Thesis

Novel applications of a 4-DMAP-derived organic electron donor under photoactivation

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  • University of Strathclyde
Date of award
  • 2013
Thesis identifier
  • T13461
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Abstract
  • 4-DMAP-derived neutral organic super-electron-donor 1 has previously been successfully applied to a number of electron transfer reactions under thermal activation in the Murphy group. The research programme focuses on the photoactivation of the donor 1 and its novel applications in achieving more demanding electron-transfer reactions. Chapter Two highlights the enhanced reducing power of the donor 1 under photoactivation. The reduction of the most challenging unactivated arenesulfonamides, where the nitrogen leaving group will be an unstabilised dialkylamide anion, e.g. 2 and 3, had not been achieved under thermal activation of 1. In contrast, photoactivated donor 1 has now provided the reduction of 2 and 3 in good yields, while activated sulfonamides e.g. 4 were reduced in excellent yields with lower loadings of 1 at room temperature compared with the thermal activation. Chapter Three investigates metal-free reductive C-O bond fragmentations of benzylic esters and ethers with photoactivated donor 1. Benzylic esters e.g. 8 were reduced to acids e.g. 10 via SET process and the complete absence of isolated toluenes e.g. 9 was attributed to trapping of benzyl radical intermediates with the radical-cation of 1. Interestingly, benzylic ethers e.g. 11 provided reduced products 9 and 12 from either side of the ether moiety, and the generation of 9 indicated the presence of benzyl anion intermediates that were arising via a DET process. [IMAGES UNAVAILABLE - SEE FULLTEXT].;To test whether the benzylic radicals were mandatory intermediates in these cleavages, cyclopropyl substrates 13 and 14 were prepared and tested. Cyclopropyl ester 13 afforded acid product 10 only and this supported the trapping of radical intermediates formed via a SET process. On the other hand, cyclopropyl ether 14 afforded intact cyclopropane 15 that could not have arisen from a benzylic radical intermediate; rather the benzylic anion must be its precursor, supporting a DET process in reaction of 14. To probe further for benzylic anion intermediates, substrates 16 and 17, containing not only a benzylic leaving group but also a potential leaving group (pivalate) in the adjacent β- position, have been designed and synthesised. Upon photoactivation, substrate 17 provided a considerably better yield of α-methylstyrene 18 compared with substrate 16 and supported a DET process in the ether cleavage seen in the challenging substrate 17. Chapter Four describes briefly the discovery of benzylic C-N bond cleavages e.g. 20 and provides a detailed account of challenging benzylic C-C σ-bond fragmentations under photoactivation of 1. This chemistry is perfectly general as seen when it was extended to substrates including e.g. electron-rich 22, electron-poor 24, dicinnamyl 26 (for an example of homologous cleavage) and cyanoester 28. [IMAGES UNAVAILABLE - SEE FULLTEXT].;All these reactions proceeded through a SET process as evident from the complete absence of toluene products. However, B-ketoester 30 provided fragmentation of the acyl group and this was further supported by Spartan calculations. Chapter Five discusses the decyanation of malononitriles e.g. 32 and cyanoacetates e.g. 34 under photoactivation of the donor 1 and provided an alternative to the traditional methods involving metal reagents such as SmI2 and carcinogenic solvents e.g. HMPA. Decyanation of malononitriles was faster than that of cyanoacetates and required lower loadings of 1. Chapter Six provides an insight into regioselective ArO-C bond cleavages of orthodialkoxybenzenes, under Birch conditions. The site of fragmentation, generally, depends upon the combined stability of the fragmented species. It was also found that the greater the stability of the charge on the alkyl group, the greater was its ease of formation. Chapter Seven provides the detailed experimental procedures and data for the compounds discussed in the Chapters two to six. [IMAGES UNAVAILABLE - SEE FULLTEXT].
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  • Strathclyde theses - ask staff. Thesis no. : T13461
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Date Created
  • 2013
Former identifier
  • 991156

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