The Modification of Ligand Reactivity by Complex Formation

We shall consider the ways in which a metal may influence a reaction. These are listed in Table 6.1. The effects of the metal and the reactivity of the coordinated ligand are interrelated since invariably at least one of the reactants becomes coordinated to the metal during a catalyzed reaction. 

Serve as a eollecting point for reactants. Sec. 6.1 Promote electron shifts in the metal-ligand system. Secs. 6.2-6.S 

Alter the strain within or the conformational characteristics of the reacting ligand. Sec. 6.8. 

Neighboring group and template effects. Promotion of nucleophilic substitution. 

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The modifications of the Gilman-Speeter reaction include the activation of zinc by tri-methylsilyl chloride (TMSCl) and the application of lithium ester enolate" or lithium thioester enolate as the substitute for the traditional Reformatsky reagent. In these modifications, it was found that TMSCl-activated zinc is much more effective in promoting the reaction between ethyl bromoacetate and Schiff bases. In addition, in the presence of a chiral ether ligand, the reaction between lithium ester enolate and imines affords 0-lactams of high enantiomeric excess, probably due to the formation of a ternary complex reagent. " The enantioselectivity and reactivity of the ternary complex depend on the size and nature of the lithium amide used. For example, the lithium amide from 2,2,6,6-tetramethylpiperidine (LTMP) is unfavorable for this reaction." ... 

In the other two mechanisms (homolytic splitting and insertion) hydride formation is accompanied by formal oxidation of the metal, and reactivity is closely linked to the susceptibility of the latter to oxidation. Thus, the high reactivity of Co(CN).-, toward Ho, compared with that of Co(CNCH i)- reflects the tendency of CN" to stabilize preferentially the higher oxidation state, and CNCH the lower oxidation state, of cobalt. For square planar complexes the expected order of the tendency toward oxidation, and hence of reactivity toward Ho, is (subject to some modification by ligand variation),... 

Since its introduction almost four decades ago, a tremendous number of modifications of Sonogashira s original protocol have been reported to improve yields, to increase the reactivity of unactivated electrophiles or stericaUy demanding substrates under mild reaction conditions, or to suppress the formation of homo-coupled products. Significant improvements have mostly been achieved by tailoring the catalytic and cocatalytic metal species, metal-complexing ligands, and amine bases used (see later discussion). 

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