title: Sterically congested tripodal phosphites: conformational analysis, solid-state polymorphism, metal complexation, and application to the asymmetric hydrosilation of ketones.
abstract: 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 P2(1)/n with the unit-cell parameters a = 20.070(10) A, b = 17.477(2) A, c = 27.620(3) A, and beta = 93.050(10) degrees, V = 9674.5(14) A(3), and Z = 4. Form II crystallized from a mixture of acetone and toluene in the space group P1 with the unit-cell parameters a = 12.493(1) A, b = 19.701(2) A, c = 21.027(2) A, alpha = 116.23(1) degrees, beta = 100.15(1) degrees, and gamma = 91.07(1) degrees, V = 4542 A(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 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.
authors.affiliation: Ciba Specialty Chemicals Corporation, 540 White Plains Road, Tarrytown, New York 10591, USA. sai.shum@cibasc.com
authors.forename: Sai P
authors.initials: SP
authors.lastname: Shum
authors.forename: Stephen D
authors.initials: SD
authors.lastname: Pastor
authors.forename: Anthony D
authors.initials: AD
authors.lastname: DeBellis
authors.forename: Paul A
authors.initials: PA
authors.lastname: Odorisio
authors.forename: Lilibeth
authors.initials: L
authors.lastname: Burke
authors.forename: Frank H
authors.initials: FH
authors.lastname: Clarke
authors.forename: Grety
authors.initials: G
authors.lastname: Rihs
authors.forename: Bogdan
authors.initials: B
authors.lastname: Piatek
authors.forename: Ronald K
authors.initials: RK
authors.lastname: Rodebaugh