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Charge-Tagged Cyclopentadienone Iron Complexes: Mechanistic Studies Using Mass Spectrometry for Catalytic Hydrogenation Reactions
Abstract
Cyclopentadienone iron complexes, also referred to as Knölker catalysts, have emerged as
promising catalysts for the reduction of ketones, aldehydes, and imines. They constitute the
iron analogues to the ruthenium based Shvo catalysts and offer the possibility to enable the
same chemistry as the ruthenium based system with use of a widely available first-row
transition metal.
While there is a consensus on the operating reaction mechanism, detailed, real-time
monitoring of the catalytic cycle has not been described in the literature. To this end, we have
investigated the reaction mechanism by mass-spectrometry. To make the cyclopentadienone
iron complex system amenable to electrospray ionization-mass spectrometry, prosthetic
charge-tags, both positive and negative ones, were introduced. These charge-tags, in
combination with conventional kinetic experiments, allowed for the observation of the catalyst
in the working state. The mass spectrometric studies led to the identification of a major catalyst
decomposition pathway that takes place in aqueous solvent. In particular, hydrolysis of
trimethylsilyl groups in the catalyst was observed, which led to dimerization, radical formation
and, ultimately, to catalysts death. Replacement of the trimethylsilyl groups by nonhydrolysable tert-butyl groups lead to a significant boost in rate and to an increase in turn-over
number from 65 to over 1000.
In addition to the mass spectrometric and kinetic studies to investigate the reaction
mechanism, an experimental study investigating the influence of the charge-tag on catalysis is
presented. It was observed that negatively charge-tagged complexes generally show slower
rates than the classical Knölker catalyst. Using a combination of NMR and kinetic
measurements, it could be experimentally shown that binding of the charge-tag to the
catalytically active site and electric field effects are not responsible for the difference in catalytic
performance. The presence of a tertiary amine in the catalyst structure was identified as a
possible source of the difference. Furthermore, it was found that negatively charge-tagged iron
catalysts form micelles in aqueous solvent. These micelles were found to enhance catalytic
performance compared to other amine containing catalysts.
Neutral, isocyanide ligand-bearing cyclopentadienone iron complexes are presented in this
work. These complexes generally show lower catalytic performance than the corresponding
carbonyl complexes. However, the isocyanides allow for the tuning of steric and electronic
properties by varying the R group on the CNR ligand, offering a new handle to tune reactivity.
Furthermore, since the isocyanides can be prepared from primary amine precursors, for which
many chiral ones are available, chiral iron complexes could be prepared. The complexes show
very modest stereoinduction. Their syntheses, characterizations and catalytic performances
will be presented and discussed. Show more
Publication status
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Organisational unit
03425 - Chen, Peter / Chen, Peter
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