Arenium ions isotope effects

Isotope Effects. If the hydrogen ion departs before the arrival of the electrophile (SeI mechanism) or if the arrival and departure are simultaneous, there should be a substantial isotope effect (i.e., deuterated substrates should undergo substitution more slowly than nondeuterated compounds) because, in each case, the C—H bond is broken in the rate-determining step. However, in the arenium ion mechanism, the C—H bond is not broken in the rate-... 

However, in many instances, isotope effects have been found. Since the values are generally much lower than expected for either the Sgl or the simultaneous mechanisms (e.g., 1-3 for instead of 6-7), we must look elsewhere for the explanation. For the case where hydrogen is the leaving group, the arenium ion mechanism can be summarized ... 

Evidence for the arenium ion mechanism has also been obtained from other kinds of isotope-effect experiments, involving substitutions of the type... 

In the framework of equation 41, it may be observed that the desilylation of the intermediate arenium ion competes with the deprotonation process, so that a H/D kinetic isotope effect (KIE) is expected to arise when the MejSi"1" transfer competes with either H+ or D+ transfer. Indeed, kinetic isotope effects for the formation of silylated products arising from the different rates of H+ vs D+ transfer have been reported from the reaction of selectively D-labelled toluene and 1,2-diphenylethane or from mixtures of unlabelled and labelled substrates (Table l)121 —123. The kinetic isotope effects listed in Table 1 are the ones reported when the base used is Et3N. The use of bases of different strength to effect the H+ or D+ transfer should have an influence on the observed kinetic isotope effect. The role of the base on the values of the KIE was indeed verified in the competitive silylation of CH3C6H5/CD3C6D5 mixtures122. 

The evidence for the first step of Figure 6.34 being rate determining and the intermediacy of the resonance-stabifized (Scheme 6.82) a-complex or arenium ion following that transition state comes from (a) the isolation of some suitably substituted arenium ion intermediates that then go on to product, (b) low-temperature studies of the NMR spectra of arenium ions, and (c) the presence or lack of a primary isotope effect when the carbon-hydrogen bond is broken. If the bond-breaking step is rate determining then an isotope effect on the rate will be seen. 

Reactions with toluene-ds and anisole-ds to form ring-deuterated derivatives of 22a and 22b (Scheme 9.20) were found to show primary deuterium isotope effects (kn/fco) of 1.18 + 0.04 and 1.52 + 0.05, respectively, demonstrating that proton elimination from an arenium ion intermediate 23 is a rate-determining step [47]. 

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