Imbalanced transition state

A Bronsted fl]t value of 0.5 and occh value of 1.31 have been calculated for deprotonation reactions of (3,5-dinitrophenyl)nitromethane promoted by substituted benzoate ions and of substituted (3-nitro-, 4-nitro- and 3,5-dinitro-)phenyhiitro-methanes promoted by benzoate ion, respectively, in methanol.146 The intrinsic rate constants are (2.0-6.3)x 104 tunes lower than for the same reactions in acetonitrile solution, and this has been attributed to commensurate reduction of strength of the hydrogen bond between the carbon acid and benzoate ion in the imbalanced transition state. The transfer activity coefficient (logMyAN) from methanol to acetonitrile solution have been calculated for (// -nitrophenyl)nitromcthyl anion (3.6) and (m-nitrophenyl)-nitromethane (—1.0). 

Based on secondary kinetic deuterium isotope effects and some ab initio calculations, Alston et al.47 calculated that the transition state in the deprotonation of acetaldehyde by HO is imbalanced in the sense shown in Equation (3). 

One of the consequences of the imbalanced nature of the transition state is that the polar effect of a remote substituent may either increase or decrease the intrinsic barrier whether there is an increase or decrease depends on the location of the substituent with respect to the site of charge development. Let us consider a reaction of the type shown in Equation (4). In this situation an electron-withdrawing substituent Z will decrease AG or increase ka. This is because there is a disproportionately strong stabilization of the transition state compared to that of the product anion due to the closer proximity of Z to the charge at the transition state than in the anion. As discussed earlier, this also leads to an exalted BrlAnsted aCH value and is the reason why aCH > Pb for the deprotonation of carbon acids such as 11-13 and others (Table 2). 

Fig. 2 Modified More O Ferrall-Jencks diagram for the CH3N02/CH2=N02 system. The curved lines represent the reaction coordinates through the optimized and constrained transition state, respectively. The constrained transition state is less imbalanced as indicated by its location to the left of the optimized transition state.

The conclusions reached by Costentin and SavOant are in fact quite consistent with our own. The main difference is that, according to these authors, the notion of an imbalanced transition state should be placed within the context of charge localization-delocalization heavy-atom intramolecular reorganization rather than of synchronization (or lack thereof) between charge delocalization and proton transfer. ... 

Perrin s group220 concluded that, for aromatic alkenes (R = aryl), the transition state, schematically represented as 89, may be imbalanced in that the delocalization of the positive... 

McClelland et al.221 have suggested that the general acid-catalyzed decomposition of a hemiacetal anion, Equation (63), proceeds through an imbalanced transition state where sp3 to sp2... 

Table 27 The effect of substituents and charges on AGj and kQ for reactions with imbalanced transition states...

In an effort to examine whether reaction (56) or similar reactions in fact have an imbalanced transition state, substituent effects on the rate and equilibrium constants of the reaction of 78 with alkoxide ions (equation 61) were determined/... 

A more reasonable hypothesis is that the transition state is imbalanced, as shown in equation (102), but that there is a structural feature characteristic of carbene complexes, absent from other carbon acids, that masks the imbalance by reducing a. The most likely candidate is the 7r-donor effect of the methoxy group. Inasmuch as the contribution of 151 leads to resonance stabilization of the carbene complexes, this resonance is expected to add to the intrinsic barrier of proton transfer. This is because, as is true for resonance effects in general, its loss at the transition state should be ahead of the proton transfer. As Z becomes more electron... 

Study of structure-activity relationships in nucleophilic vinylic substitution reactions (S nV) that proceed by the addition-elimination mechanism has been furthered by the study of acid-catalysed breakdown of alkoxide and thiolate ion adducts of benzylidene Meldrum s acid (36a), methoxybenzylidene Meldrum s acid (36b), and thiomethoxybenzylidene Meldrum s acid (36c). Catalysis of alkoxide or thiolate expulsion is in competition with protonation of the a-carbon or one of the carbonyl oxygens to form an enolate. The positive Ag values for H+-catalysed RO and RS departure imply an imbalanced transition state in which proton transfer leads C-O or C-S bond cleavage. The pA values of the various adducts are much lower than for Meldrum s acid, in view of the stabilizing influence of OR, SR, and OMe groups on the carbanion. 

In the scientific literature one finds an intense and interesting scrutiny concerning the applicability of TST versions for solution reactions " (compare also the concept of imbalanced transition states")... 

The displays are very useful in interpreting concerted vs step-wise RP or in discussing imbalanced transition states, etc. They are particularly suited for representing potential energy surfaces for... 

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