Reactivity biradicals

Unusual fragmentation reactions for thietanium salts have been observed. Their analysis may reveal more information about the influence of d-orbitals in the reaction mechanisms of organosulfur compounds. Alkylation of certain thietanes leads to 5-methylthietanonium salts. The thietanium salt 193, which is formed from 2,4-dimethylthietane and (CH3)30" BF4, breaks up when treated with n-butyllithium into a reactive biradical and its resulting cyclopropane and a thioether. 

Synthetic applications of the reaction are somewhat limited as the highly reactive biradicals and radical pairs tend to undergo reactions that compete with C—C bond formation. As in previous cases, the reaction may have synthetic value for the synthesis of strained structures involving small rings. For example, the preparation of the simplest [2]-ladderane 55 by photodecarbonylation of bicyclo[3.2.0]heptan-3-one 54 gave the bicyclic structure in 5% yield with a ring-opened 1,5-heptadiene being the dominant product (Scheme 2.14) [41]. 

Loss of two hydrogen atoms from benzene results in the reactive biradical species called benzynes or atynes. There are three possible isomers o-benzyne (41), m-benzyne (42), and p-benzyne (43). Of these three, o-benzyne is the best known and studied. It was first demonstrated to be a reactive intermediate by Roberts in 1953 and later characterized by UV, IR, and... 

Disulfur Monooxide. This is produced when a glow discharge is passed through S02 and in other ways and was long thought to be SO. However, gases of this composition are equimolar mixtures of S20 and S02. The extremely reactive biradical, SO, can be detected as an intermediate in the reactions, however, and (SO)2 has also been detected by mass spectrometry. 

The endiyne and enyne-allene have attracted the attention, as they undergo facile cycloaromatization to produce reactive biradicals [1], which could mimic the DNA-cleaving mechanism and properties of the new class of very potent antitumor antibiotics calicheamicins [2], esperamicins [3], neocarzinostatin [4], and dynemicins [5]. 

The resonance form allows the existence of several different intermediate reactive biradicals ... 

Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

The combination is in this case an out-of-phase one (Section I). This biradical was calculated to be at an energy of 39.6 kcal/mol above CHDN (Table ni), and to lie in a real local minimum on the So potential energy surface. A normal mode analysis showed that all frequencies were real. (Compare with the prebenzvalene intermediate, discussed above. The computational finding that these species are bound moieties is difficult to confimi experimentally, as they are highly reactive.)... 

The problem of the synthesis of highly substituted olefins from ketones according to this principle was solved by D.H.R. Barton. The ketones are first connected to azines by hydrazine and secondly treated with hydrogen sulfide to yield 1,3,4-thiadiazolidines. In this heterocycle the substituents of the prospective olefin are too far from each other to produce problems. Mild oxidation of the hydrazine nitrogens produces d -l,3,4-thiadiazolines. The decisive step of carbon-carbon bond formation is achieved in a thermal reaction a nitrogen molecule is cleaved off and the biradical formed recombines immediately since its two reactive centers are hold together by the sulfur atom. The thiirane (episulfide) can be finally desulfurized by phosphines or phosphites, and the desired olefin is formed. With very large substituents the 1,3,4-thiadiazolidines do not form with hydrazine. In such cases, however, direct thiadiazoline formation from thiones and diazo compounds is often possible, or a thermal reaction between alkylideneazinophosphoranes and thiones may be successful (D.H.R. Barton, 1972, 1974, 1975). 

The 1,2-bond is homolytically cleaved by both thermolytic and photolytic means to generate a biradical (17) which in the absence of reactive groups generally forms a 2//-azirine (79AHC(25)147). No direct evidence for the biradical has been presented, but indirect evidence points to such a species. Acylpyrazines have in some instances been isolated, and these would arise by dimerization of the biradical (70JCS(C)1825, 7UCS(C)2644). 

The extended Hiickel method has been used in a discussion of properties and reactivity of radicals and biradicals (75). We have found it possible to correlate the basicity constants, pKbh. of radical anions with extended Hiickel data (76). 

Although this mechanism could explain the inertness of di-t-butyl sulphide towards oxidation due to the absence of a-hydrogen atoms, it was later ruled out by Tezuka and coworkers They found that diphenyl sulphoxide was also formed when diphenyl sulphide was photolyzed in the presence of oxygen in methylene chloride or in benzene as a solvent. This implies that a-hydrogen is not necessary for the formation of the sulphoxide. It was proposed that a possible reactive intermediate arising from the excited complex 64 would be either a singlet oxygen, a pair of superoxide anion radical and the cation radical of sulphide 68 or zwitterionic and/or biradical species such as 69 or 70 (equation 35). 

Photoelectron spectroscopy has routinely been used to determine the ionization energies of stable molecules. It has also been adapted for the investigation of reactive species, including radicals, biradicals, and carbenes, usually generated chemically or by using pyrolysis. ... 

Hehre and co-workers have used this approach for the investigation of biradicals and other reactive neutral molecules. For example, by using the bracketing approach, they were able to determine the proton affinities of o- and p-xylylene (o- and p-quinodimethane (lo and Ip) Figure 5.3), from which they were able to determine the enthalpies of formation of the reactive, Kekule molecules. They found the proton affinity of the meta isomer to be too high to be measured directly by bracketing, but were able to assign a lower limit, and subsequently a lower limit to the enthalpy of formation of the m-xylylene diradicals. 

Again two products were obtained from a reactive triplet state with a rate constant for photoisomerization of 2 x 1011. This rate, however, is over six times greater than that obtained for the unsubstituted phenyl derivative and indicates that the p-cyano-phenyl group facilitates reaction by stabilization of the intermediate biradical. 

Another study16 investigates the effect of benzene ring fusion on the reactivity of 1,2-Oxathietane. Ab initio calculations were performed using the 3-21G and the 6-31G basis sets, at Hartree-Fock and MP2 calculational levels. It was found that the allowed (8s + 2s) cycloreversion is unfavorable energetically. A subsequent experimental and theoretical study17 favors biradical intermediates in the valence tautomerism of benzoxathiete to monothio-o-benzoquinone. 

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