posted on 2015-09-23, 00:00authored byLaura Rodríguez-García, Konrad Hungerbühler, Alfons Baiker, Fabian Meemken
In
the (S)-proline-mediated asymmetric hydrogenation
of isophorone (IP) on supported Pd catalyst, excellent enantioselectivity
is achieved, with an enantiomeric excess of up to 99%. The role
of the heterogeneous catalyst has been the subject of a controversial
debate, and the current mechanistic understanding cannot explain the
observed enantioselectivity of this catalytic system. The lack
of in situ information about the role of the heterogeneous
catalyst has prompted us to investigate the surface processes occurring
at the methanol–Pd catalyst interface using attenuated total
reflection infrared spectroscopy. Time-resolved monitoring of the
homogeneous solution and of the catalytic solid–liquid
interface coupled with catalytic data provides crucial information
on the catalytically relevant enantiodifferentiating processes.
While the condensation of IP and the corresponding chiral product
3,3,5-trimethylcyclohexanone with the chiral amine is connected
to the enantiodifferentiation, it was found that the crucial
enantioselectivity-controlling steps take place on the metal
surface, and the reaction has to be classified as heterogeneous
asymmetric hydrogenation. The presented spectroscopic and catalytic
results provide strong evidence for the existence of two competing
enantioselective processes leading to opposing enantioselection.
Depending on surface coverage of the Pd catalyst, the reaction is
controlled either by kinetic resolution ((S)-pathway)
or by chiral catalysis ((R)-pathway). Steering the
hydrogenation on the (R)-reaction pathway requires
sufficient concentration of IP-(S)-proline condensate,
as this chiral reactive intermediate becomes the most abundant surface
species, inhibiting the competing kinetic resolution. The unraveled
(R)-reaction pathway emphasizes an intriguing strategy
for inducing chirality in heterogeneous asymmetric catalysis.