Abstract
Generation of alkenes through decarboxyolefination of alkane carboxylates has significant synthetic value in view of the easy availability of a variety of carboxylic acids and the synthetic versatility of alkenes. Herein we report that palladium catalysts under irradiation with blue LEDs (440 nm) catalyze decarboxylative desaturation of a variety of aliphatic carboxylates to generate aliphatic alkenes, styrenes, enol ethers, enamides, and peptide enamides under mild conditions. The selection of a dual phosphine ligand system is the key enabler for the successful development of this reaction. The Pd-catalyzed decarboxylative desaturation is utilized to achieve a three-step divergent synthesis of Chondriamide A and Chondriamide C in overall 68% yield from simple starting materials. Mechanistic studies suggest that, distinct from palladium catalysis under thermal condition, irradiation-induced palladium catalysis involves irradiation-induced single-electron transfer and dynamic ligand-dissociation/association process to allow two phosphine ligand to work synergistically.
Highlights
Generation of alkenes through decarboxyolefination of alkane carboxylates has significant synthetic value in view of the easy availability of a variety of carboxylic acids and the synthetic versatility of alkenes
P roduction of alkenes from carboxylic acids through extrusion of the carboxyl group is a highly significant process1–5 considering the easy availability of carboxylic acids6–11 and the synthetic versatility of alkene products12
The synthetic significance of this transformation is further enhanced when it is applicable to α-hydroxy acids and α-amino acids to produce enol ethers and enamides, which are important intermediates for synthesis13,14 and privileged structures in bioactive compounds15,16
Summary
Generation of alkenes through decarboxyolefination of alkane carboxylates has significant synthetic value in view of the easy availability of a variety of carboxylic acids and the synthetic versatility of alkenes. We report that palladium catalysts under irradiation with blue LEDs (440 nm) catalyze decarboxylative desaturation of a variety of aliphatic carboxylates to generate aliphatic alkenes, styrenes, enol ethers, enamides, and peptide enamides under mild conditions. Reported methods for the transformation of aliphatic carboxylic acids into alkenes include transition-metalcatalyzed decarbonylative dehydration, decarboxylative oxidation using Pb(IV), and enzymatic processes24,25 These methods require either harsh reaction conditions or toxic reagents, or lack generality for synthetic utility. The reaction demonstrated provides access to olefins, enol ethers, and enamides from available carboxylates, which is of high synthetic value, and reveals that palladium catalysis under irradiation excitation has distinct ligand requirements compared with traditional thermal systems, and that optimizing a dual ligand combination provides the opportunity to achieve new reactivity
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