Visible Light-Induced Decarboxylation To Construct Aryl Aldehydes,Aryl Ketones And Diarylmethane Compounds

"Green chemistry" is also known as "sustainable chemistry",and its main meaning is to reduce or eliminate the use and production of harmful substances in the chemical conversion process.Current environmental chemists'focus mainly includes reducing the consumption of non-renewable resources and the development and application of green synthetic technology to minimize the environmental pollution caused by the chemical synthesis process.Light energy is a source of abundant,clean and renewable energy in nature.At present,more and more researches in the field of chemistry apply visible light to organic synthesis reactions to replace traditional organic chemical heating method,compared with conventional heating,the chemical reaction promoted by light is to directly convert light energy into chemical energy,and the whole process is mild and green.Carboxylic acid is widely present in nature.As the starting material of organic reaction,carboxylic acid has the advantages of high stability,easy storage,low cost and easy availability.At the same time,the decarboxylation of carboxylic acid is a way to generate active hydrocarbon radical,and the carbon dioxide(CO2)released during decarboxylation process is non-toxic and harmless.In addition,the use of oxygen in air to replace the oxidants used in the traditional synthesis and the abandonment of the widely used noble metal photocatalysts(iridium or ruthenium complexes)are also advocated in the current photocatalytic green synthesis.Therefore,the development of new metal-free green synthesis using carboxylic acids as feedstocks catalyzed by visible light has important practical application value.In this paper,starting from arylacetic acid,two new photocatalytic reaction strategies were developed to realize the synthesis of aryl aldehyde,aryl ketone and diaryl methane compounds under green and mild conditions,the specific contents are as follows:1)Visible light-induced oxidative decarboxylation of arylacetic acids to aryl aldehydes or ketones.Using arylacetic acid 1 as substrate,in the presence of 1 mol%of the photocatalyst(1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene,4CzIPN)and 50 mol%of organic base(1,1,3,3-tetramethyl guanidine,TMG),irradiated with blue LEDs for 6 hours,and the aryl aldehyde or ketone compound 2 was obtained by one-pot synthesis in an open system at room temperature.In-depth research on the mechanism had been carried out,and the results showed that the reaction was firstly induced by visible light to decarboxylation of arylacetic acid to generate active benzyl radical C,which then combined with air oxygen and further dehydrated to produce target product 2.Experiments such as fluorescence quenching and electron paramagnetic resonance(EPR)had verified the process of oxygen participation,which indicated that oxygen in air acted as a reaction oxidant.2)Visible light-induced decarboxylation/decyanation cross coupling to synthesize diarylmethanes.Using arylacetic acid 1 and cyano aromatic ring 2 as the substrates,4CzIPN as the photocatalyst,in the presence of potassium carbonate(K2CO3),irradiated with 10-watt blue LED in the nitrogen atmosphere at 30-40? to generate a series of diarylmethane compounds 3.In-depth study of the reaction mechanism showed that the arylacetic acid la first deprotonated under the action of the base,and then underwent a single electron transfer process with the excited state photocatalyst 4CzIPN*,and further extruded carbon dioxide to produce the corresponding benzyl radical A,at the same time 4CzIPN*was converted into the radical anion 4CzIPN·-.Subsequently,the 4cyanopyridine 2a was reduced by 4CzIPN·-to give radical anion B.Then,the radical intermediates A and B formed in the system provided intermediate anion C through radical coupling,finally,C removed the cyano group to obtain the target product 3a.