Understanding the enantioselectivity of a heterogeneous catalyst: the influence of ligand loading and of silica passivation

One of the major drawbacks of heterogeneous catalysts is an inferior catalytic performance relative to their homogeneous counterparts. This is often attributed to high local concentration of the catalyst, and in certain cases to various active groups of the heterogeneous support and to its steric effects. We tested the influence of these factors in the case of a silica-grafted bis(oxazoline) catalyst used in the Diels-Alder reaction, by varying the ligand loading and the degrees of passivation of the silica. We show that, in the present case, the enantioselectivity of the catalyst is linearly correlated with the passivation of the silica, and high ligand loadings can be used without damaging the performance of the catalyst. This is, to the best of our knowledge, the first example of a correlation between silica passivation and the enantioselectivity of a heterogeneous catalyst. (c) 2005 Elsevier Ltd. All rights reserved.