Cobalt radical generation

Anipsitakis GP, DD Dionysiou (2003) Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ Sci Technol 37 4790-4897. 

The first term on the right-hand side denotes the rate of dioxygen reaction with styrene (see Chapter 4) and the second term is the rate of catalytic free radical generation via the reaction of styrene with dioxygen catalyzed by cobaltous stearate or cobaltous acetylacetonate. The rate constants were found to be ki = 7.45 x 10-6 L mol-1 s-1, k2 = 6.30 x 10 2 L2 mol 2 s 1 (cobaltous acetylacetonate), and k2 = 0.31L2 mol-2 s 2 (cobaltous stearate) (T = 388 K, solvent = PhCl [169]). The mechanism with intermediate complex formation was proposed. 

Intermolecular addition of radicals, generated by photo-electrochemical catalysis, to activated alkenes can also be brought about. The reaction of 66 is used as a key step in one synthesis of the insect pheromone, brevicomin [219]. The reaction of a secondary radical from 67 occurs at low cathode potentials and without photochemical assistance [219]. This illustrates the equiibrium between a secondary al-kylcobalt(m) species and the radical - cobalt(ii) pair. The carbon radical is eventually captured by reaction with the alkene. Further steps in the synthesis lead to four isomers of the pheromone, multistriatin, each of which is a pure enantiomer since... 

A second group of ribonucleotide reductases (Class II), found in many bacteria, depend upon the cobalt-containing vitamin B12 coenzyme which is discussed in Section B. These enzymes are monomeric or homodimeric proteins of about the size of the larger a subunits of the Class I enzymes. The radical generating center is the 5 -deoxyadenosyl coenzyme.350 364 365... 

Chen S.CopeVW, Floffmann MZ(1973) Behavior of C03 radicals generated in the flash photolysis of carbonatoamine complexes of Cobalt(lll) in aqueous solution. J Phys Chem 77 1111-1116 Czapski G, Lymar SV, Schwarz HA (1999) Acidity of the carbonate radical. J Phys Chem A 103 3447-3450... 

Scheme 14.2 Mechanism for the hydrogen abstraction from aldehydes by PINO radical, generated in situ from NHPI in the presence of cobalt salts.

In parallel the cocatalysis of cobalt compounds with A7-hydroxyphthalimide 398 was developed extensively for oxidative radical reactions [434]. Ishii and colleagues showed that these conditions can be used in radical additions (Fig. 92). Ketyl radicals generated by hydrogen abstraction from secondary alcohols 396 add to a.p-unsaturated esters 397 affording 2,4-dihydroxy esters, which cyclized to lactones 399 under the reactions conditions [435]. Using 0.1 mol% of Co(OAc)2, 1 mol% of Co(acac)3, and 10 mol% of 398 under 1 atm of oxygen, the cyclic products 399 were isolated in 14—90% yield. As observed for similar reactions, Co(III) alone needs an induction period (see below). 

Several synthetic targets have been attacked by exploitation of this methodology examples include an enantiospecific synthesis of acromelic acid A and a formal synthesis of physovenine (equations 186 and 187)362. Recent work has focused on rendering the radical generation and addition processes catalytic in cobalt successes have been achieved by using more readily reduced cobaloxime complexes and carefully controlled conditions363. 

Organic peroxo compounds are also obtained by autooxidation of ethers, unsaturated hydrocarbons, and other organic materials on exposure to air. A free-radical chain reaction is initiated almost certainly by radicals generated by the interaction of oxygen and traces of metals such as copper, cobalt, or iron. The attack on specific reactive C—H bonds by a radical X" gives first R, then hydroperoxides, which can react further ... 

Tertiary acetylenic chlorides undergo self-coupling when they are treated with methylmagnesium bromide and cobalt(n) chloride, i.e. radical-generating conditions . Under these conditions, 4-chloro-4-methyl-2-pentyne (60) gave mainly the diyne 62 along with smaller amounts of another hydrocarbon tentatively identified... 

Safety requirements must be satisfied before a cobalt-60 source can be installed and licensed. Radiation-trained personnel must be on the staffbefore a license can be issued. At least 500,000-1,000,000 Ci (curies) of cobalt-60 are required in a production source for making wood-polymers, and at 1.00 or more per curie, a considerable investment must be made before production can begin. Besides cost considerations, the cobalt-60 radiation process does have some distinct advantages in making wood-polymer composites. Because the monomer is not catalyzed it can be stored at ambient conditions as long as the proper amount of inhibitor is maintained. The rate of free radical generation is constant for a given amount of cobalt-60 and does not increase with temperature. 

Chowdhury, S., Banerjee, R. (2000) Evidence for quantum mechanical tunneling in the coupled cobalt-carbon bond homolysis-substrate radical generation reaction catalyzed by mefhylmalonyl-CoA mutase, /. Am. Chem. Soc. 122, 5417-5418. 

Chen, S., Cope, V. W., and Hoffman, M. Z., Behavior of CO radicals generated in the flash photolysis of carbonatoamines complexes of cobalt (III) in aqueous solution. J. Phys. Chem. 77, 1111 (1973). 

As described earlier carbon-centered radicals can be efficiently generated by homolysis of an alkylcobalt(III) species. This species can be synthesized by a reductive process from an alkyl halide and a nucleophilic Co(I) reagent [1-6). This chapter describes the recent advances in cobalt-initiated carbon-centered free radicals (generated via a reductive process) in organic synthesis. The cobalt-mediated free-radical reactions generated via this protocol can be broadly divided into the following two categories. 

Scheme 1. Cobalt-mediated generation of radicals by a reductive process...

As exemplified by Eqn. 19, cobalt(II) complexes can efficiently trap organic radicals to form organocobalt complexes. Hence, organic radicals generated by means other than the atom-transfer step of Eqn. 18 may also be used to alkylate cobalt(II) complexes. Thus, Roche and Endicott [63] generated organic radicals by photolysis of carboxylatopentaammine-cobalt(III) complexes in acidic media (Eqn. 20)... 

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