walruslover69 - 12-8-2020 at 04:40
Not necessarily an amateur/home chemistry but I was wondering if there were any alternative reagents capable of reducing all types of carbonyl and
carboxyl functional groups besides lithium aluminum hydride. I am working with some large polyaromatic molecules and I have read reports in literature
of LAH partially hydrogenating parts of the aromatic system.
karlos³ - 12-8-2020 at 04:49
Red-Al works just the same, except that it doesn't result in dehalogenation(but it can achieve that too, with some CoCl2 added).
It is also easier to work with, less dangerous(i.e. barely pyrophoric) and can be handled quite easily, because its sold as toluene solution.
So much for the advantages, the downside is that it is more expensive, and depending on your substrate, it is possible you also need to use more of it
in comparison to LAH.
I worked with that stuff a few times and it is really nice.
draculic acid69 - 12-8-2020 at 06:54
I've never used red-al but I definitely want to try it.id definitely recommend it is worth looking into bcoz it's like lah without the problems
macckone - 12-8-2020 at 08:33
LAH is the most versatile.
Sodium Borohydride, phosphorous/iodine, aluminum amalgam can all reduce a subset of compounds.
There are many possible reducing agents and it is going to depend on the compound.
walruslover69 - 12-8-2020 at 09:44
Red-al definetly sounds like something I will look into and potentially try. especially considering the solubility versatility
The compound I am working with has ketone, alcohol, ester, carboxylic acid and epoxide groups. I am looking to either eliminate or reduce all groups
to alcohols.
Corrosive Joeseph - 12-8-2020 at 10:35
Can you get NaBH4.....?
https://chemistry.mdma.ch/hiveboard/novel/000460118.html
/CJ
karlos³ - 12-8-2020 at 14:21
NaBH4, modified with various other "co-reagents", might be able to reduce almost anything as well.
But the issue is, that only one or two of these combinations are able to reduce all the functional groups the OP wants to reduce to alcohols, like LAH
or Red-Al would do.
Also, the only reducing agent made by modifying borohydride I can think of that could achieve that, is diborane.
Thats it, I can imagine another modification that can achieve the same result, but I don't know another one besides diborane.
And I am not sure of that either, I know it can reduce most of the functional groups in question, for sure, but maybe there is one among them where it
doesn't work with? 
I would prefer Red-Al, that is less toxic than diborane, and neither a gas, makes it all more comfortable.
draculic acid69 - 12-8-2020 at 16:35
Can't stirring zinc or nickel chlorides with nabh4 produce the corresponding more reactive compounds of nibh4 or znbh4 which will reduce things nabh4
won't?
The diborane seems kind of toxic and unnecessary
karlos³ - 12-8-2020 at 17:45
Preferably nickel boride, not zinc borohydride, as the first doesn't need anhydrous conditions or an ether as solvent.
Well, it can reduce ketones, carboxylic acids, and surely epoxides to their alcohols.
Don't know about the ester though? I think that depends on the ester, i.e. only works for some, or something like this?
In general this is a good reducing agent, the only downside is just the large excess of borohydride used.
If all the groups can be reduced successful, its probably a good choice.
Easier to handle, least dangerous compared to everything else suggested, and simple to prepare and use, as alcohols are sufficient to run the
reduction in.
draculic acid69 - 12-8-2020 at 21:05
I'm guessing nickel borohydride is cheaper than red Al too
karlos³ - 12-8-2020 at 21:45
Definitely.
Its actually made in-situ, and the large excess of NaBH4 that is used still is a whole lot cheaper than Red-Al, and the other reagent is of course
just a Ni(II) salt.
I used the acetate and chloride for this, both working well, and since we all prefer OTC stuff, I should add that I made those salts from pottery
grade nickel carbonate.
Although with the recent and unexpected rise of the cost for NaBH4... might not be so much cheaper anymore?
We'll see.
Still, Red-Al is a cool and noble reducing agent, it gets the same job done without all the issues that LAH brings with it.
Some things just require a strong reductant like that, for example I had great success reducing indolylglyoxylamides to tryptamines with it already,
and I doubt that LAH would have been that easily and safe handled as I experienced it with this stuff.
clearly_not_atara - 13-8-2020 at 05:40
Red-Al solution sells for cheaper by weight than NaBH4 powder, but when you calculate the cost per mole you realize quickly that sodium
bis-methoxyethoxy aluminium dihydride is 200 g/mol while NaBH4 is just 37 g/mol and the latter contains twice as many electrons per mole -- so NaBH4
carries some ten times as many electrons per gram!
In other words, yes, borohydrides are cheaper than Red-Al, but it may not be immediately obvious.
On that note, LiBH4 will reduce most of the FG you mention, although carboxylic acids require the right conditions:
https://pubs.acs.org/doi/abs/10.1021/jo00371a017
It costs more than NaBH4, but can be made by salt metathesis in methanol:
LiCl + NaBH4 >> NaCl (s) + LiBH4
More interesting is that LiBH4 does a good job of reducing primary and tertiary -- but not secondary? -- amides.
draculic acid69 - 13-8-2020 at 18:33
So libh4 is that easily made. How long does the rxn between licl+ nabh4 take?