Reactivity effects group 4 metal substituents, positive

Kinetic studies of the substitution reaction of 2-chloro-l-methylpyridinium iodide with phenoxides are consistent with the SnAt mechanism, with rate-determining nucleophilic attack.38 The effects of a variety of ring substituents on the reactivities of 2-fluoro- and 2-chloro-pyridines in reactions with sodium ethoxide in ethanol have been examined. The results were discussed in terms of the combination of steric, inductive, and repulsive interactions.39 Substitution in 2,4,6-trihalopyridines normally occurs preferentially at the 4-position. However, the presence of a trialkylsilyl group at the 3-position has been shown to suppress reaction at adjacent positions, allowing substitution at the 6-position.40 Methods have been reported for the introduction and removal of fluorine atoms for polyfluoropyridines. Additional fluorine atoms were introduced by metallation, chlorination, and then fluorodechlorination, while selective removal of fluorine was achieved by reduction with either metals or complex hydrides or alternatively by substitution by hydrazine followed by dehydrogena-tion-dediazotization.41... 

The reactions can either be centered at any of the atoms of the CNO moiety (Scheme 3), where the substituent groups are omitted for clarity), leading to oxime (or oximate), imine or other types of complexes (in some cases with metallacycles and in others with formation of unsaturated organic N,0-products) or occur at another part of the oxime molecule. In fact, coordination does not always lead to the activation of the oxime functional group, but instead a protecting effect can result as observed (see Section 1.29.3.6) in the chlorination of some salicylaldoxime Pt11 complexes which occurs exclusively at the metal and at the benzene ring (ortho and para positions) of the oxime in contrast with the known reactivity of free oximes. 

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