Rapid microwave heating and fast quenching for the highly efficient production of long-term stable supported Ag nanoclusters

Given the exciting potential of metallic clusters in a variety of fields, the development of novel preparation methods to accurately controlling the cluster size has become a research priority. Specifically, for catalytic applications, the synthesis and deployment of metallic nanoclusters on a proper substrate is perhaps the main bottleneck. Here, we have adopted an alternative reactor that uses simultaneous ice cooling and microwave heating (unlike water ice is a low microwave absorber) for the synthesis of Ag nanoclusters directly over a support with ordered mesopores (SBA-15). The reactor design exploits the selectivity of microwave heating, assuring a rapid localized nucleation followed by a nearly instantaneous quenching that largely avoids the aggregation of nascent clusters as well as Ostwald ripening mechanisms. We have compared this new synthesis approach with some previously reported methods for the production of supported silver nanoclusters: conventional batch reactor and also a continuous flow microreactor. The resulting Ag clusters were initially analyzed in terms of size distribution, textural properties and catalytic activity in the reduction of 4-nitrophenol. Finally, encouraged by the good results obtained, these nanoclusters were also employed in the production of different cyclic organic compounds, building blocks for pharmaceutical and photochemical applications. The nanoclusters displayed a high catalytic activity, lowering the metal loading required to achieve high yield and selectivity. Furthermore, the stabilization of the clusters over the mesoporous substrate allowed their reuse in several reaction cycles. In fact, the method produced exceptionally stable Ag clusters, whose catalytic properties were preserved even after one year of storage

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. © 2020 Wiley-VCH Gmb

Rhodium(III)-catalyzed intermolecular cascade annulation through C-H activation: Concise synthesis of rosettacin

An intermolecular annulation of 2-acetylenic aldehydes or ketones with O-substituted N-hydroxybenzamides or N-hydroxyacrylamides through rhodium(III)-catalyzed C–H activation for the synthesis of isoquinolones and indolizinones is developed. This reaction features excellent functional-group tolerance and broad substrate scope, including annulation of various heterocyclic substrates. This approach evaluates the chemoselectivity of the reaction when sterically hindered diaryl-substituted alkynes are incorporated. This method also furnishes an efficient approach for the total synthesis of rosettacin and a topoisomerase I inhibitor. © 2018 Elsevier B.V

Intramolecular cascade annulation triggered by rhodium(III)catalyzed sequential C(sp 2 )–H activation and C(sp 3 )–H amination

A rhodium(III)-catalyzed intramolecular oxidative annulation of O-substituted N-hydroxyacrylamides for the construction of indolizinones via sequential C(sp 2 )–H activation and C(sp 3 )–H amination has been developed. This approach shows excellent functional-group tolerance. The synthesized scaffold forms the core of many natural products with pharmacological relevance. © 2019 Song et al

Hypervalent Iodine(III)-Mediated Cascade Cyclization of Propargylguanidines and Total Syntheses of Kealiinine B and C

An oxidative cascade cyclization of propargylguanidines promoted by phenyliodonium diacetate (PIDA) was developed. The protocol provides an efficient route for the synthesis of the alkaloids kealiinines B and C as well as homologues. The difference in the electronic nature of the acetylene substituent resulted in two ways of the cyclization. A plausible mechanism is proposed based on the experimental results. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

Recent advances in spirocyclization of indole derivatives

Spiroindolines and spiroindoles are an important class of spirocyclic compounds present in a wide range of pharmaceuticals and biologically important natural alkaloids. Various spiroindolines and spiroindoles possess versatile reactivity which enables them to act as precursors for other privileged heterocycles. In view of the importance of this scaffold, many researchers focused their efforts to develop facile and mild synthetic methods for spirocyclization of indoles. However, the synthesis of spiroindolines and spiroindoles is known to be difficult due to rapid 1,2-migration to restore aromaticity. This review aims to briefly discuss the latest developments to access highly functionalized spiroindolines and spiroindoles to stimulate further research in the field to find new and efficient methodologies for accessing new spiroindolines and spiroindoles. © 2018 The Royal Society of Chemistry

Hypervalent Iodine(III)-Mediated Cascade Cyclization of Propargylguanidines and Total Syntheses of Kealiinine B and C

An oxidative cascade cyclization of propargylguanidines promoted by phenyliodonium diacetate (PIDA) was developed. The protocol provides an efficient route for the synthesis of the alkaloids kealiinines B and C as well as homologues. The difference in the electronic nature of the acetylene substituent resulted in two ways of the cyclization. A plausible mechanism is proposed based on the experimental results. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

Microwave-Assisted Cu(I)-Catalyzed Synthesis of Unsymmetrical 1,4-Diamino-2-butynes via Cross-A³-Coupling/Decarboxylative A³-Coupling

1,4-Diamino-2-butynes display both chemical and physiological properties. Here a highly efficient synthesis avenue to generate unsymmetric 1,4-diamino-2-butynes has been developed by microwave-assisted Cu(I)-catalyzed cross-A3-coupling/decarboxylative coupling of two different amines, formaldehyde, and propiolic acid through a domino process. This multicomponent reaction provides a series of target products in moderate to good yields with high chemoselectivity