Phenyl substituent effects

To understand the unpredictable nature of the Pictet-Gams reaction, Hartwig and Whaley conducted the first mechanistic studies in 1949. Their work focused on substituent effects when directly attached to the ethylamine side chain. They also investigated a variety of dehydration agents in order to identify optimal reaction conditions. It was determined that formation of the isoquinoline structure was virtually impossible when alkyl or phenyl substituents were placed in the 4-position of the ethylamine side chain. 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

It is well-known that para substituents on the phenyl groups of H2TPP have no influence on the tautomerism rates in the ground state (see Section III,A,1). In the case of PHB, there seems to be only a small substituent effect on <1>phb (the quantum efficiency for hole burning) through modification of the relative energy of Ti (93CM366). 

The wide variation in the entropy factors for both the substituted phenyl and heterocyclic compounds and in particular for the methoxyphenyl and furan derivatives was considered to be strong evidence for solvent effects being predominant in determining the activation entropy. Consequently, discussion of the substituent effects in terms of electronic factors alone requires caution in this reaction. Caution is also needed since rates for the substituted phenyl compounds were only determined over a 20 °C range. The significance of entropy factors has also been indicated by the poor correlation of the data of the electrophilic reactivities of the heterocyclic compounds, as derived from protodemercuration, with the data for other electrophilic substitutions and related reactions572. 

The magnitude of the electrical effect is comparable to that of the trans-heterovinylene sets. The difference in magnitude between the a values for the syn and anti phenyl ketoximes is significant and suggests that the inductive effect alone cannot account for the observed substituent effect. If the inductive effect were operating by itself, the a values for syn and anti sets would be the same. 

The substituent effect in the phenyl group at the 3-position is also observed in the benzothiazoline series.2,17 The thermal fading rate increases with the bulky group at the 3-position, but the colored forms of 3-methoxy and 3-phenoxy derivatives are largely stabilized by an intramolecular interaction with 5-hydrogen atom.2... 

Absorption spectra of formazans have been studied in detail. Almost all formazans exhibit UV/visible spectra between 300 and 600 nm.1,2,12,13,40,62,325 326 The absorption maxima are very sensitive to substituent effects. For example, the 1,5-diphenyl formazan 185 when X is hydrogen, methyl, phenyl, cyano, and mercapto shows a band at 420, 410, 470, 504, and 590nm in ethanol, respectively. The 3-chloro derivative 186 when X is hydrogen, iodine, bromine, chlorine, and fluorine has a band at 433,433,430,421, and 417 nm, respectively. Table 13 shows the influence of substituents on the absorption maxima in the trisubstituted formazans 3. Table 14 shows the influence of substituents on the absorption maxima of... 

An example of this reveals an additional substituent effect (Fig. 2.18).121 Ordinarily, the phenyl and carboxyl groups anchor the double bond approximately equally (notice the sixth entry in Table 2.3 and cinnamic acid in Fig. 

The substituent effects on the alkene were investigated in the reaction of enyne 12 and chromium carbene complex 2c [8]. In the reaction of enyne -12a having a phenyl group on the alkene with Fischer chromium carbene complex 2c, metathesis product 13a was obtained as a main product along with cyclopropane 14 and cyclobutanone 15 (Eq.4). The reaction of Z-12a with 2c gave only... 

The solvent effect on the azo-hydrazone equilibrium of 4-phenylazo-l-naphthol has been modelled using ab initio quantum-chemical calculations. The hydrazone form is more stable in water and in methylene chloride, whereas methanol and iso-octane stabilise the azo form, The calculated results were in good agreement with the experimental data in these solvents. Similar studies of l-phenylazo-2-naphthol and 2-phenylazo-l-naphthol provided confirmation. Substituent effects in the phenyl ring were rationalised in terms of the HOMO-LUMO orbital diagrams of both tautomeric forms [53]. 

The range of SCS values (P-CH3O to p-NC>2) for nitrostyrenes is about 13 ppm. The correlation of the 170 chemical shift data for nitrostyrenes with that for nitrobenzenes57 gives a slope of 0.58 (Figure 3), which indicates that a comparable reduction in substituent effects results when the nitro function is separated from a p-substituted phenyl group by a carbon-carbon double bond. 

This property is relatively rare in the very large number of reactions for which substituent effects were evaluated quantitatively106. It seems to be common, however, for all dediazoniations of arenediazonium ions and of related compounds, e.g. of substituted phenyl azides forming nitrenes, as well as for additions of carbenes to alkenes. 

There follows in Sect. Ill a detailed discussion of substituent effects and their dependence on different geometric relationships of carbon atoms with respect to the perturbing group. This, however, is strictly confined to sp3 carbon atoms, though allowing for all conceivable kinds of substituents, including unsaturated ones (phenyl, cyano, etc.). 

Individual substituent effects of phenyl groups in substituted methanes and ethanes are not additive either (278), but decrease with progressive substitution In methane derivatives the first a-SCS (144) is 23.5, file second (144 — 145) 20.7, and the third (145 — 146) 14.7 (278). Although these values differ markedly from those of corresponding methylated compounds (methyl instead of phenyl) (169), the tendencies are in good qualitative agreement in both cases. 

Alkyl- and aryl-hydrazones of aldehydes and ketones readily peroxidise in solution and rearrange to azo hydroperoxides [1], some of which are explosively unstable [2], Dry samples of the p-bromo- and p-fluoro-hydroperoxybenzylazobenzenes, prepared by oxygenation of benzene solutions of the phenylhydrazones, exploded while on filter paper in the dark, initiated by vibration of the table or tapping the paper. Samples were later stored moist with benzene at —60°C to prevent explosion [3], A series of a-phenylazo hydroperoxides derived from the phenyl-or p-bromophcnyl-hydrazones of acetone, acetophenone or cyclohexanone, and useful for epoxidation of alkenes, are all explosive [4], The stability of several substituted phenylazo hydroperoxides was found to be strongly controlled by novel substituent effects [5],... 

Establishing that a smaller substituent in the peri position can raise the barrier, Oki and co-workers (149) were interested in finding the peri substituent effect on the barrier to rotation, and prepared a series of 9-(l,l-dimethyl-2-phenyl-ethyl)triptycenes (104). Data in Table 22 indicate that the barrier to rotation... 

The foregoing examples of differential reactivities of rotamers may be summarized by saying that the reactivity is controlled by the steric factor. The difference in the reactivities of rotamers of 9-(2-bromomethyl-6-methyl-phenyl)fluorene (56) in SN2 type reactions falls in the same category (176). However, the substituent effect is not limited to a steric one there can be conformation-dependent electronic effects of substituents as well. A pertinent example is found in the reactivity of the bromomethyl compound (56) when the rotamers are heated in a trifluoroacetic acid solution (Scheme 10). The ap form gives rise to a cyclized product, whereas the sp form remains intact (176). The former must be reacting by participation of the it system of the fluorene ring. 

In connection with the captodative effect, Riichardt (Zamkanei et al., 1983) has determined the BDE of the tertiary C—H bond in [20] and compared it with the tertiary bond in isobutane. He concludes that the stabilization of 12.8 kcal mol which he derives from this comparison falls 4kcal mol short of the value of 16.5 kcal mol which he calculates for the sum of the substituent effects for phenyl (9 kcal mol ), cyano- (5.5 kcal moP ) and methoxyl (1.5kcal mol ) groups. The latter values were derived from studies on C—C BDEs. Not even additivity of the substituent effects is observed. The existence of a captodative stabilization of radical [21] is denied (see, however, the studies on the thermolysis of [24]). 

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