Search

Diiodosamarium prepared in tetrahydropyran reduces allylic, benzylic and alkyl halides at 0 or −15°C to give organosamariums which are stable under these conditions. The reactivity of these organometallics has been explored, showing that they are very selective reagents.

Fast optimization of a chiral catalyst and chiral ligands prepared by combinatorial chemistry are reviewed. The case of a one-pot multi-substrate screening of a chiral catalyst has been also discussed.

α-Diphenylphosphino carboxaldehyde 4, prepared by asymmetric synthesis, was reacted with ethane diamine to give bisimine bisphosphine 5. Ruthenium(II) complex 6 and copper(I) complex 8 were easily formed and their electrochemical properties studied.

The structure of IrCOD((+)-diop)Cl was determined by X-ray analysis. The complex is a distorted trigonal bipyramid in which diop(isopropylidene-2,3-dihydroxy-l,4-bis(diphenyl phosphino)butane) behaves as an apical-equatorial bidentate ligand. The general features of the chelated diop are given. The origin of asymmetric induction in reductions catalysed by Rh/diop systems is discussed, in relation to the structure of IrCOD((+)diop)Cl.

Allylation of β-diketone, β-ketoesters and methine active hydrogen compounds by allyl phenyl ethers or allyl esters with palladium/phosphine catalytic systems has been studied, and a mechanism is suggested. The use of DIOP (2,3- O -isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane) as chiral phosphine ligand produces optically active allylated compounds with up to 10% e.e.

Various 2-substituted 1,3-dithianes have been oxidized to the corresponding 1-oxides by cumene hydroperoxide in presence of a titanium complex (Ti(O-i-Pr)4/(+)-DET/H2O = 1/2/1) used in stoichiometric or lesser amounts. Enantioselectivity of up to 80% and excellent diastereoselectivity were achieved by a careful control of experimental conditions. The reaction temperature was found to be an important parameter. The relative stereochemistries of the new substituted monosulfoxides were determined by NMR spectroscopy.

SmCp 2 , which is easily prepared from SmI 2 , has been screened as a reducing agent for organic chemistry. In particular, SmCp 2 promotes the pseudo-Barbier reaction between carbonyl compounds (aldehydes and ketones) and aliphatic or allylic halides more efficiently than does SmI 2 .

The catalytic activity of Wilkinson's rhodium complexes with a diphosphine is studied in the hydrogenation of styrene and α-acetamido cinnamic acid. There is a large effect of the chain linking the two diphenylphosphino groups, which can change the selectivity of the obtained catalyst.

The crystal structure of [FeCp((-)diop)I] has been determined by x-ray analysis and refined by least squares methods to R = 0.091 for 1603 independent reflections. The complex crystallises in the orthorhombic space group P212121 with a unit cell of dimensions a = 16.920 (3), b = 19.902(4), c = 10.649(6)». The conformation of the chelated (-)DIOP is analysed and compared with those occuridng in other complexes of known structures.

Syntheses of (−)-menthylcyclopentadiene (MCp) and (+)-neomenthyl cyclopentadiene (NMCp) from (−)-menthol are described. These chiral ligands have been used to prepare (η5-MCp)2TiCl2, (η5-NMCp)2, TiCl2, (η5-MCp)2ZrCl2, (η5-NMCp)2ZrCl2, (η5-Cp)(η5-MCp)TiCl2 and (η5-Cp)(η5-NMCp)TiCl2. The structure and absolute configuration of (η5-Cp)(η5-MCp)TiCl2 has been established by X-ray analysis.

Physical and chemical properties of a stable complex [Rh(COD)(+)Diop]+ClO4− are described. This complex is a very active and stereoselective catalyst in hydrogenation. An optical yield of 92% is attained in asymmetric reduction of N-acetyl-1-phenyl-1-aminopropene.

Various homoallylic alcohols and homobenzylic alcohols were prepared by the reaction between aldehydes and allylic or benzylic halides in the presence of samarium diiodide. This iodide is also a very good reagent for formation of pinacols from aldehydes or ketones. The reactions are especially fast and selective in the case of substituted benzaldehydes. The reactivities of various nitrogen functional groups (imine, oxime, nitro, azo, cyano) towards SmI2 were also examined.

A chiral rhodium complex with (+)-diop as ligand is used as a catalyst in asymmetric synthesis leading to amines and N -acyl derivatives. Two kinds of reactions are investigated: asymetric hydrogenation of enamides and hydrosilylation of imines. Good optical yields are observed in the synthesis of N -acetyl α-phnenylethylamine (e.e. 45%) and N -acetyl α-phenylpropylamine (e.e. 83%). A strong solvent effect is observed, which can reverse the absolute configuration of tN -acetyl-α-phenylethylamine obtained by reduction of its enamide precursor. N -Benzyl-α-phenyl ethylamine is prepared by hydrosilylation with 65% optical purity. An asymmetric synthesis of 1,2,3,4-tetrahydropapaverine (e.e 38%) and related compounds is described.

Molybdenum(VI)(oxo-diperoxo) complexes bearing bidentate ligands with hydroxy and carbonyl functions show asymmetric induction in the stoichiometric epoxidation of simple prochiral olefins. The influence of the ligand and alkene structure on the enantiomeric excess of the oxiranes has been investigated. The addition of chiral diols to the molybdenum(VI)(oxo-diperoxo) reagents gives high enantiomeric excesses probably because of an efficient kinetic resolution of the oxiranes formed.

Various chiral diphosphines related to diop, isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane, have been prepared. Substitution of a methyl group at the meta position of each aromatic ring increases optical yields, which can reach 90% (asymmetric synthesis of a 3-(3,4-dihydroxyphenyl)alanine derivative). Excellent results from asymmetric catalysis have also been obtained using structural analogues of diop in which the acetonide ring is placed by a carbon ring.

A chiral 1,2-diphosphine was prepared in two steps from ()-α-phellandrene. This phosphine phellanphos gives a cationic rhodium complex (phellanphos-cyclooctadienerhodium hexafluorophosphate) which catalyses asymmetric reductions. N -Acetylphenylalanine and N -acetylalanine have been prepared in 94–95% enantiomeric excess.

Terminal epoxides of the type have been selectively converted into methyl ketones by various catalysts. Some lanthanide derivatives, MnI 2 , and Co 2 (CO) 8 gave the best results. The rearrangement of internal epoxides into ketones is much slower, allowing specific transformation of terminal epoxides. The scope of the reaction and tentative mechanisms are discussed.