Michihiro Ishimori

Synthesis of optically active cobalt(salen) type complexes and their asymmetric reactivity toward propylene oxide

Abstract An optically active Co(I)(salen) type complex, lithium N,N′-bis(salicylaldehyde)-1(R), 2(R)-1,2-trans-cyclohexanediiminatocobalt(I), was prepared by reducing the CoII complex, N,N′-bis(salicylaldehyde)-1(R),2(R)-1,2-trans-cyclohexanediiminatocobalt(II), with LiAlH4. The structure of the CoI complex was determined on the basis of the structure of the corresponding CoII complex and was confirmed by usual physicochemical methods. Furthermore, characteristics of the absorption and circular dichroism(CD) spectra of the CoI complex were compared with those of the reported structure of Na+[Co(I)(salen)]−. Highly asymmetric selectivity was found in a resolution reaction of DL-propylene oxide by use of the above optically active lithium cobalt(I) complex as a catalyst.

Asymmetric cyclizations of some chlorohydrins catalyzed by optically active cobalt (salen) type complexes

Abstract Optically active chloromethyloxirane was obtained from 1,3-dichloro-2-propanol by a process of asymmetric synthesis. The highest enantiomenc excess (e. e. ) of chloromethyloxirane that could be obtained was 67%, using Co(II) (3,5-Cl,Cl-sal)2(S-CHXDA) and K2CO3 as the catalyst and base, respectively. For purpose of comparison, asymmetric cyclizations of racemic 2,3-dichloro-1-propanol and 2-chloro-1-propanol were examined; optically active chloromethyloxirane and methyloxirane were obtained according to kinetic resolution mechanisms, although the optical purities of oxiranes formed were not so high. The mechanisms for the asymmetric reactions were investigated by circular dichroism and absorption spectroscopies. It was found that the cobalt (salen) type complex forms a new complex with alkali metal carbonate, similarly to the function of crown ether. The substrate interacts with the newly formed chiral complex, followed by cylization to give optically active oxiranes.

1H- and13 C-NMR Studies of Polymerization of 2,3-Dideuteromethyloxirane with Zinc Dialkoxide

Abstract The deuterium-decoupled 100 MHz 1H-NMR spectra of 2,3-dideuteromethyloxirane and its polymer prepared with a lattice-disordered zinc dialkoxide were analyzed with consideration of the deuterium isotope effects on proton chemical shifts. The results of 1H- and l3C-NMR studies indicated that the initiator for methyloxirane polymerization catalyzed by the diethylzinc-alcohol system was a lattice-disordered zinc dialkoxide, from the aspect of stereoregularity of the polymers obtained.

Molecular Level Elucidation of Stereospecific Polymerization of Methyloxirane Using a Well-Defined Organozinc Complex

Since Professor Giulio Natta postulated the enantiomorphic structure for active sites of Ziegler-Natta catalyst in the propylene polymerization1, it has widely been believed that the steric regulation in the isotactic propagation stage is effectuated by the chiral structure around the catalyst sites. A number of proposals concerning possible structures of the catalyst site have been presented. Most of the proposed mechanism for the steric course of polymerization were based on the chiral structure around titanium atoms which were located at the surface crystal lattice of a- or g-type of TiCl3 2. The lattice structure of the TiCl3 crystal has well been elucidated in the field of crystallography3,4,5 but isolation and characterization of a catalyst species in a well- defined form has not yet been successful in any of studies on the stereospecific polymerization of propylene in which the formation of isotactic enchainments of the monomeric units is brought about. Owing to “the third generation catalyst”, polypropylene industries are achieving dramatic innovation in terms of productivity and of energy conservation. Mechanistic elucidation of the role of the catalyst components, such as MgCl2 or ethyl benzoate, in determining reactivity and stereospecificity of “the third generation catalyst” will be one of the most important unsolved problems both in science and technology of the olefin polymerization.