Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen atoms.
The counterpart of demethylation is methylation.
- R2N-CH3 + O → R2N-H + CH2O
In organic chemistry
Demethylation typically refers to cleavage of ethers, especially aryl ethers, although there are some exceptions, for instance cf. "desipramine".
The reaction typically requires harsh conditions or harsh reagents. For example, the methyl ether in vanillin can be removed by heating near 250 °C with strong base. Stronger nucleophiles such as diorganophosphides (LiPPh2) also cleave aryl ethers under milder conditions.
Boron tribromide is a classical reagent for the dealkylation of methyl arylethers. The mechanism of ether dealkylation proceeds via the formation of a complex between the boron center and the ether oxygen followed by the elimination of an alkyl bromide to yield a dibromo(organo)borane. The dibromo(organo)borane can then undergo hydrolysis to give the alcohol or phenol, boric acid, and hydrogen bromide as products.
- ROR + BBr3 → RO+(−BBr3)R
- RO+(−BBr3)R → ROBBr2 + RBr
- ROBBr2 + 3 H2O → ROH + B(OH)3 + 2 HBr
N-demethylation of 3° amines is by the Von Braun reaction, which uses BrCN as the reagent to give the corresponding nor derivatives. The etymological meaning of the word nor is "N ohne R". A modern variation of the Von Braun reaction was developed where BrCN was superseded by ethyl chloroformate. The preparation of Paxil from arecoline is an application of this reaction, as well as the synthesis of GSK-372,475, for example.
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