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Modification of chiral dimethyl tartrate through transesterification: Immobilisation on POSS and enantioselectivity reversion in Sharpless asymmetric epoxidation

García, R.A. and Van Grieken, R. and Iglesias, J. and Sherrington, D.C. and Gibson, C.L. (2010) Modification of chiral dimethyl tartrate through transesterification: Immobilisation on POSS and enantioselectivity reversion in Sharpless asymmetric epoxidation. Chirality, 22 (7). pp. 675-683. ISSN 0899-0042

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Abstract

Modification of dimethyl tartrate has been investigated through transesterification with aminoalcohols to provide reactive functionalities for the covalent bonding of chiral tartrate to polyhedral oligomeric silsesquioxanes. The transesterification of dimethyl tartrate has been widely studied by means of using different catalytic systems and reaction conditions. Through the proper selection of both, the catalytic system and the reaction conditions, it is possible to achieve the mono- or the bis-substituted tartrate derivative as sole products. All the intermediate chiral tartrate-derived ligands were successfully used in the homogeneous enantioselective epoxidation of allylic alcohols providing moderate enantiomeric excess over the products. Attached amine groups have been used to support the modified tartrate ligands onto a haloaryl-functionalized silsesquioxane moiety. This final chiral tartrate ligand displays enantioselectivity reversion in the asymmetric epoxidation of allylic alcohols with regards to the starting dimethyl tartrate ligand, having both molecules them the same chiral sign. However, the POSS-containing ligand can be easily recovered in almost quantitative yield and reused in asymmetric epoxidation reactions. In addition, recovered silsesquioxane-pendant ligand, though displaying decreasing catalytic activity in recycling epoxidation tests, showed very stable enantioselective behavior.