Carbocations, stability orbital calculation

The ability to promote j8 elimination and the electron donor capacity of the jS-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they promote addition and strongly control the regiochemistry. A silyl or stannyl substituent strongly stabilizes carbocation character at the j3-carbon atom and thus directs the electrophile to the a-carbon. Molecular orbital calculations indicate a stabilization of 38 kcal/mol, which is about the same as the value calculated for an a-methyl group. The reaction is then completed by the elimination step, in which the carbon-silicon or carbon-tin bond is broken. 

The Stabilities of Bridged Carbocations—Some Molecular Orbital Calculations ... 

Because the nonbonding orbital is occupied, stability increases with s character, the converse of the situation for carbocations. The order of stability of carbanions is sp < sp < sp. The relative stability of gas phase carbanions can be assessed by the energy of their reaction with a proton, which is called proton affinity. The proton affinities of the prototypical hydrocarbons methane, ethene, and ethyne have been calculated at the MP4/6-31+G level/ The order is consistent with the electronegativity trends discussed in Section 1.1.5, and the larger gap between sp and sp, as compared to sp and sp, is also evident. The relative acidity of the hydrogen in terminal alkynes is one of the most characteristic features of this group of compounds. 

The cycloheptatrienyl carbocation (20) is also a six n electron system, and all six electrons can go into bonding orbitals. Its delocalization energy is calculated to be 2.99)8 (Figure 4.26), which is close to the value of 50 kcal/mol of delocalization stabilization calculated by other methods. Therefore, the cycloheptatrienyl cation is an especially stable carbocation, although it is still a cation and is certainly not as stable as benzene. On the other hand, the cycloheptatrienyl anion is a 4n Ji system and thus is predicted to be antiaromatic by HMO theory. More advanced calculations suggest that any energy consequences of electron delocalization in the anion must be very small. 

Hyperconjugative stabilization has also been considered to be important for bridgehead carbocations since 10-tricyclo[5.2.1.0 ]decyl tosylate (40) reacts nearly 10 times slower than expected on the basis of molecular mechanics calculations. The carbocation from this compound has none of the P-y C—C or C—H bonds antiperiplanar to the vacant p orbital, and so hyperconjugative stabilization should be absent, whereas with most... 

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