Ligand exchange reagents

The use of sohd phase ion (or ligand) exchange reagents is a particularly promising area of research for spectrophotometric methods. 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

Bis(trifluoromethyl)cadinium reagent undergoes a related ligand-exchange process [S] (equation 6)... 

The phosphoranes (14) readily undergo ligand exchange on treatment with organolithium reagents. Thus, (14 R = Ph) with butyl-lithium gave an almost quantitative yield of (14 R = Bu) whereas with the less basic methyl-lithium equilibrium was established between (14 R = Ph) and (14 R = Me). 

Table 11.5. Substituted cyclopropanols 31 from carboxylic acid esters and alkenes via ligand-exchanged titanium intermediates generated from Grignard reagents and XTi(OiPr)3 (X = OiPr, Cl, Me).

The reaction has been incorporated into a synthetic approach to enediynes [77]. Structural and mechanistic studies by Oehlschlager established the reversibility of these stannylcupration reactions [25cj. Although the resultant vinylcopper reagents were thermodynamically favored, crossover experiments found facile ligand exchange processes. Efforts to control the regiochemistry of the addition were met... 

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