- The reactivity of 2-fluoro- and 2-chloropyridines toward sodium ethoxide: Factors governing the rates of nucleophilic (Het) aromatic substitutions
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The relative displacement rates of the halide substituent from 2-fluoro- and 2-chloropyridines by EtONa in EtOH at +25° were assessed by competition kinetics. The 2-fluoropyridine reacts 320 times faster than the chloro analog. A CF3 group increases the reactivity more than single halogen atoms do, whatever the element, and the latter are superior to Me3Si groups. Substituents accommodated at the 4-position operate through their inductive effect, whereas at the 3-position, this action may be attenuated by steric hindrance. Almost all 5-substituents enhance the rate of the nucleophilic substitution occurring at the 2-position. The sole exception concerns the F-atom at the 5-position which retards the reaction, presumably by lone-pair/lone-pair repulsion with the negative charge building up at the central C-atom of the intermediate Meisenheimer complex. The substituent effects are additive. Therefore, by using the increments derived from the present work, the rates of future reactions should be predictable with fair accuracy.
- Schlosser, Manfred,Rausis, Thierry
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- Ortho-selectivity in SNAr substitutions of 2,4-dihaloaromatic compounds. Reactions with anionic nucleophiles
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The nucleophilic addition of organic anions to aromatic compounds with halogens positioned both ortho and para to activating groups was studied in a variety of solvents. Substrates showed strong preferences for ortho substitution in most cases. Evidence is presented for activating group-dependent coordination, which contributes to very high ortho-selectivity in nonpolar solvents. This also drives the overall reaction rate in these solvents, and is of close to the same magnitude of rate increase derived from polar solvents. para-Products are maximized by using crown ethers in protic solvents. Solvent effects overall are very different from corresponding reactions with amine nucleophiles due primarily to the different charges present in the transition states, and to solvation of the nucleophile.
- Wendt, Michael D.,Kunzer, Aaron R.
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p. 3041 - 3044
(2010/07/18)
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