Oxophilicity
Encyclopedia
Oxophilicity is the tendency of certain chemical compounds to form oxide
s by hydrolysis or abstraction of oxygen, often from organic compounds. The term is usually used to describe metal centers, commonly the early transition metals such as titanium, niobium, and tungsten. Oxophilic metals are classified as "hard" within the HSAB concept. Many main group compounds are also oxophilic, such as derivatives of aluminium, silicon, and phosphorus(V). The handling of oxophilic compounds often requires air-free technique
s.
. For example, the high valent chlorides hydrolyze rapidly to give oxides:
These reactions proceed via oxychloride intermediates. For example WOCl4
results from the partial hydrolysis of tungsten hexachloride
. Hydroxide-containing intermediates are rarely observed for oxophilic metals. In contrast, the anhydrous
halides of the later metals tend to hydrate, not hydrolyze, and they often form hydroxides.
Reduced complexes of oxophilic metals tend to generate oxides by reaction with oxygen. Typically the oxide-ligand is bridging
, e.g.
Only in rare cases do the products of oxygenation feature terminal oxo ligands.
and butyl lithium
is useful for the deoxygenation of epoxide
s. Such conversions are sometimes valuable in organic synthesis
. In the McMurry reaction
, ketones are converted into alkenes using oxophilic reagents:
Similarly, Tebbe's reagent
is used in olefination reactions:
Oxophilic main group compounds are also well known and useful. The highly oxophilic reagent Si2Cl6 stereospecifically deoxygenates phosphine oxide
s. Phosphorus pentasulfide
and the related Lawesson's reagent
convert certain organic carbonyls to the corresponding sulfur derivatives:
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s by hydrolysis or abstraction of oxygen, often from organic compounds. The term is usually used to describe metal centers, commonly the early transition metals such as titanium, niobium, and tungsten. Oxophilic metals are classified as "hard" within the HSAB concept. Many main group compounds are also oxophilic, such as derivatives of aluminium, silicon, and phosphorus(V). The handling of oxophilic compounds often requires air-free technique
Air-free technique
Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less commonly carbon dioxide and nitrogen...
s.
Typical reactivity
Complexes of oxophilic metals typically are prone to hydrolysisHydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
. For example, the high valent chlorides hydrolyze rapidly to give oxides:
- TiCl4 + 2 H2O → TiO2Titanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
+ 4 HCl
These reactions proceed via oxychloride intermediates. For example WOCl4
Tungsten(VI) oxytetrachloride
Tungsten oxytetrachloride,also known as Tungsten tetrachloride oxide, is an inorganic compound with the formula WOCl4. This diamagnetic solid is used to prepare other complexes of tungsten. The orange-coloured compound is soluble in nonpolar solvents but it reacts with alcohols and water and...
results from the partial hydrolysis of tungsten hexachloride
Tungsten hexachloride
Tungsten hexachloride is the chemical compound of tungsten and chlorine with the formula WCl6. This dark violet blue species exists as a volatile solid under standard conditions. It is an important starting reagent in the preparation of tungsten compounds. WCl6 is a rare example of a...
. Hydroxide-containing intermediates are rarely observed for oxophilic metals. In contrast, the anhydrous
Anhydrous
As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
halides of the later metals tend to hydrate, not hydrolyze, and they often form hydroxides.
Reduced complexes of oxophilic metals tend to generate oxides by reaction with oxygen. Typically the oxide-ligand is bridging
Bridging ligand
A bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually restricted to small ligands such as pseudohalides or to ligands that are...
, e.g.
- 2 (C5H5)2TiCl + 1/2 O2 → {(C5H5)2TiCl}2O
Only in rare cases do the products of oxygenation feature terminal oxo ligands.
Applications of oxophilicity in synthesis
Oxophilic reagents are often used to extract or exchange oxygen centers in organic substrates, especially carbonyls (esters, ketones, amides) and epoxides. The highly oxophilic reagent generated from tungsten hexachlorideTungsten hexachloride
Tungsten hexachloride is the chemical compound of tungsten and chlorine with the formula WCl6. This dark violet blue species exists as a volatile solid under standard conditions. It is an important starting reagent in the preparation of tungsten compounds. WCl6 is a rare example of a...
and butyl lithium
Butyl lithium
Butyllithium may refer to one of three isomeric organolithium reagents used in chemical synthesis:*n-Butyllithium, abbreviated BuLi or nBuLi*sec-Butyllithium, abbreviated sec-BuLi or sBuLi...
is useful for the deoxygenation of epoxide
Epoxide
An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...
s. Such conversions are sometimes valuable in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
. In the McMurry reaction
McMurry reaction
The McMurry reaction is an organic reaction in which two ketone or aldehyde groups are coupled to an alkene using titanium chloride compound such as titanium chloride and a reducing agent. The reaction is named after its co-discoverer, John E. McMurry. The McMurry reaction originally involved the...
, ketones are converted into alkenes using oxophilic reagents:
- 2 R2CO + "Ti" → R2C=CR2 + TiO2
Similarly, Tebbe's reagent
Tebbe's reagent
The Tebbe reagent is the organometallic compound with the formula 2TiCH2ClAl2. It used in the methylenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative...
is used in olefination reactions:
- Cp2TiCH2AlCl(CH3)2 + R2C=O → "Cp2TiOAlCl(CH3)2" + R2C=CH2
Oxophilic main group compounds are also well known and useful. The highly oxophilic reagent Si2Cl6 stereospecifically deoxygenates phosphine oxide
Phosphine oxide
Phosphine oxides are either inorganic phosphorus compounds such as phosphoryl trichloride or organophosphorus compounds with the formula OPR3, where R = alkyl or aryl...
s. Phosphorus pentasulfide
Phosphorus pentasulfide
Phosphorus pentasulfide is the inorganic compound with the formula P4S10. This yellow solid is the one of two phosphorus sulfides of commercial value...
and the related Lawesson's reagent
Lawesson's reagent
Lawesson's reagent, or LR, is a chemical compound used in organic synthesis as a thiation agent. Lawesson's reagent was first made popular by Sven-Olov Lawesson, who did not, however, invent it. Lawesson's reagent was first made in 1956 during a systematic study of the reactions of arenes with...
convert certain organic carbonyls to the corresponding sulfur derivatives:
- P4S10 + n R2C=O → P4S10-nOn + n R2C=S