Freccero M, Gandolfi R, Rastelli A
Dipartimento di Chimica Organica, Universita di Pavia, V.le Taramelli 10, 27100 Pavia, Italy, and Dipartimento di Chimica, Universita di Modena, Via Campi 183, 41100 Modena, Italy.
J Org Chem. 2000 Sep 22;65(19):6112-20. doi: 10.1021/jo000569i.
The problem of competition between concerted and stepwise diradical mechanisms in 1,3-dipolar cycloadditions was addressed by studying the reaction between nitrone and ethene with DFT (R(U)B3LYP/6-31G) and post HF methods. According to calculations this reaction should take place via the concerted cycloaddition path. The stepwise process is a viable but not competitive alternative. The R(U)B3LYP/6-31G study was extended to the reaction of the same 1, 3-dipole with cyclobutadiene and benzocyclobutadiene. The very reactive antiaromatic cyclobutadiene has an electronic structure that is particularly disposed to promote stepwise diradical pathways. Calculations suggest that its reaction with nitrone represents a borderline case in which the stepwise process can compete with the concerted one on similar footing. Attenuation of the antiaromatic character of the dipolarophile, i.e., on passing from cyclobutadiene to benzocyclobutadiene, causes the concerted 1,3-dipolar cycloaddition to become once again prevalent over the two-step path. Thus, our results suggest that, in 1,3-dipolar cycloadditions that involve normal dipolarophiles, the concerted path (Huisgen's mechanism) should clearly overwhelm its stepwise diradical (Firestone's mechanism) counterpart.
通过使用密度泛函理论（R(U)B3LYP/6 - 31G）和后哈特里 - 福克方法研究硝酮与乙烯之间的反应，探讨了1,3 - 偶极环加成反应中协同和逐步双自由基机理之间的竞争问题。根据计算，该反应应通过协同环加成路径进行。逐步过程是一种可行但不具竞争力的替代途径。R(U)B3LYP/6 - 31G研究扩展到了同一1,3 - 偶极体与环丁二烯和苯并环丁二烯的反应。极具反应性的反芳香性环丁二烯具有一种电子结构，特别倾向于促进逐步双自由基途径。计算表明，其与硝酮的反应代表了一种临界情况，即逐步过程可以在类似基础上与协同过程竞争。亲偶极体反芳香性的减弱，即从环丁二烯转变为苯并环丁二烯时，会使协同的1,3 - 偶极环加成反应再次比两步路径更为普遍。因此，我们的结果表明，在涉及正常亲偶极体的1,3 - 偶极环加成反应中，协同路径（惠根斯机理）应明显胜过其逐步双自由基（费尔斯通机理）对应物。
