Wheland intermediate, formation protonation

Accordingly, the role of water might be explained in the following way in the absence of water, protonation of toluene can induce arylation, whereas, in the presence of water, the acidity of the clay is just sufficient to protonate nitric acid and to favour the formation of an ipsosubstituted Wheland intermediate. The most reasonable reaction sequence compatible with our observations is depicted in scheme 1 and eq. 4. 

It corresponds to addition followed by elimination, and is symbolised by AE + De. The departing X+ is often a proton, while Z is a general substituent. The key step in this scheme is the formation of an intermediate arenium ion (Wheland intermediate, a complex), and the relative stability of this species is crucial to the outcome of the reaction. Isolable arenium ions are known, and the benzenonium ion itself C6Hy has been inferred from NMR of strongly acidic solutions [255],... 

The formation of the Wheland intermediate results in the heterolytic cleavage of the chlorine/chlorine bond to form the A1C14 complex, which later acts as the base to help remove the proton that is to be substituted. 

The rate-determining step is the formation of a cr-complex, C, also called the Wheland intermediate. This is followed by the rapid loss of a proton. The activated complex for the formation of C is thought to resemble C very closely. The possible formation of a 7r-complex, prior to C, is usually of little consequence. 

Applications of MO calculations to studies of reaction mechanisms are of limited value because of the considerable demands placed on the computational method to guarantee reliable results. It is not always easy to formulate the task properly <89CHE1321>. Nevertheless, efforts to elucidate the diazo coupling reaction of imidazole have been assisted by MNDO calculations of the heats of formation of the possible Wheland intermediates <89JCS(P2)2055> (see Section 3.02.5.3.7), and an FMO analysis of the effects of methyl- and benzo-substitution on the facility of proton transfer from C-2 of imidazole, benzimidazole, and their 1-methyl derivatives has thrown light on the mechanism <86CJC1240>. 

A significant aspect of the chemistry of furans is the occurrence of 2,5-additions initiated by electrophilic attack a Wheland intermediate is formed normally, but then adds a nucleophile, when a sufficiently reactive one is present, instead of then losing a proton. Conditions can, however, usually be chosen to allow the formation of a normal a-substitution product. The occurrence of such processes in the case of furan is generally considered to be associated with its lower aromatic resonance stabilisation energy - there is less to regain by loss of a proton and the consequent return to an aromatic furan. 

The mechanism of Friedel-Crafts acylation is believed to involve ratedetermining exo attack of the acylating species generating an intermediate 11, analogous to the Wheland intermediates generated during electrophilic substitution of arenes. Rapid loss of a proton from 11 results in formation of the neutral product 7. ... 

For the isomerization of toluene via methoxy and benzene intermediate, the similar proton activation, which results in the formation of a phenoxy intermediate, is achieved (see Figure 9). As previously, the bond between the zeolitic oxygen atom and the aromatic carbon atom stretches out. A "free" Wheland complex is eventually reached which can reorient to favor the position of the toluene methyl group with the demethylation transition state. 

The formation of the phenoxy intermediate turns to be unlikely to occur as this intermediate remains at similar energy level as observed from the cluster proach (i.e. + 150 kJ/mol), whereas protonation step and Wheland complex energy levels are around +110 kJ/mol with respect to physisorbed toluene. 

Route I (oxidative addition) involves a concerted oxidative addition process with the formation of metal-hydride species A. Alternatively, an electrophilic attack by the metal center on the aryl ip o-carbon may afford a metal arenium (Wheland) complex B followed by proton loss. In the agostic C-H bond activation route, the six-membered transition state C including a hydrogen-metal interaction has been found to initiate the C-H activation process, leading to an agostic intermediate D and acting simultaneously as an intramolecular base for deprotonation. 

This was interpreted in terms of the electrophilic substitution mechanism outlined in Scheme 2, involving rapid preequilibrium formation (iCi) of a 7T complex, followed by rate-determining rearrangement (k2) to a Wheland-type cr-complex intermediate 18. Rapid proton loss then leads to products 17. 

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