[Qfbale]

I have tried to reduce a methyl esther with LiAlH4 and  I have repeatedly gotten yield of 42%. I tried to increased the mmol of LiAlH4, dried better the ether, inert atmosphere, I´ve changed THF instead ether, reflux, increase the time, but always I have gotten 42%.

I've tried also using NaBH4 / I2 system did not work me . I believe the underperformance is how to assemble the reaction system , which consists of a three-necked flask in a mouth connect to reflux condenser , nitrogen injected into another , and another I inject solvent. My system is open , by flowing nitrogen so the ether evaporates quickly and must be injected quantities of ethyl ether . And maybe this will make the reducing power of LiAlH4 decrease. How to mount their reaction with nitrogen systems without being dry your system? Thank you.
Please

[zarhym]

I suggest you demethylate your ester first and then use BH3DMS to reduce your compound.
This does not require super dry ether as a solvent nor high temperature.
R.T. overnight should be enough. Quenching BH3DMS is a little bit tricky, slow quenching at low temperature is necessary.

Here is the detail about what I did.

I used ester to synthesis aldehyde with 3 steps.

The ester was converted to carboxilic acid with LiOH.
The carboxilic acid was reduced to alcohol using BH3DMS.
The alcohol was finally oxidized to aldehyde using MnO2.

All these reaction was about a kilogram scale.

The demethylation of ester is around 95% yield.

The carboxilic acid to aldehyde is about 52% for overall yield.

The main problem was MnO2 obsorbing my product. I believe if Dess-Martin was used, I should have better yield.

[kriggy]

Why not reduce the ester directly to alcohol using NaBH4-MeOH in THF (Ie. running the reaction in THF, add NaBH4 then add slowly methanol) Might find reference for this if you want

[zarhym]

Why not reduce the ester directly to alcohol using NaBH4-MeOH in THF (Ie. running the reaction in THF, add NaBH4 then add slowly methanol) Might find reference for this if you want

There are tons of reference for this reduction. However, it depends on your starting material.
Sometimes, when the ester or the carboxylic acid is attached to something special, such as an aromatic group, the same condition you used may not work anymore.

Perhaps, this is the reason that there are so many ways to do this type of reaction.   

[Doc Oc]

Is 42% your isolated yield or reaction conversion? Are you using solid LAH or a solution of it?

[orgopete]

If you draw the mechanism, then you should realize the 'hard' step is the elimination of the O-AlR_n from a tetrahedral intermediate. Therefore, it seems reasonable that different esters may result in greater or lower yields. The alternative to use a borohydride is a good option as the elimination of a O-BR_n may be easier and may explain the higher yields. Certainly, reductions of carboxylic acids with diborane is often a quite good reaction.

Now, if we try to thread the needle here, you could try LiBH4 as this is often a better reagent for ester reductions.

[Qfbale]

Why not reduce the ester directly to alcohol using NaBH4-MeOH in THF (Ie. running the reaction in THF, add NaBH4 then add slowly methanol) Might find reference for this if you want

Yes, I accept your methodology was NaBH4 and MeOH
sincerely thanks