Tetraalkyl analogs of DIOP, (-)-EtDIOP, (-)-i-PrDIOP, and (-)-CyDIOP, have been prepared for the first time by the reaction of (+)-2,3-0-isopropylidene-2, 3-dihydroxy-1,4-difluorobutane and the corresponding lithium dialkylphosphides.
Our search for highly active hydrogenation catalysts for carbonyl compounds, starting with neutral Rh(I) hydride complexes, [RhH(PR3)n] (R=i-Pr, n=3; R=Cy, n=2), has led to the discovery of cationic Rh(I) complexes with fully alkylated diphosphine ligands, [Rh{(i-Pr)2P(CH2)nP(i-Pr )2}(NBD)]C104 (n=3,4). These compounds prove to be versatile and efficient for hydrogenation of a variety of carbonyl compounds, including aldehydes.A number of rhodium complexes, e.g., neutral rhodium(I) complexes of Wilkinson type, [RhXL3] (L=triarylphosphines) and cationic rhodium(I) complexes containing an aryl-substituted diphosphines, were found to be active catalysts for olefin hydrogenation.)) These metal complexes, however, are not very active for ketone hydrogenation.)) An enhancement of the activity of rhodium complex catalysts was observed on the addition of strong alkali, i.e., [RhCI(C8H12)(PPh3)]-NaBH4-KOH2a) or [RhCl2(bpy)2]Cl-NaOH.2b) In this paper we wish to report that the catalytic activity of rhodium(I) species for hydrogenation of carbonyl compounds can be markedly improved with electron-donating fully alkylated phosphines.Our principal strategy was to increase the electron density of the metal center by utilizing the electron-donating trialkylphosphines together with a hydrido ligand. Since [RhH{P(i-Pr),},]their activity was tested for several ketones (Table 1). The results were encouraging.[RhH(PCy3)2] was even active for PhCOPh which could not readily be hydrogenated by Osborn's system, [Rh-(PPhMe2)H2(S)2]+-H20 (S=solvent), under ambient conditions. 4) This is rather remarkable since the Rh(I) hydride containing triarylphosphine, e.g., [RhH(DBP)4] (DBP=5-phenyl-5H-dibenzophosphole) is known to be totally inactive as a hydrogenation catalyst for ketones.5) Table 1 also includes results of transfer hydrogenation of some ketones effected with [RhHhexanone is mainly (94%) trans-4-tert-butylcyclohexanol in contrast to the predominant formation of the cis-product effected by IrCl6 -P(OMe)3 in acidic medium.6) In the latter case an incipient protonation of the carbonyl oxygen atom may probably be involved, followed by the hydride transfer to the less hindered carbonyl face. The predominant formation of trans-alcohol in the present case suggests an extensive thermodynamic control which in turn implies a rapid reverse the overall catalytic rate with Rh(I) complexes of monodentate trialkylphosphines should have been impaired by rapid dehydrogenation of the product alcohol.
The chiral diphosphines (‐)‐(IV) or (‐)‐(IX), prepared as shown in the reaction scheme, are employed as ligands in neutral and cationic rhodium complexes which are tested as catalysts for the asymmetric hydrogenation of various carbonyl compounds, e.g. the α‐keto amides (X) or (S)‐(XII).
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