Synthesis of Indene Frameworks via Rhodium-Catalyzed Cascade Cyclization of Aromatic Ketone and Unsaturated Carbonyl Compounds

Synthesis of silyl indenes by ruthenium-catalyzed aldehyde- and acylsilane-enabled C-H alkylation/cyclization

A ruthenium-catalyzed C-H alkylation/cyclization sequence is presented to prepare silyl indenes with atom and step-economy. This domino reaction is triggered by acyl silane-directed C-H activation, and an aldehyde controlled the following enol cyclization/condensation other than β-H elimination. The protocol tolerates a broad substitution pattern, and the further synthetic elaboration of silyl indenes allows access to a diverse range of interesting indene and indanone derivatives.

Acylsilanes in Transition-Metal-Catalyzed and Photochemical Reactions: Clarifying Product Formation

Acylsilanes are able to react as nucleophilic carbene precursors, electrophiles, and directing groups in C-H functionalization. To date, some of the products reportedly formed during transition-metal-catalyzed and photochemical reactions involving acylsilanes have been incorrectly assigned. To provide clarity, we herein address these structural misassignments and detail the revised structures. New insights into the reactivity of acylsilanes were also afforded via the discovery that light-induced siloxy carbenes participate in intramolecular 1,2-carbonyl addition to proximal esters.

Rhodium(iii)-catalyzed successive C(sp2)–H and C(sp2)–C(sp2) bond activation of aryl oximes: synthetic and mechanistic studies

A rhodium(iii)-catalyzed redox-neutral reaction of aryl oximes and internal alkynes to generate novel N-(2-cyanoaryl) indanone imines.

Acyl silane directed Cp*Rh(III)-catalysed alkylation/annulation reactions

Studies into the Cp*Rh(iii)-catalysed hydroarylation of alkenes with aryl acyl silanes led to the discovery of a new synthetic strategy to access unique silicon derived indene frameworks. Rather than protodemetalation of the metal enolate formed following insertion of an alkene into the aryl C-H bond, intramolecular aldol condensation of the acyl silane occurred to generate a series of 2-formyl- and 2-acetyl-3-silyl indenes. This represents only the second example of rhodium-catalysed C-H functionalisation employing acyl silanes as weakly coordinating directing groups and the intramolecular aldol condensation strategy was extended to access analogous silicon derived benzofurans.

Rhodium-Catalyzed C-H Activation-Based Construction of Axially and Centrally Chiral Indenes through Two Discrete Insertions

Reported herein is asymmetric [3+2] annulation of arylnitrones with different classes of alkynes catalyzed by chiral rhodium(III) complexes, with the nitrone acting as an electrophilic directing group. Three classes of chiral indenes/indenones have been effectively constructed, depending on the nature of the substrates. The coupling system features mild reaction conditions, excellent enantioselectivity, and high atom-economy. In particular, the coupling of N-benzylnitrones and different classes of sterically hindered alkynes afforded C-C or C-N atropochiral pentatomic biaryls with a C-centered point-chirality in excellent enantio- and diastereoselectivity (45 examples, average 95.6 % ee). These chiral center and axis are disposed in a distal fashion and they are constructed via two distinct migratory insertions that are stereo-determining and are under catalyst control.

Synthesis of C3-sulfone substituted naphthols via rhodium(III)-catalyzed annulation of sulfoxonium ylides with alkynylsulfones

C-H activation of sulfoxonium ylides catalyzed by rhodium(iii) with subsequent annulation by alkynylsulfones was accomplished. This methodology offers a step-economical approach for assembling C3-sulfone-substituted naphthols with a high level of regioselectivity that is complementary to previous protocols. The approach has an extensive substrate spectrum and broad functional group tolerance.

Ruthenium-Catalyzed Brook Rearrangement Involved Domino Sequence Enabled by Acylsilane-Aldehyde Corporation

A ruthenium-catalyzed [1,2]-Brook rearrangement involved domino sequence is presented to prepare highly functionalized silyloxy indenes with atomic- and step-economy. This domino reaction is triggered by acylsilane-directed C-H activation, and the aldehyde controlled the subsequent enol cyclization/Brook Rearrangement other than β-H elimination. The protocol tolerates a broad substitution pattern, and the further synthetic elaboration of silyloxy indenes allows access to a diverse range of interesting indene and indanone derivatives.

Cp*Co(III)-Catalyzed Ketone-Directed ortho-C-H Activation for the Synthesis of Indene Derivatives

A weakly coordinating, carbonyl-assisted C-H activation of aromatic systems with α,β-unsaturated ketone and subsequent aldol condensation has been developed using a Cp*Co(CO)I₂ catalyst. The developed method is the first example of indene synthesis by cobalt-catalyzed C-H activation. In addition, the reaction requires mild reaction conditions and easily accessible starting materials, and it shows excellent functional group compatibility.

Experimental and computational studies on H2O-promoted, Rh-catalyzed transient-ligand-free ortho-C(sp2)–H amidation of benzaldehydes with dioxazolones

We develop an efficient and convenient ligand-free, rhodium-catalyzed ortho-C(sp²)–H amidation of benzaldehydes with dioxazolones using H₂O as the key promoter.

Palladium-catalyzed dehydrogenative coupling of cyclic enones with thiophenes: a rapid access to β-heteroarylated cyclic enones

A dehydrogenative coupling reaction of cyclic enones with thiophenes through palladium-catalyzed C–H functionalization is reported herein. Further molecular transformations of the coupling products to synthetically useful meta-heteroarylated phenols are also demonstrated.

Co(iii)-Catalyzed N-chloroamide-directed C–H activation for 3,4-dihydroisoquinolone synthesis

The first time synthesis of 3,4-dihydroisoquinolones via Co(iii)-catalyzed C–H activation at room temperature is described herein.

A concise synthesis of indene-based polycyclic compounds via FeCl3-catalyzed cascade cyclization

An efficient and straightforward protocol to complex indene-based polycyclic compounds via the cascade cyclization of propargylic alcohols and alkenes was developed. The catalyst FeCl₃, as an Earth-abundant and environment friendly transition metal salt, is attractive. This reaction proceeded through unusual C–C bond cleavage.

From Simple to Complex: Rhodium(III)-Catalyzed C-C Bond Cleavage and C-H Bond Functionalization for the Synthesis of 3a,8b-Dihydro-1H-cyclopenta[b]benzofuran-1-ones

A rhodium(III)-catalyzed strategy for the one-step synthesis of polysubstituted cis-3a,8b-dihydro-1H-cyclopenta[b]benzofuran-1-ones from simple 2'-hydroxychalcones and alkynes is developed. This novel transformation involves a sequential C-C bond cleavage and dehydrogenative annulation, leading to the product bearing a quaternary and a tertiary carbon center. ¹³C labeling experiments revealed that C-C bond cleavage takes place not only at the C-C(C═O) bond but also at the C≡C bond. This study provides an alternative strategy using C-C bond cleavage thus demonstrating the power of this strategy combined with C-H bond functionalization for assembling complex structures from simple starting materials.

Rh(III)-Catalyzed Aryl and Alkenyl C-H Bond Addition to Diverse Nitroalkenes

The transition metal catalyzed C-H bond addition to nitroalkenes has been developed. Very broad nitroalkene scope was observed for this Rh(III)-catalyzed method, including for aliphatic, aromatic and β,β-disubstituted derivatives. Additionally, various directing groups and both aromatic and alkenyl C-H bonds were effective in this transformation. Representative nitroalkane products were converted to dihydroisoquinolones and dihydropyridones in a single step and in high yield by iron mediated reduction and in situ cyclization. Moreover, preliminary success in enantioselective Rh(III)-catalyzed C-H bond addition to nitroalkenes was achieved as was X-ray structural characterization of a nitronate intermediate.

