Download pptx

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
1
or
• removal of hydrogen
• removal of oxygen
or
• removal of one electron
or
• addition of one electron
Chemistry 335 Supplemental Slides: Interlude 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
Chemistry 335 Supplemental Slides: Interlude 1
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 = CrO3
Dichromate = Cr2O72Chromium trioxide will oxidize 2o alcohols to ketones. A specific cocktail of CrO3 and H2SO4 in
acetone is called the Jones reagent, and is particularly effective for this transformation.
1o 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
Chemistry 335 Supplemental Slides: Interlude 1
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, 2o alcohols will be oxidized to ketones. 1o 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
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
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 Et3NHCl. 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
Chemistry 335 Supplemental Slides: Interlude 1
2. Oxidation of Alcohols to Aldehydes, Ketones or Carboxylic Acids (cont’d)
C. Selective oxidations.
notes: MnO2 is an insoluble brown powder: large excesses often required…
… and Ag2O is kind of expensive!
7
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
4. Reductions
Lots of functional groups can be reduced: alkenes, alkynes, C=O, C=N, and even alkyl halides.
9
Chemistry 335 Supplemental Slides: Interlude 1
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
Notes:
- requires a polarized substrate, eg.
- usually not stereospecific
- asymmetric additions are sometimes possible (e.g. CBS cat.)
C. Electrophilic reduction
Notes:
- good for reducing carboxylic acids; mechanism is via:
D. Radical reduction
10
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
6. Nucleophilic Reductions: Hydride Reagents
more reactive
*
more selective
• LiAlH4 and Red-Al will reduce almost anything: aldehydes, ketones, alcohols and esters (to
provide the corresponding alcohols), nitriles (to 1o 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).
• NaBH4 will only reduce more reactive species: aldehydes, ketones, imines, etc.
• NaBH4 is compatible with water (and therefore other alcohols) whereas LiAlH4 (and to a
lesser extent DIBAL) will catch fire if exposed to water!
*note: DIBAL is properly described as an electrophilic
reducing agent but it’s convenient to discuss it here.
16
Chemistry 335 Supplemental Slides: Interlude 1
6. Nucleophilic Reductions: Hydride Reagents
Lithium or potassium trialkyl borohydrides are stronger reducing agents than NaBH4
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
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1
7. Functional Group Interconversions:
21
A. Aldehydes in Organic Synthesis
Chemistry 335 Supplemental Slides: Interlude 1
7. Functional Group Interconversions:
22
B. Alkenes in Organic Synthesis
Chemistry 335 Supplemental Slides: Interlude 1
7. Functional Group Interconversions:
23
C. Alcohols and Alkyl Halides
Chemistry 335 Supplemental Slides: Interlude 1
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
Chemistry 335 Supplemental Slides: Interlude 1