Chiral catalysis is an area of much interest given the need to synthesize specific enantiomers for pharmaceutical purposes. Although most commercial catalysts consist of homogeneous transition metal complexes, there is a clear advantage for heterogeneous catalysts if they can attain reusability and ease of handling while maintaining the specificity and activity of their homogeneous counterparts.
In this presentation, we discuss some of the strategies that we are employing to synthesize chiral catalysts, both homogeneous and supported. In particular, we report the synthesis and use of (S)-MAB-Ru, an air-stable complex, and a highly active catalyst for the hydrogenation of a,b-unsaturated carboxylic acids at 25 °C. The homogeneous hydrogenations of itaconic acid and a-acetamidocinnamic acid had conversions above 97 % in each case, and the enantiomeric excesses (e.e.) were 80 and 70 % to the R-products, respectively. When (S)-MAB-Ru was anchored to MCM-41 the conversion of both acids was complete and both e.e. were practically 97%. The solid-bound catalyst was successfully reutilized in the hydrogenation of itaconic acid and the drop in asymmetric induction was only 3%.
The anchored catalyst was characterized using 13C MAS-NMR, together with X-ray diffraction and N2 adsorption. We determined that the structure of the ruthenium complex was preserved during anchoring and that the complex was primarily located inside the pores of MCM-41. The mechanism of the asymmetric hydrogenation is discussed briefly on the light of molecular modeling studies.
In this presentation we discuss primarily the reason why anchoring the complex to MCM-41 provides such a large increase in e.e., a result that we have found occurs also with other supports and other complexes. Besides the fundamental character of these results, there is a clear implication for improving commercial processes when anchoring is properly implemented.