Study On The Photocyclodehydrogenation Reaction Of O-diaryl Compounds Containing α, β-unsaturated Ketone Or Indole Structural Unit

Fused heteroaromatic compounds was an important component of organic compounds,which has already been applied to the areas of functional materials,medicine and biology.Hence,the efficient synthesis of fused heteroaromatic compounds has important research value.Recently,the synthetic methods of fused heteroaromatic via intramolecular photocyclization reactions usually can be summarized in three strategies:Mallory reaction,photocatalysis cyclization,and elimination photocyclization reaction.In these three reactions,the Mallory reaction always requires iodine as the oxidant;the photocatalysis cyclization reaction was usually limited to employ some expensive metals as the catalyst,such as the Ru,Rh,Ir,etc,and sometimes it needs to add additional ligand;Last but not least,the presence of leaving groups,such as chlorine,bromide and hydroxyl in the ortho-position of phenyl ring of stilbene analogues,is necessary for the successful elimination during photocyclization.Therefore,how to synthesize the fused heteroaromatic compounds efficiently and environmentally is one of the hotspots in organic chemistry.In this paper,a series of fused heteroaromatic compounds were synthesized by constructing carbon（sp²）-carbon bonds through the dehydrogenative process of 6π-electrocyclization,thermal[1,5]-H shift,keto-enol tautomerism or nitrogen atom induced proton isomerization without the addition of acid/base and transition metal catalysts.In the first part of this paper,the main intramolecular photocyclization reactions for synthesizing fused heteroaromatic compounds were reviewed,including several methods such as Mallory reaction,photocatalysts reaction,elimination photocyclization and photocyclization reactions of without additives.In the second part,14H-dibenzo[a,c]xanthen-14-one derivatives were synthesized by the photocyclization reactions.2,3-diphenyl-4H-chromen-4-one derivatives were obtained via Suzuki coupling reaction of 3-iodoflavone and phenylboronic.Then,the solution of 2,3-diphenyl-4H-chromen-4-one in 95%ethanol(5×10^-3 mol/L)was irradiated with a 500 W highpressure mercury lamp at room temperature under an argon atmosphere to give 26 kinds of 14H-dibenzo[a,c]xanthen-14-one derivatives in the yield of 54-81%.The structures of desired products were characterized by ¹H NMR,¹³C NMR,HRMS and IR.Finally,the reaction mechanism was analyzed and discussed through the hydrogen detecting and reductive carbonyl experiment.The results showed that the presence of the intramolecular carbonyl group in 2,3-diphenyl-4H-chromen-4-one could result in the keto-enol isomerization with release of the hydrogen gas.In the third part,π-expanded maleimide derivatives were synthesized from 3,4-diarylfuran-2（5H）-one compounds by the“two-step”reactions.The reaction took the phenylacetic acid and phenacyl bromide as the initial materials and was allowed to react in acetonitrile under the presence of triethylamine and 1,8-diazabicyclo[5.4.0]undec-7-ene（DBU）to offer 3,4-diphenylfuran-2（5H）-one compounds.Then,lactones were obtained via a photoinduced dehydrogenative annulation of 3,4-diphenylfuran-2（5H）-ones in ethanol at the room temperature,followed by interacted with primary amines in the presence of DBU under an oxygen atmosphere,givingπ-expanded maleimide derivatives in 52-81%yield.This reaction could achieve the synthesis of a highly conjugated system maleimide compounds in the absence of metal catalyst,which has the advantages of highly step-economic and broad substrate scope.Additionally,the fluorescence spectra and quantum yields of the highly conjugated maleimides were characterized and were found to possess high fluorescence quantum yields of 95%in dichloromethane solution.In the fourth part,3,4-diphenyl-3-pyrrolin-2-ones was irradiated at room temperature to provide dibenzo[e,g]isoindole-1-ones.N-acetyl-3,4-diphenyl-1,5-dihydropyrrol-2-one could deacetylate and form 3,4-diphenyl-3-pyrrolin-2-ones in methanol using sodium methoxide（ω=30%）as base.Then,irradiation of 3,4-diphenyl-3-pyrrolin-2-ones in ethanol solution（10 m M）at room temperature was performed to afford dibenzo[e,g]isoindole-1-ones with the yield of 52-78%.The reaction mechanism research found that this reaction could undergo the route of[1,5]-hydrogen shift or[1,3]-hydrogen shift.Compared with the Scholl-type oxidative reaction,the described method eliminates the usage of any strong oxidants/acids and has good functional group tolerance and electron-withdrawing groups（-CN,-F）,give the corresponding dehydrogenation products.In addition,the annulation products dibenzo[e,g]isoindole-1-ones,were successfully oxidized into dibenzo[e,g]isoindole-1,3（2H）-diones in dimethyl sulfoxide in the presence sodium methoxide.In the five part,dibenzo[a,c]carbazole derivatives were obtained via photoinduce dehydrogenative under transition-metal-free conditionds.1,2-diarylethan-1-ones were obtained by the Friedel-Crafts reaction of 2-phenylacetyl chloride with benzene in the presence of Al Cl₃,followed by treated with phenylhydrazine via Fischer reaction to afford2,3-diphenyl-1H-indoles.Finally,it was irradiated by UV lamp（30 Watt,λ=365 nm）in ethanol and gave 20 kinds of dibenzo[a,c]carbazoles with the yield of 50-92%.A novel mechanism of dehydrogenation reaction was proved through the mechanism verification experiments,containing four steps:6π-electrocyclization,thermal[1,5]-H shift,nitrogen atom induced proton tautomerism,and cyclization with hydrogen evolution.In this paper,a novel reaction for synthesizing a series of nitrogen and oxygen-contained polycyclic aromatic compounds was established via photoinduced intramolecular dehydrogenative cyclization of 2,3-diphenyl-4H-chromen-4-ones,3,4-diarylfuran-2（5H）-ones,3,4-diphenyl-1H-pyrrol-2（5H）-ones and 2,3-diphenyl-1H-indoles compounds.Compared with reported methods,the described approach does not require acid/base and metal catalyst and offers several notable advantages such as simple operation,mild reaction conditions,which provides a simple and green access to synthesize fused aromatic heterocylic derivatives.