Phosphine-oxazoline

A highly thermostable palladium-phosphine oxazoline catalytic system, shown in Eq. (11.37), has recently been reported to yield high ee under single-mode micro-wave irradiation [54, 55]. The use of this P,N-ligand catalytic system resulted in even higher ee than the P,P-ligand BINAP [56], The reactions were performed in acetonitrile (b.p. 81-82 °C) and superheating increased the temperatures up to 145 °C, as measured by means of a fluoroptic probe. 

Burgess and co-workers (38) recently reported a similar approach to the evaluation of a phosphine-oxazoline ligand system. In this paper, a series of phosphine-oxazoline ligands (132) were synthesized individually. Palladium complexes of... 

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... 

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... 

Naud, F., Malan, C., Spindler, F., Ruggeberg, C., Schmidt, A.T. and Blaser, H.-U. (2006) Ru-(phosphine-oxazoline) complexes as effective, industrially viable catalysts for the enantioselective hydrogenation of aryl ketones. Adv. Synth. Catal., 348, 47. 

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%. 

A plethora of ligands has been reported in the literature but the most effective ones are 1,2-amino alcohols, monotosylated diamines and selected phosphine-oxazoline ligands. The active structures of the complexes reported are half-sand-... 

The N,P phosphine-oxazoline chelate (59) is chiral, and complexes can act as homogeneous catalysts for asymmetric synthesis the Ir(l) and Pd(II) complexes promote enantioselective olefin hydrogenation and allylic substitution respectively. An N,P analog of the N,N didentate ligand 2,2 -bipyridine is (60), the soft P donor helping to stabilize low-valent metals. Further, 2,2-bipyridine derivatives such as (61) can bind metals such as Ir and Ru as N,C chelates with one pyridine nitrogen rotated to the opposite side, away from the metal ion. 

The intramolecular asymmetric Heck reaction has featured in the synthesis of complex heterocyclic compounds. Bidentate ligands, such as diphosphines (especially BINAP) and phosphine—oxazolines, have been used as a chiral ligand of the asymmetric Heck reaction. Imbos et al. demonstrated that the mono-dentate phosphoramidite 513 was an effective ligand for the asymmetric Heck reaction of the prochiral cyclohexadienone 511 (Scheme 153).227 The reaction of 511 in the presence of catalytic amounts of Pd-(OAc)2 and the chiral phosphoramidite 513 gave the 4a-methoxy-4a//-benzo[c]chromen-2(6E/)-one 512 in 71% yield with 96% ee. By contrast, the reaction of... 

Phosphine oxazoline ligands 179 were synthesised through three dilTcrent methodologies and employed in Pd-catalysed allylic substitution reactions <01JOC206>. 

Allylic substitution 1,3-diphenylpropenyl acetate plus dimethylmalonate 13 Phosphine oxazolines Pd Array of 1 ml polypropylene tubes, 430 pi, RT Chiral HPLC Pd catalyst identified, ee values up to 94% 1998 117d... 

Further studies on AHR of 2,3-dihydrofuran (286) with phenyl triflate afforded different products with high % ee depending on the ligands used. The isomer 287 with 96 % ee was obtained selectively when the phosphine—oxazoline ligand 291 was used [119]. On the other hand, the isomer (288) with 98% ee was the major product when (5)-MeO-BIPHEP (292) was used [120]. AHR of 283 with 289 using (R)-BITIANP (293, XV-7) in the presence of proton sponge afforded the coupling product 290 with 91 % ee selectively [121]. 

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). 

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