Oxazoline based ligands

The success of bis(oxazolines) (43) in the copper-catalyzed cyclopropanation reaction has prompted numerous researchers to modify these structures in an attempt to improve the catalysts. To date, none have approached the success and generality exhibited by /erf-butyl bis(oxazoline) (55c) although some afford improved selectivities in specific cases. As a corollary to its success in this reaction, the copper-catalyzed cyclopropanation has taken on the aspects of a testing ground for new bis(oxazoline)-based ligands. The plethora of publications in this area will be summarily condensed in the rest of this section, and emphasis will be placed only on those ligands that provide improvements over 55c and those publications that deal with structural or mechanistic advances. 

Asymmetric hydrosilylation of ketones has developed enormously since these early reports and probably there are few catalytic reactions for which the variety in ligand structure is so immense as for this reaction. Numerous reports have been published and in general oxazoline-based ligand systems seem to give the highest enantioselectivities. In the following we will mention a few... 

The copper-catalyzed addition of butyl Grignard reagent to cyclohexenone was reported by using selenium-oxazoline-based ligand 35 which gave butylcy-clohexanone 36 in 62% ee (Scheme 4) [24]. 

Serrano and co-workers have reported the synthesis of two interesting series of chiral, hexacatenar metal complexes, that is, mono- and dinuclear, derived from chiral oxazoline-based ligands.None of the pure dinuclear compounds (136 X = OAc, Cl) was mesomorphic and most were room-temperature oils or glassy materials. This is likely due to the sterically demanding central chiral unit preventing molecular stacking and, hence, mesophase formation. 

In conclusion, many chiral pyridine-based ligands have been prepared from the chiral pool and have been successfully tested as ligands for the copper- or rhodium-catalyzed cyclopropanation of olefins. Alfhough efficient systems have been described, sometimes leading interestingly to the major cis isomer, the enantioselectivities usually remained lower than those obtained with the copper-bis(oxazoline) system. 

In contrast with salen ligands, ionic liquids were used earlier than fluorinated solvents for biphasic liquid systems with bis(oxazoline)-based complexes. In... 

The relationship between selectivity vs. ligand topography was determined for some ligands based on the x-ray crystal structure of the corresponding iridium complexes." " " The stmctures of the respective oxazoline-carbene ligands 599a-d are shown in Figure 8.24." ... 

Qiiial oxazoline ligands c ntinue to be among those most widely used for asymmetric catalysis. The phosphines 61 ate superior to other oxazoline based phosphines for palladium catalysed as rmmetric allylations <990L1745>. New naphthyl substituted oxazoline ligands include the compounds 62 <99JOC1620> and 63 <98T15721>. ... 

The chiral nitrogen-phosphine ligands represent the most flourishing bitopic auxiliaries. They can be divided into five classes closely related to their skeletons, which are binaphtyl-, pyridine-, metallocene-, amine-, or oxazoline-based. Much attention has been given to the last class, which has led to a spectacular... 

The extraction concept is also applicable to sophisticated syntheses of fine-chemicals as recently shown by Ohe, Uemura and co-workers [35], They prepared a novel amphiphilic phosphinite-oxazoline chiral ligand based on D-glucosamine. The corresponding palladium complex was an efficient catalyst for asymmetric allylic substitution reactions and could be recycled by simple acid/base extraction and reused in the second reaction without loss of enantioselectivity. 

Hemilability of hybrid ligands and the coordination chemistry of oxazoline-based systems 01AG(E)680. 

Complete reversal of enantioselection using oxazoline-containing Schiff base ligand 07Y969. 

Alkyl/alkyl ketones are challenging substrates for rhodium-catalysed asymmetric hydrosilylation and are generally reduced with low enantioselectivities using oxazoline-based hgands. However a number of alternative ligand systems have been used successfidly in the rhodium-catalysed hydrosilylation of alkyl/alkyl ketones. For example, the 8-keto ester (3.158) undergoes enantioselective... 

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