Oxazoline backbone

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. 

Pd-catalyzed allylic substitutions such as the Tsuji-Trost reaction have been investigated widely, essentially in their asymmetric version [44]. This represents a valuable tool in organic synthesis since the catalyst can accommodate various functionalities on the substrate and it is possible to tune the coordination sphere through the electronic and steric effects of the ligands. Those which contain a sulfur atom are based on an oxazoline backbone, and an ee as high as 96 % has been... 

New catalyst systems that display two hydrogen bond donating arms from a rigid oxazoline backbone of type 158 were prepared and applied to activate aldehydes in hydrogen bond promoted enantioselective hetero Diels-Alder reactions (ee up to 90%) <05OL5473>. 

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 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... 

Scheme 1.27 Test reaction with axial sulfur-oxazoline ligands with biphenyl backbone.

In 2004, Ricci et al. reported the synthesis of several novel ligands based on an oxazoline-cyclopenta[Z)]thiophene backbone, as depicted in Scheme 2.15. Moderate yields combined with enantioselectivities of up to 79% ee were obtained for the 1,4-product resulting from the copper-catalysed enantioselective... 

Polyamides containing a-aminoacid units are readily obtained by reaction of bisazlactones (2-oxazolin-5-ones) with diamines. When polyamines such as diethylenetriamine (DETA) or triethylenetetramine (TETA) are used as the diamine component, the resultant polyamides readily cyclodehydrate above 200°C to produce polymers containing 2-imidazolin-5-one units in the backbone. Polyamides derived from simple diamines (e.g. 1,6-hexanedi amine) cyclodehydrate only in the presence of a suitable catalyst. Carboxylate salts and certain Lewis acids have been found to be efficient catalysts for this transformation. 

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]. 

A library of 96 possible phosphite oxazoline ligands was synthesized with systematic changes to the backbone and with special attention to the biaryl phosphite group. 

Zn(OTf) and afforded product consistent with reaction via an octahedral model, whereas l,3-bis(oxazoline) ligands B were superior to A and C for Mg(OTf)2 and Cu(OTf)2. In addition, among several 1,4-bis(oxazoline) ligands, a non-C2-symmetric bis(oxazoline) D bearing a meso backbone was also found to be highly efficient [35],... 

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... 

---