Cyclopropylcarbinyl cation reactivity

The reactive cyclopropylidene cyclopropylcarbinyl cation 37 was generated from the corresponding bromide and silver hexafluoroantimonate, and was reacted in situ with olefins such as cyclohexene to form the addition products (equation 33). On the other hand, under the same conditions, the isopropylidene cyclopropylcarbinyl cation 41 rearranged spontaneously by ring expansion (equation 34)71. 

The cyclopropylcarbinyl radical (4), the cyclopropylcarbinyl cation (5) and the cyclopropylcarbinyl anion (6) all dominate the expected reactivity of vinylcyclopropanes since one of these forms will be expected to be a major contributor to either a radical or a polar transition state. It is important to consider the various reactive subunits in some detail in order to understand the big picture reactivity of a vinylcyclopropane system, especially as perturbed by additional substituents. 

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... 

Two cationic structures, protonated cyclopropane and the cyclopropylcarbinyl cation, were among the earliest for which non-classical structures were proposed. Structure elucidation for both of these reactive intermediates owes much to the use of labeled cyclopropanes they will be the focus of attention in this section. 

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. ... 

These models represent 4 variants of the cyclopropane ring position relative to the departing group at C . All of these positions, however, are far different from the situation in the a-cyclopropylcarbinyl cations since in all the models the position of the cyclopropane ring with respect to the vacant p-orbital at C is perpendicular rather than parallel. For the compounds studied the authors have observed the following reactivity series endo-anti 565 > exo-syn 564 > endo-syn 566 > exo-anti 563 with a rate ratio of 10 10 10 1 (100 °C). 

One of the most challenging molecules for organic electronic structure theory is cyclopropane. It is difficult to understand C-C-C angles of 60° with conventional bonding models. Also, in Section 11.5.14 we examined the unusual reactivity of the cyclopropylcarbinyl cation. Much of the rearrangement chemistry of this cation, as well as its special thermodynamic stability (Chapter 2), can be understood to arise from the uniquely strong donor character of the a bonds in cyclopropane. Here we provide a more detailed description of the bonding in cyclopropane that nicely rationalizes these observations. 

Transient Three-membered-Ring Compounds.—A review on protonated cyclopropanes as reactive intermediates has appeared. The reaction of methylcyclo-propane with proton is predicted, by EH-MO calculations, to involve two dominant paths (i) removal of hydride ion to generate cyclopropylcarbinyl cation, and (ii)... 

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. 

A bicyclobutonium ion, which is the resonance hybrid of a cyclopropylcarbinyl, a cyclobutyl and a homoallyl cation is considered to account for the reactivity of this system (equation 72) . 

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