Why adding sodium bicarbonate quenches a reductive amination using sodium triacetoxyborohydride?

I'm planning to do a reductive amination reaction between methyl pyruvate and benzylamine using sodium triacetoxyborohydride, $\ce{NaBH(OAc)3}.$ Acetic acid is used as a catalyst for protonating the iminium ion intermediate. I know that at the end of the reaction, $\ce{NaHCO3}$ is added to quench the reaction by neutralizing any unreacted acetic acid.


However, $\ce{NaHCO3}$ also reacts with any excess $\ce{NaBH(OAc)3}.$ I'm stuck on the mechanism of how this step works. This step should end up with $\ce{B(OAc)3}$ so a hydride ion is released, but I don't think hydride ions react with bicarbonate ions. References and videos I've watched used $\ce{NaHCO3}$ for quenching anyways, which confuses me. Any help?




According to Grice et al. [[1](https://doi.org/10.1016/j.fuel.2015.02.007)], the addition of the bicarbonate anions to the reaction mixture containing excess sodium triacetoxyborohydride and acetic acid may trigger an attack of the hydride anion on the carbon of the bicarbonate ions forming formate ions with the expulsion of the hydroxide group from the bicarbonate to give sodium hydroxide.


They describe the reaction of sodium borohydride with $\ce{CO2}$ or $\ce{HCO3-}$ and although $\ce{NaBH4}$ is a stronger reducing agent, the reaction with $\ce{NaBH(OAc)3}$ still yields the formate anions. Since this reaction occurs in addition to the neutralization reaction with acetic acid, I think it may also help in quenching the excess reducing agent.


### Reference


1. Grice, K. A.; Groenenboom, M. C.; Manuel, J. D. A.; Sovereign, M. A.; Keith, J. A. Examining the Selectivity of Borohydride for Carbon Dioxide and Bicarbonate Reduction in Protic Conditions. *Fuel* **2015**, 150, 139–145. DOI: [10.1016/j.fuel.2015.02.007](https://doi.org/10.1016/j.fuel.2015.02.007).


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