Palladium-Catalyzed Tandem Ester Dance/Decarbonylative Coupling Reactions

Catalyst-controlled directing group translocation in the site selective C-H functionalization of 3-carboxamide indoles and metallocarbenes

Complementary methods toward the selective functionalization of indole and oxindole frameworks employing an alternative strategy in heteroaryl C-H functionalizations are presented herein. This work focuses on a catalyst-controlled, site selective C-H activation/functionalization of 3-acyl indoles, wherein an amide serves as a robust and versatile directing group capable of undergoing concomitant 1,2-acyl translocation/C-H functionalization in the presence of a RhI/AgI co-catalysts to provide the cross-coupled adducts in high yields. In contrast, the use of IrIII/AgI catalysts subverted the 1,2-acyl migration to afford the corresponding C2-functionalized products in good to excellent yields. A notable feature of the catalyst systems was the exceptional level of site selectivity observed in which the corresponding C-H functionalized indoles were obtained exclusively. Mechanistic experiments indicate a concerted 1,2-acyl migration step and indole metallation occurring through an electrophilic aromatic substitution process.

Direct C4 and C2 C-H Amination of Heteroarenes Using I(III) Reagents via a Cross Azine Coupling

Aminated nitrogen heterocycles are valuable motifs across numerous chemical industries, perhaps most notably in small molecule drug discovery. While numerous strategies for installing nitrogen atoms onto azaarenes exist, most require prefunctionalization and methods for direct C-H amination are almost entirely limited to position C2. Herein, we report a method for the direct C2 and C4 C-H amination of fused azaarenes via in situ activation with a bispyridine-ligated I(III) reagent, [(Py)2IPh]2OTf, or Py-HVI. Unlike commonly used N-oxide chemistry, the method requires no preoxidation of the azaarene and provides unprecedented direct access to C4 amination products. The resulting N-heterocyclic pyridinium salts can be isolated via simple trituration. The free amine can be liberated under mild Zincke aminolysis, or the amination and cleavage can be telescoped to a one-pot process. The scope of the method is broad; the conditions are mild and operationally simple, and the aminated products are produced in good to excellent yields. Computational studies provide insights into the mechanism of activation, which involves an unusual direct nucleophilic functionalization of an I(III) ligand, as well as a kinetic basis for the observed C2 and C4 amination products.

Divergent Transformations of Aromatic Esters: Decarbonylative Coupling, Ester Dance, Aryl Exchange, and Deoxygenative Coupling

ConspectusAromatic esters are cost-effective, versatile, and commonly used scaffolds that are readily synthesized or encountered as synthetic intermediates. While most conventional reactions involving these esters are nucleophilic acyl substitutions or 1,2-nucleophilic additions─where a nucleophile attacks the carbonyl group, decarbonylative transformations offer an alternative pathway by using the carbonyl group as a leaving group. This transition-metal-catalyzed process typically begins with oxidative addition of the C(acyl)-O bond to the metal. Subsequently, the reaction involves the migration of CO to the metal center, the reaction with a nucleophile, and reductive elimination to yield the final product. Pioneering work by Yamamoto on nickel complexes and the development of decarbonylative reactions (such as Mizoroki-Heck-type olefination) using aromatic carboxylic anhydrides catalyzed by palladium were conducted by de Vries and Stephan. Furthermore, reports have surfaced of decarbonylative hydrogenation of pyridyl methyl esters by Murai using ruthenium catalysts as well as Mizoroki-Heck-type reactions of nitro phenyl esters by Gooßen under palladium catalysis. Our group has been at the forefront of developing decarbonylative C-H arylations of phenyl esters with 1,3-azoles and aryl boronic acids using nickel catalysts. The key to this reaction is the use of phenyl esters, which are easy to synthesize, stabilize, and handle, allowing oxidative addition of the C(acyl)-O bond; nickel, which facilitates oxidative addition of the C(acyl)-O bond; and suitable bidentate phosphine ligands that can stabilize the intermediate. By modification of the nucleophiles, esters have been effectively utilized as electrophiles in cross-coupling reactions, encouraging the development of these nucleophiles among researchers. This Account summarizes our advancements in nucleophile development for decarbonylative coupling reactions, particularly highlighting the utilization of aromatic esters in diverse reactions such as alkenylation, intramolecular etherification, α-arylation of ketones, C-H arylation, methylation, and intramolecular C-H arylation for dibenzofuran synthesis, along with cyanation and reductive coupling. We also delve into reaction types that are distinct from typical decarbonylative reactions, including ester dance reactions, aromatic ring exchanges, and deoxygenative transformations, by focusing on the oxidative addition of the C(acyl)-O bond of the aromatic esters to the metal complex. For example, the ester dance reaction is hypothesized to undergo 1,2-translocation starting with oxidative addition to a palladium complex, leading to a sequence of ortho-deprotonation/decarbonylation, followed by protonation, carbonylation, and reductive elimination. The aromatic exchange reaction likely involves oxidative addition of complexes of different aryl electrophiles with a nickel complex. In deoxygenative coupling, an oxidative addition complex with palladium engages with a nucleophile, forming an acyl intermediate that undergoes reductive elimination in the presence of an appropriate reducing agent. These methodologies are poised to captivate the interest of synthetic chemists by offering unconventional and emerging approaches for transforming aromatic esters. Moreover, we demonstrated the potential to transform readily available basic chemicals into new compounds through organic synthesis.

Mechanistic Approach Toward the C4-Selective Amination of Pyridines via Nucleophilic Substitution of Hydrogen

The development of site-selective functionalization of N-heteroarenes is highly desirable in streamlined synthesis. In this context, direct amination of pyridines stands as an important synthetic methodology, with particular emphasis on accessing 4-aminopyridines, a versatile pharmacophore in medicinal chemistry. Herein, we report a reaction manifold for the C4-selective amination of pyridines by employing nucleophilic substitution of hydrogen (SNH). Through 4-pyridyl pyridinium salt intermediates, 4-aminopyridine products are obtained in reaction with aqueous ammonia without intermediate isolation. The notable regioselectivity was achieved by the electronic tuning of the external pyridine reagents along with the maximization of polarizability in the proton elimination stage. Further mechanistic investigations provided a guiding principle for the selective C-H pyridination of additional N-heteroarenes, presenting a strategic avenue for installation of diverse functional groups.

Ring Walking Mediated by Ni-Ni Species as a Vehicle for Enabling Distal C(sp2)-H Functionalization of Aryl Pivalates

Herein, we report the utilization of Ni-Ni species as a manifold for enabling a "ring-walking" event by dynamic translocation of the metal center over the arene backbone. Experimental and computational studies support a translocation occurring via a 1,2-hydride shift. The synthetic applicability of the method is illustrated in a series of C-C bond formations that occur at distal C(sp2)-H sites of simple aryl pivalates.

Aryl Dance Reaction of Arylbenzoheteroles

We report a 1,2-migration (aryl dance reaction) of the aryl group on heteroles. AlCl3 can efficiently convert C3-arylheteroles to C2-arylheteroles. Depending on the electron density of the substrate, conversion from C2- to C3-arylheteroles was also possible with catalytic Zn(OTf)2. A one-pot aryl dance/acylation or bromination and arylation/aryl dance cascade was also demonstrated.