Organocatalysis By Oxidative N-Heterocyclic Carbenes

N-heterocyclic carbene is a new class of catalysts and ligands.It's unique properties,not only can be used to catalyze organic reactions,but also as an electron-rich metal ligand,and has a strong metal complexing ability and structural characteristics easily modified.Especially in terms of small organic molecules as a nucleophilic catalyst has become a hot area of chemists' research.In recent years,the use of which catalyzed cycloaddition reaction has achieved rapid development.Of course,this all started in 1991 that Arduengo separated the first stable N-heterocyclic carbenes-imidazole-2-ene successfully.Rencently,there are four types of NHCs that have been synthesized:thiazole?triazole?imidazole and imidazolines.Cyclic compounds are very important in organic chemistry synthesis,as many compounds'synthetic intermediates.And It is also key pharmacophore groups of many drugs,active substances.Currently,a series of ring-type compounds has been synthezied by NHC.Basing on previous work,we have used(1)oxidative-NHC instead of NHC;(2)a catalytic instead of a stoichiometric amount of oxidant;(3)Some simple unmodified aldehydes instead of functional aldehydes as substrate,and with another substrate 5-ene-thiazolone,generating a series of chiral thiazol-fused-pyrone compounds in moderate to excellent yields and high stereoselectivity through asymmetric[4+2]cycloaddition.Hydroxy compounds play an important role in natural products and medicinal chemistry,for example Hamigeran A extracted from the original sponge;synthetic Dnunorubicin.Although there are some reports about ?-keto esters' ?-hydroxylation reaction by metal or some small organic molecules,but no one by oxidative NHC.On this basis,our group made the following optimization(1)using some NHC of simple structure and easy to synthesize;(2)air as the oxidant(direct exposure reaction);(3)the reaction not only applies to ?-keto esters,isatin-derived,1,3-dicarbonyl substances,also applies to ?-dicalcium phosphate esters;(4)the reaction can even be in an aqueous solvent.Eventually won a series of hydroxylated product.