Transamination and transcarboxylation reactions of tetranuclear and dinuclear zinc carbamates.

摘要

There has been great interest in metal carbamate complexes for their catalytic properties in enzymes and for their involvement in polymer synthesis. Based on this interest, two sets of zinc carbamates have been synthesized for the purpose of studying their reactivity as it relates to carbamate chemistry. The first series of complexes was tetranuclear zinc(II) carbamates of the form (Zn₄O)L₆ with a pseudo-octahedral arrangement of ligands surrounding a zinc tetramer core (L = diethylcarbamate, piperidinecarbamate or pyrrolidinecarbamate). The second set of complexes synthesized was dinuclear zinc(II) carbamate complexes which consist of two zinc(II) ions bridged by three carbamate ligands, with an unmodified amine and a chloride ion completing the coordination tetrahedra of zinc(II) ions. These complexes were also formed with diethyl-carbamate, piperidinecarbamate and pyrrolidinecarbamate ligands. Organic carbamates are typically stable against nucleophilic attack except under harsh conditions. However, these zinc carbamate complexes were shown to exchange amino groups via transamination at room temperature and pressure. Additionally, transamination between these complexes is very quick, taking less than three minutes to reach completion. In a related reaction, these complexes were shown to exchange carbon dioxide (transcarboxylation). This latter reaction is as facile as the transamination reaction.; The novel reactivity of these complexes makes them useful models for the study of bonding and reactivity in carbamate-dependent biological systems and CO₂-dependent polymer synthesis. For example, the biological function of biotin is the transfer of CO₂ between substrates via a carbamate intermediate, N1-carboxybiotin which is analogous to the transamination of inorganic zinc carbamates. In at least three other enzymes, metal carbamates play a key role in the enzyme active sites. CO ₂-dependent polymer synthesis involves formation of carbamate and carbonate intermediates by insertion of CO₂ into M-X bonds (X = N, O). It is hoped that discoveries made by this study of simple zinc carbamate complexes will aid in developing better catalysts and safer, less toxic polymer synthesis. The results from the synthesis, mechanism and kinetics on zinc carbamates is related to carbomate-based enzymes and CO₂-dependent polymer synthesis.