Electron transfer vinylcyclopropane

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

The nature of vinylcyclopropane radical cations was elucidated via the electron transfer induced photochemistry of a simple vinylcyclopropane system, in which the two functionalities are locked in the anri-configuration, viz., 4-methylene-l-isopropylbicyclo[3.1.0]hexane (sabinene, 39). Substrates, 39 and 47 are related, except for the orientation of the olefinic group relative to the cyclopropane function trans for 39 versus cis for 47. The product distribution and stereochemistry obtained from 39 elucidate various facets of the mechanism and reveal details of the reactivity and structure of the vinylcyclopropane radical cation 19 . 

Several interesting rearrangements (stereomutations) were observed for a series of l-anisyl-2-vinylcyclopropanes (32, Ar = CgH -OCty. These reorganizations have been elaborated in greater detail than most other radical cation reactions. For this reason, we will consider them in somewhat greater detail, even though they were initiated primarily by chemical, rather than photochemical electron transfer. Treatment of the m-isomer (cis-32) with aminium or dioxygenyl salts at... 

The vinylcyclopropane radical cation, is another radical cation of structure type B, which is stabilized by conjugation. Its proposed structure was based exclusively on ab initio calculations (B3LYP/6-31G ) because the electron-transfer photochemistry of this species failed to provide clear-cut CIDNP effects [128], In this context it is worth noting that product studies cannot, in principle, establish the cyclopropane radical cation structure type. Irrespective of the structure, nucleophilic capture is expected to result in the cleavage of the strained ring. 

Radical cations of three terpenes, sabinene 47, a-thujene 48, and j6-thujene 49, containing vinylcyclopropane functions held rigidly in either an anti- or syn-orientation, are elucidated by CIDNP results observed during there electron transfer reactions with photoexcited (triplet) chloranil [131]. 

The photochemical reactivity of vinylcyclopropane under electron-transfer conditions has been studied The reactions were carried out using DCB/ phenanthrene as the sensitiser system and in a solvent mixture of acetonitrile/ methanol (3 1). The three products (89)-(91) result from ring opening and trapping by solvent. The radical cation of (92) can be formed photochemically using DCB as the electron-accepting sensitiser. " When the reaction is carried out in acetonitrile/methanol as the solvent mixture the two products (93) and... 

According to Nishida ° the [27t-l-27r] cycloadditions proceed via dipolar intermediates, whereas the [2n + 2(7] cycloadditions start with a single-electron-transfer process. In [27i-l-27t] additions a deeply colored charge-transfer complex is initially formed and the reaction is favored by polar solvents (as is usually the case with [2jt-l-2jt] additions of TCNE to electron-rich alkenes). Vinylcyclopropanes with a very low ionization potential afford the [2 t-f 2(t] product they do not form a charge-transfer complex with TCNE and the reactions have to be performed under oxygen-free conditions (Table 5). 

When donor-substituted and acceptor-substituted alkenes complex and suffer a one-electron transfer, the cation radical of the radical cation-radical anion pair may suffer rearrangement at rates competitive with back electron transfer or with radical ion pair recombination. Some vinylcyclopropanes show this phenomenon (127 —> 128 —> 129 —> 130) 2 TCNE and the diphenylbenzocyclobutene (131) react similarly to form (134) by way of (132) and (133).2 ... 

It has been suggested that the recently observed 1,4-addition of trimethylsilyl-lithium to a/S-unsaturated ketones and the 1,4-addition of cuprous methyl-trialkylborates to l-acyl-2-vinylcyclopropanes also occur by a one-electron-transfer mechanism the latter reaction provides a useful synthetic route to (E)-a -unsaturated ketones. 

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