[andeau]

Hi everyone,

I really need help with an organic chemistry exercise:
I have to write the mechanism of reaction for a reduction of an ester using trimethylaluminum in THF.
The final product is a primary alcohol.

Have you any idea about how the TMA is working in this case?

[Babcock_Hall]

According to the forum rules (red link above) you must show an attempt before we can help you.

[andeau]

Well, I have thought about the possibility that aluminum would complex the carbonyilic oxygen, but the thing is I don't actually understand how trimethylaluminium can donate an H.
I based my attempt on my knowledge on DIBAL and LiAlH4 in which the aluminum is just a catalyzer and complex the carbonyl in order to help the reduction.

My question is, can TMA donate an H (maybe from one of the methyls) in a complex? or there is another mechanism of reaction I don't see? or there is the THF involved as well (but for that I don't have a possible explanation)?

[Babcock_Hall]

I am not at all certain, but you may be on the right track in thinking in terms of catalysis.  Sorry that I cannot be of more help.

[rolnor]

Do you have an example where TMA is used as reducing agent?

[andeau]

Well, in my example I have to reduce 1H-indole-2-carboxylic acid, 1-methyl-5-nitro, ethyl ester (CAS 71056-57-0) to 1-methyl-5-nitro-1H-indole-2-methanol (CAS 500014-74-4)

I could not find any other reaction similar to that as an example!   

Thank you for trying, by any chance!

[phth]

https://en.wikipedia.org/wiki/Oppenauer_oxidation
https://en.wikipedia.org/wiki/Meerwein%E2%80%93Ponndorf%E2%80%93Verley_reduction

THF may be the hydride donor.  But this is one of the older redox equilibria.  Electron transfer is possible, so I am not 100% sure.  Try searching for the answer using a resource like scifinder.  Being able to find the answer is just as good as knowing it.

[andeau]

I have already saw that reductions, but they seems not to fit with my exercise!
And I probably read all the articles published on aluminum as a reducing agent on scifinder.
Well, I guess I will have to invent something