Current Mechanistic Understanding Based on Kinetic and Spectroscopic Studies

2 Current Mechanistic Understanding Based on Kinetic and Spectroscopic Studies 

This principal reaction mechanism is widely believed to apply to most S Ar reactions irrespective of the electrophilic reagent. There are however a number of experimental observations that indicate exceptions to this mechanism. There are examples of thermodynamically controlled Friedel-Crafts reactions, when using reaction conditions like polyphosphoric acid and elevated temperatures [27,28]. In iodination and some cases of Friedel-Crafts acylation, the last step of the reaction, the proton abstraction, has been shown to have a substantial kinetic isotope effect, which indicates that this step is at least partially rate limiting [29-31]. There are also still open questions regarding the exact nature of the reaction intermediates, and we will focus on these issues in the remaining part of the chapter. 

The o-complexes present in the S Ar mechanism are even more difficult to detect than x-complexes due to their ultrafast deprotonation. This problem can be circumvented by the preparation of o-complexes without aromatic hydrogens from reactions between electrophiles and hexasubsti-tuted benzenes. The first such example was the preparation of the heptamethylbenzenium cation, which was characterized by NMR [47]. Using the same principle, o-complex intermediates of nitration and chlorinations have been prepared and subsequently characterized by NMR and UV-VIS spectroscopy [48]. The o-complexes formed from the interaction of hexamethylbenzene and the cations silyl, methyl, bromonium, and chloronium were crystallized and analyzed by X-ray diffraction [43]. 

In 2000, Hubig and Kochi claimed to have made the first observation of a true Wheland intermediate of an S Ar when studying nitrosation of methyl-substituted benzenes by time-resolved spectroscopy [49]. Irradiation of a suspension of NOBF in dichloromethane containing mesitylene with a laser pulse of 355 nm generated a transient species, which was identified as the radical ion pair. This species rapidly converted to another transient, which was assigned as the Wheland intermediate. However, theoretical studies have indicated that the reaction does not proceed via a radical ion pair and that the Wheland structure is a transition state rather than an intermediate of the nitrosation reaction [50]. This interpretation also found support in the analysis of the x-complex of nitrosonium ion and henzene hy IRMPD [44]. 

2 THE SjAr MECHANISM QUANTUM CHEMICAL CHARACTERIZATION IN GAS PHASE AND SOLUTION 

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