Carbocations stability with cyclopropyl groups

The rate acceleration for the 2,2-dimethyl derivative in the reaction of equation 4 was a factor of 92 relative to the unsubstituted derivative". The smaller ratio for 17a may be partly due to an attenuation of the substituent effect in the more stabilized carbocation from 17a, and also to steric interactions in the ion from 17a which interfere with the preferred geometry for conjugation by both the aryl and cyclopropyl groups. 

The 13 C NMR chemical shifts of the para carbon for various phenyl-substituted sp2-and sp-hybridized carbocations (Figure 15) indicate that the demand for rr-aryl delocalization of the positive charge for the -silyl-substituted vinyl cation 393 is lower compared with that in a-phenylethyl (78), a-methyl-a-phenylpropyl (397) and cumyl (292) cations150 153. The stabilizing effect of the /)-silyl group in 393 is comparable to that of the cyclopropyl substituent in 1-cyclopropylbenzyl cation (398). The para carbon shift in the /)-cr-silyl... 

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