Study On The Synthesis And Photocycloaddition Reaction Of Diarylacetylenes

In this paper,three 2,6-substituted styrylbenzoxazoles were synthesized from 4-methylbenzoxazole,2,6-substituted benzaldehyde,and KOH in N,N-dimethylformamide.Three 2,6-substituted phenylethynylbenzoxazoles,namely 2,6-difluorophenylethynylbenzenes,2,6-dichlorophenylethynylbenzoxazole and 2,4,6-trimethylphenylethynyl benzoxazole,were synthesized from the corresponding diarylethenes by the bromination with Br2 and then debromination with KO-tBu.In addition,two 2-[naphthylethynyl]benzoxazoles were also synthesized from 1-naphthaldehyde and 2-naphthaldehyde as reactant by the similar method.Ten symmetric and asymmetric diphenylacetylenes were synthesized by the Sonogashira coupling reaction using various aryl halides and trimethylsilylacetylene in one-pot or separated step reaction.The photochemical properties of these diphenylacetylenes were investigated in acetonitrile under the irradiation of UV light.It was found that diphenylacetylenes are photoactive,the isolated main photoproducts with prolonged irradiation are benzoic acid derivatives with the photolysis and oxidation of the carbon-carbon triple bond.The single crystal structure of the p-methoxybenzoic acid was determined.We are interested in the cross-photo-dehydro Diels-Alder(PDDA)reaction between the 2,6-substituted phenylethynylbenzoxazole act as dienophile and diaiylacetylene act as diene.The cross photoreactions were carried out in methanol or in acetonitrile and the mechanism were also studied.Nine unreported new crossdimers,namely 1-(2,6-Dichlorophenyl)-2,3-bis(2-benzoxazolyl)phenanthrene,1-(2,4,6-trimethylphenyl)-2,3-di(2-Benzoxazolyl)phenanthrene,4-(2,4,6-trimethylphenyl)-2,3-di(2-benzoxazolyl)phenanthrene,1-(2,4,6-trimethylphenyl)-2-(2-benzoxazolyl)-3-phenylnaphthalene,1-(2,4,6-trimethylphenyl)-2-(2-benzoyl)Oxazolyl)-3-(4-methylphenyl)-7-methylnaphthalene,1-(2,4,6-trimethylphenyl)-2-(2-benzoxazolyl)-3-(4-chlorophenyl)-7-chloronaphthalene,1-(2,4,6-trimethylpheny l)-2-(2-benzoxazolyl)-3-(4-chlorobenzene)-7-Chloronaphthalene,1-(2,4,6-trimethy lpheny l)-2-(2-benzoxazolyl)-3-naphthy lphenanthrene,1-(2,4,6)-trimethylphenyl)-2-(2-benzoxazolyl)-3-(4-trifluoromethylphenyl)-7-trifl uoromethylnaphthalene,were isolated and fully characterized by 1H NMR?13C NMR and HRMS(ESI).And the structures of four photocycloaddition products were confirmed by single crystal X-ray diffraction method.The cross-PDDA reaction between the 2,6-substituted phenylethynylbenzoxazole and diarylacetylene extends the photoreaction of the diarylacetylenes.Since the Diels-Alder reaction plays an important role in the construction of new carbon structures.We believe that the cross-PDDA reaction is also an emerging new reaction with important research value.