[orgopete]

Two points. Without wishing to debate the use of the terms kinetic and thermodynamic, I brought up Ex. 5 from the MSU ref because I hoped you would think, "that is a different reaction." I hoped you might also think the reactions of dilute acid and concentrated acid might also differ, even if only by concentration.

Although I hadn't looked at the MSU mechanisms, until disputed,  I thought it fit the data and seemed plausible. Protonation of aldehydes and ketones are common intermediates in many mechanisms, for example ketalization or enolization reactions. How would you propose an acid catalyzed enol formation without protonation?

[souro10]

Well, Enol formation can take place under acid catalysis, and in this case that is precisely what is happening ( in the mechanism I proposed )

However, my point is different. What I meant was, the carbonyl oxygen can get protonated. It can form resonance structures, and can lead to enolization/ketalization. However, none of those reactions, that I've heard of which occur via protonation of carbonyl oxygen, result in a 1,2 carbocation rearrangement. A carbocation rearrangement required sufficient , full positive charge - an actual carbocation to be formed first. Only then it can undergo rearrangement (if it has scope). I don't see how protonation , in this case would lead to carbocation formation.

Can you? 

[orgopete]

The difference between enol formation and the rearrangement is in both instances, the neighboring electrons are attracted to the protonated carbonyl carbon, but if a proton is attached, it can become lost while if it is a carbon, it cannot.

[souro10]

@Orgopete - Sorry, I could not understand what you meant.
My question is, precisely , is - is it plausible to think/how is it justified that the protonation of a carbonyl oxygen can lead to sufficient positive charge on the carbonyl carbon for a carbocation 1,2 shift to take place, as depicted on MSU website? In my opinion it is not plausible because there would not be sufficient energy for the rearrangement to take place. Recall why rearrangements do not occur in SN2 reactions. Why? Because there is delta plus- not sufficient energy.

Can you post some other literature/ reference which depicts the mechanism shown on MSU website ( for example 1 ) , hopefully with more clarity and reasoning?

[orgopete]

@Orgopete - Sorry, I could not understand what you meant.
My question is, precisely , is - is it plausible to think/how is it justified that the protonation of a carbonyl oxygen can lead to sufficient positive charge on the carbonyl carbon for a carbocation 1,2 shift to take place, as depicted on MSU website? In my opinion it is not plausible because there would not be sufficient energy for the rearrangement to take place. Recall why rearrangements do not occur in SN2 reactions. Why? Because there is delta plus- not sufficient energy.

Can you post some other literature/ reference which depicts the mechanism shown on MSU website ( for example 1 ) , hopefully with more clarity and reasoning?

Re: rearrangement reaction in SN2 reactions
Remember, SN2 reactions are on the nucleophile initiated end of the substitution spectrum. In SN1 reactions, bond breaking occurs before bond formation. In SN2 reactions, it is the opposite, bond formation precedes bond cleavage. The bond formation increases electron density on the target carbon, it would be unusual for a rearrangement to occur. If a reaction were to be more SN1-like, then you may see this happen. Reactions can be thought to occur somewhere between the two extremes I have noted.

Re: ability of a carbonyl carbon to attract electrons, rearrangement or addition
My argument was simple, although it is unusual for this reaction to take place, it should be taken in context of all carbocations. Granted that non-bonded electrons of an attached oxygen will limit the attractiveness to that carbon, but this should also be taken in context of are there any attacks? Yes, ketalization and hydrolysis involve direct attack upon a carbonyl carbon. Even an enolization must be an attraction of the C-H electrons to the carbon.

Perhaps it is unusual for a carbon to migrate, so the instances are limited to groups that can provide greater stabilization that normal, such as the diphenyl case. I also noted that Reusch suggested that a protonated epoxide may be considered. I'd also concede that though Reusch did not give any compelling examples to argue this point.

This is my perspective upon this question and chemistry in general. Our models are not complete. It isn't possible to have partial charges, for example. Electrons are negative and protons positive and only involve full charges. If you compare the oxygen of hydroxide, water, and hydronium ion, the oxygen is +8 in all instances. What changes is the number of protons attached to the electrons. They have an effect upon the electrons and their availability, but no effect upon their charge. If you use this perspective, then you may look at reactions a little differently. It isn't whether a functional group should react in a certain way, it is how the assemblage of atoms may affect the electrons. For example, you may find some books write a hydroboration reaction as though it were a (symmetry forbidden) 2+2 reaction and HCl does not. Borane is a Lewis acid and is attracted to the electrons of an alkene. What I ponder is how this should affect a reaction, for example, boron holds its electrons much more weakly and its bond lengths are greater.

