Cyclopentane radical cation

In contrast to the radical cations of strained-ring cycloalkanes, the cyclopentane radical cation, c-CsHio , formed by electron transfer to radiolysis-induced holes in halocarbon matrices, had a simpler spectrum. A triplet with uh = 2.5 mT (2H) was attributed to a localized species with Cj symmetry. The unpaired electron was assigned to a W-shaped cr-orbital, involving C5-C1-C2, and the two equatorial protons at C5 and C2 [80, 88, 89]. At temperatures above 77 K, all ring protons become equivalent, most probably as a result of processes such as ring inversion, or pseudo-rotation around the C5-axis [89]. 

Figure 14. Conformers of cyclopentane and schematic representation of possible singly occupied molecular orbitals (SOMOs) of cyclopentane radical cation.

The third class of organic donor molecules are a-donors, viz., alkanes and cycloalkanes. These substrates have inherently high ionization and oxidation potentials. Therefore, their radical cations are not readily available by photoinduced electron transfer, but typically require radiolysis and electron impact in the condensed phases or the gas phase, respectively. Thus, radical cations of simple alkanes (methane [206], ethane [207]) or unstrained cycloalkanes (cyclopentane, cyclohexane) [208] were identified and characterized following radiolysis in frozen matrices. In contrast, strained ring compounds have significantly lower oxidation potentials so that the radical cations of appropriate derivatives can be generated by photoinduced electron transfer. 

Fragmentation of 2,3-diazabicyclo[2.2.1]hept-2-enes via photoinduced electron transfer offers a convenient route to cyclopentane-1,3-diyl radical cations, in turn undergoing interesting and selective rearrangements [42]. 

Zimmerman and Hofacker have studied the photochemically induced SET reactivity of the 1,4-dienes (74). The sensitizers used were dicyanoanthracene and dicyanonaphthalene. The radical cations of the 1,4-dienes undergo regioselective cyclization to the cyclic radical cations (75) which ultimately afford the final products (76). The SET-induced photochemistry of other non-conjugated dienes such as geraniol (77) has been studied. The results demonstrate that with DCA as the sensitizer in methylene chloride a contact radical-ion pair is involved and this yields the cyclopentane derivatives (78) and (79) in the yields shown. The cyclization is the result of a five-centre cyclization. With the more powerful oxidant dicyanobenzene as the sensitizer and in acetonitrile as solvent, separated radical-ion pairs are involved and this leads to the formation of the bicyclic ethers (80) and (81). DCA-sensitized reactions of the dienes (82) and E,E-(S3) and the bicyclohexane (84) have been studied. At low conversion the irradiation of (84) under these conditions affords a mixture of the dienes (82) and , -(83) in ratios that are independent of temperature. 

There are many examples of such reactivity and some of these have been reviewed by Roth and coworkers, a research group that is extremely active in this area. An example that is typical of the processes encountered involves the cyclization of the diene geraniol (1). In this case the sensitizer is 9,10-dicyanoanthracene (DCA) and the reactions are carried out in methylene chloride. The authors state that a contact radical-ion parr is involved, i.e. the radical cation of the diene is in close proximity to the radical anion of the DCA. Reaction within this yields the cyclopentane derivatives 2 and 3 in the yields shown. The ring formation is the result of a five centre CC cyclization within the radical cation of 1. When a more powerful oxidant such as p-dicyanobenzene is used as the sensitizer in acetonitrile as solvent, separated radical-ion pairs are involved. This leads to intramolecular trapping and the formation of the bicyclic ethers 4 and 5 . The bicyclic ether incorporates an aryl group by reaction of the radical cation of the diene with the radical anion of the sensitizer (DCB). This type of reactivity is referred to later. Other naturally occurring compounds such as (/fj-f-bj-a-terpineol (6) and (R)-(- -)-limonene (7)... 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

A-[(A, A -Dialkylamino)alkenyl]benzotriazoles have been shown to be useful precursors to oc-amino radicals, which can subsequently undergo both 5-exo- and 6-exo ring closure on to a,p-unsaturated esters or nitriles.85 For example, exposure of benzotriazole 79 to Sml2 afforded cis-cyclopentane 80 in 70% yield (Scheme 5.52). The mechanism of the cyclisation is believed to involve dissociation of the benzotriazole, thereby generating an iminium cation, which undergoes rapid reduction to the a-amino radical in the presence of Sml2 (Scheme 5.52).85... 

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