QuinoxP* ligands

It is possible to prepare allylic boronates by a Cu-catalyzed reaction of allylic carbonates with bis(pinacolato)diboron. A chiral version of the reaction uses a QuinoxP ligand (36)/ ... 

Imamoto T, Nishimura M, Koide A, Yoshida K (2007) t-Bu-QuinoxP ligand applications in asymmetric Pd-catalyzed allylic substitution and Ru-catalyzed hydrogenation. J Org Chem 72 7413-7416... 

The electron-rich t-Bu-MiniPHOS ligand provided good yield but moderate enantioselectivity (entry 1). Much better results have been obtained with the related ligand t-Bu-BisP (entry 2) and with the QuinoxP ligands (entries 3-5). The diphosphine of entries 6 and 7, containing an unsubstituted ethynyl group, leads to excellent yields and almost perfect enantioselectivities. 

The sense of enantioselection in other reactions can be also analyzed using the conclusions made above. Thus, the structurally rigid Rh complex of (R, R)-QuinoxP ligand (147) always has the bulky tert-hutyl substituent in the upper left quadrant, and the sense of enantioselection of asymmetric hydrogenation (stereoselection in octahedral Rh(III) complexes) is consistent with that of asymmetric addition of arylboronic acids to enones (stereoselection in square planar Rh(I) complexes) (Scheme 1.35). ... 

R,R)-2,3-B1S(TERT-BUTYLMETHYLPH0SPH1N0)QUIN0XALINE (QUINOXP ) AS A LIGAND FOR RHODIUM-CATALYZED ASYMMETRIC... 

In 2009, Tanaka and co-workers achieved Rh-catalyzed highly enantioselective intermolecular hydroacylation reactions of aliphatic aldehydes 48 with 1,1-substituted acrylamides 49 by using a cationic Rh/QuinoxP complex as the catalyst (Scheme 8.24a). Unfortunately, the reaction of simple benzaldehyde with acrylamide 49a was sluggish and the enantioselectivity was moderate, but utilizing (i ,R)-Me-DuPhos as the ligand could improve both yield and enantioselectivity (Scheme 8.24b). When cyclopentene-substituted amide 49b was subjected to the standard reaction conditions, a thermodynamically stable hydroacylation product 50b was generated with excellent diastereoselectivity (>99 1 dr) and enantioselectivity (97% ee), although dramatically reduced reactivity was observed (5% yield) (Scheme 8.24c). This report represents the first example of an asymmetric hydroacylation reaction of a trisubstituted alkene. 

The phosphide borane anions are also reactive towards haloarenes, giving substitution products via SNAr. This fact was used by Imamoto and co-workers to prepare two ligands known as QuinoxP (Scheme 5.23). 

This is probably due to the significant electron-withdrawing character of the quinoxaline backbone, which reduces the electron density at the phosphorus atoms. Indeed, t-Bu-QuinoxP is nowadays one of the very few commercially available P-stereogenic ligands. ... 

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