Interlude 1: Oxidations, Reductions & Other Functional Group Interconversions (FGI) 1. Definition of Oxidation and Reduction For practical purposes in organic chemistry, oxidation and reduction are defined as follows: Oxidation: addition of oxygen to the substrate Reduction: addition of hydrogen to the substrate or removal of hydrogen removal of oxygen or or removal of one electron or addition of one electron 1
1. Definition of Oxidation and Reduction (cont d) Oxidation states in organic species are not always easy to assign, due to the covalent nature of bonding for organic molecules. Instead of oxidation state, we define an oxidation number, using rules derived from Hendrickson, Cram, Hammond and Pine: The contribution to each carbon is: 1 for H 0 for C + 1 for electronegative elements 2
2. Oxidation of Alcohols to Aldehydes, Ketones or Carboxylic Acids A. The most widely employed transition metal oxidants for alcohols are based on Cr(VI). Chromium trioxide = CrO 3 Dichromate = Cr 2 O 7 2- Chromium trioxide will oxidize 2 o alcohols to ketones. A specific cocktail of CrO 3 and H 2 SO 4 in acetone is called the Jones reagent, and is particularly effective for this transformation. 1 o alcohols are (transiently) oxidized to aldehydes, but in the presence of water form the hydrate which can be further oxidized to the carboxylic acid. 3
2. Oxidation of Alcohols to Aldehydes, Ketones or Carboxylic Acids (cont d) The Jones reagent is (obviously!) very acidic and so cannot be used in the presence of acidlabile functional groups. In these instances, a modified reagent is employed. Once again, 2 o alcohols will be oxidized to ketones. 1 o alcohols can be oxidized to aldehydes, provided that rigorously anhydrous conditions are employed. One disadvantage of the Collins reagent is that a large excess is often required in order to achieve good yields. This is particularly problematic given the toxicity and disposal issues surrounding Cr (VI) reagents. 4
2. Oxidation of Alcohols to Aldehydes, Ketones or Carboxylic Acids (cont d) Pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC) are also useful reagents for stopping at the aldehyde oxidation state. PCC is less acidic than the Jones reagent and PDC is even further reduced in acidity. 5
2. Oxidation of Alcohols to Aldehydes, Ketones or Carboxylic Acids (cont d) B. All these chromium reagents lead to a lot of toxic metal waste. And they re also not always perfect at stopping at the aldehyde oxidation state. The Swern oxidation is often a much cleaner reaction this is what REAL organic chemists use! To carry out a Swern reaction: Combine dimethyl sulfoxide (DMSO) and oxalyl chloride at 78 C. Add your alcohol, in solvent. Mix for awhile. Add triethyl amine, and warm to 0 C. Work up with water to remove the Et 3 NHCl. The other products are (smelly) gasses. Behold! Nice clean aldehyde! Non-acidic variants are also known. So what s the mechanism? Back to the board! 6
2. Oxidation of Alcohols to Aldehydes, Ketones or Carboxylic Acids (cont d) C. Selective oxidations. notes: MnO 2 is an insoluble brown powder: large excesses often required and Ag 2 O is kind of expensive! 7
3. A Brief (But Useful!) Survey of Other Important Oxidations A. Allylic Oxidation B. Epoxidation* D. Ozonolysis C. Dihydroxylation* * asymmetric versions are well-known and very robust! 8
4. Reductions Lots of functional groups can be reduced: alkenes, alkynes, C=O, C=N, and even alkyl halides. 9
4. Reductions Broadly speaking, reductions can be grouped into 4 mechanistic classes: A. Catalytic hydrogenation Notes: - often uses neutral, nonpolar substrates - usually gives syn addition if a heterogeneous catalyst is used - asymmetric hydrogenations with chiral metal complexes B. Nucleophilic reduction C. Electrophilic reduction Notes: - requires a polarized substrate, eg. - usually not stereospecific - asymmetric additions are sometimes possible (e.g. CBS cat.) Notes: - good for reducing carboxylic acids; mechanism is via: D. Radical reduction 10
5. Catalytic Hydrogenation Can be applied to alkenes, alkynes and imines (but not yet very useful for ketones). Use of a heterogeneous catalyst (e.g. Pd on carbon) generally affords a syn addition of hydrogen. In other words the reaction is stereospecific. 11
