Catalyst-Controlled Regiodivergent Alkyne Insertion in the Context of C−H Activation and Diels-Alder Reactions: Synthesis of Fused and Bridged Cycles

Cp*Co(III)-Catalyzed C(8)-Nucleophilic Cascade Cyclization of Quinoline N-Oxide with 1,6-Enyne

The C(8)-selective nucleophilic cascade cyclization of quinoline N-oxide with easily derived 1,6-enyne from phenol derivatives is demonstrated. A variety of quinoline N-oxide and alkynes are discovered to be suitable for producing a library of quinoline N-oxide tethered cis-hydrobenzofurans with high yields and excellent functional group tolerance. The utility of the protocol has been accomplished by post-synthetic modification of the cyclized product. The mechanistic studies indicate a base-assisted internal electrophilic-type substitution (BIES)-type pathway for C-H bond activation, and electrospray ionization mass spectrometry (ESI-MS) analysis of the stoichiometric reaction confirmed the formation of a key five-membered cobaltacycle.

Transition-Metal Catalyzed Stereoselective Desymmetrization of Prochiral Cyclohexadienones

The development of transition-metal catalyzed enantioselective and diastereoselective transformations has contributed many advances in the field of synthetic organic chemistry. Particularly, stereoselective desymmetrization of prochiral cyclohexadienones represents a powerful strategy for accessing highly functionalized and stereochemically enriched scaffolds, which are often found in biologically active compounds and natural products. In recent years, several research groups including our group have made a significant progress on transition-metal catalyzed stereoselective desymmetrizations of 2,5-cyclohexadienones. In this account, we will provide an overview of the recent developments in this area employing Pd, Cu, Rh, Au, Ag, Ni, Co, and Mn-catalysts.

Mechanochemical Solvent-Free Catalytic C-H Methylation

The mechanochemical, solvent-free, highly regioselective, rhodium-catalyzed C-H methylation of (hetero)arenes is reported. The reaction shows excellent functional-group compatibility and is demonstrated to work for the late-stage C-H methylation of biologically active compounds. The method requires no external heating and benefits from considerably shorter reaction times than previous solution-based C-H methylation protocols. Additionally, the mechanochemical approach is shown to enable the efficient synthesis of organometallic complexes that are difficult to generate conventionally.

Transition Metal-Catalyzed Intermolecular Cascade C-H Activation/Annulation Processes for the Synthesis of Polycycles

Polycycles are abundantly present in numerous advanced chemicals, functional materials, bioactive molecules and natural products. However, the strategies for the synthesis of polycycles are limited to classical reactions and transition metal-catalyzed cross-coupling reactions, requiring pre-functionalized starting materials and lengthy synthetic operations. The emergence of novel approaches shows great promise for the fields of organic/medicinal/materials chemistry. Among them, transition metal-catalyzed C-H activation followed by intermolecular annulation reactions prevail, due to their straightforward manner with high atom- and step-economy, providing rapid, concise and efficient methods for the construction of diverse polycycles. Several strategies have been developed for the synthesis of polycycles, relying on sequential multiple C-H activation/annulation, or combination of C-H activation/annulation and further interaction with a proximal group, or merger of C-H activation with a cycloaddition reaction, or in situ formation of the directing group. These are attractive, efficient, step- and atom-economic methods starting from commercially available materials. This Minireview will provide an introduction to transition metal-catalyzed C-H activation for the synthesis of polycycles, helping researchers to discover indirect connections and reveal hidden opportunities. It will also promote the discovery of novel synthetic strategies relying on C-H activation.

Rhodium-Catalyzed C(sp2 )- or C(sp3 )-H Bond Functionalization Assisted by Removable Directing Groups

In recent years, transition-metal-catalyzed C-H activation has become a key strategy in the field of organic synthesis. Rhodium complexes are widely used as catalysts in a variety of C-H functionalization reactions because of their high reactivity and selectivity. The availability of a number of rhodium complexes in various oxidation states enables diverse reaction patterns to be obtained. Regioselectivity, an important issue in C-H activation chemistry, can be accomplished by using a directing group to assist the reaction. However, to obtain the target functionalized compounds, it is also necessary to use a directing group that can be easily removed. A wide range of directed C-H functionalization reactions catalyzed by rhodium complexes have been reported to date. In this Review, we discuss Rh-catalyzed C-H functionalization reactions that are aided by the use of a removable directing group such as phenol, amine, aldehyde, ketones, ester, acid, sulfonic acid, and N-heteroaromatic derivatives.