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Free keywords:
Azoarene; Main Group Catalysis; Organobismuth; Reaction Mechanism; Transfer Hydrogenation
Abstract:
Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1a (2,6-bis[N-(tert-butyl)imino]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1a and 4, an equilibrium between 1a and 1a·[OAr]2 (Ar = p-CF3-C6H4) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pKa-gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1a·[OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1a in the presence of 5. DFT calculations indicate a rate-limiting transition state in which the initial N–H bond is formed via concerted proton transfer upon nucleophilic addition of 1a to a hydrogen-bonded adduct of azobenzene and 4. These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.
