Author Topic: alcohols to aldehydes w/ K2Cr2O7 ; solvent-free  (Read 2027 times)

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psyloxy

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alcohols to aldehydes w/ K2Cr2O7 ; solvent-free
« on: July 14, 2004, 10:23:00 AM »

Tetrahedron Lett.; 43,49.2002;8843-8844

Selective solvent-free oxidation of alcohols with potassium dichromate

The oxidation of primary alcohols to the corresponding aldehydes has been accomplished with large number of hexavalent chromium compounds. 1-4

Potassium dichromate (K2Cr2O7), a readily available and inexpensive reagent, has for some times been used as an oxidant in this reaction. Unfortunately, the traditional K2Cr2O7 oxidation methods in this transformation are limited by performing in acidic aqueous solution. Under the reaction condition, the oxidising reagent can further oxidise the aldehyde to the acid, since in an aqueous medium, the aldehyde product can be hydrated to the geminal diol, which is further oxidised. Therefore, to isolate the aldehyde, it should be continuously removed as it is formed, usually by distillation. This technique can be used successfully only with relatively volatile aldehydes, and yields are moderate. In addition, under acidic medium, the aldehyde product and unreacted alcohol react to give a hemiacetal that is rapidly oxidised to an ester. Since oxidation with K2Cr2O7 under anhydrous neutral media would appear to be a way to circumvent the overoxidation and the ester formation, in recent years the traditional K2Cr2O7 oxidation has been modified. Improved K2Cr2O7 oxidation methods, such as oxidation under phase-transfer catalysis,5 oxidation with supported reagents,6,7 and oxidation in a polar aprotic media,8,9 have been developed that allow the reaction to be conducted in organic media and under neutral conditions. Most of existing K2Cr2O7 oxidations are performed at temperatures above ambient.

We now report here that the oxidation of primary alcohols with K2Cr2O7 at room temperature under solvent-free conditions is a new system that offers a very simple and efficient selective oxidation method for the preparation of aldehydes. In the present method, overoxidation and formation of an ester can be prevented by the use of solvent-free conditions under which the aldehyde produced is stable. On the other hand, in comparison with the methods described above, the advantages of the present method are that it does not require the use of phase-transfer catalysis, the preparation of supported reagents, the usage of expensive polar aprotic solvents, and therefore it offers special promise for the preparation of aldehydes by the oxidation of alcohols into aldehydes. In addition, the present method can also successfully oxidise secondary alcohols to the corresponding ketones.

In the present experiments, commercial K2Cr2O7 with grinding to a fine powder and a 1:1 molar ratio of K2Cr2O7:substrate is employed. First, the oxidant is carefully added to the substrate and the mixtrue is stirred magnetically at room temperature until TLC analysis indicates a complete reaction. In general, the oxidations are complete within 5 h. The reaction mixture is then worked up by dilution with dichloromethane or diethyl ether and filtration. Distillation of the solvent gives a product that is of acceptable purity for most purposes. For silid substrates, the best method for doing this oxidation is first to dissolve them with a minimum amount of dichloromethane or diethyl ether at room temperature and then to perform the reaction. The results, which are shown in table 1, show that the method is generally applicable to a wide range of alcohols, and gives the corresponding products in good yields.



In conclusion, this solvent-free oxidation of alcohols using K2Cr2O7 as an oxidant at room temperature is a new and efficient method for the preparation of the corresponding aldehydes and ketones.

Caution: K2Cr2O7 is a toxic agent. All chromium(VI) reagents must be handled with care. The mutagenicity of chromium(VI) compounds is well documented.10 We have worked with quantities of 0.5-1g of alcohols. Any mixture of potassium dichromate and an organic substance must be viewed as potentially unstable. Special care must always be exercised in adding K2Cr2O7 to organic media.

