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首页> 外文期刊>Chemistry: A European journal >Thermal Rearrangements of 2-Vinylcyclopropylidene to Cyclopentadiene and Vinylallene: A Theoretical Investigation
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Thermal Rearrangements of 2-Vinylcyclopropylidene to Cyclopentadiene and Vinylallene: A Theoretical Investigation

机译：2-乙烯基环亚丙基热重排成环戊二烯和乙烯基烯丙基的理论研究

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In an attempt to clarify the favored rearrangement reaction of vinylcyclopropylidenes, the prototype thermal rearrangements of singlet 2-vinylcyclopropylidene (1) leading to 1,3-cyclopentadiene (20 and 1,2,4-pentatriene (vinylallene) (3) were investigated by means of ab initio quantum-mechanical electronic-structure calculations. The B3LYP functional with the 6-31G(d) basis set was employed for geometry optimization of the equilibrium and transition-state structures relevant to the two reaction pathways and for computing their harmonic vibration-al frequencies. Final energies were evaluated by single-point calculations at the CCSD(T) level of theory with the 6-311+G(3df, 2p) basis set. The rearrangement of s-cis 1 to 2 is found to occur by a three-step pathway. The first step involves the formation of a non-classical carbene (5), which is an internal #pi# complex between the #pi# molecular orbital of the double bond and the empty p atomic orbital of the carbene carbon. In the second step, the non-planar five-membered ring geometry of 5 flattens to reach the planar structure of 3-cyclopentenylidene (4). The last step is the 1,2-migration of a #alpha#-hydrogen atom to the carbene center in 4. The rate-determining step for the rearrangement of s-cis 1to 2 is the formation of 5, with a predicted global triangle openG~not=(220K) of only 0.6 kcal mol~(-1). The rearrangement of s-trans 1 to 2 requires an initial conversion of s-trans 1 to the s-cis conformer, with a predicted triangle openG~not=(220K) of 1.8 kcal mol~(-1). The transition structure for the ring-opening of s-trans 1 into s-trans 3 (not=(220K))=4.7 kcal mol~(-1)) is more energetic than that for the ring-opening of s-cis 1 into s-cis 3 (triangle openG~not=(220K)=2.5kcal mol~(-1)) due to larger repulsive nonbonded H…H interactions in the former transition structure. On the basis of these results, it is suggested that if the reaction of 1,1-dibromo-2-vinylcyclopropane with methyllithium at -78deg C leads to the initial formation of carbene 1, then the reaction should yield 2 as the main product together with small amounts of 3.This theoretical prediction nicely agrees with experimental findings.
机译：为了阐明乙烯基环丙叉基的有利重排反应，通过以下方法研究了单重态2-乙烯基环丙叉基（1）导致1,3-环戊二烯（20和1,2,4-戊三烯（乙烯基丙二烯）（3））的原型热重排。从头开始进行量子力学电子结构计算的方法，将具有6-31G（d）基集的B3LYP函数用于优化与这两个反应路径相关的平衡和过渡态结构的几何形状，并计算其谐波振动-al频率，通过在CCSD（T）的理论水平上以6-311 + G（3df，2p）为基础进行单点计算来评估最终能量，发现发生了s-顺式1至2的重排。第一步涉及非经典卡宾（5）的形成，它是双键的＃pi＃分子轨道与分子的空p原子轨道之间的内部＃pi＃配合物。卡宾碳。在第二个ep，5的非平面五元环几何形状会展平以达到3-环戊烯基的平面结构（4）。最后一步是4中＃alpha＃氢原子从1,2迁移到碳烯中心。s-顺式1至2重排的速率确定步骤是5的形成，具有预测的整体三角形openG_not =（220K）仅为0.6 kcal mol〜（-1）。 s-trans 1到2的重排需要将s-trans 1初步转化为s-cis构象异构体，预测的三角形openG_not =（220K）为1.8 kcal mol〜（-1）。 s-trans 1开环成s-trans 3的过渡结构（not =（220K））= 4.7 kcal mol〜（-1））比s-cis 1开环的过渡结构更有活力由于前一个过渡结构中较大的排斥性非键H…H相互作用，导致S顺式3（三角形openG〜not =（220K）= 2.5kcal mol〜（-1））。根据这些结果，建议如果-1，1-二溴-2-乙烯基环丙烷与甲基锂在-78℃下的反应导致卡宾1的初始形成，则该反应应一起产生2作为主要产物少量的3。该理论预测与实验结果非常吻合。

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