Ketones iron complex catalysis

With respect to the mechanism of the iron catalysis, the activity of FeCl3 -6H20 is closely related to its ability to give dionato chelate complexes 3 with [i-dicarbonyl compounds. Without prior deprotonation - even in Bronsted acidic media - these deeply colored iron complexes are instantly formed. With this property, Fe(III) is unique among all other transition metals, which require a stoichiometric amount of base for dionato complex formation. Known for over 100 years, the significant color of the complexes has been utilized for the detection of [i-oxo esters and [i-di ketones. 

A cooperative catalysis with the non-chiral Knolker iron complex (227) and a chiral Bronsted acid (223) has also been utilised for asymmetric reductive amination of ketones. Various ketones including aromatic, heteroaromatic and aliphatic ketones (228) have been transformed to the corresponding chiral amines (229) with high enantioselectivities up to 99% ee (Scheme 61). ... 

The proper amount of bromine can be determined by a series of identical parallel experiments. Iron(III) chloride is added as an indicator to the equilibrium mixtures consisting of the ketone and the enol form of the acetoacetic ester and various amounts of bromine are added. If too much bromine has been added, the reaction mixture turns brown. If too little bromine has been added, the reaction mixture remians yellow, as the added iron(III) chloride reacts with the enol iso-B extremely fast, hut reversibly to form the yellow octahedral complex C. This way the enol tautomer is made visible. Only if the proper amount of bromine has been identified does the reaction mixture turn colorless—only for a few seconds, of course, until by HBr catalysis enough of the enol form (iso-B) has been regenerated from the ketone form (B) of the acetoacetic ester and complexed to give the yellow C. 

The identity of active catalytic species for the TH of ketones with our iron carbonyl [6.5.6]-P-N-N-P complexes was still unclear. Did the imine or imines on the ligand get reduced in situ, allowing catalysis to occur through a bifunctional outer sphere mechanism, as seen with the analogous ruthenium systems This question drove us to further investigate the mechanism of transfer hydrogenation with our first generation [6.5.6]-P-N-N-P systems. 

Recently, iron(II) complexes with chiral N2P2 and N2P4 macrocychc hgands were introduced by Mazzetti and Gao for apphcations in ATH catalysis. In 2004, Gao and co-workers reported the ATH of ketones with the catalysts in situ generated from iron(ll) complexes and chiral PNNP ligands [154], ligands that were later incorporated into the well-defined Fe-PNNP complexes by Morris et al., vide supra. The 22-membered macrocycles 174 and 175 (Fig. 55) synthesised by Gao et al. were used to in situ generate precatalysts from different iron sources (e.g. 

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