Biomedical Engineering Reference
In-Depth Information
could be performed. On the basis of these seminal results, homogeneous gold
catalysis finally entered a new dimension during the last decade, with the
appearance of a plethora of new transformations. The impressive exponentially
growing number of studies related to the use of electrophilic gold species in
homogeneous catalysis has not only allowed pushing aside the aforementioned
assumption of gold inertness, but also highlighted an unsuspected synthetic potential.
The ongoing interest for gold catalysis is reflected today by the appearance of
numerous recent review articles covering the broad spectrum of its applications in
organic synthesis [3].
The recent chemistry of Au(I) and Au(III) species is based on their superior
Lewis acidity and exploits, as the major principle of reactivity, their capacity to
activate
p
-systems, such as alkynes, allenes, and alkenes toward the addition of a
large variety of nucleophiles [3s,3t]. The simplest reaction pathway found in gold
catalysis is shown in Scheme 4.3 in the case of a nucleophile possessing a hydrogen
atom, but a wide variety of other more complex reactivity patterns exist [3]. The
activation of the unsaturation by the electrophilic gold species induces the addition
of the nucleophile. A subsequent protodemetalation step delivers the product while
regenerating the gold catalyst.
Advantageously, gold catalysis exhibits several characteristics, such as
tolerance to oxygen, insensitivity to aqueous conditions, compatibility with a wide
range of functional groups, mildness of the reaction conditions, and ease of catalyst
manipulation, which make it a useful tool for organic synthesis. However, given
the fact that homogeneous gold catalysis is a recent field of research, interest has
been mainly focused on the development of new synthetic methods and relatively
few applications in total synthesis have been reported to date (around 5% of the
total number of publications concerning gold-catalyzed organic reactions).
However, these ones are highly representative of the synthetic value of gold catalysis
in terms of efficiency, selectivity, structural diversity, and molecular complexity.
In most cases, these total syntheses or synthetic approaches have been performed
to illustrate the synthetic potential of a method and since they generally involve the
gold-catalyzed formation of a cycle, their presentation in this chapter is organized
as a function of the nature of this cycle: Section 4.2 covers the formation of
oxygen-containing heterocycles, Section 4.3 the formation of nitrogen-containing
heterocycles, and Section 4.4 the formation of carbocycles. The final section is
Nu
[Au]
Nu
[Au]
H
H
Nu
Nu
[Au]
[Au]
[Au]
H
H
[Au]
H
Nu
Nu H
Nu H
Nu
H
[Au]
[Au]
SCHEME 4.3
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