The Art of Writing Reasonable Organic Reaction Mechanisms (PDF)

6 Transition-Metal-Catalyzed and -Mediated Reactions (p. 270)

6.1 Introduction to the Chemistry of Transition Metals

Many widely used organic reactions require the use of transition metals. You have probably already learned about metal-catalyzed hydrogenations of alkenes and alkynes, dihydroxylation of alkenes using OsO and the use of lithium dialkylcuprates (Gilman reagents) as "soft" nucleophiles. Although the mechanisms of many of these reactions appear mysterious, in fact they are quite easy to understand (in most cases) using some very basic principles. This chapter discusses some of the typical reactions of transition metals. These principles will then be applied to understanding some of the organic transformations mediated by these metals.

The reactions in this chapter are organized according to the overall transformation achieved. An organization by mechanistic type (insertions, transmetallations, etc.) was considered and rejected. The purpose of this text is to teach the student how to go about drawing a mechanism for an unfamiliar reaction. A student who does not already know the mechanism for a reaction will find it easier to narrow down the possible mechanisms by considering the overall transformation rather than by trying to determine the mechanistic type. An organization by metal (early, middle, or late) was also considered and rejected, because it would obscure the important similarities between the reactions of different metals.

No attempt is made to cover every aspect of organometallic chemistry. Many interesting organometallic reactions, such as the reactions of Cr arene complexes, have been discovered and even investigated extensively but have not been widely adopted by organic chemists for one reason or another. These reactions are not discussed. Moreover, as in previous chapters, questions of stereochemistry are deemphasized in favor of concentration on electron-pushing. The reader is urged to consult any of the