Isotope effects, halogen

As shown by frequency dependence studies [53 303], comparisons of T and T2 [305 307], the halogen isotope effect [53 30 307] and the temperature dependence (see above), the extreme narrowing situation applies and exchange between different halide ion environments... 

Karpfen A (2007) Theoretical Characterization of the Trends in Halogen Bonding. 126 1-15 Keller H (2005) Unconventional Isotope Effects in Cuprate Superconductors 114 143-169 Keller H, see Bussmann-Holder A (2005) 114 367-386 Khan AI, see Williams GR (2005) 119 161-192... 

Kinetic isotope effects have not been observed for chlorination, and only rarely for bromination, i.e. the reactions normally follow pathway [2a] like nitration. In iodination, which only takes place with iodine itself on activated species, kinetic isotope effects are the rule. This presumably arises because the reaction is readily reversible (unlike other halogenations), loss of I occurring more often from the a complex (14) than loss of H, i.e. k, > k2 ... 

The ratio of products (36) and (37) from VNS of hydrogen (Pe) and substimtion of halogen (Px), respectively (Scheme 4), will depend on the strength and concentration of base, provided that the elimination is a kinetically important step in the VNS reaction, namely Pr/Px = kikE[B]/k-ikx. The influence of base will decrease until a constant value Ph/Px = k /kx is reached as kslB] k i. This has been demonstrated for 4-chloronitrobenzene, which undergoes exclusive substimtion of chlorine unless strong base is present to favour the VNS process. The deuterium isotope effect for VNS hydroxylation by Bu OOH, determined as me ratio of H versus D substitution of l-deutero-2,4-dinitrobenzene, varied from 7.0 0.3 to 0.98 0.01 as the base in NH3 was changed from NaOH to Bu OK me former value is consistent with a rate determining E2 process. 

The kinetic data for halogenated carbon-centred radicals with silicon hydrides are also numerous and a few examples are shown in Table 3.2. The kinetic data for perfluoroalkyl radicals were obtained by competition of the appropriate silane with the addition to an olefin [16-18]. The kinetic deuterium isotope effects (/ h/ d) on the attack of on the Si—D bond of... 

As assumed, the small and positive valne of H/D kinetic isotope effect may be used as a criterion for an electron-transfer pathway. For example, anion-radicals of a-benzoyl-co-haloalkanes can react in two routes (Kimura and Takamnkn 1994). The first ronte is the common one—an electron is transferred from the oxygen anion of the carbonyl gronp to a terminal halogen. The transfer provokes fission of the carbon-halogen bond. The second ronte is the S 2 reaction, leading to a cyclic product as shown in Scheme 2.37. 

Problem 11.6 Sulfonation resembles nitration and halogenation in being an electrophilic substitution, but differs in being reversible and in having a moderate primary kinetic isotope effect. Illustrate with diagrams of enthalpy H) versus reaction coordinate. ... 

A less well developed, but potentially less intrusive alternative is to block acidic sites with deuterium and to use the kinetic isotope effect to prevent lithiation.35 43 If the substrate can be polylithiated, the use of dianions (with the last formed anion being likely to be the most reactive) may give the required selectivity this method works well in the halogen-metal exchange area (section 3.1). 

Deuterium Isotope Effects ( h/ d) hi the Side Chain Halogenation of Toluenes"... 

Table 3. Isotope effects involving halogen atoms...

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