Cobalt complexes radical reactions

The scope of this chapter is based on the analogous one by Naso and Marchese published in 19831. Since the time of their review, the focus of work in this area has shifted somewhat, and sections have been added or omitted to reflect that focus. New sections covering chromium(II)-mediated reactions of halides and covering cobalt-mediated radical reactions of halides have been added. In view of the relatively mature nature of the areas, sections dealing with 7c-allylnickel complexes, iron oxyallyl cations and cyanation reactions have been omitted. 

For an early overview on the use of stoichiometric amounts of cobalt complexes, see Pattenden, G. (1988) Cobalt-mediated radical reactions inorganic synthesis. Chem. Soc. Rev., 17, 361-82. 

A series of papers have also reported the coupling of alkyl and aryl electrophiles with aryl Grignard reagents catalyzed by iron (Equation 19.15) and cobalt complexes. These reactions build upon Kochi s and Molander s early results on coupling reactions catalyzed by complexes of these metals. The recent reactions have been conducted with simple metal salts in many cases and with discrete metal complexes as catalyst precursors in others. Although little recent mechanistic data is available on these reactions, they have earlier been shown to involve radical intermediates. Kochi concluded that the catalytic process occurs by an Fe(I)-Fe(III) couple reactions of optically active alkyl halides generate racemic coupled products and reactions of diastereomerically pure aUcyl halides generate equal ratios of diastereomeric products, as depicted in Equations 19.16 and 19.17. ... 

This may be of significance in connetion with the biosynthesis of acetate from carbon dioxide, because the next step, the fixation of carbon monoxide, was demonstrated by B. Krautler. He irradiated methyl cobalamin under Co at 30 atm and obtained the acyl cobalamin as the product. Interestingly, a radical mechanism was iproposed, involving the reaction of methyl radicals with CO to give acyl radicals, which then recombine with the cobalt complex /55/. 

Use of CD30D or methyl tetrahydrofuran solvents to encourage electron capture, resulted in a complex set of reactions for methyl cobalamine. Initial addition occurred into the w corrin orbital, but on annealing a cobalt centred radical was obtained, the e.s.r. spectrum of which was characteristic of an electron in a d z.y orbital (involving the corrin ring) rather than the expected d2z orbital. However, the final product was the normal Co species formed by loss of methyl. Formally, this requires loss of CH3 , but this step seems highly unlikely. Some form of assisted loss, such as protonation, seems probable. 

Cobalt complexes are used for the living radical polymerization of acrylates to give a high molecular weight polymer with a narrow molecular weight distribution (Mw/Mn 1.2) (Eq. 71), whereas the complex is applied to the introduction of an unsaturated group into the methacrylate polymers with a high efficiency via a reaction mechanism illustrated in Eq. (72) [27,28,267,268]. 

The next five transition metals iron, cobalt, nickel, copper and zinc are of undisputed importance in the living world, as we know it. The multiple roles that iron can play will be presented in more detail later in Chapter 13, but we can already point out that, with very few exceptions, iron is essential for almost all living organisms, most probably because of its role in forming the amino acid radicals required for the conversion of ribonucleotides to deoxyribonucleotides in the Fe-dependent ribonucleotide reductases. In those organisms, such as Lactobacilli6, which do not have access to iron, their ribonucleotide reductases use a cobalt-based cofactor, related to vitamin B12. Cobalt is also used in a number of other enzymes, some of which catalyse complex isomerization reactions. Like cobalt, nickel appears to be much more extensively utilized by anaerobic bacteria, in reactions involving chemicals such as CH4, CO and H2, the metabolism of which was important... 

M s 1). Consideration of reaction (6) suggests that these reactions fail, despite the stability of the 0-P< >2 an< SO3 products, because the primary product cobalt species is an unstable cobalt(III)-radical complex, Co-(0 ) This, in turn, suggests that oxygen atom transfer could succeed if accompanied by elec-... 

Two types of intermediates, i.e., radicals or carbanions or their organometallic equivalents, can be used to perform addition reactions to Michael acceptors. The free-radical route has already been investigated with nickel or cobalt complexes as catalysts [62-64]. These studies have been reinvestigated recently with the aim of improving the turn-over of the catalyst and/or using easily prepared cheap complexes. 

In Reaction (7.20) is reported the cyclization of a thermally unstable propar-gyl bromide cobalt complex mediated by Ph2SiH2 at room temperature and Et3B/02 as the radical initiator. However, a mixture of reduced and bromine atom-transfer products (1 1.8 ratio) are isolated due to the low hydrogen donation of the employed silane [31]. 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

The catalysis of the selective oxidation of alkanes is a commercially important process that utilizes cobalt carboxylate catalysts at elevated (165°C, 10 atm air) temperatures and pressures (98). Recently, it has been demonstrated that [Co(NCCH3)4][(PF6)2], prepared in situ from CoCl2 and AgPF6 in acetonitrile, was active in the selective oxidation of alkanes (adamantane and cyclohexane) under somewhat milder conditions (75°C, 3 atm air) (99). Further, under these milder conditions, the commercial catalyst system exhibited no measurable activity. Experiments were reported that indicated that the mechanism of the reaction involves a free radical chain mechanism in which the cobalt complex acts both as a chain initiator and as a hydroperoxide decomposition catalyst. 

The reaction is thought to involve free-radical intermediates rather than organo-cobalt complexes.175... 

Another possible precursor to conduct free radical reactions is the glycosyl-cobait(III) dimethylglyoximato complex 33 [22,23], These organometallic compounds can readily be prepared by the displacement of the halide atom in 17 with the highly nucleophilic cobalt(I) anion 32. The latter can be generated from the dimeric Co(II) complex 31 under reducing conditions. 

To achieve low radical concentrations, most radical reactions are traditionally performed as chain reactions. Atom or group transfer reactions are one of the two basic chain modes. In this process the atom or group X is the chain carrier. A metal complex can promote such chain reactions in two ways. On one hand, the catalyst acts only to initiate the chain process by generating the initial radical 29A from substrate 29 (Fig. 10). This intermediate undergoes the typical radical reactions, such as additions or cyclizations leading to radical 29B, which stabilizes to product 30 by abstracting the group X from 29. A typical example is the use of catalytic amounts of cobalt(II) salts in oxidative radical reactions catalyzed by /V-hydroxyphthalimide (NHPI), which is the chain carrier [102]. 

Nature demonstrates that transition metals can be very effective in catalyzing transformations, which are impossible to accomplish otherwise under physiological conditions. The prime example is vitamin B12, whose resting state is adenosylco-balamine(III) (reviews [267-273]). On homolysis it triggers a variety of radical reactions crucial to the living world. This inspired the interest of chemists and led to a number of applications. More recently, interest shifted to catalysis by low-valent cobalt complexes. 

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