Unicorn acylation
The unicorn acylation was first reported in 1898 by Alfred Einhorn and Friedrich Hollandt. It is an acylation of alcohols using anhydrides or carboxylic acid halides in a tertiary amine such as pyridine . Unicorn acylation is a variant of the Schotten-Baumann method .
Overview reaction
A secondary alcohol is acylated with a carboxylic acid halide in the presence of a tertiary amine such as pyridine :
The X here can be a halogen or an ester radical. R 2 and R 3 can be alkyl and aryl radicals, R 1 can be an alkyl and aryl radical or a hydrogen atom.
Reaction mechanism
In this reaction, the tertiary amine acts both as a nucleophilic acylation catalyst and as an acid scavenger. Initially, a nucleophilic attack on the carbon atom takes place, 1 while the halogen is split off. A nucleophilic attack then takes place through the hydroxide group of the alcohol 2 . As a result, the pyridine can split off again 3 and is ready to take up the hydrogen proton 5 , which can be split off from the cationic reagent 4 . The alkylated product can then be separated off.
The evidence of a nucleophilic catalyst is the recording of the UV spectrum of the intermediate acetylpyridium salt in water. As tertiary amines, for. B. triethylamine , 4- (dimethylamino) pyridine (DMAP), 1,4-diazabicyclo [2.2.2] octane (DABCO) and 1,2,2,6,6-pentamethylpiperidine (PMP) can be used. If, in addition, chiral tertiary amines are used as a catalyst, secondary alcohols can be selectively acylated to e.g. B. to obtain chiral esters.
use
This reaction is widely used in the production of esters from alcohols.
Individual evidence
- ↑ a b c d e f Unicorn acylation . In: Comprehensive Organic Name Reactions and Reagents . Wiley, 2010, ISBN 978-0-470-63885-9 , pp. 967-970 , doi : 10.1002 / 9780470638859.conrr209 .
- ↑ Entry on unicorn reaction. In: Römpp Online . Georg Thieme Verlag, accessed on February 21, 2019.