Palladium dimeric complex isomerization

In recent years, ionic liquids have become extremely popular as a green alternative to conventional reaction solvents.Since a number of the most popular ionic liquids used are A, A -dialkylated imidazolium salts, their true role in transition metal catalysed reactions, particularly in the presence of base, must be scrutinized. Following the initial report of a successful Heck reaction performed in an ionic liquid by Earle and co-workers,Xiao and co-workers were able to isolate a number of isomeric [(NHC)2PdBr2] complexes from the reaction mixture (Equation (3.3)). Under stoichiometric conditions, heating [Pd(OAc)2] and NaOAc in the ionic liquid, [BMIM]Br, the dimeric palladium carbene complex, [(IBuMe)PdBr2]2 could be readily obtained, which, upon further heating, generated bis-NHC-Pd complexes. 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

Musco has also investigated the dimerization of butadiene. He found that COj enhances the catalytic effect of tertiary phosphine-palladium complexes in the syntheses of 1,3, 7-octatriene and in the subsequent isomerization to 2, 4, 6-octauiene 1299]. 

Steroidal 7t-allyl complexes can be oxidatively cleaved to allyl alcohols with high stereoselectivity (Jones and Knox, 1975b). For example, the n-allyl palladium chloride dimer, XIV, derived from cholest-4-ene, undergoes bridge cleavage by pyridine and then oxidation by w-chloroperbenzoic acid to give XVI as the sole product. Similar reaction of the isomeric Jt-allyl... 

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