[g-bones]

Hello all,

I recently ran an oxidation of the attached 3-bromo-2-methylpyridine in hydrogen peroxide (40%) in the presence of rhenium catalyst (MeReO₃)  I am not too sure about the mechanism so I have attached the one i purpose which regenerates the catalyst and water.  I was originally looking to have the rhenium do the oxidation and the hydrogen peroxide reoxidize the metal but the mechanism i developed looks as though the rhenium does not undergo an oxidation or reduction but the reduced portion is simply the hydrogen peroxide becoming water.  Any comments or suggestions about the mechanism. 

thanks for your help

[Dan]

I was originally looking to have the rhenium do the oxidation and the hydrogen peroxide reoxidize the metal

Yeah, this was my first thought as it is the "usual" scenario for these catalytic oxidations.

I suppose we need to know whether stoichiometric MeReO₃ will do this oxidation in the absence of hydrogen peroxide...

A quick glance at the following Synlett:

http://www.organic-chemistry.org/abstracts/literature/501.shtm

Suggests a peroxo-ruthenium species. If you take HO-Ru(Me)(O)₂-O-OH, instead of attacking the terminal O of the peroxide with the pyridine, attack the O bonded to the Ru with the other OH (ejecting water) instead to give:

  Ru(Me)(O)₂
 /  \
O--O

This is suggested by the authors as plausible, I vaguely remember these sorts of peroxo Ru complexes from my lectures a few years back, but the Letter does not include detailed mechanistic studies.

[stewie griffin]

I was going to post but wanted to do some lit checking first, then forgot...
Anyway, I agree with Dan about the three membered metallocycle. It's similar to allylic epoxidations with Al, Ti, and V reagents used in conjunction with peroxide. I have those on the brain b/c I ran one of them just a week or two ago. However, I have no lit refs for Re doing the same type of chemistry as those other metals.

[g-bones]

Thanks for your help guys.  I do remember many other metals proceeding via the metallodioxerane intermediate so I agree.  In combination with how strong of an acid rhenic acid is, if that were in large quantities in the reaction it would probably protonate the pyridine and prevent oxidation.