Transition-Metal-Catalyzed and -Mediated Mechanisms

Some very widely used organic reactions are catalyzed or mediated by transition metals. For example, catalytic hydrogenation of alkenes, dihydroxylation of alkenes, and the Pauson-Khand reaction require Pd, Os, and Co complexes, respectively. The d orbitals of the transition metals allow the metals to undergo all sorts of reactions that have no equivalents among main-group elements. This doesn t mean that the mechanisms of transition-metal-mediated reactions are difficult to understand. In fact, in some ways they are easier to understand than standard organic reactions. A transition-metal-catalyzed or -mediated reaction is identified by the presence of a transition metal in the reaction mixture. 

Getting started is usually the most difficult part of drawing a mechanism. Follow these simple steps, and you will be well on your way. 

Once you have classified the overall reaction and the mechanism, you may have a short series of mechanistic choices. For example, substitution of an aromatic ring under basic conditions generally occurs by one of three mechanisms. 

Indicate which of each pair of compounds is likely to be more acidic and why. 

Classify each of the following reactions as polar, free-radical, pericyclic, or transition-metal-catalyzed or -mediated. For the polar reactions, determine whether the conditions are basic or acidic. 

Getting started is usually the most difficult part of drawing a mechanism. Follow 

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