NHC-Catalyzed Oxidative Cyclization Reactions of 2-Alkynylbenzaldehydes under Aerobic Conditions: Synthesis of O-Heterocycles

Abstract

An NHC-catalyzed, regio- and stereoselective oxidative cyclization of o-alkynylbenzaldehydes bearing an unactivated alkyne moiety as an internal electrophile has been developed to afford phthalides and isocoumarins. A single organocatalytic system enabled two sequential C-O bond formations to take place in an atom economical manner via highly efficient dual activation. Molecular oxygen in air could be utilized as a source of an oxygen atom for the oxidation of aldehydes to the corresponding benzoic acids under our newly developed reagent system.

Iridium‐Catalyzed Cycloisomerization of Alkynoic Acids: Synthesis of Unsaturated Lactones

“…Owing to the great importance of these intriguing structures, much attention was paid to exploring synthetic methods, and tremendous progress have been achieved over the last decades. Many synthetic methodologies were well established to prepare lactones, such as oxidative cyclization reaction of alkynylbenzaldehydes catalyzed by NHC (N‐heterocyclic carbenes), bromolactonization of long‐chain olefinic acids promoted by sulfur‐based zwitterionic organocatalyst, intermolecular [6+2] cyclization of amphoteric molecules with siloxy alkynes, oxetene ring‐opening reaction, borylated lactones from esters with electrophilic oxyboration, and intramolecular addition of carboxylic acids or esters to alkynes or alkenes promoted by transition metal, organic acids or bases –. Among these synthetic routes to lactones, the cycloisomerization of alkynoic acids is a straightforward and attractive synthetic method to access lactones with great atom economy .…”

NHC-catalysed highly selective aerobic oxidation of nonactivated aldehydes

“…The highly activated intermediate I , which is formed by the addition of the NHC to the aldehyde, reacts with dioxygen to form the peroxo-species [36–37]. …”

Cyclizations of phenylethyl-substituted pyridinecarboxaldehydes

“…Sonogashira coupling provides good yields of the hetrocyclic alkynes (i.e., 4 ). 10 Although reduction of the triple bond was achieved in quantitative yield, it occured with reduction of the carboxaldehyde group to give product 5 . Subsequent oxidation to the aldehyde is accomplished readily with MnO 2 providing the desired substrate 6 .…”