Salens cobalt complex

Lee, K. Y. Kawthekar, R. B. Kim, G. J. (2007) Synthesis of chiral intermediates eatalyzed by new ehiral polymeric (salen) cobalt complexes bearing Lewis acidic metal halides., Korean Chem. Soc., 28 1553-1561. 

The Direct Enantioselective Synthesis of Diols from Olefins using Hybrid Catalysts of Chiral Salen Cobalt Complexes Immobilized on MCM-41 and Titanium-containing Mesoporous Zeolite... 

A salen-cobalt complex has been appended to the PASSflow monolith system to form catalyst 42 and used for the dynamic kinetic resolution of epibromohydrin, 43. Because 43 undergoes rapid racemization under the conditions used, all the starting materials can theoretically be converted to the desired diol 44 (Scheme 4.75). The... 

Pattenden and coworkers studied the cyclization of allyl ort/io-iodophenyl ether 293a (R=H) catalyzed by (salen)cobalt complex 294 (Fig. 70) [317], Dihydro-benzofuran 295a (R=H) was obtained in a moderate 45% yield. 

The Salen motif has been widely utilized as a ligand for transition metals. Jacobsen et al. reported that chiral salen-cobalt complex (Co-salen) could be utilized as a Lewis acid catalyst for hydrolytic kinetic optical resolution of racemic... 

Trifluorodiazomethane 280 was generated in situ from trifluoroethylamine hydrochloride 281 in the presence of FeTPP 282 [200] or chiral salen-cobalt complex 283. Chiral trifluoromethyl-substimted cyclopropane 284 was isolated in good yield with high optical purity (Scheme 1.137) [201]. 

In 2010, a cooperative dual-catalyst system was reported to promote the highly enantioselective fluoride ring opening of various meso epoxides having alkene, ester, and protected amine functionalities. The reactions were conducted with a chiral (Salen)cobalt complex, (-)-tetramisole, benzoyl fluoride as a latent source of fluoride in the presence of HFIP. The efficient catalytic enantioselective reaction is explained by the generation of a (Salen)Co(III) fluoride under the cocatalytic conditions that occurred in good yields with up to 95% ee (Scheme 44.32). Racemic terminal epoxides, such as styrene oxide, were also studied, but they almost exclusively lead to the fluorine in the primary position therefore, the fluorine atom was not introduced on a stereogenic center. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

Chiral salen-cobalt(III) complexes can also catalyze the reaction of glyoxylates with activated dienes to give the cycloaddition product in moderate yield and ee [29]. 

The principle cost determinant in typical hydrolytic or phenolic resolutions is the cobalt catalyst, despite the relatively low catalyst loadings used in most cases and the demonstrated recyclability with key substrates. From this standpoint, recently developed oligomeric (salen)Co complexes, discussed earlier in this chapter in the context of the hydrolytic desymmetrization of meso-epoxides (Scheme 7.16), offer significant advantages for kinetic resolutions of racemic terminal epoxides (Table 7.3) [29-31]. For the hydrolytic and phenolic kinetic resolutions, the oligo-... 

Many of these cobalt complexes will catalyze the reduction of organic compounds by borohydride, hydrazine, thiols, etc. Cobalt cyanide complexes will catalyze the reduction of a,j8-unsaturated acids by borohydride (105) DMG complexes the reduction of butadiene and isoprene by borohydride, but not by H2 (124) Co(II) salen, the reduction of CHCI3 and CH3CCI3 to the dichloro compounds by borohydride (116) and cyanocobalamin, the selective reduction of -CCI2- by borohydride to -CHCl- in compounds such as aldrin, isodrin, dieldrin, and endrin without... 

Another application of salen ligands is the hydrolytic kinetic resolution of epoxides (Scheme 3). For this purpose cobalt complexes are efficient, and fiu-... 

One example of outer-sphere electron transfer is the reaction between the dipotassium cycloocta-tetraene (KjCgHg) and the cobalt complex of bis(salicylidenediamine) (Co Salen) (Levitin et al. 1971). 

Cobalt complexes with square planar tetradentate ligands, including salen, cor-rin, and porphyrin types, all catalyse the reduction of alkyl bromides and iodides. Most preparative and mechanistic work with these reactions has used cobalamines, including vitamin-B,. A generalised catalytic cycle is depicted in Scheme 4.10 [219]. At potentials around -0.9 V vs. see, the parent ligated Co(lll) compound un-... 

Abstract This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. 

If a chiral cobalt complex, instead of Co(salen)2, is used, enantioselectivity during the ring-opening process is induced (at —20°C the optimal ee was 76%) <2002CC28>. The application of stabilized Horner-Wadsworth-Emmons phosphonates represents a viable alternative to ylides in the cyclopropanation reaction <2002JOC3142>. 

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. ... 

A combination of chiral cobalt-catalyst and sodium borohydride was successfully applied to the asymmetric reduction of aromatic ketones. A chiral cobalt complex 164 (5 mol%), prepared from the corresponding salen-type chiral bisketoaldimine and cobalt(II) chloride, catalyzed the reduction of dimethylchromanone 165 in the presence of sodium borohydride (1.5 equiv to ketone) in chloroform, including a small amount of ethanol at -20°C for 120 h to give alcohol 166 92% ee (S ) in 94% yield (Scheme 2.18) [94], Addition of tetrahydrofurfuryl alcohol (THFFA) to the reaction system or the use of pre-modified borohydride, NaBH2(THFFA)2, improved the catalyst activity, that is, using this protocol, the reactions of ketone 165 and... 

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