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

A series of calix[4]arenes bearing two phosphorus pendent groups p-Bu-t-cal ix[4]arene-25,26-[CH2P(O)Ph-2](2)-27-(OR1)-28-(OR2) (R-1 or 2 = CH2CO2Et. R -2 or 1 = H, 2 R-1 = R-2 = CH2CO2Et, 3; R-1 or 2 = C(O)Ocholesteryl, R-2 or 1 = H. 4; R-1 = R-2 = 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) (R-1 = (R)-CH2C(O)NHCMe(Ph) H, R-2 = H, 7; R-1 = (R)-CH2C(O)NHCMe(Ph) H, R-2 = SiMe3, 8; R-t = (R)-CH2C(O)NHCMe(Ph)H. R-2 = (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 = tetra hydrofuran) afforded the chelate complexes [Pd(2-Me-allyl)(diphosphine)]BF4 11-13, respectively, while reaction with [Rh(NBD)(THF)(2)]BF4 (NBD = norbo rnadiene) resulted in quantitative formation of the complexes [Rh(NBD)(diph osphine)]BF4 14-16, respectively. As a result of allyl rotation, the pallad ium complexes 11-13 exist in solution as two interconverting species. These complexes efficiently catalyse the alkylation of 1.3-diphenylprop-2-enyl a cetate with dimethyl malonate. the turnovers being ca. 30 h(-1). Enantiosel ectivity was shown to depend on the size difference between the B and C sub stituents. Thus. while virtually no induction was observed with the chiral calixarene 10 bearing two identical substituents, ee's of 45% and 67% respe ctively were observed with 12 and 11, which have a more marked dissymmetry. Similar trends were observed in the catalytic hydrogenation of dimethyl it aconate 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 cons iderably less effective in catalysis of both allylic alkylation and hydroge nation.