1431138-93-0Relevant articles and documents
P-chirogenic phosphines supported by calix[4]arene: New Insight into palladium-catalyzed asymmetric allylic substitution
Khiri-Meribout, Naima,Bertrand, Etienne,Bayardon, Jeroime,Eymin, Marie-Joelle,Rousselin, Yoann,Cattey, Helene,Fortin, Daniel,Harvey, Pierre D.,Juge, Sylvain
, p. 2827 - 2839 (2013/06/27)
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 31P 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 bis-methylphenylphosphino 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 C2 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 η1-allyl moiety and with the regio- and enantioselectivity of the Pd-catalyzed allylations.
