Synthesis of Naphtho[1′,2′:4,5]imidazo[1,2‑<i>a</i>]pyridines and Imidazo[5,1,2-<i>cd</i>]indolizines Through Pd-Catalyzed Cycloaromatization of 2‑Phenylimidazo[1,2‑<i>a</i>]pyridines with Alkynes

In this paper, palladium-catalyzed oxidative cycloaromatization of 2-phenylimidazo­[1,2-<i>a</i>]­pyridine (PIP) with internal alkyne is studied. From this reaction, two classes of fused <i>N</i>-heterocycle, naphtho­[1′,2′:4,5]­imidazo­[1,2-<i>a</i>]­pyridine (NIP) and imidazo­[5,1,2-<i>cd</i>]­indolizine (IID), were formed through dehydrogenative coupling featured with cleavage of the C–H bonds located on different moiety of the PIP substrates. Moreover, when 5-methyl-2-phenylimidazo [1,2-<i>a</i>]­pyridine or 2-mesitylimidazo­[1,2-<i>a</i>]­pyridine was used, either NIP or IID could be obtained as an exclusive product with good efficiency. Intriguingly, Pd­(II) showed different action mode in promoting this reaction compared with Rh­(III) and led to the formation of NIP with reversed regio-selectivity for the reaction of asymmetrical alkyne

Synthesis of Indeno[1′,2′:4,5]imidazo[1,2‑<i>a</i>]pyridin-11-ones and Chromeno[4′,3′:4,5]imidazo[1,2‑<i>a</i>]pyridin-6-ones through Palladium-Catalyzed Cascade Reactions of 2‑(2-Bromophenyl)imidazo[1,2‑<i>a</i>]pyridines

A novel and efficient synthesis of 11<i>H</i>-indeno­[1′,2′:4,5]­imidazo­[1,2-<i>a</i>]­pyridin-11-one, a hybrid structure of indenone with imidazo­[1,2-<i>a</i>]­pyridine, from the reaction of 2-(2-bromophenyl)­imidazo­[1,2-<i>a</i>]­pyridine with carbon monoxide through palladium-catalyzed CO insertion and C–H bond activation, has been developed. Intriguingly, under similar conditions but in the presence of Cu­(OAc)₂, the reaction selectively afforded 6<i>H</i>-chromeno­[4′,3′:4,5]­imidazo­[1,2-<i>a</i>]­pyridin-6-one, a hybrid structure of chromenone with imidazo­[1,2-<i>a</i>]­pyridine, via a more sophisticated cascade process including acetoxylation, deacetylation, CO insertion, and C–H bond activation

Metal-Free Synthesis of 2‑Aminobenzothiazoles via Iodine-Catalyzed and Oxygen-Promoted Cascade Reactions of Isothiocyanatobenzenes with Amines

In this paper, a highly efficient and sustainable synthesis of 2-aminobenzothiazoles through the cascade reactions of isothiocyanatobenzenes with primary or secondary amines by using iodine as a catalyst and oxygen as an oxidant is presented. Mechanistically, the formation of the title compounds involves the in situ formation of the required benzothiourea intermediate followed by its intramolecular cross dehydrogenative coupling of a C­(sp²)–H bond and a S–H bond. To our knowledge, this should be the first example in which 2-aminobenzothiazoles are efficiently prepared from simple and cheap isothiocyanates and amines under metal-free conditions by using iodine as a catalyst and molecular oxygen as an oxidant with water as the byproduct. Compared with literature protocols, this method eliminates the use of <i>ortho</i>-halo-substituted precursors, expensive transition-metal catalysts, and hazardous oxidants

<i>t</i>‑BuOK-Catalyzed Regio- and Stereoselective Intramolecular Hydroamination Reaction Leading to Phthalazinoquinazolinone Derivatives

We report herein an efficient and practical strategy for the preparation of 5H-phthalazino[1,2-b]quinazolin-8(6H)-one derivatives through a t-BuOK-catalyzed intramolecular hydroamination reaction of functionalized quinazolinones under extremely mild reaction conditions. A variety of quinazolinone substrates are well tolerated to furnish the corresponding products in good to high yields via an exclusive 6-exo-dig cyclization process. The present protocol has the advantages of readily obtainable starting materials, broad substrate scope, and high regio- and stereoselectivity

Synthesis of Naphthoquinolizinones through Rh(III)-Catalyzed Double C(sp²)–H Bond Carbenoid Insertion and Annulation of 2‑Aryl-3-cyanopyridines with α‑Diazo Carbonyl Compounds