References
1. Fieser, L.F.; Fieser, M. Reagents for Organic Synthesis; John Wiley & Sons: New Yorck, 1967; Vol. 1, pp. 14-147 and subsequent volumes in the series.
2. Freeman, F. In Organic Synthesis by Oxidation with Metal Compounds; Mijs, W. J.; de Jonge, C. R. H. I., Eds.; Plenum Press: New Yorck, 1986; pp. 68-81.
3. Haines, A. H. Methods for the Oxidation of Organic Compounds, Alcohols, Alcohol Derivatives, Alkyl halides, Nitroalkanes, Alkyl Azides, Carbonyl Compounds, Hydroxyarenes and Aminoarens; Academic Press: London, 1988; pp. 17-41.
4. Ley, S. V.; Madin, A. in Comprehensive Organic Synthesis; Trost, B. M.; Fleming, I., Eds.; Pergamon Press: London, 1991; Vol. 7, pp. 251-289.
5. Hutchins, R. O.; Natale, N. R.; Cook, W. J.; Ohr, J. Tetrahedron Lett. 1977, 18, 4167.
6. Santaniello, E.; Ferraboschi, P. Nouv. J. Chim. 1980, 4, 279.
7. Climent, M.S.; Marinas, J. M.; Sinisterra, J. V. React. Kinet. Catal. Lett. 1989, 38, 13.
8. Santaniello, E.; Ferraboschi, P.; Sozzani, P. Synthesis 1980, 646.
9. Lou, J.-D.; Lu, L.-H.; Liu, W. Synth. Commun. 1997, 27, 3701.
10. Cupo, D. Y.; Wetterhahn, K. E. Cancer Res. 1985, 45, 1146

--psyloxy--

starlight

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very interesting
« Reply #1 on: July 14, 2004, 11:17:00 AM »
It raises the question in my mind as to whether a similar reaction (solvent free conditions) can be carried out on propenylbenzenes to make aldehydes without over-oxidation using dichromate or permanganate.

Or would the low solubility of dichromate in the propenylbenzene may make this impractical and hence the need for THF/H2O cosolvent?

Vitus_Verdegast

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telepathy ;-)
« Reply #2 on: July 14, 2004, 11:20:00 AM »
I was just asking myself the same thing!  :)

Although the double bonds in these examples don't seem to be affected. Maybe because of the 1:1 ratio it will always favor the alcohol..


java

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Alcohols to Aldehydes........
« Reply #3 on: July 14, 2004, 12:47:00 PM »
A very good find , this may be  good way to make Hydratropic Aldehyde........also the benzaldehyde for the L -PAC  project Org. has been spending lots of time on......

Congratulations Psyloxy.........java


OcoteaCymbarum

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Any idea about scalability?
« Reply #4 on: July 16, 2004, 06:53:00 AM »
If anyone has tried that procedure, what was it like?
My real question would be about scalability. In the paper they use 0.5 to 1g alcohol.
I wonder because if some some reason a bee was able to get benzyl alcohol, the cromate and nitroethane, would that oxidation at a biggerscale feasible to get to the nitropropene?

The boiling point of benzaldehyde is 179. BEnzylalcohol is 205 celsius. Would distilling out the benzaldehyde as it is formed work? Would that reduce or prevent any possible overoxidation to benzoic acid(Bp 249 Celsius)? Im curious about this method, it seems easy and straight forward. I wish the papers had a little more details about possible scalability. After all, it would be of little interest IMHO to the chemistry community if it wasnt at least a bit scalable. I dont see why not. Any opinions?

armageddon

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Removal of the formed benzaldehyde during the...
« Reply #5 on: July 16, 2004, 02:17:00 PM »
"to isolate the aldehyde, it should be continuously removed as it is formed, usually by distillation. This technique can be used successfully only with relatively volatile aldehydes, and yields are moderate. In addition, under acidic medium, the aldehyde product and unreacted alcohol react to give a hemiacetal that is rapidly oxidised to an ester."
(from first post in this thread - but there are no acidic conditions present, so...)

