Substituent effects hyperconjugative stabilization

Substituent effects Carbocations are formed in the S l reactions. The more stable the carbocation, the faster it is formed. Thus, the rate depends on carbocation stability, since alkyl groups are known to stabilize carbocations through inductive effects and hyperconjugation (see Section 5.2.1). The reactivities of SnI reachons decrease in the order of 3° carbocation > 2° carbocation > 1° carbocation > methyl cation. Primary carbocation and methyl cation are so unstable that primary alkyl halide and methyl halide do not undergo SnI reachons. This is the opposite of Sn2 reactivity. 

The /3-silyl effect is attenuated in a-ferrocenyI-substituted vinyl cations because the strong electron-donating effect of the a-ferrocenyl group renders the cation centre less electron-deficient and thus lowers the demand for hyperconjugative stabilization by substituents at the /1-carbon149. 

The 1 -(para-methoxyphenyl )-2-(triisopropylsilyl)vinyl cation 131 has also been generated, characterized, and studied323,333 to elucidate the importance of a-7t aryl and (3-0 hyperconjugative stabilization. A comparison was made with the para-methoxyphenylvinyl cation 124, which has a higher demand for a-aryl 7t-stabilization. The para carbon of cation 131 is 7 ppm more shielded compared to that of cation 124, indicating that the (3-silyl stabilization effect is operative. The experimentally determined rotation barrier of the para-methoxy substituent for 131 (<8 kcal mol-1) and 124 (9.0 kcal mol-1) shows a less significant double-bond character of the MeO-C(4)... 

The major reaction product formed from the solvolysis of the trans y-silyl ester was cyclohexene formed from the carbocation 138 (Scheme 18) by 1,2-hydrogen migration to give the p-silyl cation 139 followed by loss of the silicon substituent. In contrast to the behavior shown by 136, the cis y-silyl ester 134 exhibited a small inverse p-d4 isotope effect (kH/kn = 0.97) attributed to the inductive effect of the P-deuteriums and implies that there is very little hyperconjugative stabilization of... 

Effect of alkyl substituent on carbocation stability. A carbocation is stabilized by overlap of filled orbitals on an adjacent alkyl group with the vacant p orbital of the carbocation. Overlap between a cr bond and a p orbital is called hyperconjugation. 

Summary 1-Ary 1-2-trialkyIsilyl-substituted vinyl cations are characterized in solution by NMR spectroscopy. The NMR chemical shift data reveal the stabilization of the positive charge by a (5-silyl substituent. The order of hyperconjugative stabilization of a positive charge by P-substituents is H < alkyl < silyl. The P-silyl effect is dependent on the electron demand of the carbocation and decreases with better electron donating a-substituents. NMR spectroscopy is a suitable tool to investigate the competition between 7i-resonance and CT-hyperconjugation in these type of carbocations. 

In p-silyl substituted benzyl cations with different a-aryl substituents the stabilizing effect of a P-silyl group depends on the electron donating ability of the a-aryl group. The better the a-aryl substituent can stabilize the positive charge, the lower is the need for P-o-hyperconjugative stabilization by the silyl group and the no-bond resonance structure becomes less important (Fig. 6). 

Scheme 2.112 The regioselectivity of fluoride addition and the relative stabilities of the carba-nions derived from different perfluoroolefins are governed by the number of -fluorine substituents which can stabilize the negative charge by their combined -/ effects and by negative hyperconjugation.

The Cieplak model emphasizes an alternative interaction, between the ct orbital of the C-X bond and the antibonding orbital to the nucleophile.In this case, a better donor X should be the most stabilizing. Li and le Noble pointed out that both of these hyperconjugative interactions will be present in the transition structures There is also general agreement that addition of reactive nucleophiles have early transition states, which would suggest that substituent effects might best be examined in the reactant. 

The issues that remain under discussion are (1) the relative importance of the acceptor (Felkin-Ahn) or donor (Cieplak) hyperconjugation capacity of a substituents (2) the relative importance of electrostatic effects and (3) the role of reactant pyramidalization in transmitting the substituent effects. Arguments have been offered regarding the importance of electrostatic effects in all the systems we have discussed. Consideration of electrostatic effects appears to be important in the analysis of stereoselective reduction of cyclic ketones. Orbital interactions (hyperconjugation) are also involved, but whether they are primarily ground state (e.g., reactant pyramidalization) or transition state (e.g., orbital stabilization) effects is uncertain. 

The TT-donor and (3-hyperconjugation effects are present for both alkyl and vinyl carbocations. The vinyl carbocations exhibit somewhat larger stabilizations because they are inherently less stable. The vinyl cations also appear to be more sensitive to the a-polar effect. Note that the F, OH, and NH2 substituents are less stabilizing of vinyl cations than of alkyl cations. This is believed to be due to the greater sensitivity to the polar effect of the more electronegative sp carbon in the vinyl cations. The NPA and bond order characterizations of the carbocations are shown in Table 3.13. The values give the charge associated with the cationic carbon and the bond order between the cationic carbon and the substituent X. 

The greater the number of alkyl substituents bonded to the positively charged carbon, the more stable the carbocation will be. The order of relative stability of carbocations is tertiary benzyhc > allylic secondary > primary vinyl> phenyl. The nature of electron release by alkyl groups is not very clear. It may be an inductive effect, a resonance effect (hyperconjugation), or a combination of the two. When we refer to the inductive effect of the alkyl groups, it should be clear that this might well include a contribution from hyperconjugation. 

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