Electra
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Catalytic Transfer Hydrogenation with an Aqueous Metal Salt & Phase Transfer Catalyst
I'm planning to do a little experiment in a week when some supplies arrive, since I haven't found too much information on this topic. I plan to use
Nickel (II) Chloride(and CoCl2 in another experiment), Cetyl trimethylammonium bromide (CTAB), and Sodium Formate to reduce Nitrobenzene. I also plan
to test the effectiveness of this system against Ammonium Formate as the hydrogen donor. I was initially skeptical to why there hasn't been much
application of this specific technique, wondering if the dissolved metal ion may have some counter-effect on the hydrogenation, possibly due to lack
of substrate-ion interaction, but further findings have me starting to think this not be the case (other than potential solubility issues with cross
phase interactions with the lack of a PTC as mentioned below). Perhaps the field is simply under investigated. The PTC should act to solubize the
metal ions in both phases so that interaction with the hydrogen donor and the organic substrate is possible.
One potential downside and/or possible reason for lack of practice of this method is that the soluble metal-salt-catalyst cannot be easily recovered
and separated from the hydrogen donors which are typically water soluble themselves, thus making the long-term economic reuse of the catalyst a
challenge.
I have been reading heavily about this for some time before purchasing the supplies, since I haven't found all too much documentation on this specific
application. In the attached paper, of the few incidences of such utilizes Nickel Bromide in Alkaline PrOH (with no PTC) in high yields (12-99%+) to
hydrogenate various compounds. In the paper they also cite the use of Cobalt Bromide and Cobalt Iodide for a similar CTH, but with only ~60% yields.
Since these are water soluble salts, it is a wonder yields were this high without the use of a phase transfer catalyst. Though, simultaneously not
such a mystery seeing as some of the substrates that had lower yields also had lower solubility in water. The authors of the paper also make reference
about Organic soluble complexes of- and Water soluble NiCl2 for this purpose.
Additionally organic-phase soluble catalytic complexes of Nickel, Ruthenium, Iridium, and others, both chiral and non-chiral, in the past have been
well studied and used for such a CTH without the use of a PTC. These solubized systems offer the obvious advantage of that the work-up is much easier
with no metallic water-insoluble dust to deal with, though separation is still a potential problem. Additionally, metal-contact with the substrate is
potential maximized in these organic soluble systems, allowing for very efficient and quick hydrogenations.
Can anyone care to comment on this particular water-soluble application? Also, if available, can anyone provide any papers/reviews on this subject?
The bulk of what I have been able to find on the topic discusses the use of water-insoluble metals. Additionally, as of the last 10-15 years, papers
discussing the use of various organic-soluble (chiral) catalytic complexes with such asymmetric reductions. While research regarding the use of PTC in
such applications is not entirely absent, it is much less in number than the aforementioned systems.
The use of cheap soluble metal salts and affordable phase transfer catalysts in laboratory scale CTH could prove to be one of the most affordable and
applicable methods for the amateur chemist. Given the cheapness of PTC like CTAB ($15/kg) due to its wide use in the industry, this method is even
more seductive.
Attachment: Nickel BromidePDF.pdf (36kB)
This file has been downloaded 389 times
[Edited on 4-3-2014 by Electra]
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Electra
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An Update:
Still waiting for some needed substrates to arrive. I continued my research/reading on this subject out of my growing skepticism of how well this
reaction would work. I have found much history of transfer hydrogenations done with Nickel (II) Salts and Sodium Borohydride, LAH, or another Hydride
donor. These are obviously messy hydrides for the average chemist to get their hands on. The use of Formate anions/salts such as Sodium Formate and
Ammonium Formate have been utilized many times with Raney Nickel, but there is barely any usage of them with Nickel (II) Salts, or other nobel metal
salts.
With some creative google searching and use of the undocumented very useful "AROUND(##)" Search operator, I was able to comparatively find the word hydride donor around my target substrates.... confirming their usability
in my intended reaction.
| Quote: |
"The formate anion acts as hydride donor"
"Formate ion is apparently a sufficiently good reducing agent (hydride donor)"
"... cofactors NAD(P)(+) into NAD(P)H with formate as a hydride donor."
