镍配合物催化N-烯丙基酰胺异构化反应机理

摘要

Reaction mechanism of Ni(PPh3)2-catalyzed isomerization of N-allylamides to generate N-propenylamides was studied theoretically in detail by using density functional theory(DFT)method.The C—H bond activation,isomerization,and reductive elimination to form new C—H bond steps were involved.For C—H bond activation and isomerization steps,Ni(PPh3)2and Ni(PPh3)with only one PPh3ligand were considered and found that the former was more active than the later for these two steps.Both π-allyl and σ-al-lyl mechanisms were calculated for isomerization and found that the π-allyl mechanism was preferred kinetical-ly to the σ-allyl one.The rate-determining energy barrier to generate the E isomer of product is 141.8 kJ/mol,close to that of 141.1 kJ/mol to generate the Z isomer,in agreement with the experimental result that E/Z=56/44.Considering Pd(PPh3)2as the catalyst active species,it is found that the rate-determining energy bar-riers for the formation of E and Z isomers are more than 175 kJ/mol,consistent with the experimental observa-tion that Pd(PPh3)4showed no reactivity.The difference of reactivity between Ni(PPh3)2and Pd(PPh3)2 can be understood from the more strong back-donation of d electrons from Ni to π?anti-bonding orbital of allyl anion comparing with Pd.In addition,the influence of substituents in reactants on E/Z selectivity has been analyzed and found that the steric repulsion between substituents and Ph of PPh3in the rate-determining tran-sition states to generate E and Z isomers induces the difference of E/Z selectivity.%采用密度泛函理论(DFT)方法,对镍配合物Ni(PPh3)2催化N-烯丙基酰胺异构化生成N-丙烯基酰胺的微观反应机理进行了计算.反应涉及了C—H键活化、异构化及还原消除生成新的C—H键等步骤.对C—H键活化和异构化步骤,分别考虑了Ni(PPh3)2和Ni(PPh3)的催化活性,发现均为前者对应的能垒更低;对异构化步骤,分别考虑了π-烯丙基和σ-烯丙基机理,发现前者能垒更低.在整个反应路径中,生成产物E异构体的决速能垒为141.8 kJ/mol,与生成Z异构体的决速能垒(141.1 kJ/mol)仅差0.7 kJ/mol,与实验上E/Z选择性不高(56/44)一致.Pd(PPh3)2催化的决速中间体和过渡态的计算表明,生成E和Z异构体的决速能垒较高,均超过175 kJ/mol,与实验上Pd(PPh3)4没有催化活性一致.Ni(PPh3)2和Pd(PPh3)2催化活性不同,可由Ni的d电子对烯丙基阴离子π?反键的反馈作用较Pd更强来解释.此外,通过反应物中不同取代基对产物E/Z选择性影响的分析,发现E/Z选择性不同是由各取代基在生成E和Z异构体的决速过渡态中所受空间位阻不同所导致.