Catalytic Asymmetric Synthesis of Pyrroloindolines via a Rhodium(II)-Catalyzed Annulation of Indoles

Synthesis of ortho-arylated and alkenylated benzamides by palladium-catalyzed denitrogenative cross-coupling reactions of 1,2,3-benzotriazin-4(3H)-ones with organoboronic acids

An efficient palladium-catalyzed denitrogenative cross-coupling reaction of 1,2,3-benzotriazin-4(3H)-ones with organoboronic acids is described. The reaction affords various ortho-aryl and alkenylated benzamides in good to high yields.

Co-Catalyzed Transannulation of Pyridotriazoles with Isothiocyanates and Xanthate Esters

An efficient radical transannulation reaction of pyridotriazoles with isothiocyanates and xanthate esters was developed. This method features conversion of pyridotriazoles into two N-fused heterocyclic aromatic systems-imino-thiazolopyridines and oxo-thiazolopyridine derivatives-via one-step Co(II)-catalyzed transannulation reaction proceeding via a radical mechanism. The synthetic usefulness of the developed method was illustrated in the synthesis of amino acid derivatives and further transformations of obtained reaction products.

Aziridinium Ylides: Underutilized Intermediates for Complex Amine Synthesis

Harnessing the chemistry of onium ylide intermediates generated from transition metal catalysis is a powerful strategy to convert simple precursors into complex scaffolds. While the chemistry of onium ylides has been studied for over three decades, transformations of aziridinium ylides have just recently emerged as a versatile way to exploit the strain of these reactive intermediates to furnish densely functionalized N-heterocycles in a highly stereocontrolled manner. Herein, we provide a short overview of the key concepts and recent developments in this area, with a focus on how mechanistic studies to delineate the factors controlling the reactivity of aziridinium ylides can stimulate fruitful future investigations.

Silver-Catalyzed, Chemo- and Enantioselective Intramolecular Dearomatization of Indoles to Access Sterically Congested Azaspiro Frameworks

An asymmetric dearomatization of indoles bearing α-diazoacetamide functionalities was developed for synthesizing high-value spiro scaffolds. A silver phosphate chemoselectively catalyzed the sterically challenging dearomatization, whereas more typically used metal catalysts for carbene transfer reactions, such as a rhodium complex, were not effective and instead resulted in a Büchner ring expansion or cyclopropanation. Mechanistic studies indicated that the spirocyclization occurred through a silver-assisted asynchronous concerted process and not via a silver-carbene intermediate. Analyses based on natural bond orbital population and a distortion/interaction model indicated that the degree of C-Ag mutual interaction is crucial for achieving a high level of enantiocontrol. In addition, an oxidative disconnection of a C(sp³)-C(sp²) bond in the product provided unconventional access to the corresponding chiral spirooxindole.

Synthesis of Polycyclic Fused Indoline Scaffolds through a Substrate-Guided Reactivity Switch

Zn(II)-catalyzed divergent synthesis of functionalized polycyclic indolines through formal [3 + 2] and [4 + 2] cycloadditions of indoles with 1,2-diaza-1,3-dienes (DDs) is reported. The nature and type of substituents of substrates are found to act as a chemical switch to trigger two distinct reaction pathways and to obtain two different types of products upon the influence of the same catalyst. The mechanism of both [4 + 2] and [3 + 2] cycloadditions was investigated and fully rationalized by density functional theory (DFT) calculations.

Synthesis of 2,5-diaryloxazoles through rhodium-catalyzed annulation of triazoles and aldehydes

An efficient synthesis of a variety of 2,5-diaryloxazole derivatives via a rhodium-catalyzed annulation of triazoles and aldehydes is achieved. Various oxazole derivatives could be obtained in good to excellent yields. A concise synthesis of antimycobaterial natural products balsoxin and texamine has been achieved using this method.