"...using sodium formate as the hydride donor..."
" In this reaction, the formate ion formed acts as hydride donor"
"In synthetic organic chemistry, formic acid is often used as a source of hydride ion."
"A pH-independent asymmetric transfer hydrogenation of β-keto esters in water with formic acid/sodium formate can be conducted open to air and
gives access to β-hydroxy esters in excellent yields and selectivities."
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compared with (the obvious)
| Quote: |
"with a hydride donor (NaBH4, super hydride derivatives, etc...)"
"...hydride donor like NaBH4 or LiAlH4 give..."
"Reduction of an ester with a hydride donor such as NaBH4..."
"NaBH4: a very mild hydride donor"
"LAH is a very reactive hydride donor"
"LAH, alane, and borane were used as hydride-donor reducing agents. "
"Lithium aluminum hydride is a much more powerful hydride donor"
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And some brief results on the NaBH4/Ni (II) system
| Quote: |
"Reduction with sodium borohydride coupled with transition metal salts and transfer hydrogenation"
"The best results were obtained on the hydrogenation.... by means of sodium borohydride with the addition both of nickel chloride"
"The catalytic use of nickel(II) chloride in combination with excess sodium borohydride is environmental benign and tolerates air and moisture"
"The first step involves chemical reduction of nickel chloride in an aqueous medium with sodium borohydride"
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Some sources have claimed that hydrogenation of various Nitro Groups with Ni(II) Salt/Hydride system can sometimes be unsatisfactory. This could
theoretically be attributed to lack of solubility of the Nickel Salts in the organic phase. Insoluble metals such as Raney Nickel, or Pd/C can act as
more efficient transfer hydrogenation agents compared to water-soluble metal salts, due to the organics reacting much more easily with the metal since
it is not suspended in the aqueous phase. My initial skepticism of this reaction not working stemmed from the fact that Raney Nickel is in the Nickel
(0) reduced form, allowing it to reduce molecule H2, and Nickel (II) salts are not. For transfer hydrogenation, the reduced form of the metal appears
to not matter since the hydrogen is already in its most reduced hydride form in the hydride donor (HCOONa, NH4HCO2, LAH, NaBH4, and others)
Both Sodium and Ammonium formate are commonly used in transfer hydrogenation with Raney Nickel, Pd/C, and other transition/nobel metals, while LAH and
NaBH4 are frequently reported in use with both the aforementioned insoluble metal-reducing-agents, and the water-soluble salt-forms. Since the
formates appear to be regarded as hydride donors in a similar light as LAH and NaBH4, with regard to transfer hydrogenation, then it makes sense to
reason that Nickel (II) Salts and Formate anions can be used together for transfer hydrogenation reactions. And to the point of this
post/topic, Phase Transfer Catalysts should increase the efficiency and speed of this reaction by a great deal due to the inherent mechanism of phase
transfer catalysts
I hope to confirm these results in the coming week. If formate salts can be used in place of LAH and NaBH4 with metal-salt transfer hydrogenations,
then this would prove to be a great step forward for amateur chemists, due to how cheap, safe, and readily available formate salts are in comparison
to LAH and NaBH4.
Edit:
The NaBH4/NiCl2 system for various compounds yields are less that desirable, under 60% for some compounds (aliphati nitro). These reactions are
sometimes conducted in water-absent mediums, which arguably reduces the solubility of the catalysts/reducing agents thus preventing their facile
interaction with the organics. In theory, a PTC in an biphasic liquid Aq/Organic system should be able to maximize those yields, since the reaction
without a doubt does take place, albeit moderately, without the PTC.
I am getting antsy. Can't wait till my supplies arrive.
