploitation of the novel bulky di-1-adamantyl-substituted bipyrazolylphosphine ligand L6. Reaction of aryl halides including activated, nonactivated, and (hetero)aryl bromides as well as aryl chlorides with primary alcohols gave the corresponding alkyl aryl ethers in high yield. Noteworthy, functionalizations of primary alcohols in the presence of secondary and tertiary alcohols proceed with excellent regioselectivity.

Palladium-Catalyzed Formylation of Aryl Bromides: Elucidation of the Catalytic Cycle of an Industrially Applied Coupling Reaction

The first comprehensive study of the catalytic cycle of the palladium-catalyzed formylation of aryl bromides with synthesis gas (CO/H2, 1:1) is presented. The formylation in the presence of efficient (Pd/PR2(n)Bu, R = 1-Ad, (t)Bu) and nonefficient (Pd/P(t)Bu3) catalysts was investigated. The main organometallic complexes involved in the catalytic cycle were synthesized and characterized, and their solution chemistry was studied in detail. Comparison of stoichiometric and catalytic reactions using P(1-Ad)2(n)Bu, the most efficient ligand known for the formylation of aryl halides, led to two pivotal results: (1) The corresponding carbonylpalladium(0) complex [Pd(n)(CO)(m)L(n)] and the respective hydrobromide complex [Pd(Br)(H)L2] are resting states of the active catalyst, and they are not directly involved in the catalytic cycle. These complexes maintain the concentration of most active [PdL] species at a low level throughout the reaction, making oxidative addition the rate-determining step, and provide high catalyst longevity. (2) The product-forming step proceeds via base-mediated hydrogenolysis of the corresponding acyl complex, e.g., [Pd(Br)(p-CF3C6H4CO){P(1-Ad)2(n)Bu}]2 (8), under mild conditions (25-50 degrees C, 5 bar). Stoichiometric studies using the less efficient Pd/P(t)Bu3 catalyst resulted in the isolation and characterization of the first stable three-coordinated neutral acylpalladium complex, [Pd(Br)(p-CF3C6H4CO)(P(t)Bu3)] (10). Hydrogenolysis of 10 needed significantly more drastic conditions compared to that of dimeric 8. In the presence of amine base, complex 10 gave a catalytically inactive diamino acyl complex, which explains the low activity of the Pd/P(t)Bu3 catalyst formylation of aryl bromides.

Linear-Selective Hydroarylation of Unactivated Terminal and Internal Olefins with Trifluoromethyl-Substituted Arenes

We report a series of hydroarylations of unactivated olefins with trifluoromethyl-substituted arenes that occur with high selectivity for the linear product without directing groups on the arene. We also show that hydroarylations occur with internal, acyclic olefins to yield linear alkylarene products. Experimental mechanistic data provide evidence for reversible formation of an alkylnickel-aryl intermediate and rate-determining reductive elimination to form the carbon-carbon bond. Labeling studies show that formation of terminal alkylarenes from internal alkenes occurs by initial establishment of an equilibrating mixture of alkene isomers, followed by addition of the arene to the terminal alkene. Computational (DFT) studies imply that the aryl C-H bond transfers to a coordinated alkene without oxidative addition and support the conclusion from experiment that reductive elimination is rate-determining and forms the anti-Markovnikov product. The reactions are inverse order in α-olefin; thus the catalytic reaction occurs, in part, because isomerization creates a low concentration of the reactant α-olefin.

Palladium-catalyzed reaction of aryl halides with ureas

A new method for the palladium-catalyzed arylation of ureas is described. The coupling reaction of urea and phenylurea with aryl halides containing electron-withdrawing groups in the p-position in dioxane in the presence of 0.5–1.0 mol% of Pd2dba3·CHCl3, Xantphos and Cs2CO3 as a base gives N,N′-diarylureas in yields of 64–92%.

Variation of xanthene-based bidentate ligands in the palladium-catalyzed arylation of ureas

A series of xanthene-based bidentate ligands containing various substituents on diphenylphosphino groups were synthesized and tested in the palladium-catalyzed arylation reaction of urea with unactivated aryl bromides. It was found that both steric and electronic properties of the ligands have a pronounced effect on the yields and ratios of the products. Arylation of urea and phenylurea with unactivated aryl bromides in the presence of Pd2dba3·CHCl3/3,5-(CF3)2Xantphos and Cs2CO3 as base in dioxane at 100°C gave the corresponding N,N′-diarylureas in 62–98% yields.

Palladium-catalyzed Arylation of Ureas

Urea arylation with aryl halides in the presence of catalyst precursor Pd2dba3-CHCl3/Xantphos and Cs2CO3 in dioxane at 100°C affords symmetrical N,N-diarylureas in 64-92% yield. With the same catalytic system the reaction between N-phenylurea and aryl bromides containing electron-withdrawing substituents in the para-position provided N-aryl-N-phenylureas in 64-91% yields.

Variation of xantphos-based ligands in the palladium-catalyzed reaction of aryl halides with ureas

A series of Xantphos-based ligands containing various substituents in the diphenylphosphino groups were synthesized, and their effect on the product yield and ratio in the palladium-catalyzed arylation of ureas with nonactivated aryl halides was studied. The arylation of urea and phenylurea in the presence of Pd2(dba)3-CHCl3, 3,5-(CF3)2Xantphos, and Cs2CO3 in dioxane at 100°C gave the corresponding N,N-diarylureas in 62-98% yield.

Urea as ammonia equivalent in aryl halides amination catalyzed by palladium complexes

Urea reaction with nonactivated aryl bromides and chlorides under catalysis with palladium complexes led to the formation in 65–95% yield of triarylamines from para-and meta-substituted aryl halides and of diarylamines from ortho-isomers.