2 research outputs found
Zirconium-Catalyzed Imine Hydrogenation via a Frustrated Lewis Pair Mechanism
Zirconium-based
frustrated Lewis pairs (FLPs) are active imine
hydrogenation catalysts under mild conditions. Complexes of the type
[Cp<sup>R</sup><sub>2</sub>ZrOMes]Â[BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] utilize the imine substrate itself as the Lewis base component
of the FLP. Catalyst performance is a function of ligand structure;
in general more bulky, more electron rich cyclopentadienyl derivatives
give the best results. However, Cp* derivatives are not catalytically
active, being stable after initial heterolytic cleavage of H<sub>2</sub>; this allows experimental verification of the competence of the
zirconocene–imine pair in FLP-type heterolytic H<sub>2</sub> cleavage. Enamines and protected nitriles are also hydrogenated
if an additional internal phosphine base is used
Small Molecule Activation by Intermolecular Zr(IV)-Phosphine Frustrated Lewis Pairs
We
report intermolecular transition metal frustrated Lewis pairs
(FLPs) based on zirconocene aryloxide and phosphine moieties that
exhibit a broad range of small molecule activation chemistry that
has previously been the preserve of only intramolecular pairs. Reactions
with D<sub>2</sub>, CO<sub>2</sub>, THF, and PhCCH are reported. By
contrast with previous intramolecular examples, these systems allow
facile access to a variety of steric and electronic characteristics
at the Lewis acidic and Lewis basic components, with the three-step
syntheses of 10 new intermolecular transition metal FLPs being reported.
Systematic variation to the phosphine Lewis base is used to unravel
steric considerations, with the surprising conclusion that phosphines
with relatively small Tolman steric parameters not only give highly
reactive FLPs but are often seen to have the highest selectivity for
the desired product. DOSY NMR spectroscopic studies on these systems
reveal for the first time the nature of the Lewis acid/Lewis base
interactions in transition metal FLPs of this type