Cobalt-initiated radical reactions

Rhodium and cobalt carbonyls have long been known as thermally active hydroformylation catalysts. With thermal activation alone, however, they require higher temperatures and pressures than in the photocatalytic reaction. Iron carbonyl, on the other hand, is a poor hydroformylation catalyst at all temperatures under thermal activation. When irradiated under synthesis gas at 100 atm, the iron carbonyl catalyzes the hydroformylation of terminal olefins even at room temperatures, as was first discovered by P. Krusic. ESR studies suggested the formation of HFe9(C0) radicals as the active catalyst, /25, 26/. Our own results support this idea, 111,28/. Light is necessary to start the hydroformylation of 1-octene with the iron carbonyl catalyst. Once initiated, the reaction proceeds even in the... 

The hypothesis that the cobalt carbonyl radicals are the carriers of catalytic activity was disproved by a high pressure photochemistry experiment /32/, in which the Co(CO), radical was prepared under hydroformylation conditions by photolysis of dicobalt octacarbonyl in hydrocarbon solvents. The catalytic reaction was not enhanced by the irradiation, as would be expected if the radicals were the active catalyst. On the contrary, the Co(C0)4 radicals were found to inhibit the hydroformylation. They initiate the decomposition of the real active catalyst, HCo(C0)4, in a radical chain process /32, 33/. 

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. 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

The cyclization reactions of organocobalt complexes are very useful, and they offer an excellent alternative to the tin hydride method when reduced products are not desired. Most cobalt cyclizations have been conducted with nucleophilic radicals. Precursors are prepared by alkylation of cobalt(I) anions, and are usually (but not always) isolated. One suspects that alkylcobalt precursors should be useful for slow cyclizations because there are no rapid competing reactions that would consume the initial radical (coupling of the initial radical with cobalt(II) regenerates the starting complex). 

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]. 

The reactions are thought to proceed by initial formation of a cobalt(III)-oxygen complex 416 from CoX2 and 02 (Fig. 97). The induction period observed in many reactions involving Co(acac)3 can be traced to its initial slow reduction to Co(acac)2. Complex 416 initiates the radical reaction by hydrogen abstraction... 

Nikishin and co-workers have carried out extensive studies of the reactions of aliphatic aldehydes with Mn(III) and Co(III) acetates in acetic acid in the presence of olefins. Depending on the reaction conditions, a variety of interesting products are formed. In the presence of catalytic amounts of cobalt(II) acetate and a limited oxygen supply, ketones are formed via the cobalt-initiated addition of acyl radicals to the olefin,324 326b e.g.,... 

Benzoyl peroxide, C HsCO-O-O-COCtHg, or methyl ethyl ketone peroxide, CH3C2Hs-C(0-0) CCzH5 CH3 is used as a catalyst. To initiate the reaction at room temperature it is necessary to add another substance ouch as cobalt naphthenate, (Cdimethyl aniline, C H (CH3), to produce an initiator, e.g. free radicals C6H C 00(benzoil radical). 

Because oxidations with oxygen are free-radical reactions, free radicals should be good initiators. Indeed, in the presence of hydrogen bromide at high enough temperatures, lower molecular weight alkanes are oxidized to alcohols, ketones, or acids [5 7]. Much more practical are oxidations catalyzed by transition metals, such as platinum [5, 6, 55, 56], or, more often, metal oxides and salts, especially salts soluble in organic solvents (acetates, acetylacetonates, etc.). The favored catalysts are vanadium pent-oxide [3] and chlorides or acetates of copper [2, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66], iron [67], cobalt [68, 69], palladium [60, 70], rhodium [10], iridium [10], and platinum [5, 6, 56, 57]. 

The formed hydroperoxide initiates the radical crosslinking reaction. Metal salts are used frequently as catalysts, to accelerate crosslinking reactions (for example with cobalt(II) compounds, reaction 17.14). 

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. 

Propargyl)Co2(CO)g radicals presumably are also involved in the Mn(III)-mediated addition of P-dicarbonyl compounds to complexed 1,3-enynes, which produces highly functionalized dihydrofiuan derivatives 122 (Scheme 4-65) [222, 223]. The chemo-, regio-, and stereoselectivity of these reactions stands in contrast to the variable selectivity associated with the corresponding reactions of free enynes [224]. The formation of ethers 123 in methanol (Scheme 4-65) suggests that the cobalt-propargyl radicals initially produced are rapidly oxidized by Mn(III) to the stabilized carbocations. 

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 reaction is absolutely dependent on the formation of a thiyl radical (located on Cys-461) on the p face of the ribonucleotide. The model postulates that the enzyme catalyzes the homolysis of the cobalt-carbon bond of AdoCbl to generate, in a concerted fashion, this thiyl radical together with 5 -deoxyadenosine and cob(II)alamin. The radical initiates the reaction by abstraction of Ha the hydrogen atom at the 3 position of the substrate ribose, to give a 3 -radical. The... 

CCTP has its origins in biochemistry where coenzyme B12 is used to conduct many free-radical reactions. Enikolopyan et al. were the first who used analogues of B12 for polymerization [257,258]. Methacrylate was polymerized by a catalyzed chain transfer using a cobalt porphyrine. AIBN was used as initiator. Two possible reaction sequences for the catalytic aspect of CCT are described in the following scheme ... 

Fiikuzumi and Cuo have very recently concluded that the termination reaction for oxidation of c miene with manganese dioxide or cobalt oxide supported on silica ( ) and during autoxidation of cumene initiated by reaction of cumene hydroperoxide with lead oxide ( ) is strictly first-order with respect to the concentration of cumylperoxy radicals. These workers proposed an unprecedented 1,3-methyl shift followed by 0-0 bond cleavage to account for these inusual kinetics. 

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