Inclusion water suspension medium

Some solid-solid reactions were shown to proceed efficiently in a water suspension medium in Sect. 2.1. When this reaction, which gives a racemic product, is combined with an enantioselective inclusion complexation with a chiral host in a water suspension medium, a unique one-pot preparative method of optically active product in a water medium can be constructed. Some such successful examples are described. 

When a mixture of methyl phenyl sulfide (69a) (1 g, 8.1 mmol), 30% H2O2 (1.84 g, 16.2 mmol), and water (10 ml) was stirred at room temperature for 24 h, rac-lOa was produced (Scheme 11). To the water suspension medium of rac-70a was added 10c (2 g, 4 mmol), and the mixture was stirred for 15 h to give a 1 1 inclusion complex of 10c with (+)-70a. Heating the filtered complex in vacuo gave (+)-70a of 57% ee (0.45 g, 82% yield). From the filtrate left after separation of the inclusion complex, (-)-70a of 54% ee (0.4 g, 73% yield) was obtained by extraction with ether. By the same procedure, optically active 70b-d were also prepared (Table 11). In the case of (+)-70b and (-)-70c,the efficiency of the enantiomeric resolution was very high. 

Optically active Diels-Alder adducts were also prepared by using a one-pot preparative method and enantioselective formation of inclusion complex with optically active hosts in a water suspension medium.68 For example, A-ethylmaleimide reacts with 2-methyl-1,3-butadiene in water to give the racemic adduct 1. Racemic 1 and the optically active host 2 form enantioselectively a 1 1 inclusion complex of 2 with (+)-l in a water suspension. The inclusion complex can be filtered and heated to release (+)-l with 94% ee (Eq. 12.23). 

Although some kinds of optically active compounds can be prepared by an asymmetric synthesis using a chiral catalyst, this method is not applicable for preparation of all kinds of compounds. Furthermore, optical yields of the product are not always very high. On the contrary, optical resolution method by inclusion complexation with a chiral host is applicable to various kinds of guest compounds as described in this chapter. When optically pure product cannot be obtained by one resolution procedure, perfect resolution can be accomplished by repeating the process, although asymmetric synthetic process cannot be repeated. Especially, optical resolutions by inclusion complexation with a chiral host in a water suspension medium and by fractional distillation in the presence of a chiral host are valuable as green and sustainable processes. 

Host-guest inclusion complexations are usually carried out in organic solvents. As a green process, inclusion complexation can be performed in a water suspension medium or in the solid state. When the solid-state reaction in a water suspension medium is combined with an enantioselective inclusion complexation in the same water medium, a one-pot green preparative method for obtaining optically active compounds can be designed. In all these cases, enantiomers separated as inclusion complexes are recovered by distillation of the inclusion complex. When enantioselective inclusion complexation in the solid state is combined with the distillation technique, a unique green process for enantiomeric separation can result. 

Enantiomeric separations of bicyclic acid anhydride 69, lactones 70 and 71 and carboximides 72 and 73 by complexation with la-c in organic solvents were also successful (Table 3.3-3) [26]. These complexations can probably be carried out in a water suspension medium and hence be described as green processes. rac-Panto-lactone (74) was separated to produce (S)-(-)-74 of 99% ee in 30% yield by complexation with Ic [27]. Enantiomerically impure monoterpenes were purified by inclusion complexation with a chiral host compound. For example, (lS,5S)-(-)-verbe-none (75a) of 78% ee gave 99% ee enantiomer by complexation with la. By similar treatment of 75b of 91% ee with la as above, (lR,5R)-(-i-)-75b of 98% ee was obtained [28]. 

In this section, one-pot preparations of optically active compounds by a combination of solid-state reaction and enantioselective inclusion complexation in a water suspension medium are described. In order to establish the suspension procedure as a general enantiomeric separation method, enantiomeric separations of various compounds by complexation in hexane and water suspension media were studied. Furthermore, by combining enantioselective inclusion complexation with a chiral host in the solid state with distillation, a fascinating enantiomeric separation method by fractional distillation was established. 

In the case of volatile racemic guest, optical resolution can be carried out by using distillation technique in the presence of a non-volatile chiral host compound. The resolution by distillation is summarized in the section of 5. In the section 5, optical resolution by inclusion crystallization in a suspension medium in hexane or water is also described. 

Bissessur and coworkers explored the inclusion of poly(2-ethylaniline) (PEA) and poly(2-propylaniline) (PPA) into GO, in addition to polyaniline [90]. The technique of intercalation differed from previously reported methods. They showed that polyaniUnes can be directly inserted into GO without the preparation of precursor phases. The polymers were first prepared from the monomers by oxidation with ammonium peroxydisulfate in acidic medium. GO, synthesized by using the Hummers method, was dispersed in deionized water with the aid of sonication. Colloidal suspensions of the polymers in NMF were then added to the aqueous GO suspensions. The reaction mixtures were then acidified and heated at 60 °C for 90 min. The intercalated phases were isolated via freezedrying. A similar process was used to intercalate polypyrrole into GO [91]. 

Pallay and Berghmans used starch as the emulsifier and the water-swellable phase. Prepolymerization of styrene-starch mixture was carried out to a conversion of approximately 30%. The viscous mixture was subsequently transferred to a water medium containing suitable suspension agents. Suspension polymerization was carried out to achieve complete conversion of styrene and the water was directly absorbed into the starch inclusions. 

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