Intramolecular selectivity

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

Evidently S, is a measure of intramolecular selectivity because it involves a ratio, the contribution of the benzene substitution rate disappears, and the selectivity factor expresses the selectivity of the reagent X in Eq. (7-83) for the para position relative to the meta position. Each individual partial rate factor, on the other hand, is expressive of an inteimolecular selectivity thus p is a measure of the selectivity of the reagent for the para position in CgHsY relative to benzene. It was observed that Eq. (7-85), where Cmc is a constant, is satisfied for a large number of electrophilic substitutions of toluene. 

Methyl cation affinities of benzene and some substituted benzenes have been calculated. These follow a simple additivity rule and the value for benzene shows good agreement with the experimental estimate. Conclusive evidence is presented that these values are linearly related to the corresponding proton affinities. The competition between deuteriation and alkylation in the reaction of radiolytically formed perdeuterio ethyl cations with iV-methylpyrrole and with thiophene has been studied. Deuteriation, the Brpnsted acid pathway, predominates and intramolecular selectivities have been determined for each reaction. ... 

Changes in intramolecular selectivity in the bromination and nitration of alkyl-benzenes in acidic media have been attributed to changes in medium polarity or changes in electrophile solvation. Mass spectrometric studies of the first stage in the gas-phase reactions of halobenzenes, furan, thiophene and pyrrole with alkyl cations have been rationalized in terms of co-existing a- and tt-complexes. The extent of... 

Although the intermolecular selectivity of the nitration of alkylbenzenes by nitric acid in trifluoroacetic acid is controlled by both electronic and steric factors, it is argued that intramolecular selectivity is controlled by steric effects on transition state solvation. 

Let us first consider the intramolecular selectivity of out-of-ring substitution at 2 and 1 methyl groups in 1,2,3-trimethylbenzene derivatives. The 2/ 1 ratio in Table 4.1 correlates with of the corresponding cation-radicals. Scheme 4.16 compares cation-radical and pure radical pathways of substitution in 1,2,3-trimethylbenzenes bearing a group Z in position 5 (Baciocchi 1995). 

The data in Table 5 show that concave pyridines 3 are able to differentiate alcohols. In synthesis, this capability is important when OH-groups within one molecule shall be differentiate. Therefore, intramolecular selectivities were also determined. Table 6 lists the results of intramolecular competition reactions for 1,2-propanediol (64) 172°, tra- Again, the small concave pyridine 3c only showed a small selectivity whereas the large concave pyridines 3d and 3r were the most selective catalysts. With a selectivity of 15 (3r), a selectivity range is reached which may be useful for applications because more than 93% of the functionalized OH-groups are primary ones. 

Comparison of results from the gas-phase proton-induced unimolecular isomerization of (R)- -d -3-(p-fluorophcnyl )bulanc (11) with the positional selectivity of the corresponding gas-phase bimolecular arene alkylation confirms the presence of non-covalent j-type intermediates and their important role in determining the intramolecular selectivity of gas-phase electrophilic aromatic substitutions.20... 

As mentioned in Sec. 15.2.2, hydrogen abstraction of benzylic hydrogens by the /77i triplet state of ketones is generally efficient and offers an access to benzylic radicals. However, the method is rarely of preparative interest since benzylic and ketyl radicals couple statistically giving a mixture of bibenzyls, alcohols, and pinacols, although the initial hydrogen abstraction may show some interesting intramolecular selectivity with polymethylbenzenes and substituted alkylbenzenes [220,221]. However, in... 

The intramolecular selectivity in the deprotonation of alkylaromatic radical cations has been investigated in a series of 5-X-l,2,3-trimethylbenzene radical cations the results showed that spin and/or charge density at the scissible C H bond can strongly influence the kinetic acidity, which is, accordingly, very sensitive to the nature and position meta or para) of ring substituents (Scheme 32) [149]. 

Table 4. Intramolecular selectivities ki/kx) in the deprotonation of 5-X-l,2,3-trimethylbenzene radical cations generated by anodic or CAN-promoted oxidation in acetic acid.

For example, Attina and Cacace122 investigated the intramolecular selectivity of electrophilic reactions of radiolytically formed C2H5+, i-C3H7+, f-C4Hg+, (CH3)2F+ and CH3CO+ ions with aniline in the gas phase at nearly atmospheric pressure. Under conditions of kinetic control, the reactivity of the amino group and of the aromatic... 

Relative intramolecular selectivity only. Values not relative to cyclohexane bR=neopentyl. 

Aromatic nitration reactions exhibit, under all conditions, important Intramolecular selectivity. The isomer distributions for the reactions of toluene under a variety of conditions. 

By analogy with literature discussions ( ) of diffusion-controlled processes it was implied that there was no interaction within the encounter pair. If this were the case, as k i can be estimated ( 7) as about 10 -10 ° s and k2 can in the limit approach 10 - 10 s , it would be possible for considerable intramolecular selectivity to remain between positions in a substrate which are sufficiently reactive to be nitrated at the limiting rate.( )... 

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