[zsinger]

All,
Was flipping through some old notes from lab, and somewhere, I dropped a little acid (I think it was phosphoric or sulfuric) in a mix with 2-phenylpropanal and ended up with a bit of phenyl acetone (I Think based on NMR)………Is this some sort of methyl shift, as I have drawn out the mechanism and cannot see a driving force for the methyl to shift?  Any help appreciated!

[zsinger]

Any help appreciated.  Thanks.

[orgopete]

There is an example of an unusual rearrangement posted on Reusch's MSU website. This resulted in some discussion in this forum, http://www.chemicalforums.com/index.php?topic=64473.msg232682#msg232682 (which I completely screwed up). But, none the less, I think the rearrangement can be found in Reusch's website.

[zsinger]

Ok, I think I understand this re-arrangement.  PLEASE correct me if I'm wrong.  Once we pour an acid into 2-Phenylpropanal, we should naturally protonate the Oxygen on the aldehyde.  This gives it a "+" Charge, which, PERHAPS, wants a methyl group to hyper conjugate with for electron density RATHER than a little old Hydrogen.  Just a postulate, and I am not sure if it correct, however it seems to make sense.  With a "CH3" group, the aldehyde becomes a MORE STABLE ketone in this case, as a result of the said methyl shift.  Gosh, usually the mechanism tells me all I need to know!!!!!!!  Thoughts?

[orgopete]

Ok, I think I understand this re-arrangement.  PLEASE correct me if I'm wrong.  Once we pour an acid into 2-Phenylpropanal, we should naturally protonate the Oxygen on the aldehyde.  This gives it a "+" Charge, which, PERHAPS, wants a methyl group to hyper conjugate with for electron density RATHER than a little old Hydrogen.  Just a postulate, and I am not sure if it correct, however it seems to make sense.  With a "CH3" group, the aldehyde becomes a MORE STABLE ketone in this case, as a result of the said methyl shift.  Gosh, usually the mechanism tells me all I need to know!!!!!!!  Thoughts?

I am not going to research this, so this is what I recall. The original question was virtually about reversing a pinacol rearrangement. Normally this gives the more stable carbocation to give the product. I think this went something like this. 1,1-Diphenylethane-1,2-diol gives diphenyl acetaldehyde as the product, as expected. However, diphenylacetaldehyde when treated with H2SO4 will rearrange to give 1,2-diphenylethanone. In this case protonation of the C=O group gives a carbocation which can rearrange to a phenyl stabilized carbocation. Loss of a proton or hydride shift gives the protonated ketone. There were some labeling experiments reported in the Reusch's website to support the rearrangement (which I hope I got right).

If so, then this problem should be similar. Methyl migration would give a phenyl stabilized carbocation, loss of the proton will lead to the methyl ketone.

[zsinger]

Is this SIMPLY a reverse pinacol re-arrangement?  I am still having trouble with the mech, but I KNOW it works in the lab, as I have seen it work!!!!!
         -Z

[orgopete]

Go to http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/rearrang.htm, scroll down to pinacol rearrangements and click on example #2.