Observation of Borane–Olefin Proximity Interaction Governing the Structure and Reactivity of Boron‐Containing Macrocycles

• Published in: Angewandte Chemie Vol. 133; no. 26; pp. 14751 - 14756
• Main Authors: Murata, Yukihiro, Matsunagi, Kenta, Kashida, Junki, Shoji, Yoshiaki, Özen, Cihan, Maeda, Satoshi, Fukushima, Takanori
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 21.06.2021
• Subjects: Acidity; Alkynes; Boranes; Boron; Chemistry; density functional calculations; Functional groups; macrocycles; noncovalent interactions; reaction mechanisms
• ISSN: 0044-8249; 1521-3757
• DOI: 10.1002/ange.202103512

While attractive interactions between borane and olefin have been postulated to trigger various boron‐mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane–olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi‐step 1,2‐carboboration between a strained alkyne and 9‐borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi‐step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron‐mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes. With borane‐containing macrocyclic polyenes obtained by multi‐step insertion of a strained alkyne into 9‐borafluorene, the first direct crystallographic evidence of attractive borane–olefin interaction, which is beyond van der Waals force and governs the reactivity of the molecules, is provided.