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Hard versus soft reactivity of Lewis acidic boranes

Wilkins, Lewis 2017. Hard versus soft reactivity of Lewis acidic boranes. PhD Thesis, Cardiff University.
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Abstract

This work outlines the use of Lewis acidic boranes in a variety of different reactions, mainly in the activation of unsaturated substrates such as allenes, alkynes, ketones, aldehydes and imines. The activation of allenes toward frustrated Lewis pairs showed that a formal 1,4-addition product prevails, trapping the product as the zwitterionic s-cis diene species. Conversely, reacting these same substrates with B(C6F5)3 alone allows two distinct reactive pathways; either σ-activation to yield the scarcely observed 1,2- carboboration product or π-activation followed by dealkylation to generate the γ-lactone. Following this second activation mode, the reaction between propargyl esters, amides, carbamates and carbonates with a variety of homo- and hetero-leptic boranes as well as borocations promotes the 5-exo-dig cyclisation which, depending on the substrate, was either isolated as the oxazolium or dioxaborinine heterocycle. In addition, utilising the Lewis acid PhBCl2 in this reaction interestingly produces the hitherto unreported 1,3-haloboration of alkynes. B(C6F5)3 was also shown to be hugely successful in a number of other ring closing reactions, specifically the 6-endo-dig cyclisation of alkynyl ester derivatives to form various pyrones, isocoumarins and pyryliums. Of particular interest is the development of a new methodology for catalytic carbon-carbon bond formation through alkyl group transfer. In this case, benzyl, α-methylbenzyl and benzhydryl functionalised alkynyl esters underwent the expected cyclisation using catalytic B(C6F5)3 to form the lactone however, 1,5-migration occurs from oxygen to carbon to generate the corresponding γ-functionalised pyrones and isocoumarins. Finally, a diverse array of reduction chemistry was performed by attenuating the Lewis acidity at boron to specialise its function in catalysis. Other work focused on the exploitation of novel processing technologies to improve conventional batch-type main group reactivity via continuous-flow chemistry. Additionally, biologically inspired dihydropyridines were used in the formation of thermally stable borohydrides as well as elemental hydrogen surrogates in boron mediated transfer hydrogenation pathways

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 7 February 2018
Last Modified: 07 Feb 2018 11:03
URI: http://orca.cf.ac.uk/id/eprint/108928

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