Solvation organometallic addition

Senn HM, Ziegler T (2004) Oxidative addition of aryl halides to palladium(O) complexes A density-functional study including solvation. Organometallics 23 2980-2988... 

Reaction conditions can be modified to accelerate the rate of lithiation when necessary. Addition of tertiary amines, especially TMEDA, facilitates lithiation53 by coordination at the lithium and promoting dissociation of aggregated structures. Kinetic and spectroscopic evidence indicates that in the presence of TMEDA lithiation of methoxybenzene involves the solvated dimeric species (BuLi)2(TMEDA)2.54 The reaction shows an isotope effect for the o-hydrogcn, establishing that proton abstraction is rate determining.55 It is likely that there is a precomplexation between the methoxybenzene and organometallic dimer. 

This is also a field of chemistry, where biphasic and phase transfer-assisted organometallic catalysis [11-12] are very close and sometimes may even overlap. One reason for this closeness is in that inorganic bases are often used in aqueous solutions. Of them, OH is so strongly solvated in water that it will practically not transfer to non-polar organic solvents without a phase transfer (FT) agent, e.g. a quaternary ammonium cation. However, some reactions proceed readily with H2O dissolved in the organic phase, or can take place with reasonable rates at the liquid-liquid interface, and in these cases addition of FT catalysts is not essential. 

As most organometallic compounds, lithium enolates are highly polar entities susceptible to combine in various types of (eventually solvated) aggregates that undergo dynamic equilibria in solution. This phenomenon explains why enolate solutions are difficult to describe by the classical spectroscopic, physicochemical or theoretical methods, a difficulty enhanced by the sensitivity of these equilibria to many physicochemical factors such as the concentration, the temperature or the presence of complexing additives (lithium halides, amides, amines, HMPA,. ..). The problems due to dynamics are avoided in the solid state where many clusters of lithium enolates, alone or co-crystallized with exogenous partners, have been identified by X-ray crystallography. 

Further detailed investigations of the solvation of carbonyl compounds as the predominant factor influencing the diastereoselectivity of nucleophilic additions of various organometallics (n-BuLi, t-BuLi, n-BuMgBr) to a-RsSiO-substituted aldehydes can be found in reference [853]. 

The reaction rates depend to a great extent on the nature of the organometallic compounds, such as polarity of the bonds and the degree of solvation. In polar solvents, where free solvated ion pairs predominate, the mechanism of initiation may simply consist of a direct addition of the anion to the monomer. If the solvents are nonpolar, on the other hand, the initiation is more complex. In these solvents the metal cation coordinates with the monomer first. This is followed by a rearrangement ... 

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