Diphosphine Catalysts Research Articles

ChemInform Abstract: Asymmetric Hydroformylation of N‐Acyl‐1‐aminoacrylic Acid Derivatives by Rhodium/Chiral Diphosphine Catalysts.

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Asymmetric hydroformylation of N-acyl 1-aminoacrylic acid derivatives by rhodium/chiral diphosphine catalysts

The asymmetric hydroformylation of N-acylaminoacrylic acid esters 1 is efficiently catalysed by HRh(CO)(PPh 3) 3 in the presence of DIOP and related diphosphines as chiral ligands. The reaction is completely regioselective and affords in high yield the more branched aldehyde with the formyl group linked to a quaternary stereocenter. Irrespective of the structure of 1, preferential insertion of carbon monoxide always occurs onto the Re face of the substrate giving the (R)-antipodes in up to 60% enantioselectivity. Due to self enrichment during crystallisation, aldehyde 2b is readily isolated in 70–75% e.e.

A highly enantioselective rhodium catalyst for the hydrogenation of aliphatic α‐keto esters

AbstractThe enantioselective hydrogenation of several α‐keto acid derivatives with rhodium diphosphine catalysts has been investigated using a random screening approach. The neutral rhodium catalyst prepared in situ from bis(2,5‐norbornadiene rhodium chloride) and NORPHOS has been found to be an excellent catalyst for preparing aliphatic α‐hydroxy esters in high optical purities. The reaction parameters for the hydrogenation of ethyl 2‐oxo‐4‐phenyl‐butyrate, an intermediate for the ACE inhibitor Benazepril, were optimized and the best optical yields obtained were 96%.

Synthesis of endo,endo-2,5-bis[(diphenylphosphino)methyl]bicyclo[2.2.1]heptane, a chelating diphosphine with a natural bite angle of 120.degree.

One of the key intermediates in rhodium-catalyzed hydroformylation reactions is a trigonal-bipyramidal rhodium diphosphine species. The two phosphines in (R{sub 3}P){sub 2}(CO){sub 2}RhH have the possibility of occupying two axial, two equatorial, or one axial and one equatorial position. Use of chelating ligands such as DIPHOS, which has a preferred bite angle (P-M-P) of about 90{degree} allows study of locked axial, equatorial diphosphine catalysts. Because of the authors' interest in hydroformylation, they have initiated a program to design and synthesize chelating diphosphines which have a constrained bite angle near 120{degree} for selective diequatorial coordination. Here they describe the synthesis and characterization of the racemic diphosphine.

ChemInform Abstract: Homogeneous Catalysis. Conversion of 4‐Pentenals to Cyclopentanones by Efficient Rhodium‐Catalyzed Hydroacylation.

A variety of complexes of the type [Rh(diphosphine)]+ have been investigated as catalysts for hydroacylation, the intramolecular cyclization of 4-pentenals to cyclopentanones. All of the complexes studied effect this conversion in weakly or noncoordinating solvents at 20 °C, but the most effective catalyst was found to be the rhodium(I) species containing diphos ((C6H5)P(CH2)2P(C6H5)2). This complex converts 4-pentenal at a remarkably fast rate of one turnover every 6 s at 20 °C in CH3NO2 and CH2Cl2 solutions. These catalysts are effective for 4-pentenals bearing mono substituents at the 2-, 3-, 4-, and 5-positions and disubstitution at the 3-position. Disubstitution at the 2-position slows the rate and effectiveness of catalysis considerably, and substrates having disubstitution at the terminal 5-position are not turned over by these catalysts. For the diphos catalyst, between 100 and 800 rapid turnovers are observed at 1 molar percent catalyst depending on the substrate. After this, the catalysis becomes sluggish because of substrate decarbonylation leading to the catalytically inactive [Rh(diphos)(CO)2]+ species. Even when the dicarbonylated species is present, catalysis continues because of substrate-induced dissociation of the carbonyl ligands. Unlike the case of hydroacylation with the [Rh(PPh3)3Cl] complex no cyclopropanes are produced with these catalysts. Double-bond migration is a competing reaction, the extent of which depends on the substrate and the diphosphine catalyst, but in general it is a minor side reaction and it is not rate-limiting.