Radicals and Diradicals

The ability of thiols to trap the diradical intermediates 104 and the partial rearrangement of a-allylbutyrophenone to 2-phenyl-2-norbomanol 105 strongly 

The actual reactions of the diradicals and radical pairs formed by triplet state hydrogen abstraction are those expected of radical pairs coupling (cyclization)  

Inductive effects of substituents on diradical behavior are most pronounced in the disproportionation reaction. With phenyl ketones, electron-donating substituents in the para position and electron-withdrawing substituents in the y-position enhance the probability for product formation from the diradical intermediate, at the expense of disproportionation. This disproportionation is now seen as a normal, low activation energy, ground state reaction 29 94 . It has been 

The mere fact that so much diradical disproportionation occurs in triplet type II reactions reflects the acidity of the hydroxyl proton. The ability of even very weak Lewis bases to suppress this disproportionation 115 is further evidence. It often happens that added Lewis base causes Pp to equal unity, such that quantum yields are determined solely by competing triplet reactions. Unfortunately, in sterically crowded ketones, such is not always the case. 

Since a-hydroxyradicals are the primary photoproducts in any hydrogen abstraction by ketones, their strong reducing power should be noted. The photo-reduction of benzophenone in secondary alcohols provided the first example of the reducing capabilities of hydroxyradicals 116 . 

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Mendiara, S. N., E. Ghibaudi, L. J. Perissinotti, and A. J. Colussi, Free Radicals and Diradicals in the Reaction between Nitrous Acid and Bisulfite in Acid Aqueous Media, . /. Phys. Chem., 96, 8089-8091 (1992). 

Figure 11 Examples of Mataga[117] systems involving radicals and diradicals expected to be in the high-spin ground state.

West et al. 4, 5) have suggested a number of possible reaction intermediates, which include anions, radical anions, radicals, and diradicals. Zeig-ler (6, 7) has proposed, on the basis of some radical-trapping experiments, that the intermediate is, at least at some point, a radical. He showed that the diradical silylene was not an intermediate, and he also stressed the importance of bulk solvent composition on the course of the reaction. The bulk solvent composition determines the expansion of the polymer coil as it interacts with the sodium surface. Miller et al. (8) initially suggested that the reaction, which is promoted by the addition of diethylene glycol dimethyl ether, proceeds by an anionic process, although they later (9) accepted Zeigler s bulk-solvent model. 

At the beginning of this section the crucial difference between radicals and diradicals needs to be discussed. Whereas the former has only one spin state, the latter may exist in two distinct spin states, the singlet and the triplet state. The following discussion is limited to diradicals that are formally derived from cr-bond homolysis and are not concerned with broken (twisted) 7c-bonds. 

Such reactions, however, can also be used for synthetic purposes. For example, p-diisopropyl benzene has two easily transferable hydrogen atoms. The hydrogen atoms are transferred in the reaction with initiator free radicals, and diradicals are formed. The monomer diradicals then recombine in what is called polyrecombination to poly(diradicals) ... 

Lebegue, E., T. Brousse, J. Gaubicher, and C. Cougnon. Chemical functionalization of activated carbon through radical and diradical intermediates. Electrochem. Comm. 34, 2013 14-17. 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

Many of the problems and misconceptions occurring for dithiolene compounds are related to the fact that the ligands are redox-active and can be oxidized to monoanionic radicals. Typical examples for this phenomenon are the mono and diradical complexes [Fe ( "bdt )( "bdt)(PMe3)] (9) and [Fe ( "bdf)2(PMe3)]-" (10) for which bdt and bdt are tcrt-butyl-dithiolene and its one-electron oxidized form. Originally, these and other bdt derivatives had been described as... 

The ntility of the experimental methods are illnstrated in this chapter by considering their applications to the stndy of reactive molecules, including radicals, car-benes and diradicals, carbynes and triradicals, and even transition states. These are provided in Section 5.4, which inclndes resnlts for representative bond dissociation energies and an extensive list of thermochemical results for carbenes, diradicals, carbynes, and triradicals. Section 5.5 provides a comparison and assessment of the resnlts obtained for selected carbenes and diradicals, whereas spectroscopic considerations are addressed in Section 5.6. 

From the point of view of both synthetic and mechanistic interest, much attention has been focused on the addition reaction between carbenes and alkenes to give cyclopropanes. Characterization of the reactivity of substituted carbenes in addition reactions has emphasized stereochemistry and selectivity. The reactivities of singlet and triplet states are expected to be different. The triplet state is a diradical, and would be expected to exhibit a selectivity similar to free radicals and other species with unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition... 

In Scheme 7, the peroxidic 0-0 bond of the hydroperoxyl group is broken together with /1-scission of the formed alkoxyl radical, and, further, ring closure of alkyl peroxyl diradical may occur. The process generates a hydroxyl radical, methylcarbonyl terminal groups (-CH2-CO-CH3) and dioxetane. The latter is unstable and decomposes into an excited triplet state of formaldehyde and/or excited triplet state of methylcarbonyls (Scheme 8). 

When the polymerization of St was carried out with 51 under conditions identical to those in Fig. 3, i.e., [7]/4=[8]/2=51=2X 10-3 mol/1, the formation of benzene-insoluble polymers was observed from the initial stage of the polymerization. Although 7 and 8 induced living radical mono and diradical polymerization similar to that previously mentioned, benzene-insoluble polymers were formed in the polymerization with 51, and the molecular weight of the soluble polymers separated decreased with the reaction time. This suggests that a part of the propagating polymer radicals underwent ordinary bimolecular termination by recombination, leading to the formation of the cross-linked polymer, which was prevented by the addition of 13. 

All these transformations except for reversion of radical to cation have been accomplished with the benzobisdithiazolium cation (60) which has given rise to radical cation (61) and diradical (62) <86SM233,86SM239,88JCR(S)358>. The diradical (62) is a dimer in solution (cryoscopical measurements in hexane) <86SM233> the radical cation (61) is loosely dimeric in the crystal (Section 4.12.3.1). 

A new wave of interest and productive research was aroused by synthesis of tricyclic condensed derivative benzobis(l,3,2-dithiazole) (BBDTA) which was shown experimentally to afford at least three distinct oxidation states dication, radical cation, and diradical <86SM233, 86SM239). This has been helpful for developing an approach to ferromagnetic organic metals in which homomolecular stacks are formed from donor-acceptor complexes in which the donor is a triplet and the acceptor a radical ion derived from the donor <86SM233>. Initial experiments showed conductivities lO " -10 S for complexes of BBDTA radicals with TCNQ <86SM239>. 

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