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P‑Chirogenic Phosphines Supported by Calix[4]arene: New Insight into Palladium-Catalyzed Asymmetric Allylic Substitution
journal contribution
posted on 2013-05-13, 00:00 authored by Naima Khiri-Meribout, Etienne Bertrand, Jérôme Bayardon, Marie-Joëlle Eymin, Yoann Rousselin, Hélène Cattey, Daniel Fortin, Pierre D. Harvey, Sylvain JugéThe
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.