[stewie griffin]

Hey all,
I'm trying to do an oxymercuration/demercuration reaction and I'm just not having any luck with my substrate. I have done simple controls like oxymercuration/demercuration of styrene and also of 1-methylcyclohexene with no troubles. So I know my mercury acetate is good and I must have at least some clue how to properly run/work-up these reactions. However my substrate just won't react at all. Initially my substrate was a trisubstituted alkene. After that showed no activity I gave up on it and changed the structure of my substrate to a 1,1-disubstituted alkene, which are supposed to react significantly faster than a trisubstituted olefin. Still no reaction though 
I am doing 1:1 THF/water, though with the trisubstituted olefin system I had also tried acetone/water as well as ether/water. I have read that adding perchloric acid is supposed to lead to oxymercuration of "even stubborn" olefins within 1 hr. I tried that today with my 1,1-disubstituted olefin... all I got was an 82% yield of silyl deprotected starting material (took my TBS and TIPS right off). Absolutely no oxymercuration product found.
Does anyone know whether the color of the reaction provides any info? When you first dissolve the mercury acetate in water and then add THF, the solution turns bright yellow. Upon addition of substrates that work (like styrene) the solution rapidly changes to clear/colorless. However with my actual substrate, the yellow color never dissipates. Thus that's why today I decided to add some perchloric acid after 1 hr of stirring with mercury acetate. Although the yellow color dissipated quickly, as I said no desired product was formed.
Any ideas how to perhaps troubleshoot this?

[bromidewind]

If you're working with an alkene, then maybe the reason it's not reacting well is due to its inability to rotate about the double bond and the substitution.

As for the color, I would wager that it has something to do with the size of the molecules involved; a highly conjugated and substituted double bond(s) is going to give you some pretty good absorption.

If neither of those are right, I'm not sure where else to go.

[orgopete]

This is a reaction that I have thought about in the past as it seems paradoxical. In a competition of water or acetate or an alkene with mercury, an alkene would seem to be the least basic. Why then should an alkene react? The success of the reaction cannot simply be the strength of the complexation, yet the reaction can work.

I've not done an oxymercuration reaction, but from reading the descriptions, I infer the reaction is not instantaneous, like an acid-base neutralization. Therefore, the kinetics of the reaction must be one factor. The other factor that I have thought about is that mercury would prefer a bond to a carbon than to an oxygen. If mercury is behaving as a Lewis acid, then sharing electrons with a carbon would be better than sharing electrons with an oxygen. Therefore, one might think that the reaction would like to occur to give a more stable product.

This suggests that mercury is an in equilibrium with acetate, water, and the alkene. If the reaction of the complex is reversible, then its success would depend entirely on the rates of the forward and reverse reactions. I don't know if that is why some alkenes react and other do not, it is just a guess. Given than, I would try a different mercury salt, such as a trifluoroacetate.

[onawire]

Did you end up getting the reaction to work?  Did you try the Hg(TFA)2 instead of the acetate?

I'm working on a rather BULKY cyclic alkene trying to form the ether.  So I have Hg(OAc)2 with Anhydrous MeOH.  Tho, my Substrate has a difficult time dissolving in most (ALL!!!) solvents except for chlorinated solvents.  So I had to put some DCM into the reaction, making the solvent medium to be 2:1 (MeOH:DCM).

I'm monitoring by TLC and NMR, but after 6 hours, NOTHING!!  Starting material just sitting there.  I haven't been able to find out if chlorinated solvents are bad for this reaction?


Also,  I started out with just MeOH (no DCM), I was hoping that my SM would get pulled into the reaction, but it didn't.  When I removed two small amts to test for solubility with DCM and CHCl3 - both went into solution but they both turned a yellowish color (almost bright yellow)?  The CHCl3 turned yellow much faster, but i'm not sure if that was just because the aliquot was bigger??

Anybody have an suggestions?

Thanks,
paul

REFs:

J. Org. Chem., Vol. 49, No. 14, 1984
Eur. J. Org. Chem 2006 3181 - Following this method!

[stewie griffin]

Sorry for the late response...I graduated and got a job so it's been a while since I've thought of this reaction. Long story short I eventually got some rxn (about 20%) but the stereochemistry ended up being the undesired product so we gave up anyway. I don't remember what I did magically to get that 20% yield....I think I used the Hg(TFA)2 but I really never had much luck with this (stupid) reaction. I'm still bitter about it 

[Halogenator]

This is a reaction that I have thought about in the past as it seems paradoxical. In a competition of water or acetate or an alkene with mercury, an alkene would seem to be the least basic. Why then should an alkene react? The success of the reaction cannot simply be the strength of the complexation, yet the reaction can work.

Water and acetate are hard. Hg²⁺ and olefins are soft. This is why Hg²⁺ works better than H⁺.