Experimental and computational studies on the role of single electron transfer in selected organic reactions

• Main Author: Emery, Katie J.
• Published: University of Strathclyde 2017
• Subjects: 541
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.723040

The transition metal-free reaction conditions that couple haloarenes to benzene using a combination of KOtBu and an organic additive is proposed to be initiated by a single electron transfer (SET) into the haloarene. It was initially believed that KOtBu was the electron donor, however mounting experimental evidence suggests that electron donors are formed in situ. Within this thesis, both of these potential initiation pathways are investigated. Experimental evidence is presented to support the proposal that the electron-rich enolate anion 6 (formed by deprotonation of the additive N,N’-dipropylketopiperazine 5) acts as an electron donor (Scheme 1). When the additive 7 was used in the transition metal-free reactions, the cyclised product 10 was isolated, which arose by SET from the enolate anion 8. Computational analysis has shown that when DMF is used as the additive in these transition metal-free reaction conditions, the electron donors, 13 or 14, can form from the dimerisation of the carbamoyl anion of DMF. This work lead towards the development of new additives for these transition metal-free reaction conditions. Computational analysis and experimental evidence is presented that puts into question the previously proposed mechanism that KOtBu can donate a single electron to CBr4 in the bromination of adamantane (Scheme 2). It is proposed that alkoxides, like 16, form hypobromite intermediates when reacted with CBr4 15 (rather than undergoing SET) and these hypohalites have been shown to successfully achieve this transformation. These transition metal-free reaction conditions have been used to achieve SRN1 reactions to couple aryl halides with enolate anions (Scheme 3). A thorough study of the reaction mechanism and possible selectivity of reaction pathways, between SRN1 and aryl-aryl bond formations, was performed. The conclusion is that neither photoactivation nor transition metal-induced activation is needed, and solvent isshown to influence the product selectivity. (Scheme 1 The proposal of the formation electron donors from N,N’-dipropylketopiperazine 5 and DMF 11 that can donate a single electron to haloarenes, such as 1, in the transitionmetal-free coupling reactions. Scheme 2 The formation of radical species from the reaction of alkoxides, like 16, with tetrabromomethane occurs through the formation of hypobromites like 17, as opposed to SET from alkoxide 16, as previously proposed. Scheme 3 Transition metal-free reaction conditions used to activate haloarenes to form arylradicals in SRN1 cyclisation of substrate 22 and analogues).