Conjugate Addition (The Michael Reaction)

We learned that the aldol reaction is reversible. The following shows a mechanism for the reverse process, the hydrolysis of an alpha,beta-unsaturated aldehyde.

The key new feature of this mechanism is the conjugate addition shown in red. In an a,b-unsaturated aldehyde (or ketone, or ester etc.) the pi orbital of the alkene can interact with the carbonyl pi orbital of the carbonyl. These two pi bonds are said to be conjugated, hence a conjugate addition. A conjugate addition can occur under acidic conditions (see above) or basic conditions:

What kinds of things are good nucleophiles for the Michael Reaction? Gilman reagents and nucleophiles with lots of charge delocalization, such as enamines or enolates of the following:

b-Diesters, b-ketoesters and b-diesters are more acidic than regular esters and ketones: check out their pka’s.

It’s possible to combine both a Michael reaction and an aldol reaction in a Robinson annulation!

a,b-Unsaturated Carbonyl Compounds: Electronic Considerations

Because of conjugation some of the positive charge we associate with carbonyl carbons can be shared on the beta carbon:

An electrostatic potential map suggests little electron density on the beta carbon:

Note how much different the charge distribution looks in butadiene:

Clearly, the oxygen in propenal is an electron hog. Without conjugation the alkene pi bonds keep their electron density separate.

A conjugate addition reaction is also called a “Michael Reaction,” and an electrophile in the Michael reaction (the a,b-unsaturated aldehyde) is called a “Michael Acceptor”.