Deoxygenation is a chemical reaction involving the removal of oxygen atoms from a molecule. The term also refers to the removal of molecular oxygen (O2) from gases and solvents, a step in air-free technique and gas purifiers. As applied to organic compounds, deoxygenation is a component of fuels production as well a type of reaction employed in organic synthesis, e.g. of pharmaceuticals.

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.[1]

Other routes

Oxygen groups can also be removed by the reductive coupling of ketones, as illustrated by the McMurry reaction.

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.[2][3]

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.[4]

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.[5][6]

S=O bonds

A chemical reagent for the deoxygenation of many sulfur and nitrogen oxo compounds is the combination trifluoroacetic anhydride/sodium iodide.[7] For example, in the deoxygenation of the sulfoxide diphenylsulfoxide to the sulfide diphenylsulfide:

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.

TFAA NaI deoxygenation mechanism

See also

References

  1. ↑ Sheldon, Roger A. (2014). "Green and sustainable manufacture of chemicals from biomass: state of the art". Green Chemistry. 16 (3): 950–963. doi:10.1039/C3GC41935E.
  2. ↑ K. Barry Sharpless, Martha A. Umbreit (1981). "Deoxygenation of Epoxides with Lower Valent Tungsten Halides: trans-Cyclododecene". Org. Synth. 60: 29. doi:10.15227/orgsyn.060.0029.
  3. ↑ Takuya Nakagiri; Masahito Murai; Kazuhiko Takai (2015). "Stereospecific Deoxygenation of Aliphatic Epoxides to Alkenes under Rhenium Catalysis". Org. Lett. 17 (13): 3346–3349. doi:10.1021/acs.orglett.5b01583. PMID 26065934.
  4. ↑ Tsurugi, Hayato; Mashima, Kazushi (2019). "Salt-Free Reduction of Transition Metal Complexes by Bis(trimethylsilyl)cyclohexadiene, -dihydropyrazine, and -4,4′-bipyridinylidene Derivatives". Accounts of Chemical Research. 52 (3): 769–779. doi:10.1021/acs.accounts.8b00638. PMID 30794373. S2CID 73505603.
  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. ↑ Podyacheva, Evgeniya; Kuchuk, Ekaterina; Chusov, Denis (2019). "Reduction of phosphine oxides to phosphines". Tetrahedron Letters. 60 (8): 575–582. doi:10.1016/j.tetlet.2018.12.070. S2CID 104364715.
  7. ↑ Trifluoroacetic anhydride-sodium iodide reagent. Nature and applications Arkivoc 2007 (JE-2136MR) Zbigniew H. Kudzin, Marcin H. Kudzin, Józef Drabowicz, and Andrzej Kotyński Link
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