Photo-dehydro Diels-alder Reaction Of 2,6-positions Blocked Phenylethynylpyridine

Photo-dehydro Diels-Alder(PDDA)reaction,reported by Pablo Wessig in the past decade,is a new photocycloaddition reaction.The PDDA reaction of arylacetylene ethers,arylacetylene ketones and arylacetylene esters was studied in turn.Arylsubstituted naphthalene and naphthalene derivatives were isolated.In this paper,three 4-(2,6-di-R-styryl)pyridines were synthesized from the 2,6-positions blocked benzaldehyde and 4-methylpyridine,two 2-(2,6-di-R-styryl)pyridines from 2,6-positions blocked benzaldehyde and 2-methylpyridine.Five 4-(2,6-di-R1-R2-phenethynyl)pyridines(R1=C1,R2=H;R1=F,R2=H;R1?R2=Me)and 2-(2,6-di-R1-R2-phenethynyl)pyridines(R1=Cl,R2=H;R1?R2=Me)were synthesized from the corresponding diarylethenes by bromination with Br2 and then debromination with alkali.The photocycloaddition reactions of the phenylethynylpyridine monomers were carried out in the 1 mol/L aqueous hydrochloric acid solution under the irradiation of 300W high-pressure mercury lamp.The photocycloaddition reaction mechanism was discussed.Seven unreported halo addition products with the structure of 7-(4-pyridyl)-6,8-bis(2,6-dichlorophenyl)isoquinoline,3,5-bis(4-pyridyl)-2,4-bis(2,6-dichlorophenyl)furan,7-(4-pyridyl)-6,8-bis(2,6-difluorophenyl)isoquinoline,5,7,8-trimethyl-l,3-bis(4-pyridyl)-2-(2,4,6-trimethylphenyl)naphthalene,3,5-bis(4-pyridyl)-2,4-bis(2,4,6-trimethylphenyl)furan,2-(2,6-dichlorophenyl)-1-[(2,6-dichlorophenyl)(methyloxy)methyl]-3-(2-pyridyl)indolizine,5,7,8-trimethyl-1,3-bis(2-pyridyl)-2-(2,4,6-trimethylphenyl)naphthalene were synthesized.All the eight unreported halo addition products were determined by 1H NMR?13C NMR and HRMS(ESI).And the structures of the new halo addition products were confirmed by single crystal X-ray diffraction method.In this paper,we found that the key diradical intermediates were formed by the monomer upon the UV irridiation.The results indicated that not only the triarylisoquinolines via a PDDA reaction pathway were generated,but also the tetraarylsubstituted furans from the same key diradical intermediate of the PDDA reaction with O2 were generated.The PDDA photoproducts of 2,4,6-trimethyl substituted phenylethynylpyridines were triaryl naphthalenes rather than the triarylisoquinolines,it was ascribed to the migration of the methyl on the 2-position of the phenyl ring which was activated by the three substituted methyl groups.The presence of the key diradical intermediates were demonstrated in this paper.The PDDA reaction of diarylacetylenes is proved to be an important photocycloaddition reaction.The studies in this thesis expanded the scope of diarylacetylenes photocycloaddition reaction.It was found that the pyridinium group could be involved in the PDDA reaction and led to the formation of triaryl substituted isoquinolines.