An unprecedented Rh­(III)-catalyzed double C­(sp²)–H bond carbenoid insertion and annulation of 2-aryl-3-cyanopyridines with α-diazo carbonyl compounds is presented. Through this cascade reaction, a series of naphthoquinolizinone derivatives with a large π-system were efficiently prepared. The reactions could selectively afford naphthoquinolizinones with either an amine or an amide unit attached on the 11-position depending on the nature of the solvent and the additive used. Compared with literature methods, this is a more efficient, convenient, and atom-economic way to provide polycyclic heteroaromatic compounds through direct π-extension of simple aromatics via inert C–H bond activation and functionalization

Synthesis of 4‑Acylpyrazoles from Saturated Ketones and Hydrazones Featured with Multiple C(sp³)–H Bond Functionalization and C–C Bond Cleavage and Reorganization

In this paper, an efficient and convenient one-pot synthesis of diversely substituted 4-acylpyrazole derivatives via copper-catalyzed one-pot cascade reactions of saturated ketones with hydrazones is reported. Mechanistically, the formation of the title compounds involves the in situ formation of an enone intermediate through the dehydrogenation of a saturated ketone and the [2 + 3] cyclization of the enone with hydrazone followed by an aromatization-driven C–C bond cleavage and reorganization. To our knowledge, this is the first example in which the biologically and pharmaceutically important yet otherwise difficult-to-obtain 4-acylpyrazole derivatives are directly prepared from saturated ketones and hydrazones featured with multiple aliphatic C–H bond functionalization and C–C bond cleavage and reorganization. Compared with literature methods, this novel process has advantages such as simple and economical starting materials, a sustainable oxidant, excellent regioselectivity, and good efficiency

Synthesis of Functionalized Pyridines via Cu(II)-Catalyzed One-Pot Cascade Reactions of Inactivated Saturated Ketones with Electron-Deficient Enamines

In this paper, a novel and efficient synthesis of 3-acylpyridines and pyridine-3-carboxylates through the oxidative one-pot sequential reactions of inactivated saturated ketones with electron-deficient enamines is presented. Mechanistically, the formation of the title compounds involves the in situ formation of an enone intermediate through an oxidative dehydrogenation of the saturated ketone substrate, followed by its [3+3] annulation with β-enaminone or β-enaminoester via a cascade process, including Michael addition, aldol type condensation, and oxidative aromatization

Rhodium(III)-Catalyzed Redox-Neutral Synthesis of Isoquinolinium Salts via C–H Activation of Imines

Redox-neutral synthesis of isoquinolinium salts via C–H activation of presynthesized or in situ formed imines and coupling with α-diazo ketoesters has been realized, where a zinc salt promotes cyclization as well as provides a counteranion. Under three-component conditions, both ketone and aldehydes are viable arene sources. The coupling of imines with diazo malonates under similar conditions afforded isoquinolin-3-ones as the coupling product

One-Pot Cascade Reactions Leading to Pyrido[2′,1′:2,3]imidazo[4,5‑<i>c</i>][1,2,3]triazolo[1,5‑<i>a</i>]quinolines under Bimetallic Relay Catalysis with Air as the Oxidant

In this paper, we report an efficient one-pot synthesis of 1,2,3-triazole/quinoline-fused imidazo­[1,2-<i>a</i>]­pyridines starting from 2-(2-bromophenyl)­imidazo­[1,2-<i>a</i>]­pyridines, alkynes, and sodium azide. This novel method involves a one-pot bimetallic relay-catalyzed cascade process combining azide–alkyne cycloaddition, C–N coupling between 1,2,3-triazole and aryl bromide, and intramolecular cross dehydrogenative C–C coupling between 1,2,3-triazole and imidazo­[1,2-<i>a</i>]­pyridine. Notable features of this protocol include simple starting materials, sustainable oxidants, reduced synthetic steps, and high efficiency

Tunable Synthesis of Functionalized Cyclohexa-1,3-dienes and 2‑Aminobenzophenones/Benzoate from the Cascade Reactions of Allenic Ketones/Allenoate with Amines and Enones

A TEMPO-dependent tunable synthesis of functionalized cyclohexa-1,3-dienes and 2-aminobenzophenones/benzoate from the one-pot cascade reactions of allenic ketones/allenoate with amines and enones is presented. Mechanistically, the construction of the entitled six-membered carbocycles involves the in situ generation of an enaminone intermediate via the conjugate addition of allenic ketone with amine followed by its catalyst- and base-free [3+3] annulation with enone along with the simultaneous introduction of the valuable amino and carbonyl groups