镍催化氮杂环丁酮和丁二烯环加成反应机制的理论研究

采用密度泛函理论(DFT)方法对镍催化1-Boc-3-氮杂环丁酮和2,3-二甲基-1,3-丁二烯的环加成反应进行了理论研究. 计算结果表明, 该反应采用氧化加成机制而非实验推测的β-碳消除机制. 氧化加成机制主要由3个基元反应步骤组成, 分别为氮杂环丁酮底物中C—C(=O)键的氧化加成、 二烯顺式插入Ni—C(=O)键、 以及还原消除生成八元氮杂环产物, 其中烯烃插入是整个反应的决速步骤, 反应能垒为86.74 kJ/mol. 通过探讨烯烃分别插入到Ni—C(=O) 键和Ni—C(sp³) 键的2种反应途径分析了烯烃插入步骤的区域选择性, 得到了与实验数据基本一致的结果.

Theoretical Mechanistic Study on Nickel-Catalyzed Cycloaddition of Azetidinone with Butadiene†

Density functional theory(DFT) calculations were carried out on the nickel-catalyzed cycloaddition of 1-Boc-3-azetidinone and 2,3-dimethyl-1,3-butadiene. The calculation results indicate that the reaction occurs via an oxidative addition mechanism rather than experimentally proposed β-carbon elimination mechanism. The important elementary steps consist of the oxidative addition of C—C(=O) bond of azetidinone to Ni(PPh₃)₂, cis-diene insertion into the Ni—C(=O) bond and the reductive elimination of an eight-membered heterocyclic product. The rate-determining step is the olefin insertion with an acceptable Gibbs activation energy of 86.74 kJ/mol. In addition, the regioselectivity of olefin insertion was investigated. The olefin insertion into Ni—C(=O) bond is more favorable than that into Ni—C(sp³) bond, which agrees well with the experimental observation.