Abstract: A study on the influence of acidic and non-acidic promoters for Pd-catalyzed methoxycarbonylation of vinyl acetate with triphenylphosphine as ligand was conducted in order to find an efficient pathway for the synthesis of lactic acid. Some new promoters/catalytical systems were successfully tested. Aluminium triflate is the most active promoter.
Abstract: The epimerization of chiral lactides was studied in the presence of various homogeneous and heterogeneous bases. Some solvent/base systems were found to promote epimerization at room temperature efficiently. Side reactions such as polymerization were not observed or occurred only slowly. This new protocol offers the opportunity of the transformation of meso-lactide into rac-lactide. Therefore it can reduce the overall manufacturing costs of polylactides, a problem which currently hampers the broad application of those environmentally friendly polymers in a large scale.
Abstract: Oxidative coupling of 3-(3-tert-butyl-4-hydroxyphenyl)propionic acid methyl ester (2) gave dimethyl 3,3`-(5,5`-di-tert-butyl-6,6´-dihydroxybiphenyl-3,3´-diyl)-dipropionate (1c), which upon phosphorylation/transesterification with a phosphochloridite derived from (R)-binaphthol, formed the new unsymmetrical binaphthol-bridged diphosphite 4. A rhodium catalyst based on 4 as ligand gave predominantly iso-selectivity in the hydroformylation of selected styrenes but opposite regioselectivity with 2,6-disubstituted derivatives. New chelate metal complexes (acac)RhL, PdCl¬2L and PtCl2L have been synthesized by reacting 4 with (acac)Rh(CO)2, PdCl2(MeCN)2 and PtCl2(COD), respectively. The structure of obtained compounds is determined based on 1H, 13C, 31P and 195Pt NMR spectroscopy and mass spectrometry data.
Abstract: A new and facile method for the synthesis of 6H-dinaphtho[c,e][1,2]oxaphosphinines starting from dinaphthol (BINOL) is described. The ring-opening of an intermediary dinaphtho[2,1-b;1,2-d]furan proceeds with extremely high diastereoselectivity and forms the thermodynamically most stable product. The stereochemistry was elucidated by 31P NMR spectroscopy and X-ray structural analysis. Epimerization at the stereogenic P-centre did not take place. DFT calculations were performed to determine the dihedral angles of several dinaphtho[c,e][1,2]oxaphosphinines and to explain the observed loss of stereochemistry during the total synthesis from the starting enantiopure BINOL. The synthetic potential of 6-chloro-6H-dinaphtho[c,e][1,2]oxaphosphinine was corroborated in its reactions with phenols to afford the corresponding phosphonites. These were successfully applied as ligands in the Rh-catalysed hydroformylation of three terminal olefins.
Abstract: Three techniques for the resolution of 4-formyl[2.2]paracyclophane were examined with the separation of the diastereomeric derivatives with (R)- and (S)-2-hydrazino-2-oxo-N-(1-phenylethyl)acetamide followed by their hydrolysis being found to be the most efficient, allowing (Rp)- and (Sp)-4-formyl[2.2]paracyclophanes to be obtained with 99.5% and 98.7% ee, respectively.
Abstract: For the first time the hydrogen bond based structure of self-aggregated Rh-phosphine complexes in fluorinated alcohols was directly determined which gives a rationale for the high enantioselectivity observed in the asymmetric hydrogenation.
Abstract: The use of fluorinated alcohols as solvents, cosolvents or additives in homogeneous catalysis is reviewed. The effect of these particular compounds on efficiency, regioselectivity and stereoselectivity of metal-catalyzed reactions, as well as on reactions mediated by organocatalysts, is described. Where available in several cases, the effect of fluorous alcohols on the catalytic mechanism is analyzed in detail and rationalized by their unique physical and chemical properties. In most instances, a comparison to the effect of commonly employed solvents is elaborated.
Abstract: The synthesis of the aliphatic subunit 9 of the macrolide LL-Z1640-2 (I), starting from a 4-deoxy-D-mannose derivative 2a, is described. The procedure includes the first successful application of a Vasella ring opening reaction for a 4-deoxypyranoside. Nucleophilic addition of an alkynyllithium reagent to the aldehyde 4 led to the propargylic alcohol 7, which was converted to the advanced building block 9 in two further steps.
Abstract: We herein describe the beneficial effect of fluorinated alcohols on asymmetric hydrogenation using chiral self-assembling rhodium complexes. Previously, the application of these catalysts has been hampered by low reaction rates in nonpolar solvents, which are essential for establishing the self-assembling architecture via hydrogen bonding. Excellent reaction rates are usually observed in alcohols as solvents, but the characteristic hydrogen bonds are cleaved in those media, resulting in poor ee values. We now show for the first time that the disadvantageous properties of both solvent classes on the catalytic reaction can be overcome by using fluorinated alcohols. Due to this key finding, homogeneous catalysis with self-assembling catalysts is much closer to practical application.
Abstract: New chiral phospholanes were prepared by coupling of bromo-substituted heterocycles with the enantiopure, building block (2R,5R)-2,5-dimethyl-1-chlorophospholane. Some of the new phosphine ligands have the potential for self-assembling via hydrogen bondings in a metal complex. By application of these and related ligands in the palladium catalyzed allylic amination reaction, high enantioselectivities (up to 99%) were achieved. The influence of the construction of the cyclic phosphine ligands on the enantioselectivity is analyzed.
Abstract: The formation of four differently protected 4-deoxy-D-lyxo-hexose derivatives 7, 8, 12, and 14 is described. In the first procedure, a nucleophilic displacement of the allylic mesylate 4 by hydride was combined with a highly stereoselective osmylation of olefin 6 to afford diol 7. In the second radical procedure, tributyl tin hydride was substituted by the cheap and environmentally friendly hypophosphorous acid as a hydrogen donor in the reduction of xanthate 13 to 4-deoxy lyxo-hexose 14.
Abstract: Efficient syntheses of different enantiomerically and diastereomerically pure N,O-ligands with alkylamino and phenol groups attached to the [2.2]paracyclophane framework are described. Several transformations of the ortho-lithiated [2.2]paracyclophan-4-yl diethylcarbamate 7 and the reduction of the [2.2]paracyclophane imino derivatives 3, 4, 17, 18, and 21 allow the preparation of a wide range of compounds in which the chiral environment can be controlled by the planar chiral fragment and modified by the presence of one or two additional chiral centers in the alkylamino group
