Substituent effects transmission

Some differences in the change of 13C NMR chemical shifts of nitrobenzothiazoles compared with nitrobenzimidazoles and nitrobenzoxazoles have been discussed. In Table 3.30 the 13C NMR chemical shifts of nitrobenzothiazoles are presented. The nitro group introduction into position 2 leads to a 10 ppm down field shift of the ipso-carbon resonance, whereas a similar effect of the ipso-substitution in the phenylene fragment of benzothiazoles is 20 ppm [778-781], The results of regression analysis of the 13C NMR chemical shifts of benzothiazoles in terms of the inductive and resonance constants of substituents (F and R, c, and cR, c, and aR°) provide evidence for the fact that the substituent effect transmission from positions 2-6 is approximately 30% weaker than in the opposite direction [779], As stated previously, an analogous picture is observed for benzimidazoles. 

Therefore the electron substituent effects in radical dianions of 2-substituted 5(6)-nitrobenzimidazoles are transmitted with approximately equal contributions of inductive and resonance components. A similar picture is observed for para-substituted nitrobenzene radical anions also [689, 883], Actually, the aN(N02) correlation of 2-substituted 5(6)-nitrobenzimidazoles RDA (RDA BI) with the aN(N02) of para-substituted nitrobenzene RA (RA Bz) indicates the same mechanism of substituent effects transmission but with different intensity, as shown in equation (3.4) ... 

These and other studies of the relative substituent effects of X and CH X in nitration were considered in terms of the transmission factor a of the methylene group. To avoid complications from conjugative interactions, attention was focussed mainly on substitution at the meta-position, and ct was defined in terms of partial rate factors by the equation ... 

In the second, which belongs to a systematic study of the transmission of substituent effects in heterocyclic systems, Noyce and Forsyth (384-386) showed that for thiazole, as for other simple heterocyclic systems, the rate of solvolysis of substituted hetero-arylethyl chlorides in 80% ethanol could be correlated with a constants of the substituent X only when there is mutual conjugation between X and the reaction center. In the case of thiazole this situation corresponds to l-(2-X-5-thiazolyl)ethyl chlorides (262) and l-(5-X-2-thiazolyl)ethyl chlorides (263). 

Although Noyce and Fike have recently found for the solvolysis of 2-thiazolyl-ethyl chlorides analogous modality of substituent electronic effect transmission from position 2 toward position 5 and from position 5 toward position 2(60). a more general conclusion indicates that the... 

Next we turn to the magnitudes of the p constants. Evidently if p = 0, there is no substituent effect on reactivity. Moreover because p = -I-1.000 by definition for the aqueous ionization of benzoic acids, we have a scale calibration of sorts. Wiberg gives examples of p as a measure of the extent of charge development in the transition state. McLennan" has pointed out that p values must first be adjusted for the transmission factor before they can be taken as measures of charge devel-... 

IV. Transmission of Substituent Effects through Heterocyclic Systems 236... 

In the manner outlined, a few attempts have been made to apply the Hammett equation to the transmission of substituent effects in the pyridine series. In the alkaline hydrolysis of 5-substituted ethyl picolinates (5-R-2-COOEt) in 85% ethanol at 25, 35, and 45°, the reaction constants are about 60% as large as those in the corresponding benzene series the overall fit to the Hammett equation, however, is at best fair, since out of four points (R = Et, H, I, Ac) one (Ac) deviates widely. 

Reaction Series Involving the Transmission of Substituent Effects Through Heterocyclic Rings... 

The paucity of data for substituted ethynylene sets is remarkable. Before any definitive conclusions concerning the transmission of substituent effects by the triple bond can be made, much further investigation is required. This field is one which is certainly worthy of further inquiry. 

In the application of the extended Hammett equation to the system XGY, where X is a substituent and Y is a reaction site, both of which are attached to the skeletal group G, the question of the degree of transmission of substituent effects through G arises. The transmitivity of G is measured by the magnitude of a and (3. Now,... 

The quantity 7ig is therefore a measure of the transmission of localized electrical substituent effects through the group G relative to the transmission of localized electrical substituent effects through the group Go. Similarly, for the transmission of the delocalized electrical effect, we may write... 

However, not all of the vinyloxyphosphazene monomers will undergo radical polymerization) those with amino substituents are unreac-tive. The i C nmr data indicate that these species electronically resemble vinyl ethers (which do not undergo radical polymerization) whereas the reactive derivatives resemble vinyl acetate. These data demonstrate an excellent example of electronic effect transmission in cyclophosphazene systems. 

