Cobalt -salen

We have seen, in the previous section (and section III.A), that cobalt (Salen) and its active derivatives normally form diamagnetic peroxo type I dioxygen adducts. However, a pyridine solution of Co(3-methoxy Salen) has been shown 137) to take up dioxygen with 1 1 (Co O2) stoichiometry it was found 137) that there was no significant IR absorption band, attributable to the 0—0 stretch, for the oxygenated complex, and this suggested that the dioxygen is symmetrically bonded in an unidentate manner,... 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Kureshy, R. I. Singh, S. Khan, N. H. Abdi, S. H. R. Ahmad, I. Bhatt, A. Jasra R. V. (2005) Improved catalytic activity of homochiral dimeric cobalt salen complex in hydrolytie kinetic resolution of terminal racemic epoxides.. Chirality, 17 590-594. 

In recent research, Shimakoshi etal. [147] synthesized a novel complex consisting of two cobalt salen moieties joined by a methylene bridge (28). 

In a fashion similar to that of cobalt salen, distinct redox chemistry can be observed for each of the two cobalt centers of (28). Coulometric studies showed that the complex can undergo a one-electron reduction as well as a one-electron oxidation of each cobalt center and that there is no interaction between those cobalt centers. Thus, the two metal centers can be used as separate catalytic sites for the reduction of halogenated organic compounds. In the same article [147] is a hst of literature citations of work done over the past 20 years by the group of Hisaeda on the catalytic behavior of electroreduced vitamin B12 derivatives. 

In the realm of hydrolytic reactions, Jacobsen has applied his work with chiral salen complexes to advantage for the kinetic resolution of racemic epoxides. For example, the cobalt salen catalyst 59 gave the chiral bromohydrin 61 in excellent ee (>99%) and good yield (74%) from the racemic bromo-epoxide 60. The higher than 50% yield, unusual for a kinetic resolution, is attributed to a bromide-induced dynamic equilibrium with the dibromo alcohol 62, which allows for conversion of unused substrate into the active enantiomer <99JA6086>. Even the recalcitrant 2,2-disubstituted epoxides e.g., 64) succumbed to smooth kinetic resolution upon treatment with... 

Addition of a quaternary ammonium salt to a cobalt-salen catalyst drastically enhances the catalytic performance for the co-polymerization of PO with GO2. Lu and Wang investigated the binary catalyst system that consisted of a cobalt-salen complex and a tetrabutylammonium salt (Table 6). This binary catalyst system is able to promote... 

Table 6 Alternating co-polymerization of terminal epoxides with CO2 catalyzed by cobalt-salen complexes...

Enantiomer-differentiating co-polymerization of terminal epoxides is achieved by chiral chromium and cobalt complexes. Jacobsen etal. reported the co-polymerization of 1-hexene oxide with GO2 by using complex 35a. The reaction proceeds with kinetic resolution at 90% conversion, the unreacted epoxide is found to be enriched in the (i )-enantiomer of 90% ee. Detailed information about the resultant polymer, however, is not described. As discussed in the previous section, chiral cobalt-salen complex 34c co-polymerizes PO and GO2 (Table 3). When 34c with /r<3 / j--(li ,2i )-diaminocyclohexane backbone is applied to the co-polymerization, (A)-PO is consumed preferentially over (i )-enantiomer with a of 2.8 to give optically active PPG (Equation (8)). In a similar manner, a binary catalyst system, 34d/Bu4NGl, preferentially consumes (A)-PO over R)-PO with = 2.8-3.5. ... 

Since both nickel(II) and copper(II)(salen) complexes have been found to form asymmetric phase-transfer catalysts, the use of other metal(salen) complexes was investigated. Cobalt(salen) complexes 42a-d provided an opportunity to probe the influence of the oxidation state of the metal on the catalytic activity of the complex [42]. Hence, each of these complexes was prepared and tested as a catalyst for the benzylation of substrate 16a, according to the conditions specified in Scheme 8.18. 

Fig. 25 Cobalt-salen complex 13 with cationic arms and proposed mechanism for prevention of cyclic formation...

To avoid the limitations of the small apertures and cavities provided by zeolites, mesopores have been created inside zeolites X, Y and DAY.[140] By dealumination of the zeolite structure, mesoporous regions that are completely surrounded by micropores were obtained, and these intrazeolitic cavities were then used as the space in which to assemble metal complexes. The preparation of a cobalt-salen-5 complex provided a catalytic material that shows improvements in the conversion,... 

A number of cyclic and sugar-derived halo acetals 273 were subjected to radical 5-exo cyclizations catalyzed by a cobalt salen catalyst 274 with NaB H4 as the stoichiometric reductant but in the presence of air (entry 11) [321, 322]. Under these conditions, bicyclic oxygenated tetrahydrofurans 275a were obtained in 50-84% yield. Diastereomeric isomers 275b were isolated as the minor components. The yields were similar to those obtained with tributyltin hydride. The oxygen concentration proved to be important, since air gave better yields... 

In the event, Mitsunobu-type displacement of the unprotected hydroxyl of 24 delivered the iodinated sugar 25, which was then elaborated into acyclic iodohexenitol 26 via ring opening using activated zinc in refluxing ethanol, followed by reduction, tosylation and iodination. The pivotal radical cyclization/oxygenation of 26 was finally carried out in the presence of a catalytic amount of cobalt(salen) complex in air. This... 

However, even a large-pore zeolite such as zeolite Y, whose structure consists of almost spherical 13 A supercages interconnected tetrahedrally through smaller apertures of 7.4 A in diameter, is limited regarding the size of guest molecules by the space available in these. Indeed, organometallic complexes as they are widely used for stereo controlled reactions in the production of fine chemical production are often too spacious to enter those pores. This was previously demonstrated for Rhodium diphosphine or Cobalt salen complexes12-13. 

We can conclude that a comparison of the respective catalytic results of these new heterogeneous catalysts and their homogeneous counterparts showed that the entrapment of the organometallic complex was achieved without considerable loss of activity and selectivity. The immobilised catalysts are reusable and do not leach. The oxidation system applies only O2 at RT instead of sodium hypochloride at 0°C. A disadvantage is the use of pivalic aldehyde for oxygen transformation via the corresponding peracid. This results in the formation of pivalic acid which has to be separated from the reaction mixture. The best results so far - 100 % conversion, 96 % selectivity and 91 % de - were achieved with the immobilised Cobalt(salen-5) complex in the epoxidation of (-)-a-pinene. 

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