Bis(a-camphorquinone dioximato)cobalt

[67770-21-2]  · C20H32CoN4O5  · Bis(a-camphorquinone dioximato)cobalt  · (MW 467.43)

(catalyst for asymmetric cyclopropanation, especially for styrene, butadiene, and conjugated alkenes such as acrylates)

Physical Data: mp 240 °C (under nitrogen).

Solubility: sol hydrocarbons or similar organic solvents.

Form Supplied in: ligand may be available but the catalyst is sensitive to air and must be prepared just before use.

Analysis of Reagent Purity: a change in color from brown to pale brown or yellow indicates decomposition. Only the soluble part should be used.

Preparative Method: Co(a-cqd)2 is prepared from Cobalt(II) Chloride hexahydrate and the corresponding isomer of camphorquinone dioxime (cqd) (from optically active natural camphor) in ethanol with addition of an aqueous solution of NaOH under nitrogen or argon.1 This complex is best when freshly prepared before use under nitrogen or preferably under argon. The starting material, L-camphor, is easily obtained in optically pure form.

Handling, Storage, and Precautions: the dry solid must be kept under nitrogen or argon, preferably in the cold. The solution of the catalyst in organic solvents such as benzene, acetone, ethyl acetate, hexanes, or acetophenone deteriorates on standing even under nitrogen. Therefore use just after preparation is recommended.

Although many chiral cyclopropanation catalysts are known, this class of complexes is superior for the alkenes containing vinyl, phenyl, or alkoxycarbonyl groups. Some relevant examples are shown in eqs 1-5. In eq 5, the enantiomeric excess of the product is not known due to the absence of enantiomerically pure isomer. The absolute configuration is not known.

The related vicinal dioximatocobalt(II) complexes such as Co(dmg)2 and Co(nqd)2 (dmg = dimethylglyoximato, nqd = nopinoquinone dioximato), are also catalytically active but the enantioselectivity varies with the structure of the alkenes.

The steric bulk of the ester alkyl group generally enhances the ee values (eqs 6-8). Thus, the neopentyl ester of diazoacetate gives the highest ee value (88%) for the reaction with styrene (eq 6).

Other than diazoacetates, diazoacetophenone and diazodicyanomethane may also be used for cyclopropanation. The ee values are, however, lower than those obtained for diazoacetates.

Various organic solvents can be used, e.g. benzene, toluene, hexanes, acetone, acetophenone, diethyl ether. However, the substrate must be in large excess to the diazo compounds. The effect of additives has been examined. Pyridine or similar donor molecules retard the catalytic rates and decrease the optical yields.3

The enantioselectivity increases upon decreasing temperature. Thus the reaction between 0 and about -30 °C gives the best enantioselectivity in neat alkene.

Related chiral dioximato ligands have also been prepared. Isomeric nopinoquinone dioximato ligands (b- and d-nqd) are prepared from 1-pinene.4 The d-isomer has been found to work to give an enantiomerically opposite isomer relative to the isomer obtainable with cqd by the cyclopropanation.

The enantiomeric purity of the chiral cyclopropanes may be enhanced by recrystallization of the acid obtained after mild alkaline hydrolysis of the chiral ester when the ee values are over 60%.

Related Reagents.

(S,S)-2,2-(Dimethylmethylene)bis(4-t-butyl-2-oxazoline); Bis(dimethylglyoximato)(methyl)(pyridine)cobalt(III); Copper(II) Trifluoromethanesulfonate; (1S,9S)-1,9-Bis{[(t-butyl)dimethylsilyloxy]methyl}-5-cyanosemicorrin; (S)-2-[2-(Diphenylphosphino)phenyl]-4-phenyloxazoline; Ethyl Diazoacetate.

1. Nakamura, A.; Konishi, A.; Tatsuno, Y.; Otsuka, S. JACS 1978, 100, 3443.
2. Nakamura, A.; Konishi, A.; Tsujitani, R.; Kudo, M.; Otsuka, S. JACS 1978, 100, 3449.
3. Nakamura, A. PAC 1978, 50, 37.
4. Tatsuno, Y.; Konishi, A.; Nakamura, A.; Otsuka, S. CC 1974, 588.
5. Nakamura, A.; Konishi, A.; Otsuka, S. JCS(D) 1979, 488.

Akira Nakamura

Osaka University, Japan

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