Two-Step Racemization of the 1,2-Bis methoxycarbonyl ethyl Group

The cobaloxime complexes with a bulky / -l,2-bis(methoxycarbonyl)ethyl group, (/ -bmce) group, were prepared to examine whether the CTystalline-state racemization would occur or not, because the powdered samples of the complexes were found to 

respectively, and only one molecule in the asymmetric unit, showed no racemization with retention of the single crystal form [4-6]. The crystal with diphenylmeth-ylphosphine (dpmp, 4) was racemized at 298 K [6], and the crystal with pyridine (py, 5) was racemized only at 358 K [4]. Those crystals have two crystallographically independent molecules, A and B, in the asymmetric unit of the P2i cell. 

When the dpmp crystal was irradiated with the xenon lamp at room temperature, the complex crystal was racemized without degradation of the crystallinity. After about 10 h exposure, the change of the cell dimensions became within experimental error. The change of the a axis length is shown in Fig. 4.14. Although the change of 

The reaction cavities for the A and B bmce groups were drawn and the cavity volumes were calculated to be 42.1 and 56.8 A respectively. It is clear that the B bmce group with larger cavity is inverted to the opposite configuration. This is also the same as that observed in the cobaloxime complexes with the chiral 1-ce and 1-mce groups. 

The conformational and configurational changes of the B bmce group are shown in Fig. 4.17. The configuration of the chiral carbon atom is completely inverted to the opposite configuration. Moreover, the conformation around the perpendicular methoxyl group rotates by about 100° anticlockwise around the C-C bond [7]. 

---