Transition states carbocationic

Evidently, the transition state for acid-catalyzed epoxide opening has an Sn2 -like geometry but also has a large amount of S]v-l-like carbocationic character- Since the positive charge in the protonated epoxide is shared by the more highly substituted carbon atom, backside attack of Br- occurs at the more highly substituted site. 

The normal effect of NO2 on S l solvolysis of substrates such as benzhydryl chloride is to retard reaction. Thus in the ethanolysis of XQdLjCIIPhCI, logfc values (first-order rate coefficients, s 1) are as follows (50 °C) H, —3.05 m-N()2, —5.64 p-NCT. —5.99198. The p value for this reaction is about —3.7, so the logfc values for the m-NCT and p-NCT derivatives correspond fairly closely to the a values of 0.71 (or 0.73) and 0.78, respectively (Section III.B). This reaction is, of course, strongly accelerated by —R para-substituents through cross-conjugation in the carbocationic transition state (Section n.A). 

The mechanism of the acid-catalysed breakdown of hemiorthoesters Electron releasing substituents when attached to the pro-acyl carbon have a smaller effect on fcH+ for the breakdown of hemiorthoesters than of orthoesters (see p. 67 above) which suggest that these two classes of compounds react by different mechanisms and that the transition state for breakdown of the hemiorthoesters has less carbocationic character. Jencks and his coworkers (Funderburk et al., 1978) proposed mechanism (5) for the breakdown of hemiacetals and a similar mechanism (6) can be written for the breakdown of hemiorthoesters. This would explain (i) the general acid catalysis observed... 

The predictions of the CM model are exactly the same. In line with a simple Bell-Evans-Polanyi diagram (e.g. Fig. 18), stabilization of the product configuration leads to an earlier transition state, while stabilization of an intermediate configuration leads it increasingly to mix into the transition-state wave-function. For example, stabilization of the carbocationic configuration [36] results in the transition state acquiring more of that character so that an E2 process becomes more El-like (Fig. 266). 

Indeed, it has recently been shown by Rozen [13] that tertiary carbon-hydrogen bonds can be selectively replaced by carbon-fluorine bonds when the reaction is carried out in a polar solvent at low temperature,but it was suggested that an electrophilic process involving a carbocationic transition state is occuring in these instances (see 3.1.1.1). 

After discussing the dehydration of methanol and formation of DME, we are able to illustrate a number of key theoretical concepts. The first is that carbocation fragments are found in transition states, rather than as stable intermediates. Furthermore, the nature of these species is different from what is predicted from gas-phase studies, experimental or theoretical. The cluster, i.e., the zeolite, controls the stabilization of this carbocationic fragment. Second, we see that each different reaction requires a different transition state, rather than formation of a transition state that can be converted in a number of possible reactions. (This latter view received some support as a result of different processes possessing very similar activation barriers.)... 

Even the alkylation of isobutane by the ferf-butyl cation 4 despite the highly unfavorable steric interaction has been demonstrated142 by the formation of small amounts of 2,2,3,3-tetramethylbutane 36. This result also indicates that the related five-coordinate carbocationic transition state (or high-lying intermediate) 35 of the degenerate isobutylene-terf-butyl cation hydride transfer reaction is not entirely linear, despite the highly crowded nature of the system (Scheme 5.21). 

A quantum chemical investigation of the biosynthesis of farnesyl pyrophosphate through the condensation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate suggests that the mechanism is concerted, although the transition state has carbocationic character.164 Quantum chemical calculations were performed on the cyclization of the farnesyl cation to the sesquiterpene pentalenene.165 Two distinct pathways with similar activation barriers were identified, each differing from previous proposed mechanisms, and each involving unusual carbocationic intermediates. Mechanisms previously proposed for enzyme-catalysed formation of the sesquiterpene trichodiene involve carbocation intermediates with a 1,4-hydride transfer as the key step, e.g. (89) -> (90) - (91).166 Quantum chemical calculations, however, show a... 

The reaction is suggested to proceed via a two-electron three-center bound carbocationic transition state (218) formed by insertion of dication 214 into the C-H cr-bond of methane. 

This combination of significant C—O bond cleavage and the requirement for sp3 hybridization served to rule out the classic symmetric concerted transition state. The stereospecificity observed was used to rule out radical and carbocationic intermediates, and the slight increase in A H on increasing methyl substitution also argued against these electron-deficient carbon centers being present in the necessary intermediate. 

The r value should be a measure of the positive charge stabilization through the TT-delocalization interaction with the aryl rr-system in the incipient carbocation or the carbocationic transition state and will vary significantly with the a-R-substituents. Mishima et al. (1990c) found that the r values for all these systems [IC" ], ArC R R, can be correlated using (32) in terms of both resonance Act r and inductive polar a° substituent constants of R and R, ... 

In agreement with these considerations, a fairly good correlation between the rate constants for reaction (Eq. 21) and the solvolysis rates of the addition products has been observed (Fig. 13). Taking into account that approximately 90% of carbocationic character is manifest in the solvolysis transition states [175], the slope of the correlation in Figure 13 (slope x 2.3 RT = 0.45) indicates that roughly half of the character of the new carbocations is developed in the transition states of the addition reactions. 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

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