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Asymmetric Organocatalysis (Paperback, 2009 ed.)
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Asymmetric Organocatalysis (Paperback, 2009 ed.)
Series: Topics in Current Chemistry, 291
Expected to ship within 10 - 15 working days
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As nucleophiles, simple alkenes are typically so unreactive that
only highly active electrophiles, such as carbocations, peroxides,
and halogens will react with them. For the generation of
carbon-carbon bonds, milder methods will often be required.
Fortunately, it is possible to increase the reactivity of
alkene-type p-nucleophiles by introducing electron-donating
substituents. Substitution of one H with an OH or OR gives an enol
or a vinyl ether, which are already much better nucleophiles. Using
nitrogen instead of oxygen, one obtains even better nucleophiles,
enamines. Enamines are among the most reactive neutral carbon
nucleophiles, exhibiting rates that are even comparable to some
charged nucleophiles, such as enolates [1, 2]. Most enamines,
unfortunately, are sensitive to hydrolysis. The parent enamine,
N,N-dimethylvinylamine, has in fact been prepared [3], but appears
to be uns- ble. Enamines of cyclic ketones and many aldehydes can
readily be isolated, however [4-7]. The instability of enamines
might at first appear to diminish the utility of enamines as
nucleophiles, but actually this property can be viewed as an added
benefit: enamines can be readily and rapidly generated
catalytically by using a suitable amine and a carbonyl compound.
The condensation of aldehydes or ketones with amines initially
affords an imine or iminium ion, which then rapidly loses a proton
to afford the corresponding enamine (Scheme 1).
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