One-pot synthesis of quaternary carbon centered cyclobutanes via Pd(ii)-catalyzed cascade C(sp3)–H activations

A novel approach toward quaternary carbon centered cyclobutanes through Pd(ii)-catalyzed sequential intramolecular methylene C–H alkylation and intermolecular methine C–H bond arylation, alkenylation, alkylation, alkynylation, allylation, benzylation or alkoxylation is described.

Cp*Rh(iii)-catalyzed C(sp3)–H alkylation of 8-methylquinolines in aqueous media

A mild and efficient approach for the cross-coupling reaction of 8-methylquinolines with a range of allylic alcohols in water as a solvent under rhodium(iii) catalysis is described.

A cascade C–H functionalization/cyclization reaction of N-arylpyridin-2-amines with α,β-unsaturated aldehydes for the synthesis of dihydroquinolinone derivatives under rhodium catalysis

A novel rhodium catalyzed cascade C–H functionalization/cyclization reaction of N-arylpyridin-2-amines with α,β-unsaturated aldehydes has been developed.

Naphthol synthesis: annulation of nitrones with alkynes via rhodium(iii)-catalyzed C–H activation

An efficient and redox-neutral naphthol synthesis has been realized via rhodium(iii) catalyzed C–H activation of α-carbonyl nitrones and annulation with alkynes, where the nitrone group functioned as a traceless directing group.

Organocatalytic regiospecific synthesis of 1H-indene-2-carbaldehyde derivatives: suppression of cycloolefin isomerisation by employing sterically demanding catalysts

Extremely high regioselectivity (>99%) for indene-aldehyde products was achieved by using sterically demanding catalysts.

Transition-Metal-Catalyzed C-H Bond Addition to Carbonyls, Imines, and Related Polarized π Bonds

The transition-metal-catalyzed addition of C-H bonds to carbonyls, imines, and related polarized π bonds has emerged as a particularly efficient and powerful approach for the construction of an incredibly diverse array of heteroatom-substituted products. Readily available and stable inputs are typically employed, and reactions often proceed with very high functional group compatibility and without the production of waste byproducts. Additionally, many transition-metal-catalyzed C-H bond additions to polarized π bonds occur within cascade reaction sequences to provide rapid access to a diverse array of different heterocyclic as well as carbocyclic products. This review highlights the diversity of transformations that have been achieved, catalysts that have been used, and types of products that have been prepared through the transition-metal-catalyzed addition of C-H bonds to carbonyls, imines, and related polarized π bonds.

Rh(III)-Catalyzed C-H Bond Addition/Amine-Mediated Cyclization of Bis-Michael Acceptors

A Rh(III)-catalyzed C-H bond addition/primary amine-promoted cyclization of bis-Michael acceptors is reported. The C-H bond addition step occurs with high chemoselectivity, and the subsequent intramolecular Michael addition, mediated by a primary amine catalyst, sets three contiguous stereocenters with high diastereoselectivity. A broad range of directing groups and both aromatic and alkenyl C-H bonds were shown to be effective in this transformation, affording functionalized piperidines, tetrahydropyrans, and cyclohexanes.

Convergent Synthesis of Diverse Nitrogen Heterocycles via Rh(III)-Catalyzed C-H Conjugate Addition/Cyclization Reactions

The development of Rh(III)-catalyzed C-H conjugate addition/cyclization reactions that provide access to synthetically useful fused bi- and tricyclic nitrogen heterocycles is reported. A broad scope of C-H functionalization substrates and electrophilic olefin coupling partners is effective, and depending on the nature of the directing group, cyclic imide, amide, or heteroaromatic products are obtained. An efficient synthesis of a pyrrolophenanthridine alkaloid natural product, oxoassoanine, highlights the utility of this method.