Huaiqing Zhao

Recent advances in directed C–H functionalizations using monodentate nitrogen-based directing groups

The use of directing groups has proven to be a successful strategy to enhance reactivity and control selectivity in C–H functionalization reactions. In the past decade, a multitude of new transformations and new directing groups have been explored, and several recent reviews have discussed directing group approaches for C–H functionalization. This review focuses specifically on the use of monodentate nitrogen-based directing groups published during the past two years, with the aim of covering a body of literature that is complementary to existing reviews.

Copper-catalyzed direct alkynylation of electron-deficient polyfluoroarenes with terminal alkynes using O2 as an oxidant.

A copper-catalyzed direct alkynylation of electron-deficient polyfluoroarenes with a wide range of terminal alkynes is realized for the first time. The reaction proceeds under mild conditions with O(2) as an oxidant.

Carboxylic Acids as Traceless Directing Groups for the Rhodium(III)-Catalyzed Decarboxylative CH Arylation of Thiophenes†

A rhodium(III)-catalyzed carboxylic acid directed decarboxylative C-H/C-H cross-coupling of carboxylic acids with thiophenes has been developed. With a slight adjustment of the reaction conditions based on the nature of the substrates, aryl carboxylic acids with a variety of substituents could serve as suitable coupling partners, and a broad variety of functional groups were tolerated. This method provides straightforward access to biaryl scaffolds with diverse substitution patterns, many of which have conventionally been synthesized through lengthy synthetic sequences. An illustrative example is the one-step gram-scale synthesis of a biologically active 3,5-substituted 2-arylthiophene by way of the current method.

Pd/PR3-catalyzed cross-coupling of aromatic carboxylic acids with electron-deficient polyfluoroarenes via combination of decarboxylation with sp2 C-H cleavage.

By using Pd(TFA)(2)/PCy(3) as a catalyst, a broad range of aromatic carboxylic acids, including heteroaromatic carboxylic acids, efficiently underwent decarboxylative coupling with an array of polyfluoroarenes in the presence of stoichiometric amount of silver salts to generate biaryls. Silver salts were adjusted to the reactivity of aromatic carboxylic acids to efficiently suppress the protodecarboxylation and therefore improve decarboxylative cross-couplings. It was established that the palladium complex containing the PCy(3) ligand was capable of catalyzing the decarboxylation of electron-rich aromatic carboxylic acids, and silver salts promoted the decarboxylation of both electron-rich and -deficient ones. To explain the two different decarboxylation processes, two possible reaction pathways are proposed, which were further supported by the facts that the stoichiometric arylpalladium complex can directly arylate pentafluorobenzene in the presence of PCy(3) and the arylpalladium complex can catalyze the decarboxylative coupling of 2,4-dimethoxybenzoic acid with pentafluorobenzene. The kinetic isotope effect of 4.0 clearly showed that the C-H bond cleavage of polyfluoroarenes is involved in the rate-determining step.

Rhodium(III)-Catalyzed Intermolecular N-Chelator-Directed Aromatic C–H Amidation with Amides

Rh(III)-catalyzed intermolecular direct aromatic C-H bond amidation with amides has been accomplished under mild reaction conditions. This protocol is applicable to a broad range of N-chelator-containing arenes amidated with aromatic and aliphatic sulfonamides. A possible mechanism is proposed according to the experimental results.

Rh(III)-Catalyzed Amide-Directed Cross-Dehydrogenative Heteroarylation of Pyridines

A new catalytic methodology has been developed for the synthesis of heteroaryled pyridines via a rhodium(III)-catalyzed dehydrogenative cross-coupling reaction. This protocol features a good substrate scope with a broad range of functional group tolerance and high regioselectivity of the pyridyl C-H activation.

Ambient-Temperature Ortho C–H Arylation of Benzoic Acids with Aryl Iodides with Ligand-Supported Palladium Catalyst

The ambient-temperature ortho C-H arylation of electron-deficient benzoic acids with aryl iodides has been achieved by using an Ac-Ile-OH-supported Pd catalyst. A wide range of unactivated benzoic acids could cross-couple an array of aryl iodides in moderate to excellent yields. The choice of HFIP as a solvent is crucial to realizing the mild C-H arylation, and the beneficial effect of the ligand on the reaction likely stems from the accelerated C-H activation process and the improved catalyst lifetime.

Photoinduced C-H direct arylation of unactivated arenes

Two general protocols have been developed for the photoinduced, metal-free direct arylation of unactivated arenes with aryl iodide. Both methods gave good to excellent yields for most substrates. Notably, the C-F bond in method A and the C-F, C-Cl, and C-Br bonds in method B could survive the arylation reaction. These methods offered excellent options for syntheses of biaryls.