P‑Chirogenic Phosphines Supported by Calixarene: New Insight into Palladium-Catalyzed Asymmetric Allylic Substitution
2013-05-13T00:00:00Z (GMT) by
The first P-chirogenic mono- and diphosphine ligands supported on the upper rim of a calixarene moiety were synthesized using the ephedrine methodology. The lithiated calixarene mono- and dianions both react with the oxazaphospholidine–borane, prepared from ephedrine, to afford regio- and stereoselectively the corresponding calixarenyl 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 calixarenyl mono- or diphosphines. Both enantiomers of the calixarenyl 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 <sup>31</sup>P NMR in the presence of a chiral palladium complex. The absolute configurations of the mono- and diphosphinocalixarenes were assigned by X-ray analysis of their crystalline borane complexes. The P-chirogenic calixarenyl phosphines were tested for asymmetric palladium-catalyzed allylic substitution of (<i>E</i>)-1,3-diphenylprop-2-en-1-yl acetate, by dimethyl malonate or benzylamine. When the bis-methylphenylphosphino calixarene 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 <i>C</i><sub>2</sub> symmetry of the P-chirogenic diphosphine calixarene 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<sup>1</sup>-allyl moiety and with the regio- and enantioselectivity of the Pd-catalyzed allylations.