Low-valent cobalt complex

The cobalt mediated homo Diels-Alder reaction of norbomadiene (560) with phenyl acetylene (568a), affording a phenyl substituted deltacyclene, demonstrated the potential of low-valent cobalt complexes as catalysts332. Lautens and coworkers327 extended the scope of this reaction and were able to synthesize a wide range of substituted deltacyclenes from alkynes 568 (equation 164, Table 33). The low-valent cobalt or cobalt(O) species to be used was prepared in situ by reduction of Co(acac)3 with Et2AlCl. Monosubstituted... 

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. 

Fig. 58 Cyclizations of aryl iodides catalyzed by low-valent cobalt complexes...

Improved hydrocyanation, as with catalysts discussed elsewhere in this section, follows from the use of Lewis acid promoters and excess phosphorus ligand. Low-valent cobalt complexes may be prepared by reduction of cobalt(II) chloride with zinc metal in the presence of phosphorus ligand generating in situ the requisite Lewis acid promoter, ZnClj. 

Once we became interested in determining the utility of the homo Diels-Alder (HDA) in synthesis, the search for a highly active catalyst was undertaken. In the late 1970s, Lyons showed that unactivated acetylenes react with norbomadiene in the presence of low-valent cobalt complexes to yield a mixture of cycloadducts in which the major product arises via a [2 + 2 + 2] coupling (Eq. 

Previous reports on some low-valent cobalt complexes with NBD showed that the two olefins in NBD occupy an axial and equatorial position (with bite angle -90°) in the trigonal bipyramid. For chelat-... 

A similar reaction occurs with low valent cobalt complexes with diarsines and tetra-arsines [38]. For example, 1 1 peroxo complexes can be prepared by reaction of cobalt(I) arsine complexes with molecular oxygen, equation (6) [38]. The same type of complex is formed by reaction of a Co(III) arsine complex with hydrogen peroxide [38]. Treatment of a Co(II) arsine complex with dioxygen, however, leads to the formation of a /x-peroxo species, equation (6) [38]. 

At this point it might be well to review those factors which are important in determining which type of dioxygen compound is formed from cobalt complexes in solution. Clearly low valent cobalt complexes, (Co(I)), stabilized by good 7r-acceptor... 

The oxidative addition step in this reaction is postulated to involve singleelectron transfer from a low-valent cobalt complex to the alkyl hahde with the intermediate generation of alkyl radicals. This procedure enables sequential radical cychzation-alkynylation reactions of 6-halo-1-hexene derivatives [102]. 

Interest in the synthesis and reactivity of coordinatively unsaturated low-valent metal complexes has led to the use of an o-carboranedithiolato ligand in the formation of metalladithiolene ring complexes. Recently, we69 70 and Wrackmeyer et al.1 72 have reported on the synthesis of the 16e cobalt, rhodium, and iridium... 

Low-valent cobalt pyridine complexes, electrogenerated from CoCl2 in DMF containing pyridine and associated with a sacrificial zinc anode, are also able to activate aryl halides to form arylzinc halides.223 This electrocatalytic system has also been applied to the addition of aryl bromides containing an electron-withdrawing group onto activated alkenes224 and to the synthesis of 4-phenylquinoline derivatives from phenyl halides and 4-chloroquinoline.225 Since the use of iron as anode appeared necessary, the role of iron ions in the catalytic system remains to be elucidated. 

Ruthenium compounds are widely used as catalysts for hydrogen-transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. For example, Backvall and coworkers [85] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt Schiff s base complex. The proposed mechanism is shown in Fig. 14. A low-valent ruthenium complex reacts with the alcohol to afford the aldehyde or ketone product and a ruthenium dihydride. The latter undergoes hydrogen transfer to the benzoquinone to give hydroquinone with concomitant... 

Due to the low solubility of cobalt(II) fluoride in most solvents, formation of cobalt fluoro N-donor complexes (which are the only low-valent cobalt fluorides which are reliably reported) features a variety of starting materials. A common theme that runs throughout this work has been the use of [Co(BF4)2] as the fluoride source, and the subsequent controlled decomposition to obtain a metal-bound fluoride. This has been done, for example, with tris- (3,5-dimethyl-pyrazol-l-yl)methyl amine (amtd) to give [M2(amtd)2F(BF4)3(EtOH)Y(H20)] (M = Co, Cu, Zn x = 0-1.5, y = 1-2). The cobalt complex has been structurally characterised by X-ray diffraction [Fig. 3] [57]. Similarly, the combination of [M(BF4)2] (M = Mn, Co, Ni), [M(N03)2], NH4(NCS) and 3,5-diethyl-1,2,4-triazole (detrH) produces... 

While the majority ofmetal-mediatedproteinmodifications covered in this survey involve activation of small molecules (e g. O2, H2O) by metal complexes, there are some instances in which complexation of the metal ion itself serves as the mechanism of modification. Complexes with low-valent cobalt and chromium undergo relatively rapid hgand exchange, permitting solution complexes to coordinate with exposed amino acid side chains (e g. histidine) on the surface of proteins. Oxidation of this adduct converts the complex to a substitution-inert species. The extreme inertness of Cr(III) and Co(III) complexes toward hgand-substitution... 

Low-valent cobalt [Co, Co° and Co" ] are known to complex with unconjugated dienes (e.g., 1,5-COD, NBD, etc.) and acetylenes. The Co or Co " precatalyst is presumably reduced by EtjAlCl to either Co° or Co. Co, which has the electronic configuration [Ar]3d and commonly forms five-coordinate complexes in a trigonal bipyramidal form. Thus, although very little is known about the structure of the catalytic intermediates, an intermediate with structures Ij and/or seems reasonable (Fig. 2). 

The individual steps in the cycloaddition are not known but several proposals have been put forward. Our studies build on these proposals. Following the complexation of NBD and the acetylene to the low-valent cobalt, formation of metallacycle IV is likely to occur (Scheme 5). Isolation of 5-10% of side product N (when R = TMS) provides further support for the formation of such a metallacycle, but insertion into the... 

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. 

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