Removal of the formed benzaldehyde during the reaction (as it is formed) should present no problem - but be sure to use a column, because the bp's of BA and benzylOH are closer than 30°C together, so for proper separation, a vigreux or similar is necessary...

(I don't think the yields would be lower, compared to other isolation techniques, if continuous BA removal through distillation is done - just think about LeChatelier. The equation contains BA on the product side, so if it is removed, more starting material reacts..)

Greetz A


WizardX

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Balanced Equation
« Reply #6 on: July 18, 2004, 08:18:00 PM »
2 KMnO4 + 3 H2SO4 + 5 Benzylethylalcohol ==> K2SO4 + 2 MnSO4 + 8 H2O + 5 benzaldehyde

longimanus

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LeChatelier(-Brown) Principle...
« Reply #7 on: July 19, 2004, 08:59:00 AM »
Hmm, since benzaldehyde could not oxidize Cr(III) to Cr(VI) I don`t see any equilibrium and the LeChatelier principle is not applicable for this reaction. Maybe armageddon meant that if benzaldehyde is continuously removed from the mixture the dichromate reacts only with benzyl alcohol.

And some questions, WizardX. How are you planning to obtain benzaldehyde from benzylethylalcohol (or isn`t it 3-phenylpropanol?) without any C-atoms detached from its molecule (or maybe you meant phenylmethanol)? What happened with Cr? ;)

K2Cr2O7 + 4H2SO4 + 3C6H5CH2OH -> 3C6H5CHO + Cr2(SO4)3 + K2SO4 + 7H2O

WizardX

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Sorry all!
« Reply #8 on: July 20, 2004, 03:19:00 AM »
longimanus: Yes, your correct. I was incorrect in saying benzylethylalcohol. I meant benzyl alcohol, C6H5CH2OH

Sorry all!

Rhodium

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NaBr-Oxone: Benzyl Alcohols to Benzaldehydes
« Reply #9 on: September 10, 2004, 06:19:00 PM »
Oxidation of Benzyl Alcohols to Benzaldehydes with Oxone and Sodium Bromide
Bon-Suk Koo, Chang Keun Lee, Kee-Jung Lee

Synth. Commun. 32(14), 2115-2123 (2002)

(https://www.thevespiary.org/rhodium/Rhodium/chemistry/alcohol2aldehyde.oxone-nabr.html)

Abstract
Reaction of benzyl alcohols with Oxone and sodium bromide in aqueous acetonitrile gave the corresponding benzaldehydes in excellent yields. However, electron-rich benzyl alcohols afforded ring bromination products via bromodecarbonylation of the resulting benzaldehydes.




amine

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Here you go Rhodium.
« Reply #10 on: September 12, 2004, 12:45:00 PM »
Heres the PDF which includes the missing chart.





indole_amine

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benzyl alcohols to aldehydes using hypohalites
« Reply #11 on: October 20, 2004, 07:19:00 AM »
Hi!

I hope this wasn't posted before; if it was, I apologize in advance for crossposting... :)


Aromatic Aldehydes from Benzyl Alcohols via Inorganic Hypochlorite Oxidation
(C.Y. Meyers)
JOC 26, 1046 (1961)



Abstract
The action of inorganic aequous hypochlorite on several benzyl alcohols has been studied. Benzaldehyde and ortho-methoxybenzaldehyde were thus obtained in good yield from their respective alcohols. Under the conditions employed there was no evidence of reaction between the hypochlorite and these aldehydes. o-Hydroxybenzyl alcohol, under identical conditions, provided no aromatic aldehyde but was chlorinated in the nucleus with a concurrent elimination of formaldehyde. Possible mechanisms are considered.



(also referenced to under (11) in 

Post 444048 (missing)

(Aurelius: "Tetrahedron letters 23, 1, 35-38, (1982)", Methods Discourse)
)


indole_amine