Chiral bisphosphine

The Gabriel synthesis represents another indirect but highly valuable approach to amines. Trost has demonstrated a method for the asymmetric ring-opening of butadiene monoepoxide by use of one equivalent of phthalimide, 7t-allylpalladium chloride dimer, and the chiral bisphosphine 22 (Scheme 7.37). The dynamic kinetic asymmetric transformation proceeded through a putative achiral intermedi-... 

Riant et al. in 2006 reported an enantioselective reductive aldol reaction of acetophenone and methyl acrylate mediated by PhSiH3 (140 mol %) and catalyzed by a complex generated in situ from [CuF(Ph3P)3]2MeOH (1-3 mol %) and a chiral bisphosphine (1-3 mol %) [57]. According to Mori s... 

The seven-membered ring containing chiral bisphosphine 121 (n = 1) was made as part of a series of bisphosphines (n = 1-6) to study the influence of ligand dihedral angles on the enantioselectivity of Ru-catalysed asymmetric hydrogenation of p-ketoesters . 

A number of chiral phosphine ligands has also been reported (Figure 4). Zhang and co-workers described binaphthalene phosphine 44 with a pyridine moiety to afford the addition product with up to 92% ee.50,50a With chiral bisphosphine 45, Imamoto et al. got only moderate enantioselectivity for the addition of cyclohexenone,... 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

Chiral Bisphosphine Ligands Based on DIOP Modifications... 

The first highly enantioselective asymmetric hydroformylation was the asymmetric hydroformylation of styrene.120 In 1991, Stille et al.121 reported the achievement of up to 96% ee using a chiral bisphosphine complex of PtCl2 as the catalyst in combination with SnCl2. However, the Pt(II)-catalyzed hydroformylation of arylethenes and some functionalized olefins has several disadvantages, such as low reaction rates, a tendency for the substrates to undergo hydrogenation, and poor branched-to-linear ratio. 

K. Inoguchi, S. Sakuraba, K. Achiwa, Design Concept for Developing Highly Efficient Chiral Bisphosphine Ligands in Rhodium-Catalysed Asymmetric Hydrogenations, Synlett, 1992,169-178. 

Hydroxy-phosphines undergo benzoylation with o-sulfobenzoic anhydride in the presence of bases (Na2C03 or BuLi) affording sulfobenzoylated phosphine products. In such a way several mono- and dihydroxy phosphines could be made soluble in water, exemplified by the chiral bisphosphines 53. It should be noted, that this general method allows the preparation of water-soluble sulfonated derivatives of acid-sensitive phosphines, such as DIOP, too, which are not accessible via direct sulfonation [56]. 

Pye and Rossen have developed a planar chiral bisphosphine ligand, [2.2]PHANE-PHOS, based on a paracyclophane backbone (Scheme 1.6) [69]. Moreover, the ortho-phenyl substituted NAPHOS ligand, Ph-o-NAPHOS, has been successfully applied for the rhodium-catalyzed hydrogenation of a-dehydroamino acid derivatives [70]. 

Figure 9.4 A selection of successful chiral bisphosphine ligands for enantioselective hydrogenation.

H2, chiral Cp2Ti(BINAP)-n-BuLi-PhSiH3 H2, cat Ir-chiral bisphosphine (enantioselective) Hj, cat [Ir(diphosphine)Hl2]2 (enantioselective) H2, Raney nickel... 

The initial study in this area employed the meso-1,3-dimethylallyl ligand.432 The chloride dimer was generated from 2-pentene, and a variety of optically active phosphine ligands were added to form the chiral bisphosphine complexes. Reaction of these allyl complexes with sodiodiethyl malonate resulted in optical yields in the range of 2-29% (equation 351). 

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