An experimental and computational investigation of transition metal-free reactions

  • Mark Allison

Student thesis: Doctoral Thesis

Abstract

Over recent decades transition metal-catalysed cross-coupling has been developed significantly, opening up new areas of reactivity in both an academic and industrial environment. However, over recent years transition metal-free coupling reactions of haloarenes to arenes, proceeding by the Base-promoted Homolytic Aromatic Substitution (BHAS) mechanism have been widely reported in the literature. These reactions work well when initiated by organic electron donors that are formed in situ from organic additives of various types. The work in this thesis builds on previous work carried out within the Murphy group and other groups working in this field.Scheme 1 - BHAS mechanism proposed by Studer and Curran as responsible for transition metal-free cross coupling reactions. [Graphic Item]The work in this thesis looks at transition metal-free reactions in two forms; firstly looking at new additives that could form electron donors to initiate the BHAS mechanism and secondly, looking at where coupling reactions are occurring when there is no electron donor present. The final aspect of this thesis looks at exploring observations seen in experiments with haloarenes and potassium hydride.The first set of work was looking at a series of additives that were designed to form electron-rich alkenes, a common feature of electron donors, under basic conditions. These were studied both computationally and experimentally and this work showed these can work together effectively in studying electron donors.Scheme 2 - Additives used in Chapter 4 produced high yields of coupled products. [Graphic Item]While these reactions proceed in good yield when there is an electron donor present in the reaction, when there is no electron donor a measurable amount of cross coupling still occurs. A plausible mechanism, shown in scheme 3, has been proposed for the initiation of the BHAS pathway. This thesis shows that biradicals, generated in Bergman cyclisations and hexadehydro-Diels-Alder reactions, can indeed act as a source of initiation for this pathway. This thesis now shows that additives that afford arenediyls, provide independent initiation of the coupling reactions for this substrate in the absence of electron donors, supporting a similar capability for benzyne generated by base-dependent means.Scheme 3 - Proposed benzyne initiation of the BHAS cycle. [Graphic Item]In reactions performed by other members of the Murphy group involving haloarenes and KH, some interesting reaction products were observed. In this thesis these have been rationalised using computational studies.Scheme 4 - This thesis proposes formation of 7.7 proceeds via the four-memberedtransition state calculated. [Graphic Item]
Date of Award1 Apr 2018
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorJohn Murphy (Supervisor) & Christopher Tuttle (Supervisor)

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