C3‐Chiral Tripodal Amido Complexes

A comprehensive study into the coordination chemistry of two C3-chiral tripodal amido ligands has been carried out. The amido ligands contain a trisilylmethane backbone and chiral peripheral substituents. The amine precursors, HC{SiMe2NH[(S)-1-phenylethyl]}3 (1) and HC{SiMe2NH[(R)-1-indanyl]}3 (2) were found to be in equilibrium in solution with the cyclic diamines HC{SiMe2N[(S)-1-phenylethyl]2}{SiMe2NH[(S)-1-phenylethyl]} (3) and HC{SiMe2NH[(R)-1-indanyl]}{SiMe2NH[(R)-1-indanyl]} (4), which are generated upon ejection of one molecule 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 three aryl groups of the chiral peripheral substituents, the Li−C contacts being in the range of 2.65–2.73 A. 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), [ZrCl{HC{SiMe2N[(S)-1-phenylethyl]}3}] (9) and [ZrCl{HC{SiMe2N[(R)-1-indanyl]}3}] (10), respectively. Of these, compound 7 was structurally characterised by X-ray structure analysis and was shown to possess a C3-symmetrical arrangement of the tripod ligand. The chiral anionic dinuclear complex [Li(OEt2)4][Zr2Cl3{HC{SiMe2N[(S)-1-phenylethyl]}3}2] (11) was isolated from reaction mixtures leading to 9. An X-ray diffraction study established its dimeric structure, in which the chiral amido ligands cap the two metal centres, which are linked through three symmetrically arranged, bridging chloro ligands. Reaction of 9 and 10 with a series of alkyl Grignard and alkyllithium reagents yielded the corresponding alkylzirconium complexes. X-ray structure analyses of [Zr(CH3){HC{SiMe2N[(S)-1-phenylethyl]}3}] (12) and [Zr(CH3){HC{SiMe2N)[(R)-1-indanyl]}3}] (20) established their detailed molecular arrangements. While the reaction of 12 with the aryl ketones PhC(O)R (R=CH=CHPh, iPr, Et) gave the corresponding C−O insertion products, which contain an additional chiral centre in the alkoxy group, with low stereoselectivity (0–40 % de). The corresponding conversions with several aryl aldehydes yielded the alkoxo complexes with high stereoselectivity. Upon hydrolysis, the chiral alcohols were isolated and shown to have enantiomeric excesses between 68 and 82 %. High stereodiscrimination 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 analogous experiments with carbonyl compounds, the ethyl- and butyl-zirconium analogues of 12 did not undergo CO insertion into the metal–alkyl bond. Instead, β-elimination and formal insertion into the metal-hydride bond occurred. It was found that the elimination of the alkene was induced by coordination of the carbonyl substrate to the metal centre.