Carbocations, stabilization cyclopropyl groups

Quantitative estimates of the thermodynamic stabilities of various phenyl and cyclopropyl substituted cyclopropenium ions were carried out by their pATR+ measurements18. The pKg+ values for 1,2,3-tricyclopropylcyclopropenium, l,2-dicyclopropyl-3-phenylcy-clopropenium ion and 1 -cyclopropyl-2,3-diphenylcyclopropenium ion were determined to be 10.0, 7.09 and 5.04, respectively. Thus, replacement of each phenyl group by a cyclopropyl group enhances the stability of the carbocation by two pATR+ units (2.74 kcal mol1). These results were also supported by the isodesmic reaction of equation 72 for which the energies were optimized at the HF/3-21G //HF/3-21G level18. 

The stability of the carbocation increases by the presence of each additional cyclopropyl group. 

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. 

Reactivity studies of conformationally fixed substrates has been an important technique for the study of the conjugative stabilization of carbocation centers afforded by cyclopropyl groups, and although Chapter 8 of this monograph is devoted to this topic, brief mention will be made of the subject here because of the direct relevance to the topic at hand. Examination of the long-lived ions themselves, principally by NMR, provides a further dimension to the analysis. 

The reactivity of a large number of geometrically constrained cyclopropylcarbinyl cations have been examined so that the angular dependence of the ability of the cyclopropyl group to stabilize the developing carbocation center could be assessed. Overlap is optimal when the angle 0 in Figure 1, Section II.C, is 0°, as in 7b, and is at a minimum when 6 is 90°, as in 7d. Relative reactivities for conformationally fixed systems... 

Stabilizing interactions of developing carbocations were observed for one but not both of the )S-cyclopropyl groups in 94a-94c . 

Thus, it is seen that additional cyclopropyl groups are effective at further stabilizing an adjacent carbocation center. 

A wide range of studies55,60-62) have indicated that a cyclopropyl group is equal or more effective than a phenyl group in stabilizing an adjacent carbocation center. 

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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