(536f) Understanding the Enhanced Enantioselectivity during Hydrogenation Using a Supported Ruthenium Mab Complex

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.