Chiral oxazoline backbone

In 1999, Ikeda et al. reported a new type of sulfur-oxazoline ligands with an axis-fixed or -unfixed biphenyl backbone prepared in good yields by coupling reactions of methoxybenzene derivatives substituted with a chiral oxazoline and a sulfur-containing Grignard reagent. These ligands were subsequently evaluated for the test palladium-catalysed asymmetric allylic alkylation... 

Chiral bis(oxazolines) 51 with an oxalylic acid backbone were used for the Ru-catalyzed enantioselective epoxidation of tran5-stilbene yielding franx-l,2-diphenyloxirane in up to 69% ee [24]. The asymmetric addition of diethylzinc to several aldehydes has been examined with ferrocene-based oxazoline ligand 52 [25], resulting in optical yields from 78-93% ec. The imide 53 derived from Kemp s triacid containing a chiral oxazoline moiety was used for the asymmetric protonation of prochiral enolates [26]. Starting from racemic cyclopentanone- and cyclohexanone derivatives, the enantioenriched isomers were obtained in 77-98 % ee. 

If the ligand has a backbone chirality like in 16, computations are required to determine, whether the stereo-discriminating substituent on the oxazoline ring lies "below" or "above the ligand plane. ... 

In 1997, Uozumi and Hayashi found high enantioselective Wacker-type cycUza-tion of o-allylphenols or o-homoaUylphenols by using Pd(II) catalysts coordinated with chiral bis(oxazoline) ligands based on the 1,1 -binaphthyl backbone (Eq. 6.36)... 

In 2003, Bonini et al. reported a new synthesis of ferrocenyloxazolines based on an iodide-mediated ring expansion of A-ferrocenoyl-aziridine-2-carboxylic esters. The thus-formed ligands were successfully employed as palladium chelates for the test reaction, since they allowed the product to be formed in quantitative yields and good to high enantioselectivities (Scheme 1.69). According to the results, it seemed that the additional chiral centre present in the oxazoline backbone of these ligands did not play a major role for the asymmetric induction and the activity of the corresponding catalysts. 

Recently, carbene-oxazoline catalysts 33 and carbene-phosphine catalysts 34 (Fig. 29.19) with a chiral paracyclophane backbone have been synthesized and used to hydrogenate a variety of alkenes, with modest selectivity [41]. 

Encouraged by these successful results, Saigo and co-workers tested ligand 78 in the rhodium-catalyzed hydrosilylation of ketones.56 Indeed, asymmetric hydrosilylation of acetophenone and tetralone using 78 as a chiral source led to considerably improved enantioselectivities (94% and 89% ee, respectively) compared to reactions performed with valinol-derived phosphorous-containing oxazoline 66 (82% and 59%, respectively).59,60 The equal accessibility of the two enantiomers of the m-2-amino-3,3-dimethyl-l-indanol backbone in 78 represented an additional advantage over oxazoline 66, which is derived from an amino alcohol of the chiral pool because (5)-tetralol could easily be obtained using (-)-78 in 97% yield and 92% ee (Scheme 17.30).56... 

Inspired by the chiral phosphine/oxazoline ligands developed by Helmchen and Pfaltz [131], Crudden and coworkers, have prepared a chiral NHC-oxazoline possessing a rigid backbone (Fig. 14) [ 132 ]. The rhodium complex 74 has been used in the catalytic hydroboration of olefins and the hydrosilylation of prochiral ketones with enantiomeric excesses that did not exceed 10%. 

Several miscellaneous polymer-supported chiral phosphine ligands and oxazoline ligands are depicted in Figure 6.8 and Figure 6.9, respectively. Binaphthyl monophosphine MOP [36] was anchored onto PS-PEG resin at the 2 -, 6- and O -positions of its binaphthyl backbone (compounds 84, 85 and 86) [37]. The polymeric phos-... 

The last group of chiral NHC ligand precursors is comprised of oxazoline-substituted imidazolium salts. The first derivative of this type 63 was reported by Herrmann et al. in 1998. A slight modification of the ligand backbone and use of the oxazoline C4 atom for bridging gave the imidazolium salt 64. The direct connection of the two heterocycles led to 65. Imidazolium salts built from oxazoline heterocycles like 66 were prepared by Glorius et alP ... 

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