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{2,2‘,2‘ ‘-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) Å, b = 17.477(2) Å, c = 27.620(3) Å, and β = 93.050(10)°, V = 9674.5(14) Å3, and Z = 4. Form II crystallized from a mixture of acetone and toluene in the space group P1̄ with the unit-cell parameters a = 12.493(1) Å, b = 19.701(2) Å, c = 21.027(2) Å, α = 116.23(1)°, β = 100.15(1)°, and γ = 91.07(1)°, V = 4542 Å3, 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 X-ray characterization of enantiomerically pure (S,S,S)-tri{2,2‘,2‘ ‘-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.