Rh-Catalyzed Aziridine Ring Expansions to Dehydropiperazines

Piperazines are prevalent in pharmaceuticals and natural products, but traditional methods do not typically introduce stereochemical complexity into the ring. To expand access to these scaffolds, we report Rh-catalyzed ring expansions of aziridines and N-sulfonyl-1,2,3-triazoles to furnish dehydropiperazines with excellent diastereocontrol. Productive ring expansion proceeds via a pseudo-1,4-sigmatropic rearrangement of an aziridinium ylide species. However, the structural features of the carbene precursor are important, as pyridotriazoles undergo competing cheletropic extrusion to furnish ketimines.

Copper-Catalyzed Asymmetric Reaction of Alkenyl Diynes with Styrenes by Formal [3 + 2] Cycloaddition via Cu-Containing All-Carbon 1,3-Dipoles: Access to Chiral Pyrrole-Fused Bridged [2.2.1] Skeletons

The generation of metal-containing 1,3-dipoles from metal carbenes represents a significant advance in 1,3-dipolar cycloaddition reactions. However, these transformations have so far been limited to reactions based on diazo compounds or triazoles as precursors. Herein, we disclose a copper-catalyzed enantioselective reaction of alkenyl N-propargyl ynamides with styrene derivatives by formal [3 + 2] cycloaddition via Cu-containing all-carbon 1,3-dipoles, which constitutes a novel way for the generation of metal-containing 1,3-dipoles via metal carbenes. This protocol allows the practical and atom-economical synthesis of valuable chiral pyrrole-fused bridged [2.2.1] skeletons in moderate to good yields (up to 90% yield) with excellent diastereoselectivities (dr > 50/1) and generally excellent enantioselectivities (up to >99% ee).

Synthesis of Bridged Azacycles and Propellanes via Nitrene/Alkyne Cascades

A nitrene/alkyne cascade reaction terminating in C-H bond insertion to form functionalized bridged azacycles from carbonazidates is presented. Due to an initial Huisgen cyclization, all carbonazidates reacted with the alkyne in an exo mode in contrast to the use of sulfamate esters, which react predominately in an endo mode. Substrates with different ring sizes as well as different aryl and heteroaryl groups were also explored. Variation of the nitrene tether showed that 7-membered rings were the maximum ring size to be formed by nitrene attack on the alkyne. Examples incorporating stereocenters on the carbonazidate's tether induced diasteroselectivity in the formation of the bridged ring and two new stereocenters. Additionally, propellanes containing aminals, hemiaminals, and thioaminals formed from the bridged azacycles in the same reaction via an acid-promoted rearrangement.

Straightforward access to novel indolo[2,3-b]indoles via aerobic copper-catalyzed [3+2] annulation of diarylamines and indoles

Herein, we report an unprecedented aerobic copper-catalyzed [3+2] annulation reaction of diarylamines with indoles, which allows direct access to novel 2-diarylaminoindolo[2,3-b]indoles, a class of potential photoelectric device molecules. The developed transformation proceeds with broad substrate scope, good functional group tolerance, high chemo-selectivity, and no need for pre-preparation of specific agents, which offers a practical route for diverse and atom-economic synthesis of the desired products that are difficult to prepare with the conventional approaches.

A rhodium(ii) catalysed domino synthesis of azepino fused diindoles from isatin tethered N-sulfonyl-1,2,3-triazoles and indoles

An efficient and convenient protocol for the synthesis of a novel class of azepino fused diindoles from isatin tethered N-sulfonyl-1,2,3-triazoles and indoles has been disclosed. The reaction proceeds via denitrogenative aza-vinyl rhodium carbene formation to give a carbonyl ylide, which with indole results in 1,3-dipolar cycloaddition followed by sequential semipinacol rearrangement/ring expansion/oxidation to produce azepino fused diindoles. The reaction shows a broad substrate scope giving up to 81% yield. Furthermore, reversible catalytic hydrogenation and photophysical studies were carried out to demonstrate the application of these molecules.

General and efficient synthesis of 1,2-dihydropyrrolo[3,4-b]indol-3-ones via a formal [3 + 2] cycloaddition initiated by C–H activation

A [Cp*RhCl₂]₂-catalyzed formal [3 + 2] cycloaddition involving a sequential coupling reaction initiated by C–H activation and aza-Michael addition has been developed for the general and efficient synthesis of 1,2-dihydropyrrolo[3,4-b]indol-3-ones.

