The first P-chirogenic aminophosphane-phosphinite (AMP*P) ligand (4a) supported on the upper rim of a calix [4]arene moiety was synthesized in two steps using the ephedrine methodology. Ligand 4a was used for the preparation of the corresponding rhodium complex [Rh(COD)-(AMP*P)]BF 4 (5a) (COD=1,8-cyclooctadienyl), which was tested for asymmetric catalyzed hydrogenation of various substrates. The structures of the AMP*P ligand as diborane and rhodium complexes 3a and 5a were established by X-ray analysis. The asymmetric hydrogenation catalyzed with the Rh complex 5a exhibits excellent enantioselectivities up to 98%. Investigation of modified P-chirogenic aminophosphane-phosphinite ligands 4b,c, bearing an isoelectronic or a sterically similar substituent on the P-chirogenic aminophosphane unit, demonstrates that the calix[4]arene substituent of the aminophosphane moiety plays a major role in the better asymmetric induction. The enantioselectivity of the catalyzed hydrogenation was weakly influenced by the hydrogen pressure, which is in good agreement with a stereodetermining step involving the substrate-rhodium complexes. Computer modeling indicated the presence of two conformers for the active AMP*P rhodium species, according to whether the rhodium metal is outside or inside the calix[4]arene cavity (called outer and inner). It is obvious that the complexation of the substrate with the active rhodium species forces this complex to adopt fully the outer conformation and hence explains why the calixarene fragment plays a key role in the stereodetermining step. † This paper is dedicated to Prof. H. Kagan for his 80th birthday.
The first P-chirogenic mono-and diphosphine ligands supported on the upper rim of a calix[4]arene moiety were synthesized using the ephedrine methodology. The lithiated calix[4]arene mono-and dianions both react with the oxazaphospholidine−borane, prepared from ephedrine, to afford regio-and stereoselectively the corresponding calix[4]arenyl aminophosphine−boranes, by cleavage of the heterocyclic ring at the P−O bond position. Subsequent reactions with HCl and then organolithium reagent and finally decomplexation with DABCO lead to the corresponding calix[4]arenyl mono-or diphosphines. Both enantiomers of the calix[4]arenyl phosphines were obtained either by using (+)-or (−)-ephedrine or by changing the addition order of the organolithium reagents during the synthesis. The enantiomeric excesses of the phosphines were determined either by HPLC on a chiral column of their borane complexes or by 31 P NMR in the presence of a chiral palladium complex. The absolute configurations of the mono-and diphosphinocalix[4]arenes were assigned by X-ray analysis of their crystalline borane complexes. The P-chirogenic calix[4]arenyl phosphines were tested for asymmetric palladium-catalyzed allylic substitution of (E)-1,3-diphenylprop-2-en-1-yl acetate, by dimethyl malonate or benzylamine. When the bismethylphenylphosphino calix[4]arene was used, the allylic products were obtained with 82% and 79% ee, respectively. In both cases, the use of a diphosphine affords better results than using 2 equivalents of monophosphine. Despite the C 2 symmetry of the P-chirogenic diphosphine calix[4]arene ligand, computer modeling of the corresponding Pd(allyl) complex shows a clear dissymmetry of the LUMO, which is in good agreement with a complexed η 1 -allyl moiety and with the regio-and enantioselectivity of the Pd-catalyzed allylations.
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