Chiral concepts in s-block metal chemistry
Given the nature of the work undertaken in this project, this thesis will be divided into six chapters. The topics discussed within this thesis primarily focus on the study of chiral bimetallic complexes both from a synthetic and a structural approach. The chemistry of magnesiate complexes will be covered in Chapters 1-3. Chapter 1 focuses on the synthesis and characterisation of five different adducts of the synthetically important lithium tris(n-butyl)magnesiate LiMg(nBu)3 (1). Complex 1 has been co-complexed with different ligands to form the complexes [(TMPDA)·Li(u-nBu)2Mg(u-nBu)]2 (48), [(PMDETA)·Li(u-nBu)(nBu)Mg(u-nBu)]2 (49) [{(R,R)-TMCDA}·Li(u-nBu)2Mg(u-nBu)]2 (51), [(TMEDA)·Li(u-nBu)(u-OnBu)Mg(nBu)]2 (52) and [[(dioxane)·Li(u-nBu)2Mg(u-nBu)]2]∞ (56) In Chapter 2, the reactivity of lithium magnesiates complexes for the asymmetric deprotonation of prochiral ketones is studied. Homoleptic and heteroleptic lithium magnesiate bases have been evaluated and the results highlight that good conversions and enantioselectivities can be achieved when lithium amide LiPEA (lithium (+)-bis-(R)-1-phenylethylamide, Li-57) and a second organometallic reagent are combined. The chemistry of chiral sodium magnesiates is studied in Chapter 3. Solid state and solution structural studies of complexes [NaMg(PEA)2nBu]∞ (103), [TMEDA·Na(PEA)2MgnBu] (104) and inverse crown ether [NaMg(PEA)4O] (105) have been performed. Compounds 103 and 104 have also been tested as deprotonating reagents towards 4-tert-butylcyclohexanone. Chapter 4 focuses on the synthesis of chiral homoleptic and heteroleptic lithium zincates and their applications in enantioselective addition reactions towards ketones. Two new lithium zincates have been fully characterised, TMEDA·Li(u-PEA)(u-Me)ZnMe (147) and (R,R)-TMCDA·Li(u-Me)2ZnMe (149). The alkyl addition to ketones with this monomer is successful although the enantiomeric excess is relatively low. Chapter 5 describes the synthesis, characterization and reactivity studies of a series of mixed alkali-metal complexes: [LiNa(PEA)2]2 (168), [(THF)2LiNaPEA2] (169) and [LiK(PEA)2]2 (170). A preliminary study on the reactivity of these amides as initiators of the asymmetric rearrangement of epoxides has been performed. Each chapter contains a self-contained experimental section where the synthesis and solution characterisation of the complexes are included. Chapter 6 summarises the synthetic procedures for starting materials, as well as a description of general experimental techniques. Finally, an appendix with the crystallographic data for compounds synthesised in this work is included after Chapter 6.
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