Diradicals triplet

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 reaction is ordinarily stereoselective, favoring the more stable adduct for either alkene isomer, and a long-lived triplet diradical intermediate is implicated. ... 

Scheme 8 Electron configuration and polarization in the triplet diradicals...

There is a degree in the continuity and discontinuity of the orbital phase [20]. 2-Oxopropane-l,3-diyl (Scheme 10) is a hetero analog of trimethylenemethane (TMM) where the orbital phase is continuous in the triplet diradical (Sect. 2.1.5) and discontinuous in the singlet diradical (Sect. 2.1.6). The n and orbitals of carbonyl bonds are lower in energy than those of C=C bonds. The lowering strengthens the interaction of the radical orbitals (a, b) with and weakens that... 

Most of the triplet diradicals have disrotatory conformations, in which the cyclic orbital interactions are favored by the phase continuity. 1,3-Diradicals with the second-row substituents, 25-28, prefer the conformer a with the central C-H bond in conjugation (except for the conrotatory conformation b in 27), whereas those substituted by the third-row groups, 29-32 favor the disrotatory conformations c... 

As demonstrated above, the most stable singlet siUcon-centered monocyclic diradicals are the o-diradicals where the radicals interact with each other through the Si-Ge bonds, whereas the most stable triplet diradicals are jt-diradicals where the radicals interact with each other through the Ge-X bonds. However, for the bicyclic diradicals, 60 and 61, the conformations are fixed to exclude possibility of Jt-diradicals (Fig. 22). 

High stereospecificity is observed when the rotation of the diradical intermediate is slow in comparison with cyclization to cycloadduct or reversion to reactants. With the presence of external heavy atoms, it could facilitate the intersystem crossing (ISC) of the first-formed singlet diradical to the longer-lived triplet counterpart. The triplet diradical will have a chance to undergo rotation before it reverts back to singlet and cyclizes or cleaves to reactants. This then accounts for the reduced stereospecificity. The alternative possibility of a zwitterionic intermediate is considered unlikely because there is no interception of zwitterions by water. 

In addition to the photoxygenation/diimide (equation 6),16) silver salt (Eq. 22), 36) and triflate (Eq. 44)s6> routes, 9 has also been prepared by benzophenone-sensitized photodecomposition of the corresponding azo compound 59 and trapping of the resultant triplet diradical with oxygen (Eq. 45) 57). 

Although no other examples have been reported, oxygen trapping of azo-derived triplet diradicals provides a potentially versatile strategy for the synthesis of bicyclic peroxides under neutral conditions. 

Electron spin resonance (ESR) studies of the urazole-bridged 1,3 diradicals 64 derived from the azoalkanes 63 confirm a triplet ground state for these species. The nearly zero symmetry parameter, that is, Elhc= 0.0004 0.0001 cm-1, for the triplet diradical 64 of the diphenyl azoalkane 63 establishes a planar conformation <1995JOC308, 1997JA10673>. 

No similar meta substituent effect exists for ions since resonance between a singlet and triplet (diradical) state is prohibited. The dipolar ion theory would suggest that suitably disubstituted radicals should be especially stable, particularly in polar solvents, because of structures like III. 

An elimination-addition mechanism has been suggested for the substitution of amino for chloro in chlorobenzene.95 The neutral and symmetrical intermediate is called benzyne although it can not contain an ordinary linear bond system like that of acetylene. It is not known whether it is a distorted acetylene, a triplet (diradical), or a zwitterion. 

Reaction leading to the triplet diradical is exothermic by 1.0 kcal mol-1 but singlet formation would be exothermic by 27.4 kcal mol-1. 

The [2 + 2] photocycloaddition reaction of enones with allenes was first reported in 1966. A diradical intermediate is formed from a triplet enone via an exciplex. The triplet diradical cyclizes to the product after spin inversion to the singlet state [31,32]. 

A new triplet diradical is detected by ESR from the photolysis of 2-nitrobiphenyl106 (equation 85). The spectrum shows a temperature dependence which implies that the observed triplet state is a ground state. 

The photolysis of the diazobicyclo[2.2.2]heptene derivative (142) was studied at different temperatures and was found to give mixtures of syn (143) and anti (144) products. The experimental data support the homolytic (Xe2) pathway as the prevalent reaction channel at elevated temperatures for the generation of the sterically encumbered syn product, whereas at low temperatures the triplet pathway operates and loss of the syn selectivity is observed. The loss of syn selectivity at low temperatures is due to efficient intersystem crossing in the singlet-excited azoalkane to afford the planar, nitrogen-free triplet diradical which unselectively ring closes. 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

In summary, although the computed structural details of the reaction profile depend on the method used for calculations, the general salient mechanistic conclusion is that the dioxetane thermolysis starts with the 0—0 bond rupture to generate the 0C(H2)—C(H2)0 triplet diradical, which is followed by C—C bond cleavage to afford the final ketone products one of them is formed preferentially in its triplet excited state. Since even simple 1,2-dioxetanes still present a computational challenge to resolve the controversial thermolysis mechanism, the theoretical elucidation of complex dioxetanes constitutes to date a formidable task. 

The observed products correspond to the formation of the most stable intermediate by the addition of the photoexcited triplet (diradical) state of acetophenone to the alkene. 

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