Biradicals interconversions

Other 2-thiazolines, on irradiation in acetonitrile, undergo fragmentation with the formation of the corresponding nitrile and thiiran.142 A thiiran (167) was also obtained on irradiation of the 1,3-oxathiole (168),143 and the allylic biradical (169) has been proposed as an intermediate in the photochemically induced interconversion of the cyclobutenes (170 and 171).144 Surprisingly,... 

Nitroxides are the most common of the oxygen-centred biradicals to be reported. As a model for spin-crossover molecules, the nitronyl nitroxide (105) was prepared and by oxidation with PbC>2 afforded the triplet biradical (106) which was characterized by ESR (Scheme 15).242 The one-electron oxidation of (105) afforded the singlet cation (107) which was seen to exist in equilibrium with (106) in solution. The authors claim that pH-controlled interconversion between two species of different spin multiplicities in this way may provide die basis for novel magnetic switches or pH sensors. The N.N-dialkylamino nitronyl nitroxides (108) were prepared and afforded die diplet-state biradical cation species by one-electron oxidation with iodine.243 The authors propose that, by the similarity of die electronic structures, diese structures can be regarded as hetero-analogues of trimethylenemethane. A paper confirms die conversion of 3,3-dimethyldioxetane into die corresponding ring-opened 1,2-diol but refutes the... 

However, if the nature (e.g. electronic configuration) of the first singlet excited state. S, and the first triplet excited state 7 , are known, the electron-transfer is easy to handle in a mechanistic sense. An important preequisite is to know if the Si and/ or 7 , are the exclusive precursor states for the electron-transfer product or if intermediates such as upper excited states, biradicals or ground-state intermediates might be involved. In addition, structural properties have to be considered as well because the states of a chemical system are parametrized in terms of distinct structures. For instance, not all structural interconversions are allowed under a given set of reactions conditions (e.g. solvent or temperature) [88]. 

This chapter is concerned with chemical reactions that occur while the system is still in the paramagnetic world. After an explanation of the radical pair mechanism and a brief treatment of experimental details, three case studies are presented that illustrate the application of CIDNP to transformations of radicals into other radicals and to interconversions of biradicals. 

The low-temperature formation of C +-P-C6o and its recombination to a triplet state allow some interesting and potentially useful magnetic field effects. In the presence of a small (20 mT) static magnetic field, the lifetime of the C +-P-C6o charge-separated state in 43 is increased by 50 % [155]. This is ascribed to the effect of the magnetic field on interconversion of the singlet and triplet biradicals. At zero field, the initially formed singlet biradical state is in equilibrium with the three triplet biradical sublevels, and all four states have comparable populations. Decay to the carotenoid triplet only occurs from the three triplet sublevels. In the presence... 

Fig. 22A-E. The C2H4 + CH2 (triplet) system A) coordinates B) activated complex for the addition C) biradical at the end of addition D) diradical ground state E) activated complex for interconversion (Interatomic distances in A)...

A biradical intermediate has been suggested for the interconversion of 85 and 86 however, recent work has suggested that while the biradical 87 may well be a transition state for the conversion of 88 into 89, it is unlikely to be an intermediate. 

Because both 1-methylenespiropentane (89) and 1-cyclo-propylidenecyclopropane (95) include methylenecyclopropane moiety in their structures, the methylenecyclopropane-type reversible interconversion between 89 and 95 is expected to occur upon pyrolysis, involving a trimethylenemethane biradical intermediate. However, such a rearrangement formally does not take place, though 95 rearranges to 89. Instead, on pyrolysis at 320 C, 89 rearranges to dimethylenecyclobutanes (91 and 93) through the tetramethylene-ethane biradical 90 and the vinylic-allylic biradical 92, respectively. Presumably, biradical 94 formed by C-2-C-5 bond fission... 

If the reaction is concerted then there should be a high level of stereoselectivity, as is indeed observed. However, this does not rule out a two-step mechanism should rotation about the bonds in the intermediate be slow compared with the rate of ring-closure. In this connection, it is noteworthy that cycloaddition of trans-and c/5-l,2-dichloroethene to cyclopentadiene is completely stereospecific (3.5). A two-step mechanism via a biradical intermediate might have been expected to be sufficiently long-lived to allow some interconversion, resulting in a mixture of products. Addition of dichlorodifluoroethene to cis,cis- and / ran5,/ ra 5-2,4-hexadiene is, however, non-stereospecific and is thought to proceed by a two-step mechanism with a biradical intermediate. 

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