[Edited on 8-3-2014 by Electra]
[Edited on 8-3-2014 by Electra]
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AvBaeyer
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Electra:
From your second post: "then it makes sense to reason that Nickel (II) Salts and Formate anions can be used together for transfer hydrogenation
reactions"
I doubt that this is true. Nickel formate, which will form in an equilibrium with nickel chloride and your formate salt, is a stable compound under
normal conditions. (I have some in a bottle.) Nickel formate does decompose at high temperature which is illustrated on Wikipedia (under "formates"):
"For example, hydrated nickel formate, a salt, decarboxylates at about 200 °C to give finely powdered nickel metal:
Ni(O2CH)2(H2O)x → Ni + 2 CO2 + x H2O + H2
Such fine powders are useful as hydrogenation catalysts."
The reactions of LAH and NaBH4 with nickel salts forms either nickel metal or substances such as nickel boride. Reactions involving these species are
quite different from what we typically call transfer hydogenations.
In any event, I remain intrigued by the mechanistic aspects of the paper you attached in your first post. The reaction reported is very similar to the
Meerwein-Ponndorf reduction in which aluminum isopropoxide is the reducing agent. Of potential interest is that the Meerwein-Ponndorf reaction can be
run in reverse as an oxidation reaction which is known as the Oppenauer oxidation. Could that be done with the nickel salt-isopropanol system?
I do wish you success in your experiments. Keep us posted.
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Electra
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You put forth an interesting idea, but a few google searches lead me to some various patents/papers ... now I haven't read these in depth so forgive
me if they are out of context. It does seem a number of them refer to the heating of Nickel Formate to form Nickel Metal. Your idea about Nickel
Formate forming an unreactive intermediate in equilibrium has me skeptical due to the use of Metal-Cl-Amine complexes commonly used with formates for
hydrogenation... which I touched on in the second paragraph from the bottom.
http://www.google.es/patents/US2576072
| Quote: |
This invention relates to a method of producing nickel formate by dissolving nickel metal in formic acid.
Nickel fcrmate is used in large quantities in the catalytic hydrogenation of oils, especially vegetable oils in the manufacture of
margarine and vegetable shortenings. Nickel metal is the ultimate catalyst, but the formate is reduced to metallic state in the hydrogenation reaction
and may therefore be added, instead of reduced nickel, to the charge of oil to be hydrogenated. Notwithstanding the large use of nickel
formate for hydrogenation, the prevailin method of preparation has continued for perhaps thirty years to involve precipitation of nickel
carbonate from nickel sulfate solution by the use of soda ash and reaction of the carbonate with formic acid. Nickel formate thus commercially
prepared is contaminated with by-product salts to an extent that it is seldom much, if any, above 99% purity.
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http://www.google.com/patents/US1296496
| Quote: |
This invention relates to the of nickel formate and to its conversion. into a catalytic body suitable for the hydrogena-v the following is a specition
of fatty oils and the invention will be described beginning with the preparation of nickel nitrate from metallic nickel and the is precipitated and
an, excess of ammonia conversion of the-nitrate into the formate
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Also found on google...document wont load but from googles brief summary of it:
https://www.jstage.jst.go.jp/article/bcsj1926/31/6/31_6_775/...
| Quote: |
"...nickel formate is effective for hydrogenation of many organic substances."
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The Nickel Metal is the ultimate catalyst, but the formate, through the nickel, can give up its hydride, to form Nickel Carbonate. Just as all
formates form carbonates upon giving up their hydrides.
You may indeed be right, that nickel formate forms in equilibrium, but this may be an important intermediate in the hydrogenation process. It seems to
have ready applications in the hydrogenation of "oils", whatever that may mean, broadly. I would assume this form of hydrogenation is by hydride
transfer as the papers imply, since there is no other obvious source of reducing ability other than the Nickel Formate.
You say that Nickel Formate is relatively stable in solution, but sodium Formate and ammonium Formate are also stable in solution, and not naturally
reducing on their own, in the absence of a hydrogen transfer catalyst(nickel/palladium/platinum/iridium/rhodium/etc). Unlike the metal
hydrides(LAH/NaBH4), the formates will not spontaneously release hydrogen. The ability of formates to reduce organic compounds may be indeed limited
by the amount of contact with the organic phase.
