Reactivity-Selectivity postulate

Lewis and co-workers also are concerned with the reactivity-selectivity postulate (RSP), which can also be derived from the Marcus expression. In the examples given here, selectivity does not vary with reactivity, in apparent contradiction to Marcus theory. This result can be explained on the basis that the intrinsic barriers are not constant and by assuming that the quadratic term of the Marcus equation contributes very little when the identity barriers are high (as they are when rates are well below diffusion control). Other important contributions to understanding the RSP have been made recently (9a, 9b). 

Leffler and reactivity-selectivity postulates, which predict that the selectivity should decrease and the Brpnsted (5 approach unity as the reactions become more endergonic. The curvature in this plot is much greater than predicted by the Marcus equation (equation 6) (22), however, and is believed to be an artifact caused by enhanced solvation of 7r-acceptor para substituents such as CN, COC6H5, and N02 (21). [The Marcus equation, which has gained wide acceptance in the interpretation of electron-transfer reactions, is represented in equation 6 as a Brpnsted relationship with an exponential term added to take into account curvature (23)]. 

Chemists have intuitively assumed for some time that selectivity is inversely related to reactivity. Thus a very fast reaction is likely to be less affected by substituents than is a slower one. There is at present considerable discussion as to the general validity of the reactivity-selectivity postulate (RSP) and Johnson [91] has indicated in his book that the linearity of the Hammett correlation is directly contrary to the postulate. The validity of the reactivity-selectivity postulate is directly related to that of Hammond (see later) which states that the structure of the transition state is closest to the state (ground or product) which has the highest potential energy. 

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. 

In other words, under these restrictive conditions, outer sphere electron-transfer reactions obeying the Marcus-Hush model are typical examples where the Hammond-Leffler postulate and the reactivity-selectivity principle (see, for example, Pross, 1977, and references cited therein, for the definition of these notions) are expected to apply. 

On the basis of the Leffler-Hammond postulate the theoretical justification for the reactivity-selectivity principle may be observed. First, let us define the selectivity 5, of a species A, in its reaction with two competing reagents X and Y, as indicated by (4), where k ... 

It is important to note that the above presentation, justifying the reactivity-selectivity principle, is based on a number of fundamental assumptions. First, it is assumed that the Leffler-Hammond postulate is valid, which in turn implies that the reaction under consideration obeys a rate-equilibrium relationship [eqn (2)]. This assumption often cannot be verified since for reactions of highly active species such as carbenes, free radicals, carbonium ions, etc., equilibrium constants are generally not measurable. However it follows that for reactions which do not conform to a rate-equilibrium relationship, no reactivity-selectivity relationship is expected. Also, in Fig. 4, the difference in the free energy of the... 

In line with the Leffler-Hammond postulate, which is a differential analogue of the Hammond postulate, the rationale for a differential reactivity-selectivity principle has been demonstrated, i.e., the marginal stabilization of a particular species will result in a corresponding increase in its selectivity. Recently, the view has been expressed that there is no substantive evidence for such behaviour... 

Reactivity-selectivity relationships are obtainable in carbene chemistry provided an independent selectivity parameter is found. Under such circumstances the relative rate data then serve as a measure of relative reactivity. This has been done with a number of Hammett studies. Thus the addition of dichlorocarbene to compounds [ 1 ] — [3] (listed in Table 15) indicates that a reactivity-selectivity relationship is obtained using p as a measure of selectivity. A more reactant-like transition state is obtained for the more reactive substrate in accordance with the Leffler- Hammond postulate. 

In spite of these uncertainties, however, the utility of the reactivity-selectivity principle has been illustrated for a number of diverse areas of mechanistic interest. Such applications are being extended to other areas as well. For example, Olah has recently studied the mechanism of electrophilic addition to multiple bonds using selectivity data and concluded that the transition states of the bromine addition to alkenes are of a 7r-complex nature (Olah and Hockswender, 1974). Finally the large number of reactivity-selectivity relationships which have been discovered offer considerable experimental support for the various expressions and formulations of the Hammond postulate whose profound effect on modem mechanistic chemistry is now beyond question. 

The Hammond postulate is often accepted as a general principle an increase in reactivity is accompanied by a decrease in selectivity because the transition structure becomes closer to that of the reactant state as the energy barrier decreases. This idea has some truth for a hypothetical A to B reaction model where it is implicit that only a single bond change occurs moreover the Hammond postulate is predicted by the Marcus theory (above). The postulate often breaks down for reactions where more than one major bond change results in product formation. It should be emphasised that any discussion of the reactivity-selectivity relationship has to be confined to those reactions where there is no change in rate-limiting step or mechanism. 

Abstract A review is presented of synthetic methods for the preparation of biaryls by the rhodium-catalyzed C-H bond arylation of arenes with aryl halides (C-H/ C-X couplings), arylmetal reagents (C-H/C-M couplings) and arenes (C-H/C-H couplings), with an emphasis on postulated mechanisms and their implications on reactivity, selectivity and substrate scope. 

The reactivity-selectivity relationship can be analyzed by postulating the following with respect to the reactants reactant A is designated as if it is highly... 

Abstract Site-selective peptide/protein degradation through chemical cleavage methods is an important modification of biologically relevant macromolecules which complements enzymatic hydrolysis. In this review, recent progress in chemical, site-selective peptide btuid cleavage is overviewed, with an emphasis oti postulated mechanisms and their implications on reactivity, selectivity, and substrate scope. 

Physical organic chemistry has been built around LFER, which is still a matter of active research from experiment to theoretical simulation [53,54]. We ourselves have also addressed these problems under the perspective of the ISM [10,36a,42]. Here we will only deal with the interpretation of the Brpnsted relations, the postulate of Hammond and the Reactivity-Selectivity Principle (RSP). 

The reactivity-selectivity relationship can be analyzed by making the following postulations with respect to the reactants Reactant A is designated as Afj if it is highly reactive (hot) and as Ac if it is less reactive (cold), and the second reactant (which is also referred to as the substrate) is designated as B if it is more reactive and as 5 if less. Thus, the selectivity will vary depending on different combinations of A and B. 

In so far as Westheimer s treatment correlates isotope effects with changes in force constants and the structure of the transition state, it cannot be tested without some experimental measure of these properties, and usually it has been assumed that, within a family of related reactions, the structure of the transition state varies smoothly with the rate constants and equilibrium constants of the reactions, with reactant-like transition states associated with reactive substrates and exothermic reactions. This assumption, which derives from observations of rate-equilibrium and reactivity-selectivity correlations [2, 3, 42], as well as calculations of semiempirical potential energy surfaces [43], is generally known (not quite accurately) as Hammond s Postulate [44]. It should be noted that while the postulate probably applies more generally to proton transfers than to other reactions, recent considerations of its scope and limitations [45, 46], based on extensive experimental experience, strongly suggest that departures... 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

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