Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: tropane on April 19, 2011, 11:26:09 AM
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Can anyone tell me what the mechanism and reaction name of the cyclisation of ortho-methylbenzenediazonium tetrafluoroborate to 1H indazole using the phase transfer catalyst, crown ether and potassium acetate? I think its a little like Richter but without water.
If anyone can help I would much appreciate it.
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18-crown-6 is known to dramatically increase the reactivity of anions by complexation of the cation.
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Yes I agree with you thanks for that but I need to know the mechanism
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Well lets try another hint then: the 18-crown-6 is probably not acting strictly as a phase transfer catalyst.
Have you tried to solve this? What do you have so far?
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Well I am assuming that the 18 crown-6 and poatssium acetate act as nucleophile and so abstract a H from the methyl group. This inturn creates a -CH2- nucleophile which attacks the azo species resulting in a 5 membered ring. However I am unsure of the subsequent electronic configuration and whether there is a unstable positive centre at the 1 postion in the benzene ring and how this inturn creates a stable indazole product.
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Bases remove protons, not nucleophiles.
However, things seem reasonable so far. What I was getting at before is that normally acetate is a pretty mild base without much hope of attacking that benzyl site. However, activation by the crown increases its basicity by effectively removing potassium from the scene. From there, I believe this reaction has more in common with azo coupling than the von Richter reaction, i.e carbon nucleophile attacking a diazonium. I've no idea if this reaction has a name or not.
Not sure how you're getting a positive charge at the 1-position on the benzene. After the initial deprotonation, the net charge should be neutral. You've also got to go from the 3H-indazole to the 1H-indazole.
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Ok thanks for your help so far... im assuming that the remaining potassium acetate base abstracts a hydrogen at the 3 position forming a double bond which in turn causes the azo double bond to attck a a near by proton or would it be the same proton it removed at the 3 position? see diagram
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Bases remove protons, not nucleophiles.
all bases are nucleophiles, but not all nucleophiles are bases.
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Or something like this also makes sense:
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all bases are nucleophiles, but not all nucleophiles are bases.
azmanam brings up a good point and I should clarify my previous statement. I meant that when a species is abstracting a proton, it should be referred to as a base for this very reason.
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Thanks guys for all your input I have a couple more which I will post with mechanisms and I will try to figure out tomorrow.