Trimethylene type radicals

Structural isomerizations of arylcyclopropane cation radicals to propene derivatives are also frequently observed. Trimethylene cation radicals are the most probable intermediates in these processes. However, in some cases the nature of the mode used to promote the SET event is crucial in determining the type of rearrangement sequence that is followed. For instance, DCN-sensitized photoreaction of 24 gives 25, whereas photoexcitation of the EDA complex of 24 and TCNE leads to production of 26. Oxygenation reactions to form dioxolanes are also typical reactions of arylcyclopropane cation radicals formed under aerobic conditions. Of particular interest are the DCA-sensitized oxygenation reactions of cis-27 and trans-27, both of which give cis-2S as a major product. Addition of biphenyl as a cosensitizer or magnesium perchlorate is known to accelerate these processes. Trimethylene cation radicals are plausible intermedi-... 

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. 

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... 

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]. 

As noted in the introduction, nonconjugated diradicals are those in which the partially filled orbitals reside on two different carbons that are connected by one or more saturated carbons or by unsaturated carbons in which the it bonds do not overlap the two radical centers. Two types are discussed here 1,3-diradicals, in which the radical centers are separated by a single carbon, and 1.4-diradicals, in which two carbons separate the radical centers. The archetypal 1,3-diradical is trimethylene (TM), the diradical formed by cleavage of a C-C bond in cyclopropane and tetramethylene, the diradical formed by breaUng a C-C bond in cyclobutane, is the archetypal 1,4-diradical. 

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