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Author Topic: oxidation with MnO2  (Read 21504 times)

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g-bones

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oxidation with MnO2
« on: October 20, 2009, 02:21:26 PM »

Hello everyone,

I am currently running an oxidation of a propargylic (and benzylic) alcohol with MnO2 and it is really sluggish.  I was wondering if anybody had any experience with this type of reaction before?  It maybe possible that the MnO2 is bad (its older) and I may need to prepare some of it fresh.  does anybody know any methods for the fresh prep of MnO2?
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Heory

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Re: oxidation with MnO2
« Reply #1 on: October 20, 2009, 05:10:49 PM »

Preparation of Active MnO2 from KMnO4 under Basic Conditions (Attenburrow).
A solution of MnSO4.4H2O (110 g) in H2O (1.5 L) and a solution of NaOH (40%; 1.17 L) were added simultaneously during 1 h to a hot stirred solution of KMnO4 (960 g) in H2O (6 L). MnO2 precipitated soon after as a fine brown solid. Stirring was continued for an additional hour and the solid was then collected with a centrifuge and washed with water until the washings were colorless. The solid was dried in an oven at 100-120 °C and ground to a fine powder (960 g) before use.

Preparation of Active MnO2 from KMnO4 under Acidic Conditions.
Active MnO2 was made by mixing hot solutions of MnSO4 and KMnO4, maintaining a slight excess of the latter for several hours, washing the product thoroughly with water and drying at 110-120 °C. Its activity was unchanged after storage for many months, but it was deactivated by H2O, MeOH, thiols, or excessive heat (500 °C). MnO2 was less active when prepared in the presence of alkali and ineffective when precipitated from hot solutions containing a large excess of MnSO4.

Preparation of Highly Active MnO2 from KMnO4.
A solution of MnCl2.4H2O (200 g) in H2O (2 L) at 70 °C was gradually added during 10 min, with stirring, to a solution of KMnO4 (160 g) in H2O (2 L) at 60 °C in a hood. A vigorous reaction ensued with evolution of chlorine; the suspension was stirred for 2 h and kept overnight at rt. The precipitate was filtered off, washed thoroughly with H2O (4 L) until pH 6.5-7 and the washing gave a negligible chloride test. The filter cake was then dried at 120-130 °C for 18 h; this gave a chocolate-brown, amorphous powder; yield 195-200 g. Alternatively, the wet cake was mixed with benzene (1.2 L) and H2O was removed by azeotropic distillation giving a chocolate-brown, amorphous powder; yield 195 g. The last procedure gave a slightly less active material.

Preparation of Active MnO2 by Pyrolysis of MnCO3.
Powdered MnCO3 was spread in a one-inch thick layer in a Pyrex glass and heated at 220-280 °C for about 18 h in an oven in which air circulated by convection. The initially tan powder turned darker at about 180 °C, and black when maintained at over 220 °C. No attempt was made to determine lower temperature or time limits, nor the upper limit of temperature. The MnO2 prepared as above was stirred with about 1 L of a solution made up of 15% HNO3 in H2O. The slurry was filtered with suction, the solid was washed on the Buchner funnel with distilled water until the washes were about pH 5, and finally was dried at 220-250 °C. The caked, black solid was readily crushed to a powder which retained its oxidizing ability even after having been stored for several months in a loosely stoppered container.

Preparation of g-MnO2.
To a solution of MnSO4 (151 g) in H2O (2.87 L) at 60 °C was added, with stirring, a solution of KMnO4 (105 g) in H2O (2 L), and the suspension was stirred at 60 °C for 1 h, filtered, and the precipitate washed with water until free of sulfate ions. The precipitate was dried to constant weight at 60 °C; yield 120 g (dark-brown, amorphous powder).

