Wheland-type intermediate

The obtained l3C and 29Si NMR data do not vary significantly with the solvent as long as aromatic hydrocarbons are used. That is the maximum solvent effect on l3C NMR chemical shift, AS l3C w/v, for cation 9a is AS I3C w/v = 0.5 when the solvent is changed from benzene to toluene and the position of the 29Si resonance remains even unchanged. This indicates negligible interaction between the cation and solvent molecules, in particular no Wheland-type intermediates are formed. (38) Solvents other than aromatic hydrocarbons are however reactive towards vinyl cations 8-10 (see below). 

In the case of silylium ions the formation of Wheland-type intermediates is shown by appreciable solvent effects on the 29Si NMR chemical shifts. 

Scheme 6 Evidence for a rate-determining Wheland-type intermediate...

Electrophilic substitution can be categorized into two subsets. One class of electrophilic substitution reaction involves electrophilic addition of the metal to the n system of an aromatic substrate, to form a Wheland-type intermediate, followed by proton loss (Scheme 11.35). This is the metal analogue of the classic organic... 

SCHEME 11.35 Net electrophilic substitution of C—H bond via a Wheland-type intermediate. 

The acylation reaction should proceed through a Wheland type intermediate formed presumably within the intersection of the pores. If we compared the size of these intermediates with the intersection volume (Table 4) we can draw the following conclusion ... 

This section will expand the theme of reactions between a carbocation and a nucleophilic species such as an alkene or an alcohol. In this section, however, the carbocation will be attacked by an aromatic ring to form a Wheland-type intermediate, which leads to a substituted aromatic system. These are the Friedel-Crafts reactions and they are among the most important reactions in organic chemistry. 

A detailed study of the orientation and reactivity in the nucleophilic replacement reactions of aromatic polyhalo-compounds has been published . This involves study of the stability of the Wheland type intermediates (17, 18) where Nu is a nucleophile. The formation of meta... 

As noted earlier in this chapter, charge densities have been calculated for 6a-thiathiophthen and some of its derivatives localization energies for Wheland-type intermediates in substitution reactions have also been estimated. The results indicate, as has been inferred previously, that the 3-position should be attacked by electrophiles. The localization energies for attack at sulphur are very similar to those for attack at C(3) thus, attack at sulphur may become competitive, and indeed examples of this mode of reaction are known. The theoretical treatment further predicts that attack by nucleophiles should occur at the 2- (or 5-) position, in agreement with the experimental evidence. 

ArSiMcg by trifluoroacetic acid has been reported. High values of the product isotope effect, using deuteriated reagents, are found, 5—6.5, and it is suggested that proton transfer to form a Wheland-type intermediate is more than half complete in each case. The solvent isotope effect has also been used as a mechanistic... 

The influence of the Brpnsted and Lewis acidity on OBP was investigated using a ZSM-5 zeolite. A maximum in catalytic activity and selectivity was reached for steamed samples under mild conditions (30% conversion, 94% selectivity). The Brpnsted acid sites play a role of primary importance in OBP as they work in combination with the Lewis acidity. These sites, which are present in the vicinity of the A1 framework, are formed during the dealumination of the zeolite. A Langmuir-Hinshelwood mechanism seems to operate. An acid-catalyzed mechanism is proposed, passing through a Wheland-type intermediate stabilized within... 

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