The Research Of PEG-functionalized Pyridine Triazoles/Copper/ABNO-Catalyzed Aerobic Oxidation Of Secondary Alcohol

Oxidation of alcohols to aldehydes and/or ketones is one of classic organic transformations in synthetic chemistry with wide applications in chemical industry.It was undoubted that the use of oxygen as oxidant for such reactions was the best choice due to its high atom economy,minimal environmental effect as well as low cost.Therefore,over the past two decades,the development of high efficient,economic,safe catalytic systems for alcohol oxidation has been one of the hot research topics in the field of organic chemistry which has important theoretical and practical values.4-Pyridyl-1,2,3-triazole(Py Ta)was a novel N,N-bidentate ligand with many coordination sites,strong coordination ability and easy to be prepared.In this thesis,polyethylene glycol PEG2000(average molecular weight of 2000)functionalized pyridine triazole(PEG-Py Ta)was prepared and applied as a water-soluble N,N-bidentate ligand in the copper catalyzed aerobic oxidation of secondary alcohols to ketones in combination of ABNO as co-catalyst in water.Firstly,with commercially available polyethylene glycol PEG2000 as starting material,PEG-functionalized pyridine triazole ligand(PEG-Py Ta)was synthesized by three reaction sequences including mesylation with Ms Cl,azidation with Na N3 and [3+2] cycloaddition with 2-ethynylpyridinethe.Its structure was characterized by 1H NMR,13 C NMR and MALDL-TOF-MS.Then,PEG-Py Ta was used as a N,N-bidentate ligand for the copper salt/ABNO-catalyzed aerobic oxidation reaction of secondary alcohols.With the oxidation reaction of 1-phenylethanol to acetophenone as a model reaction.a series of reaction conditions were optimized and the optimal reaction conditions was obtained as: 5 mol% Cu SO4 as a catalyst,2.5 mol% PEG-Py Ta as a ligand,3 mol% ABNO as a co-catalyst,an aqueous KHCO3/KOH buffer solution(p H = 10)as reaction medium,at room temperature(25 °C)for 6 h,oxygen as terminal oxidant.After that,the substrate scope of the developed catalytic system was studied.The experimental results showed that a wide range of substituted aromatic(including O-,S-contained heterocyclic)and allylic ?-secondary alcohols could totally converted to corresponding ketones with absolute selections under the typical reaction conditions.For the totally conversion of some several sterically hindered alcohols,such as 1-phenylpropan-1-ol,1-(p-tolyl)propan-1-ol,1-phenylbutan-1-ol,2-methyl-1-phenyl-propan-1-ol,9H-fluoren-9-ol,more basic aqueous reaction condition(p H =11)was required.We also were delighted to find that the catalytic system was capable to oxidize more difficult aliphatic secondary alcohols,such as,4-phenylbutan-2-ol,2-octanol.After increasing amount of ABNO to 8% and prolonging reaction time to 24 h,these substrates could totally converted to corresponding ketones in aqueous KHCO3/KOH buffer solution(p H = 11).All of these ketone products were isolated in 90-99% yields.Unfortunately,we found that the catalytic system was inefficient in the oxidation of N-and CN-containing substrates which maybe ascribed to their competing coordination to catalytic Cu(II)cation.Finally,the reusability of the catalytic systems was studied.With the oxidation reaction of 1-phenylethanol to acetophenone as a model reaction,we found that the conversion of 1-phenylethanol still could be reached to 96% at the 12 th recycles,which indicated that the catalytic system could be reused at lest for more than 12 times without obvious loss of its activity.As a conclusion,we developed a novel catalytic system for the aerobic oxidation of secondary alcohol to ketones in water.The catalytic system has advantages of mild reaction conditions,broad substrate scopes,simple product separation,and robust reusability.In addittion,since the reaction process did not require the addition of any organic co-solvents,it was very safe and has potential for practical applications.