Polymer-supported reactions epoxide ring-openings

ARO reaction with phenols and alcohols as nucleophiles is a logical extension of HKR of epoxides to synthesize libraries of stereochemically defined ring-opened products in high optical purity. To this effect Annis and Jacobsen [69] used their polymer-supported Co(salen) complex 36 as catalyst for kinetic resolution of epoxides with phenols to give l-aiyloxy-2-alcohols in high yield, purity and ee (Scheme 17). Conducting the same reaction in the presence of tris(trifluoromethyl)methanol, a volatile, nonnucleophilic protic acid additive accelerates KR reaction with no compromise with enantioselectivity and yield. Presumably the additive helped in maintaining the Co(III) oxidation state of the catalyst. 

Chiral amino alcohols can be prepared by reaction of chiral epoxides with amines. Enantiopure (25, 3.R)-2,3-epoxy-3-phenylpropanol anchored to Merrifield resin has been used for ring-opening with secondary amines in the presence of lithium perchlorate to afford polymer-supported chiral amino alcohols 47 (Eq. 18) [56], By analogy, (2i ,35)-3-(cis-2,6-dimethylpiperidino)-3-phenyl-l,2-propanediol has been anchored to a 2-chlorotrityl chloride resin (48). Although this polymer had high catalytic activity in the enantioselective addition of diethylzinc to aldehydes, the selectivity of the corresponding monomeric catalyst was higher (97 % ee) in the same reaction. 

Besides biomimetic complexes, Jacobsen described particularly efficient bis (chromium-salen) catalyst 9 for the asymmetric ring-opening reaction of epoxides with azide (Scheme 9) [42]. The efficiency of this class of catalysts is attributed to a cooperative mechanism, both substrates being activated toward each other by their respective chromium atom. Of note, a less pronounced cooperative effect was initially demonstrated in an intermolecular manner using monomeric Cr(N3)-salen catalyst [43]. Jacobsen also showed that an analogous cooperative mechanism takes place using polymer-supported chiral Co(salen) complexes for the hydrolytic kinetic resolution of terminal epoxides [44, 45]. 

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