Oxidative activation tridentate ligands

This paper validates the assumption that ligands for asymmetric catalysis can be obtained by simple functionalization of common carbohydrates. Condensation of 2-aminoglucose with substituted-2-hydroxybenzaldehydes affords 0,N,0 -tridentate ligands whose activity in the V-catalyzed asymmetric oxidation of thioanisole is reported in Table 9.6. 

Tridentate ligands (Figure 3.1) have been reported by various groups [31-34] but the oxidation performance was moderate. One has to keep in mind that two tridentate ligands (or one hexadentate ligand) on one iron core will occupy all coordination sites, thereby hampering the reaction [32]. That means that oxidation reactions can only occur if one of the iron-ligand bonds dissociates beforehand. The catalytic activity remains low or even absent if this requirement is not met. 

Synthetic complexes modeling a-keto carboxylate-dependent enzymes have played a key role in furthering our understanding of these enzymes. Several [Fe (L)(a -keto acid)] complexes have been reported as functional models using tetradentate and tridentate ligands. All of the model complexes that react with O2 afford quantitative yields of the decarboxylated a-keto acid, but in only two cases was the active oxidant trapped. Intermolecular olefin epoxidation has been observed in the case of [Fe (Tp )(BF)j complex. This complex reacts with O2 to form a species capable of stereospecific oxidation of cA-stilbene to its oxide as the product. However trans -stilbene is not epoxidized, suggesting that the active oxidant is capable of steric discrimination. 

A tridentate ligand based on 1,8-naphthyridine (NP), which bears a ferroceneyl amide pendant arm, was synthesized and used to support dipalladium(I) and diruthenium(I) compounds [69]. The synthesis of the dipalladium(I) compound 104 started with a Pd(II) precursor, but the detail of the redox reaction was not discussed in the original article (Scheme 10.54). Complex 104 is diamagnetic and the Pd(I)-Pd(I) bond is short at 2.3952(8) A (Entry 14, Table 10.7). Compound 104 was proven to be an active catalyst for Suzuki and Heck coupling, and a bimetallic-synergy mechanism was proposed for the pivotal oxidative addition/reductive elimination steps. 

Asymmetric sulfoxidation with tridentate ligand Ti complexes and 30% H2O2 is quite efficient in terms of catalytic activity with yields of up to 96%, high selectivity to sulfoxide (90%), minimal over-oxidation to sulfone and rather low catalyst loading, usually <1% mol. Nevertheless, the enantioselectivity was not very high with a maximum ee of 60% after optimization of the chiral ligand. ... 

Chiral induction was also observed in lanthanide(III)-alkoxide-mediated MPV reductions. The optically active ligand (Fig. 35C) was used in enantioselec-tive samarium-catalyzed MPV reductions of arylmethyl ketones (Scheme 30) [253], The resulting mixed alkoxide-iodide complex shows higher reactivity than (rBuO)SmI2. It was pointed out that the tridentate, secondary alkoxide ligand is not oxidized under the reaction conditions and that tridendate ligands... 

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