If I am not mistaken, NaBH4 on its own does not reduce nitro groups, but coupled with a nickel salt catalyst this can indeed happen, presumably by
catalytic hydrogen transfer. If you search NaBH4 and Nickel Salt, the words 'Transfer Hydrogenation' are commonly found in the same sentence.
I am personally of the opinion that hydrogenation with these various salts is not researched heavily due to solubility issues and poor organic phase
interaction. In addition to that, I have found that the use of Phase Transfer Catalysts is heavily under-studied in such fields, if not ignored all
together. It would be no surprise to me if Nickel Formate was poor at reducing non-water soluble organic compounds in the absense of a PTC, but
efficient in the presence of one.
I will continue to look into this. I am intrigued by your nickel formate idea. In any case, if this reaction does not work even with
the phase transfer catalyst, it has been proven that simply by reacting my nickel salt with an amine for a number of hours, whether chiral or not, an
organic-phase soluble complex will form that can indeed be used with formate transfer hydrogenations. This sort of hydrogenation is well documented
with Iridium Chloride, Ruthenium Chloride, and other precious/transition metal amine-complexes. The complex is not permanently bonded, the amine
aspect of it acts to effect the solubility and introduce chiral information into the reaction, but it does not interfer with the [transfer]
hydrogenation properties of the metal at the core. This fact is something that we need to consider here. If Nickel Formate can form from Nickel
Chlorides, then other M-Formates should form with the other various lewis acidic catalysts that are used to form amine-complexes with asymmetric
hydrogenations.
Additionally I have found this:
http://www.primaryinfo.com/scope/nickel-formate.htm
| Quote: |
"Nickel formate is a chemical compound of nickel. Water-soluble green crystals; used in hydrogenation catalysts."
"Nickel Catalyst for the hydrogenation of vegetable oils."
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Additionally, in any scenario, in a room temperature stirred mix with Nickel Chloride, Sodium Formate, Water, Organics, and the PTC: Nickel Formate
may form in equilibrium, but will never "permanently" form. There will inevitably be collision of the formate anion with the nickel metal and the
organics, at which some point the hydride should inevitably transfer.
[Edited on 8-3-2014 by Electra]
[Edited on 8-3-2014 by Electra]
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Electra
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Sorry to double post but I feel this idea deserves its own post. Definitely worth looking into.
I did some googling....Since Pd/C is used just like Raney Nickel in Transfer Hydrogenations...
For all I know these could merely be referring to the interaction between reduced Palladium (0) and formate, but I figure it was worth posting.
Though, the last example is interesting because it uses Palladium Acetate.
| Quote: | "Palladium formate" AROUND(20) "hydride"
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Link
| Quote: | n addition, ammonium formates [174, 175] afford a transient palladium-formate complex and the delivery of the hydride occurs via a concerted mechanism
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Link 2
| Quote: |
a palladium formate cation. The formate complex loses carbon dioxide reforming the cationic palladium hydride catalyst
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http://downloads.hindawi.com/journals/jcat/2013/614829.pdf
| Quote: |
The combination of formic acid and palladium acetate is known to undergo anionic ligand exchange to form a palladium diformate complex, eventually
producing Pd(0)through decarboxylation and loss of molecular hydrogen.The high degree of chemoselectivity in palladium-catalyzed transfer
hydrogenation using HCOOH or its salts has been explained on the basis that the hydrogen is delivered directly from a palladium formate species, which
has much stronger hydridic nature as compared to that of a palladium hydride species (Scheme16)
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The mechanisms of transfer hydrogenation have been unclear, but it seems to rely on a simultaneously interaction between the Metal, the Hydride agent
(formate), and the organic species. Where electron charges are seemingly balanced out through contact, with the hydrogenated form of the compound
being the most energy stable state of the tri-complex in solution. Just a guess.
