The Nature of Catalyst Systems

The catalytic system may also require a cocatalyst. By cocatalyst we mean a compound added to the system to improve the effectiveness of the catalyst. These are usually nontransition-metal compounds and would 

It should be noted that the solid arrows in the loop are drawn in one direction to indicate the major flow of the reactions actually, the individual reaction steps can be, and often are, reversible, so that an equilibrium mixture of reactants and products can be approached from either direction. A catalyst system increases the rate of approach to equilibrium but, of course, cannot affect the equilibrium distribution. 

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Iridium-catalyzed intramolecular l,n-enyne metathesis has been studied as a unique tool for the synthesis of various types of cyclic compounds. Reactions of this type depend on both the structure of substrates and the nature of catalyst systems used (411). Recently, the cycloisomerization of various 1,6-enynes have been shown to be catalyzed by [Ir(cod)Cl]2/dppf (494). These reactions are highly stereoselective, and generate the (Z)-isomer preferentially over the ( )-isomer (Scheme 63). The proposed mechanism (Scheme 64) involves oxidative cyclization of the enyne at Ir(I) to give the trivalent iridacyclopentene. The intermediate undergoes (3-hydride elimination to give the irida-1,3-diene, which experiences steric repulsion between the metal fragment and the cis substituent on the... 

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