Structural variations on chiral ligands : new chelating diphosphines from optically active (1,4)-(Z)-2-butenes

The asymmetric catalysis by transition metal complexes is recognized an alternative strategy for the preparation of compounds which were believed to be impossible to carry out by conventional methods. The greater part of the transition metals known as excellent catalysts for asymmetric synthesis have optically active diphosphines as source of chirality but in spite of the great variety of chiral ligands available, there is still demand for the development of catalysts which display the requirements of high enantioselectivity, diastereoselectivity and productivity in fine chemicals, fragrances, flavours, agrochemicals. The development of new catalysts is usually closely connected to the development of new chiral ligands among which optically active diphoshines take the lion’s share but, even if up to now thousands of chiral chelating phosphines are available, only few of them give catalysts with satisfactory enantioselectivity, productivity and process robusteness to find application on industrial scale. We wish to present a new class of chiral chelating diphosphines derived from (1,4)-(Z)-2-butenes; these ligands would gather the conformational rigidity of the atropisomeric ligands like BINAP or BITIOP with the flexibility and the possibility to modulate the electronic and steric properties typical of the ligands based on chiral sp3 carbon. The ligands are prepared from the corresponding (1,4)-(Z)-2-butenes ( R = CH3-, C6H5-, (CH3)2CH- ) according to the scheme: The structure of the palladium(II)Cl2 complexes of the cis,2,5-bis-diphenylphosphino-3-hexene are reported in the following figures The ruthenium complexes with the optically pure ligands give promising results in the asymmetric hydrogenation of prochiral ketones. Work is in progress to investigate other catalytic reactions like asymmetric hydroformylation with rhodium complexes.