Study On CO₂-promoted Palladium-catalyzed Dehydration Of Allyl Alcohols To Prepare Conjugated Dienes

Because of its unique structure and reactivity,conjugated diene is widely used in the production of functional polymers and the synthesis of fine chemicals.Compared with the traditional methods for the synthesis of substituted conjugated dienes,such as Wittig reaction,alkenyl cross-coupling reaction,and nucleophilic substitution of propargyl carbonate,the dehydration reaction of allyl alcohol attracts wide attention since the substrates are easy to obtain and the reactions are easy to operate.However,the reaction still needs to use heterogeneous catalysts or carbon monoxide and be carried out at high temperature and other rigorous reaction conditions.In addition to being used as a renewable C1 raw material to synthesize value-added chemicals,carbon dioxide can also serve as an effective accelerator to promote a variety of reactions under milder conditions by forming more reactive intermediates.For example,when alcohol reacts with carbon dioxide to in situ form carbonate,the carbon-oxygen bond would be activated and thus easier to break.Based on the above insight,this thesis will design and construct a carbon dioxide-promoted palladium-catalyzed reaction system for the dehydration of allyl alcohol to synthesize a series of substituted conjugated dienes with high efficiency and selectivity under mild conditions.Firstly,the dehydration reaction of（E）-3-（2-naphthyl）-2-buten-1-ol was used as a model reaction,and through the investigation of the reaction conditions such as palladium catalyst,ligand,solvent,temperature and pressure of carbon dioxide,the optimal reaction conditions were determined as follow:5 mol%Pd（OAc）₂ as the catalyst,6 mol%（S）-BINAP as the ligand,2.0 MPa of CO₂,CH₃CN as the solvent,80 ^oC,12 h.Then,under optimal reaction conditions,more than twenty allyl alcohol compounds containing different substituents participate in dehydration reaction to give corresponding conjugated diene products in medium to high yields.Carbon dioxide can not only promote the reaction efficiency,but also increase the selectivity of the products.The reaction can be scaled up to the gram level and the yield is not affected.Compared with the dehydration reaction of allyl alcohol under reaction conditions above 100^oC using a heterogeneous catalyst or carbon monoxide as a promotor,the dehydration reaction of allyl alcohol promoted by carbon dioxide conducts safely under milder conditions.Finally,the synthesized conjugated diene compounds can be converted into other important synthetic intermediates through D-A reaction and transition metal-catalyzed addition reactions.Mechanism studies showed that the promotion of carbon dioxide and the catalysis of palladium catalyst are indispensable for the dehydration reaction.Isotope labeling experiments verified that carbon dioxide could react with allyl alcohol to form an allyl carbonate intermediate.The possible mechanism for the reaction is as follows:carbon dioxide would firstly react with allylic alcohol to form the corresponding allyl carbonate intermediate in which the C–O bond was activated,then the oxidative addition reaction of a palladium catalyst would generate aσ-type intermediate or[π-allyl-Pd]⁺complex,β-hydrogen elimination of[π-allyl-Pd]⁺complex would finally yield the substituted conjugated diene product and release carbon dioxide,meanwhile regenerating the palladium catalyst.In conclusion,this thesis developed a carbon dioxide-promoted palladium-catalyzed dehydration reaction of allyl alcohols,which provides a simple and efficient method for the synthesis of conjugated diene compounds under mild conditions.