Pre-association

Diffusion control and pre-association in nitrosation, nitration, and halogenation, 16,1... 

Halogenation, nitrosation, and nitration, diffusion control and pre-association in, 16, I... 

Neighbouring group participation by carbonyl groups in ester hydrolysis, 28, 171 Nitration, nitrosation, and halogenation, diffusion control and pre-association in, 16, 1 Nitrosation, mechanisms, 19, 381... 

Electrostatically-controlled pre-association interactions have an important effect on rates for [Pd(dien)Cl]+ reacting with thione-containing nucleosides, nucleotides and oligonucleotides, as is often the case for reactions between metal complexes and this type of biological ligand. Interaction between the charged complex and the polyanionic oligonucleotide surface leads to an increase in both enthalpy and entropy of activation in the DNA or model environment (252). 

A mechanistic scheme, (Scheme 4) similar to that previously reported (72) for nucleophilic substitution, in which pre-association, free-ion and ion-pairs pathways compete, can therefore also be proposed for bromination. Based on this scheme, the stereochemical behaviour depends on both solvent and substituents. In trifluoroethanol, the reaction occurs independently of the substituents on the double bond, via free ions ... 

Analogously, bromine bridging is not the only factor affecting the elimination-nucleophilic trapping ratio. Pre-association with a nucleophilic solvent can explain the increased selectivity towards addition products with respect to elimination products, as observed in bromination of 1,1-diphenylethylene in methanol. 

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