Palladium-Catalyzed Allylation And Allenylation Of Benzyl Chlorides

The synthesis of phenylbutenes and benzylallenes has always been a hot topic in organic chemistry.The phenylbutenes and benzylallenes play an important role in the synthesis of medicine and natural products.In the traditional synthesis method,Lindlar catalyst is not in line with the concept of green chemistry because of its toxic heavy metal lead.The Grignard reaction method cannot react with raw materials containing active groups,so the substrate adaptability is poor.New synthesis methods include electrochemical coupling,dehydration and reduction of carboxylic acid,but there are also some disadvantages,such as unstable reaction conditions,difficult to obtain raw materials and so on.Therefore,it is of great significance to develop new green and simple synthetic methods.In this paper,the coupling reaction of benzyl chloride with allyl pinacolborate and allenyl pinacolborate catalyzed by palladium catalyst was realized under the synergistic effect of ligand and heat,which has the advantages of mild reaction conditions and wide substrate adaptability.The reactions of benzyl chloride and 1-chloromethylnaphthalene with allyl pinacolborate were used as model reactions.The effects of catalyst,ligand,solvent and temperature on the reactions were systematically investigated.The reaction conditions were as follows:When benzyl chloride was used as substrate,the reaction was conducted in the presence of Pd（OAc）₂（5 mol%）as catalyst,Xantphos（7.5 mol%）as ligand,Cs F（3.0 equiv.）as additive,THF（2 m L）as solvent at 80oC under nitrogen atmosphere for 12 h.Menwhile,when1-chloromethylnaphthalene was used as substrate,the reaction was carried out in the presence of Pd（PPh₃）₂Cl₂（5 mol%）as catalyst,Xantphos（7.5 mol%）as ligand,Cs F（3.0 equiv.）as additive and DCE（2 m L）as solvent at 80oC under nitrogen atmosphere for 12 h.Under the optimized reaction conditions,allyl pinacolborate can react smoothly with benzyl chlorides containing electron-donating groups,electron-withdrawing groups,large steric hindrance groups and active groups,giving phenylbutenes as terminal products.The yield of p-methylbenzyl chloride is the highest,which can reach 83%.The positions of substituents include ortho,meta and para.Allenyl pinacolborate can also react with benzyl chlorides containing electron-donating groups,steric hindrance groups and active groups,giving benzylallenes as terminal products.The yield of p-methylbenzyl chloride is the highest,which can reach 76%.The control experiments proved that the reaction needs to be regulated by heat and bidentate phosphine ligand.The products were characterized by ¹H NMR,¹³C NMR,HRMS and IR spectrum.