Transition state imbalance

Nucleophilic addition of phenolic nucleophiles to l,l-dicyano-2-arylethenes in the gas phase and in water has been studied theoretically" using the semiempirical AMI method and the Cramer-Truhlar solvation model SM2.1. The difference between the Brpnsted coefficients (a" = 0.81 and P" =0.65) determined for the gas-phase reaction is indicative of a small positive transition state imbalance of / = 0.16. For reaction in water the estimates (a" = 0.61 and P" = 0.36, giving I = 0.25) are close to the experimental values (a" = 0.55 and P" = 0.35) obtained with amine bases, and the small imbalance is as expected for a reaction involving no hybridization change at the incipient carbanion site. 

Transition state imbalance in the deprotonation of substituted 2-tetralones by hydroxide ions has been described. A Brpnsted plot of logA versus substrate pAfa is linear, with slope —a) of —0.60 0.01 but the negative deviation of the point for the 6-nitro derivative suggests that delocalization of charge lags behind proton transfer. ... 

AMI molecular orbital studies of gas-phase deprotonations of A-methyl-4-(Y-phcnylsulfonylmcthyl)pyridinium cations and A-(Y-benzyl)-4-(phenylsulfonylmethyl)-pyridinium cations promoted by NH3 reveal negative transition state imbalances, I (= a — (1) < 0 (where a = 0.17-0.18 and /i = 0.53), which are a consequence of the difference in distance between the substituents and the anionic charge centre in the transition state and products.149... 

The corresponding relationship between substituent effects in forward, reverse, and equilibrium reactions and transition state imbalance in carba-nion reactions, of which the nitroalkane anomaly235 is a prime example, has been discussed in detail by Bernasconi.233... 

Transition state imbalance and the PNS 224 Scope of this chapter 225... 

The effect of resonance on intrinsic barriers and transition state imbalances 226... 

This year marks the 25th anniversary of the principle of nonperfect synchronization (PNS) it was introduced in 19851 as the principle of imperfect synchronization (PIS) but in later papers and reviews2 4 the name was changed due to the awkwardness of the acronym PIS. The foundations of the PNS rest mainly on a marriage between two fundamental concepts of physical organic chemistry, i.e., the concept of intrinsic barriers and that of transition state imbalances. 

The determination of BrVinsted coefficients provides the most transparent tool for the evaluation of imbalances, but there are other ways to probe transition structure in search for evidence of transition state imbalances. Terrier et al.45 reported that the change to a more electron-withdrawing Z-... 

Amyes and Richard49 deduced the presence of a transition state imbalance in the deprotonation of methyl and benzylic mono carbonyl compounds by HO- from the linearity of the BrlTnsted plot of the rate constants versus the pK of these carbonyl compounds. They argued that because of the large reactivity range the Br Ansted plot should have shown Marcus curvature 5 8 if the intrinsic barriers for these reactions were all the same and hence the absence of such curvature indicates changes in the intrinsic barriers. They... 

There are a number of factors that affect intrinsic barriers and/or transition state imbalances. Many of these may be viewed as derived effects because they are a consequence of the imbalance caused by the presence of Jt-receptors, i.e., in the absence of this imbalance they would not affect the intrinsic barriers even if they affect actual barriers and equilibria. 

A complementary aspect of solvation is that it affects the magnitude of the transition state imbalance. This can be seen for the reactions of ArCH2N02 in DMSO and MeCN where the imbalances are much smaller than in water (Table 2, entries 4 and 6). Again we see the connection between imbalance and intrinsic barriers the greater imbalance induced by solvation leads to an enhanced intrinsic barrier. 

Table 15 includes two geometric parameters that provide complementary information about transition state imbalances. They are the % progress of the... 

The resonance effect increases the barriers by 1.0 to 3.2kcalmol 1. The reason for this small increase is that the resonance stabilization of the transition state which is given by p aR+pjenhancing effect of resonance is consistent with the rather small transition state imbalances (Table 15). 

The fact that the intrinsic rate constants for nucleophilic addition to Fischer carbene complexes are relatively low, for example, much lower than for most reactions with comparable vinylic substrates or carboxylic esters,188 constitutes strong evidence for the presence of substantial transition state imbalances. However, there have only been a few studies of substituent effects that demonstrate the imbalance directly by showing a uc > p uc or by providing an estimate of its magnitude from the difference a uc - p uc. One such study is the reactions of 76-Cr-Z and 76-W-Z with HC CCII20 and C.F3CH20, 183 It yielded a Llc 0.59 and p ]uc< 0.46 for 76-Cr-Z, and a[juc 0.56 and 0 42 for 76-W-Z, i.e., a ue > p uc as expected. 

Another type of transition state imbalance was noted by determining substituent effects on the rate and equilibrium constants for buffer-catalyzed MeOH addition to 78a (equation 62). 

As already mentioned in the above referred sections, the A o-reducing effect of resonance stabilization of the carbanion resulting from the deprotonation of organic carbon acids is the consequence of transition state imbalances. These imbalances manifest themselves in the Br0nsted a value being greater than the /3 value, with the... 

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