And diradicals

The mechanisms of these ring expansions are not certain. Both concerted" and diradical" pathways have been proposed, and it is possible that both pathways operate, in different systems. 

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... 

Singlet-triplet Energy Splittings in Carbenes and Diradicals 229... 

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. 

Different methods have been developed for the generation of carbene and diradical negative ions. One of the most commonly used approaches involves the reaction of an organic substrate with atomic oxygen ion, O , to form water by H2 abstraction (Eq. 5.7). "... 

Alternative negative ion-based methods for measuring carbene and diradical enthalpies of formation have been developed, which can give BDEs indirectly. A common approach for this involves the use of halide affinity measurements. The relationship between enthalpy of formation and halide affinity is illustrated by Eq. 5.14. 

Enthalpies of formation and BDEs for the formation of carbenes and diradicals measured by using negative ion approaches are included in Table 5.3. 

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. 

In connection with Chichibabin s hydrocarbon it is appropriate to mention that there is little or no resonance between states of different multiplicity.4 Thus the singlet-triplet transition represented by the covalent and diradical structures of the hydrocarbon is a true equilibrium and not an example of resonance. 

In addition to mono and diradicals, the conjugate base of a nitrogen positive ion is a possible intermediate in the above reactions. 

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>. 

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). 

The thermal reactions of furans merit only brief mention. Ligularone (5) is isomerized to isoligularone (6) without a catalyst at 260 °C and diradical intermediates have been postulated (73TL3999). The double bond of furan can be forced to participate in electrocyclic... 

To avoid Norrish II photolysis of pyranosid-2-uloses derivatives containing aglycones derived from primary and secondary alcohols, t-butoxy derivatives 58 can be successfully photolyzed by a Norrish I reaction to yield, by C-l, C-2 cleavage and diradical formation, compounds 59 [30 %) and 60 [40 %) (Scheme 32) [62],... 

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. 

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