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Abstract

Diborane(4) compounds, exhibiting an electron-precise boron-boron bond, are extremely versatile reagents in organic synthesis, therefore the preparation and reactivity of these compounds is of great interest in modern boron chemistry. The introduction of such boron-boron bonds is limited to very few examples in contrast to the variety of carbon-carbon coupling reactions. Among the established strategies to boron-boron bond formation, dehydrocoupling reactions constitute an elegant, “atom- economic” pathway. However, these reactions are still restricted to a very few selected examples. The aim of this thesis was to establish new strategies to boron-boron bond formation via dehydrocoupling reactions; therefore various bidentate ligands, such as neutral bisphosphanes or anionic amidine or guanidine compounds were introduced. The respective bisphosphane borane adducts were isolated and fully characterized. Furthermore, the reactivity of these diboranes towards carbonyl complexes, hydrid abstraction and potential dehydrocoupling catalysts was investigated. Reaction with iodine selectively replaced one hydride in each borane by iodine, leading to the isolation and characterization of mono iodoboranes, as well as a doubly phosphane stabilized boron dication. However, boron-boron dehydrocoupling could not be achieved from this approach. In contrast, introduction of anionic ligands gave rise to a new route to boron-boron bond formation. The dehydrocoupling strategy developed in this thesis enables access to new nucleophilic diborane(4) compounds. Reaction of the borane adducts (L+H)-BH3 (L+H = 1,2,3,4,4a,5,6,7-octahydro-1,8- naphtyridine) as well as hppH-BH3 (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidine) with iodine at room temperature lead to formation of [IB(μ-L)]2 and [IB(μ-hpp)]2. Both diboranes(4) were isolated and fully characterized. Based on a strong set of experimental data and quantum chemical computations, a reaction pathway for this unusual reaction was proposed. In difference to traditional pathways using reducing reagents, the reduction from BIII to BII is paradoxically achieved by the addition of the oxidation reagent iodine. Furthermore, quantum chemical computations were carried out on the diboranes(4) [IB(μ-L)]2 and [IB(μ-hpp)]2, as well as the hydrogen analogues [HB(μ-L)]2 and [HB(μ-hpp)]2, to determine various parameters, such as proton affinity and HOMO energies. All compounds were classified due to their nucleophilicity. The well documented, intensely studied [HB(μ-hpp)]2 constitutes the strongest nucleophilic character among the analysed compounds while the nucleophilicity of [IB(μ- L)]2 is quite low. Overall this thesis established access to an efficient and unprecedented boron-boron dehydrocoupling reaction under mild conditions, applicable to diverse borane adducts.