I have included a scheme and I suggest it should not be unusual to see attack upon a protonated carbonyl at the carbon atom. Enolization can be thought as a neighboring group reaction. The rearrangement may not be a general reaction (hence my error in assumption about it). I think the examples given are consistent with other chemistry. For example, protonation of an enol ether can be written with the same intermediate as the enolization step. If that is correct, then migration of a phenyl group may also occur similarly, especially if it were to lead to a "stabilized carbocation". (I too found it difficult to think of an carbocation alpha to a carbonyl group to be more stable.)

Granted, mechanisms are rationalizations. I agreed with Reusch's mechanism. It seemed plausible and consistent.

[souro10]

Well, I really appreciate your efforts.

I agree to this mechanism of yours, however there still remains a doubt. In the second rearrangement in your scheme, that is the one with three phenyl substituents, how does the third intermediate( from the left ) reach the final( or the fourth) product ? By loss of a proton? That is by enol formation? The mechanism for this particular rearrangement on the MSU website ( example 2 ) is slightly different, which I considered not plausible. For both Examples 1  and 2 , they are showing that a rearrangement or hydride shift is occurring instead of a proton loss which would lead to enol formation.

Secondly, I'd like to know how you'd account for the fact that the C-label is scrambled, using your mechanism? Would you go for an epoxide intermediate ? How would you accommodate an epoxide intermediate with this mechanism, for the triphenyl case?

And lastly, can we now conclude - in Pinacol-Pinacolone rearrangements, if the initially formed product under mild acid treatment is an aldehyde, it can rearrange itself into a more stable ketone ( often, a phenyl-ketone conjugated system ), although rearrangement of ketones into more stable ketones have not been reported. How can we account for the fact that ketones ( as in my original question ) do not usually rearrange into ore stable ketones? Is it because the hybridization changes from sp2 to sp3( and bond angle decreases from 120degrees to 109.5 degrees) in the migration-transition-state and hence for ketones steric factor kicks in? I think there could be further factors which does makes ketones rearranging into more stable ketones upon protonation uncommon. We must keep in mind the fact that when a Nucleophile attacks the carbonyl-carbon , it does so from an angle of approximately 107 degrees with the oxygen atom. Now during rearrangement of ketones due to the very nature of the rearrangement (anti-periplanar) , keeping such an angle of attack becomes more difficult due to sterric congestion by the already existing substituent.

I was going through " Acid catalyzed rearrangement of Aldehydes and ketones " from Jerry March's Advanced Organic Chemistry. From this book, and also the reference research papers mentioned at the bottom of the pages, it is obvious that the reaction involves an epoxide intermediate. And in the first step, it seems that there is simultaneous Neighboring Group Participation by oxygen and Phenyl !

[orgopete]

Okay, I hadn't read the Reusch comments completely. In order to scramble the label, an epoxide or a protonated epoxide intermediate makes the most logical sense.

Re: hydride shift
I was expecting enol formation via deprotonation. Then I was looking at an abstract of a paper for the rearrangement of glyceraldehyde to pyruvaldehyde and dihydroxy acetone. Proton exchange with solvent did not occur to the methyl group, the label showed a hydride shift occurred. Although this could have been enol formation, the lack of incorporation of a label shows a hydride had migrated. Mechanistically, this should be the same reaction.

Re: ketone rearrangement reactions
I expect electronic effects to be controlling. As noted, diphenyl carbocations could form a reaction sink. None the less, migration may not occur to a neighboring tertiary carbon. The non-bonded electrons of oxygen would stabilize a carbocation. If the other substituent was a carbon rather than a hydrogen, it too would stabilze the carbocation and thus be less attractive for migration.

Re: stereo electronic effects
We already know a migration occurs to triphenylacetaldehyde, so any stereoelectronic barrier should be equal.

Returning to the original question, I think the alternate product could be formed. Whether it does or not may be another question. I know I have done reactions in which I had enough by-products to support many additional reactions. In order to answer that question, I'd look at the papers Reusch was citing. How good were the reactions, yield, by-products, etc. Low yields could account allow the reaction originally posted (in addition to others). If one wanted to find alternate products, obviously the pinacol is one low energy path, one can bias the conditions toward another reaction, as was the case here.

Sorry for the long-winded coming to an agreement with your original proposal. A case of too much ignorance and sloppiness.