Metathesis of carbon dioxide and phenyl isocyanate catalysed by group(IV) metal alkoxides: An experimental and computational study

Abstract

The insertion reactions of zirconium(IV) n-butoxide and titanium(IV) n-butoxide with a heterocumulene like carbodiimide, carbon dioxide or phenyl isocyanate are compared. Both give an intermediate which carries out metathesis at elevated temperatures by inserting a second heterocumulene in a head-to-head fashion. The intermediate metallacycle extrudes a new heterocumulene, different from the two that have inserted leading to metathesis. As the reaction is reversible, catalytic metathesis is feasible. In stoichiometric reactions heterocumulene insertion, metathesis and metathesis cum insertion products are observed. However, catalytic amounts of the metal alkoxide primarily led to metathesis products. It is shown that zirconium alkoxides promote catalytic metathesis (isocyanates, carbon dioxide) more efficiently than the corresponding titanium alkoxide. The difference in the metathetic activity of these alkoxides has been explained by a computational study using model complexes Ti(OMe)4 (1bTi) and Zr(OMe)4 (1bZr). The computation was carried out at the B3LYP/LANL2DZ level of theory. Zirconium(IV) n-butoxide promotes heterocumulene metathesis more efficiently than titanium(IV) n-butoxide. Computational studies indicate that double insertion in a head-to-head fashion, which is a key for metathesis to occur, is thermodynamically favoured for zirconium(IV) alkoxide compared to titanium(IV) alkoxide.