Cobaloxime complexes

Microwave-assisted Diels-Alder cycloaddition reactions using water-soluble aquo-cobaloxime complexes have been reported by Welker and coworkers [198]. Many more examples of microwave-assisted cycloaddition processes leading to heterocycles are described in Section 6.24. 

Steric and Electronic Effects in Cobaloxime Complexes of Phosphines and Phosphites... 

Ohashi et al. reported a fine example of racemic-to-chiral transformation in cobaloxime complex crystal by irradiation. [36] A racemic mixture of (1-... 

Ohashi also reported the asymmetric induction by the irradiation of chiral crystals of achiral (2-cyanoethyl)(pyrrolidine)cobaloxime complex 62. [37] In this case, the achiral cobaloxime complex crystallized in P2 2 2 space group, and the solid-state photolysis... 

Koura, T. and Ohashi, Y. (2000) Asymmetric Induction in Cobaloxime Complex Crystals Due to Chiral Crystal Environment, Tetrahedron, 56, 6769-6779. 

The development of new radical initiators is an area of active investigation. Tada et at. have described a (triphenyltin)cobaloxime complex as a reagent for radical generation from bromides [95JOC6635]. This is an alternative to reductive processes involving tin hydride and allows for the introduction of an additional functional group in the product. 

Several different types of reactions have been observed to occur in single crystals of cobaloxime complexes [120,121]. The axial ligand C(CH3)(H)(CN) shown in Scheme 26 for chiral cobaloxime 41 racemizes in a single crystal when exposed... 

Several synthetic targets have been attacked by exploitation of this methodology examples include an enantiospecific synthesis of acromelic acid A and a formal synthesis of physovenine (equations 186 and 187)362. Recent work has focused on rendering the radical generation and addition processes catalytic in cobalt successes have been achieved by using more readily reduced cobaloxime complexes and carefully controlled conditions363. 

Scheme 28 Asymmetric transformation of racemic cobaloxime complex in the chiral crystalline environment.

Scheme 29 Absolute asymmetric transformation of cobaloxime complex crystal.

Racemic-to-Chiral Transformation and the Chirality Inversion Process in Cobaloxime Complex Crystals Only by Photoirradiation... 

On the other hand, there has been no report on the latter method until we found that the racemic crystal of a cobaloxime complex showed optical rotation after the crystal was exposed to visible light [11]. This indicates that only photoirradiation produced chiral substances in a racemic crystal. 

Recently we found that the chiral alkyl group bonded to the cobalt atom in a cobaloxime complex crystal is almost (82%) inverted to the opposite configuration only by photoirradiation [12]. In this chapter we describe the processes of the racemic-to-chiral transformation and also the chirality inversion in the... 

II. CRYSTALLINE-STATE REACTION OF COBALOXIME COMPLEXES BY PHOTOIRRADIATION... 

Before the mechanism of the racemic-to-chiral transformation and inversion processes are examined, it may be better to explain the solid-state photoreaction with retention of the single crystal form, which is called a crystalline-state reaction. We found that the chiral 1-cyanoethyl group, bonded to the cobalt atom in 4 cobaloxime complex crystal, was racemized on exposure to x-rays or visible light [13]. Since the crystallinity was kept in the whole process of the racemization the intensity data were collected at any stage of the reaction, and the process of the structural change was observed by x-ray crystal structure analysis. The change of the unit cell dimensions with time, which was well explained by first-order kinetics, corresponded to the rate of racemization [14] (Scheme 1). ... 

Such crystalline-state racemizations by photoirradiation were observed in the cobaloxime complexes not only with the chiral 1-cyanoethyl group but also with the chiral l-(methoxycarbonyl)ethyl [18-20], 1-(ethoxycarbonyl)ethyl [12,21], 1,2-bis(methoxycarbonyl)ethyl [22], l,2-bis(ethoxycarbonyl)ethyl [23], and l,2-bis(allyloxycarbonyl)ethyl [24] groups as axial alkyl groups. The concept of a reaction cavity was also applicable to the racemization of the cobaloxime complexes with such bulky chiral alkyl groups. 

As shown in Fig. 2, the chiral 1-cyanoethyl group bonded to the cobalt atom was produced from the prochiral 2-cyanoethyl group in some cobaloxime complex crystals with retention of the single-crystal form [31]. Only one enantiomer was observed in the crystal structure after the photoirradiation [32]. The chiral 1-... 

For the cobaloxime complex with pyrrolidine as an axial base ligand, the crystalline state reaction was also observed when the crystal was exposed to a xenon lamp. Four kinds of crystals with different R S compositions were prepared. The crystals of pyrr-1 were obtained from the racemic solution. The pyrr-2 crystals were obtained from a solution that has the complex with an R S ratio of 75 25. The crystals of pyrr-3 and pyrr-4 were also obtained from solutions with the R S ratio of 80 20 and 90 10, respectively. The x-ray crystal analysis indicated that the four crystals are isostructural to each other. From solutions with the R S ratio greater than 9 1, pure enantiomeric crystals were obtained, which are not isostructural to the above crystals pyrr-1 to pyrr-4 [40]. 

Recently a cobaloxime complex with a racemic 1-cyanoethyl group and methyl" (S)-alaninate as axial ligands was prepared [41]. In order to resolve the diastereo-meric pair, fractional crystallization was performed several times. However, the, optical rotation of the complex did not increase after several times of crystalliza-S tion. One of the crystals suitable for x-ray work was picked up and the structure was analyzed. To our surprise, there are two crystallographically independent molecules, which are a pair of diastereomers, in a chiral unit cell. This explains why the fractional resolution of the diastereomeric pair was impossible in the usual way. f... 

In the study of the four conformational polymorphs of a cobaloxime complex, Sawada et al. (1996) were able to establish a quantitative relationship between the size of the reaction cavity for an intramolecular photoisomerization reaction and the rate constant for the process in three of the modifications. Such studies provide a great deal of direct detailed information about the relationship between the environmental influences on the mechanism of a reaction at essentially the atomic level, information which is much more difficult to obtain from studies on a single crystalline form. 

Sawada, K., Hashizume, D., Sekine, A., Uekusa, H., Kato, K., Ohashi, Y., Kakinuma, K. and Ohgo, Y. (1996). Four polymorphs of a cobaloxime complex with different solid-state photoisomerization rates. Acta Crystallogr. B, 52, 303-13. [237]... 

Danno, M., Uchida, A., Ohashi, Y., Sasada, Y., Ohgo, Y., and Baba, S. Crystalline-state reactions of cobaloxime complexes by X-ray exposure. XIV. Uneven racemization at the independent reaction sites. Acta Cryst. B43, 266-271 (1987). 

Cobaloxime complexes of cobalt, (DMG)Con) 72, give Co1—cobaloximes (DMG)CoI in the presence of amides by reaction with tertiary free radicals. This... 

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