What is the purpose of NaBH4 in this coboxalamine alkylation?

[Mayhem]

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.

 

[TeethWhitener]

The borohydride reduces the cobalt from Co(III) to Co(I), which is strongly nucleophilic and displaces the iodide in the alkylation step.

 

[Mayhem]

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?

 

[TeethWhitener]

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.

 

[Mayhem]

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.

 

[TeethWhitener]

Homework problems are supposed to be posted in the homework sections.

 

[Mayhem]

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.