Analysis of the palladium-catalyzed (aromatic)C-H bond metalation-deprotonation mechanism spanning the entire spectrum of arenes.

A comprehensive understanding of the C-H bond cleavage step by the concerted metalation-deprotonation (CMD) pathway is important in further development of cross-coupling reactions using different catalysts. Distortion-interaction analysis of the C-H bond cleavage over a wide range of (hetero)aromatics has been performed in an attempt to quantify the various contributions to the CMD transition state (TS). The (hetero)aromatics evaluated were divided in different categories to allow an easier understanding of their reactivity and to quantify activation characteristics of different arene substituents. The CMD pathway to the C-H bond cleavage for different classes of arenes is also presented, including the formation of pre-CMD intermediates and the analysis of bonding interactions in TS structures. The effects of remote C2 substituents on the reactivity of thiophenes were evaluated computationally and were corroborated experimentally with competition studies. We show that nucleophilicity of thiophenes, evaluated by Hammett σ(p) parameters, correlates with each of the distortion-interaction parameters. In the final part of this manuscript, we set the initial equations that can assist in the development of predictive guidelines for the functionalization of C-H bonds catalyzed by transition metal catalysts.

Rhodium(III)-catalyzed heterocycle synthesis using an internal oxidant: improved reactivity and mechanistic studies.

Directing groups that can act as internal oxidants have recently been shown to be beneficial in metal-catalyzed heterocycle syntheses that undergo C-H functionalization. Pursuant to the rhodium(III)-catalyzed redox-neutral isoquinolone synthesis that we recently reported, we present in this article the development of a more reactive internal oxidant/directing group that can promote the formation of a wide variety of isoquinolones at room temperature while employing low catalyst loadings (0.5 mol %). In contrast to previously reported oxidative rhodium(III)-catalyzed heterocycle syntheses, the new conditions allow for the first time the use of terminal alkynes. Also, it is shown that the use of alkenes, including ethylene, instead of alkynes leads to the room temperature formation of 3,4-dihydroisoquinolones. Mechanistic investigations of this new system point to a change in the turnover limiting step of the catalytic cycle relative to the previously reported conditions. Concerted metalation-deprotonation (CMD) is now proposed to be the turnover limiting step. In addition, DFT calculations conducted on this system agree with a stepwise C-N bond reductive elimination/N-O bond oxidative addition mechanism to afford the desired heterocycle. Concepts highlighted by the calculations were found to be consistent with experimental results.

Predictable and site-selective functionalization of poly(hetero)arene compounds by palladium catalysis.

The challenge of achieving selective and predictable functionalizations at C-H bonds with complex poly(hetero)aromatic substrates was addressed by two different approaches. Site-selectivity can be obtained by applying various reaction conditions that are (hetero)arene specific to substrates that contain indoles, pyridine N-oxide, and polyfluorinated benzenes. An experimental classification of electron-rich heteroarenes based on their reactivity toward palladium-catalyzed C-H functionalization was established, the result of which correlated well with the order of reactivity predicted by the DFT-calculated concerted metalation-deprotonation (CMD) pathway. Model substrates containing two reactive heteroarenes were then reacted under general reaction conditions to demonstrate the applicability this reactivity chart in predicting the regioselectivity of the palladium-catalyzed direct arylation and benzylation reactions.

Rhodium(III)-catalyzed arene and alkene C-H bond functionalization leading to indoles and pyrroles.

Recently, the rhodium(III)-complex [Cp*RhCl(2)](2) 1 has provided exciting opportunities for the efficient synthesis of aromatic heterocycles based on a rhodium-catalyzed C-H bond functionalization event. In the present report, the use of complexes 1 and its dicationic analogue [Cp*Rh(MeCN)(3)][SbF(6)](2) 2 have been employed in the formation of indoles via the oxidative annulation of acetanilides with internal alkynes. The optimized reaction conditions allow for molecular oxygen to be used as the terminal oxidant in this process, and the reaction may be carried out under mild temperatures (60 °C). These conditions have resulted in an expanded compatibility of the reaction to include a range of new internal alkynes bearing synthetically useful functional groups in moderate to excellent yields. The applicability of the method is exemplified in an efficient synthesis of paullone 3, a tetracyclic indole derivative with established biological activity. A mechanistic investigation of the reaction, employing deuterium labeling experiments and kinetic analysis, has provided insight into issues of reactivity for both coupling partners as well as aided in the development of conditions for improved regioselectivity with respect to meta-substituted acetanilides. This reaction class has also been extended to include the synthesis of pyrroles. Catalyst 2 efficiently couples substituted enamides with internal alkynes at room temperature to form trisubstituted pyrroles in good to excellent yields. The high functional group compatibility of this reaction enables the elaboration of the pyrrole products into a variety of differentially substituted pyrroles.

