Conjugate Addition with
Chapter 18: Enols
Reaction type : Nucleophilic Addition
- Organolithium cuprates, R2CuLi
are particularly useful for conjugate or 1,4-addition to α,β-unsaturated
aldehydes and ketones.
- Lithium dialkylcuprates are formed
from organolithium compounds (review)
organometallic reagents such as alkyl lithiums tend to undergo direct
or 1,2-addition, while Grignard reagents may give mixtures of 1,2- and 1,4-addition
depending on the system.
- Remember that like enolates,
organometallic compounds are a source of nucleophilic C systems
e.g. RMgX is equivalent to R-
- So it is "useful" to impose
ionic character on organometallics e.g. RMgX is equivalent
to R- Mg2+ X- or R2CuLi
is equivalent to (R-)2 Cu+ Li+
MECHANISM OF THE ORGANOCUPRATE
The nucleophilic C in the cuprate attacks the conjugated ketone
at the electrophilic alkene C in a nucleophilic
addition type process with the electrons being pushed through to the
electronegative O, giving an intermediate enolate.
On acidic work-up, the enolate is protonated at the α-C
creating the more favourable carbonyl group.
* This is a simplified representation of the mechanism