Interconversions of Biradicals

FIGURE 9.6 Photosensitized hydrogen abstraction from triethylamine DH by 9,10-anthra-quinone A (for the formulas, see Chart 9.3). The rate constant of in-cage deprotonation as obtained from the polarity pattern, is shown as a function of the relativity permittivity e of the reaction medium (mixtures of acetonitrile and chloroform). The timescale of the CIDNP effect provides a kinetic window, within which such a quantitative treatment is apphcable. Further explanation, see text. 

As far as CIDNP is concerned, the most important difference between radical pairs and biradicals is that the interdiffusion of the paramagnetic centres is restricted in the latter case. This has two implications. 

the exchange interaction often does not fall off to zero. On the one hand, this reduces the efficiency of intersystem crossing between IS) and ITq). On the other hand, it opens up another intersystem crossing pathway, namely, between IS) and IT j) (or, rarely, IT + i)) because the potential energy curves of these states intersect at some point, and the system spends more time in that region if diffusion is not free. 

Second, there is usually no escape channel because the two radical termini cannot separate completely. This usually causes a fundamental difference between CIDNP from radical pairs and CIDNP from biradicals Being nuclear spin sorting as described above, intersystem crossing between IS) and ITq) crucially relies on both exit channels leading to different products, whereas intersystem crossing between IS) and IT i) occurs by simultaneous electron-nuclear spin flips, and so creates net nuclear polarizations without the need of different exit channels. 

in most cases, biradicals do not exhibit radical-pair type CIDNP but S-T i -type CIDNP. The two variants are easily distinguishable because the former gives rise to both absorptive and emissive polarizations with, ideally, a grand total of zero (compare Fig. 9.3), while the latter manifests itself by the same phase, usually emission, of aU CIDNP signals. 

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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. 

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,... 

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... 

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. 

When either method of preparation was carried out in the presence of methyl acrylate products derived from a 1 1 mixture oF the 1,2-and 1,4-isomers were obtained. On the other hand, in the presence of the more reactive dienophile methyl B-cyanoacrylate the products derived from the complex were again a mixture, but from the diaza--compound only adducts derived from 1,4-dideuteriocyclobutadiene were obtained. The intermediate cyclobutadiene cannot therefore have been square or again products apparently derived frbm the 1,2- or 1,4-isomers would have resulted since there would be no distinction between the isomers. It may also be deduced that both the decomposition of the diaza-compound and the trapping of the resultant cyclo-butadiene are concerted pericycllc processes and that a biradical intermediate cannot be involved. Apparently in the presence of the less reactive dienophile interconversion of the valence isomers is more rapid than the trapping cycloaddition reaction whereas with the more reactive dienophile the converse obtains. A lower limit of... 

In order to answer the question posed in Scheme 8.8 as to whether or not external cyclopropane bond homolysis was reversible in norcaradiene, Willcott and Berson examined the pyrolysis of 3,7,7-trimethyl-cycloheptatriene. " Remarkably, the 2,7,7- and 1,7,7-isomers were formed in a 10 1 ratio, respectively, along with small amounts of m- and p-cymene. Importantly, substantial quantities of l-methyl-3-isopropenyl-1,4-cyclohexadiene and 2-methyl-3-isopropenyl-1,4-cyclohexadiene were also found, and these reverted to the cycloheptatrienes under the reaction conditions. Therefore, the interconversion could be the result of biradical formation or reversible homodieny 1-1,5-hydrogen shifts (Scheme 8.10). 

Spiro[2.4]heptadiene rearranges to 6-methylfulvene and 5-vinylcyclopentadiene in a 5 4 ratio, respectively, with log k = 12.89 — 43 600/23RT Subsequently, it was shown that interconversion of cis- and 1,2-dimethyl derivatives preceded the structural isomerization." The structural and geometric isomerizations would appear to involve cleavage to a biradical followed by hydrogen shift or reclosure after bond rotation (Scheme 8.23). 

Details of the coding system developed to handle the interconversions of 1,2-dimethylenecyclobutanes and the related biradicals have now appeared. Gajewski has presented stereochemical evidence for the interconversion of planar and orthogonal bisallyl radicals in the thermal rearrangement of trans-3,4-dimethyl-l,2-dimethylenecyclobutane (442). Pyrolysis of (442) at 230°C for 80 minutes gave a 9 % conversion into an 18 1 1 mixture of the isomers (443), (444), and (445). The recovered (442) was 19 % racemized, and the isomer (443) also appeared to be racemic. The pyrolysis products show the expected preference for conrotatory motions about the 1,2- and 3,4-bonds. The conrotatory out motion leads to the anti-anti-bismethallyl radical (446) which can only close to anri,anri-diethylidene-cyclobutane (445) or anti-l-ethylidene-2-methylene-3-methylcyclobutane (443). The small percentage of the syn-isomer (444) found in the products could arise from the... 

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