Cations cyclopropane ring opening

A novel iron-catalyzed, cationic cyclopropane ring opening was reported. Aryl-substituted cyclopropyl acetylenes gave ring-opened allene intermediates by a cationic rearrangement. In some cases, this gave functionalized naphthalene products. 

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... 

The details of the electron-transfer processes involved on irradiation of mixtures of tetrachloro-p-benzoquinone with the cyclopropane derivatives (58) and (59) have been reported. The intermolecular electron-transfer processes lead to the formation of the radical cation of the cyclopropanes. Ring-opening of the cyclopropanes results from these species. ... 

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. ... 

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

Almost all reactions of alkylidenecycloproparenes lead to opening of the cyclopropane ring. A notable exception to this is the reversible electrochemical reduction of237 and 240 which leads to the stablfe radical anions 396 and 397, with half-wave potentials of -2.32 and -1.93 V, respectively, and their oxidation to the quasi-stable radical cations 398 and 399 ( i/2(ox) = +0.68 and +0.81). The cations may be further oxidized to the corresponding very short-lived dications. In contrast, the photoelectron spectra of 237 and 240 reveal practically identical first-oxidation potentials of both compounds, which indicates that the difference in half-wave potentials for oxidation (in condensed phase) of 237 and 240 does not exist in the gas phase. This has been attributed to structure-specific solvation energies in the radical cations 398 and 399. °... 

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