Study On Etherification And Reduction Of Alcohols Catalyzed By Metal-free System

Alcohols are one of the most abundant organic compounds,and hydroxyl groups are widely present in natural products.Alcohols are important synthetic intermediates and can be easily introduced or transformed into products containing many functional groups.Although the conversion of alcohols has always been a hot issue in organic synthesis,it is difficult to achieve the direct reduction and dehydration etherification of alcohols due to the high dissociation energy and high kinetic energy barrier of C-O bond.Therefore,the activation of alcohols into active intermediates and then conversion is a widely used path,but the two-step conversion method often leads to problems such as low atomic utilization rate,increased waste and higher cost.In the developed direct catalytic conversion system of alcohol,metal complexes or Lewis acids are the main catalysts,which are easy to cause metal pollution.Therefore,it is of great significance to construct a metal-free green catalytic system to realize the conversion of alcohol.In this paper,the catalytic effect of Br?nsted acid and acid functionalized ionic liquid as catalysts on C-O bond breaking was investigated.Three metal-free catalytic systems for direct conversion of alcohols were constructed:（1）Using Br?nsted acids as metal-free catalyst and two different alcohols as reaction substrates,by changing the condition such as temperature,time,solvent and additives,in the green solvent CPME,the efficient dehydration etherification between different aromatic alcohols,aromatic alcohols and fatty alcohols was realized,and the highest could almost quantitatively obtain asymmetric etherification products.The catalytic system avoids the use of strong base and competitive elimination reaction,water was generated as the only by-product.The mechanism of reaction was preliminarily explored through controlled experiments,and the possible mechanism involving carbocation was proposed.（2）Direct metal-free reduction of highly active allyl alcohols was catalyzed by Br?nsted acids,avoiding the complicated reaction steps and metal contamination.Ts OH·H₂O was used as catalyst and p-methyl benzyl alcohol was used as reducing agent,olefins with 94%yield were obtained at 80 ^oC for 2 h.The system was mainly suitable for aromatic allyl alcohols with different substituents,and gram-scale experiment could be carried out smoothly.When allyl alcohols with different substituents at 1-and 3-positions were used as substrates,the double bond of the main product was close to the benzene side of the electron-donating group.Deuterium labeling experiments clearly indicated that the hydrogen was derived from methylene of p-methyl benzyl alcohol,and other controlled experiments indicated the presence of two ether intermediates in the reaction.The kinetic isotope effect（k _H/k _D=1.28）shows that C-H bond fracture is not the decisive step of the reaction.A possible mechanism involving carbocation,ether intermediates and hydrogen transfer was proposed.（3）A metal-free and recyclable catalyst for the direct reduction of aryl allyl alcohol by ionic liquids（ILs）was developed using p-methyl benzyl alcohol as reducing agent and DMC as green solvent.The catalyst could be recycled and reused,and its catalytic activity basically remained unchanged after 5 cycles,which was of great significance for environmental protection and efficient utilization.Infrared spectroscopy（ATR-FTIR）showed that ILs interact with substrates and reductants through multiple hydrogen bonds.On the basis of controlled experiments and spectrogram characterization,a possible reaction mechanism including hydrogen bond,hydrogen transfer and ether intermediates was proposed.