Visible Light-Driven Flexible Synthesis of α-Alkylated Glycine Derivatives Catalyzed by Reusable Covalent Organic Frameworks.

Herein, we report a heterogeneous visible light-driven preparation of α-alkylated glycine derivatives. This approach employed a ß-ketoenamine-linked covalent organic framework (2D-COF-4) as the heterogeneous photocatalyst and N-hydroxy phthalimide (NHPI) esters as the alkyl radical sources. Numerous glycine derivatives, including dipeptides, were precisely and efficiently alkylated under visible light-driven reaction conditions. Based on the excellent photoactivity and organic reaction compatibility of 2D-COF-4, this alkylation could proceed flexibly in a green solvent (ethanol) without any other additives. The photocatalyst and phthalimide were fruitfully recycled with a simple workup procedure, revealing a high ecoscale value and low environmental factor (E-factor).

Synthesis of naphthalene-substituted aromatic esters via Rh(III)-catalyzed C-H bond naphthylation and cascade directing group transformation.

A regioselective Rh(III)-catalyzed C-H bond naphthylation and cascade directing group (DG) transformation has been realized with aryl imidates and oxa bicyclic alkenes. Diverse naphthalene-substituted aromatic esters were synthesized fruitfully. The DG transformation into ester endows the strategy with chances of further C-H bond functionalization and derivatization. Preliminary mechanistic studies demonstrate that the ester carbonyl oxygen possibly originates from the corresponding oxa bicyclic alkene.

Benzocarbazole Synthesis via Visible-Light-Accelerated Rh(III)-Catalyzed C-H Annulation of Aromatic Amines with Bicyclic Alkenes.

A visible-light-accelerated Rh(III)-catalyzed C-H annulation of aromatic amines with bicyclic alkenes for the synthesis of benzocarbazole derivatives was developed. In this approach, with the cooperation of rhodium catalysis and visible-light irradiation, various aromatic amines reacted with oxabicyclic alkenes and azabicyclic alkenes smoothly at room temperature, delivering the corresponding bridged oxa or aza tetrahydro benzocarbazoles in good to excellent yields. Moreover, a series of benzo[b]carbazoles were synthesized conveniently through further aromatization in one pot. The potential of this method was demonstrated via directing-group removal, derivatization, a scale-up reaction, and fluorescence investigations.

Covalent Organic Frameworks toward Diverse Photocatalytic Aerobic Oxidations.

Photoactive two-dimensional covalent organic frameworks (2D-COFs) have become promising heterogenous photocatalysts in visible-light-driven organic transformations. Herein, a visible-light-driven selective aerobic oxidation of various small organic molecules by using 2D-COFs as the photocatalyst was developed. In this protocol, due to the remarkable photocatalytic capability of hydrazone-based 2D-COF-1 on molecular oxygen activation, a wide range of amides, quinolones, heterocyclic compounds, and sulfoxides were obtained with high efficiency and excellent functional group tolerance under very mild reaction conditions. Furthermore, benefiting from the inherent advantage of heterogenous photocatalysis, prominent sustainability and easy photocatalyst recyclability, a drug molecule (modafinil) and an oxidized mustard gas simulant (2-chloroethyl ethyl sulfoxide) were selectively and easily obtained in scale-up reactions. Mechanistic investigations were conducted using radical quenching experiments and in situ ESR spectroscopy, all corroborating the proposed role of 2D-COF-1 in photocatalytic cycle.

γ-Carboline synthesis enabled by Rh(iii)-catalysed regioselective C-H annulation.

A redox-neutral Rh(iii)-catalyzed C-H annulation of indolyl oximes was developed. Relying on the use of various alkynyl silanes as the terminal alkyne surrogates, the reaction exhibited a reverse regioselectivity, thus giving an exclusive and easy way for the synthesis of a wide range of substituent free γ-carbolines at C3 position with high efficiency. Deuterium-labelling experiments and kinetic analysis have preliminarily shed light on the working mode of this catalytic system.

Covalent Organic Frameworks: A Sustainable Photocatalyst toward Visible-Light-Accelerated C3 Arylation and Alkylation of Quinoxalin-2(1H)-ones.

A practical and scalable protocol for visible-light-accelerated arylation and alkylation of quinoxalin-2(1H)-ones with hydrazines is reported. In this protocol, a hydrazone-based two-dimensional covalent organic frameworks (2D-COF-1) was employed as the heterogeneous photocatalyst (PC). Due to its excellent photocatalytic properties, good chemical stability and heterogeneous nature, the present method exhibits high efficiency, good functional group tolerance, easy scalability and remarkable catalyst reusability. More importantly, it provides an alternative way that allows rapid access to various C3 arylated or alkylated quinoxalin-2(1H)-ones in a greener and sustainable manner.

Covalent Organic Frameworks: A Sustainable Photocatalyst toward Visible-Light-Accelerated C3 Arylation and Alkylation of Quinoxalin-2(1H)-Ones.

Invited for the cover of this issue is the group of Xiaobo Yang at the Shenyang Normal University. The image depicts the covalent organic frameworks photocatalyst (2D-COF-1) acting as a bridge between the substrate and product. Read the full text of the article at 10.1002/chem.201903523.

Rhodium-Catalyzed Tandem Reaction of Isocyanides with Azides and Oxygen Nucleophiles: Synthesis of Isoureas.

A novel rhodium-catalyzed tandem reaction of isocyanides with azides and various oxygen nucleophiles has been developed. The reaction provides a simple and highly efficient one-pot synthesis of various N-sulfonyl/acylisoureas with broad substrate scope in an atom-economical manner from readily available starting materials in a highly stereoselective manner.

Activation of α-diazocarbonyls by organic catalysts: diazo group acting as a strong N-terminal electrophile.

For the first time α-diazocarbonyls have been used as highly active N-terminal electrophiles in the presence of bicyclic amidine catalysts. The CN bond-forming reactions of active methylene compounds as C nucleophiles with α-diazocarbonyls as N-terminal electrophiles proceed quickly under ambient conditions, in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), because of the formation of the reactive N-terminal electrophilic intermediates. DBU activates both the active methylene and α-diazocarbonyl. Importantly, this reaction is general for both active methylenes and α-diazocarbonyls, and the activation mode will lead to new synthetic applications of α-diazocarbonyls.

A base-catalyzed cycloisomerization of 5-cyano-pentyne derivatives: an efficient synthesis of 3-cyano-4,5-dihydro-1H-pyrroles.

A base-catalyzed cycloisomerization of 5-cyano-pentyne bearing a terminal alkyne group has been developed under metal-free conditions. This reaction involves a tandem process providing efficient access to 3-cyano-4,5-dihydro-1H-pyrroles in good to excellent yields in an atom-economic manner with 1,3-cyano migration as the key transformation.