Intermediates Wheland sigma complex

Another class of gitonic superelectrophiles (based on the 1,3-carbodica-tion structure) are the Wheland intermediates or sigma complexes derived from electrophilic aromatic substitution of carbocationic systems (eq 8). 

Sigma-complexes, observed by NMR, are analagous to the well-known Wheland intermediates in hydrocarbon chemistry, is possible upon the addition of CsF to perfluoro-s-triazine derivatives [101] (Figure 4.41) but direct observation of similar o-complexes derived from less-activated perfluoroaromatic systems has not yet been reported. 

Parker and Bethell expressed doubt concerning the conclusions that the Wheland intermediates are not involved in aromatic brominations which differ principally from, say nitration with nitronium ion, by the fact that a strong nucleophile (bromide ion) is present under the former conditions and not the latter. It seemed reasonable that computations would not reveal the presence of the positive sigma complex since its formation might be expected to be very slow compared to the very rapid subsequent reactions. It is commonly observed that it is difficult to observe an intermediate of the latter nature by computation if the transition state for its formation is not found. Several transition states were found in the previous study but all of them were depicted as complex processes (see Table 2 in Ref 157) involving the simultaneous making and breaking of more than one bond. 

Structures of the nine TS are shown in Scheme 1.24. At the level of theory used, TSl and TS2 appear to be very similar. Three parameters are of interest as to whether or not these structures are derived from the bromo—sigma complex. The fint is the distance between bromide ion and the bromine atom associated to the benzene molecule, while the second is the Br—C distance of the latter. The third parameter recorded on the images of the structures is the Br-C-H angle at the C atom to which Br is attached. The importance of the Br—Br distance relative to the question of whether or not the Wheland intermediate is involved relates to the separation of charge in the bromine molecule. This requires considerable energy and is not expected to take place without compensation from bond formation of the resulting Br moiety. The distribution of charge on the... 

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