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Sandmeyer
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I have no such example, 1 eq. NiS is irreversibly formed so I don't understand how the process could be catalytic. Anyway, I should have been more
specific couple posts up, by a thiocompound I mean a thione (a keton with sulfur instead of oxygen) because deactivated RaNi failed to
hydrodesulfurize it - I got back the thione again and again. I tried to partly deactivate it by boiling it in acetone but it is very difficult to do
this, sometimes I got RaNi unable to hydrodesulfurize C=S, sometimes I got RaNi being able to do this, but it also started reducing other functional
group - so I gave up. Well, I wonder if there is an example of completely dehydrogenated RaNi being able to dehydrosulfurize thiols? Maybe with thiols
it does work, but for thiones it dosen't.
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Nicodem
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Quote: Originally posted by Sandmeyer  | The hydrogen atom on that C-S-H is just acidic and AFAIK you can't treat dead RaNi with an acid to make it active - it has to be hydrogen gas, which
is absorbed onto the surface of nickel by providing one electron per hydrogen atom to the d-orbital (where the juicy valence electrons are) of nickel.
So for example, I think that attempted hydrodesulfurization with dead RaNi conducted in acetic acid would fail. However, I should also add that I'm
just guessing.  |
Interesting bone to chew on that Barium served us. I must admit that I completely missed the point in his post about deactivating Raney nickel first,
so I did not understood what DJF90 was asking about.
If you think about it, it is perfectly plausible that hydrogenless nickel can desulfurize thiols if you consider that the electrons for the reduction
come from Ni(0) and H from the thiol group.
I don't know the mechanism of the desulfurization or whether if any was ever demonstrated, but already if you chose just any of the most likely ones
you can see this is possible. For example, if you take an organometalic mechanism and chose either oxidative insertion of Ni(0) into the Hetaryl-S
bond or S-H bond, you get a Ni(II) intermediate which can rearrange in a Hetaryl-NiS-H intermediate which upon reductive elimination gives Hetaryl-H
and NiS. If you take a radical anion formation folowed by the capturing of aryl radical by the metal (similar to the proposed mechanism of the Ullmann
condensations) you still get a viable route to the desulfurization, because at the last stage you get to the same reductive elimination as in the
purely organometalic mechanism.
The obvious question that arises from the consideration of these two mechanisms is: Does treating diaryl tioethers with such hydrogenless Raney nickel
gives biaryls? This should happen if any of the two mechanisms is valid, yet I have not heard of any such example. I was intrigued about this
possibility and went to do a SciFinder search. Indeed there are some examples with the NiCRA or other Ni(0) reagents, either as the desired reaction
or just a side reaction observed, but all cases are for the ring contraction of tricyclic systems containing endocyclic sulfur:
Journal of Heterocyclic Chemistry, 22 (1985) 1547-1550. (with Ni(cod)2)
Journal of Organometallic Chemistry, 139 (1977) C51-C55, DOI: 10.1016/S0022-328X(00)85477-7. (with Ni(cod)2)
Journal of Organic Chemistry, 54 (1989) 4848-4853. (with NiCRA, this paper also proposes the radical anion mechanism)
I already thought that there would be no paper where deactivated Raney Ni was tried when I stumbled upon a carbazole synthesis from phenothiazine
which uses degassed Raney Ni (Molecules, 13 (2008) 1345-1352) and gives reference to Aust. J. Chem., 17 (1964) 366-370. So not only it seems plausible that
deactivated/hydrogenless Raney Ni can desulfurize thiols, but also that it can induce ring contraction in some endocyclic sulfur containing systems. I
must say that I'm quite surprised about this last property.
BTW, the Molecules paper gives warning: "Caution! Degassed Raney nickel is explosive!". Anybody knows what is this about?
| Quote: Originally posted by Barium | | I'd still love to see a reference in which nickel is used as a catalyst in a desulfurization or hydrodesulfurization. |
Do you mean Raney Nickel or just any nickel catalyst? I have not found any example with Raney Ni, but there are plenty with other nickel catalysts.
Catalytic desulfurization with external hydrogen and H2S as the end product is the most common form of desulfurization, though admittedly only because
it is used in just about any petroleum processing plant. Just google "catalytic desulfurization". Ni or NiS on alumina was used in the old times (the
book Organic chemistry of bivalent sulfur. Volume 1, available in References, has a chapter on these early catalysts). Nowadays more
effective catalysts exist, particularly some cobalt molybdates and who knows what else.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Sandmeyer
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| Quote: Originally posted by Nicodem |
If you think about it, it is perfectly plausible that hydrogenless nickel can desulfurize thiols if you consider that the electrons for the reduction
come from Ni(0) and H from the thiol group.
I don't know the mechanism of the desulfurization or whether if any was ever demonstrated, but already if you chose just any of the most likely ones
you can see this is possible. For example, if you take an organometalic mechanism and chose either oxidative insertion of Ni(0) into the Hetaryl-S
bond or S-H bond, you get a Ni(II) intermediate which can rearrange in a Hetaryl-NiS-H intermediate which upon reductive elimination gives Hetaryl-H
and NiS. If you take a radical anion formation folowed by the capturing of aryl radical by the metal (similar to the proposed mechanism of the Ullmann
condensations) you still get a viable route to the desulfurization, because at the last stage you get to the same reductive elimination as in the
purely organometalic mechanism. |
Well, Barium already pointed out the intramolecular donation leading to hydrodesulfurization. Treating Ar-SH with degased RaNi might as well give
Ar-S-S-Ar and in that case no hydrodesulfurization has happened.
[Edited on 5-7-2010 by Sandmeyer]
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Sandmeyer
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| Quote: Originally posted by Sandmeyer |
Yes, I agree with you, there is enough hydrogen atoms (equal amount in reactants as well as products) in the case of thiols. The hydrogen atom on that
C-S-H is just acidic and AFAIK you can't treat dead RaNi with an acid to make it active - it has to be hydrogen gas, which is absorbed onto the
surface of nickel by providing one electron per hydrogen atom to the d-orbital (where the juicy valence electrons are) of nickel. So for example, I
think that attempted hydrodesulfurization with dead RaNi conducted in acetic acid would fail. However, I should also add that I'm just guessing.
[Edited on 2-7-2010 by Sandmeyer] |
Well, what I’m saying here is bollocks, if degassed Raney nickel can insert between H-H it might as well do the same between S-H. And indeed it
does, but if it then leads to hydrodesulfurization is the question here - and I would like to see an example that this is the case. Meanwhile, W.
Bonner has published a study entitled: “The source of hydrogen in reductive desulfuraton”. This gentleman did some interesting experiments,
notably he took naphtalene-2-thiol and refluxed together with “hydrogen-free” Raney nickel and got naphthalene disulfide (i.e Naphtyl-S-S-Naptyl),
interestingly desulfurization was not observed, and he did the experiments in both ethanol (hydrogen donor) and benzene. Most conclusions of paper can
be seen on the first page which is free of charge from AC$: http://pubs.acs.org/doi/abs/10.1021/ja01124a047
I should also correct myself in saying that I worked with a thione, in fact I attempted a chemoselective hydrodesulfurization on a highly substituted
methylthioether where sulfur is connected to a sp2 carbon (isolated), if anyone cares.
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DJF90
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Fieser shows a Raney Nickel desulfurisation similar to what is requested. However, a few pages back the book says that halogens are also displaced
from aromatic systems under alkaline conditions. Find attached the mentioned pages.
Attachment: Raney Nickel - Fieser extract.pdf (148kB)
This file has been downloaded 741 times
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