Zhuangzhi Shi

Recent advances in transition-metal catalyzed reactions using molecular oxygen as the oxidant

For green and sustainable chemistry, molecular oxygen is considered as an ideal oxidant due to its natural, inexpensive, and environmentally friendly characteristics, and therefore offers attractive academic and industrial prospects. This critical review introduces the recent advances over the past 5 years in transition-metal catalyzed reactions using molecular oxygen as the oxidant. This review highlights the scope and limitations, as well as the mechanisms of these oxidation reactions (184 references).

Indoles from Simple Anilines and Alkynes: Palladium-Catalyzed CH Activation Using Dioxygen as the Oxidant†

Pd doles it out: A palladium-catalyzed approach to indoles using the title reaction was achieved (see scheme). The oxidant used in this catalytic cycle was O(2). Both N-nonsubstituted and N-alkyl monosubstituted anilines can be successfully transformed into the corresponding indoles by this method.

Rh(III)-Catalyzed Synthesis of Multisubstituted Isoquinoline and Pyridine N-Oxides from Oximes and Diazo Compounds

Multisubstituted isoquinoline and pyridine N-oxides have been prepared by Rh(III)-catalyzed cyclization of oximes and diazo compounds via aryl and vinylic C-H activation. This intermolecular annulation involving tandem C-H activation, cyclization, and condensation steps proceeds under mild conditions, obviates the need for oxidants, releases N2 and H2O as the byproducts, and displays a broad substituent scope.

Indole Synthesis by Rhodium(III)-Catalyzed Hydrazine-Directed CH Activation: Redox-Neutral and Traceless by NN Bond Cleavage†

Fishing for complements! There is an alternative to the useful Fischer indole synthesis. The new method utilizes the same retrosynthetic disconnection but is based on a Rh(III) -catalyzed directed CH activation step and a successive coupling with alkynes.

Efficient and versatile synthesis of indoles from enamines and imines by cross-dehydrogenative coupling.

The indole unit is one of the most abundant and relevant heterocycles in natural products and pharmaceuticals. The synthesis of indoles has been a topic of research for over 100 years, and a variety of well-established classical methods are now available. The venerable Fischer indole synthesis, discovered in 1883, remains one of the most powerful and versatile routes to the indole heterocycle, although this method suffers from several drawbacks. A variety of synthetic modifications have been introduced to increase the practicality and lessen the environmental impact of the Fischer cyclization such as using mild acids, reusable catalysts, and ionic liquids. However, it is important to note that hydrazines are carcinogenic and that certain hydrazines are difficult to prepare, are unstable, or display only moderate functional-group tolerance under acidic conditions (Scheme 1a).

Synthesis of fluorenones via quaternary ammonium salt-promoted intramolecular dehydrogenative arylation of aldehydes

Biologically interesting fluorenone, xanthone and anthrone derivatives have been prepared via an intramolecular oxidative acylation process. This novel direct acylation reaction proceeded without the aid of any transition metals, acids or bases, and uses a catalytic amount of a quaternary ammonium salt in the presence of a persulfate oxidant. Initial mechanistic studies have been carried out to elucidate the reaction pathway.

Synthesis of β- and γ-Carbolinones via Pd-Catalyzed Direct Dehydrogenative Annulation (DDA) of Indole-carboxamides with Alkynes Using Air as the Oxidant

A palladium-catalyzed direct dehydrogenative annulation (DDA) of indolecarboxamides with internal alkynes via C-H and N-H bond cleavage using air as the oxidant was developed. With this method, both beta- and gamma-carbolinones can be easily prepared under the mild conditions.

Pd(II)-Catalyzed Synthesis of Carbolines by Iminoannulation of Internal Alkynes via Direct C−H Bond Cleavage Using Dioxygen as Oxidant

A palladium-catalyzed iminoannulation of internal alkynes via direct C-H bond cleavage was developed. Dioxygen was employed as a clean oxidant in this kind of catalysis. Carbolines were synthesized from tert-butylimines of N-substituted indole-2-carboxaldehydes or indole-3-carboxaldehydes.

Palladium-Catalyzed C–H Arylation of Indoles at the C7 Position

In the past decade, direct C-H arylation of indoles has been developed with high selectivity at the C2 and C3 positions via transition-metal-catalyzed cross-coupling reactions. Here we show that C-H activation can be directed to the C7 position with high selectivity in Pd-catalyzed coupling of indoles with arylboronic acids. The key to this high regioselectivity is the appropriate choice of a phosphinoyl directing group and a pyridine-type ligand in the presence of Pd(OAc)2 catalyst. This previously elusive transformation should provide insight for the design of other cross-couplings as well.

Nickel-Catalyzed Decarbonylative Borylation of Amides: Evidence for Acyl C−N Bond Activation

A nickel/N-heterocyclic carbene catalytic system has been established for decarbonylative borylation of amides with B2 nep2 by C-N bond activation. This transformation shows good functional-group compatibility and can serve as a powerful synthetic tool for late-stage borylation of amide groups in complex compounds. More importantly, as a key intermediate, the structure of an acyl nickel complex was first confirmed by X-ray analysis. Furthermore, the decarbonylative process was also observed. These findings confirm the key mechanistic features of the acyl C-N bond activation process.