Sterically Congested Tripodal Phosphites: Conformational Analysis, Solid-State Polymorphism, Metal Complexation, and Application to the Asymmetric Hydrosilation of Ketones1

The synthesis as well as isolation and crystallographic analysis of two solid-state polymorphs of the tripodal ligand tri{2,2ʹ,2ʹ ʹ-tris [(2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl)oxy]ethyl}amine (3) is described. Form I crystallized from ethyl acetate in the space group P21/n with the unit-cell parameters a = 20.070(10) (angstrom), b = 17.477(2) (angstrom), c = 27.620(3) (angstrom), and (beta) = 93.050(10), V = 9674.5(14) (angstrom)3, and Z = 4. Form II crystallized from a mixture of acetone and toluene in the space group P with the unit-cell parameters a = 12.493(1) (angstrom), b = 19.701(2) (angstrom), c = 21.027(2) (angstrom), (alpha)= 116.23(1), (beta)= 100.15(1), and (gamma)= 91.07(1), V = 4542 (angstrom)^3, and Z = 2. Differences in the relative absolute stereochemistry of the stereoaxes in the seven-membered dibenzo[d,f][1,3,2]dioxaphosphepin ring are discussed. The synthesis and Xray characterization of enantiomerically pure (S,S,S)-tri{2,2ʹ,2ʹ ʹ-tris[(2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2] dioxaphosphepin-6-yl)oxy]propyl}amine [(S,S,S)-7] are reported. Two crystallographically independent molecules exist in the unit cell that cannot be superimposed with each other by either a translation or a symmetry operation. The two solid-state conformers in the unit cell differed predominately by the absolute stereochemistry of the stereoaxes in the seven-membered dibenzo[d,f][1,3,2] dioxaphosphepin ring. The Rh(I)-catalyzed hydrosilation of acetophenone with the chiral ligands (R,R,S)-7 and (S,S,S)-7 showed significant differences in chiral induction. Chiral cooperativity between the stereoaxes and stereocenters in (S,S,S)-7 is observed. The mechanism of the communication between the stereocenters and stereoaxes leading to chiral cooperativity in the stereoselective transition state is suggested to be primarily steric in nature.