Deoxygenation

Deoxygenation of C-O bonds

With replacement by H₂

The main examples involving the replacement of an oxo group by two hydrogen atoms (A=O → AH2) are hydrogenolysis. Typical examples use metal catalysts and H2 as the reagent. Conditions are typically more forcing than hydrogenation.

Stoichiometric reactions that effect deoxygenation include the Wolff–Kishner reduction for aryl ketones. The replacement of a hydroxyl group by hydrogen (A-OH → A-H) is the point of the Barton–McCombie deoxygenation and the Markó–Lam deoxygenation.

Biomass valorization

Deoxygenation is an important goal of the conversion of biomass to useful fuels and chemicals. Partial deoxygenation is effected by dehydration and decarboxylation.

Other routes

Oxygen groups can also be removed by the reductive coupling of ketones, as illustrated by the McMurry reaction. :[[Image:McMurryBenzophenone.png|300px]]

Epoxides can be deoxygenated using the oxophilic reagent produced by combining tungsten hexachloride and n-butyllithium generates the alkene. This reaction can proceed with loss or retention of configuration. :[[File:Trans-CDene.png|thumb|center|220 px|Deoxygenation of trans-cyclododecene oxide, which occurs with retention.]]

Deoxygenation of S-O and P-O bonds

N=O bonds

Nitroaromatics are deoxygenated by strongly reducing silyl reagents such as N,N'-bis(trimethylsilyl)-4,4'-bipyridinylidene.

P=O bonds

Phosphorus occurs in nature as oxides, so to produce elemental form of the element, deoxygenation is required. The main method involves carbothermic reduction (i.e., carbon is the deoxygenation agent). :4 Ca5(PO4)3F + 18 SiO2 + 30 C → 3 P4 + 30 CO + 18 CaSiO3 + 2 CaF2

Oxophilic main group compounds are useful reagents for certain deoxygenations conducted on laboratory scale. The highly oxophilic reagent hexachlorodisilane (Si2Cl6) stereospecifically deoxygenates phosphine oxides.

S=O bonds

A chemical reagent for the deoxygenation of many sulfur and nitrogen oxo compounds is the combination trifluoroacetic anhydride/sodium iodide. For example, in the deoxygenation of the sulfoxide diphenylsulfoxide to the sulfide diphenylsulfide:

:[[Image:SulfoxideDeoxigenation.png|400px|Sulfoxide deoxygenation]]

The reaction mechanism is based on the activation of the sulfoxide by a trifluoroacetyl group and oxidation of iodine. Iodine is formed quantitatively in this reaction and therefore the reagent is used for the analytical detection of many oxo compounds. :[[Image:TFAA deoxigenationMechanism.png|400px|TFAA NaI deoxygenation mechanism]]

References

References

1. Sheldon, Roger A.. (2014). "Green and sustainable manufacture of chemicals from biomass: state of the art". Green Chemistry.
2. [[K. Barry Sharpless]], Martha A. Umbreit. (1981). "Deoxygenation of Epoxides with Lower Valent Tungsten Halides: ''trans''-Cyclododecene". [[Org. Synth.]].
3. (2015). "Stereospecific Deoxygenation of Aliphatic Epoxides to Alkenes under Rhenium Catalysis". Org. Lett..
4. (2019). "Salt-Free Reduction of Transition Metal Complexes by Bis(trimethylsilyl)cyclohexadiene, -dihydropyrazine, and -4,4′-bipyridinylidene Derivatives". Accounts of Chemical Research.
5. David P. Sebesta "Hexachlorodisilane" in Encyclopedia of Reagents for Organic Synthesis John Wiley, London, 2001. {{doi. 10.1002/047084289X.rh007 Article Online Posting Date: April 15, 2001.
6. (2019). "Reduction of phosphine oxides to phosphines". Tetrahedron Letters.
7. ''Trifluoroacetic anhydride-sodium iodide reagent. Nature and applications'' [[Arkivoc]] '''2007''' (JE-2136MR) Zbigniew H. Kudzin, Marcin H. Kudzin, Józef Drabowicz, and Andrzej Kotyński [http://content.arkat-usa.org/ARKIVOC/JOURNAL_CONTENT/manuscripts/2007/JE-2136MR%20as%20published%20mainmanuscript.pdf Link]