The preparation and characterization of Rh I , Pd II , and Pt II through conformational equilibration of the chelate ring, and/or inversion of configuration at the stereogenic sulfur. In complexes with chiral C 2 -symmetrical dithioether ligands derived from 1,1Ј-binaphthalene-2,2Ј-dithiol (BINAS) are re-the solid state the palladium derivative of Me 2 BINAS shows a twisted chair conformation of the chelate ring, with both ported. All complexes are fluxional in solution at room temp. Interconversion between the stereoisomers can take place methyl groups in equatorial positions.
Scheme 1. Atropisomeric dithioether ligands
Recent investigations by our groups have shown that, in the hydroformylation of styrene [1] , rhodium(I) complexes with the atropisomeric dithioethers 1 and 2 (Scheme 1), either preformed or prepared in situ by addition of the appropriate amount of the sulfur donor ligand to a suitable rhodium diolefin precursor [Rh(cod)(µ-X)] 2 (cod:1,5-cyclooctadiene; X:Cl; MeO), are endowed with a high catalytic activity. Quantitative conversions of the substrate, with complete chemoselectivity in the formation of the aldehydes 3 and 4 (Scheme 1), are easily attained; even at room temp. and for a fairly wide range of carbon monoxide/hydrogen (1:1) pressures.
As these catalysts favour carbon monoxide insertion into the more substituted vinylic carbon, the major product is the branched isomer 4, which, in the most favourable cases, can account for more than 95% of the reaction mixture. This regioselectivity, which is among the highest recorded in the hydroformylation of styrene, prompted us to exploit the potential of the enantiopure derivatives 1 and 2 in enantioselective hydroformylation. Transfer of chirality from using rhodium catalysts containing diphosphane ligands as chiral modifiers . the ligand to the product does take place, but the efficiency is quite low and the best enantiomeric excess (e.e.) obtained A notable feature of the dithioether ligands 1 and 2 is that upon coordination to the metal they can give rise to so far is only slightly greater than 20%. Albeit low, this value is in the range of the e.e.Јs normally obtained in this different diastereomeric complexes, since the two sulfur donors become stereogenic centers. This fact is expected to reaction after more than twenty years of experimentation