Reading a rather..well...unusual, to say the very least, if not outright WEIRD, paper..it's on modification of the reduction abilities as concern substrates, of sodium borohydride (NaBH4), NaBH4 is one of the most user-friendly hydride reducing agents around in chemistry. Metal hydrides of such nature tend to be..rather volatile, with a tendency towards extremely violent reactions (indeed often explosively so) with water, and have to be used, for the most of them, in apparatus which has been immediately flame-dried before it's used, to make absolutely CERTAIN that even the tiniest trace of atmospheric moisture has been removed from any surface that contacts the reagent, to avoid a fire, or worse, which is liable to be especially ugly considering many are used in either diethyl ether or THF (tetrahydrofuran, a cyclic ether type solvent, highly flammable, volatile, although unlike ether it will dissolve in water to form a solution), both VERY flammable solvents indeed; as in 'one spark and you've got yourself a huge fireball or explosion' kind of flammable.
Common ones like LAH (LiAlH4, or to chemists just 'LAH or 'lith-al', lithium aluminium hydride) for example, one must flame dry the glassware itself, at least baked out in a hot oven for a significant time and heat for quite some time before using suitable grease and putting them together hot, attaching drying tubes to prevent moisture suckback from the atmosphere during cooling or during the reaction, as well as sodium hydride, potassium hydride and others, as well as the solvents used all having to be first dried with conventional drying agents, sitting over them for a long time, then distilling them off fresh drying agent, THEN drying by boiling them with chips of sodium or potassium metal, for as long as two days (48 hour reflux over potassium is common), and finally third-stage desiccation using advanced tools like molecular sieves, engineered synthetic zeolite ceramics, which are REALLY neat for all kinds of uses, which have a nanoporous structure, the pore sizes being controlled to within single angstrom (atomic widths) increments, so as to be able to be 'tuned' to be able to selectively remove one liquid from another, sucking up anything smaller in molecular size than the pores, anything else, can't fit and is left behind. Even capable of removing methanol from water or from ethanol if the right kinds are used. They aren't quick but my god are they ever thorough. Iv'e got some 3A molecular sieves and they are wonderful-gentle, and being basically high-tech pottery, they can be made as beads, that can just be filtered out with great ease, like BB pellets made of ceramic pottery), and after the distillation after prolonged reflux over potassium, under an inert atmosphere for several days, then several days to a week over molecular sieves to abstract the last traces of water; with the hydrides themselves being handled in an inert gas-purged, dry, sealed glove-bag or glovebox to prevent them catching fire, whilst they are weighed out, etc. and the reductions conducted wholly under an atmosphere of helium or argon.
Borohydride isn't like that at all, it's really nice to work with. Not in the slightest pyrophoric, the solvents don't need to be dried to within the bounds of godly perfection, the glassware doesn't need to be anything but ordinarily dry, hell some reactions can actually be performed IN water or other protic polar solvents, or mixtures of them. NaBH4 can be used in water for some things, in methanol for others, although it does decompose at a moderate rate in MeOH, but not in ethanol or isopropanol which thus make excellent solvents for it, wonderful shelf life (YEARS, even in powder form, although the NaBH4 I have, came in the form of tablets, pressed just like pharmaceutical pills, each one conveniently containing exactly 1 gram of sodium borohydride so the decreased exposed surface area will make it last even longer)
It isn't as outright powerful as some of the other hydrides, but there are a multitude of other chemicals and catalysts that are usable alongside it, such as metal nanoparticles, nickel boride, cobalt boride, various Lewis acids, copper (II) chloride, molybdenum trioxide and others, forming other borohydride derivatives, altering the activity of the borohydride, or forming metal borides in-situ, or even really neat catalytic systems such as metallic copper or nickel crystals of between just 1-3 nanometers in size, surrounded by a permeable 'cage' complex of the metal boride with various surface alterations such as differing amounts of sodium borate on the surface of the cage material that are formed in-situ from borohydride and additives, and are used as co-reducing agents in hydrogen-donor solvents like isopropyl alcohol for catalytic transfer hydrogenation. A really neat lot of other borohydrides such as triethylborohydride, triacetoxyborohydride, cyanoborohydride with cyanides, etc. are also used and can be modified thus in similar sorts of tricks to alter what they can and cannot reduce.