5. Catalytic Hydrogenation Can be applied to alkenes, alkynes and imines (but not yet very useful for ketones). Use of a heterogeneous catalyst (e.g. Pd on carbon) generally affords a syn addition of hydrogen. In other words the reaction is stereospecific. Relative to the size of a molecule, the heterogeneous catalyst is also huge. This means that the hydrogen will be delivered from the most accessible face of the molecule. In other words, the reaction is stereoselective. 12
5. Catalytic Hydrogenation Catalytic hydrogenation can also be enantioselective, if a chiral metal complex is used (i.e. a metal with a chiral ligand). Lots of these are known. Some Representative Chiral Ligands (of thousands...) 13
5. Catalytic Hydrogenation This is super important industrially. Catalytic enantioselective hydrogenation accounts for over 50% of all asymmetric processes done in industry. A couple of representative syntheses: 14
5. Catalytic Hydrogenation Hydrogenation of alkynes will afford alkanes unless a special poison is introduced into the catalyst! In the Lindlar catalyst, the lead (and sometimes quinoline) poisons the palladium, allowing the skilled experimentalist to stop at the alkene. This is a particularly good way to generate cis double bonds. Note that the Wittig reaction also gives cis alkenes (usually), but most other olefinations give trans products. Therefore partial reduction from an alkyne can be a useful strategy! 15
6. Nucleophilic Reductions: Hydride Reagents more reactive * more selective LiAlH 4 and Red-Al will reduce almost anything: aldehydes, ketones, alcohols and esters (to provide the corresponding alcohols), nitriles (to 1 o amines), and even under forcing conditions carboxylic acids! These reagents can even reduce alkyl tosylates!! DIBAL will reduce aldehydes and ketones as well, but in reactions with esters and nitriles, it is possible to stop at the corresponding aldehyde or imine (which hydrolyzes to aldehyde). NaBH 4 will only reduce more reactive species: aldehydes, ketones, imines, etc. NaBH 4 is compatible with water (and therefore other alcohols) whereas LiAlH 4 (and to a lesser extent DIBAL) will catch fire if exposed to water! 16 *note: DIBAL is properly described as an electrophilic reducing agent but it s convenient to discuss it here.
6. Nucleophilic Reductions: Hydride Reagents Lithium or potassium trialkyl borohydrides are stronger reducing agents than NaBH 4 The three electron-donating R groups make the borohydride more nucleophilic! all very powerful reducing agents! These also provide extra steric bulk that can be useful in providing selectivity in reactions. 17
6. Nucleophilic Reductions: Hydride Reagents Another way to accomplish selective reduction of enones is to take advantage of electronics: Cerium is oxophilic and so binds to the oxygen of the enone. This activates the C=O toward direct nucleophilic attack. The presence of cerium almost always makes reagents harder, i.e. favours 1,2-addition over 1,4-addition. By contrast, the Cu H bond in Stryker s reagent constitutes a much softer nucleophile, and therefore favours 1,4-addition over 1,2-addition. 18
6. Nucleophilic Reductions: Hydride Reagents Just as catalytic hydrogenation* provided a convenient means to control absolute stereochemistry (i.e. enantioselectivity) so too can hydride addition: Extremely useful reaction. Often gives ee s in the 90 99% range, and good yields. Both enantiomers of CBS catalyst are commercially available (or make from proline). What s the mechanism? See you in Chem 432 *and epoxidation, and dihydroxylation... 19
7. Functional Group Interconversions: an Incomplete Summary A. Aldehydes as Central Players in Organic Synthesis B. Alkenes as Central Players in Organic Synthesis C. Alcohols and Alkyl Halides as Central Players in Organic Synthesis 20
7. Functional Group Interconversions: A. Aldehydes in Organic Synthesis 21
7. Functional Group Interconversions: B. Alkenes in Organic Synthesis 22
7. Functional Group Interconversions: C. Alcohols and Alkyl Halides 23
7. Functional Group Interconversions: an Incomplete Summary A. Aldehydes as Central Players in Organic Synthesis B. Alkenes as Central Players in Organic Synthesis C. Alcohols and Alkyl Halides as Central Players in Organic Synthesis 24