Re: KriticalMass intramolecular rxn
I agree with Heory about the missed charge on the 4-membered ring and his doubt of the mechanism. I am doubtful more from a bond angle and electrophilicity point of view. The carbonyl carbon should have a bond angle of about 120°. An intramolecular attack on the oxygen seems unlikely (unless you show some precedent). Will an alpha-halo amide should be more conducive of this reaction?
Re: other mechanisms
I concede to being at a disadvantage in that the only examples I am aware of for the Perkow reaction are those cited in Wikipedia. In the proposed mechanism with bromoacetone, how much Perkow product results in this reaction? Does the Arbusov reaction compete with it?
I am familiar with a bisulfite addition to aldehydes, but I am unaware of a phosphite equivalent. If phosphite does add to the carbonyl, then the reaction of triethylphosphite with bromo or chloroacetaldehyde should be an even better Perkow reaction. Is it?
Re: addition of electrons to the oxygen of a carbonyl
I would argue with Woodward on this one. While that may have been written, I would argue that it goes more to writing a short explanation of what must have happened, but not necessarily how. I am more inclined to think along the lines of a dissolving metal reduction or an acyloin condensation in which electrons are added to the carbon and resulting in breaking the pi-bond. If a second electron were added, it would add to the radical.
This example points to some very real quandaries in proposing mechanisms. The carbonyl group is not particularly basic in this example, so protonation by acetic acid seems an unlikely initial step. The mechanism does not involve acetic acid. Does the reaction succeed without it? Is acetic acid present to "clean" the zinc? There are other similar reductions, like a Clemmensen reduction. For it, I thought it important to protonate a carbonyl group first and then the electrons are added to a increasingly electrophilic carbon.
I do concede that upon searching, it is not difficult to find the proposed mechanism for a Reformatsky reaction. I had been taught that it inserted into the C-Br bond, but that should not be important. Since it now appears that a Zimmerman-Traxler transition state exists for addition of a Reformatsky reagent to an aldehyde, therefore the zinc is complexed with the oxygen and not the carbon. I can understand that a mechanism of addition of electrons to the carbonyl oxygen will result directly in that enolate. Is that compelling to the mechanism? How do electrons add to carbonyl groups? The reason I was skeptical was I can find many examples in which electrons are added to the carbonyl carbon, but I was not aware of a single example of an addition to the oxygen. For example, if it adds to the oxygen, then addition of a methyl Grignard to a bromoketone ought to give the same enol ether product. I don't know that none of that forms, but I don't think it is an important reaction.
Even so, I am not persuaded to believe any of the proposed mechanisms for the quinoline dione example in Wikipedia. An SN2 displacement on a tertiary trifluoroaceate does not seem likely. Direct displacement by attack on the oxygen of the trifluoroacetate does not seem likely. Perhaps addition to the oxygen of the carbonyl group would work, but I don't like that it is an aryl ketone with a vinylogous nitrogen donating electrons as well. That does not mean that one of those mechanisms may not present. It just means I am uncomfortable with matching their electron movements with other chemistry that I have knowledge of.
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Topic: what's the mechanism? (Read 17628 times)