Study On The Photo-dehydro-diels-alder Reaction Of Arylethynylbenzoxazole Derivatives

In the development of new organic photochemical reactions,the Photo-Dehydro-Diels-Alder(PDDA)reaction of arylalkynes is one of the important reactions,and it is also a new method for constructing macrocyclic aromatic systems.The study of PDDA reaction is of great significance and value.In this paper,the Sonogashira coupling method was used to synthesize fifteen terminal arylacetylenes using aryl halide and trimethylsilylacetylene as raw materials,(PPh3)2PdCl2 and Cul as catalysts,and triethylamine as solvent.Six arylethynylbenzothiazole derivatives were synthesized from iodobenzothiazole and arylacetylenes,two arylethynylbenzimidazole derivatives were synthesized from bromobenzimidazole and arylacetylenes,and eleven arylethynyl benzoxazole derivatives were synthesized from iodobenzoxazole and arylacetylenes.The photochemical properties of the synthesized six arylethynylbenzothiazoles,two arylethynylbenzimidazoles and eleven arylethynylbenzoxazole derivatives were carried out in different organic solvents upon the irradiation of UV light.It was found that arylethynylbenzothiazoles and arylethynylbenzimidazoles were inert upon irradiation in different solvents,while arylethynylbenzoxazoles were photoactive.The solvent screening showed that acetonitrile was the best solvent for the PDDA reaction with highest reaction yield.Then,the PDDA reaction of arylethynylbenzoxazole derivatives were explored in acetonitrile,and 10 photodimerization products were successfully synthesized in moderate yields.The structure of the photodimers was characterized by 1H NMR,13C NMR,and high-resolution mass spectrometry.The mechanism of the reaction was described.It also found that the yield of the PDDA reaction of arylethynylbenzoxazole with larger aryl groups was low,which was ascribed to the steric hindrance effect which decreased the photoactivity of the monomer.In addition,substituents at the 4-position of the naphthalene ring were more likely to undergo PDDA reactions than at the 6-positions,and the electron-donating substituent increased the activity of PDDA reaction while the electron-withdrawing group was the opposite.