Diphosphinites

Scheme 2.10 Cu-catalysed 1,4-additions of ZnEt2 and AlEt3 to 2-cyclohexenone with thioether-phosphinite ligands and diphosphinite ligands bearing xylo-furanose backbone.

Luminescent Ir complexes of diphosphine and diphosphinite calix[4]arene show emission maxima at 619 nm and 597 nm, respectively, at 77 K. The emission lifetimes are perturbed by addition of Li+, Na+, and U022+. 

Another class of molecules related to the metalla-/3-diketonates are the metalla-B-diphosphinites, LnM(R2PO)2 (7-10). These anionic complexes can be protonated to give neutral enolic tautomers or coordinated to main group elements or metal ions to give polynuclear complexes. The metalla-/3-diketonate anions form the same types of complexes. 

C-Chiral diphosphinites based on cyclohexane and cyclopentane rings (31) (cf. structure 17) have been used in 1 1 rhodium complexes at 50 atm H2 in unspecified solvents (259, 260). 

Synthesis of 2,3-0-bis(diphenylphosphino)-6-0-triphenylmethylcellu-lose (51), a cellulose-supported C-chiral diphosphinite analogous to the homogeneous counterpart (41) shown in Section III,A, 1, has led to a rhodium(I) catalyst giving high stereoselectivity (77% ee) for a slow hydrogenation of 2-phenylbut-l-ene at 50 atm H2, but only a 17.5% optical yield with a-acetoamidocinnamic acid (369). 

Recently, Miethchen modified diphosphinite 97 d with a crown-ether linker in the 1,4-positions in order to study the effect on enantioselectivity in Rh-cata-lyzed asymmetric hydrogenation reactions [99]. Introduction of the crown ether in the 1,4-position of the carbohydrate allows the enantioselectivity to be tuned, based on a strong effect of the formation of cryptate species with alkali ions. 

Bidentate Diphosphinite, Phosphite-Phosphinite, Diphosphite and Phosphine-Phosphoroamidite Ligands... 

Furanoside diphosphinite ligands 10 and 11 (Fig. 11) were applied in the Ir-catalyzed asymmetric hydrogenation of several dehydroaminoacid derivatives [25]. The best enantioselectivities (ee s up to 78%) were obtained in the reduction of methyl W-acetamidoacrylate with ligand 10. These results using the lr/10-11 catalysts precursor show that enantiomeric excesses are strongly dependent on the absolute configuration of the C-3 stereocenter of the carbohydrate backbone. The best enantioselectivity were therefore obtained with ligand 10 with an... 

Fig. 11 Summary of the best results obtained in the Ir-catalyzed hydrogenation using furanoside diphosphinite ligands 10 and 11...

Later, furanoside diphosphinite ligands 12, but with C2-symmetry, provided enantioselectivities up to 70% in the asymmetric hydrogenation of several N-arylimines (Fig. 12) [26]. The electronic effect of the ligand on enantioselectivity is considerable. Results were best when electron-donating groups are present on the phenyl rings (ligand 12b). On the other hand, the use of additives was detrimental to both conversion and enantioselectivity. 

The furanoside phosphite-phosphinite ligand 13, related to the previously diphosphinite ligands 12, was also applied to the asymmetric hydrogenation of N-arylimines, increasing the enantioselectivities up to 76% (Fig. 13) [26]. Precursors based on cationic [Ir(cod)2]BF4 provided better yields and enantioselectivities than... 

An interesting asymmetric transformation is the asymmetric conjugate addition to a-acetamidoacryhc ester 30 giving phenylalanine derivative 31, which has been reported by Reetz (Scheme 3.10) [10]. The addition of phenylboronic acid 2m in the presence of a rhodium complex of l,T-binaphthol-based diphosphinite ligand 32 gave a quantitative yield of 31 with up to 11% enantiomeric excess. In this asymmetric reaction the stereochemical outcome is determined at the hydrolysis step of an oxa-7r-aUylrhodium intermediate, not at the insertion step (compare Scheme 3.7). 

The procedures (2.1.1 and 2.1.2) can be applied to the preparation of a series of (S)-Hg-BINOL based diphosphinites with varying basicities. 

Asymmetric hydrogenation of dimethyl itaconate and methyl (Z)-a-acetamido cinnamate with in situ formed rhodium(I)-diphosphinite catalyst system gave the desired products with high activity and enantioselectivity (Table 2.1). The asymmetric hydrogenation may be applied to a wide range of substrates. 

Asymmetric Hydrogenation. Asymmetric hydrogenation with good enantio-selectivity of unfunctionalized prochiral alkenes is difficult to achieve.144 145 Chiral rhodium complexes, which are excellent catalysts in the hydrogenation of activated multiple bonds (first, in the synthesis of a-amino acids by the reduction of ol-N-acylamino-a-acrylic acids), give products only with low optical yields.144 146-149 The best results ( 60% ee) were achieved in the reduction of a-ethylstyrene by a rhodium catalyst with a diphosphinite ligand.150 Metallocene complexes of titanium,151-155 zirconium,155-157 and lanthanides158 were used in recent studies to reduce the disubstituted C—C double bond with medium enantioselectivity. 

The two diphosphinites 40 and 41 obtained from D-glucose by standard procedures proved to be good asymmetric hydrogenation catalysts <2000CCL587>. 

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