Reduction_Reactions - II Objectives Reduction Reactions By...

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II Reduction Reactions Objectives By the end of this section you will: 1) be able to exploit the differences in reactivity of various reducing agents (hydride vs neutral reductants) in chemoselective reductions and be able to provide a mechanistic rationale to account for their differing reactivities; 2) be able to use the inherent chirality in a substrate to control the outcome of a reduction of proximal ketones to generate selectively syn and anti 1,3- and 1,2-diols; 3) be able to rationalise the outcome of these diastereoselective reactions using T.S. diagrams; 4) have gained an appreciation of the versatility of transition metals in reduction reactions; 5) have gained an appreciation of the synthetic utility of dissolving metal reductions; 6) be able to use radical chemistry for deoxygenation and reduction of halides. II.A Reduction of Carboxylic Acid Derivatives and Related Functionality RO H H OR' primary alcohol aldehyde carboxylic acid derivatives N R RN H 2 O 2
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Issues of Reactivity and Selectivity Similar issues of selectivity and reactivity to those we encountered in the case of oxidation reactions also arise in reduction reactions. 1. Chemoselectivity . Many different functional groups can be reduced in a variety of ways. We often need to selectively reduce one functional group whilst leaving others intact (remember year 1 practical!). O 2 N O Sn, HCl O 2 N OH H 2 N O NaBH 4 Chemoselective reductions from a practical in CHM1C3 2. In the case of carboxylic acid derivatives there are two possible reduction products: an aldehyde and an alcohol. Ideally we need methods for selectively accessing either product. Q? Why is it often difficult to stop the reduction of an ester at the aldehyde (consider the relative electrophilicities of the starting material and intermediate product. 3. Stereoselectivity . Asymmetrically substituted ketones provide secondary alcohols on reduction, which introduces a new stereogenic centre into the molecule. We need methods for controlling the stereochemical outcome (relative and absolute) of this reduction using substrate- or reagent- (or both) control. In this course we will only consider substrate-controlled diastereoselective reductions. 4. Regioselectivity . Ambident electrophiles such as α , β -unsaturated ketones can give a variety of reduction products. We need methods for obtaining only the one that we want.
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II.A.1 Hydride Reducing Agents Some of the most important reducing agents are hydrides derived from aluminium and boron. There are numerous varieties differing principally in their reactivity. They all act as sources of nucleophilic hydride and therefore are most reactive towards electrophilic species. Some of the most widely used hydride reagents are discussed below: II.A.1.i Lithium Aluminium Hydride (LiAlH 4 ) one of the most powerful reductants highly flammable reagent and therefore must be used with care reactions are normally carried out in ethereal solvents ( e.g.
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This note was uploaded on 03/26/2011 for the course PSYCH 166A taught by Professor Peng during the Spring '11 term at University of California, Berkeley.

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Reduction_Reactions - II Objectives Reduction Reactions By...

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