Vinylcyclopropane cation radical

Roth et aJ [10] have used CIDNP to study the structures of vinylcyclopropane radical cations fomied from precursors such as sabinene (1). 

Roth H D, Weng H and Herbertz T 1997 CIDNP study and ab initio calculations of rigid vinylcyclopropane systems evidence for delocalized ring-closed radical cations Tetrahedron 53 10 051-70... 

As a consequence of the above observations, the intramolecular capture of the vinylcyclopropane radical cation was attempted next. Thereby, chrysanthemol and homochrysanthemol both yielded the expected cyclization products (Scheme 35) [55],... 

Scheme 35 Cyclization by intramolecular capture of vinylcyclopropane radical cations.

A more detailed evaluation of the diverse structures proposed for the secondary species goes beyond the scope of this review. We mwely emphasize that the ESR results provide detailed evidence for the nature of the radical center, but fail to elucidate the cationic site. The identity of this center is left to secondary considerations or speculation. We also note that any alternative structure has the virtue of not contradicting the ab irutio calculations the potential c ture of chloride ion has precedent in the nucleophilic substitution at a cyclopropane carbon (see Section 7). Another type of ring-opened structure has been postulated as an intermediate in the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes (see Section 5). 

Figure 7. Calculated bond lengths (pm) and hyperfine couplings (G) for the syn- and ant/-conformers of vinylcyclopropane radical cationic states (MP4SDQ/6-31G )." ...

On the other hand, the CIDNP approach and recent ab initio calculations have provided what we consider unambiguous evidence for the nature of the vinylcyclo-propane radical cation as well as for some simple derivatives. We have carried out ab initio calculations on the prototype 19 and several simple derivatives, and probed by CIDNP the structures of three simple vinylcyclopropane radical cations in which the two functionalities are locked in a syn configuration. 

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... 

In general, radical cations of alkenes or cyclopropanes produce nonconjugated radicals, while those of dienes give rise to allyl radicals, and those of vinylcyclopropane or vinylcyclobutane systems generate either allylic or nonconjugated radicals with an additional element of unsaturation. In contrast to the most thoroughly characterized nucleophilic substitution of appropriate neutral molecules. 

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 . 

Phenylcyclopropane radical cation (9 ) has divergent hyperfine coupling constants for the secondary cyclopropane protons (aptrans = 0.78 mT upcis = 0.07 mT CIDNP, B3LYP/6-31G calculations), apparently because the cis protons are located in a nodal plane. " Similarly, vinylcyclopropane radical cation is... 

The vinylcyclopropane radical cation, generated at 77 K by X-irradiation of (139) in a Freon-113 matrix, was shown to rearrange at 105-110 K to afford two ring-opened distonic radical cationic species.300 The rearrangement reactions of the radical cations of 1,3- and 1,4-pentadiene and cyclopentene and the formation of spin adducts with 2.4.6-tri-/-butylnitrosobenzene (BNB) are discussed. The pulse radiolysis of 1,1 -binaphthyl-2,2,-diyl hydrogenphosphate (BiNPCUH) (140) in deaerated f-butanol at... 

Vinylcyclopropane to cyclopentene rearrangement 184 Dissociation of acetone radical cation 186 The wolff rearrangement 187... 

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 final topic to be discussed in this section concerns the prospect of opening or breaking the weakened bond in the radical cations of the 2Aj structure type. This topic has been the subject of some controversy. As mentioned above, ab initio calculations fail to support the existence of such a species. Yet, they have been postulated to rationalize some ESR observations and also to explain the geometric isomerizations of 1-aryl-2-vinylcyclopropanes upon reaction with ami-nium radical cations [225]. 

Ring-opened cyclopropane radical cations have also been postulated to account for the stereochemistry of the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes [225]. These systems, of course, contain substituents that may veil the true nature of the cyclopropane radical cations by delocalizing spin and charge. In addition, we note that the experimental findings allow some latitude in their interpretations. 

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. 

Although bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane are modestly efficient electron donors, the main value of these ring systems lies in the rigid framework they offer for the elucidation of the stereochemical course of radical cation reactions. We will mention several vinylcyclopropane systems and several tricycUc cyclopropane derivatives. 

Methylene-l-isopropylbicyclo[3.1.0]hexane (sabinene, 34) contains an alkene and a cyclopropane function locked in the anti orientation. CIDNP effects for 34 support a radical cation, in which the electron spin is delocalized between the olefinic 7t-system and the internal cyclopropane bond. The results are compatible with a vinylcyclopropane radical cation, 34 , with one weakened cyclopropane bond, which has retained the steric integrity of the parent molecule [219]. 

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]. 

Oxgaard, J., Wiest, O. The Vinylcyclopropane Radical Cation Rearrangement and Related Reactions on the C5H8.bul.+ Hypersurface. J. Am. Chem. Soc. 1999,121,11531-11537. 

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