Mechanistic analysis of azine N-oxide direct arylation: evidence for a critical role of acetate in the Pd(OAc)2 precatalyst.

Detailed mechanistic studies on the palladium-catalyzed direct arylation of pyridine N-oxides are presented. The order of each reaction component is determined to provide a general mechanistic picture. The C-H bond cleaving step is examined in further detail through computational studies, and the calculated results are in support of an inner-sphere concerted metalation-deprotonation (CMD) pathway. Competition experiments were conducted with N-oxides of varying electronic characters, and results revealed an enhancement of rate when using a more electron-deficient species, which is in support of a CMD transition state. The effect of base on reaction rate was also examined and it was found that a carboxylate base was required for the reaction to proceed. This led to the conclusion that Pd(OAc)(2) plays a pivotal role in the reaction mechanism as more than merely a precatalyst, but also as a source of acetate base required for the C-H bond cleavage step.

Investigation of the mechanism of C(sp3)-H bond cleavage in Pd(0)-catalyzed intramolecular alkane arylation adjacent to amides and sulfonamides.

The reactivity of C(sp(3))-H bonds adjacent to a nitrogen atom can be tuned to allow intramolecular alkane arylation under Pd(0) catalysis. Diminishing the Lewis basicity of the nitrogen lone pair is crucial for this catalytic activity. A range of N-methylamides and sulfonamides react exclusively at primary C(sp(3))-H bonds to afford the products of alkane arylation in good yields. The isolation of a Pd(II) reaction intermediate has enabled an evaluation of the reaction mechanism with a focus on the role of the bases in the C(sp(3))-H bond cleaving step. The results of these stoichiometric studies, together with kinetic isotope effect experiments, provide rare experimental support for a concerted metalation-deprotonation (CMD) transition state, which has previously been proposed in alkane C(sp(3))-H arylation. Moreover, DFT calculations have uncovered the additional role of the pivalate additive as a promoter of phosphine dissociation from the Pd(II) intermediate, enabling the CMD transition state. Finally, kinetic studies were performed, revealing the reaction rate expression and its relationship with the concentration of pivalate.

Intramolecular palladium-catalyzed alkane C-H arylation from aryl chlorides.

The first examples of efficient and general palladium-catalyzed intramolecular C(sp(3))-H arylation of (hetero)aryl chlorides, giving rise to a variety of valuable cyclobutarenes, indanes, indolines, dihydrobenzofurans, and indanones, are described. The use of aryl and heteroaryl chlorides significantly improves the scope of C(sp(3))-H arylation by facilitating the preparation of reaction substrates. Careful optimization studies have shown that the palladium ligand and the base/solvent combination are crucial to obtaining the desired class of product in high yields. Overall, three sets of reaction conditions employing P(t)Bu(3), PCyp(3), or PCy(3) as the palladium ligand and K(2)CO(3)/DMF or Cs(2)CO(3)/pivalic acid/mesitylene as the base/solvent combination allowed five different classes of products to be accessed using this methodology. In total, more than 40 examples of C-H arylation have been performed successfully. When several types of C(sp(3))-H bond were present in the substrate, the arylation was found to occur regioselectively at primary C-H bonds vs secondary or tertiary positions. In addition, in the presence of several primary C-H bonds, selectivity trends correlate with the size of the palladacyclic intermediate, with five-membered rings being favored over six- and seven-membered rings. Regio- and diastereoselectivity issues were studied computationally in the prototypal case of indane formation. DFT(B3PW91) calculations demonstrated that C-H activation is the rate-determining step and that the creation of a C-H agostic interaction, increasing the acidity of a geminal C-H bond, is a critical factor for the regiochemistry control.

Rhodium(III)-catalyzed intermolecular hydroarylation of alkynes.

The rhodium(III)-catalyzed hydroarylation of internal alkynes is described. Good yields are obtained for a variety of alkynes, and excellent regioselectivity is observed for unsymmetrically substituted alkynes. The reaction also gives good yields for a range of arenes. Preliminary mechanistic investigations suggest that this reaction proceeds through arene metalation with the cationic rhodium catalyst, which enables challenging intermolecular reactivity.

Rhodium(III)-catalyzed isoquinolone synthesis: the N-O bond as a handle for C-N bond formation and catalyst turnover.

