CM2006 Carbonyl Lec3 - Lecture 3 Practical consequences...

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Lecture 3
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Practical consequences - enantiopure enolisable aldehydes or ketones in which the stereogenic centre is α - to the carbonyl group must be carefully protected from even traces of acid or base - otherwise racemisation is rapid. Stereogenic centres which are not α - to a carbonyl group are not racemised by enolisation: Me O H Ph H + or OH - Me OH H Ph No racemisation Stereogenic centres which are α - to a carbonyl group but which do not bear an enolisable hydrogen are not racemised by enolisation: O Ph Me OH Ph Me H + or OH - No racemisation
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Keto-Enol Tautomerism - summary of some important points : C C C O C C C OH Keto tautomer Enol tautomer α - H •Structural isomerism of carbonyl compounds - an equilibrium reaction and not resonance. •Requires at least one hydrogen α - ( i.e. next to) the carbonyl group •Enol tautomer (isomer) only present in very low concentration •Enol formation very slow under neutral conditions •Enol formation catalysed by either acid or base •Catalysis involves either - (Acid) Protonation on O followed by deprotonation at the α -carbon
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or (Base) Deprotonation at the α -carbon followed by re-protonation on O C C C OH C C C OH + Š Nucleophilic site •Resonance stabilisation of enol tautomer renders α -carbon nucleophilic •Two equivalent ways of showing an enol acting as a carbon nucleophile: C C C OH C C C OH + Š E + C C C OH E + C C C OH E + E + •Enol tautomer is very reactive despite its low equilibrium concentration so that many important reactions of carbonyl compounds involve enols as intermediates, among them: Deuteration at the α -carbon
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Acid- or base-catalysed racemisation of an α - stereocentre Halogenation at the α -carbon Enols as reaction intermediates - α -halogenation of ketones - the most extensively studied example of enol reactivity.
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