Cyclopropyl group stabilizing effect

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

The selective oxidation of hydrocarbons by dry ozonation is not restricted to reaction at tertiary C atoms the CH2 groups adjacent to a cyclopropane ring are smoothly converted into carbonyl groups. This cyclopropyl activation can be explained by the well-known ability of the cyclopropyl group to effectively stabilize a neighboring R3C+ center. Although in the oxidation of alkanes with ozone the first intermediate, the hydrotrioxide, [11] does not arise from carbenium ions, the effect of polarity upon attack of the electronegative ozone molecule, either as a radical or by 1,3-dipolar... 

The spatial position of the vacant p-orbital relative to the cyclopropane ring influences the efficiency of its participation in the stabilization of the cation centre In a-cyclopropylcarbinyl ions the cyclopropyl group interacts effectively with the cation centre if the vacant p-orbital is parallel to the plane of the cyclopropane ring ... 

If this conclusion is correct, it means that the effect by which a cyclopropyl group stabilizes an adjacent cationic center in cyclopropylcarbinyl cations is not neighboring group participation as defined initially in Chapter 1 and that the rate enhancement should not be referred to as anchimeric assistance. 

In ion D, in which the phenyl group would be expected to be coplanar with the cationic center to maximize delocalization, the observed angle is 25-30°. This should permit effective benzylic stabilization. The planes of the cyclopropyl groups in both structures are at 85° to file plane of file trigonal carbon, in agreement with expectation for the bisected... 

In contrast to the stability of cyclopropylmethyl cations (p. 222), the cyclopropyl group exerts only a weak stabilizing effect on an adjacent carbanionic carbon. ... 

Cyclopropyl carbinyl cation1". The most dramatic example for a 7i-acceptor/7i-attractor substituent effect is provided by the cyclopropyl carbinyl cation. In its bisected conformation, the vicinal CC bonds are considerably longer (1.65 A) and the distal CC bond considerably shorter (1.41 A) than the bonds in 1 (Table 13). The CH2+ group stabilizes 1 considerably more in the bisected form than in the perpendicular form (13 kcal mol1). Since the CC+ bond possesses partial double bond character, it is shorter (1.35-1.36 A, Table 13) than a normal CC+ bond (1.51 A). 

The relationship between charge density and the NMR chemical shifts is, however, only qualitative and should be used with caution. Other factors, such as neighboring anisotropic effects of the substituents, should also be considered. The cationic center of triphenylmethyl cation (<5I3C 211.2 ppm), for example, is much shielded from that of tricyclopropylmethyl cation (270.9 ppm), which may erroneously lead to the conclusion that a phenyl is more stabilizing than the cyclopropyl group. 

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... 

Perkins, M. J. Peynircioglu, N. B. The rates of deprotonation of some cyclopropyltoluene derivatives in search for the effect of the conformation of cyclopropyl groups on the stabilization of the carbanionic centers. Tetrahedron 1985, 41, 225-227. 

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. 

Ree and Martin" used the data of Brown and Cleveland to calculate a value of — 0.21 for a cyclopropyl group twisted out of conjugation with the aryl ring by two ortho-methyl groups. The cr value of — 0.03 for cyclopropyl also indicates a cation-stabilizing ability intermediate between H and CH3 for a non-conjugated cyclopropyl group. The inductive effect of cyclopropyl is discussed further in Section VI.D but is yet to be fully resolved. 

The effect of j5-cyclopropyl groups on hydration according to equation 40 was demonstrated to be decelerating relative to hydrogen or methyl, as shown by the data in Table 21. This effect was interpreted in terms of the transition state 174, in which the rate effect of P-c-Ft was correlated with a term proportional to the cr parameter of cyclopropyl (— 0.04) and another term proportional to the estimated stabilizing effect of cyclopropyl on a double bond (4.4 kcal mol ) . 

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. 

From solvolytic studies Hart inferred in 1959 the extreme stabilizing effect of a cyclopropyl group on an adjacent positively charged center20. In the early 60 s Deno21 and Olah22 reported the first spectroscopic data of long-lived cyclopropyl substituted carbonium ions. 

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