Metal-Catalyzed Carbon-Carbon Bond Formation

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.