S-Chiraphos

The solid (12.8 g) is placed in a 250-mL, two-necked, round-bottomed flask containing a Teflon-coated magnetic stirring bar. One neck is capped with a rubber septum and the other is equipped with a reflux condenser with a gas inlet adapter connected to an argon flow line and an exit-gas bubbler. 

1 The checkers suggested that cooling to — 25°C was unnecessary. The authors have also cooled the solution to 0°C with similar results. 

Bastos, Tetrahedron, 45, 6901 (1989) and references cited therein. 

SUBMITTED BY DOMINIQUE MATT, MICHAEL HUHN, and PIERRE BRAUNSTEIN  

---

Recently, a series of chiral diphosphines (S. -Me-Duphos, (S. -chiraphos, (R,R)-diop and (+)-Norphos were grafted after an ionic exchange onto Al-MCM-41 134 complexes of the form [Rh(cod)(diphosphine)]+ were tested for the hydrogenation of dimethylitaconate. The supported complex with (S,S)-methyl-Duphos reached an activity for the formation of dimethyl ( -methyl-succinate as high as TON = 4000 with an ee close to 92%. Both (R,R)-diop and (,S S )-chiraphos give lower enantioselectivities (ee = 34% and 47% respectively). With (+)-Norphos, dimethyl-([Pg.457]

Figure 3.43. Scope of Rh/(S,S)-chiraphos-catalyzed asymmetric 1,4-addition of aryl-boronic acids to a,P-unsaturated 2-p5ridyl sulfones.

A resolution of racemic CHIRAPHOS ligand has been achieved using a chiral iridium amide complex (Scheme 8.3). The chiral iridium complex (- -)-l reacts selectively with (S.S -CHIRAPHOS to form the inactive iridium complex 2. The remaining (R,R)-CHIRAPHOS affords the catalytically active chiral rhodium complex 3. The system catalyzes asymmetric hydrogenation to give the (5)-product with 87% ee. The opposite enantiomer (—)-l gives the (R)-product with 89.5% ee, which is almost the same level of enantioselectivity obtained by using optically pure (5,5)-CHlRAPHOS. 

Figure 1.29 EXSYspectrum of [Rh(S,S-chiraphos)(mac)]+ in CD3OD with (a) a mixing time of 200 ms in the presence of mac, (b) a mixing time of 40 ms and a defiency of mac.

SCHEME 8. Reactivity of diastereomeric substrate-Rh complexes and ciystalline structure of the major [Rh((5,S)-chiraphos)(EAC)]+Cl04 complex. [A. S. C. Chan, J. J. Pluth, and I. Halpem, J. Am. Chem. Soc., 102, 5952 (1980). Reproduced by permission of the American Chemical Society.]... 

Racemic diphosphines may be resolved by using transition metal complexes that contain optically active olefinic substrates (Scheme 11) (24). When racemic CHIRAPHOS is mixed with an enantiomerically pure Ir(I) complex that has two ( —)-menthyl (Z)-a-(acetam-ido)cinnamate ligands, (S,5)-CHIRAPHOS forms the Ir complex selectively and leaves the R,R enantiomer uncomplexed in solution. Addition of 0.8 equiv of [Rh(norbomadiene)2]BF4 forms a catalyst system for the enantioselective hydrogenation of methyl (Z)-a-(acetamido)cinnamate to produce the S amino ester with 87% ee. Use of the enantiomerically pure CHIRAPHOS-Rh complex produces the hydrogenation product in 90% ee. These data indicate that, in the solution containing both (S,S)-CHIRAPHOS-Ir and (/ ,/ )-CHIRAPHOS-Rh complexes, hydrogenation is catalyzed by the Rh complex only. 

A combined system formed from Co(acac)3, 4 equiv of diethylalu-minum chloride, and chiral diphosphines such as (S,S)-CHIRAPHOS or (/ )-PROPHOS catalyzes homo-Diels-Alder reaction of norbomadiene and terminal acetylenes to give the adducts in reasonable ee (Scheme 109). Use of NORPHOS in the reaction of phenylacetylene affords the cycloadduct in 98.4% ee (268). It has been postulated that the structure of the active metal species involves noibomadiene, acetylene, and the chelating phosphine. The catalyzed cycloaddition may proceed by a metallacycle mechanism (269) rather than via simple [2+2 + 2] pericyclic transition state. 

---