| 1,2,3-triazole and its derivatives are an important kind of five membered nitrogen-containing heterocyclic compounds in organic chemistry.Due to their advantages of many coordination sites,strong coordination ability and small size,they could coordinate with a lot of transition metals to form corresponding complexes,which exhibited unique structures and properties.In recent years,as a novel N,N-bidentate ligand,4-pyridiyl-1,2,3-triazole（PyTa）has been received increasingly interest in organometal-catalyzed reactions.However,most of these reactions were carried out in organic solvents.In this paper,we prepared two water soluble PEG-functionalized pyridine triazole ligands（MeO-PEG1900-PyTa and PyTa-PEG2000-PyTa）and applied them in aqueous palladium-catalyzed Suzuki reaction and copper-catalyzed aerobic oxidation of alcohols to aldehydes.The contents were as follows:Part Ⅰ,firstly,with monomethyl polyethylene glycol（MeO-PEG1900-OH）as starting material,PEG-functionalized pyridine triazole ligand（MeO-PEG1900-PyTa）was synthesized by three step sequence including methylsulfonation,azidation and[3+2]click cycloaddition with 2-acetylene pyridine.After that,the resulted MeO-PEG1900-PyTa ligand was mixed with different palladium sources（PdCl2 and Pd（OAc）2）in toluene at room temperature for 24 h to obtain the corresponding palladium complexes（Cat-1 and Cat-2）which was found to be easily soluble in water.The Cat-1 and Cat-2 were also characterized by NMR and MALDI-TOF-MS.Then,the catalytic activities of Cat-1 and Cat-2 were investigated in the aqueous Suzuki coupling reaction.With the cross-coupling of bromobenzene and 4-methyl phenylboronic acid to form 4-methyl biphenyl as a model reaction,a series of optimization experiments were performed and the optimized reaction conditions were obtained as follows:at 50 oC of reaction temperature,H2O:EtOH（9:1）as solvent,K2CO3（3.0 equiv.）as base,and 0.01 mol%catalyst loadings of Cat-1.Finally,under the optimized reaction conditions,a wide range of arylboronic acids and aryl bromides were reacted to form corresponding biphenyls in good to excellent yields with the catalyst TON numbers up to 1.0×104.Part Ⅱ,firstly,with the polyethylene glycol with the average molecular weight of2000 as starting material,PEG supported pyridine triazole ligand（PyTa-PEG2000-PyTa）was obtained by three steps sequence including methylsulfonation,azidation,[3+2]click cycloaddition with 2-acetylene pyridine.The resulted PyTa-PEG2000-PyTa ligand was thermo-stable and water soluble.It was also characterized by NMR and MALDI-TOF-MS.Then,the PyTa-PEG2000-PyTa was used as N,N-bidentate ligand in the Cu-catalyzed aerobic oxidation of primary alcohols to corresponding aldehydes.With the oxidation of benzyl alcohol to benzaldehyde as the model reaction,a series of reaction conditions such as copper sources,bases,piperidine nitroxyl radicals and ligands were screened.The optimized reaction conditions were obtained as followings:5 mol%of Cu（OTf）2 as catalyst,5mol%of TEMPO as co-catalyst,2.5 mol%of PyTa-PEG2000-PyTa as ligand,20mol%K2CO3 as base,oxygen balloon as oxygen source,in pure water and at room temperature（25 oC）.Under the optimized reaction condition,a wide range of aromatic,heteroaromatic and allylic primary alcohols were smoothly converted to corresponding aldehydes in good to excellent yields（91%99%）.Moreover,the ligand could be recycled for at least 7 times without significant loss of activity.The PEG-functionalized pyridine triazole ligands proposed in this study are readily available,inexpensive and easy to prepare.They could be used as water-soluble ligand in aqueous organometal-catalyzed reactions. |
