1,4-addition of enolates to enones (“The Michael Reaction”)
Enolates of carbonyl compounds will add to an α,β-unsaturated carbonyl compounds to give 1,5-dicarbonyl compounds. This is called the Michael reaction.
Enolates are excellent nucleophiles; although a second resonance form is present with the negative charge on oxygen, the resonance form with the negative charge on carbon tends to be more important for determining reactivity.
Notes: Note that in each case a carbon-carbon bond is being formed
Removal of a proton from the ketone with strong base (Step 1, arrows A and B) results in an enolate, which then performs a 1,4-addition on the α,β-unsaturated carbonyl compound (Step 2, arrows C and D), which is then protonated (Step 3, arrows E and F).
Sodium here isn’t specifically important, it just balances out the negative charge.
(Advanced) References and Further Reading
The Michael Reaction (or Michael addition) refers to the general addition of nucleophiles to a,b-unsaturated systems in a 1,4 manner rather than 1,2.
- Ueber die Addition von Natriumacetessig‐ und Natriummalonsäureäthern zu den Aethern ungesättigter Säuren
A. Michael, J. Prakt. Chem. 1887 35 (1), 349-356
The original paper by Arthur Michael on the addition of carbon nucleophiles to a,b-unsaturated systems. Note that even though Michael was an American chemist at Tufts College (see the very end of the paper), this was still written in German and published in a German journal since German chemistry journals were the best at the time.
- The Michael Reaction
Bergmann, E. D.; Ginsburg, D.; Pappo, R. React. 1959, 10, 179
Classic review on the Michael Reaction. This also has detailed experimental procedures towards the end, similar to what you might find in Organic Syntheses.
- The asymmetric Michael addition reactions using chiral imines
Jean d’Angelo, Didier Desmaële, Françoise Dumas, André Guingant, Vedejs, E.
Tetrahedron Asymmetry 1992, 3 (4), 459
Michael reactions can also be asymmetric; if you have a chiral imine instead of a carbonyl, then stereoselective 1,4-addition is possible.
- The Enantioselective Organocatalytic 1,4-Addition of Electron-Rich Benzenes to α,β-Unsaturated Aldehydes
Nick A. Paras and and David W. C. MacMillan
Journal of the American Chemical Society 2002, 124 (27), 7894-7895
This is a modern twist on the previous reference by Prof. MacMillan, who has made important contributions to organic chemistry. This paper shows that asymmetric Michael additions can be done through chiral imine intermediates by generating the chiral imine in situ using a chiral amine catalyst. The concept is simple and elegant.