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 H<sub>3</sub>B∙NMeH<sub>2</sub> to yield the polyaminoborane [H<sub>2</sub>BNMeH]<sub>n</sub>. Chapter 2 describes the synthesis of suitable Rh<sup>III</sup> and Rh<sup>I</sup> Xantphos precursors to be used in this investigation. Moreover, the first example of the dehydrogenative B—B homocoupling of the tertiary amine-borane H<sub>3</sub>B∙NMe<sub>3</sub> to form H<sub>4</sub>B<sub>2</sub>•2NMe<sub>3</sub> is reported. The synthesis of the Rh<sup>I</sup> precatalyst introduced in Chapter 2 entails the hydroboration of tert-butylethylene by H<sub>3</sub>B∙NMe<sub>3</sub>. 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 H<sub>3</sub>B∙NMe<sub>2</sub>H and H<sub>3</sub>B∙NMeH<sub>2</sub> with {Rh(Xantphos)}+ to form the dehydrocoupling products [H<sub>2</sub>BNMe<sub>2</sub>]<sub>2</sub> and [H<sub>2</sub>BNMeH]<sub>n</sub>, 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. H<sub>2</sub> is shown to inhibit the dehydrocoupling, and lead to production of shorter chain [H<sub>2</sub>BNMeH]<sub>n</sub>. Conversely, using THF as the dehydropolymerisation solvent instead of C<sub>6</sub>H<sub>5</sub>F results in longer chain [H<sub>2</sub>BNMeH]<sub>n</sub>. 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 H<sub>3</sub>B∙NMe<sub>3</sub>, or dehydrocoupling of H<sub>3</sub>B∙NMe<sub>2</sub>H. One of these dimers was tested as a catalyst for the dehydrocoupling of H<sub>3</sub>B∙NMe<sub>2</sub>H, and the reaction kinetics appear closely related those obtained using {Rh(Xantphos)}+, suggesting that the active catalysts in each system may be related.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:655129 |
| Date | January 2015 |
| Creators | Johnson, Heather C. |
| Contributors | Weller, Andrew S. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:fee1a668-bd18-474e-8457-1f9fd16ec5bd |
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