Control of Differential Inclusion Complexation by Seed Crystals

When a mixture of powdered 12 and an equimolar amount of 80 was kept at room temperature for 170 h or contacted with MeOH vapor at room temperature for 1 h, 79 and 81 were obtained, respectively [44], The easy formation of 81 in solution suggests a possibility of successful optical resolution of 12. By using preferential crystallization of the chiral complex by adding a seed crystal prepared from optically active 12 and 80 to a solution of 12 and 80, optical resolution of 12 was accomplished very successfully [44]. 

Direct formation of 81 from 79 by a phase transition in the solid state was also discovered. Heating of 79 at 160 °C for 5 min or contact of 79 with MeOH vapor at room temperature for 30 min gave 81 [44, 45], Microscopic observation of the phase transition by MeOH vapor is indicated in Fig. 2.2.8 [44, 45]. Only one other similar phase transition has been reported [46]. 

Control of Differential Inclusion Complexation by Seed Crystals 

Differential inclusion complexation in solution can be controlled by addition of seed crystals during the recrystallization process. It was also found that the control by seed crystals can be accomplished in the solid state (see below). This seeding experiment was carried out for inclusion complexation between 12 and ethyl ether. 

When 12 (0.2 g) was recrystallized from 3 ml and 5 ml ethyl ether, the 3 1 82 and the 1 2 inclusion complex 83 were obtained, respectively. This is the first discovery that differential inclusion complexation is controlled by a concentration of solution. When 12 (0.2 g) was recrystallized from 4 ml of ethyl ether, 82 or 83 was formed, depending on slightly different conditions. In this case, however, control of the differential formation of 82 or 83 can be accomplished completely by addition of their seed crystals. X-ray crystal structures of 82 and 83 are very different, as shown in Figs. 2.2.9 and 2.2.10, respectively. Although both complexes are labile and lose ethyl ether easily, 82 is slightly more stable than 83, probably because the ether molecule which is surrounded by three host molecules in 82 is only released with difficulty (Fig. 2.2.9) but the ether molecule which is accommodated in the channel of 83 can easily be released through the channel (Fig. 2.2.10) [47]. 

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