C-3-chiral tripodal amido complexes

A comprehensive study into the coordination chemistry of two C-3-chiral tri podal amido ligands has been carried out. The amido ligands contain a trisi lylmethane backbone and chiral peripheral substituents. The amine precursor s, HC{SihMe(2)NH[(S)-1-phenyicthyl])(3) (1) and HC{SiMe2NH[(R)-1-indanyl]}( 3) (2) were found to be in equilibrium in solution with the cyclic diamines HC{Si-Me2N[(S)-1-phenylethyl](2)} (3) and HC{Si-Mc(2)NH[(R)-1-indanyl]}{Si Me2NH[(R)-1-indanyl]} (4), which are generated upon ejection of one molecul e of the chiral primary amine. Reaction of these equilibrium mixtures with three molar equivalents of butyllithium instantaneously gave the trilithium triamides HC{SiMe2N(Li)[(S)-1-phenylethyl]}(3) (5) and HC{SiMe2N(Li)[(R)-1 -indanyl])(3) (6), both of which were characterised by an X-ray diffraction study. Both lithium compounds possess a central heteroadamantane core, in which the two-coordinate Li atoms are additionally weakly solvated by the t hree aryl groups of the chiral peripheral substituents, the LI-C contacts b ring in the range of 2.65 - 2.73 Angstrom. Reaction of 5 and 6 with [TiCl4( thf)(2)] and ZrCl4 gave the corresponding amido complexes [TiCl-{HC{SiMe2N[ (S)-1-phenylethyl}(3)}] (7), [TiCl{HC{SiMe2N[(R)-1-indanyl]}(3)}] (8), [ZrC l{HC{SiMe2N[(S)-1-phenyethyl]}(3)}] (9) and [ZrCl{HC{SiMe2N[(R)-1-indanyl]} (3)}] (10), respectively. Of these, compound 7 was structurally characteris ed by X-ray structure analysis and was shown to possess a C-3-symmetrical a rrangement of the tripod ligand. The chiral anionic dinuclear complex [Li(O Et2)(4)][Zr2Cl3{HC{SiMe2N[(S)-1-phenylethyl]}(3)}(2)) (11) was isolated fro m reaction mixtures leading to 9. An X-ray diffraction study established it s dimeric structure, in which the chiral amido ligands cap the two metal ce ntres, which are linked through three symmetrically arranged, bridging chlo ro ligands. Reaction of 9 and 10 with a series of alkyl Grignard and alkyll ithium reagents yielded the corresponding alkylzirconium complexes. X-ray s tructure analyses of [Zr(CH3){HC{SiMe2N[(S)-1-phenylethyll}(3)}] (12) and [ Zr(CH3)-{HC{SiMe2N[(R)-1-indanyl]}(3)}] (20) established their detailed mol ecular arrangements. While the reaction of 12 with the aryl ketones PhC(O)R (R=CH=CHPh, iPr, Et) gave the cor responding C-O insertion products, which contain an additional chiral centre in the alkoxy group, with low stereose lectivity (0-40% de). The corresponding conversions with several aryl aldeh ydes yielded the alkoxo complexes with high stereoselectivity. Upon hydroly sis, the chiral alcohols were isolated and shown to have enantiomeric exces ses between 68 and 82%. High stereo-discrimination was also observed in the insertion reactions of several chiral ketones and aldehydes. However, this was shown to originate primarily from the chirality of the substrate. In a nalogous experiments with carbonyl compounds, the ethyl- and butyl-zirconiu m analogues of 12 did not undergo CO insertion into the meta-alkyl bond. In stead, beta -elimination and formal insertion into the metal-hydride bond o ccurred. It was found that the elimination of the alkene was induced by.