Biradical species

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). 

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)... 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

The electron spin resonance (ESR) spectra of the radical anion of 2,2 -bipyridine, sometimes in the form of its alkali metal com-plgx, 71.175,177.299-304 radical anion of 3,3 -bipyridine, ° and the radical anion of 4,4 -bipyridine, ° ° usually obtained by reduction of the bipyridines with an alkali metal, have been measured, and hyperfine splitting constants were assigned. Related biradical species have also been investigated. The ESR spectrum of the 4,4 -bipyridinium radical cation, of which... 

The results are shown in Figure 5 and the expressions are confirmed. The ratio k13jk%0 was found to be about 12 (M sec) % where this value may be somewhat low (perhaps a factor of two) because of the nonuniformity of the concentration of biradical species in the reaction cell. 

Several alternatives are open to the reactive nitrene species residing adjacent to the nucleus. The singlet nitrene can attack a suitable proximate, electron-rich centre or it may insert in the thiophene double bond to form an azirine which can subsequently undergo further transformations. A third alternative is for the singlet to cross over to the triplet state and then undergo the reactions characteristic of this biradical species. 

Since 2,4,6-trisubstituted phenols are readily oxidized to fairly stable phenoxy radicals, this system has been used to construct various biradical species. The bisphenol 6 below loses three hydrogens to form the very stable crystalline radical galvinoxyl.86... 

Such a species has the charge distribution expected of a state. It allows a simple series of arrow pushes to lead to a charge-separated bicyclohexane[3.1.0] structure analogous to 32. One of the attractions of this scheme is that it is well known in ground-state chemistry that 1,2 shifts of alkyl groups are quite facile in carbonium-ion species but highly improbable in free-radical species. However, it is not apparent that such restrictions need apply to excited biradical species. 

If a biradical intermediate is involved in the reaction path, it must be the one shown above and clearly is not the most stable possibility. However, the initial addition to the isolated double bond does proceed to form a five-membered ring as would appear to be common with radical and biradical species. 

If biradical species are intermediates in these intramolecular cyclizations, products which represent radical-radical disproportionation should appear occasionally. The photocyclization of citral423 probably is a fitting example. 

The quenching cross section for cyclopropane is considerably smaller than those for olefins.497 However, the fact that cyclopropane-de is, if anything, a more effective quencher of triplet mercury than cyclopropane-/ 6497 favors an energy transfer process over a hydrogen abstraction process. Recently Gunning has reported that methylene-cyclobutane is the major high-pressure product in the mercury-sensitized decomposition of spiropentane.509 This result provides very compelling support for the intermediacy of a biradical species. 

The gas-phase ionization of 2,4,6-tribromobenzene in the presence of m-fluoropyridine afforded the A -aryI -m-fluoropyridinc adduct from which the biradical cation was generated by loss of two bromine radicals.232 This biradical species was isolated and characterized using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and its chemical properties are discussed. FT-ICR was also used to isolate and characterize the products of electron ionization of fluorinated acetyl compounds, which included a biradical anion.233... 

The tandem radical cyclization of tetrayne (97) and its derivatives has been performed to generate the polycyclic pyran (98) via a biradical intermediate.238 The cycloaddition reaction of a biradical species (or diyl) and a multiply bonded species (the diylophile)239 has been observed with unique allene diylophiles.240 The short-lived biradical fonned by the irradiation of the diazene (99) is trapped by an allene diester to form a second biradical species (100). Intramolecular cyclization occurs such that all steric interactions are minimized and so enforces stereocontrol in the formation of the cycloadduct (101) see Scheme 14. A paper reports the rearrangement of 2-vinyhnethylenecyclopropane (102) to 3-methylcyclopentene (103) via the triplet biradical (104), which has been characterized for the first tune by IR spectroscopy.241... 

The very long endocyclic C—C bond in 362 (1.66 A) indicates already relatively facile homolytic C—C bond cleavage38. The solution of the dimer and the irradiated pivaloyl-acylsilane 147 gave rise to a strong broad ESR signal without a fine structure. Attempts to intercept a putative biradical like 363 failed, but they do not rule out the possible existence as an intermediate in low concentrations. A biradical species like 363 would also account for dimerization products isolated from other silenes (see below)119. 

The biradical benzo-l,2 4,5-bis(l,3,2-dithiazolyl) (BBDTA) is known in the literature but characterization is incomplete. A new study reports the electronic, molecular, and solid-state structure of BBDTA.224 The lifetime of an alkyl phenylglyoxalate-derived 1,4-biradical has been estimated, using the cyclopropylmethyl radical clock , to be in the range 35—40 ns.225 The indanols (88) and their C(3) methyl and trideuteromethyl analogues have been prepared from phenyl benzyl ketone via photo-cyclization of an intermediate 1,5-biradical species.226,227 Selectivity for these products over their C(l) epimers is high but is profoundly effected by substitution in the benzyl ring or the alkyl side-chain. The findings are rationalized in terms of the conformational preference of the intermediate 1,5-biradicals. 

It is necessary to mention the ongoing discussion about the hypothesis of the involvement of a biradical species resulting from breaking of one of the Ge—C bonds in the process described by equation 23. It has been found that the thermal decomposition of 17... 

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