Diradicals reaction

Experimentally, the kinetic data obtained from the pyrolysis of cyclobutane to ethene between 696 and 1007"C using a very low-pressure pyrolysis technique is consistent with a 1,4-diradical reaction route.82 A diradical process is also favored over a concerted pathway for the formation of nearly 1 1 mixtures of cis- and tranj-but-2-enes and [l,2-2H2]ethene from the thermal rupture of cb-l,2-dimethyl[cw,firn /-3,4-2H2]cyclobutane (5),83... 

While it is in the triplet state a molecule may undergo typical diradical reactions. This provides a plausible mechanism for radical-like reactions of substances that are largely diamagnetic. They are partly converted to the triplet state by light, or in the case of low lying triplet states, by heat. Probable examples of this mechanism are the photooxidation of rubrene and the photooxidation and dimerization of anthracene and higher members of the acene series.76... 

The explanation for these diradical reactions can not be quite so simple as the one hinted at above however, since the quantum yield of rubrene peroxide does not appear to depend only on the concentration of photoactivated rubrene triplet molecules but also on the concentrations of rubrene itself and of oxygen.77... 

The formation of l,4-dihydro-2,3-benzodioxin 5 from the benzocyclobutene 141/o-quinodimethane 142 equilibrium has been utilized as a trapping experiment for the kinetic analysis of diradical reactions (Scheme 38) <2002CC1594, 1988CB1357>. 

The hydroperoxide radical reacts with another molecule of oxygen (Reaction 5) to give the hydroperoxide-peroxy radical. This radical in turn reacts with a molecule of dihydroanthracene (Reaction 6), to give the dihydroperoxide and generate a radical to propagate the chain. However, the hydroperoxide radical formed in Reaction 4 may be decomposed by a carbanion to the anthracene diradical (Reaction 7). [An example of the decomposition of an unstable hydroperoxide by reaction with an anion is found in the basic autoxidation of 2-nitropropane (3).]... 

Dithiazolium cations can be readily reduced to the stable mono- and diradicals. Reaction of the disalt 43 could be effected, on a milligram scale, by electrolysis in an acetonitrile solution at 50 pA onto Pt wire cathode <1997JA2633>. Larger quantities could be obtained by chemical reduction. Attempts to reduce cation 43 directly with silver or zinc powder were unsuccessful. The most successful approach involved the use of triphenylantimony as reducing agent and bis(triphenylphosphine)iminium chloride ((PPN)Cl Equation (5)). The product obtained (7) is remarkably stable in the solid state, in air, and in organic solutions. 

Fig. The diradical reaction shown shcmaticallyin an energy diagram. After elimination of carbon ntortoxidefrom the parent cyclopentanonephtochemicaUy the tetramethylene diradical is created this can undergo ring closure to yield cyclobutane or can produce two ethylenes.

No charge and spin separations appear in the case of AN because of closed-shell character, but spin separation (SS) ( 1) is significant for FR case, although the charge separation (CS) ( ) is rather weak, as in the case of homolytic diradical. On the other hand, the zwitterionic ( ) CS is remarkable for FN case, and both SS and CS ( +, 1) become important for AR case such as electron-transfer diradical reactions. In recent papers, we have performed the symmetry-stability analyses of chemi- and bio-luminescence reactions from these theoretical view points.4,5... 

Alko l radicals and chlorine atoms are highly reactive in hydrogen abstraction leading to the formation of macroalkyl radicals (P), which in turn react with almost zero activation energy with ground state oxygen which is itself a diradical (reaction 2) ... 

Chapter 22 Free-Radical and Spin-Paired Diradical Reactions.283... 

Several studies have demonstrated the successful incoriDoration of [60]fullerene into polymeric stmctures by following two general concepts (i) in-chain addition, so called pearl necklace type polymers or (ii) on-chain addition pendant polymers. Pendant copolymers emerge predominantly from the controlled mono- and multiple functionalization of the fullerene core with different amine-, azide-, ethylene propylene terjDolymer, polystyrene, poly(oxyethylene) and poly(oxypropylene) precursors [63,64,65,66,62 and 66]. On the other hand, (-CggPd-) polymers of the pearl necklace type were fonned via the periodic linkage of [60]fullerene and Pd monomer units after their initial reaction with thep-xy y ene diradical [69,70 and 71]. 

The concept of biradicals and biradicaloids was often used in attempts to account for the mechanism of photochemical reactions [2,20,129-131]. A biradical (or diradical) may be defined as [132] an even-electron molecule that has one bond less than the number permitted by the standard rules of valence. 

The Flory and Mayo proposals can be combined by the common diradical -D-, which collapses to either DH or 1,2-diphenylcyclobutane. Nonconcerted Diels-Alder reactions are permissible for two nonpolar reactants (122). 

A new type of rearrangement has been reported for certain l-(o-nitrophenyl)pyrazoles (169), giving cis- and trans-benzotriazole 1-oxides (170 Scheme 12) (73TL891). The reaction was rationalized in terms of an intermediate azo compound (171 formed in turn either from the diradical species (172) or from the intramolecular 1,4-adduct (173). Subsequently... 

The electrolysis of adamantylideneadamantane solutions affords the radical cation, which can add molecular (triplet) oxygen to give the peroxide radical anion, which can react with adamantylideneadamantane to give the 1,4-diradical and another molecule of adamantylideneadamantane radical cation. The latter reacts with oxygen, to continue the chain of the reaction, while the former cyclizes to the corresponding 1,2-dioxetane (Scheme 18) (81JA2098). 

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

Thiirene intermediates in the photolysis of 1,2,3-thiadiazoIes readily undergo ring opening to a diradical which can be trapped by reaction with an alkyne (Scheme 25) (79CB1769). Significant polymerization is not observed. 

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