panziandi
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Direct Iodination of Arenes with CrO3/I2
I would like to share with you a reference a friend gave me some time ago that at the time didn't appeal to me.
Lulinski, P. Skulski, L. The Direct Iodination of Arenes with Chromium (VI) Oxide as the Oxidant. 1997, 70, Bull. Chem. Soc. Jpn., 1665-1669
It basically describes the iodination of both activated and deactivated arenes using iodine and chromium (VI) oxide in the acidic anhydrous medium
AcOH/Ac2O/H2SO4. Yields are very high 60-80% for most of the arenes tested. Below is the method given for monoiodination of deactivated arenes.
CrO3 (6.25g, 62.5mmol, 25% excess) added slowly to a mixture of AcOH (40mL) and Ac2O (20mL, 212mmol) the temperature being kept below 40*C. Once
dissolved I2 (7.94g, 31.2mmol, 25% excess) added slowly temperature being kept below 40*C. H2SO4 (18.66mL, 350mmol) (note 1) added dropwise being kept
below 30*C. Next the arene is added (50mmol). The mixture is left for 3-6hrs at room temperature then raised to 45*C for 1hr. The reaction mixtures
are then poured into a solution of Na2SO3 (12.6g, 100mmol) and (NH4)2CO3 (50g, 520mmol) in 150g iced-water. This is left for 1hr and the solids
filtered washed with cold water, dried and recrystalised.
1) quantity of H2SO4 depends on which arene is being used, this is for benzoic acid (giving 3-iodobenzoic acid as the product)
When I attempted this I scaled it down by a factor of 10. I left the mixture overnight (12hrs) with no heating to 45*C. The mixture starts dark
brown/red and ends up dark green. I had plenty of ppt but I fear my water washings dissolved some of product and I used i-PrOH and not chloroform to
recrystalise which I think lead to a lot of loss, as a result my yield was a crappy 10%-ish (paper quotes 86%). Had I followed the paper results would
have been much better. I shall be repeating this on the scale they used in the paper.
If anybody else tries similar reactions from it perhaps they could post results here? Enjoy 
Attachment: Cr6+ iodination of arenes.pdf (954kB)
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panziandi
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I had my sample analysed by a friend at university using IR and NMR it appears that what I isolated was most likely the anhydride of 3-iodobenzoic
acid  . What likely happened is that my water washings washed most of the free
acid away and into the chromium waste which I had disposed of before realizing what had happened, leaving the more insoluble and fairly water-stable
anhydride in my filter. I am currently in the process of repeating this experiment hopefully with better results.
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smuv
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I don't have a specific reference, but in the past I have seen aromatic iodation with Iodine in nitric acid; same idea, I think it goes through the
formation of an I+ species.
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panziandi
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Well this paper says I+3 species and then I+1 species in the second step of the reaction are present in this method. In fact it describes the
diacetoxyiodoarenes can be isolated from the reaction mixtures.
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