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

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.

Chiral rhodium complexes containing bidentate ligands derived from (R,R)-1,2-diaminocyclohexane for catalytic enantioselective hydrosilylation of acetophenone

“…for C 50 H 68 ClN 2 O 8 P 2 Rh: C,58.57;H,6.68;Cl,3.46;N,2.73. Found: C,58.81;H,6.75;Cl,3.41;N,2.46 6 (12) Ligand 3a (106 mg, 0.2 mmol) was added to a stirred solution of [Rh(cod)Cl] 2 (49 mg, 0.1 mmol) in toluene (5 mL). The yellow solution was stirred for 30 min, and NH 4 PF 6 (42.4 mg, 0.26 mmol) was added.…”

Conformational Polymorphism of a Simple Tripodal Podand Bearing Nitro Functionality

“…16 However, these compounds have not received any attention in the context of polymorphism regardless of their intrinsic conformational flexibility. 17 During our systematic investigation of tripodal receptors, 18 we have obtained three conformational polymorphs (I, II, and III) of a podand L (Scheme 1) from different crystallization conditions. Differences in molecular conformation of the polymorphs are very likely a result of torsion angle differences.…”

“…The structures and properties of tripodal ligands have been well studied . However, these compounds have not received any attention in the context of polymorphism regardless of their intrinsic conformational flexibility . During our systematic investigation of tripodal receptors, we have obtained three conformational polymorphs (I, II, and III) of a podand L (Scheme ) from different crystallization conditions.…”

Asymmetric Synthesis and Metal Complexes of aC3-Symmetric P-Stereogenic Triphosphine, (R)-MeSi(CH2PMe(t-Bu))3(MT-Siliphos)

“…Testing this hypothesis has been limited by the lack of synthetic routes to C 3 -symmetric chiral triphosphines . Very few of these ligands are known, and their use in coordination chemistry or catalysis has been little explored . The most successful application, Rh-catalyzed asymmetric hydrogenation using tris­(phospholane) 1 (Chart ), was slow at room temperature; Burk proposed that dissociation of one arm of the triphosphine or of a chelating olefin substrate was required to generate a vacant coordination site .…”