Ligands aryloxides

The alkoxides and aryloxides, particularly of yttrium have excited recent interest. This is because of their potential use in the production of electronic and ceramic materials,in particular high temperature superconductors, by the deposition of pure oxides (metallo-organic chemical vapour deposition, MOCVD). They are moisture sensitive but mostly polymeric and involatile and so attempts have been made to inhibit polymerization and produce the required volatility by using bulky alkoxide ligands. M(OR)3, R = 2,6-di-terr-butyl-4-methylphenoxide, are indeed 3-coordinate (pyramidal) monomers but still not sufficiently volatile. More success has been achieved with fluorinated alkoxides, prepared by reacting the parent alcohols with the metal tris-(bis-trimethylsilylamides) ... 

Sone, T., Iwata, M., Kasuga, N. and Komiya, S. (1991) Drmethylgod(III) aryloxides and alkoxides having a triphenylphosphrne ligand. Chemistry Letters, 20, 1949. 

Tripodal Carbene and Aryloxide Ligands for Small-Molecule Activation at Electron-Rich Uranium and Transition Metal Centers Karsten Meyer and Suzanne C. Bart... 

Fig. 16.2 The Nbvand Tav hydride complexes containing bulky aryloxide ligands (as described by Rothwell).

Arene hydrogenation catalysts based on other metals than late transition ones are less numerous. Of particular relevance are the results reported by Rothwell, who found that Nbv and Tav hydride complexes containing bulky aryloxide ligands (Fig. 16.2) are active for the homogeneous hydrogenation of arenes [30]. 

Lewis acids of chiral metal aryloxides prepared from metal reagents and optically active binaphthol derivatives have played a significant role in asymmetric synthesis and have been extensively studied.23 However, in Diels-Alder reactions, the asymmetric induction with chiral metal aryloxides is, in most cases, controlled by steric interaction between a dienophile and a chiral ligand. This kind of interaction is sometimes insufficient to provide a high level of enantioselectivity. 

Cai et al. (71) examined the use of dinuclear copper complexes as catalysts in the cyclopropanation reaction. Their ligand design, based on the success exhibited by the Aratani system, incorporates a diimine aryloxide. A comparison of the mononuclear catalyst 99 with the corresponding dinuclear catalyst 100 showed certain modest benefits conferred by the latter, Eq. 52. The authors note that these catalysts are effective at ambient temperature but isolated yields are higher at 50°C with no loss in enantioselectivity. 

In a study published concurrently with the Evans bis(oxazoline) results, Jacobsen and co-workers (82) demonstrated that diimine complexes of Cu(I) are effective catalysts for the asymmetric aziridination of cis alkenes, Eq. 66. These authors found that salen-Cu [salen = bis(salicylidene)ethylenediamine] complexes such as 88b Cu are ineffective in the aziridination reaction, in spite of the success of these ligands in oxo-transfer reactions. Alkylation of the aryloxides provided catalysts that exhibit good selectivities but no turnover. The optimal catalyst was found to involve ligands that were capable only of bidentate coordination to copper. 

Bolm et al. (130) reported the asymmetric Baeyer-Villiger reaction catalyzed by Cu(II) complexes. Aerobic oxidation of racemic cyclic ketones in the presence of pivalaldehyde effects a kinetic resolution to afford lactones in moderate enan-tioselectivity. Aryloxide oxazolines are the most effective ligands among those examined. Sterically demanding substituents ortho to the phenoxide are necessary for high yields. Several neutral bis(oxazolines) provide poor selectivities and yields in this reaction. Cycloheptanones and cyclohexanones lacking an aryl group on the a carbon do not react under these conditions. 

Aryloxide ligands, thorium, 24 770 Aryloxides, uranium complexation with, 25 436-437... 

TRIPODAL CARBENE AND ARYLOXIDE LIGANDS FOR SMALL-MOLECULE ACTIVATION AT ELECTRON-RICH URANIUM AND TRANSITION METAL CENTERS... 

Fig. 5. Tris-aryloxide triazacyclononane ligand for uranium coordination chemistry.

As with the starting complex, 4 -U, the uranium ion is displaced below the plane formed by the aryloxide ligands by 0.66 A for... 

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