Cyclopropylethyl cation

From the dynamic NMR studies it was shown that the activation barrier for the degenerate equilibrium is 7.4 kcalmoT at -112 This activation barrier is similar to those observed for other 1-cyclopropylethyl cations. The l-(c/.s-l,2-dimethylcyclo-propyl)ethyl and l-(2-methylcyclopropyl)ethyl cations 51 and 55 have an activation barrier for the degenerate rearrangement of less than 5 kcalmol The degenerate equilibrium in all these cases may proceed through the substituted cations as intermediates vide suprdf. ... 

Theoretical calculations, such as those performed in this work, lead to the structure presented in Fig. 16, in which the plane dehned by the carbons of the 2-propyl moiety is a perpendicular bisector of the cyclopropyl ring, as in the case of the 1-cyclopropylethyl cation. This is an important result, in full agreement with the NMR spectrum of the ion. Furthermore, rotation around the C(l)-C(a) bond involves a barrier of ca. 13.7 kcal mol These structural features, as well as those presented by 15 and 17, are consequences of the stabilizing interaction between the strained C-C bonds of the cyclopropane moiety and the formally empty p orbital of the alkyl moiety. 

Tertiary 1-aryl-l-cyclopropylethyl cations 27 show the expected deshielding of the cationic center chemical shift with increasing electron demand, as measured by aryl rings58. Thus, <513C(C+) values of227.8,247.6 and 251.1 were observed for thep-OMe,p-H and p-CF3 substituents, respectively. 

The stabilization of the vinyl cation by the a-cyclopropyl group was calculated to be significantly less than that by the phenyl group. The theoretical rotational barrier of the a-cyclopropylvinyl cation is less than that of the cyclopropylethyl cation, presumably due to the stabilization of the intermediate perpendicular conformation by the overlap of the ff-bonds with the n-electrons of the C=C bond72. 

Tertiary 1-aryl-l-cyclopropylethyl cations 27 show the expected deshielding of the cationic center chemical shift with increasing electron demand, as measured by or... 

Evidence for the conjugative interaction of cyclopropyl with adjacent vinyl cations comes from a number of sources. Theoretical calculations for a variety of substituted vinyl cations (109), including the case where R = c-Pr, indicated that substituent effects in 109 were similar to those of the corresponding ethyl cations (110) . The rotational barrier calculated for cyclopropylvinyl cation was half that of cyclopropylethyl cation, a result tentatively attributed to conjugation of cyclopropyl with the 7r-bond of the vinyl cation when the cyclopropyl was twisted perpendicular to the vacant p orbitaP . ... 

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