Triflate-functionalized calixarenes as versatile building-blocks: application to the synthesis of an inherently chiral Zn(ii) complex†
Cavity-based metal complexes can find many applications notably in the fields of catalysis and biomimicry. In this context, it was shown that metal complexes of calixarenes bearing three aza-coordinating arms at the small rim provide excellent structural models of the poly-imidazole sites found in the active site of many metallo-enzymes. All these N-donor ligands were synthesized from the 1,3,5-tris-methoxy-p-tBu-calixarene platform, which presents some limitations in terms of functionalization. Therefore, there is a need for the development of new calixarene-based building-blocks selectively protected at the small rim. Herein we describe the regioselective one step synthesis of two calixarenes decorated with triflate groups, i.e. X6H4Tf22 and X6H3Tf33, from the parent calixarene X6H61. It is shown that the triflate groups can either act as protecting or deactivating groups, allowing the elaboration of sophisticated calixarene-based systems selectively functionalized at the large and/or at the small rim. In addition, X6H3Tf33 is functionalized on the A, B, and D rings and thus gives access to inherently chiral compounds, as demonstrated by the synthesis of a rare example of inherently chiral cavity-based metal complex.