1.3- Diradical

Pedersen S, Herek J L and Zewail A H 1994 The validity of the Diradical hypothesis direct femtosecond studies of the transition-state structures Science 266 1359-64... 

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. 

Chemical Evidence for PX Monomer. Estabhshing early on that PX is indeed the pyrolysis product, rather than the molecule formed by breaking only one of the original diben2yl bonds, the dimer diradical (5), would prove to be an important development. 

Upon exposure to uv light, ground-state benzophenone is excited to the ttiplet state (a diradical) which abstracts an alpha H atom from the alcohol, resulting in the formation of two separate initiating radicals. With amine H atom donors, an electron transfer may precede the H-transfer, as in ttiplet exciplex formation between benzophenone and amine (eq. 43) ... 

Figure 4c also describes the spontaneous polymerisation ofpara- s.yX en.e diradicals on the surface of soHd particles dispersed in a gas phase that contains this reactive monomer (16) (see XylylenePOLYMERS). The poly -xylylene) polymer produced forms a continuous capsule sheU that is highly impermeable to transport of many penetrants including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. This process is a Type B encapsulation process, but is included here for the sake of completeness. 

The 1,3-zwitterion appears to have some diradical as weU as 1,2-zwitterionic (carbonyl oxide) character ... 

There is evidence that dioxirane is an intermediate product in the low temperature ozonization of ethylene and is probably formed from the diradical resonance isomer of the 1,3-zwitterion (164). 

Cychc diacyl peroxides decompose thermally and photolyticaHy to yield products derived from diradical intermediates (188,198,205) (eq. 31). 

P-Peroxylactones undergo thermal decarboxylation to carbonyl compounds by the initial formation of a 1,5-diradical (238). a-Peroxylactones undergo similar decarboxylation, emitting light since the ketone is generated in the triplet excited state (85,239,240) ... 

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). 

Only a small fraction of reactant is iavolved ia this step. When naphthenes are iavolved, diradicals are produced. Eor aromatics with side chains, H radicals are produced. 

For isoxazoles the first step is the fission of the weak N—O bond to give the diradical (51) which is in equilibrium with the vinylnitrene (52). Recyclization now gives the substituted 2//-azirine (53) which via the carbonyl-stabilized nitrile ylide (54) can give the oxazole (55). In some cases the 2H-azirine, which is formed both photochemically and thermally, has been isolated in other cases it is transformed quickly into the oxazole (79AHC(2.5)U7). 

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... 

Photolysis of spiro[fluorene-9,3 -indazole] (384) to the tribenzopentalene (385) has been rationalized in terms of the initial formation of triplet diradical (386) (76JOC2120). The spiroindazole (387) behaves differently and on irradiation in THF is converted into the dimer (388) and the stable iV-ylide (389) (76CB2596). 

The thermal and photolytic processes have been extensively studied in connection with the mechanism (concerted or diradical) and the stereochemistry of the decomposition. For a classical paper see (66JA3963) and for more recent studies, Table 33. 

Cis- and trans-3,5-diphenyl derivatives 1,3-Diradical mechanism 76T619... 

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). 

The loss of one or two (or sometimes more) ring members from heterocyclics, concerted with or followed by formation of a new ring, is a highly versatile method for heterocyclic synthesis. Loss of N2, CO, CO2, S, SO, SO2, H2C=CH2, etc. is common. Diradical or dipolar intermediates are often encountered, and valence isomerization before the actual fragmentation is characteristic for some systems. 

An interesting variation was uncovered on thermolysis of the A-ethoxycarbonyi-substituted aikyiideneaziridine (323) (75JOC224). Heating this material at 130 °C gave oxazoline (325) as the exclusive product. In this case cyclization of the diradical occurs via a five-membered ring. 

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