Zinc catalysed cross coupling and dehydroboration reactions

• Main Author: Procter, Richard
• Format: Dissertation
• Language: English
• Published: University of Manchester 2019
• Subjects: Boron; C-H activation; Carbene; Cross Coupling; Dehydroboration; Earth Abundant catalysis; NHC; Suzuki-Miyaura; Zinc; Zinc hydride; Zincate

Two of the most important classes of reaction for synthesis, in both academia and industry, are the Suzuki - Miyaura reaction and C - H borylation reactions. The current state of the art methodologies require expensive and toxic catalysts, and as such research into viable new catalysts is an area of great interest. To this end, this thesis presents the development of a range of zinc compounds and explores their reactivity with a focus on development of catalytic reactions. The first strategy employed centres on transmetallation from nucleophilic aryl boron reagents to simple zinc salts for cross coupling with organic halides. Alkyllithium activated borates [Li][(Bu)PhBPin] were found to generate zincates LiZnAr3 in reaction with zinc salts, and these zincates could react with benzyl halides to form diarylmethane products. This enabled a highly selective zinc catalysed reaction between the borates and benzyl halides, which represents the first example of a zinc catalysed Suzuki - Miyaura cross coupling reaction. No ligands were required and a range of functional groups were tolerated, and the reaction was optimised by a statistical Design of Experiments method. The second strategy utilised highly sterically encumbered NHC ligands, in particular 7DiPP, to stabilise a range of highly electrophilic, low co-ordinate zinc compounds. The synthesis of complexes of zinc halides, zinc-aryls, and zinc hydride is presented alongside investigation into their reactivity with benzyl halides, hydrogen activation, and their reduction. Of particular note is the successful application of two of these complexes, [(7DiPP)ZnPhNTf2] and [(7DiPP)Zn(H)NTf2], as catalysts for the dehydroboration of a range terminal alkynes, again based on a key boron to zinc transmetallation process. Key intermediates, mechanisms, and a catalyst degradation pathway by NHC ring opening were identified by experimental and computational investigations. This reaction demonstrates the potential of these compounds to enable useful organic transformations, the range of which is currently being investigated and expanded within the Ingleson group.