2-Oxazolines ligand classes

Glos and co-workers introduced the aza-bis(oxazolines) 258 and 259 (Fig. 9.78) as a new class of chiral C2-symmetric bis(oxazoline) ligands.These catalysts were used in various reactions such as enantioselective allylic substitution and cyclopropanation it was also shown that these new catalysts could easily be tethered to a polymeric support, as shown in structure 259, allowing for facile recovery of the catalyst. There have been other examples of bis(oxazoline) ligands immobilized on solid supports and their use in catalysis.These methods have shown mixed results. 

Mechanistic aspects of the hydrogenation of alkenes catalysed by iridium complexes with a new class of chiral phosphine-oxazoline ligands have been discussed and a selectivity model to help rationalize the results obtained has been presented.344... 

A new class of phosphaferrocene-oxazoline ligands has been disclosed by Fu and co-workers30 and applied to the copper-catalyzed asymmetric conjugate addition of diethylzinc to acyclic enones with good enantioselectivity. The substitution pattern on the phosphoryl ring as well as on the oxazoline was shown to have an enormous impact on the selectivity. Ligands 26 and 27, which share the same absolute configuration in the oxazoline, provide the (5)-l,4-adduct preferentially. This observation led to the conclusion that the stereochemistry at the oxazoline, and not the planar... 

Gilbertson et al. [40] reported the new class of phosphine-oxazoline ligands 65 and 66, bearing a chiral phosphanobomadienyl and an oxazoline and their application to the cyclohexenylation of 2,3-dihydrofuran (1). 

Liu, D., Dai, Q. and Zhang, X. (2005) A new class of readily available and conformationally rigid phosphino-oxazoline ligands for asymmetric catalysis. Tetrahedron, 61, 6460-71. 

Thanks to the fundamental studies of Tsuji, Trost, and others, palladium-catalyzed allylic substitution has become a versatile, widely used process in organic synthesis [40]. The search for efficient enantioselective catalysts for this class of reactions is an important goal of current research in this field [41]. It has been shown that chiral phosphine ligands can induce substantial enantiomeric excesses in Pd-catalyzed reactions of racemic or achiral allylic substrates with nucleophiles [42]. Recently, promising results have also been obtained with chiral bidentate nitrogen ligands [43]. We have found that palladium complexes of neutral aza-semicorrin or methylene-bis(oxazoline) ligands are effective catalysts for the enantioselective allylic alkylation of l,3-diphenyl-2-propenyl acetate or related substrates with dimethyl malonate (Schemes 18 [25,30] and 19 [44]). 

The 1-t-butylphospholane sulfide intermediate to TangPhos was also used to prepare the P,N ligands 48 by reacting the lithium complex with C02 and then oxazoline formation with a range of chiral amino alcohols [69b, 74]. The Ir complexes of these ligands have been successfully used in the reduction of / -methylcinnamic esters (80-99% ee) and methylstilbene derivatives (75-95% ee), a particularly challenging class of unfunctionalized olefins [4 c]. 

Cyanogen bromide reacts with amino alcohols to give 2-aminooxazolines that condense in the presence of benzaldehyde and p-toluenesulfonic acid with loss of ammonia to give aza-bis(oxazolines) 173. Aza-bis(oxazohnes) such as 173 are emerging as a new and important class of chiral ligands for asymmetric processes with the added advantage that they can be immobilized on solid supports (Scheme 8.50). 

A new class of chiral phosphine-oxazolines act as ligands in iridium-catalysed asymmetric hydrogenation of imines, and of alkenes, giving ees up to 99%.51... 

Oxazolines are another important class of ligands with a nitrogen donor group [87]. They can be regarded as cychc imines [70]. Phosphinooxazoline ligands [38,88-92] are of course the... 

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