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Would it be possible to reduce a α,β-unsaturated nitro compound to a saturated amine using for example $\ce{NaBH4}$? I know these kind of compounds will be reduced by hydrogenation using a $\ce{Pt/C}$ catalyst, $\ce{LiAlH4}$ or freshly prepared Raney Nickel, but I'm looking for a reducing agent that is easier to handle than those.

The book from which I'm learning, Clayden et al., does not really go into much detail on reduction of nitro compounds or unsaturated alkanes, other than that hydrogenation is a good approach to both and that the nitro group is one of the most easily reduceable functional groups. But how about a α,β unsaturated compounds then? How would I asses whether some reducing agent is able to reduce both simultaneously?

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  • $\begingroup$ Perhaps you could have a look at : sciencedirect.com/science/article/pii/S004040390060148X $\endgroup$
    – user11314
    Dec 15, 2014 at 13:51
  • $\begingroup$ @jori I feel that the electron-withdrawing nature of the nitro group would make reduction with electrophilic reducing agents less likely but reduction with nucleophilic reducing agents more likely. Perhaps something strong such as LAH would do the trick. $\endgroup$
    – Dissenter
    Dec 18, 2014 at 2:19

3 Answers 3

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I'm not aware of a $\ce{NaBH4}$-based reduction of nitroalkenes to saturated amines, such as in:

nitroalkene-to-sat.amine

It is however possible to reduce nitroalkenes to saturated hydroxylamines using $\ce{BH3*THF}$ in the presence of catalytic amounts of $\ce{NaBH4}$. (DOI)

If there's a chance to perform the desired transformation in one step via catalytic hydrogenation at normal pressure: go for it!

The handling of these reactions is nothing to be afraid of.

Typically, I performed hydrogenations in a standard 2- or 3-neck flasks and used ordinary balloons (think childrens' birthday) as gas storage. Advice: Stuff a piece of rubber tube into the inlet of the balloon and secure it with teflon tape.

  • Either attach this to the glas olive of a valve attached to your flask
  • or take a plastic syringe, cut off the base, stuff the the plastic cylinder into the rubber tube. Fill the balloon and attach a needle. Hydrogen may now be fed to your reaction mixture through a rubber septum cap.

UPDATE

I still doubt that the intended reduction by $\ce{NaBH4}$ is possible.

There is however another established route from $\alpha,\beta$-unsaturated nitroalkenes to amines which is worth to be mentioned:

Condensation products of aromatic aldehydes and nitroalkenes are reduced by iron in hydrochloric (or acetic) acid to the corresponding arylalkanones.

reduction of arylalkanone

These ketones can be converted to primary amines by reductive amination in methanol (or ethanol) in the presence of (excess) ammonia and, of course, hydrogen over Raney Nickel or Urushibara Nickel.

reductive amination

In the course of this reaction, the imine intermediate is in situ reduced to the desired primary amine.

The resulting amines are valuable precursors in the synthesis of natural products: Their conversion to imines or amides and subsequent Pictet-Spengler or Bischler-Napieralski cyclisations furnish tetrahydro- and 3,4-dihydroisoquinolines.

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  • $\begingroup$ You mean like this: youtube.com/watch?v=PC4DT4ier2Q ? $\ce{BH3}$ is impossible to handle safely in amateur setting, way worse I think than Raney Nickel or even LAH. Perhaps Urushibara nickel will do the job, which is said to be more easy to handle (read non-pyrophoric). For small scale reactions your idea with the balloon might work well indeed. Do you have a link to a somewhat more extensive description of the technique? $\endgroup$
    – Jori
    Dec 15, 2014 at 23:29
  • $\begingroup$ I agree with this, you should not avoid catalytic hydrogenation... Nitro groups are easily reduced, but NaBH4 alone won't make it. Another nice feature of sodium borohydride! $\endgroup$
    – Altered State
    Dec 15, 2014 at 23:31
  • $\begingroup$ I also saw relatively cheap Parr hydrogenators on eBay (some in the range of 400-600$). These things seem pretty heavy and loud and I'm a bit weary of the explosion hazards, but they can be used for larger amounts and require less work. What level of expertise is required to handle such things anyway? Do you need special training? (see e.g. ebay.to/1BMFHHP) $\endgroup$
    – Jori
    Dec 15, 2014 at 23:36
  • $\begingroup$ @Jori Yes, like in the video, although I usually evacuated the flask through a larger diameter using a piece like this. In the video, the tube attached to the balloon looks like a commercially available piece, I always chopped something together from material from the lab drawers ;) The balloon technique works nice, but I never did this on a kilogram scale. Btw, Raney Nickel isn't that bad as long as you handle it with care and keep it wet. It really is pyrophoric :D $\endgroup$
    – Klaus-Dieter Warzecha
    Dec 16, 2014 at 2:52
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    $\begingroup$ @Jori >Can you comment on why Fe/HCl reduces the α,β-unsaturated nitroalkene to a keton instead of a saturated nitroalkane or a (un)saturated amine? || because enamines tend to isomerize into imines that react with water producing ammonia and ketone. You should reduce the double bond first. $\endgroup$
    – permeakra
    Dec 19, 2014 at 14:04
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There are numerous reports on transfer hydrogenation, where hydrogen is transferred from strongly reducing agent onto double bond or other groups.

In particular, Raney nickel was utilized in reduction of both nitrogroups to amines and alkenes to alkanes, using formic acid for the former and isopropanol for the latter. I guess, that combined process is also likely to work.

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$\ce{NaBH4}$ will only reduce the $\ce{RCH=CHNO2}$ to $\ce{RCH2CHNO2}$. An additional reducing agent, such as $\ce{Zn/HCl}$, can reduce the nitro group to the amine. A catalyst + $\ce{NaBH4}$ could do the whole thing, but you'll have to look at the literature.

Catalytic hydrogenation, using $\ce{Pd/C}$ under moderate conditions (around 5 bar $\ce{H2}$, 30-90 °C), can reduce $\ce{RCH=CHNO2}$ directly to $\ce{RCH2CHNH2}$.

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  • $\begingroup$ I have done the reduction of RCH=CHNO2 to RCH2CHNO2 using NaBH4 in the presence of column chromatography grade silica gel. It works well. Hydrogenation over Pd/C is poor giving a mixture unless you do it in the presence of ammonia. $\endgroup$
    – Waylander
    Nov 3, 2021 at 8:17

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