Operando spectroscopy shows a transition from dehydrogenation to hydrogen transfer during the reaction, and allows measuring optimal conditions for maximum rate and efficiency.
Noyori–Ikariya
type [(arene)RuCl(TsDPEN)] (TsDPEN, sulfonated
diphenyl ethylenediamine) complexes are widely used C=O and
C=N reduction catalysts that produce chiral alcohols and amines
via a key ruthenium–hydride intermediate that determines the
stereochemistry of the product. Whereas many details about the interactions
of the pro-chiral substrate with the hydride complex and the nature
of the hydrogen transfer from the latter to the former have been investigated
over the past 25 years, the role of the stereochemical configuration
at the stereogenic ruthenium center in the catalysis has not been
elucidated so far. Using
operando
FlowNMR spectroscopy
and nuclear Overhauser effect spectroscopy, we show the existence
of two diastereomeric hydride complexes under reaction conditions,
assign their absolute configurations in solution, and monitor their
interconversion during transfer hydrogenation catalysis. Configurational
analysis and multifunctional density functional theory (DFT) calculations
show the λ-(
R
,
R
)
S
Ru
configured [(mesitylene)RuH(TsDPEN)] complex to be
both thermodynamically and kinetically favored over its λ-(
R
,
R
)
R
Ru
isomer
with the opposite configuration at the metal. Computational analysis
of both diastereomeric catalytic manifolds show the major λ-(
R
,
R
)
S
Ru
configured
[(mesitylene)RuH(TsDPEN)] complex to dominate asymmetric ketone reduction
catalysis with the minor λ-(
R
,
R
)
R
Ru
[(mesitylene)RuH(TsDPEN)] stereoisomer
being both less active and less enantioselective. These findings also
hold true for a tethered catalyst derivative with a propyl linker
between the arene and TsDPEN ligands and thus show enantioselective
transfer hydrogenation catalysis with Noyori–Ikariya complexes
to proceed via a lock-and-key mechanism.
Chemical solution-phase equilibria such as acid/base reactions and complex formation are typically investigated by titration studies that either use in situ analysis of a continuously changing sample with techniques that...
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