Diphosphines based on an inherently chiral calix[4]arene scaffold: synthesis and use in enantioselective catalysis

Abstract

A series of calix[4]arenes bearing two phosphorus pendent groups p-Bu^t-calix[4]arene-25,26-[CH₂P(O)Ph₂]₂-27-(OR¹)-28-(OR²) (R^{1 or 2} = CH₂CO₂Et, R^{2 or 1} = H, 2; R¹ = R² = CH₂CO₂Et, 3; R^{1 or 2} = C(O)Ocholesteryl, R^{2 or 1} = H, 4; R¹ = R² = C(O)Ocholesteryl, 5; R^{1 or 2} = (R)-CH₂C(O)NHCMe(Ph)H, R^{2 or 1} = H, 6) and p-Bu^t-calix[4]arene-25,26-(CH₂PPh₂)₂-27-(OR¹)-28-(OR²) (R¹ = (R)-CH₂C(O)NHCMe(Ph) H, R² = H, 7; R¹ = (R)-CH₂C(O)NHCMe(Ph)H, R² = SiMe₃, 8; R¹ = (R)-CH₂C(O)NHCMe(Ph)H, R² = (R)-CH₂C(O)NHCMe(Ph)H, 10) have been synthesised. The enantiomerically pure calixarenes 7, 8 and 10 having an AABC substitution pattern are inherently chiral. Reaction of the latter three diphosphines with [Pd(2-Me-allyl)(THF)₂]BF₄ (THF = tetrahydrofuran) afforded the chelate complexes [Pd(2-Me-allyl)(diphosphine)]BF₄  11–13, respectively, while reaction with [Rh(NBD)(THF)₂]BF₄ (NBD = norbornadiene) resulted in quantitative formation of the complexes [Rh(NBD)(diphosphine)]BF₄  14–16, respectively. As a result of allyl rotation, the palladium complexes 11–13 exist in solution as two interconverting species. These complexes efficiently catalyse the alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate, the turnovers being ca. 30 h⁻¹. Enantioselectivity was shown to depend on the size difference between the B and C substituents. Thus, while virtually no induction was observed with the chiral calixarene 10 bearing two identical substituents, ee’s of 45% and 67% respectively were observed with 12 and 11, which have a more marked dissymmetry. Similar trends were observed in the catalytic hydrogenation of dimethyl itaconate with the rhodium complexes 14–16, leading to ee’s of 48%, 25% and 0%, respectively. Related chiral calixarenes in which the two phosphine arms occupy proximal instead of distal phenolic positions were found to be considerably less effective in catalysis of both allylic alkylation and hydrogenation.