An external-oxidant-free process to access the isoquinolone motif via cross-coupling/cyclization of benzhydroxamic acid with alkynes is described. The reaction features a regioselective cleavage of a C-H bond on the benzhydroxamic acid coupling partner as well as a regioselective alkyne insertion. Mechanistic studies point out the important involvement of a N-O bond as a tool for C-N bond formation and catalyst turnover.

Room-temperature direct arylation of polyfluorinated arenes under biphasic conditions.

New biphasic conditions for the palladium-catalyzed direct arylation of electron-poor fluorinated arenes have been developed. Taking advantage of biphasic chemistry, the use of an immiscible mixture of water and an organic solvent allows complete solubilization of all components of the system, enabling the reaction to proceed at room temperature in yields up to 99%.

Modulating reactivity and diverting selectivity in palladium-catalyzed heteroaromatic direct arylation through the use of a chloride activating/blocking group.

Through the introduction of an aryl chloride substituent, the selectivity of palladium-catalyzed direct arylation may be diverted to provide alternative regioisomeric products in high yields. In cases where low reactivity is typically observed, the presence of the carbon-chlorine bond can serve to enhance reactivity and provide superior outcomes. From a strategic perspective, the C-Cl bond is easily introduced and can be employed in a variety of subsequent transformations to provide a wealth of highly functionalized heterocycles with minimal substrate preactivation. The impact of the C-Cl functional group on direct arylation reactivity has also been evaluated mechanistically, and the observed reactivity profiles correlate very well with that predicted by a concerted metalation-deprotonation pathway.

Mechanistic considerations in the development and use of azine, diazine and azole N-oxides in palladium-catalyzed direct arylation.

Azines, diazines or thiazole N-oxides are highly reactive substrates in palladium-catalyzed direct arylation reaction. For these reactions, the results are inconsistent with an SEAr reaction pathway and may best fit with a concerted metalation-deprotonation-like (CMD) mechanism.

Domino palladium-catalyzed Heck-intermolecular direct arylation reactions.

A domino palladium-catalyzed Heck-intermolecular direct arylation reaction has been developed, giving access to a variety of dihydrobenzofurans, indolines, and oxindoles. A variety of sulfur-containing heterocycles such as thiazoles, thiophenes, and benzothiophene can be employed as the direct arylation coupling partner in yields up to 99%.

Palladium-catalyzed benzylation of heterocyclic aromatic compounds.

Broadly applicable palladium-catalyzed heteroarene benzylation reactions are described with a focus on the most challenging heterocyclic classes under traditional benzylation techniques such as sulfur-containing heterocycles and those bearing functional groups that would be incompatible with reactions requiring Lewis acids and/or strong bases.

Isoquinoline synthesis via rhodium-catalyzed oxidative cross-coupling/cyclization of aryl aldimines and alkynes.

A general rhodium-catalyzed oxidative coupling reaction between internal alkynes and aryl aldimines is described. This process affords 3,4-disubstituted isoquinolines in good yield and high regioselectivity. Preliminary mechanistic studies suggest that the C-N bond formation arises from the reductive elimination of a rhodium(III) species.

Establishment of broadly applicable reaction conditions for the palladium-catalyzed direct arylation of heteroatom-containing aromatic compounds.

Conditions for the palladium-catalyzed direct arylation of a wide range of heterocycles with aryl bromides are reported. Those conditions employ a stoichiometric ratio of both coupling partners, as well as a substoichiometric quantity of pivalic acid, which results in significantly faster reactions. An evaluation of the influence of the nature of the aryl halide has also been carried out.

Palladium-catalyzed direct arylation of azine and azole N-oxides: reaction development, scope and applications in synthesis.

Palladium-catalyzed direct arylation reactions are described with a broad range of azine and azole N-oxides. In addition to aspects of functional group compatibility, issues of regioselectivity have been explored when nonsymmetrical azine N-oxides are used. In these cases, both the choice of ligand and the nature of the azine substituents play important roles in determining the regioisomeric distribution. When azole N-oxides are employed, preferential reaction is observed for arylation at C2 which occurs under very mild conditions. Subsequent reactions are observed to occur at C5 followed by arylation at C4. The potential utility of this methodology is illustrated by its use in the synthesis of a potent sodium channel inhibitor 1 and a Tie2 Tyrosine Kinase inhibitor 2.

Site-selective azaindole arylation at the azine and azole rings via N-oxide activation.

Subjection of N-methyl 6- and 7-azaindole N-oxides to a Pd(OAc)2/DavePhos catalyst system enables regioselective direct arylation of the azine ring. Following deoxygenation, 7-azaindole substrates undergo an additional regioselective azole direct arylation event in good yield.