Xiangguang Li

Amide-assisted radical strategy: metal-free direct fluorination of arenes in aqueous media

A metal- and initiator-free direct fluorination of arenes with the assistance of an amide group is developed. This reaction proceeded under simple aqueous conditions with good functional group tolerance and ortho–para selectivity, and is highly practical because it could be readily scaled up to a multigram-scale. At this stage, an exclusive mechanism could not be proposed, and several possibilities have been discussed. The possibility of the amide-assisted radical chain mechanism has been supported by experimental and computational investigations as well as a seminal work.

Br2-Catalyzed regioselective dehydrative coupling of indoles with acyloins: direct synthesis of α-(3-indolyl) ketones

An efficient Br2-catalyzed synthesis of α-(3-indolyl) ketones via dehydrative coupling of simple indoles with acyloins is presented. This reaction proceeded with high C-3 selectivity and a wide substrate scope, and without any metal catalyst. Both the activation of alcohols by carbonyl groups and the catalysis of Br2 were essential. Density functional theory (DFT) calculations indicated that carbonyl groups not only enhanced the electrophilicities of the resulting carbocations, but acted as stabilizers for them as well, and that the activation of the catalyst by hydrogen bonding between Br2 and the hydroxyl hydrogen of acyloins was the key process for this transformation.

Catalyst-free synthesis of 3-(1-arylsulfonylalkyl)indoles via three-component reaction of indoles, carbonyls, and arenesulfinic acids

ABSTRACT A catalyst-free three-component reaction of indoles, carbonyls, and arenesulfinic acids performed at room temperature provides direct access to biologically important 3-(1-arylsulfonylalkyl)indoles. This process features mild conditions, low cost, broad substrate scope, and high yields, and mechanistically bis(indolyl)methanes were identified as the key intermediates. GRAPHICAL ABSTRACT

Experimental and theoretical study of I2-catalyzed dialkenyl oxindoles synthesis from isatins and α-cyano ketene ethylene dithioacetal

ABSTRACT An I2-catalyzed synthesis of dialkenyl oxindoles from isatins and α-cyano ketene ethylene dithioacetal is described. Both electron-withdrawing groups (EWGs) and alkylthio groups exert effects on the reactivities of ketene dithioacetals. Density functional theory (DFT) calculations suggested that the highest negative charge density on the α-carbon of α-cyano ketene ethylene dithioacetal and the largest positive charge on C(3) of the related key intermediate are both responsible for the superior activity of α-cyano ketene ethylene dithioacetal. The cationic intermediate derived from 2-(1,3-dithian-2-ylidene)acetonitrile is the most stable but the least positive, thus the corresponding alkenylhydroxyoxindole is the thermally stable and separable product. Other ketene dithioacetals are less nucleophilic, and their corresponding cationic intermediates are probably not positive enough to enable further transformation. GRAPHICAL ABSTRACT

Metal-free and selective cleavage of unstrained carbon–carbon single bonds: Synthesis of β-ketosulfones from β-chlorohydrins and sodium sulfinates

ABSTRACT A metal-free protocol for the selective cleavage of unstrained C–C single bonds was developed. Under the catalysis of KI and in the presence of NaHCO3, the readily available α-chloro-β-hydroxy ketones underwent bond breaking and sulfonylation smoothly to afford β-ketosulfones with high efficiency and broad substrate scope. Mechanism investigations, both experimental and theoretical, showed that a retro-aldol cleavage/nucleophilic substitution sequence might be involved. GRAPHICAL ABSTRACT

Br2- or HBr-catalyzed synthesis of asymmetric 3,3-di(indolyl)indolin-2-ones

Abstract Under the catalysis of 1 mol% of Br2 or HBr at room temperature, indoles undergo a rapid reaction with 3-hydroxy-3-(indol-3-yl)indolin-2-ones to give asymmetric 3,3-di(indol-3-yl)indolin-2-ones with high efficiency and wide substrate scope. This is a rare example of Br2 acting as a Lewis acid catalyst. Theoretical calculations suggest that both the catalytic activity of the catalysts and the stability of reaction intermediates are responsible for the high efficiency of this reaction.