Asymmetric bisphosphines

Much effort has been placed in the synthesis of compounds possessing a chiral center at the phosphoms atom, particularly three- and four-coordinate compounds such as tertiary phosphines, phosphine oxides, phosphonates, phosphinates, and phosphate esters (11). Some enantiomers are known to exhibit a variety of biological activities and are therefore of interest Oas agricultural chemicals, pharmaceuticals (qv), etc. Homochiral bisphosphines are commonly used in catalytic asymmetric syntheses providing good enantioselectivities (see also Nucleic acids). Excellent reviews of low coordinate (coordination numbers 1 and 2) phosphoms compounds are available (12). 

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-... 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

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 . 

Chan has discovered a completely atropdiasteroselective synthesis of a biaryl diphosphine by asymmetric intramolecular Ullmann coupling or Fe(m)-promoted oxidative coupling. A chiral atropisomeric biaryl bisphosphine ligand 2 was synthesized through this central-to-axial chirality transfer.38 Recently, a xylyl-biaryl bisphosphine ligand, Xyl-TetraPHEMP, was introduced by Moran, and is found to be effective for the Ru-catalyzed hydrogenation of aryl ketone.39... 

Mezzetti91 and Leeuwen/Widhal92 have independently reported P-chiral ferrocenyl bisphosphines 42 and 43. These two ligands have shown excellent enantioselectivity (up to 99% ee) for the asymmetric hydrogenation of... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

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. 

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. 

Kuwano, R., Uemura, T., Saitoh, M. and Ito, Y., A trans-chelating bisphosphine possessing only planar chirality and its application to catalytic asymmetric reactions. Tetrahedron Asymm.flWSA,... 

Carbohydrates remain an attractive source of chirality in preparation of ligands for asymmetric catalysis. Functionalized phospholanes, 192 [167], and chiral bisphosphinites 193 [168] with an attached crown ether unit were obtained recently from D-mannitol and from phenyl 2,3-di-0-allyl-4,6-0-benzylidene-p-D-glucopyranoside, respectively (Figure 18). Compounds 194 and 195 were obtained in the photochemical addition of H2P(CH2)3PPH2 onto the crresponding alkenes - Pd-complexes of these new bisphosphines were successfully applied as catalysts in the copolymerization of CO and... 

The chiral phosphine 31 or 32-rhodium complex catalyzed the addition of arystannanes 30 to N-sulfonylimines 29 to give diarylmethylamines 33 with high enantioselectivity (75-96% ee) [21]. The choice of the chiral monoden-tate phosphine ligand is essential for their catalytic asymmetric arylation. With chelating bisphosphine ligands the arylation was very slow. The authors hypoth-... 

The first successful example of asymmetric copolymerization of propene with carbon monoxide was reported in 1992 and used a Pd complex with a chiral electron-rich bisphosphine... 

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