I also just found this paper:
A Simple Protocol for Direct Reductive Amination of Aldehydes and Ketones Using Potassium Formate and Catalytic Palladium Acetate
This is interesting because the hydride transfer seems to effectively take place using the Palladium (II) salt with a formate salt that happens to be
soluble in the organic phase and aq. phase. It could be reasoned that other formate salts can also be used if a PTC is used in addition. Though this
study seems to indicate that nitro-groups did not seem to be reduced. Although, the one compound containing a nitrogroup that was put through the
reduction only yielded 56% of the reduced-imine, leaving to question whether the rest of it was by-product(although otherwise indicated), or simply
unreacted.
| Quote: |
It therefore appeared reasonable to investigate whether potassium formate, which is soluble inpolar organic solvents and in water, with activation by
palladium salt could significantly reduce the C-N double bond of the imine formed in the direct reductive amination reaction. We report herein our
observation, which constitutes a one-pot reductive amination protocol for aldehydes and ketones, including conjugated ones, with the aid of potassium
formate and catalytic palladium acetate.
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[Edited on 8-3-2014 by Electra]
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AvBaeyer
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Electra,
You have done some nice searching and I will take the time, just not right now, to give your hits some attention. I think that we can both agree that
in the realm of metal catalyzed reductions the reaction mechanisms are still shrouded in a good deal of mystery. Most of the transition metals that
are used in one manner or another for reduction of organic compounds tend to shuttle between different oxidation states, ie Ni(0) and Ni(+2), Pd(0)
and Pd(+2), etc. Thus, trying to predict what will happen in a novel reducing system is difficult. The answer needs to be derived by experiment and I
encourage you to persue this avenue. We can argue all day about why something should not work but in the end the careful experimentalist rules the
day. I do hope that we can continue this discussion when results are in hand. I will also admit that I am intrigued enough by what you have found and
proposed to try some experiments along these lines myself.
AvB
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Electra
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Excellent points. There is indeed no purpose in arguing over theory. Experimental results are what matters.
There appears to be much recorded investigation on the mechanism of transfer hydrogenation with transition metal lewis acid diamine or diphosphine
complexes. RuCl2 chiral Diamine complex definitely is effective in transfer hydrogenation, but from what I have read the Diamine constituents play an
important role in the asymmetric transfer of the hydrogen from the formate. I am curious to see if the Lewis Acid aspect of it can be used on its own
with a PTC. These Diamine complexes are generally lipophilic which makes them fairly effective for transfer hydrogenation purposes, where as the lewis
acid on their own are not. Formate salts used with PTC's in these complex-based transfer hydrogenations are very effective since as soon as the
formate is transfered to the organic phase the transfer of the hydride can take place, allowing very high 95%+ yields in just a few hours. This has
been experimentally tested time and time again.
In any case, if my experiment does not work, then forming a lipophilic Diamine complex or Diphosphine complex will indeed work, which is in fact so
surprisingly easy that you will be hard pressed to find any how-to guides on how to do it. Those of you smart people out there should know how to form
these fairly easily 
[Edited on 9-3-2014 by Electra]
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Dr.Bob
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I have not ever tried aqueous reductions, but I do know that Raney nickel can be used in EtOH to do H2 type reductions. Not sure if water would be
bad or not. And I suspect that transfer hydrog. would work fine in Raney Ni as well, maybe a mix of EtOH and water might be better for solubility of
substrate if nothing else. Plus there are lower limits of flammability for alcohol in water. Best of luck.
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Electra
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Raney Nickel is well documented in working for transfer hydrogenations. Water is considered a big obstacle in these sorts of reactions because it
usually further decreases the ability of the water soluble reactants to interact with the organic phase. This is why insoluble catalysts such as Raney
Nickel, Pd/C, and PtO2 are generally so effective, because the water is not an obstacle for them. A phase transfer catalyst can make it so the water
does not interfere with water-soluble reactants reacting with the organic phase, if those soluble reactants are ionic.
The only thing I really need to figure out is whether the positive charge of the Nickel Chloride would hinder the transfer hydrogenation. This could
be the case. Transfer hydrogenations work well with NiCl2, RuCl2, and other transition metal complexes, but I believe this may be due to the net
charge of the complex being 0 (compared to +2), since the amine or phosphine ligands acts as a base to neutralize the lewis acid. Pd/C, Raney Nickel
and other insoluble reagents are in the +0 oxidation state. These transition metal complexes also being in the +0 oxidation state, yet also being
soluble, allows them to function in the same manner (with additional chiral features), as the solid catalysts, but without the mess of clean up, gunk,
and flammability issues.
