Hydroxyl-Directed Rhodium-Catalyzed C−H Bond Activation and Cyclization Leading to Naphtho[1,8-bc ]pyran Derivatives and its Analogues

Synthesis of 3,4,5-trisubstituted phenols via Rh(III)-catalyzed alkenyl C-H activation assisted by phosphonium cations

An efficient method for the construction of various 3,4,5-trisubstituted phenol derivatives has been achieved via the Rh(III)-catalyzed coupling of phosphonium cations with internal alkynes. This protocol shows good substrate compatibility, as an array of structurally and electronically diverse phosphonium compounds react efficiently with up to 87% yield.

Hydroxyl-Directed Rh(III)-Catalyzed C-H Functionalization: Access to Benzo[de]chromenes

A cascade oxidative annulation reaction of heterocyclic ketene aminals (HKAs) with internal alkynes catalyzed by [Cp*RhCl₂]₂ and oxidized by Cu(OAc)₂·H₂O was developed to efficiently synthesize highly functionalized benzo[de]chromene derivatives in good to excellent yields. The reaction proceeded by the sequential cleavage of C(sp²)-H/O-H and C(sp²)-H/C(sp²)-H bonds. These multicomponent cascade reactions were highly regioselective. In addition, all of the benzo[de]chromene products exhibited intense fluorescence emission in the solid state, and they demonstrated concentration-dependent quenching in the presence of Fe³⁺, indicating that these compounds could be used in the recognition of Fe³⁺.

Rh(III)-catalyzed [4 + 1] cyclization of aryl substituted pyrazoles with cyclopropanols via C-H activation

A rhodium-catalyzed formal [4 + 1]-cyclization reaction of aryl substituted pyrazoles with cyclopropanols via C-H bond activation/cyclization processes to selectively construct a series of carbonyl functionalized pyrazolo[5,1-a]isoindoles is described. The reaction features good functional group compatibility and a broad substrate scope with respect to both cyclization components with up to 84% yields. Mechanistic studies indicated that the C-H cleavage might be the rate-determining step in this transformation.

Ruthenium-Catalyzed Hydroxyl-Directed peri-Selective C-H Activation and Annulation of 1-Naphthols with CF3-Imidoyl Sulfoxonium Ylides for the Synthesis of 2-(Trifluoromethyl)-2,3-dihydrobenzo[de]chromen-2-amines

A ruthenium-catalyzed peri-selective C-H activation and annulation of 1-naphthols with CF₃-substituted imidoyl sulfoxonium ylides that uses hydroxyl as a weakly coordinating directing group is disclosed. The strategy provides a facile and practical route to diverse trifluoromethyl-containing 2,3-dihydrobenzo[de]chromen-2-amines with high efficiency. Notable advantages of this protocol include readily available materials, excellent regioselectivity, good functional group compatibility, and scalability.

Transition-Metal-Catalyzed C-H Bond Functionalization of Arenes/Heteroarenes via Tandem C-H Activation and Subsequent Carbene Migratory Insertion Strategy

The past decade has witnessed tremendous developments in transition-metal-catalyzed C-H bond activation and subsequent carbene migratory insertion reactions, thus assisting in the construction of diverse arene/heteroarene scaffolds. Various transition-metal catalysts serve this purpose and provide efficient pathways for an easy access to substituted heterocycles. A brief introduction to metal-carbenes has been provided along with key mechanistic pathways underlying the coupling reactions. The purpose of this review is to provide a concise knowledge about diverse directing group-assisted coupling of varied arenes/heteroarenes and acceptor-acceptor/donor-acceptor diazo compounds. The review also highlights the synthesis of various carbocycles and fused heterocycles through diazo insertion pathways, via C-C, C-N and C-O bond forming reactions. The mechanism usually involves a C-H activation process, followed by diazo insertion leading to subsequent coupling.

Programmed Multiple C-H Bond Functionalization of the Privileged 4-hydroxyquinoline Template

The introduction of substituents on bare heterocyclic scaffolds can selectively be achieved by directed C-H functionalization. However, such methods have only occasionally been used, in an iterative manner, to decorate various positions of a medicinal scaffold to build chemical libraries. We herein report the multiple, site selective, metal-catalyzed C-H functionalization of a "programmed" 4-hydroxyquinoline. This medicinally privileged template indeed possesses multiple reactive sites for diversity-oriented functionalization, of which four were targeted. The C-2 and C-8 decorations were directed by an N-oxide, before taking benefit of an O-carbamoyl protection at C-4 to perform a Fries rearrangement and install a carboxamide at C-3. This also released the carbonyl group of 4-quinolones, the ultimate directing group to functionalize position 5. Our study highlights the power of multiple C-H functionalization to generate diversity in a biologically relevant library, after showing its strong antimalarial potential.

Directed Cobalt-Catalyzed C-H Activation to Form C-C and C-O Bonds in One Pot via Three-Component Coupling

Herein, we disclose an efficient cobalt-catalyzed three-component coupling of benzamides, diazo compounds, and tert-butyl hydroperoxide, which provides an efficient approach to construct C(sp²)-C(sp³) and C-O bonds in one-pot accompanied with C-H activation. This protocol features low catalyst loading (4 mol %), the avoidance of additives, and excellent functional group compatibility, providing three-component coupling adducts with high yields under mild conditions (up to 88%). Mechanism studies show that the reaction may involve a radical process.

Synthesis of naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines via Rh(iii)-catalyzed C–H functionalization of 2-arylimidazo[1,2-a]pyridines with cyclic 2-diazo-1,3-diketones featuring with a ring opening and reannulation

An unprecedented synthesis of functionalized naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines via rhodium-catalyzed cascade reactions of 2-arylimidazo[1,2-a]pyridine-3-carbaldehydes with cyclic α-diazo-1,3-diketones is presented.

Sulfhydryl-Directed Iridium-Catalyzed C-H/Diazo Coupling and Tandem Annulation of Naphthalene-1-thiols

The first sulfhydryl-directed iridium-catalyzed C-H/diazo coupling and tandem annulation of naphthalene-1-thiols has been developed. The framework of naphtho[1,8-bc]thiopyrans was constructed in a one-step reaction with good yields. This transformation provides a practical synthetic route for the widely used naphtho[1,8-bc]thiopyran derivatives.

Ruthenium(ii)-catalyzed synthesis of indazolone-fused cinnolines via C-H coupling with diazo compounds

A robust, efficient and scalable method for the synthesis of 12H-indazolo[2,1-a]cinnolin-12-ones was developed. Significantly, a less developed cationic complex [Ru(p-cymene)(MeCN)3(SbF6)2] was found to be effective for this transformation. In this reaction, a tandem pathway of C-H ruthenation, Ru(ii)-carbene formation, migratory insertion and condensation was involved. The results of a primary mechanistic study suggested that the C-H activation process might follow an electrophilic-type metalation/deprotonation mechanism.