Dipole‐bound states and substituent effects of Breslow intermediates in the enolate form

• Published in: Journal of the Chinese Chemical Society (Taipei) 2021-11, Vol.68 (11), p.2060-2070
• Main Authors: Hsieh, Ming‐Hsiu, Huang, Gou‐Tao, Yu, Jen‐Shiang K.
• Format: Article
• Language: English
• Subjects: Breslow intermediate; Dipoles; dipole‐bound state; Electronic structure; Electrons; Imidazole; N‐heterocyclic carbene; Oxidizing agents; Potential energy; radical; Redox reactions
• ISSN: 0009-4536; 2192-6549
• DOI: 10.1002/jccs.202100310

Breslow intermediates play crucial roles in both umpolung and redox reactions in N‐heterocyclic carbene catalysis. Compared to the well‐known nucleophilic character, the electronic structure of Breslow intermediates on the radical route is still unclear. We investigate the potential energy surfaces with high‐level ab initio methods for four typical Breslow intermediates in both of their enol and enolate forms. In the enol form, high energies of around 60 kcal/mol to the Rydberg‐like states and those higher than 120 kcal/mol to remove an electron demonstrate that the enol Breslow intermediates tend not to generate radicals unless strong oxidants are present. The low‐lying dipole‐bound states and small electron detachment energies in the enolate form in contrast show that the enolate Breslow intermediates are possible precursors to radicals. More importantly, metastable dipole‐bound states exist in the imidazole‐ and the triazole‐based enolate Breslow intermediates. Energies to detach one electron of several enolate Breslow intermediates reveal that the bulky and electron‐withdrawing groups stabilize the singlet ground states, which explains that the utilization of such substituents can lead to successful isolation for Breslow intermediates in experiments. Electronic structure of the enolate Breslow intermediates demonstrates that metastable dipole‐bound states could act as doorways to form radicals. Bulky and electron‐withdrawing substituents tend to stabilize the anionic ground states.