Rode Joanna E, Dobrowolski Jan Cz
Industrial Chemistry Research Institute, 01-793 Warsaw, 8 Rydygiera Street, Poland.
J Phys Chem A. 2006 Mar 16;110(10):3723-37. doi: 10.1021/jp055073p.
The reaction paths of [2 + 2] cycloadditions of allene (H2C=C=CH2) to isocyanic acid (HN=C=O) and ketene (H2C=C=O) to vinylimine (H2C=C=NH), leading to all the possible 14 four-membered ring molecules, were investigated by the MP2/aug-cc-pVDZ method. In the two considered reactions, the 2-azetidinone (beta-lactam) ring compounds were predicted to be the most stable thermodynamically in the absence of an environment. Although 4-methylene-2-azetidinone is the most stable product of the ketene-vinylimine cycloaddition, its activation barrier is higher than that for 4-methylene-2-iminooxetane by ca. 6 kcal/mol. Therefore, the latter product can be obtained owing to kinetic control. The activation barriers in the allene-isocyanic acid reactions are quite high, 50-70 kcal/mol, whereas in the course of the ketene-vinylimine cycloaddition they are equal to ca. 30-55 kcal/mol. All the reactions studied were found to be concerted and mostly asynchronous. Simulation of the solvent environment (toluene, tetrahydrofuran, acetonitrile, and water) by using Tomasi's polarized continuum model with the integral equation formalism (IEF-PCM) method showed the allene-isocyanic reactions remained concerted, yet the activation barriers were somewhat higher than those in the gas phase, whereas the ketene-vinylimine reactions became stepwise. The larger the solvent dielectric constant, the lower the activation barriers found. The lowest-energy pathways in the gas phase and in solvent were confirmed by intrinsic reaction coordinate (IRC) calculations. The atoms in molecules (AIM) analysis of the electron density distribution in the transition-state (TS) structures allowed us to distinguish pericyclic from pseudopericyclic from nonplanar-pseudopericyclic types of reactions.
采用MP2/aug-cc-pVDZ方法研究了丙二烯（H2C=C=CH2）与异氰酸（HN=C=O）以及乙烯酮（H2C=C=O）与乙烯亚胺（H2C=C=NH）的[2 + 2]环加成反应路径，这些反应会生成所有可能的14种四元环分子。在这两个所考虑的反应中，预测2-氮杂环丁酮（β-内酰胺）环化合物在无环境影响时在热力学上最稳定。虽然4-亚甲基-2-氮杂环丁酮是乙烯酮-乙烯亚胺环加成反应中最稳定的产物，但其活化能垒比4-亚甲基-2-亚氨基氧杂环丁烷的活化能垒高约6 kcal/mol。因此，由于动力学控制，可以得到后一种产物。丙二烯-异氰酸反应中的活化能垒相当高，为50 - 70 kcal/mol，而在乙烯酮-乙烯亚胺环加成过程中，它们约为30 - 55 kcal/mol。研究发现所有这些反应都是协同的且大多是非同步的。使用带有积分方程形式（IEF-PCM）方法的Tomasi极化连续介质模型对溶剂环境（甲苯、四氢呋喃、乙腈和水）进行模拟表明，丙二烯-异氰酸反应仍然是协同的，但活化能垒比气相中的略高，而乙烯酮-乙烯亚胺反应则变为分步反应。溶剂介电常数越大，发现的活化能垒越低。通过内禀反应坐标（IRC）计算确定了气相和溶剂中能量最低路径。对过渡态（TS）结构中电子密度分布进行的分子中的原子（AIM）分析使我们能够区分周环反应、假周环反应和非平面假周环反应类型。
