Palladium‐Catalyzed Cross‐Dehydrogenative Coupling of <i>o</i>‐Xylene: Evidence of a New Rate‐Limiting Step in the Search for Industrially Relevant Conditions

Álvarez‐Casao, Yolanda; Slagmaat, Christian A. M. R. van; Verzijl, Gerard K. M.; Lefort, Laurent; Alsters, Paul L.; Fernández-Ibáñez, M. Á́ngeles

doi:10.1002/cctc.201701973

Cited by 13 publications

(8 citation statements)

References 39 publications

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“…It was shown by Stahl and Carrow that the Pd‐complexes can activate o ‐xylene and thiophene in the presence of pyridine and thioether ligands respectively, and subsequently exchange the activated (hetero)aryl ligands in a transmetalation step. Which step is rate‐limiting in the overall mechanistic pathway strongly depends on the applied reaction conditions . Yet, the catalytic cycle involves a single mechanism for both C−H activation steps, which has not provided an explanation for the remarkable shift in selectivity observed in the Pd(carboxylate) 2 ‐catalyzed C−H/C−H cross‐coupling of benzene and heteroaromatics.…”

Section: Methodsmentioning

confidence: 99%

“…Which step is rate-limiting in the overall mechanistic pathway strongly depends on the applied reaction conditions. [13] Yet, the catalytic cycle involves a single mechanism for both CÀ H activation steps, which has not provided an explanation for the remarkable shift in selectivity observed in the Pd(carboxylate) 2 -catalyzed CÀ H/CÀ H cross-coupling of benzene and heteroaromatics.…”

Section: The Dual Effect Of the Acetate Ligand On The Mechanism Of Thmentioning

confidence: 99%

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The Dual Effect of the Acetate Ligand on the Mechanism of the Pd‐Catalyzed C−H/C−H Coupling of Benzene

2019

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Pd(II)carboxylates have emerged as potent catalysts for the direct coupling of (hetero)aromatics, enabling a two‐fold C−H activation with various oxidants. Yet, prior research on the mechanism of the C−H/C−H coupling without directing group(s) has led to contrasting insights. In this work, a direct effect of acetic acid (AcOH) concentration on the mechanism of Pd‐catalyzed benzene coupling to biphenyl is uncovered. The catalytic cycle was investigated via kinetic isotope effects, H/D exchange experiments and by assessing the reactivity of the Pd(aryl)‐intermediates. The study revealed that the catalytic reaction makes a transition between two dissimilar C−H activation steps: a “concerted metalation‐deprotonation” (CMD) mechanism followed by an electrophilic substitution (SEAr). A dual role of the acetate ligand explains the transition between these mechanisms, mediating the deprotonation via CMD and coordinating on the Pd(II) ion to preclude a SEAr mechanism for the first C−H activation.

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Section: Methodsmentioning

confidence: 99%

Section: The Dual Effect Of the Acetate Ligand On The Mechanism Of Thmentioning

confidence: 99%

The Dual Effect of the Acetate Ligand on the Mechanism of the Pd‐Catalyzed C−H/C−H Coupling of Benzene

2019

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“…Using a cheap and air-stable manganese carbonyl catalyst, MnBr(CO) 5 , allylic carbonates and ethers can be synthesized from alkynes bearing these functionalities through C − H Another class of cross-dehydrogenative coupling (CDC) reaction in which two unfunctionalized aryl rings are coupled via C − H activation under aerobic oxidation was extensively investigated by Stahl and co-workers [160][161][162]. The coupling of o-xylene, specifically, holds promise in the preparation of monomers used in the production of the polyimide resin Upilex [163] and metalorganic frameworks [164].…”

Section: − H Alkenylationmentioning

confidence: 99%

Pushing the boundaries of C–H bond functionalization chemistry using flow technology

