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Title: Exploring the reactivity of cationic rhodium xantphos complexes with amine-boranes
Author: Johnson, Heather C.
ISNI:       0000 0004 4991 7733
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2015
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This thesis explores the reactivity of amine-boranes with the {Rh(Xantphos)}+ fragment, with the aim of gaining mechanistic insight into the catalytic dehydropolymerisation of the amine-borane H3B∙NMeH2 to yield the polyaminoborane [H2BNMeH]n. Chapter 2 describes the synthesis of suitable RhIII and RhI Xantphos precursors to be used in this investigation. Moreover, the first example of the dehydrogenative B—B homocoupling of the tertiary amine-borane H3B∙NMe3 to form H4B2•2NMe3 is reported. The synthesis of the RhI precatalyst introduced in Chapter 2 entails the hydroboration of tert-butylethylene by H3B∙NMe3. In Chapter 3, the ability of the {Rh(Xantphos)}+ fragment to mediate this hydroboration in a catalytic manner is explored, and a mechanism is presented in which reductive elimination is proposed to be turnover-limiting. Other alkenes and phosphine-boranes are also trialled to determine the scope of the hydroboration. Chapter 4 investigates the catalytic dehydrocoupling of H3B∙NMe2H and H3B∙NMeH2 with {Rh(Xantphos)}+ to form the dehydrocoupling products [H2BNMe2]2 and [H2BNMeH]n, respectively, and the dehydrocoupling mechanisms are shown to be similar. Both involve an induction period in which the active catalyst is formed (thought to involve N—H activation), and saturation kinetics operate during the productive phase of catalysis. H2 is shown to inhibit the dehydrocoupling, and lead to production of shorter chain [H2BNMeH]n. Conversely, using THF as the dehydropolymerisation solvent instead of C6H5F results in longer chain [H2BNMeH]n. Finally, Chapter 5 presents new dicationic {Rh(Xantphos)}-based dimers, the formation of which involves loss of a phenyl group from the Xantphos ligands by P—C activation. The dimers are produced by routes involving either dehydrogenative homocoupling of H3B∙NMe3, or dehydrocoupling of H3B∙NMe2H. One of these dimers was tested as a catalyst for the dehydrocoupling of H3B∙NMe2H, and the reaction kinetics appear closely related those obtained using {Rh(Xantphos)}+, suggesting that the active catalysts in each system may be related.
Supervisor: Weller, Andrew S. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Organometallic Chemistry ; Inorganic chemistry ; Amine-borane ; rhodium ; dehydrocoupling ; Xantphos