Phenyl substituent contributions

Phenyl Substituent Contributions in Circular Dichroism Spectra of Cobalt(HI) Complexes of Ethylenediamine-A A -diacetate Ion... 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

We were concerned that we had overcompensated for the instability of the indenyl cation with the inclusion of the stabilizing phenyl substituent. That is, did the presence of the phenyl substituent cause a change in the resonance hybrid giving an enhanced contribution of 15 over 16 ... 

The length of the oligomer chain plays a role in the stereospecificity, the presence of a few units in the initiating chain leads to an increase in diastereoselectivity, as was also found for growing propene oligomers. Thus, the hindrance between chain and metal catalyst contributes to the orientation of the phenyl substituent of the incoming styrene molecule. 

In contrast to the deactivating influence of the ra-phenyl substituent, the 3-phenyl substituent should activate the ring towards electrophilic substitution. The available evidence for simple aliphatic systems indicates that resonance contributions from phenyl substituents are large and far outweigh the inductive effect. 

Variations in the contribution of p-fluoro and p-phenyl substituents prevent the definition of a satisfactory constant. 

Structure-activity relationship (SAR) studies In this series of tetrahydrobenzazepines ( 5) indicate that the 1-phenyl, or substituted phenyl, group contributes significantly to the DA receptor agonist properties of these compounds (13, 14). As the 1-phenyl substituent in I-III provides these molecules with an asymmetric center, separation and study of the enantiomers was of particular Interest. 

Another example for an application of computed sector maps is the OR of phenylalanine, as far as contributions from the phenyl group are concerned [155]. Sector maps for benzene were computed as shown in Fig. 25. The findings of [155] were in agreement with a revised sector rule for the phenyl chromophore by Pescitelli et al. who investigated a set of chiral molecules with phenyl substituents (PhCH(Me)R, with R = Et, "Pr, Pr, and Bu) [265],... 

On addition of a second pair of phenyl substituents at C4 and C6, the interallylic distance further lengthens so that the contribution of structure C to the TS wave function is further enhanced. The ability of the second pair of phenyl substituents to increase the optimal value of R from 7 = 2.218 A to 7 = 2.649 A allows each of the phenyl substituents in l,3,4,6-tetraphenyl-l,5-hexadiene to provide more stabilization for the Cope TS than the pair of phenyl substituents in 1,3-diphenyl-1,5-hexadiene. This cooperative effect of the four phenyl groups in 1,3,4,6-tetraphenyl-1,5-hexadiene lowers AT/ for Cope rearrangement by, not twice, but by four times as much as the pair of phenyl substituents in 1,3-diphenyl-1,5-hexadiene. 

However, really large phenyl stabilization energies come, not at / o = 1-965 A, where the TS is best described by structure B in Fig. 30.1, but at values of R at which the TS has a larger contribution from either structure A or C. Small values of R enhance the contribution of diradical A, and large values of R enhance the contribution of diradical C. Thus, as shown in Tables 30.1 and 30.2, phenyl groups at C2 and C5 of 1,5-hexadiene favor a small value of R in the Cope TS, and phenyl substituents at Cl, C3, C4, and C6 favor a large value of R in the TS. 

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