There's been rather a clandestine chemistry revolution somewhat late this year, a bit of an early xmas gift from the gods of chemistry really, in-situ formation of a caged copper crystallite nanoparticle-copper boride system used alongside sodium borohydride as well as made with it, which allows one to eschew mercury salts and amalgam reductions, something I find nasty to use, and don't like to see because a lot of people don't know what to do rightly when getting rid of their mercury contaminated slops, but are still popular because of their simplistic chemical requirements, aluminium kitchen foil and mercuric chloride or other mercury (II) salts and substrate, solvent and acid, often GAA,.
I can see whats going on there, reduction of the copper salt, CuCl2 to form the boride complex nanostructured catalyst, by analogy with the same from nickel salts, although nickel (II) acetate is better than NiCl2, in that case as chloride ions can poison nickel catalysts of some kinds as lead or sulfur do to precious metal platinum group element based catalysts; and deactivate them.
But here I've been reading a paper which talks of reducing nitroalkenes to aminalkanes directly, using sodium borohydride and of all the damnably weird things, ammonium sulfate!. No metal there to form a boride, and even the researchers have no idea of a possible mechanism, but apparently they did it and it worked. There's a lot of people who want to reduce conjugated phenylnitroalkenes to phenylalkylamines, such as 1-phenyl-2-beta-nitropropene to amphetamine, previously often done using Al/Hg amalgam reduction, leading to more use of mercury by more people either not responsible with the wastes, not thorough enough in workup and purification of products to prevent mercury carryover into the final product.
This boon we had, it was addition of a solution of copper (II) chloride, cupric chloride, to NaBH4, and use of this to reduce not phenylnitroalkenes, but the corresponding phenylnitroalkanes, having reduced the double bond of the nitroalkenes first, using sodium borohydride alone, unmodified, then extraction and aqueous washing of the nitroalkane, followed by reduction of the nitro group to the amine, with the modified copper boride nanoparticle composite-NaBH4 system giving good yields of amphetamine, as well as some substituted amphetamines, although it also removes halogens from aromatic rings, meaning that certain precursor substrates are incompatible unless the halogenation is carried out AFTER forming the substituted amphetamine's close relative with hydrogen in place of the halogen and utilizing electronic and steric effects to serve as directing groups to make the halogen go where you want it to be in your end product.
This paper, its weird...ammonium sulfate? what the devil? I can't even begin to wonder...especially as it was confirmed explicitly that diborane was NOT being formed as an intermediate and reacting in-situ (diborane, B2H6, the boron analog of ethylene, where boron replaces carbon in ethylene gas, is a highly poisonous, violently pyrophoric gas with apparently, a truly heinous stench too. Never smelled it, luckily for me, although I have once encountered one of the pentaboranes, accidentally, in a trace quantity, and smelled it, making me panic and run like hell to don my mask in clean air. And it did indeed smell foul, absolutely STANK of rotting dairy produce, like rancid yoghurt or putrid milk. Vile, and poisonous as hell, and catches fire on contact with air too. They tested for its being an intermediate, as things could be reduced that diborane cannot reduce and thus it is known NOT to be the responsible ultimate intermediate reaction product of the borohydride when used with ammonium sulfate.
BUT....ammonium salts....what on earth is going on there. They claim it'll reduce conjugated nitroalkenes to aminoalkanes. Bears more research certainly given how cheap and easy to buy ammonium sulfate is, its used in the multi-megatons annually worldwide as fertilizer, dirt cheap, and totally unsuspicious, and of course, ammonia gas and sulfuric acid will make it, if one wished to do so with no difficulty)
As for what the bloody hell is going on though, I really can't even begin to guess, unless it HAD been diborane, but it has been proved that B2H6 is NOT the active reducing species in the sodium borohydride-ammonium sulfate combination. Unless some funky metathesis and weird ammonium borohydride type species is being generated with new and hitherto unexplored potentials....I really do not know. But the practical side warrants exploration and then some.
And I'm more than in a position to do so. I can get my hands on more P2NP than I know what to do with (well I sure know what can be done with it, vis a vis amphetamine, meth, N-ethylamphetamine etc. but I mean, as in 'practically got it coming out of my ears, swimming in the stuff', kilos wouldn't be a problem if I wished to make that much)
So I think some research is warranted there. I want to know if it works as described, and on nitroalkEnes as opposed to needing pre-reduction to the nitroalkanes (NaBH4 will reduce to the nitroalkane, but while it'll reduce the double bond it won't reduce the nitro group; not that it's difficult, once the double bond has been taken care of, the resulting 1-phenyl-2-beta-nitropropane can be reduced simply by using a slurry of fine iron dust in glacial acetic acid or strong hydrochloric acid with a catalytic quantity of ferric chloride)
Squirrelier and squirrelier it gets....ammonium sulfate, whatever next?
