How to Bend a Cumulene

• Published in: Chemistry : a European journal Vol. 26; no. 20; pp. 4633 - 4639
• Main Author: Barquera‐Lozada, José Enrique
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 06.04.2020
• Subjects: Acidity; Bend strength; carbenes; carbon; Chemistry; cumulenes; density functional calculations; Linearity; Mathematical analysis; Molecular chains; nitrogen heterocycles; carbenes; density functional calculations; cumulenes; carbon; nitrogen heterocycles
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202000025

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−1 more stable than the [2n−1]cumulenes. Bend it: [n]Cumulenes are known for the linearity of their carbon chains, but when the terminal groups are N‐heterocyclic carbenes, this situation changes. The even cumulenes of this family can be bent depending on the π acidity of the carbene terminal group. Poor π‐back‐donation from the carbon chain to the terminal group promotes a significant bending in the carbon chain.