Ligand exchange reactions associative mechanism

It has been suggested that intermolecular incorporation, i.e. oxidative addition and complexation of a substrate by a metal should be favored, intramolecular reactions, i.e. insertion, migration and deinsertion reactions should be invariant, and extmsion reactions such as reductive elimination or decomplexation should be disfavored by pressure [13], However, decomplexation reactions are in most cases ligand exchange reactions, which can proceed by associative mechanisms, and indeed, there is ample evidence that ligand exchange reactions can be accelerated by pressure [2]. 

A detailed mechanism of the process shown in Scheme 1.3 is unknown. Two general mechanistic pathways, dissociative and associative, have been proposed for the ligand exchange reactions of X -iodanes (Scheme 1.4) [26, 127]. The dissociative pathway seems to be less likely to occur, because of the low stability of the dico-ordinated iodonium ion [PhIL]+ involved in this mechanism [127]. Such iodonium 8-1-2 species, however, have been frequently observed in the gas phase, for example, in mass spectrometry studies of protonated iodosylbenzene, [PhIOH]+ [101], or in the mass spectra of all known iodonium salts, [ArIR]+. The presence of cationic iodonium species in aqueous solution has been confirmed by spectroscopic measurements and potentiometric titrations of PhI(OH)OTs and PhI(OH)OMs [198] however, all available experimental data show that the iodonium species in solution are coordinated with solvent molecules or with available counteranions. X-Ray diffraction analysis of the protonated iodosylbenzene aqua complexes [PhI(H20)0H]+ isolated from aqueous solutions revealed a T-shaped stmcture, ligated with one water molecule at the apical site of the iodine(III) atom of hydroxy(phenyl)iodonium ion, with a near-linear O-I-O triad (173.96°), which is in agreement with a regular X -iodane structure [178]. 

Reactions of iron and cobalt dithioline complexes, [M(mnt)jX] mnt = [SjCa-(CN)al, X = a phosphine or phosphite, with unidentate nucleophiles (L) give five-co-ordinate products, [M(mnt)aL]", and with bidentate nucleophiles (L—L) give six-co-ordinate products, [M(mnt)2(L—L)]. In each case the iron complexes react more rapidly than those of cobalt and the mechanism involves both dissociative and, unusually for five-co-ordinate substrates, associative pathways. The kinetic equations are necessarily complicated but the experimental results are consistent with the predictions in limiting cases. Kinetic studies have been carried out on the ligand-exchange reactions ... 

Transmetalation, though, requires enhanced electrophilicity of the Pd. Additionally a free coordination site may be required, which may be freed by dissociation of either a neutral or an anionic ligand. The involvement of five-coordinate species and association-dissociation ligand-exchange mechanisms in the individual steps of Pd-catalyzed reactions also cannot be neglected (Scheme 3).384... 

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