Substituent effects additivity rules

Chemical shifts of olefinic hydrogens not only reflect the inductive and mesomeric effects of substituents, but also their stereochemical position. Well-known empirical additivity rules have... 

The addition of O2 (whether or A ) to 138 or 4 has apparently not been studied theoretically although selection rules for singlet oxygen reactions/ interpretation of substituent effects and orbital correlations for homolytic fission and re-formation of the peroxide bond in 9,10-diphenyl-anthracene peroxide and rubrene peroxide have been published. 

Calculations of the 13C-13C coupling constants for all aldopyranoses of the D-series have allowed some general relationships with respect to stereochemistry and coupling constants to be postulated <2003RJ0663, 2003RJ01194>. Data have been reported for the 13C NMR substituent effects at the a, f3, it, and S positions for 2-halomethyl-2-hydroxy-tetrahydrofurans and 2-halomethyl-5,6-tetrahydro-4/7-pyrans. These studies showed that additivity rules allow the prediction of the chemical shifts for each carbon <1996SPL631>. 

An empirical substituent chemical shift (SCS) additivity rule has been established by which the shift of a particular aromatic carbon in the yS-O-4, /f-5 and 5-5 substructures of lignin can be estimated by summation of the different substituent effects (Hassi et al 1987, Drumond et al 1989) This method helps to distinguish among overlapping signals and improves assignment... 

Meisenheimer complex formation as separate reactions. Meisenheimer complexes can be considered as the anionic pseudobases derived from neutral aromatic molecules, and in this light it is clear that heterocyclic Meisenheimer complexes are appropriately considered in the current Review. By so doing, it is hoped that attention can be drawn to potentially mutual benefits that may be derived from comparative studies of neutral and anionic pseudobases. Certainly, the spectroscopic techniques applicable to the study of pseudobase and Meisenheimer complex formation are identical. Quantitative studies of substituent effects and structural effects on rates and equilibria for nucleophilic addition should be relevant both to neutral and to anionic e-complex formation. The general rules enunciated by Strauss23,318 and Fendler319 for the prediction of the relative stabilities of Meisenheimer complexes should be directly applicable to analogous pseudobases. Terrier et al.2n have made an important contribution in this area with a detailed comparison of kinetic and thermodynamic parameters for formation of a benzofuroxan Meisenheimer complex and an isoquinoline pseudobase. 

The additive nature of the substituent constants is expressed in equation 12, which states that the influence of several substituents is equal to the sum of the individual effects of each substituent alone. The additivity rule is usually reasonably well obeyed without any significant deviations (2, 5, 6, 7, 8). 

NOESY has also been used to elucidate the chain conformation of poly(styrene-a/ -MMA).220,221 2D INADEQUATE has been applied to studies of monomer sequence distribution in ethylene-propylene copolymer.223 Additivity rules for the 13C chemical shifts of ethylene-propylene copolymer were devised for configurational sequences as well as substituent effects.226... 

A great deal of effort has been expended to model the transition structures of concerted pericyclic reactions. All of the major theoretical approaches, semiempirical MO, ab initio MO, and DFT have been applied to the problem and some comparisons have been made. The conclusions drawn generally parallel the orbital symmetry rules in their prediction of reactivity and stereochemistry and provide additional insight into substituent effects. 

Apart from the local and global softness (hardness), the concept of group softness (hardness) has been introduced [19], especially for studying the electronic effect of substituents in aromatic compounds [25]. It also applies to solids and can be directly calculated according to the additivity rule for local softness, Eq. (23). Yet another related quantity can be defined, which is useful for understanding observed trends and which has considerable predictive value. 

As a result of the fortuitous simplifying circumstance mentioned earlier, a molecule can be divided into action and neutral zones. The latter zone usually occupies the bulk of the molecule s size. The word action is more appropriate than reaction as used by many authors, because the concept is applicable even where no reaction occurs. Changes in properties are assumed to occur only as a result of changes in the action zone. Rules can be formulated for the effect of different substituent groups in the action zone. These are the additivity rules, or the rules of group contributions. It must be noted, however, that in the interest of greater accuracy in properties estimation, it may often be necessary to introduce higher order approximations that violate the neutrality of the neutral zone but one pays a price for this an increase in the number of empirical parameters. 

A series of additional 3- and 5-substituted derivatives of 2-foimylpyridine thiosemicarbazone have been synthesized [19] in which the 3-substituent was COOH or F and the 5-substituent was F, Q, Br, I, Et, OCF3, CF3, N(Me2), OjSMe, and OAc. The effect of these substituents on antitumour activity was tested in several systems however, potency against a particular tumour followed no simple parametric rules. Furthermore, the order of substituent effects changed markedly from one tumour system to another [19]. 

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