Osmium Tetroxide (OsO4)

OSMIUM TETROXIDE (OsO4)

1. Osmium tetroxide (OsO4) is a colorless solid. But it appears in light yellow color because OsO4 usually impure with ‘OsO2’.

2. Osmium tetroxide (OsO4) has high vapor pressure i.e. it has volatile nature. Due to this high vapor pressure, it can gives highly toxic fumes which are extremely dangerous to human eyes, to overcome these toxicity (or) dangerous OsO4 should be handled with extreme care in a “Chemical fume hood”.

3. OsO4 is more expensive.

Preparation of OsO4

OsO2 is when treated with air under 8000c temperature then OsO4 is formed.

4. Osmium tetroxide (OsO4) is a 16 electron complex which is deficient from 2 electrons to exhibit octant rule (18 electron rule), so it is a electron deficient species. It can act as electrophile, which can accept / removes electrons from the substance or Reactants, so it can act as an oxidizing reagent.

5. In OsO4 oxidations, OsO4 is in electrophilic nature, while olefins are in nucleophilic nature, when electron releasing groups are present on olefins then the nucleophilicity of olefins are increases which results in increases rate of OsO4 oxidations. When electron withdrawing groups are present on olefins then nucleophilicity of olefins are decreases which results in decreases rate of OsO4 oxidations.

6. Osmium tetroxide (OsO4) oxidations will takes place under inert solvents / non-polar solvents / organic solvents) such as Carbon tetra chloride CCl4, Acetone, alcohols – t-BUOH, Methanol, Ether – THF, diethyl ether, dioxiranes. etc., OsO4 partially soluble in H2O. Most commonly “acetone “is used as solvent for OsO4 hydroxylation.

Applications of OSMIUM TETROXIDE (OsO4)

1. Oxidation of alkenes / Hydroxylation of alkenes

• Osmium tetroxide (OsO4) selectively oxidizes alkenes to syn – glycols.

• In the above mechanism alkenes / olefins are when treated OsO4 then five member cyclic osmate ester is formed as an intermediate. The formed cyclic osmate ester can be cleaved to cis – diols and osmic acid as by product by using reducing agents.

• The reducing agents such as Tertiary butyl alcohol + water mixture (most commonly used), Lithium aluminium hydride (LiAlH4), sodium sulfite or bisulfite in aqueous ethanol solution, Bisulfite ion in pyridine, Manitol in alkaline solution, KClO3 under acidic conditions.

• In the formed osmic acid, osmium atom is in +VI oxidation state, these osmium atom cannot be involve in further oxidations. It must be re oxidized to osmium (VIII) by using some co-oxidants. This regenerating step is helpful to reduce the expansive of osmium.

• The co-oxidants such as Metal chlorides (KClO3, NaClO4 (sodium chloride) silver chloride (AgClO3), barium chloride (BaClO3), peroxides (hydrogen peroxide t-BuOOH, t-amine oxides – N-methyl morpholine N-oxide.

• Tertiary amine-oxide, such as – N-methyl morpholine N-oxide (NMO) are very effective catalyst.

• In decomposition of osmate ester intermediate only osmium oxygen bond (Os-O) cleavage takes place preference to C-Obonding.

• So, finally the combination OsO4 /acetone / CCl4/ ethers (THF, Dioxine, diethyl ether, alcohols-t-BuOH, MeOH. Is used for hydroxylation.

• In some hydroxylation by OsO4 over oxidation products are formed.

• To overcome the over oxidation products of this reaction by the mixture of formaldehyde, ascorbic acid and Na2CO3.

2. Lemieux – Johnson reagent (OsO4- NaIO4)

• The combination of osmium tetroxide (OsO4) with sodium periodate (NaIO4) is known as Lemieux – Johnson reagent.

• Lemieux – Johnson reagent oxidizes alkenes to carbonyl compounds (aldehydes, ketones) as like in ozonolysis procedure.

• In this osmium tetroxide (catalytic amount) oxidizes alkenes to glycols and converted into corresponding carbonyl compounds followed by reaction with sodium periodate.

3. Oxidation of alkynes to carbonyl compounds

OsO4 is when treated with non-terminal acetylenes in the presence of tertiary amines, such as pyridine or isoquinoline to give osmium ester, which yields 1,2 di carbonyl compounds on hydrolysis with sodium sulfite.

OsO4 is when treated with terminal acetylene in the presence of tertiary amines, such as pyridine or isoquinoline to give osmium ester, which yields carboxylic acid compounds on hydrolysis with sodium sulfite, by loss of one acetylene carbon atom.

4. Asymmetric Hydroxylation

• The combination of osmium tetroxide (OsO4), N-methyl morpholine N-oxide (NMO), with chiral substrate i.e. cinchona alkaloids such as esters of dihydroquinidines is called as Asymmetric Hydroxylation agent, the hydroxylation which carries under this reagent is called as Asymmetric Hydroxylation.

• This was explained by sharpless, so it was named by sharpless Hydroxylation.

• In this Asymmetric Hydroxylation enantiomeric excess (%ee) of the diol products are formed.

5. Osmium tetroxide (OsO4) oxidizes sulfoxides to sulfones, but not sulfides.

6. Osmium tetroxide (OsO4) oxidizes the enol forms of carbonyl compounds (aldehydes, ketones), esters into - hydroxy carbonyl compounds or esters.