Substituent position

Substituted aromatics, eg, aLkylbenzenes, sometimes experience attack at the substituent position by NO/ (7). A cyclohexadienyl cation is formed it is unstable and the nitro group migrates on the ring to a carbon atom that is attached to a hydrogen. Loss of the proton results in a stable nitroaromatic. 

Substituent Aza positions Substituent position Substituent position Substituent position ... 

Another important component of Table 18 is substituent chemical shift (SCS) datadenvedforeachoftheSlentnes The SCS is simply thedifferencein F-NMR chemical shifts of the substituted compounds and that of unsubstituted fluoroben zene (-113 5 ppm in CDCI3) These values numerically represent the mfluence a substituent has on the shieldmg or deshieldmg of the fluorine nucleus and depend upon substituent position o, m, orp) Fluonne chemical shifts can be predicted for polysubstituted fluorobenzene systems simply by addmg the SCS value of each substituent to a base value of -113 5 ppm... 

What effect does a ring flip have on substituent positions Do all of the substituents change position or only certain ones If the latter, then which substituents are affected Obtain the energies of the two conformers. Which conformer is preferred Why ... 

Substituents Position of exchange lff ki Substituents Position of exchange 107kt... 

For any given reaction series the equation is applied to meta- and para-substituents separately, and so values of p, and pR characteristic both of reaction and of substituent position are obtained. The various crR-type scales are linearly related to each other only approximately. In any given application the scale which gives the best correlations must be found65. ... 

Figure 32-1. The porphin molecule. Rings are labeled I, II, III, and IV. Substituent positions on the rings are labeled 1,2,3,4,5,6,7, and 8. The methenyl bridges (—HC=) are labeled a, (3,y, and 5.

The structure of the parent compound 43 of the 1,1 -binaphthyl host family has been determined many times in independent laboratories and in different (racemic and resolved) crystal forms 60,61,72,73). A common feature of both types of structure is that molecules of 43 form helices. Lateral contacts between such helices play an important role in the respective crystals. These spatial arrangements also emphasize the importance of the question concerning the presence of alpha or beta substituents positioning as mentioned earlier 28 ... 

The results obtained with different amines cannot be explained merely on the effects of amine basicity. Thus, to obtain complete hydrogenation of Q to DHQ, the basicity has to be tailored by other factors such as the steric hindrance of the amine and its electronic interaction with the catalyst active sites this seems to be favored by the presence of an electron-rich aromatic ring. Of note, the positive effect of substituted aromatic amines, with a 49% DHQ yield being obtained for ethylanilines, is independent of the substituent position of the alkyl group. 

A similar reaction sequence of triisopropylphenylphosphole or mesitylphosphole (17b and 17a, respectively) with phosphorus tribromide afforded the corresponding 2-substituted products. The reaction of dibromophosphine 37 with nucleophiles followed by oxidation or hydrolysis gave phosphonic or //-phosphinic derivatives (39 or 41, respectively) (Scheme 9) [48, 49], The regioselectivity is obviously the consequence of the presence or the lack of the steric hindrance with ortho tert-butyl groups, only position 3 is available, while with the smaller triisopropyl substituent, position 2 may be the appropriate reaction site. 

Few 13C-NMR data are available (46,51,52)-, they provide a simple means to elucidate substituent positions in ring-substituted borabenzene derivatives (46,52). 

In order to further experimentally probe the hard-core model of polysilane aggregate chirality, a series of poly-[(alkyl)alkylphenylsilylene]s, differing in phenyl ring-substituent position and chain length were synthesized, designed such that the polymer chain diameter, d, and helical pitch, p, in the hard-core model were varied.343... 

Table 3 presents the experimental enthalpies of formation of polynitrobenzenes and Table 4 presents the calculated additivity values and DSEs for these same compounds. Enthalpy-of-formation values have been determined experimentally for all three dinitrobenzene isomers in the gaseous state. The enthalpy-of-formation difference between the meta and para isomers is indistinguishable from 0. Conventional wisdom suggests that the para isomer should be destabilized relative to the meta because of adjacent positive charges in key ionic or polar resonance structures. Thus it seems that electronic effects due to meta/para dinitro substituent position are small. This small enthalpy-of-formation difference is similar to that for the meta and para dicyano, difluoro and dichloro benzenes, but does not mimic the ca 22 kJ mol 1 difference for the phthalic acids with which the... 

The RDA reaction is also a typical process of flavones, naphthoflavones, and methoxynaphthoflavones. [115] In many cases it provides intact A- and B-ring fragments, and therefore, it is of high relevance for structure elucidation. The intensity ratio of A- to B-ring fragments was found to be strongly influenced by the substituent position, i.e., to be very sensitive to the charge distribution within the molecular ion. [116]... 

Galanakis D, Calder JA, Ganellin CR, Owen CS, Dunn PM. Synthesis and quantitative structure-activity relationships of dequalinium analogues as K channel blockers investigation into the role of the substituent position 4 of the quinoline ring. J Med Chem 1995 38 3536-3546. 

The results of the studies will be summarized. Details of the QSAR analyses are or will be published elsewhere, including intercorrelation matrices of the steric parameters mentioned. But relevant conclusions from e.g. intercorrelations will be dicussed. At this moment the STERIMOL method has been applied successfully in about 50 publications often with better results than other steric approaches, including MTD and MTD, especially in series with few substituent positions. A recent example is our study of DDT analogs. Brown et al. (9J analysed a series of 21 derivatives using the van de Waals (Vw) volumes as steric parameters. In Table I the equations are given in which the steric parameters are compared. 

When less substituent positions are present STERIMOL can be used but in those cases there is often hardly any difference in results if compared with the MTD method. This is illustrated by our version of the QSAR of the insecticidal activity against American cockroaches of 36 substituted benzyl chrysanthemates. 

The acyl group of an intermediate acyl-HP (e.g., 17) in principle may attack at either nitrogen atom of the pyrimidine ring. Therefore, in addition to ring isomerism another type of isomerism exists for synthesis B, caused by the position of substituents (positional isomerism). 

Since the activity seemed highly dependent on the positions of hetero-atom substituents, the compounds were stereochemically classified into two groups, meso- and (//-analogs, according to their hetero-substituent positions as shown in Fig. 12. QSAR analyses for insecticidal activity of these groups were performed separately to give Eq. 52 and 53 74). 

---