Barquera-Lozada José Enrique
Instituto de Química, Universidad Nacional Autónoma de México, Circuito exterior, Ciudad Universitaria Coyoacán, México, D.F., 04510, Mexico.
Chemistry. 2020 Apr 6;26(20):4633-4639. doi: 10.1002/chem.202000025. Epub 2020 Mar 18.
Allenes (carbodicarbenes) and [3]cumulenes are linear carbon chains that can be bent when the terminal group has a strong carbene nature. This bending can be quite pronounced in allenes but not in [3]cumulenes. In this study, how N-heterocyclic or cyclic (alkyl)(amino) carbene (NHC and CAAC, respectively) terminal groups can modify the linear structure of [n]cumulenes has been analyzed. A low π acidity of the terminal carbene affects the linearity of [2n]cumulenes. Indeed, it has been found that the NHC [4]cumulene is extremely bent, contrary to classical [4]cumulenes. The predicted NHC [4]cumulene or tricarbodicarbene has two lone pairs and the π electrons are delocalized over the whole molecule. More significantly, DFT calculations have shown that this bent [4]cumulene is very stable, considerably more so than the corresponding [3]cumulene, which has been elusive to synthesize. Remarkably, calculations have shown that all the NHC [2n]cumulenes are more than 25 kcal mol more stable than the [2n-1]cumulenes.
丙二烯（碳二卡宾）和[3]累积多烯是直链碳链，当端基具有强卡宾性质时会发生弯曲。这种弯曲在丙二烯中可能非常明显，但在[3]累积多烯中则不然。在本研究中，分析了N-杂环或环状（烷基）（氨基）卡宾（分别为NHC和CAAC）端基如何改变[n]累积多烯的线性结构。端基卡宾的低π酸度会影响[2n]累积多烯的线性度。事实上，已发现NHC [4]累积多烯极度弯曲，这与经典的[4]累积多烯不同。预测的NHC [4]累积多烯或三碳二卡宾有两个孤对电子，且π电子在整个分子中离域。更重要的是，密度泛函理论计算表明，这种弯曲的[4]累积多烯非常稳定，比相应难以合成的[3]累积多烯稳定得多。值得注意的是，计算表明所有的NHC [2n]累积多烯比[2n - 1]累积多烯稳定超过25 kcal mol。
