Most of the examples of carbon–carbon bond-forming reactions discussed earlier utilize stoichiometric quantities of the carbon nucleophile and the carbon electrophile.
METAL-CATALYZED CARBON–CARBON
BOND FORMATION
Most
of the examples of carbon–carbon bond-forming reactions discussed earlier
utilize stoichiometric quantities of the carbon nucleophile and the carbon
electrophile. Most commonly the nucleophilic species is formed in solution
(organometallic, enolate, phosphorane, etc.) and the electrophile is added.
Thus stoichiometric quantities of reagents such as bases or metals are used to
form the reactive nucleophile. These are, for the most part, classic processes
that work well and give predictable results. Many have been used quite
successfully for well over one hundred years for assembling organic structures.
In
the last several decades there has been ever-increasing interest in
discover-ing new methods for the formation of carbon–carbon bonds utilizing
transition metal complexes as catalysts. The reasons for doing this are many:
1. Many new reactions are possible which might overcome the
functional group requirements and/or limitations known for many of the older
meth-ods. The metal complex provides a structurally defined catalytic center at
which reagents are brought together and reacted. This contrasts with the random
collisions characteristic of many traditional uncatalyzed pro-cesses. Moreover
the multiple oxidation states available to transition metals provide new
mechanisms for the activation of reagents involved in the bond-forming process.
2. Different ligands complexed to the metal might be used to
modulate the reactivity of the catalyst and thus achieve increased selectivity.
This would increase the yields and efficiencies of the reaction and lead to
single prod-ucts rather than mixtures.
3. Chiral ligands would offer the opportunity to introduce
chirality into the transition state between two achiral reaction partners. In
this way stereos-election could be tuned via the choice of catalyst and
ligands.
4. High turnover numbers would allow very small amounts of
catalyst to be used, thus simplifying the workup and purification of reaction
products.
5. Finally, such processes would have high atom economy.
This means that most of the atoms put into the reaction mixture end up in the
products and not the by-products.
As
one might expect, the fairly large number of transition metals has resulted in
a very large number of organic transformations. Virtually all of the
tran-sition metals have been used to create new carbon–carbon bonds in various
reactions. However, two metals in particular have had the greatest impact on
preparative organic chemistry. The use of palladium and ruthenium complexes for
the formation of carbon–carbon (and other) bonds has led to profound changes in
synthetic planning. The reactivity patterns of complexes of these metals are
quite general and quite distinct from the reactivity patterns of traditional
car-bon–carbon bond-forming reactions. Moreover they are extraordinarily
tolerant of diverse functional groups. As a result completely new strategies
for the con-struction of carbon skeletons have emerged which complement older
methods. What results is a much richer synthetic toolkit for building
molecules.
The
following discussion will focus on the most common and general reactions these
two metals catalyze. Given the tremendous amount of information that has been
gathered, it comes as no surprise that it is impossible to cover this chemistry
fully in this text. Instead the general principles will be developed and some
common applications will be illustrated. The idea is to provide a foundation
for understanding the reactivity patterns of these complexes. Further insight
can be gained from a number of specialized monographs and review articles.
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