Enantioselective Dearomatization of Indoles by an Azoalkene-Enabled (3+2) Reaction: Access to Pyrroloindolines

The enantioselective dearomatization of indoles by an organocatalytic (3+2) reaction has been established. The reaction makes use of simple indole derivatives as substrates, and employs azoalkenes reaction partners. A wide range of pyrroloindolines containing an all-carbon quaternary stereogenic center were readily prepared in high yields and with excellent enantioselectivities.

Light-induced metal-free transformations of unactivated pyridotriazoles

A highly efficient and practical method for incorporation of the arylmethylpyridyl moiety into diverse molecules has been developed. This method features the transition metal-free light-induced room temperature transformation of pyridotriazoles into pyridyl carbenes, which are capable of smooth arylation, X-H insertion, and cyclopropanation reactions. The synthetic usefulness of the developed method was illustrated in a facile synthesis of biologically active molecules.

Catalytic asymmetric cycloaddition reactions of enoldiazo compounds

This review describes catalytic asymmetric cycloaddition reactions of silyl-protected enoldiazo compounds for the construction of highly functionalized carbo- and heterocycles which possess one or more chiral center(s). The enoldiazo compound or its derivative, donor-acceptor cyclopropene, form electrophilic vinylogous metal carbene intermediates that combine stepwise with nucleophilic dipolar reactants to form products from [3 + 1]-, [3 + 2]-, [3 + 3]-, [3 + 4]-, and [3 + 5]-cycloaddition, generally in high yield and with exceptional stereocontrol and regioselectivity.

Palladium(0)-Catalyzed Intermolecular Asymmetric Cascade Dearomatization Reaction of Indoles with Propargyl Carbonate

An intermolecular asymmetric cascade dearomatization reaction of indole derivatives with propargyl carbonate was developed. The challenges associated with both the chemoselectivity between the carbon and nitrogen nucleophile and the enantioselective control during the formation of an all-carbon quaternary stereogenic center were well addressed by a Pd catalytic system derived from the Feringa ligand. A series of enantioenriched multiply substituted fused indolenines were provided in good yields (71-86 %) with excellent enantioselectivity (91-96 % ee) and chemoselectivity (3/4>19:1 in most cases).

Rhodium(ii)-catalyzed divergent intramolecular tandem cyclization of N- or O-tethered cyclohexa-2,5-dienones with 1-sulfonyl-1,2,3-triazole: synthesis of cyclopropa[cd]indole and benzofuran derivatives

A novel rhodium(ii)-catalyzed divergent intramolecular tandem cyclization of N- or O-tethered cyclohexa-2,5-dienones and 1-sulfonyl-1,2,3-triazole is disclosed, affording cyclopropa[cd]indole and benzofuran derivatives in moderate to good yields.

Copper-catalyzed [4+1]-annulation of 2-alkenylindoles with diazoacetates: a facile access to dihydrocyclopenta[b]indoles

A copper-catalyzed [4+1]-annulation of 2-vinylindoles with diazoacetates: a facile access to the dihydrocyclopenta[b]indoles bearing two contiguous all-carbon quaternary centers.

Copper-Catalyzed Multicomponent Domino Reaction of 2-Bromobenzaldehydes, Aryl Methyl Ketones, and Sodium Azide: Access to 1 H-[1,2,3]Triazolo[4,5- c]quinoline Derivatives

A practical copper-catalyzed multicomponent reaction has been developed for the synthesis of 1 H-[1,2,3]triazolo[4,5- c]quinoline derivatives from commercially available 2-bromobenzaldehydes, aryl methyl ketones, and sodium azide. This protocol integrated consecutive base-promoted condensation, [3 + 2] cycloaddition, copper-catalyzed S_NAr, and denitrogenation cyclization sequences. Preliminary mechanistic studies revealed that CuBr₂ acted as a multifunctional catalyst to streamline this domino process. The mild catalytic system enabled effective construction of one C-C and four C-N bonds in one operation.