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Aldehydes from oxidation of primary alcohols using the Dess- Martin periodinane reagent 14.2 Preparing Aldehydes and Ketones.

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Presentation on theme: "Aldehydes from oxidation of primary alcohols using the Dess- Martin periodinane reagent 14.2 Preparing Aldehydes and Ketones."— Presentation transcript:

1 Aldehydes from oxidation of primary alcohols using the Dess- Martin periodinane reagent 14.2 Preparing Aldehydes and Ketones

2 Aldehydes from reduction of carboxylic esters using diisobutylaluminum hydride (DIBAH) Preparing Aldehydes and Ketones

3 Secondary alcohols are oxidized by variety of chromium-based reagents to give ketones Aryl ketones from Friedel-Crafts acylation reactions Preparing Aldehydes and Ketones

4 Mechanism of Grignard Reaction Nucleophilic Addition of Grignard and Hydride Reagents: Alcohol Formation

5 In an analogous manner the reaction of aldehydes and ketones with hydride reagents may be represented as proceeding through a nucleophilic addition of a hydride ion (:H – ) to the C=O carbon LiAlH 4 and NaBH 4 act as if they are donors of hydride ion Nucleophilic Addition of Grignard and Hydride Reagents: Alcohol Formation

6 Primary amines, RNH 2, add to aldehydes and ketones to yield imines, R 2 C=NR Secondary amines, R 2 NH, add similarly to yield enamines, R 2 N-CR=CR Nucleophilic Addition of Amines: Imine and Enamine Formation

7 Imines are common biological intermediates where they are often called Schiff bases Nucleophilic Addition of Amines: Imine and Enamine Formation

8 Imine and enamine formations reach maximum rate around pH = 4 to 5 Slow at pH > 5 because there is insufficient H + present in solution to protonate intermediate carbinolamine –OH to yield the better leaving group –OH 2 + Slow at pH < 4 because the basic amine nucleophile is protonated and initial nucleophilic addition cannot occur Nucleophilic Addition of Amines: Imine and Enamine Formation

9 Worked Example 14.1 Predicting the Product of Reaction between a Ketone and an Amine

10 Acetal and hemiacetal groups are common in carbohydrate chemistry Glucose, a polyhydroxy aldehyde, undergoes intramolecular nucleophilic addition Exists primarily as a cyclic hemiacetal Nucleophilic Addition of Alcohols: Acetal Formation

11 Wittig reaction Converts aldehydes and ketones into alkenes Phosphorus ylide, R 2 C–P(C 6 H 5 ) 3, adds to aldehyde or ketone to yield dipolar, alkoxide ion intermediate Ylide (pronounced ill-id) is a neutral, dipolar compound with adjacent positive and negative charges Also called a phosphorane and written in the resonance form R 2 C=P(C 6 H 5 ) 3 Dipolar intermediate spontaneously decomposes through a four-membered ring to yield alkene and triphenylphosphine oxide, (Ph) 3 P=O Wittig reaction results in replacement of carbonyl oxygen with R 2 C= group of original phosphorane Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction

12 Wittig reaction mechanism Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction

13 Phosphorus ylides are prepared by S N 2 reaction of primary and some secondary alkyl halides with triphenylphosphine, (Ph) 3 P, followed by treatment with base Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction

14 Wittig reactions used commercially to synthesize numerous pharmaceuticals Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction

15 What carbonyl compound and what phosphorus ylide might you use to prepare 3-ethylpent-2-ene? Worked Example 14.3 Synthesizing an Alkene Using a Wittig Reaction

16 Solution Worked Example 14.3 Synthesizing an Alkene Using a Wittig Reaction

17 Cannizzaro reaction is a nucleophilic acyl substitution reaction of aldehydes and ketones OH ¯ adds to aldehyde to give tetrahedral intermediate H: ¯ ion is transferred to a second aldehyde The aldehyde accepting the H: ¯ ion is reduced and the aldehyde transferring the H: ¯ is oxidized Biological Reductions

18 Cannizzaro reaction mechanism is analogous to biological reduction in living organisms by nicotinamide adenine dinucleotide, NADH NADH donates H: ¯ to aldehydes and ketones, similar to tetrahedral alkoxide intermediate in Cannizzaro reaction Biological Reductions

19 Conjugate addition occurs because the nucleophile can add to either one of two electrophilic carbons of the ,  -unsaturated aldehyde or ketone Conjugate Nucleophilic Addition to α,β- Unsaturated Aldehydes and Ketones

20 Conjugated double bond of ,  -unsaturated carbonyl is activated by carbonyl group of the aldehyde or ketone C=C double bond is not activated for addition in absence of carbonyl group Conjugate Nucleophilic Addition to α,β- Unsaturated Aldehydes and Ketones

21 Primary and secondary amines add to ,  -unsaturated aldehydes and ketones to yield  -amino aldehydes and ketones Both 1,2- and 1,4-addition occur Additions are reversible More stable conjugate addition product accumulates Conjugate Nucleophilic Addition to α,β- Unsaturated Aldehydes and Ketones

22 Conjugate addition of an alkyl or other organic group to an  - unsaturated ketone (but not aldehyde) is a useful 1,4- addition reaction Conjugate Nucleophilic Addition to α,β- Unsaturated Aldehydes and Ketones

23 Conjugate addition of alkyl groups to an  -unsaturated ketone (not aldehyde) is accomplished with a lithium diorganocopper reagent, R 2 CuLi (Gilman reagent) Lithium diorganocopper reagent is prepared by reaction of 1 equivalent of copper(I) iodide and 2 equivalents of an organolithium reagent, RLi Organolithium reagent is prepared by reaction of lithium metal with an organohalide Conjugate Nucleophilic Addition to α,β- Unsaturated Aldehydes and Ketones

24 Primary, secondary, and even tertiary alkyl groups undergo conjugate addition Alkynyl groups react poorly Grignard reagents and organolithium reagents normally give direct carbonyl addition to  -unsaturated ketones Conjugate Nucleophilic Addition to α,β- Unsaturated Aldehydes and Ketones

25 How might you use a conjugate addition reaction to prepare 2-methyl-3-propylcyclopentanone? Worked Example 14.4 Using a Conjugate Addition Reaction

26 Solution Worked Example 14.4 Using a Conjugate Addition Reaction

27 Spectroscopy of Aldehydes and Ketones

28 Aldehyde protons (RCHO) absorb near 10  in the 1 H NMR Aldehyde proton shows spin-spin coupling with protons on the neighboring carbon, with coupling constant J ≈ 3 Hz Hydrogens on carbon next to a carbonyl group are slightly deshielded and absorb near to 2.0 to 2.3  Spectroscopy of Aldehydes and Ketones

29 Carbonyl-group carbon atoms of aldehydes and ketones have characteristic 13 C NMR resonances in the range of 190 to 215  Spectroscopy of Aldehydes and Ketones


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