Whether or not this +2 / +0 charge plays a significant role in the transfer hydrogenation is something I hope to find out, as I do not believe it has
ever been explicitly stated.
There is the high chance that the +2 charge may not matter since the phase transfer catalyst can act to temporarily
neutralize the charge as it transfers the ion into the organic phase. This is where the effect of the reaction is ultimately determined by whether or
not a cationic PTC or an anionic PTC is used. An anionic phase transfer catalyst would be needed to carry the nickel cation to the organic phase,
which would also put it in the +0 state, and a cationic phase transfer catalyst would be needed to carry the formate cation into the organic phase.
This would obviously be problematic as using both PTC at once may cause them to neutralize each other, making them ineffective.
I could indeed see this reaction working if an anionic phase transfer catalyst was used with the transition metal salt, and the hydrogen donor was in
the organic phase. It may be possible for only an anionic phase transfer catalyst to be used, and perhaps the neutralized Nickel +0 ion attached to
the PTC could transfer the hydride from the formate in the aq. phase to the organic molecules in the organic phase.
Based on what I have gathered, I do believe only the use of a anionic PTC would be required to put nickel into the +0 state. The formate ion would
stay in the aq. phase, but these hydrogenations have worked fine in the past with the formate ion not traveling over. The nickel should bounce back
and forth between the phases neutralized by the PTC in the +0 state and should effectively contact both the formate ion and the organics, transfering
the hydrides.
Now my challenge is to find an affordable anionic phase transfer catalyst. Any suggestions? CTAB is cheap enough but it is
effectively a cationic ptc.
Edit:
I could also be entirely wrong about the type of PTC mattering. I am not exactly clear on the mechanism of ion transfer of these phase transfer
catalysts, whether they temporarily neutralize the charge or not, or just cling to the salt just enough to bring the entire thing into the organic
phase.
[Edited on 10-3-2014 by Electra]
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Dr.Bob
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I don;t think you will have Ni(+2) in a reducing environment. That is why many people use stable Pd(+2) salts for reactions since the Pd is reduced
to Pd(0) in situ. So I would think in most hydrogenations, any metal salts will be reduced to the metal (0) state quickly. The PTC does not
interact withth emetal in most cases of reductions, you are better with something that ligates the metal or just finely divided metal. But the
PTC-metal complex does not exist for long in a reduction, unless it complexes the metal(o) atom, like P or S, and sulfur normally kills most
reactivity.
The PTC catalysts are only useful for solubilizing the reagents, if they are soluable in water or the solvent used, they might not help. They work
best in two phase reactions where the substrate is in the organic layer and the reactant (often an ion, like Br-, CN-, or -OH) is in the aqueous
layer, such as a bromine substitution with HBr or hydroysis with base.
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Electra
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Here is a paper on the mechanism for transfer hydrogenation of the RuCl2 complexes. It seems the base part of the complex plays an important role.
Very good diagrams of these mechanisms. Judging by this, my proposed idea will not work, unless there is another mechanism at play. I will try it
anyways just to see.
Here is the relevant study on the mechanism, I've gotta go out, gonna take another look at this when I get back:
http://www.diva-portal.org/smash/get/diva2:217223/FULLTEXT02...
Taking a look at this Paper, some very interesting concepts are proposed. Look at this:
| Quote: |
In the outer-sphere mechanism, the reaction proceeds through the outer
sphere of the metal without coordination of the alcohol to the metal. This
kind of mechanism was first proposed by Noyori et al. for his TsDPEN cata-
lyst 3 (Scheme 7). 23 The alkoxide intermediate mechanism could be ruled
out since the reaction could be performed without base.
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[Edited on 10-3-2014 by Electra]
[Edited on 10-3-2014 by Electra]
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