Methods have been presented, with examples, for obtaining quantitative structure-property relationships for alternating conjugated and cross-conjugated dienes and polyenes, and for adjacent dienes and polyenes. The examples include chemical reactivities, chemical properties and physical properties. A method of estimating electrical effect substituent constants for dienyl and polyenyl substituents has been described. The nature of these substituents has been discussed, but unfortunately the discussion is very largely based on estimated values. A full understanding of structural effects on dienyl and polyenyl systems awaits much further experimental study. It would be particularly useful to have more chemical reactivity studies on their substituent effects, and it would be especially helpful if chemical reactivity studies on the transmission of electrical effects in adjacent multiply doubly bonded systems were available. Only further experimental work will show how valid our estimates and predictions are. 

The similarity observed for the correlation of the carbon shifts para to the substituent for 1 and 2 indicates that the nitrogen shift is a better probe of the transmission of the substituent effect to nitrogen. This conclusion can also be reached from a study of the polyfluorinated anilines. 

Among the earliest research on the transmission of substituent effects was the pioneering work of Lauterbur (58) and of Spiesecke and Schneider (59). For a wide series of substituted methanes and ethanes they found an approximate correlation of >3C chemical shifts with the electronegativity (E) of the substituents (X). So they concluded that inductive effects through o-bonds play an important role in determining the chemical shifts of a- and -positioned carbon atoms, and that this effect reflects the substituent s electron-withdrawing or -releasing ability. 

There are numerous other transmission mechanisms for substituent effects. Those connected with unsaturated frameworks, such as mesomeric effects and correlations with it charge densities and Hammett-type reactivity constants, and intramolecular hydrogen-bonding effects are beyond the scope of this article and are discussed elsewhere (1,8,25,57). 

Many authors have tried to identify other contributions to y-gauche substituent effects instead of or in addition to sterically induced bond polarizations. For electronegative substitutents, for example F, OR, NR2, and Cl, the most obvious interpretation is to assume a through-space electric-field transmission of yg-SCS... 

Of all substituent effects, that caused by y-anti substituents has been the hardest to understand in terms of transmission mechanisms, notably because the resulting shifts are sometimes upfield and sometimes downfield. So, for example, y -SCS(OH) values range from about +6 (208) to —6 (209). Correspondingly, the discussion about transmission mechanisms is prone to speculation and controversy. At present it is impossible to arrive at a consistent interpretation of this substituent effect there is no choice but to compile a number of molecular properties that apparently affect the transmission of substituent influences on antiperiplanar carbon atoms. 

In the previous sections the main focus of the discussion of substituent effects was their dependence on structure (especially stereochemistry) and the underlying transmission mechanisms. Admittedly, it would still be veiy difficult to predict 13C chemical shifts in more complicated and highly substituted aliphatic compounds with sufficient precision even if our knowledge about the physical back-... 

It has been found that the chemical shift of the benzylidene carbon, Sct, in ring-substituted benzaldoximes (Scheme 2) correlates well with Hammett a constants. On the other hand, it is known that the chemical shift of that atom in the NMR spectra of benzaldoximes depends mainly on the substituents inductive effect and its resonance effect is of reduced importance. Multiparameter correlations of Scr with the inductive and resonance substituents constants are of better quality. It is known that the chemical shifts of para-substituted benzaldoximes are linearly dependent on Hammett a constants. Correlations between the NMR chemical shifts of the oximino oxygen and a, a and aj substituent constants for substituted benzaldoximes are poor. These results show that coplanarity of Ar—CH=NOH creates difficulties in transmission of the substituents effect, especially the resonance effect, to the CH=NOH group. ... 

Quantum chemical calculations on substituted alkanes are in agreement with a through-space (field) transmission of substituent electrical effects . In view of the ever-increasing evidence for through-space transmission of electrical effects, it is time to make a conclusive statement. There is no inductive-effect component in electrical-effect transmission ... 

There is a means of determining the acidity site based on the extent of transmission of the electrical effect. We have shown that electrical-effect transmission in a data set is dependent on L, the coefficient of a L is a function of the number of bonds, n, intervening between the nearest atom of the substituent and the atom losing the proton. For 2 period elements in a number of acidic functional groups, the average values of L are ... 

For co-oxidation of nuclear-substituted styrenes, Table I shows a consistently decreasing rarh product as the polarity difference increases in pairs of substituted styrenes, the minimum value being about 0.4. This effect cannot be steric and must be polar. The polar effects could be caused by transmission of substituent effects through the O—O link, to some tendency of different styrenes to associate in solution—i.e., actual... 

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