What is the purpose of NaBH4 in this coboxalamine alkylation?

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Discussion Overview

The discussion centers on the role of sodium borohydride (NaBH4) in the alkylation of coboxalamine, specifically regarding its interaction with cobalt and the subsequent reaction with ethyl iodide. The scope includes theoretical and conceptual aspects of the reaction mechanism.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • Some participants propose that NaBH4 creates a reactive, nucleophilic intermediate by reducing cobalt from Co(III) to Co(I), which can then perform an SN2 reaction on ethyl iodide.
  • Others argue that the reduction of cobalt leads to a nucleophilic Co(I) that displaces iodide during the alkylation step.
  • A participant questions the fate of the bromide substituent, suggesting that hydride may bind to bromine, resulting in HBr formation, but under alkaline conditions, bromide ion may remain.
  • Another participant reiterates the idea about the bromide substituent and mentions the stabilization of NaBH4 in KOH with methanol as the solvent.
  • There are comments regarding the classification of the discussion as homework, with some participants asserting that it is not traditional homework but rather pre-lab safety schemes.

Areas of Agreement / Disagreement

Participants express differing views on the mechanism of NaBH4's action and the fate of the bromide substituent, indicating that multiple competing perspectives remain without a consensus.

Contextual Notes

Some assumptions about the reaction conditions and the specific roles of the reagents are not fully detailed, and there is an unresolved discussion regarding the classification of the topic as homework.

Mayhem
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I am unsure what its role is. My intuition is that it somehow creates a reactive, nucleophilic intermediate which can perform SN2 on ethyl iodide, but my knowledge of the Co-Br bond is sparse and I need some pointers.
 
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The borohydride reduces the cobalt from Co(III) to Co(I), which is strongly nucleophilic and displaces the iodide in the alkylation step.
 
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TeethWhitener said:
The borohydride reduces the cobalt from Co(III) to Co(I), which is strongly nucleophilic and displaces the iodide in the alkylation step.
Thank you very much. What happens to the bromide substiuent?
 
Mayhem said:
Thank you very much. What happens to the bromide substiuent?
I’ve honestly never really thought about it before. I imagine the hydride binds to the bromine, pushing the electron pair from the Co-Br bond onto the cobalt center. So you’ll get HBr out, but these reactions are often done under slightly alkaline conditions, so it’ll probably be bromide ion at the end.
 
TeethWhitener said:
I’ve honestly never really thought about it before. I imagine the hydride binds to the bromine, pushing the electron pair from the Co-Br bond onto the cobalt center. So you’ll get HBr out, but these reactions are often done under slightly alkaline conditions, so it’ll probably be bromide ion at the end.
Yes, the NaBH4 is stabilized in KOH as the solvent is MeOH in this case. I turned in the assignment the other night, so I guess we will just wait and see.
 
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Homework problems are supposed to be posted in the homework sections.
 
TeethWhitener said:
Homework problems are supposed to be posted in the homework sections.
Eh, these were pre-lab safety schemes. I didn't consider it homework in the traditional sense.
 

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