Preparation of Active MnO2 on Silica Gel.13n
KMnO4 (3.79 g) was dissolved in water (60 mL) at rt. Chromatographic grade silica gel (Merck, 70-230 mesh, 60 g) was added with stirring, and the flask connected to a rotary evaporator to strip off the water at 60 °C. The purple solid was ground to fine powder and then added with vigorous stirring to a solution of MnSO4.H2O (9.3 g) in H2O (100 mL). The resulting brown precipitate was filtered with water until no more MnII ion could be detected in the wash water by adding ammonia. After being dried at 100 °C for 2 h, each gram of this supported reagent contained 0.83 mmol of MnO2.

As shown above, a wide range of products of various activities are called active MnO2 and the results described in the literature are sometimes difficult to reproduce since the nature of the MnO2 which was used is not always well defined. Now, the commercial materials give reproducible results but they are not always convenient to perform all the oxidation reactions described in the literature. In addition, their origin (method of preparation) is not often indicated and comparison with the active MnO2 described in the literature is sometimes difficult. For all these reasons, the use of activated MnO2 has been somewhat restricted in spite of the efficiency and selectivity of its reactions since only an empirical approach and a careful examination of the literature allow selection of the suitable activity of MnO2 and the optimum reaction conditions for a defined substrate.
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movies

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Re: oxidation with MnO2
« Reply #2 on: October 22, 2009, 10:36:30 AM »

FYI, MnO2 oxidations are often really slow even with fresh stuff.  Good reagent can be the difference between complete conversion and not, though.  Are you using a big excess of MnO2?  As I recall, you usually use around 5 equiv.
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g-bones

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Re: oxidation with MnO2
« Reply #3 on: October 23, 2009, 12:05:30 PM »

Yeah, I pushed it to completion with 12 equivalents.  Very very clean reaction and quantitative yield, but unfortunately not feasible for scale-up  :-[

And thanks for your help Heory :)
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dunno260

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Re: oxidation with MnO2
« Reply #4 on: October 25, 2009, 06:26:44 PM »

I have never run either reaction, but I do know that my synthesis textbook mentioned that Barium permanganate (you prepare it in situ from some barium salt and potassium permanganate) is a good alternative to manganese dioxide.  Don't know if its less selective though.
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movies

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Re: oxidation with MnO2
« Reply #5 on: October 26, 2009, 10:34:21 AM »

Are you sure it wasn't Barium Manganate?  BaMnO4 is a different manganese oxidation state that KMnO4.
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StarvinMarvin

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Re: oxidation with MnO2
« Reply #6 on: November 17, 2009, 12:20:50 PM »

Seems we're on the same page here (almost). I've been trying to make the MnO2 oxidation (to a ketone) work and even with 12-15 eq. it takes ages to go to completion. When I decided to switch from DCM to chloroform and use 20 eq. I could get the job done in less than 30 hours but the downside is that the concentration fo starting material is usually very low. Today I decided to leave it and go for PCC and - works like a charm.
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stewie griffin

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Re: oxidation with MnO2
« Reply #7 on: November 17, 2009, 12:28:18 PM »

I do an allylic oxidation in my project. I used to use 20-30 equiv MnO2 in CH2Cl2 overnight. Sometimes it would still not be done. However, doing 20-30 equiv MnO2 in benzene at reflux does the trick in 2 hours. So if your molecule can stand it try that.
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g-bones

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Re: oxidation with MnO2
« Reply #8 on: December 01, 2009, 06:13:24 AM »

I found that, using 10 eq of MnO2 was sufficient if you set it up and just let it run in the back of your hood for about a week.  uses less metal but obviously needs more time.  something to think about if you have plenty of other chemistry to do in the meantime you might as well just set it up and forget about it for a while  :)
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Heory

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Re: oxidation with MnO2
« Reply #9 on: December 01, 2009, 09:29:53 PM »

I do an allylic oxidation in my project. I used to use 20-30 equiv MnO2 in CH2Cl2 overnight. Sometimes it would still not be done. However, doing 20-30 equiv MnO2 in benzene at reflux does the trick in 2 hours. So if your molecule can stand it try that.
Is stewie griffin fond of wine?  ;D
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