Carbocationic character

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. 

It does not acmally lean completely to one side, but it does lean a little toward one side—which gives more carbocationic character to the tertiary carbon ... 

The crucial influence on the reactivity pattern in both cases is the very high stabilization that silicon provides for carbocationic character at the p-carbon atom. This stabilization is attributed primarily to hyperconjugation with the C-Si bond (see Part A, Section 3.4.1).85... 

The classification fragmentation applies to reactions in which a carbon-carbon bond is broken. One structural feature that permits fragmentation to occur readily is the presence of a carbon that can accommodate carbocationic character (3 to a developing electron deficiency. This type of reaction, known as the Grob fragmentation, occurs particularly readily when the y-atom is a heteroatom, such as nitrogen or oxygen, that has an unshared electron pair that can stabilize the new cationic center.96... 

It is necessary for the intermediate cation or complex to bear considerable car-bocationic character at the carbon center in order for effective hydride transfer to be possible. By carbocationic character it is meant that there must be a substantial deficiency of electron density at carbon or reduction will not occur. For example, the sesquixanthydryl cation l,26 dioxolenium ion 2,27 boron-complexed imines 3, and O-alkylated amide 4,28 are apparently all too stable to receive hydride from organosilicon hydrides and are reportedly not reduced (although the behavior of 1 is in dispute29). This lack of reactivity by very stable cations toward organosilicon hydrides can enhance selectivity in ionic reductions. 

Markovnikov (M) or anti-Markovnikov (AM) regiochemistry (equation 93 and 94)12. A preferential attack of the electrophile on the least substituted double bond has often been observed13. The M adduct is the only one formed when the ionic intermediate has a high carbocationic character, and may be formed from bridged species when the nucleophilic step has a substantial Sn 1 character. The AM product arises from an Sn2 process on the bridged intermediate. 

When nonnucleophilic salts, for example LiC104, are included in the reaction medium, products indicative of a more reactive intermediate with carbocationic character are... 

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 equilibrium between the enol (5a) and the keto (5b) forms depends upon the nature of R, electropositive substituents destabilizing the keto form 5b. This destabilization is due to the adjacent carbon having some carbocationic character, as can be seen in the resonance form 5c. 

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... 

Chalcogenopyrylium ions have a marked carbocationic character illustrated by resonance structures 6b-6c, which suggest pronounced electrophilic reactivity of a and y positions. Indeed most reactions occur through... 

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. 

For carboxylic esters, the rate ratios were much smaller, although still in the same order, so that this reaction is closer to a pure E mechanism, although the transition state stiU has some carbocationic character. Other evidence for a greater initial C-O cleavage with carboxylic esters is that a series of 1-arylethyl acetates followed rather than a, showing carbocationic character at the 1 position. ... 

---