Studies on the Photoinduced Coupling Reactions between Aromatic Carbons and Unsaturated Carbons

Author:Chen Ming

Supervisor:xia wu jiong


Degree Year:2018





The couplings between aromatic carbons and unsaturated carbons are common transformations in organic synthesis.By means of these reactions,aryl moieties can be easily introduced into molecular scaffolds,and lots of compounds which are of special physical and chemical activities,bioactivities,medicinal activities,can be effectively prepared.Traditional methods on C-C coupling reactions between aromatic carbons and unsaturated carbons always employ transition-metals as catalysts,which are not only expensive but unfriendly to environment,and such disadvantages place a big limitation on their application and population.As a novel synthetic technique,organic photochemistry is attractive due to the clean,mild conditions and easily manipulated feature of photoreaction.Thus,it has penetrated into many C-C coupling reactions and made remarkable progress.In this article,we mainly introduce some photoinduced methods on coupling reactions between aryls and aryls,aryls and unsaturated carbon-heteroatom double bonds.A wide range of isothiazolones,phenanthrenes,carboxamoyl phenanthrenes,diarylmethanols and diarylmethylamines could be prepared by means of these methods.When reactions conduct under the atmosphere of air,using quartz tube as container,acetonitrile as solvent,300 nm low pressure mercury lamp as light source,a variety of polycylic aromatic hydrocarbon-fused isothiazolones could be prepared from 3-aryl-N-(arylsulfonyl)propynamides in 42%-84% yields after1.3-16.0 hours of light exposure.Substrates with diverse electron-withdrawing or electron-donating substituents are applicable with the standard reaction conditions.Para-substituent on the benzene ring of substrate yields one single product while meta-or orth-substituent gives two kinds of adducts due to the position effect of substituent.In addition,reaction on gram-scale could be successfully performed.By controlling lighting time,a range of diaryl substituted isothiazolones,which are key intermediates of hydrocarbon-fused isothiazolones,could be obtained in40%-94% yields.Therefore,we propose that the triple bond of substrate is excited first under the irradiation of ultraviolet light,then the intermediate,diaryl substituted isothiazolone,could be obtained via Smiles rearrangement.In the presence of oxygen,diaryl substituted isothiazolone could convert into the final product,polycylic aromatic hydrocarbon-fused isothiazolone via aryl-aryl crossdehydrogenative coupling and cyclization.Based on the above mentioned work,we further explore the photochemical behaviors of 3-aryl-N-(arylsulfonyl)-propynamides in the present of base.All thesubstrates could be rapidly(0.9-3.5 h)converted into phenanthrenes when 1 eq morpholine is employed as base and quartz tube as reaction vessel.Besides phenanthrene products,diphenyl acetylenes could be obtained from otho-substituted substrates.When reactions conduct under N2 atmosphere,using glass tube as reaction container,25 eq morpholine as base,substrate concentration is 1.5 m M and medium pressure mercury lamp as light source,a series of carboxamoyl phenanthrenes could be prepared in 80%-99% yields by employing 3-aryl-N-(arylsulfonyl)propynamides as substrates.By controlling illumination time,two same intermediates are isolated from the two reaction systems,respectively.Thus we suspect that substrates firstly transform into diaryl substituted isothiazolone intermediates via Smiles rearrangement,and then an aryl-aryl coupling of diaryl substituted isothiazolone intermediate takes place.With the aid of morpholine,1,3hydrogen shift proceeds twice,subsequently,ring-opening and desulfonylation occur.Under the circumstance of phenanthrene preparation,phenanthrene and diphenyl acetylene derivatives could be obtained via decarboxamidation from desulfonylated intermediates;while under the conditions of carboxamoyl phenanthrenes preparation,biradical species could be generated via 1,3 hydrogen shift,two free radicals bonding affords the final carboxamoyl phenanthrene products.Under the protection of nitrogen,by 3 eq triethylamine as reductant,1mmol% Ir(ppy)2(dtbbpy)PF6 as catalyst,2 eq 1,4-DCB(terephthalonitrile)or4-cyanopyrid-ine as arylation reagent,20 W blue LED as light source,the reductive arylation of aromatic aldehyde/ketone/imine could take place under the irradiation of visible light.By means of this method,different kinds of diarylmethanols,diarylmethyl-amines,aryl-pyridyl methanols could be prepared in 24%-96% yields.The control experiments indicate such reactions proceed via a radical pathway.Based on the investigation on the loading of trimethylamine and isolated byproducts from control experiments,we propose that reactions proceed via two photoredox cycles.In the first cycle,when trimethylamine is used as reductant,carbonyl/imino radical could be generated through proton-coupled electron transfer(PCET)from unsaturated carbon-heteroatom double bond;in the second cycle,trimethylamine is used as reductant as well,and could reduce cyanoaromatics into aryl radicals,both the radical intermediates combine with each other to yield the desired products.