The introduction of a halogen atom at any aromatic position of toluene considerably accelerates the base-promoted deprotonation of the methyl group. p-Fluorotoluene is the only exception; proton abstraction from its benzylic site occurs approximately at one tenth of the rate found with toluene (at -75°C). Lone-pair repulsion appears to be at the origin of the decrease in acidity. Chloro- and bromotoluenes instantaneously exchange benzylic hydrogen against metal when treated with solution of lithium 2,2,6,6-tetramethylpiperidide (LITMP) in diethyl ether in the presence of potassium tert-buloxide. and N,N,N′,N″,N″-pentamethyldiethylenetriamine at -100°C. Due to extensive side reactions ("aryne" formation as a consequence of concomitant deprotonation of aromatic sites adjacent to the halogen atom), products can be isolated only in moderate yield (10-35%), but they are regioisomerically pure.
|Number of pages||7|
|Journal||Chemistry - A European Journal|
|Publication status||Published - Mar 3 2000|
- Electron-electron repulsion
- Substituent effects
ASJC Scopus subject areas
- Organic Chemistry