JACS: bismuth catalyzed transfer hydrogenation

Precious metal catalysts have been playing an important role in organic synthesis for a long time, but the transition development and use of precious metals will inevitably lead to the decrease of precious metal reserves and the increase of prices Therefore, in recent years, chemists have paid more attention to the first line of transition metals with more abundant reserves to find sustainable substitutes for precious metals Recent reports show that P (III) / P (V) redox platform shows good catalytic activity for various transformations (figure 1a) This pioneering method opens the door for the search for excellent redox catalysts other than transition metals Bismuth is a non-toxic and more abundant element than ordinary transition metals At present, the use of bismuth in organic synthesis is mainly based on the stoichiometric reaction of Bi (V) or Bi (III), and the catalytic strategy mainly uses the soft Lewis acid property of Bi (III) (figure 1b) Because it is difficult to separate low valent Bi (I) compounds, they are usually prepared by in-situ dehydrogenation of highly unstable Bi (III) dihydrides coupled with ligands However, compared with the method of using high price bismuth, people pay little attention to low price Bi (I) (source: J am Chem SOC.) recently, the Josep Cornella group of Max Planck Institute of coal research in Germany reported the transfer hydrogenation with Bi (I) complex as catalyst and aminoborane (AB) as transfer reagent Compared with phosphorus catalysis, some functional groups which are sensitive to low-cost transition metals perform well under Bi (I) catalysis, and show orthogonal reactivity The mechanism study shows that bismuth species play a key role in hydrogenation and hydrogen formation The results were published in J am Chem SOC (DOI: 10.1021 / JACS 9b00594) Firstly, the transfer hydrogenation conditions were optimized with azoarene 2A as substrate and Bi (I) complex (1) as catalyst (Table 1) With THF as solvent, in the presence of 1 mol% 1, 1.0 equivalent AB and 1.0 equivalent H 2O, the target product 3A can be obtained in 99% yield Without catalyst or AB complex, the reaction can not be carried out (source: J am Chem SOC.) then, the author studied the application range of different azoarenes (Table 2) Both electron rich and electron deficient azaarenes can be reduced In addition, asymmetric azoarenes are also suitable substrates Some sensitive groups (such as bromine and iodine) can be tolerated Furthermore, the scheme is extended to the reduction of nitroaromatics In contrast to the reduction catalyzed by most transition metals, the scheme is highly selective for the formation of n-arylhydroxylamine By slightly modifying the reaction conditions, all kinds of nitroaromatics can be reduced and the corresponding products can be obtained in excellent yields When nitroaromatics with higher steric hindrance were used, the yield was lower Finally, the author studies the transformation mechanism of the reaction (scheme 1) The results show that Bi (I) can promote the slow dehydrogenation of ab In addition, the hydrogen bond between AB and water has a positive effect on hydrogen release, and the proton of AB plays a key role in the transfer hydrogenation (source: J am Chem SOC.) through the kinetic isotope effect experiment and the competition experiment between two kinds of azoarenes with different electrical properties, the author further determined that the N-H and B-H bond breaks are the critical steps of the reaction (RDS), and Bi (I) and ab participate in RDS However, when H 2 O exists in the system, the reaction rate is significantly accelerated, which indicates that H 2 O may interact with AB through hydrogen bonds and participate in RDS (Scheme 2) (source: J am Chem SOC.) (source: J am Chem SOC.) although Bi (III) - H compounds are highly unstable and reactive, the author designed a series of experiments to clarify the existence of these compounds (scheme 3) (source: J am Chem SOC.) conclusion: the Josep Cornella research group of Max Planck Coal Research Institute in Germany has developed a new strategy of catalytic oxidation-reduction conversion involving bismuth This conversion represents a unique example of nitrogen group elements in low-cost catalytic cycle, which can be used for hydrogenation of azoarenes and partial reduction of nitroarenes.