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Author: Subject: Pink/peach tinge on acidic workup of P2-type nickel boride/NaBH4 reductions of aliphatic nitroalkenes?
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[*] posted on 30-5-2017 at 13:05
Pink/peach tinge on acidic workup of P2-type nickel boride/NaBH4 reductions of aliphatic nitroalkenes?

This isn't the first time this phenomenon has occurred, it seems. Others have reported likewise, and I should start by stating that I didn't expect to get it perfect first time.

Anyhows, after reading several pretty favourable looking papers regarding the use of catalytic nickel boride/NaBH4 it was given a shot, with the substrate P2NP since it happened to be lying around, would have chosen simple phenethylamine as test substrate, but no nitromethane was to hand, only nitroethane and some preformed P2NP (the nitropropene, although a little of the saturated derivative is to hand, chose the nitropropene over the nitropropane, partly because the latter would have needed working up first whilst the nitropropene was available, and already waiting. Decided to give P2-type nickel boride (P1 is the more active of the two, somewhat more active in reductions of alkenes than W2 Raney nickel in terms of speed and lacking susceptibility to chiral inversion (3% as opposed to up to 20% according to one paper Toady has been reading whilst reportedly P2 is slower, and more sensitive to steric constraints)

And partly because it was desired to see whether Ni2B/NaBH4 would reduce a phenylnitropropene directly to the amine without having to be reduced with borohydride first to the nitroalkane, although I figured it most probably would do so given its reported capacity for reducing nitrogenous compounds such as aromatic, aliphatic nitro compounds and alkenes.

The paper being worked from at the time, didn't give much away in terms of reaction time, and at the time of the rxn being performed access to a computer was unavailable so it was all the literature available to work from.

The main difference from some of the other literature is that, owing to availability constraints, MeOH (with 5% H2O added) was used to form the fine slurry of nickel (II). And accordiing to the paper in question (apologies for not posting it, but I don't actually have an online copy, only a printout and it hasn't the DOI/PMID with it) rather than 95% EtOH, sonication was not used, again, not available and according to this paper, nickel acetate, rather than NiCl2 was the salt chosen, since it states several divalent nickel salts were tested and nickel (II) acetate was found to produce a superior catalyst than nickel chloride. And there was the hope that acetate as a counterion might just have better solubility in 95% methanol.

Anyhow, with thorough shaking and swirling, first (with hindsight it might well have been a lot of catalyst w/ respect to substrate, but references seem to vary massively in the quantity of catalyst used with respect to substrate, I've read from 5mmol to 25 mol %, so quite the wide scope for experimentation there)

10ml 1M methanolic NaBH4 was added to 50ml 95% MeOH/H2O containing either 1.58 or 1.59 (writing was a little spidery at the time, was hurriedly taking notes whilst the borohydride addition was taking place.) of nickel (II) acetate hydrate and whilst the flask containing the nickel salt had the 1M borohydride (20ml 1M soln. in 95% MeOH) added to it, copious H2 evolution was noted, although contrary to other reports of the preparation of P2-type Ni2B in EtOH, little to no foaming was observed during addition, portionwise in 4-5 aliquots, only strong bubbling off of H2.

5g of recrystallized (MeOH, water washed, washed with GAA to remove the catalyst used during the preparation of the nitropropene via knoevanagel condensation between benzaldehyde/nitroethane, TETA in this case, as its tetraacetate salt (catalyst preformed, no GAA used as solvent, 10 min microwave irradiation in pulses to temp range between 60-80 (although briefly it did shoot up to refluxing the nitroethane/aldehyde, the microwave used was rather powerful for the average household MW oven here) subsequently, pulses of MW irradiation were shortened to keep the temperature of reaction as close to 60 degrees 'C as possible, on average between 60-75, with intermittent on/off heating/cooling in water bath back to 20 'C before a new cycle, total irradiation time 10 minutes) before chilling down to freezer temperature and a final short burst of irradiation. Pretty nice results, yield isn't quite certain yet however, as after filtration out of the crystals, which were quite yellow from the start, quite free of the redder color of the mother liquors, the container was put back on ice, and at least a further two crops have reared their pretty yellow heads)

Back to the boride reduction-Not having any guidance at the time, it was conducted at reflux temperature for several hours, in hindsight that looks like almost certainly quite the overshoot. Following initial addition of the borohydride (BTW, just for others, this catalyst according to the paper I have on hand cyanoborohydride doesn't work for preparing these catalysts for whatever reason. Not of personal relevance atm since I personally am not currently employing cyanoborohydride for anything. But for anyone else, hopefully I might save somebody from wasting some)

Way way overshot the time period, and conducted at reflux rather than RT (a balloon was also employed to cap the top of the flask in order to assist in determination of whether H2 was being absorbed over time (it seemed to deflate a fair bit although how much was due to takeup of H2, how much to potential migration of H2 through the rubber/latex of the balloon itself and if utilized is not certain to me atm.

What I'd most like to know, is the nature of the peach coloration that developed on acidic workup, and also, whether Ni2B poses any risk of formation of binary boron hydrides, their being, as they are, rather nasty, toxic, pyrophoric things. The catalyst is stated to be acid-reactive.

And also, what people think of, given the strongly basic PH of the methanolic post-rxn liquor, the idea of going straight to vac distillation and then acid-base, recrystallization then?
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[*] posted on 30-5-2017 at 15:17

MeOH + NaBH4 = a very active reducing agent that doesn't do what you'd want it to do. Most of the time, substituting methanol for ethanol is okay. This is not one of those times. NaBH4 reacts far more strongly with methanol than even water, for some reason. I'm not sure why, it just does. Isopropanol works well enough if ethanol is hard and/or expensive to obtain in high purity.

edit: Link:

You can only see the first page there, but that's all you really need. Apparently a basic pH will lower NaBH4's reactivity, and when it reacts with water, the pH goes up, slowing the reaction. This does not happen with methanol though, and in fact the product of the reaction with methanol actually catalyzes the reaction.

[Edited on 5/31/17 by Melgar]
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[*] posted on 30-5-2017 at 17:46

No cookery please.

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[*] posted on 16-11-2017 at 08:05

Eh. See my post in moderators lounge.

I do wish tsathoggua would put all posts written while under the influence of "substances" on hold until he has edited them while at base line.

Some of these posts stray into territory just a BIT verbose, twisted and baroque for a science related board?

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