The aldol reaction has a chair-like transition state
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص869-870
2025-07-21
362
The aldol reaction has a chair-like transition state
These are the experimental facts: how can we explain them? Aldol reactions are another class of stereoselective process with a cyclic transition state. During the reaction, the lithium is transferred from the enolate oxygen to the oxygen of the carbonyl electrophile. This is represented in the margin both in curly arrow terms and as a transition state structure. A six-membered ring is involved, and we can expect this ring to adopt more or less a chair conformation. The easiest way to draw this is first to draw the chair, and then convert atoms to O or Li as necessary. Here it is.
In drawing this chair, we have one choice: do we allow the aldehyde to place R equatorial or axial? Both are possible but, as you should now expect, there are fewer steric interactions if R is equatorial. Note that the enolate doesn’t have the luxury of choice. If it is to have three atoms in the six-membered ring, as it must, it can do nothing but place the methyl group pseudoequatorial. The aldol formed from the favoured transition state structure, with R pseudo equatorial, is shown below—first in the conformation of the transition state, and then flattened out on to the page, and it is anti.
We can do the same for the cis enolate. The enolate has no choice but to put its methyl sub stituent pseudoaxial, but the aldehyde can choose either pseudoequatorial or pseudoaxial. Again, pseudoequatorial is better and the reaction gives the product shown—the syn aldol.
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