2020

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C-H functionalization chemistry is one of the most vibrant research areas within synthetic organic chemistry. While most researchers focus on the development of small-scale batch-type transformations, more recently such transformations have been carried out in flow reactors to explore new chemical space, to boost reactivity or to enable scalability of this important reaction class. Herein, an up-to-date overview of C-H bond functionalization reactions carried out in continuous-flow microreactors is presented. A comprehensive overview of reactions which establish the formal conversion of a C-H bond into carbon-carbon or carbonheteroatom bonds is provided; this includes metal-assisted C-H bond cleavages, hydrogen atom transfer reactions and C-H bond functionalizations which involve an S E-type process to aromatic or olefinic systems. Particular focus is devoted to showcase the advantages of flow processing to enhance C-H bond functionalization chemistry. Consequently, it is our hope that this review will serve as a guide to inspire researchers to push the boundaries of C-H functionalization chemistry using flow technology.

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“…Finally, we verified via similar initial rate measurements whether the observed phenomena in the homogeneous conditions are also applicable to the Pd-zeolites (Supplementary Figure 36). Pd was pre-loaded onto zeolite beta to avoid a rate-limiting dissociation of the Pd3(OAc)6 trimer in the Pd(OAc)2 precursor as was proposed by Fernandez-Ibanez et al 57 . Similar trends were observed.…”

Section: Kinetic Studymentioning

confidence: 99%

Shape-selective C–H activation of aromatics to biarylic compounds using molecular palladium in zeolites

Vercammen¹,

Bocus

Neale

et al. 2020

Nat Catal

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The selective activation of inert C-H bonds has emerged as a promising tool for avoiding wasteful traditional coupling reactions. Oxidative coupling of simple aromatics allows for a cost-effective synthesis of biaryls. The utilization of this technology is however severely hampered by a poor regioselectivity and by the limited stability of state of the art homogeneous Pd catalysts. Here we show that confinement of cationic Pd in the pores of a zeolite allows for a shape-selective C-H activation of simple aromatics without functional handle or electronic bias. For instance, out of 6 possible isomers, 4,4'-bitolyl is produced with high shape-selectivity (80 %) in oxidative toluene coupling on Pd-Beta. Not only is a robust, heterogeneous catalytic system obtained; a concept is also set to control the selectivity in transition metal catalyzed arene C-H activation by spatial confinement in zeolite pores. MainBiarylic bonds are important structural motifs in numerous organic chemicals. Their current industrial production relies on traditional coupling reactions (Suzuki, Ullmann etc.), which use pre-activated arenes (aryl halides, arylboronic acids etc.) 1 . The high cost of these intermediates and the associated waste generation usually direct their application towards high-end, specialty chemicals. Especially in the polymer industry there is a growing demand for biarylic monomers because of their superior chemical and physical stability, and their favorable health and safety profiles 2,3,4 . Direct functionalization of unreactive C-H bonds via transition metal mediated C-H activation has emerged as a promising alternative to traditional multi-step approaches 1 . C-H/C-H arene-arene couplings can in principle be used for the direct synthesis of biaryls from simple arenes, with e.g. Pd carboxylates as the catalysts 6,7,8,9,10 . If O2 is used as the oxidant, water is the only by-product 11 . Unfortunately, the multitude of C-H bonds present in organic reactants, and the poor differentiation between them often result in poor regioselectivity. Consequently, reactions with simple arenes (e.g. toluene) lead to useless mixtures of many isomers 12 . Ortho-selectivity can be achieved if the existing substituent exerts a directing effect, as is typical for ketone groups, carbamates, amides

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Palladium‐Catalyzed Cross‐Dehydrogenative Coupling of o‐Xylene: Evidence of a New Rate‐Limiting Step in the Search for Industrially Relevant Conditions

Cited by 13 publications

References 39 publications

The Dual Effect of the Acetate Ligand on the Mechanism of the Pd‐Catalyzed C−H/C−H Coupling of Benzene

The Dual Effect of the Acetate Ligand on the Mechanism of the Pd‐Catalyzed C−H/C−H Coupling of Benzene

Pushing the boundaries of C–H bond functionalization chemistry using flow technology

Shape-selective C–H activation of aromatics to biarylic compounds using molecular palladium in zeolites

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