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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species

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Published on: August 16, 2018

Umpolung Addition of Aldehydes to Aryl Imines

Ning Chen^1,2, Xi-Jie Dai¹, Haining Wang¹

¹Department of Chemistry and FQRNT Centre for Green Chemistry and Catalysis, McGill University, 801 Sherbrooke St. W., Montreal, Quebec, H3A 0B8, Canada.

Angewandte Chemie (International Ed. in English)

|December 28, 2016

Abstract:

One of the classical ways to synthesize amines involves the coupling of carbonyl compounds and imines, either through enolate chemistry or acyl-based carbanion equivalents. We herein report an alternative strategy that is based on the use of aldehydes as alkyl carbanion equivalents in a reductive coupling with aryl imines. A wide array of secondary amines can be synthesized in moderate to high yields. This reaction is mediated by hydrazine and catalyzed by ruthenium(II) complexes, and it tolerates various functional groups, such as esters, amides, and nitriles.

Keywords:

aldehydes hydrazine imines reductive addition ruthenium catalysis

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Aldehydes and Ketones with Amines: Imine and Enamine Formation Overview

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Primary amines react with carbonyl compounds—aldehydes and ketones—to generate imines. Imines consist of a C=N double bond and are named Schiff bases after its discoverer—the German chemist Hugo Schiff. On the other hand, secondary amines react with carbonyl compounds to give enamines. In enamines, the presence of a C=C double bond adjacent to the nitrogen atom leads to the delocalization of the lone pair.

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

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One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.

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Imine formation involves the addition of carbonyl compounds to a primary amine. It begins with the generation of carbinolamine through a series of steps involving an initial nucleophilic attack and then several proton transfer reactions. The second part includes the elimination of water, as a leaving group, to give the imine.
Imines are formed under mildly acidic conditions. A pH of 4.5 is ideal for the reaction.
If the pH is low or the solution is too acidic, the reaction slows down in the...

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