Isomerization biradical

The biradical intermediate of 3,4-dimethylenethiophene forms by intramolecular cyclization of bisallenyl sulfide or by pyrolysis of a diazene precursor and dimerization to give a variety of products (Scheme 11) <87TL4263>. Studies of transient signals in C NMR spectra show that the isomeric biradical 3,4-dimethylenethiophene forms in rigid matrices at 77 K and exhibits an intense purple... 

Bond-stretch isomerism in (8) and (9) has been studied. In (8), stretching of the C-1—C-5 bond gives an isomeric biradical which is only weakly stabilized by orbital interaction, but in ketone (9) and markedly in cation (10) an interaction as in (11) leads to a double energy minimum and both EH and CNDO/2 calculations support the existence of bond-stretch isomerism in (9) and (10). 

The low-temperature limit of the rate constant for the isomerization of the biradical... 

Also in this case the relative energy of all the possible intermediates involved in the photochemical isomerization was calculated (OOOJOC2494). The results are collected in Fig. 2. Also in this case the sensitized irradiation involves the formation of the biradicals. We have to note, however, that the fission of O—Cq, bond in the triplet state of the molecule is not so favored as in furan. The process should be quite inefficient. The corresponding biradicals show the same energy as that in the triplet state. In this case, then, the formation of a biradical should depend on the activation energy. 

In contrast, when the irradiation is performed on 2-cyanopyrrole, the isomeric products are observed. In fact, in this case, the corresponding Dewar pyrrole shows a lower energy than in the previous case, allowing the formation of the isomeric products (Fig. 6). When 2-methylpyrrole is used as substrate, the formation of the triplet state is favored, but this triplet state cannot evolve through the formation of the biradical intermediate. 

Also in this case calculation results fit the experimental data (Fig. 7) [99H(50)1115]. In fact, the singlet excited state can evolve, giving the Dewar thiophene (and then isomeric thiophenes) or the corresponding excited triplet state. This triplet state cannot be converted into the biradical intermediate because this intermediate shows a higher energy than the triplet state, thus preventing the formation of the cyclopropenyl derivatives. 

In the photochemical isomerization of isoxazoles, we have evidence for the presence of the azirine as the intermediate of this reaction. The azirine is stable and it is the actual first photoproduct of the reaction, as in the reaction of r-butylfuran derivatives. The fact that it is able to interconvert both photochemically and thermally into the oxazole could be an accident. In the case of 3,5-diphenylisoxazole, the cleavage of the O—N bond should be nearly concerted with N—C4 bond formation (8IBCJ1293) nevertheless, the formation of the biradical intermediate cannot be excluded. The results of calculations are in agreement with the formation of the azirine [9911(50)1115]. The excited singlet state can convert into a Dewar isomer or into the triplet state. The conversion into the triplet state is favored, allowing the formation of the biradical intermediate. The same results [99H(50)1115] were obtained using as substrate 3-phenyl-5-methylisoxazole (68ACR353) and... 

The irradiation of 3-phenyl-4-acetyl-5-methylisoxazole (49) gave the isomeric oxazole (50) (Scheme 22) (75JA6484 76HCA2074). The reaction can involve the formation of the biradical intermediate starting from the triplet state, in agreement... 

Computational results are reported for the isomerization of 1,4,5-trimethyl-imidazole (99MI233). They show that the isomerization occurs through the Dewar isomer arising from the excited singlet state. The formation of the triplet state is energetically favored however, the biradical intermediate cannot be produced because it has higher energy than the excited triplet state. 

Semiempirical calculations on 94 showed that it can isomerize only via the ICI mechanism in fact, the triplet state cannot evolve to give the corresponding biradical derivatives (Fig. 16) (99MI233). 

When the reaction was carried out on the phenoxy derivative 106, only 107 was obtained (Scheme 44) (88JHC1551). The formation of this product was rationalized assuming a heteroly tic cleavage of the O—N bond followed by isomerization (Scheme 44). If the reaction occurs in the excited triplet state of the molecule, the biradical is the most probable intermediate. 

A comparison of the rate constant for photoisomerization of the unsubstituted 3-phenyl derivative (kT = 3 x 1010 sec-1) to that of the 3-(p-methoxy phenyl) derivative (kr = 1.5 x 1010 sec-1) indicates that the presence of the p-methoxy groups imparts no special stability to the intermediate responsible for isomerization even though cleavage of a cyclopropane bond is predominant. Clearly these results are inconsistent with an intermediate possessing electron-poor or electron-rich species such as would be obtained from heterolytic cleavage of the cyclopropane. On the other hand, the results are consistent with a biradical species as intermediate. Further evidence consistent with this conclusion was obtained in a study of trans-3-p-cyanophenyl-/ra w-2-phenyl-1 -benzoylcyclopropane,<82)... 

Aryl ketones are often used to effect cis and tram isomerization of olefins.(118-ia0) Although this, in some cases, can be viewed as an energy transfer process where the ketone triplet transfers its energy to the olefin, which then isomerizes, the failure of noncarbonyl sensitizers of comparable triplet energy to isomerize the olefins suggests that a process other than energy transfer may be involved. Schenck and Steinmetz<118) suggested that isomerization results from decomposition of a biradical carbonyl-olefin adduct similar to that involved in oxetane formation ... 

Finally, thermally induced isomerizations which generate carbon-centered biradical organic molecules have been shown to serve as alternative for conventional chemical and photochemical methods [71]. A straightforward procedure to accomplish such biradicals was described by Myers using a thermal conversion of yne-allenes [72]. According to this scheme, Wang and coworkers [73] heated 3-178 in 1,4-cyclohexadiene to 75 °C and obtained 3-181 in 22% yield via the biradicals 3-179 and 3-180 (Scheme 3.48). 

Azidofurans lose nitrogen giving products reminiscent of those formed from furyl carbenes and biradicals.278 External nitrenes apparently add to furan double bonds just as carbenes do, and again the initial products collapse very readily to give nonheterocyclic products. The nitrene produced oxidation of N-aminophthalimide reacts as in Scheme 54. The products are relatively stable derivatives of but-2-endial and are therefore of potential synthetic value, initially the butene link is Z but easily isomerizes to E on silica columns.279... 

The cyclic enediynyl sulfide 93 is also prone to undergo prototropic rearrangement (Scheme 20.21) [57]. When the l,8-diazabicydo[5.4.0]undec-7-ene (DBU)-induced isomerization was conducted in carbon tetrachloride, three cycloaromatized products, 96 to 98, were isolated, indicating the formation of the biradical 95a as a transient intermediate. In a polar solvent, such as methanol or ethanol, the formation of 99 can best be accounted for by regarding the biradical 95a as the zwitterion ion 95b. A related process involving the oxidation of 93 with selenium dioxide has also been reported [58],... 

The propargylic alcohol 102, prepared by condensation between 100 and the lithium acetylide 101, was efficiently reduced to the hydrocarbon 103, which on treatment with potassium tert-butoxide was isomerized to the benzannulated enyne-allene 104 (Scheme 20.22) [62], At room temperature, the formation of 104 was detected. In refluxing toluene, the Schmittel cyclization occurs readily to generate the biradical 105, which then undergoes intramolecular radical-radical coupling to give 106 and, after a prototropic rearrangement, the llJ-f-benzo[fo]fluorene 107. Several other HJ-f-benzo[fo]fluorenes were likewise synthesized from cyclic aromatic ketones. 

The activation energy data suggest that the isomerization around the C=C bond occurs in a dipolar rather than a biradical transition state. 

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

---