Cyclopropane ring opening, allylic cation

The mechanism for bromination of semibullvalene has been proposed, based on quantum chemical calculations. The reaction pathway involves concerted bromine addition and cyclopropane ring opening to form an allylic cation, without the intermediacy of a bromonium or a cyclopropylcarbinyl cation.18... 

Depending on the reaction conditions, the allylic cation can be trapped by the halide ion expelled during cyclopropane ring opening or by an external nucleophile. For example, 3,3-dibromotricyclo[4.1.1.0 ]octane (14) rearranged to give 15. ... 

The direct observation of ion 46 is of particular interest in that it clearly does not involve a significantly opened cyclopropane ring, which could lead to the formation of an allylic cation. Thus, it must be considered as a bent cyclopropyl cation.164-170 It is, however, clear that the Ci-C6 ct-bond must to some degree interact with the empty p orbital at Cn and that homoconjugation between them becomes the important factor in stabilizing such a bent cyclopropyl species. 

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of fluoroolefins and other unsaturated compounds. For example, F-allyl cation 1 was proposed as a reactive intermediate in reactions of HFP with fluoroolefins catalyzed by Lewis acids [7]. The difference in stability of the corresponding allylic cations was suggested as the explanation for regio-specific electrophilic conjugated addition to CF2=CC1CF=CF2 [11]. Allylic polyfluorinated carbocations were proposed as intermediates in the reactions of terminal allenes with HF [53] and BF3 [54], ring-opening reactions of cyclopropanes [55], Carbocations are also an important part of the classic mechanism of electrophilic addition to olefins (see Eq. 2). This section deals with the questions of existence and stability of poly- and perfluorinated carbocations. 

An illustration of this divergent reactivity can be observed in BanwelTs use of gem-dibromocyclopropane 7 to construct an intermediate used in an approach to the Amaryllidaceae alkaloids (Scheme 4.2).9 When 7 was heated in the absence of Lewis acid, allyl bromide 8 was produced in quantitative yield via thermal ring opening of the cyclopropane and subsequent trapping of the resultant cation by free bromide in... 

The instability of cyclopropyl cations means that, even as they start to form as intermediates, they spring open to give allyl cation-derived products. Try nucleophilic substitution on a cyclopropane ring and this happens. 

The cycloaddition of alkenylidenecyclopropanes 5 with chlorosulfonyl isocyanate is described via electrophilic attack at C4, disrotatory opening of the cyclopropane ring to form a dipolar intermediate 8, and ring closure by nucleophilic attack of nitrogen or oxygen on either end of the allyl cation portion of the intermediate 8 which gives adducts 9-12. [27t -I- 2tc] Products 13 were also obtained. 

For a summary of early examples of cationic cyciopropyl to allyl rearrangements see Houben-Weyl, Vol. 4/3, pp 615-632 and Vol. E19c, pp 265-267. The ring-opening reactions of donor-acceptor substituted cyclopropanes that do not contain a leaving group X are discussed in Section 2.1.2. 

The third item of Table 8.7, the solvolysis of the toluenesulfonate ester of endo-bicyclo[3.1.0]hexyl-6-ol in acetic acid to produce the acetate ester of 2-cyclohexen-l-ol (Scheme 8.78), is of particular interest since it requires the opening of the cyclopropane ring in an allowed electrocycUc reaction (Chapter 4, Section G). In this case, the a bond of the three-membered ring opens in a disrotatory fashion (conserving mirror symmetry) to generate the allyl cation. ... 

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