1-Cyclopropylvinyl cation

Extended Hiickel calculations have been carried out on the 1-cyclopropylvinyl cation 156 (122). These results show that the most favorable conformation for this ion is the linear bisected structure 156a. However, Hanack et al. (166b), by means of a modified CNDO technique, calculate the most stable geometry of the intermediate ion resulting from homopropargyl participation to be a bridged cyclobutenyl cation rather than 156a. 

Many valuable reviews of the chemistry of these species are given in the new book Dicoordinated Carbocations An introduction by Grob " is followed by reviews of various theoretical studies of vinyl cations, their gas-phase chemistry, their generation by nuclear decay, and their NMR spectroscopic characterization. Vinyl cation production by addition to acetylenes and allenes, by solvolysis, and photolytically are covered, together with the chemistry of the species generated in these various ways. The next chapter deals with the synthetic applications of vinyl cations,and alkynyl and aryl cations are covered in the last chapter. A review of the NMR spectroscopic and quantum-chemical investigation of vinyl cations in superacid media (also of dienyl and 1-cyclopropylvinyl cations) is published separately,as is a review of alkynylcar-... 

Recent calculations based on the extended Hiiekel (EHT) molecular orbital method for the 1-cyclopropylvinyl cation indicate that the ion is more stable in the linear bisected conformation (Kelsey and Bergman 1971). 

The high stability of the 1-cyclopropylvinyl cation provides the driving force for a 1,2-hydride shift on solvolysis of the cis- and trans-trifluoromethanesulphonates (790). This contrast with the solvolysis of the isomer (794), in which no hydride shifts are observed. ... 

An a-cyclopropylvinyl cation, l-cyclopropyl-3-methylbuta-l,2-dienyl cation 38, has been prepared by Siehl by ionizing the corresponding acetylenic alcohol with SbF5 (equation 32)68. 

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. 

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 . ... 

Rearrangement of vinyl cations to place the positive charge adjacent to a cyclopropyl group is more effective than the corresponding reaction to place the charge adjacent to phenyP. Many other aspects of the chemistry of cyclopropylvinyl cations have been summarized in a recent review ... 

The area of cyclopropylvinyl cation chemistry has been recently reviewed in detail by Stang and coworkers " and thus will only be summarized here. The majority of the studies subsequent to the initial report have concentrated on effects of the nature and of the stereochemistry of substituents on the double bond or on the cyclopropane ring on the rates of solvolysis and product ratios °. A typical example of the product forming-behavior from a cyclopropylvinyl system is given below. ... 

Full details have now been published of the solvolysis of P-allenic tosylates, which gives rise to cyclopropyl ketones and methylenecyclobutanols as discussed in an earlier Report. The homoallenic participation may involve a cation of the bicyclobutonium type in addition to a cyclopropylvinyl cation. Similarly, a cyclopropylcarbinyl cation has been implicated in the solvolysis of the vinyl triflate (40) which gives rise to the cyclic products (41), (42), and (43 R = CHjCFg) in addition to acyclic derivatives (Scheme 4). The same cyclic products are formed in the solvolysis of the tosylate (43 R = Ts). 

In 1969 Sherrod and Bergman and Bassler and Hanack independently reported that cyclopropyl groups strongly stabilize adjacent vinyl cations generated by solvolysis. Thus, 1-cyclopropylvinyl iodide (105) reacted with silver acetate in acetic acid at room temperature at a rate which is about 10 times faster than that of the corresponding 1-isopropylvinyl iodide (106). 

From the nature of these compounds it can be easily seen that in the ring expansion the most substituted bond of the cyclopropane ring is preferentially involved. In order to account for the stereoselectivity of the rearrangement, i.e. 68 % of the cis stereochemistry of the two methyl groups of the cyclopropylvinyl chloride (104) is retained in the methylenecyclobutanols, the intermediacy of a methylenebicyclobutonium cation was suggested... 

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