Chiral cobalt complex

It is noteworthy that, as early as 1929, Shibata and Tsuchida reported a kinetic resolution of rac-3,4-dihydroxyphenylalanine by selective oxidation of one enantiomer using a chiral cobalt complex [Co(en)3NH3Cl]Br2 as a catalyst [46,47]. Figure 12 shows a highly enantioselective addition of diisopropy-Izinc to 2-(ferf-butylethynyl)pyrimidine-5-carbaldehyde via an autocatalytic process in the presence of a chiral octahedral cobalt complex with ethylenedi-... 

A desymmetrizing reduction of a dicarbonyl has also been achieved as a route to flMfi-aldol adducts. Yamada and coworkers have shown that a chiral cobalt complex catalyzes the desymmetrization of diaryl-1,3-diketones in excellent yield and enantioselectivity, greatly favoring the anti isomer [Eq. (10.65)]. Anti selectivity is rationalized using a Felkin-Anh model ... 

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>. 

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... 

The catalytic isomerization of meso-epoxides to allylic alcohols has been achieved with chiral cobalt complexes, in particular with cobalamin (vitamin B12) [47, 48]. 

Very recently Sutherland and coworkers merged the Pd-catalyzed Overman rearrangement with RCM and ATRC (Fig. 44) [256]. Treatment of dienols 179 with trichloroacetonitrile in the presence of 10 mol% Pd(MeCN)2Cl2 afforded the non-isolated trichloroacetimidate 180, which underwent the [3,3] rearrangement to /V-dienyltrichloroacetamides 174. They were immediately subjected to the RCM/ ATRC sequence using 10 mol% of 145. When chiral cobalt complex (S)- or (R)-181 was used as the catalyst for the Overman rearrangement, indolinones 177 were isolated in 51-70% yields and 89-94% ee. 

Only a few chiral catalysts based on metals other than rhodium and ruthenium have been reported. The titanocene complexes used by Buchwald et al. [109] for the highly enantioselective hydrogenation of enamines have aheady been mentioned in Section 3.4 (cf. Fig. 32). Cobalt semicorrin complexes have proven to be efficient catalysts for the enantioselective reduction of a,P-unsaturated carboxylic esters and amides using sodium borohydride as the reducing agent [ 156, 157]. Other chiral cobalt complexes have also been studied but with less success... 

The formation of chiral silyllithium and silyl-Grignard reagents was also observed in the reactions of an optically active silyl-cobalt carbonyl complex93,94. Treatment of the chiral cobalt complex with methyllithium produced optically active silyllithium which hydrolysed to the corresponding hydrosilane (equation 26). 

The intrinsic ability of homogeneous solutions of chiral cobalt complex 43 (Fig. 9) to discriminate between the enantiomers of amino acids was established for a baseline for the imprinting effect [28]. The results are shown in Table 8. The optical purity of bound phenylalanine was 50%, while the optical puritiy for bound iV-benzylvaline recovered from 43 was found to be 88%. 

In her review, Laschat summarizes the strategies that have been employed to control stereochemistry. These strategies include diastereo-selectivity from enantiopure starting materials and enantioselectivity with chiral additives. The use of enantiopure starting materials falls into three catagories The controlling stereocenter is in the tether for an intramolecular PKR, the controlling stereocenter is in a chiral auxiliary on the alkyne or alkene, or a chiral cobalt complex controls stereochemistry. 

Chiral cobalt complexes have been used to control stereochemistry. A successful strategy described by Verdaguer uses bidentate ligands... 

On the other hand, another cooperative catalysis approach was developed by Oh and Kim with a highly diastereo- and enantioselective domino aldol-cyclisation reaction occurring between aldehydes and methyl a-isocyanoacetate. The process employed a combination of a chiral cobalt complex derived from brucine amino diol and an achiral thiourea. The reaction was applicable to a range of aliphatic, aromatic and heteroaromatic aldehydes, providing the corresponding chiral oxazolines in good yields and diastereoselectivities of up to >90% de combined with good to excellent enantioselectivities of up to 98% ee, as shown in Scheme 7.12. 

Extending the scope of combined chiral selector systems beyond CDs, one may note that this mode has been used in CE for a rather long time [53-56], The very first example of combined chiral selectors in CE seems to be the report by Fanali et al. [53] in 1989 when 15 mM L-(+)-tartaric acid buffer was used in combination with 15 mM P-CD in order to resolve the enantiomers of chiral cobalt complexes. CDs have also been combined with chiral surfactants such as cholic acids [54, 55] and synthetic micelle-forming agents [56], In recent years, several studies were published on the combination of CDs with chiral [57, 58] and achiral [51, 59-61] crown ethers. The latter studies [59-62] where the achiral crown ether cannot contribute to enantioseparations independently clearly illustrate that the simplified approach described in [12, 47] may not be universally applied to all dual chiral separation systems in CE. 

The reaction of [2+2+2] cycloaddition of acetylenes to form benzene has been known since the mid-nineteenth century. The first transition metal (nickel) complex used as an intermediate in the [2+2+2] cycloaddition reaction of alkynes was published by Reppe [1]. Pioneering work by Yamazaki considered the use of cobalt complexes to initiate the trimer-ization of diphenylacetylene to produce hexasubstituted benzenes [54]. Vollhardt used cobalt complexes to catalyze the reactions of [2+2+2] cycloaddition for obtaining natural products [55]. Since then, a variety of transition complexes of 8-10 elements like rhodium, nickel, and palladium have been found to be efficient catalysts for this reaction. However, enantioselective cycloaddition is restricted to a few examples. Mori has published data on the use of a chiral nickel catalyst for the intermolecular reaction of triynes with acetylene leading to the generation of an asymmetric carbon atom [56]. Star has published data on a chiral cobalt complex catalyzing the intramolecular cycloaddition of triynes to generate a product with helical chirality [57]. 

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