Visible-Light-Mediated Decarboxylative Functionalization Of Carboxylic Acids

The decarboxylation functionalization of organic carboxylic compounds is an important research topic in the field of organic synthesis.It provides a simple way to directly convert carboxyl groups to various valuable C-C and C-X bonds?e.g.,C-halogen,C-O,C-N,C-S and C-B bonds?.Compared with the traditional metal-catalyzed decarboxylation reaction,the mild and environmentally friendly visible-light-mediated decarboxylation functionalization has shown good prospects in recent years.As reported,under visible-light-mediated conditions,carboxylic acids and their esters through decarboxylation,can not only ungdergo alkylation,arylation,vinylation,allylation,alkynylization,and cyanation,but also can effectively connect with other heteroatoms to quickly increase the complexity of the molecule in a convenient way.Because the C-S structural unit exists widely in many drugs and natural products,the introduction of sulfur-containing functional groups into the organic molecular skeleton through a green and sustainable method has attracted widespread attention over chemists.In this paper,mild and environmentally friendly visible light-mediated decarboxylative strategies were used to promote the sulfonylation of unsaturated carboxylic acids,and further used in the absence of additives to promote the thiocyanation of unsaturated carboxylic acids,providing two green and convenient methods for the rapid construction of C-S bonds.In the second chapter,we mainly studied the synthesis of?-ketosulfone with the organic dye fluorescein as a photocatalyst and atoric acid and sulfonyl hydrazide as substrates via visible-light-mediated decarboxylative pathway.The optimal reaction conditions were determined after carefully screening influencing factors including photocatalyst,solvent,alkali,additive,feed ratio,gas atmosphere and light source.Then the substrate range was investigated,and 20?-ketosulfone compounds were obtained with the highest yield of 91%.Through radical quenching experiments,we successfully isolated 1,1-diphenylethylene adducts and 2,6-di-tert-butyl-4-methylphenol?BHT?adduct,respectively confirming that the sulfonyl radical and superoxide radical are key intermediates of this reaction.When doing isotope labeling experiments by using MeCN/H₂O¹⁸ as the reaction solvents,we can not detect the existence of the ¹⁸O-labled desired products,indicating that the oxygen atom of the carbonyl group in the desired products comes from dioxygen.The results of light on/off experiments showed that the reaction may involve a free radical chain process.In addition,through fluorescence quenching and cyclic voltammetry experiments,we concluded sulfonyl hydrazides may be responsible for quenching the excited photocatalyst via single electron transfer,and that the additive potassium iodide,not capable of quenching the excited photocatalyst,may work as reductants for reducing intermediates generated in the reaction.Via this mild,green visible-light-mediated decarboxylative strategy,non-toxic,cheap and easily available atoric acid and sulfonyl hydrazide were used as starting materials,and economically ubiquitous dioxygen as oxygen source and sacrificial oxidant,not only leading to the rapid construction of C-S bond and C=O bond in one pot,but also providing an environmentally-friendly and effective route for the preparation of?-ketosulfones.In the third chapter,we further studied the visible-light-mediated decarboxylative reaction for the construction of C-S bond without any additives by using ammonium thiocyanate as sulfur sources and inner reductants.After screening influencing factors including photocatalyst,feed ratio,solvent,gas atmosphere and light source,the optimal reaction conditions for this reaction were obtained.Then,the applicable range of the unsaturated carboxylic acid was investigated.As a result,its scopes were found to include aryl-substituted acrylic acid,alkyl-substituted acrylic acid and poly-substituted acrylic acid,and 15?-thiocyanone compounds were temporarily prepared.In addition,the potential applications of the desired product were explored.In simple basic or acidic conditions,we smoothly prepared disulfide compounds,cyclic compounds or aromatic compounds from the desired products.Through a series of mechanic experiments,our pre-estimated reaction mechanism was proveed,i.e.ammonium thiocyanate serves not only as sulfur sources to participate in the construction of C-S bonds,but also as an inner reductant to promote the reduction of intermediates.Simarily via visible-light-mediated decarboxylation strategy to construt C-S bond and C=O bond at the same time,this reaction,owing to no additives included,possesses higher atom economy.Moreover,this reaction can provide a green and effective route for the preparation of important?-thiocyanone compounds.