Rui Shang

Palladium-Catalyzed Decarboxylative Couplings of 2-(2-Azaaryl)acetates with Aryl Halides and Triflates

Pd-catalyzed decarboxylative cross-couplings of 2-(2-azaaryl)acetates with aryl halides and triflates have been discovered. This reaction is potentially useful for the synthesis of some functionalized pyridines, quinolines, pyrazines, benzoxazoles, and benzothiazoles. Theoretical analysis shows that the nitrogen atom at the 2-position of the heteroaromatics directly coordinates to Pd(II) in the decarboxylation transition state.

Synthesis of aromatic esters via Pd-catalyzed decarboxylative coupling of potassium oxalate monoesters with aryl bromides and chlorides.

Pd-catalyzed decarboxylative cross-coupling of aryl iodides, bromides, and chlorides with potassium oxalate monoesters has been discovered. This reaction is potentially useful for laboratory-scale synthesis of aryl and alkenyl esters. Bulky, electron-rich bidentate phosphine ligands are preferred in the reaction, whereas Cu is not needed for decarboxylation. Theoretical calculations suggest a five-coordinate Pd(II) transition state for decarboxylation with an energy barrier of approximately 30 kcal/mol.

β-Arylation of Carboxamides via Iron-Catalyzed C(sp3)–H Bond Activation

A 2,2-disubstituted propionamide bearing an 8-aminoquinolinyl group as the amide moiety can be arylated at the β-methyl position with an organozinc reagent in the presence of an organic oxidant, a catalytic amount of an iron salt, and a biphosphine ligand at 50 °C. Various features of selectivity and reactivity suggest the formation of an organometallic intermediate via rate-determining C-H bond cleavage rather than a free-radical-type reaction pathway.

Iron-Catalyzed C–H Bond Activation

Catalytic C-H bond activation, which was an elusive subject of chemical research until the 1990s, has now become a standard synthetic method for the formation of new C-C and C-heteroatom bonds. The synthetic potential of C-H activation was first described for ruthenium catalysis and is now widely exploited by the use of various precious metals. Driven by the increasing interest in chemical utilization of ubiquitous metals that are abundant and nontoxic, iron catalysis has become a rapidly growing area of research, and iron-catalyzed C-H activation has been most actively explored in recent years. In this review, we summarize the development of stoichiometric C-H activation, which has a long history, and catalytic C-H functionalization, which emerged about 10 years ago. We focus in this review on reactions that take place via reactive organoiron intermediates, and we excluded those that use iron as a Lewis acid or radical initiator. The contents of this review are categorized by the type of C-H bond cleaved and the type of bond formed thereafter, and it covers the reactions of simple substrates and substrates possessing a directing group that anchors the catalyst to the substrate, providing an overview of iron-mediated and iron-catalyzed C-H activation reported in the literature by October 2016.

Pd-catalyzed decarboxylative cross coupling of potassium polyfluorobenzoates with aryl bromides, chlorides, and triflates.

Pd-catalyzed decarboxylative cross coupling of potassium polyfluorobenzoates with aryl bromides, chlorides, and triflates is achieved by using diglyme as the solvent. The reaction is useful for synthesis of polyfluorobiaryls from readily accessible and nonvolatile polyfluorobenzoate salts. Unlike the Cu-catalyzed decarboxylation cross coupling where oxidative addition is the rate-limiting step, in the Pd-catalyzed version decarboxylation is the rate-limiting step.

Iron-Catalyzed Directed C(sp2)–H and C(sp3)–H Functionalization with Trimethylaluminum

Conversion of a C(sp(2))-H or C(sp(3))-H bond to the corresponding C-Me bond can be achieved by using AlMe3 or its air-stable diamine complex in the presence of catalytic amounts of an inorganic iron(III) salt and a diphosphine along with 2,3-dichlorobutane as a stoichiometric oxidant. The reaction is applicable to a variety of amide substrates bearing a picolinoyl or 8-aminoquinolyl directing group, enabling methylation of a variety of (hetero)aryl, alkenyl, and alkyl amides. The use of the mild aluminum reagent prevents undesired reduction of iron and allows the reaction to proceed with catalyst turnover numbers as high as 6500.

Iron-Catalyzed C(sp2)–H Bond Functionalization with Organoboron Compounds

We report here that an iron-catalyzed directed C-H functionalization reaction allows the coupling of a variety of aromatic, heteroaromatic, and olefinic substrates with alkenyl and aryl boron compounds under mild oxidative conditions. We rationalize these results by the involvement of an organoiron(III) reactive intermediate that is responsible for the C-H bond-activation process. A zinc salt is crucial to promote the transfer of the organic group from the boron atom to the iron(III) atom.

Palladium-catalyzed decarboxylative coupling of potassium nitrophenyl acetates with aryl halides.

A palladium-catalyzed decarboxylative cross-coupling of potassium 2- and 4-nitrophenyl acetates with aryl chlorides and bromides has been developed. Because the nitro group can be readily converted to many other functional groups, the new reaction provides a useful method for the preparation of diverse 1,1-diaryl methanes and their derivatives.

Decarboxylative 1,4-Addition of α-Oxocarboxylic Acids with Michael Acceptors Enabled by Photoredox Catalysis

Enabled by iridium photoredox catalysis, 2-oxo-2-(hetero)arylacetic acids were decarboxylatively added to various Michael acceptors including α,β-unsaturated ester, ketone, amide, aldehyde, nitrile, and sulfone at room temperature. The reaction presents a new type of acyl Michael addition using stable and easily accessible carboxylic acid to formally generate acyl anion through photoredox-catalyzed radical decarboxylation.

Room-Temperature Decarboxylative Couplings of α-Oxocarboxylates with Aryl Halides by Merging Photoredox with Palladium Catalysis

Enabled by merging iridium photoredox catalysis and palladium catalysis, α-oxocarboxylate salts can be decarboxylatively coupled with aryl halides to generate aromatic ketones and amides at room temperature. DFT calculations suggest that this reaction proceeds through a Pd(0) -Pd(II) -Pd(III) pathway, in which the Pd(III) intermediate is responsible for reoxidizing Ir(II) to complete the Ir(III) -*Ir(III) -Ir(II) photoredox cycle.