Cyclopropylcarbinyl cations substituent effects

The most curious feature of these reactions is that the observed products can only be rationalized by preferential migration of the ester-bearing carbon to the electron-deficient centre of a cyclopropylcarbinyl cation. This is not at all the expected substituent effect for such an electron-withdrawing group. 

L. R. Schmitz and T. S. Sorensen, J. Am. Chem. Soc., 104,2605 (1982). This paper is concerned mostly with substituent effects on the classical cyclopropylcarbinyl cation structure, but footnote 2 in this reference provides a useful listing of all earlier calculations on the system. 

The dominant contributor to the reactivity of vinylcyclopropanes in any radical reaction is the form (4a), the cyclopropylcarbinyl radical system. The opening of a cyclopropylcarbinyl radical to a butenyl radical is among the fastest radical processes known, with a rate constant of 1.3 x 10 sec . The various stereoelectronic effects of this rearrangement have been reviewed. - The structure of (4a), deduced from its ESR spectrum - and in agreement with calculations (STO-36 basis set), is in the bisected conformation shown, predicted to be 1.4 kcal mol more stable than its perpendicularly oriented counterpart. Above -100 C only the butenyl radical (4b) can be detected. Substituent effects do not seem to operate here when the substituents are on the cyclopropane (i.e. product stabilization). The cyclopropylcarbinyl cation and anion have structures similar to (4a), bisected conformations (5) and (6), respectively. A concise summary of solvolytic and mechanistic data for system (5) has recently appeared. Reviews of cyclopropylcarbinyl anions and carbenes are also available. ... 

Thus the low reactivity of 87 certainly shows the absence of the strong conjugative stabilization found for the bisected cyclopropylcarbinyl arrangement, but does not provide a quantitative assessment as to the respective roles of ring strain and the inductive effect of cyclopropyl in giving the observed rate of deceleration relative to dimethyl substituents. Accurate molecular mechanics calculations for 87 and the derived cation would provide an assessment of the former, and a measure of o-j for c-Pr free of resonance effects is much to be desired. 

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