Copper-Catalyzed Selective Cleavage Of Carbon-Carbon Bonds In α-Arylethanols And Arylethenes To Produce Aryl Nitriles

Aryl nitriles not only constitute key components of many dyes,pharmaceuticals and agrochemicals,but also can be used as versatile intermediates for synthesizing amines,aldehydes,amides,nitrogen-containing heterocycles and so on.Thus,their synthesis plays an important role in modern organic synthesis.At present,chemists have developed many methods for synthesizing aryl nitriles.They can be synthesized based on cleavage of carbon-halogen bonds in aryl halides,condensation of aryl aldehydes with ammonia,dehydration of aryl oximes,oxidative dehydrogenation of benzylamine and cleavage of carbon-boron bonds in aryl boronic acids.On the other hand,α-arylethanols and arylethenes are common and easily available organic substances,especially for the α-arylethanol structures that widely exists in the renewable resource like lignins.If conversion of them to aromatic nitriles can come true,an effective avenue to synthesize aromatic nitrile will be obtained.Obviously,it is considerably challenging to convert α-arylethanols and arylethenes to aryl nitriles because that the selective cleavage of inert carbon-carbon bond is quite difficult.Aiming at these challenges,conversion of α-arylethanols to aryl nitriles was achieved via screening a large number of catalysts,which not only provides an efficient method for the synthesis of aryl nitriles,but also enriches the activation chemistry of carbon-carbon bond.The detailed results are as follows:(1)To the best of our knowledge,there is no report on synthesis of aryl nitriles from α-arylethanols.However,such reactions have an important value in organic synthesis(for example,it can be used for conversion of lignin to high value-added aromatic nitrile products).In the present thesis,this kind of chemical transformations was,for the first time,realized by screening a large number of catalysts.The obtained optimal conditions were as follows: 0.5 mmol α-arylethanol,50 mol% Cu(OAc)2,0.75 mmol(NH4)2CO3,2 m L DMSO,4 MPa O2 and 120°C.Under the optimal conditions,the cleavage of C(OH)-C bonds in α-arylethanol proceeded smoothly,and aryl nitrile products were obtained in up to 81% yield.The method could tolerate many functional groups including alkyl,alkoxy,amino,chlorine,bromine,iodine,ester and cyano groups.This method was also applicable into the cleavage of β-O-4 and β-1 bonds in lignins.Based on our investigation,a plausible mechanism was proposed.The reaction starts with oxidative dehydrogenation of the hydroxyl group.Then the resulting ketone undergoes oxidative cleavage of the C(O)-C bonds to form aryl aldehyde,followed by condensation between aryl aldehyde and NH3 to give aryl nitriles.(2)In previous literatures,methods for synthesizing aryl nitriles from arylethenes have suffered from a serious disadvantage where expensive diacetoxyiodobenzene is used as the oxidant.Aiming at this problem,oxygen was used as the oxidant to achieve conversion of arylethene to aryl nitriles,and to avoid using expensive diacetoxyiodobenzene.The obtained optimal conditions were as follows: 0.5 mmol arylethene,15 mol% Cu(NO3)2,1 mmol(NH4)2CO3,2 m L DMSO,2 MPa O2,140 °C and 30 h.The method could tolerate various functional groups including alkyl,alkoxy,N,N-dimethylamino,chlorine,bromine,iodine,ester and cyano groups.This method was also effectively applied into conversion of naphthylethenes and N-heterocyclic arylethenes.Based on our investigation,a plausible mechanism was proposed.Firstly,arylethenes undergo aerobic oxidative cleavage of the C=C bonds to form aryl aldehydes,which is followed by condensation between aryl aldehydes and NH3 to give aryl nitriles.