Cobalt-mediated radical

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. 

VII. COBALT-MEDIATED RADICAL REACTIONS OF ORGANIC HALIDES... 

Cobalt-mediated radical reactions of heterocycles 88CSR361. 

Comparable stereoselectivity is described in the enantiospecific synthesis of aromelic acid A via a cobalt-mediated radical cyclization76. 

In studies directed towards forskolin, Pattenden et al. have observed dichotomous reactivity in stannane- and cobalt-mediated radical cyclization. In one instance, it was shown that the radical cyclization of the bromoacetal 24 initiated by BuySn" (BusSnH, AIBN) led to predominantly (95%) the equatorial-oriented side-chain isomer 25a, whereas use of catalytic vitamin B12 (MeOH, LiC104, -1.9 V, 24 h) produced (70%) almost entirely the corresponding axial epimer 25b (Scheme 9) [13j. 

Scheme 9. Dichotomous reactivity in stannane and cobalt-mediated radical cyclization...

Scheme 18. Cobalt-mediated radical olefin couplings. Synthesis of functionalized styrene derivatives...

In a similar manner, these researchers have also synthesized [48] a C-8 side-chain analog 60 of domoic acid using a cobalt-mediated cyclization-elimination sequence on the iodide 59 (Scheme 22). They extended this methodology to an enantiospecific total synthesis of acromelic acid A 64, a potent neurotoxin obtained from poisonous mushrooms [46]. The cornerstone of their synthetic strategy was a cobalt-mediated radical cyclization of the substrate 61 which was prepared from the epoxy alcohol in optically pure form. Treatment of 61 with cobalt(I) afforded 62, which was converted to the natural product 64 via pyridone 63 using routine functional group manipulation (Scheme 23). 

Pattenden and coworkers have developed a cascade cobalt group transfer reaction by effecting consecutive cobalt-mediated radical cyclizations in a controlled... 

Scheme 1. Cobalt-mediated radical cyclization followed by functionalization by trapping with TEMPO...

As one application of cobalt-mediated radical carbon-to-carbon bond forming reactions, the reaction between 2-oxazolidinone (255) and cobaloxime(I) gave an inseparable mixture of (Z) and ( ) 3-oxo-6,7-disubstituted perhydropyrrolo[l,2-c]oxazole (256) in high yield, and variable amounts of the a,)8-unsaturated esters (257) (Equation (28)) <92T9373>. 

The synthesis of a well defined poly(vinyl alcohol)-b-poly(aciylic acid) (PVA-PAA) DHBC has been recently reported in the literature [32] by a two step synthetic scheme. First the synthesis of a poly(acrylonitrile) (PAN) block was realized via cobalt-mediated radical polymerization, using a poly(vinyl acetate) (PVAc) macroinitiator, followed by hydrolysis of both blocks. The polymerization was performed in DMF, a very good solvent for PAN, and at low temperature, where block copolymers with low polydispersity were obtained. The polymerization procedure led to well defined macromolecules with relatively high molecular weights. The obtained copolymers were transformed to the desired DHBCs by hydrolysis, using large excess of potassium hydroxide in a water/ethanol mixture. The successful completion of the hydrolysis reaction was monitored by NMR and IR spectroscopy. An additional macroscopical indication of the DHBC formation was the aqueous solubility of the reaction product. 

A recently discovered quantitative radical chain-couphng reaction of polymer precursors preformed by CMRP, which is referred to as cobalt-mediated radical couphng (CMRC), was first observed when PAN-Co(acac)2 chains were treated with a large excess of isoprene at room temperature [58]. Instead of the formation of an expected PAN-h-polyisoprene (PI) diblock copolymer, a weU-defined homo PAN sample, with an exactly twofold higher molar mass compared to the precursor, was recovered. Interestingly, this coupling reaction proved to be quantitative. 

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. 

DEB 05] Debuigne A., Caille J.-R., Jerome R., Highly efficient cobalt-mediated radical polymerization of vinyl acetate , Angewandte Chemie International Edition, vol. 44, pp. 1101-1104, 2005. 

DEC 98] Decker C., Highly efficient cobalt-mediated radical polymerization of... 

More monomers are mentioned in patent literature but details of the reaction conditions are discrete. One such example is the CCTP of a hydrophilic acrylic macromonomer. Sodium acrylate and ammonium acrylate homopolymers are mentioned and most likely to be examples of cobalt-mediated radical polymerization (CMRP) rather than of CCTP. ... 

Figure 9 Cobalt(ll) bis(p-diketonates) used in cobalt-mediated radical polymerization.

Figure 36 General strategy for polymer release and regeneration of the Co" complex in cobalt-mediated radical polymerization of VAc (the nature of the PVAc-Co" dormant species is more likely a ligand adduct, like IX, or a chelated species, like X, depending on conditions, see Figure 10 and related discussion in Section 3.11.5.5).

Besides the most common CRP methods used to control the radical polymerization of vinylic monomers, other techniques have been developed over the time, and have been tested in miniemulsion or emulsion polymerization as well. These methods are described in the corresponding chapters of this comprehensive. They are iodine transfer polymerization (ITP and the reverse method, RITP), organotelluriirm-mediated CRP (TeRP), and cobalt-mediated radical polymerization (CoMRP). 

Four-arm stars [poly(vinyl acetate) ]4C6o have been prepared by cobalt-mediated radical polymerization of vinyl acetate (VAc) followed by addition of the PVAc-Co(acetylacetonate)2 chains to the fullerene [30]. The photoactive water-soluble [poly(vinyl alcohol) hCgo was then obtained by hydrolysis of the PVAc arms. Because... 

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