Odile Samuel

Asymmetric oxidation of sulfides mediated by chiral titanium complexes : mechanistic and synthetic aspects

Abstract Asymmetric oxidation of sulfides by hydroperoxides mediated by chiral titanium complexes was further developed.The effect of water was discussed as well as steric and electronic factors of aryl groups in aryl-S-alkyl oxidation. When aryl is replaced by 1-alkyne, the sulfoxide is still obtained with high ee. The enantioselectivity of oxidation is enhanced by using cumene hydroperoxide instead of t-butyl hydroperoxide (up to 96%). The asymmetric oxidation was also run under catalytic conditions (with respect to the titanium complex) and beneficial effects were observed in the presence of molecular sieves in the reaction medium.

A convenient method for the preparation of monolithioferrocene

Abstract Monolithioferrocene has been prepared in 70% yield from the reaction of ferrocene with t -BuLi in THF, and some examples of its almost quantitative reactions are described.

A New Class of Ferrocene-Based l,2-Bis(phosphanes) Possessing only Planar Chirality

Chiral 1,2-bis(phosphanes) 13, devoid of individual chiral centers, have been prepared in three steps from chiral sulfoxide 10. Their corresponding rhodium complexes were used as catalysts for asymmetric hydrogenation, giving high ee values (⩽ 95%) in the reduction of itaconic acid or its ester. A cationic rhodium complex involving coordination of two molecules of 13b and one molecule of oxygen has been isolated and its crystal structure established.

Synthesis of Some Ferrocene-Based 1,3-Bis(phosphanes) with Planar Chirality as the Sole Source of Chirality

Several enantiopure diphosphanes 21 were synthesized in three steps from a common precursor 19. The cationic rhodium complexes 22 were screened as catalysts for asymmetric hydrogenation of various alkenes. Enantioselectivities up to 95−98% have been observed in some cases. Enantioselectivities are modest in allylic substitution catalyzed by the corresponding palladium complexes.

TOWARDS NEW FERROCENYL LIGANDS FOR ASYMMETRIC CATALYSIS

Some new approaches for asymmetric synthesis of chiral femenyl ligands are described. Many types of ferrocene compounds with planar chirality were prepared through diastereoselective ortholithiation, thanks to a chiral sulfoxide or chiral aced auxiliary. Various mono- and diphosphines may be subsequently generated. In the sulfoxide route, a key intermediate is pure monolithioferrocene. A convenient procedure was set up, involving the isolation of stable tri-n-butylstannyl-femene as a precursor of monolithioferrocene. The species FczPH (Fc = ferrocenyl) and its borane protected derivative allowed to synthesize chiral diphosphines where the usual PPh2 groups are replaced by the PFc2 fragment.

Asymmetric oxidation of some 1,3-dithianes in presence of chiral titanium complexes

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.

Asymmetric additions of α-sulfoxide carbanion on imines

Abstract The formation of the anion of methyl p-tolyl sulfoxide was studied in various experimental conditions. This allowed to get a very good control of diastereoselectivity during addition of the above carbanion on many imines. Various stereoisomerically pure aminosulfoxides have been prepared by this approach.

Highly enantioselective oxidation of ferrocenyl sulfides

Abstract Oxidation of aryl or alkyl ferrocenyl sulfides by cumene hydroperoxide was investigated in presence of chiral titanium complexes. It was found that the preparation of the combination Ti(O i Pr) 4 / DET /H 2 O = 1:2:1 is very sensitive to time and temperature. In suitable conditions some aryl ferrocenyl sulfoxides almost enantiomerically pure ( > 99% ee) could be produced.

Synthesis of phellanphos, an efficient chiral 1,2-diphosphine for asymmetric catalysis

Abstract 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.

New Approaches for Asymmetric Synthesis of Sulfoxides

Abstract Recent progress in asymmetric oxidation of sulfides is passed in review, and the various possibilities of using chiral sulfites for asymmetric synthesis of sulfoxides are discussed