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The preparation of enolates via metal catalysed allylic isomerisation

Gazzard, Lewis John; (1995) The preparation of enolates via metal catalysed allylic isomerisation. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

This thesis is divided into three chapters. As an introduction to this work. Chapter One presents an overview of current stereoselective enolate syntheses. In Chapter Two a new approach to enolate chemistry is described, involving the transition metal catalysed isomerisation of allylic alkoxides. The regio- and stereochemical consequences are investigated, and the mechanism discussed. Chapter Three details the experimental procedures employed. It is shown that treatment of 1-phenyl-2-propen-l-ol with catalytic quantities of chlorotris(triphenylphosphine)rhodium (Wilkinson's catalyst) in tetrahydrofuran at reflux effects allylic isomerisation to give exclusively the (Z)-enolate stereoisomer in good yield, as evidenced by aldol reaction or trapping as the enol acetate. Schlenk-line techniques were adopted as standard. Related substrates with increasing substitution around the double bond require longer reaction times but are effectively isomerised. Highly substituted substrates show little or no reactivity-the catalyst is deactivated by extended reflux times and starting material is isolated. It is found that with substrates for which it is possible, mixtures of enolate regioisomers are produced. Also, in the production of tetra-substituted enolates the several possible starting alcohols lead to identical ratios of enolate stereoisomers. This apparent equilibration of reaction products is thought not to be directly metal-mediated but caused by protic reaction by-products. Discussion and evidence are presented. It is found that catalysts formed in situ from bis(cyclooctadiene)nickel (0) and monophosphines are efficient catalysts, and affect the isomerisation of substrates unreactive towards the rhodium catalyst. However chiral chelating diphosphines give inactive species and this approach to asymmetric induction is therefore unfruitful. Instead it is shown that complexes with chelating nitrogenous ligands are far more active. A number of chiral ligands are synthesised and compared with the aim of generating homochiral enolate derivatives from achiral precursors.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: The preparation of enolates via metal catalysed allylic isomerisation
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Pure sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10098515
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