Hydrides As Reducing Agents
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Hydrides as Reducing Agents Lithium aluminum hydride (LiAlH4) is a strong reducing agent. It will donate hydride (“H-”) to any C=O containing functional group. Examples: + 2. H3O 1. LiAlH4 (or just H2O) aldehyde primary alcohol 1. LiAlH4 2. H2O ketone secondary alcohol Hydrides as Reducing Agents Lithium aluminum hydride (LiAlH4) is a strong reducing agent. It will reduce almost any C=O containing functional group to an alcohol. Example: 1. LiAlH4 2. H2O ester One and then another equivalent equivalent adds, of H- adds, unavoidably. Reduced by LiAlH4 to an alcohol: aldehyde ketone carboxylic acid ester acyl halide Double Addition of Hydride to Carboxylic Acids and Derivatives Why? Ketones and aldehydes are more electrophilic than acids, esters and acyl halides. As soon as a ketone or aldehyde is Lone pair donation by generated, it is immediately reduced oxygen reduces partial again. positive charge on C=O carbon. Reduced by LiAlH4 to an alcohol: aldehyde ketone carboxylic acid ester acyl halide Hydrides as Reducing Agents Exception: LiAlH4 reduces amides to amines. Examples: 1. LiAlH4 Mechanism depends slightly on whether amide has an N-H or 2. H2O not. But the result is the same. 1. LiAlH4 2. H2O Reduced by LiAlH4 to an alcohol: aldehyde ketone carboxylic acid ester acyl halide Hydrides as Reducing Agents Sodium borohydride (NaBH4) is a mild reducing agent. It is only capable of reducing aldehydes and ketones. NaBH4 NaBH4 isn’t as basic as LiAlH4, EtOH so reaction can be conducted in protic solvent, and separate workup step isn’t essential. aldehyde primary alcohol NaBH4 EtOH Reduced by NaBH4: ketone secondary alcohol aldehyde ketone Biological Cofactors as Redox Agents LiAlH4 isn’t used in biology, but biological reductants are mechanistically similar. NADH reduces by acting as an H- donor enzyme NADH NAD+ (derived from niacin, vitamin B ) 3 NAD+ oxidizes by acting as an H- acceptor Cofactor: A small-molecule “helper” that is required by an enzyme to catalyze a reaction. Many vitamins are cofactors. Sterically Hindered Reducing Agents Stop At Aldehyde 1. (Chapter 22) “DIBAL-H” 2. H2O ester carboxylic acid aldehyde (Chapter 22) LiAlH(OtBu)3 -78 °C acyl halide Nucleophiles Approach Carbonyl Groups At An Angle Nuc Nucleophiles are electron donors. As a nucleophile approaches the C=O electrophile, it will donate electrons to the most available, lowest-energy molecular orbital (the LUMO). Nuc Optimal overlap with the * LUMO is at an angle, above or below the C=O plane. Nucleophile Additions to Carbonyls Generate Racemic Mixtures (addition to top face) (addition to bottom face) 50:50 mixture of top- and bottom-face adduct is observed. Enantioselective Reduction of Ketones Using Chiral Catalysts Chiral catalysts bring reactants together in specific geometries, to force the preferential formation of one enantiomer over the other. Example: 1. BH3 (S)-CBS reagent (R) “RS” “R ” L (S) Reaction generates this enantiomer only. 2. H+ (workup) Enantioselective Reduction of Ketones Using Chiral Catalysts BH3 2. H2O product (S)-CBS reagent Ketone orients larger So H- addition occurs on group away from catalyst. one face, and not the other..