Trimethylene radical cations

Electron transfer and ring-opening—trimethylene radical cation... 

Finally, it should be noted that geometrical and structural isomerizations of substituted cyclopropanes by means of ET-catalyzed reactions, via intermediate trimethylene radical anions, is only one pathway to perform these reactions. Other possibilites are the thermal reaction via trimethylenes , the light induced reaction the photosensitized reaction via trimethylene radical cations the Pd/C-catalyzed reaction S and the base-catalyzed reaction ... 

D. A. Hrovat, P. Du, and W. T. Borden, Chem. Phys. Lett., 123, 337 (1986). Has the Trimethylene Radical Cation Been Observed An Ab Initio Molecular Orbital Study. 

The cyclopropane radical cation can be prepared by y-radiolysis in rigid matrices. At temperatures as low as 4.2 K, its ESR spectrum shows evidence for static Jahn-Teller distortion, resulting in a structure of the (ring-closed) trimethylene ( Ai) type, r " ... 

The cyclopropane radical cation can be prepared by y-radiolysis in rigid matrices its ESR spectrum at 4.2 K shows evidence for static Jahn-Teller distortion, resulting in a structure of the trimethylene (2Aj) type [294, 295], Irradiation of several methyl substituted derivatives at 77 K gave rise to a family of radical cations of the same structure type [293], which had been previously identified on the basis of CIDNP results [229, 230]. We begin with a discussion of the CIDNP investigations, since they preceded the ESR studies of all species but the prototype. 

More recently, the radical cation of the parent system has been characterized by ESR/ENDOR spectroscopy [340], These experiments revealed medium-sized hfcs for both the bridgehead (—11.4 G) and the equatorial protons (+11.4G) and large positive hfcs (+ 77 G) for the axial protons (which in 75 are replaced by the trimethylene bridge). The large difference between the couplings of the axial and equatorial protons indicates a nonplanar, puckered geometry of the radical cation. No evidence for interconversion is apparent up to 160 K this finding requires an inversion barrier of at least 12 kJ mol-1. 

Among the earliest examples of symmetrical bifunctional radical cations, the distonic trimethylene species (103) invoked by Williams and coworkers [293, 296, 297] are stabilized solely by hyperconjugation. The main rationale for their formation would be the relief of ring strain. On the other hand, the non-vertical radical cations 137 derived from cyclopentadiene dimers [386-389] are favored by two elements of allylic stabilization. This radical cation has three eonformat-... 

The lifetime of Pyr +oPyr seemed to depend on the structure of the parent dimer. It was observed that the presence of a trimethylene bridge linking both pyrimidine units at N(l) and N(l ), as in the mixed thymine-uracil dimer 4 and in the thymine dimer 5, leads to significant stabilization of the corresponding dimer radical cation, whereas in unbridged dimers a maximum lifetime of 10-1°-10-9 s was proposed for their dimer radical cations [6]. It was further shown that in bridged pyrimidines dimers of types 4 and 5 the interconversion of dimer and... 

As mentioned earlier, the cyclopropane radical cation, prepared by y-radiolysis in rigid matrices, had an ESR spectrum compatible with the trimethylene structure Ai) [78, 79]. Irradiation of several methyl-substituted derivatives at 77 K gave rise... 

The potential ring-opening of cyclopropane radical cations, breaking the weakened bond of type A radical cations (21 +), has been a subject of both interest and controversy. The ESR spectra of cyclopropane radical cation and its methyl-substituted derivatives decayed at temperatures near 100 K. They were replaced by secondary spectra, in which the protons at one cyclopropane center do not interact with the electron spin. This coupling pattern was interpreted as evidence for a ring-opened trimethylene species (22 +) in which one terminal carbon has rotated into an orthogonal orientation [105, 106, 140]. 

Pioneering work of Dowd on trimethylene methane [142], 1,8-naphthoquinodi-methane diradicals were among the first examples of non-Kekule 7r,7r-diradicals that were found to be persistent in their triplet ground state at cryogenic temperatures and were thoroughly characterized by various spectroscopic methods [143], Time resolved studies of diradicals have provided lifetimes and reactivities of numerous diradical intermediates in solution [144], y-irradiation of CAN, 68, in haloalkane glasses at 77 K yielded radical cation 68 + its electronic absorption spectrum has been obtained and represents first report of absorption spectrum of an ionized diradical [145]. Irradiation at >640 nm converted it into an isomer identical with that formed by ionization of 1,4-dihydro-l,4-ethanonaphtho[l,8-rfe][l,2]diazepine 70, and subsequent irradiation of the radical cation at A > 540 nm. Results identified the photo isomer of 68 + to be 69 + (Scheme 4). 

Structural abbreviations for examples of the nonhydrazine couples studied appear in Chart 10.2. The only two trialkylamines with radical cation lifetimes long enough to study are l,4-diazabicyclo[2.2.2]octane, N[222]N, and Alder s tris-trimethylene bridged diamine N[333]N. Three tetraaIkyl-2-tetrazenes (abbreviated N4, preceded... 

Partial reduction of the trimethylene bis-pyridinium dication, J with the pyridinyl radical 4 yields the cation radical, In addition to modified pyridinyl... 

Dinnocenzo and Conlon have described the remarkable effect of one-electron oxidation on the rate of certain vinylcyclopropane rearrangements. Exposure of several l-p-anisyl-2-vinylcyclopropane derivatives to a catalytic amount of tris(4-bromophenyl)aminium hexafluoroantimonate in acetonitrile at room temperature was found to induce ring expansion to form cyclopentenes (equation 25) temperatures in excess of 200 °C are required for the conventional thermal rearrangement cf these systems. At this time it is uncertain whether these reactions follow concerted mechanisms, or are stepwise processes involving trimethylene cation radical intermediates. 

In a series of papers Gross et provide experimental evidence that ionization of cyclopropane in the gas phase results in the formation of a trimethylene cation radical (2) isomerization of cold 2 to ionized propene (3) can be ruled out. The authors, moreover, stress that the structural changes of ionized cyclopropane in both the gas phase and in condensed phase are entirely analogous. 

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