Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Cover Picture (Angew. Chem. Int. Ed. 10/1999)
The cover picture shows two views of the structure of the naturally occuring mineral hemimorphite, Zn4Si2O7(OH)2 (Zn: blue, Si: yellow, O: red, H: white). Although all known zinc silicates have very similar structures containing SiO4 and ZnO4 tetrahedra, hemimorphite is unique as it is the only zinc silicate capable of very effectively catalyzing the addition of primary alcohols to alkynes and allenes. The proposed mechanism, which is supported by ab initio calculations, has many parallels to the mechanism accepted for the action of carbonic anhydrase, a ubiquitous zinc-containing enzyme that catalyzes the addition of water to CO2. The rate-determining activation of the alcohol at the hemimorphite surface is made possible by the presence of tetrahedrally coordinated zinc and by the presence of properly oriented silanol groups as proton shuttles. More details on this interesting catalyst can be found in the contribution by J. H. Teles et al. on pages 1401 ff.