Trimethylenemethane Radical Cation

Readers can refer to the literature [4, 7, 8, 41, 60] for further details of J-T distortion of other 7d and Z)3h molecules. 

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The second example of triplet BET in radical ion pairs involves the (ring-opened, distonic) trimethylenemethane radical cation (24 ) generated in the... 

In the second group of radical cations to be discussed here, spin and charge are stabilized in two separate fragments of the molecule. In early examples of this family the charge was stabilized by diaryl or methoxy substitution. We mention the ring-opened trimethylenemethane radical cations (152), the various di-... 

The photooxygenation of 2,2-diaryImethylenecyclopropanes proceeds via trimethylenemethane radical cation intermediates, giving 1,2-dioxolanes [160,161]. When 1,1,2,2-tetracyanoethene (TONE) is used as an electron acceptor, the (3 + 2) cycloaddition between methylenecyclopropanes and... 

Scheme 13. Reactions of a substituted trimethylenemethane radical cation.

Furthermore, we deal with a stfuctural distortion of the trimethylenemethane radical cation (TMM+) [63], The neutfal TMM has a high symmetry of and is in a triplet state with a doubly degenerate c" HOMO in the ground electronic state [64], The Z)3h structure of TMM is expected to be distorted to a Civ structure with a lower symmetry in TMM+ due to the J-T effect, so as to split the original degenerate e" orbitals into ai and b orbitals. 

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... 

Calculations have been carried out on the spectroscopy and excited-state energies and geometries of polyenes,84 the 1,5-cyclo-octadiyne radical cation,85 the visual chromophore, ll-c/s-retinal,8 polyene carbaldehydes,87 aromatic hydrocarbon anions and cations,88 trimethylenemethane,89 thiamine derivatives,80 porphyrins,91 lumiflavin,92 chlorophyll,93 nucleic acids,94 and ferrocene.95... 

Like in the diaryl-substituted trimethylenemethane cation radical (Section 3), the p-orbital at C-1 in 67" is probably perpendicular to the C-2-C-3 7t-orbital because of the bulky aryl groups. The ring closure at C-1 and C-4 thus selectively forms 52. This process operates preferentially in less polar solvents, especially in the nonpolar benzene (Tables 2 and 3). The formation of 65 and 66 in low yields under aerobic conditions can be accounted for by molecular oxygen capture of 67 which reluctantly diffuses from the contact ion radical pair [67 /sens-]. 

Photoexcitation of the EDA complexes of methylenecyclopropanes and TCNE is a convenient procedure to generate trimethylenemethane cation radical intermediates. 2,2-Diaryl-l-methylenecyclopropanes (77a-c) form colored EDA complexes when mixed with TCNE in dichloromethane. Irradiation (A, > 390 nm) of an oxygen-saturated dichloromethane solution (X max = 370 and 568 nm) of 77a ( ° = +1.35 V vs. SCE) and TCNE, leads to formation of dioxolanes 78a and 79a and the TCNE adducts 80a and 81a in respective yields of42and54%. 

Spectroscopic Evidence for Bisected Trimethylenemethane Cation Radical and Biradical Intermediates... 

Nanosecond laser flash photolyses of 77a under TCNB- and NMQ BF -sensitized conditions provide strong spectroscopic evidence not only for the intermediacy of cation radical 82a in the SET-sensitized photoreactions but also for the trimethylenemethane biradical intermediate 82a. Laser excitation (308 nm) of TCNB with 77a in acetonitrile gives rise to two intense transient absorptions with maxima at 350 and 500 nm whereas in dichloromethane only the shorter wavelength transient absorption is observed. In contrast, under the NMQ BF -sensitized conditions in acetonitrile the longer wavelength transient absorption predominates but the shorter wavelength band is extremely weak (Table 5). 

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