Cobalt diene radicals

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

A tandem radical 5-exo cyclization/radical addition/allylic substitution reaction was subsequently described [292]. Allylic ot-bromo acetal 242b cyclized cobalt-catalyzed. Addition to diene 245 and subsequent coupling with coformed organocobalt(I) species generates an allylcobalt complex, which undergoes reductive elimination to cyclic product 246 in 93% yield (cf. Fig. 56). 

The polarographic reduction of i/4-diene complexes was first noted for [M(i/4-l,4-quinone)Cp] (M = Rh or Ir, quinone = duroquinone or 2,6-di(t-butyl)quinone), which show one two-electron wave (M = Ir), or two one-electron waves (M = Rh) at potentials more negative than those of the free quinone (575). The cyclopentadienone compounds [M(i/4-C5R40)Cp] (M = CoorRh, R = Ph or C6F5) undergo reversible one-electron reduction to radical anions (e.g., M = Co, R = Ph, E° = — 1.46 V in thf). The cobalt complexes, more stable than those of rhodium, are generated by alkali metal reduction at 100 K and show ESR spectra characteristic of metal-based d9 species (374). 

The ESR spectra of the anions are particularly interesting. Both isomers of [Co(t/4-cod)Cp] are metal-based radicals but [Co(t/4-1,3-cot)Cp] - in frozen thf shows a more nearly isotropic spectrum with the singly occupied orbital possessing little cobalt d character. MO calculations are also consistent with localization of the free electron on the unbonded diene unit of [Co(r 4-l,3-cot)Cp] [cf. [Fe(CO)3(f/4-cot)], Section VI], but on the metal of [Cofa4-cod)Cp]" (255). 

Calculations of charge and spin densities for the intermediates involved suggest that the reaction of hexafluoroben2ene with a mixture of cobalt(iu) and calcium fluorides, which shows a strong preference for formation of the l,4 diene (62) rather than its 1,3-isomer, proceeds via route (a), not route (b) (Scheme 16). The favoured path involves reaction of the initial radical cation (60) (see p. 446) with fluoride ion rather than a fluorine atom (cf. Vol. 2, p. 351 this vol., p. 358) and... 

The fluorination of pentafluoro-, 3-chlorotetrafluoro-, and 3,5-dichlorotrifluoro-pyridines with a mixture of cobalt(m) and calcium fluorides gives mixtures of perhalogenoazacyclohex-enes and -adienes and perhalogeno-l-azahexenes, depending on the reaction conditions used (see p. 219) tetrafluoropyrazine gives 40% of perfluoro-l,4-diazacyclohexa-l,3-diene (288), whereas tetrafluoropyrimidine affords the dimer (289), which is evidence for the intermediacy of radicals. ... 

Cobalt(II)-bis((Z)-2-hydroxy-5,5-dimethyl-1 -(4-methylpiperazin-l -yl)hex-2-ene-1,4-dione) efficiently catalyses the aerobic oxidative 5-exo cyclization of )5,) -unsaturated oximes to give isoxazolines. Carbon-centred radicals are generated in the basic cyclization step. The predominant oxidative products of the reaction carried out in the presence of i-PrOH and TBHP are (16), whereas reductive products (17) are selectively obtained when the reaction is carried out in toluene in the presence of cyclohexa-1, 4-diene. " ... 

A basic reaction scheme of the autoxidative drying process is depicted in Scheme 16.18. A catalyst is needed to give this reaction sufficient speed usually a cobalt(II) salt is used. The diene moieties of the linolenic acids, especially, are involved in this process. After formation of a conjugated hydroperoxide with molecular oxygen from the ambient air, the hydroperoxide decomposes into oxy-radicals that can fur-... 

When 1,3-diene is the radical acceptor, a three-component coupling reaction of alkyl halide, 1,3-diene, and trimethylsilylmethyl-magnesium chloride takes place to afford homoallylsilane The final step of the three-component coupling is reductive elimination from allyl(trimethylsilylmethyl)cobalt that predominates over conceivable /3-hydride elimination. 

The mechanism of addition of the superoxide radical anion to cobalt(n) macro-cyclic complexes, [CoN4] +, has been studied in aqueous media. In the pH range 7—8, reaction of [Coii(4,ll-diene-N4)] and [Co (l,3,8,10-tetraene-N4)] with Oa yielded transient intermediates which have been characterized spectroscopically. The rate constants (1.4 x 10 and 1.6 x 10 1 mol s respectively) rule out the participation of HOa at these pH s. The transients are neither Co nor Co and it is concluded that the Oa adds to the metal centre at a labile axial site. The transient complex [OaCo(4,ll-diene-N4)]+ is also formed by the reaction of the corresponding cobalt(i) species with O2. The fate of these intermediates is still uncertain although one possibility is a dimerization to the dinuclear / -complex. 

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