Hydroarylation of enamides enabled by HFIP via a hexafluoroisopropyl ether as iminium reservoir.

Here we describe that HFIP greatly expands the scope with respect to both reaction partners of the Brønsted acid-catalyzed hydroarylation of enamides. The reaction is fast and practical and can be performed on the gram scale. A hexafluoroisopropyl ether intermediate was isolated from the reaction mixture and was shown to convert to the product when resubmitted to the reaction conditions. Extensive kinetic studies and computations reveal that the hexafluoroisopropyl ether is formed rapidly and serves as a slow-release reservoir for the key cationic intermediate, preventing the oligomerization of the substrate under the reaction conditions. Given the relatively low electrophilicity of the cationic intermediates in the present study, it seems likely that HFIP also actively participates in other reactions involving more electrophilic carbocations.

Synthesis and Reactions of 3,3-Difluoro-2-exo-methylidene Indolines.

A dearomative electrophilic fluorination of 2-methylindoles is reported, delivering 3,3-difluoroindolines bearing an exomethylidene. The model substrate was synthesized on up to a 20 mmol scale and was purified by a practical recrystallization as a crystalline bench-stable, yet reactive solid. The olefin is amphoteric and can react both as a nucleophile and as an electrophile. A wide range of metal-free, palladium, rhodium, and copper reactions was explored, forming new C-H, C-B, C-C (alkyl and aryl), C-N, C-O, C-P, and C-S bonds.

Palladium-Catalyzed Arylation/Heteroarylation of Indoles: Access to 2,3-Functionalized Indolines.

A palladium-catalyzed diastereoselective dearomatization of N-(2-bromobenzoyl)indoles by an arylation/heteroarylation sequence is reported. Diverse functionalized indolines are accessed in good to excellent yields and selectivity. Studies conducted on the effects of copper in the reaction revealed that, along with improving conversions, the additive inhibits epimerization of the product.

Palladium-Catalyzed Synthesis of 2-Cyanoindoles from 2-gem-Dihalovinylanilines.

An efficient Pd(0)-catalyzed synthesis of 2-cyanoindoles from 2-gem-dihalovinylanilines is reported. Few methods have aimed to synthesize these scaffolds, which are found in many natural products and have high bioactivity. This protocol features a robust catalyst system utilizing Zn(TFA)2 to prolong the catalytic activity. Additionally, the amount of cyanide in the reaction phase is minimized by taking advantage of the solubility of Zn(CN)2 in a two-solvent mixture.

Pd(0)-Catalyzed Dearomative Diarylation of Indoles.

We have developed a protocol for a Pd(0)-catalyzed dearomative syn 1,2-diarylation of indoles using readily available boroxines (dehydrated boronic acids) as coupling partners. This reaction proceeds efficiently using PtBu3 as the ligand to divergently access to fused indolines while minimizing the extent of direct Suzuki coupling. The scope of the reaction is remarkably broad and all products are obtained as single diastereomers in moderate to excellent yields. We have also compiled data which parallels the steric and electronic properties of both substrate and boroxine with the propensity to undergo the desired dearomative process over direct Suzuki coupling.

Dearomative Indole Bisfunctionalization via a Diastereoselective Palladium-Catalyzed Arylcyanation.

The first Pd-catalyzed dearomative indole bisfunctionalization via a diastereoselective arylcyanation is reported. This method facilitates the formation of diverse indoline scaffolds bearing congested stereocenters with high levels of diastereoselectivity. This also represents the first example of a cyanation mechanism involving a 2° benzylic Pd(II) intermediate.