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Nucleophilic conjugate addition

Reaction mechanism

Conjugate addition is the vinylogous counterpart of direct nucleophilic addition. A nucleophile reacts with a α,β-unsaturated carbonyl compound in the β position. The negative charge carried by the nucleophile is now delocalized in the alkoxide anion and the α carbon carbanion by resonance. Protonation leads through Keto-enol tautomerism to the saturated carbonyl compound. In vicinal difunctionalization the proton is replaced by another electrophile.

Reactions

Scope

Conjugate addition is effective in the formation of new carbon-carbon bonds with the aid of organometallic reagents such as the organozinc iodide reaction with methylvinylketone. An example of an asymmetric synthesis by conjugate addition is the synthesis of (R)-3-phenyl-cyclohexanone from cyclohexenone, phenylboronic acid, a rhodium acac catalyst and the chiral ligand BINAP. In another example of asymmetric synthesis the α,β-unsaturated carbonyl compound first reacts with a chiral imidazolidinone catalyst and chiral auxiliary to an iminium compound in an alkylimino-de-oxo-bisubstitution which then reacts enantioselective with the furan nucleophile. The immediate reaction product is an nucleophilic enamine and the reaction cascades with abstraction of chlorine from a chlorinated quinone. After removal of the amine catalyst the ketone is effectively functionalized with a nucleophile and an electrophile with ratio of 8:1 and 97% enantiomeric excess. This principle is also applied in an enantioselective multicomponent domino conjugated Addition of nucleophilic thiols such as benzyl mercaptan and electrophilic DEAD.

Toxicology

Suitably soluble Michael acceptors are toxic, because they alkylate DNA by conjugate addition. Such modification induces mutations, which are cytotoxic and carcinogenic. However, glutathione is also able to react with them and for example dimethyl fumarate reacts with it.

See also

References

"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/Nucleophilic_conjugate_addition, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Nucleophilic_conjugate_addition&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany