Pd(0)催化1-R-3-苯基亚丙基环丙烷(R=Me/H)与呋喃甲醛[3+2]环加成反应机理的密度泛函理论研究

Density Functional Theory Study for Exploring the Mechanisms of the[3+2]Cycloaddition Reactions between 1-R-3-Phenylpropylidenecyclopropane(R=Me/H)and Furfural Catalyzed by Pd(0)

结合相对论赝势,用密度泛函理论方法M06优化了用模型Pd(PH_(3))_(2)模拟的Pd(PPh_(3))_(4)催化两个标题反应路径中各驻点的几何结构,并计算相应的热力学函数,结合能量跨度(Energetic span)分析,以探讨其机理.结果表明,两个催化反应主要经历三个步骤:(1)亚烷基环丙烷与Pd(PH_(3))_(2)催化剂的氧化加成,形成钯环丁烷中间体;(2)钯环丁烷与呋喃甲醛的羰基经过分步加成,形成六元氧杂金属环化合物;(3)Pd(PH_(3))_(2)催化剂经过还原消去从六元氧杂金属环化合物解离,得到最终的亚甲基四氢呋喃取代产物.能量跨度分析表明,两个反应中TOF(转换频率)决速过渡态(TDTS)都是还原消去步的过渡态,但二者中TOF决速中间体(TDI)不同.在呋喃甲醛溶剂中R=Me时自由能垒(能量跨度δE)比R=H时的低9.5 kJ/mol,估算得到120℃下两个催化循环的TOF之比TOF_(Me)/TOFH约为18,表明在相同条件下,Pd(PH_(3))_(2)对R=Me的反应的催化效率要远高于对R=H的反应,可以很好地解释120℃、Pd(PPh_(3))_(4)催化下,R=Me时比R=H时反应有高得多的产率的实验观察.

The optimal geometric structures and thermodynamic properties for all stationary points(reactants,intermediates,transition states,and products)on each pathway of the[3+2]cycloaddition reactions between 1-R-3-phenylpropylidenecyclopropane(R=Me/H)and furfural with model catalyst Pd(PH_(3))_(2)have been computed by using density functional theory(DFT)method with M06 functional combined with Stuttgart/Dresden relativistic effective core potential for exploring the mechanisms of the reactions catalyzed by Pd(PH_(3))_(4).Based on the obtained free energy curves,energetic span analyses were carried out.The calculated results show that both reactions go through three steps:(1)Pd(PH_(3))_(2)catalyst oxidatively added to methylenecyclopropanes,in which Pd inserted into the distal carbon-carbon bond of methylenecyclopropanes ternary ring to form palladium cyclobutane intermediate;(2)six-membered oxa-metal ring compounds were formed by stepwise additions of palladium cyclobutane intermediate with the carbonyl of furan formaldehyde;(3)Pd(PH_(3))_(2)catalyst dissociates from the six-membered oxy-hetero-metal ring compound through reductive elimination,and the five-membered-ring final products,methylene tetrahydrofurans were obtained.In addition,with the energetic span model proposed by Kozuch and Shaik,the calculated turnover frequency(TOF)-determining transition states(TDTS)for both catalytic cycles with R=Me/H are the same as the transition state(TS)of the reductive elimination step,however the TOF-determining intermediate(TDI)in each catalytic cycle is different from each other.As a result,the apparent free energy barrier(energetic span,δE)for the catalytic cycle with R=Me is 9.5 kJ/mol lower than that for the catalytic cycle with R=H,and the estimated TOF at 120℃for the catalytic cycle with R=Me is ca 18 times larger than that for the catalytic cycle with R=H.These indicate that Pd(PH_(3))_(4)is more effective for catalytic cycle with R=Me,which accounts well for the previous experimental observation that the yield for the reaction with R=Me is much higher than that for the reaction with R=H under the catalysis of Pd(PH_(3))_(4)at 120℃.