The Origin of Substituent Effects

The possibility of calculating 33S NMR shielding constants has made it possible to obtain a better insight into the origin of substituent effects on 33S NMR in 2-substituted ethanesulphonates.35 Sulphur isotropic absolute shielding... 

These findings suggest that a substituent R at the d-position is more effective than one at the e-position in controlling facial discrimination toward the aldehyde carbonyl (Fig. 15). The origin of this effect has not been determined. 

Worthy of brief mention are the inverse effects of y-deutera-tion on solvolysis of n-propyl derivatives (Leffek et al., 1960b), as well as the effect of deuterated alkyl substituents (Lewis and Coppinger, 1954 Shiner and Verbanic, 1957) or direct ring deu-teration (Klein and Streitwieser, 1961) on solvolysis of benzyl- or benzhydryl-type compounds. The reader is referred to the original articles for discussions of the origins of these effects. 

It was also clear that the use of NMR required a different approach to problem solving than that typically used with NMR. While the latter technique could be employed with a very general approach (e.g., the Karplus equation), NMR would, at least for the immediate future, require a relatively extensive set of model systems from which the consequences of stereochemical changes could be derived for any given carbon framework. Indeed, our own research efforts required such a set of spectral data for model, monosubstituted systems and served as the impetus for the collection of the extensive data that will be found in Chapters 5-18. With this extensive data available in computer searchable form, we are beginning to assemble the equivalent of a Karplus relationship for y substituent effects in NMR and hope to make these findings available in the not too distant future. However, the origin of such effects is not at all well defined and the observed spectral shifts may be caused by the coincidence of several relatively independent factors. 

The electronic origin of substituent effects on the Fermi contact term of /hc couplings in 1-X-cyclopropanes (X = H, Cl, Br, I, CN, COOH, CHO and NH2) has been investigated by Neto et by the CLOPPA (Contributions from Localized Orbitals within the Polarization Propagator Approach) method. The results obtained have been interpreted in terms of hyperconjugative interactions. 

The preference for endo attack in 7,7-dimethylnorbomene is certainly steric in origin, with the 7-methyl substituent shielding the exo direction of approach. The origin of the preferred exo-attack in norbomene is more subject to discussion. A purely steric explanation views the endo hydrogens at C—5 and C—6 as sterically shieldihg the endo approach. There probably is also a major torsional effect Comparison of the exo and endo modes of reproach shows that greater torsional strain develops in the endo mode of... 

Finally, in this account of multiparameter extensions of the Hammett equation, we comment briefly on the origins of the a, scale. This had its beginning around 1956 in the a scale of Roberts and Moreland for substituents X in the reactions of 4-X-bicyclo[2.2.2]octane-l derivatives. However, at that time few values of o were available. A more practical basis for a scale of inductive substituent constants lay in the o values for XCHj groups derived from Taft s analysis of the reactivities of aliphatic esters into polar, steric and resonance effects . For the few o values available it was shown that o for X was related to o for XCHj by the equation o = 0.45 <7. Thereafter the factor 0.45 was used to calculate c, values of X from o values of XCH2 . ... 

It is often difficult to understand at an intuitive level the explanation for the effect of changing substituents on the rate constant ratio kjkp for partitioning of carbocations between nucleophilic addition of solvent and deprotonation. In these cases, insight into the origins of the changes in this rate constant ratio requires a systematic evaluation of substituent effects on the following ... 

In conclusion, bromination is a particularly attractive reaction for studying the origin of reactivity-selectivity effects in detail, since it is now well established that substituent and solvent effects arise not only from changes in the stability of the cationic intermediate but also from transition-state shifts, in agreement with the Bema Hapothle, i.e. RSP, Hammond postulate and Marcus effects. 

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