Carbonyl ylide radical cation

Figure 12 n MOs of allyl and of the iso-it-electronic carbonyl ylide radical cation, 0(CH2)2, obtained by replacement of the central CH in allyl by O". Note how the excited state, created by a2—>2b] electron promotion, is lower in energy and, thus, more prone to SOJT vibronic interaction with the h.2 ground state in than... 

Arnold first discovered that PET reactions of cis- and rrans-2,3-diphenyloxiranes (1 and 2) with electron-accepting sensitizers such as DCN, 1,4-dicyanobenzene, dimethyl terephthalate, and methyl 4-cyanoben-zoate produce the isomeric tetrahydrofuran derivatives 7 and 8 (Scheme 2). The proposed mechanism involves the CC bond cleavage of the oxirane radical cations to give ylide radical cations 3 and 4, followed by back electron transfer (BET) to generate carbonyl ylides 5 and 6. The resulting carbonyl ylides are trapped with dipolarophiles, such as acrylonitrile, maleonitrile, and fumaronitrile, to produce the observed products. In the absence of dipolarophiles, C)S-/ir s-isomerization (1/2 = 0.19-0.28) was observed, which is consistent with the formation of carbonyl ylides 5 and 6, as well as their radical cations 3 and 4. This method was successfully apphed to the synthesis of various tetrahydrofurans (Scheme 3) and dihydrofurans (Scheme 4). ... 

Various other [3 + 2] cycloadditions, affording chiral, anellated C6o derivatives with stereogenic centers in the addends are reported in literature. The products were generally obtained as racemates and resulted from reaction of buckminsterfullerene with species like 2,3-disubstituted 2//-azirincs (via nitrile ylides [under direct irradiation] or via 2-azaallenyl radical cations [sensitization by photoinduced electron transfer]),365 1-substituted 5-diazopentane-1,4-diones (via cyclic carbonyl ylides),366 7-alkylidene-2,3-diazabicyclo[2.2.1] hept-2-ene (via a diradicaloid trimethylenemethane derivative),367 1-benzylpy-razolidine-3-ones in the presence of aldehydes (via pyrazolidinium ylides),368 2-trifluoromethyl-2,5-dihydro-l,3-oxazol-5-ones (via nitrile ylides),369 nitro-alkanes in the presence of triethylamine and trimethylsilyl chloride (via N-silyloxynitrones),370 or dv-HOCH2 CH=C H C H 2 OCO 2 H( in the presence of... 

Oxygenation of 1,2-dinaphthyloxiranes 24 and 25 by PET was conducted without BP, as both 24 and 25 were able to quench the fluorescence of DCA. Thus, ds-trioxolane 26 was isolated as the sole stereoisomer (Scheme 8). However, the Schaap mechanisms described above, involving a highly stereoselective formation of ds-l,2,4-trioxolanes (Scheme 7 and 8), seem to be inconsistent with the previously described Arnold mechanism (Scheme 2), where two types of radical cations (3 and 4) as well as carbonyl ylides (5 and 6) are generated by the PET oxidation of 1 and 2, respectively. 

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