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Enhancement of Benzylic Basicity by a Fluorine Substituent at the para ‐Position: A Case of Lone Pair/Pair Pair Repulsion
Author(s) -
Faigl Ferenc,
Marzi Elena,
Schlosser Manfred
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/(sici)1521-3765(20000303)6:5<771::aid-chem771>3.0.co;2-g
Subject(s) - lone pair , substituent , chemistry , fluorine , position (finance) , computational chemistry , stereochemistry , organic chemistry , molecule , economics , finance
Abstract 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 ‐butoxide 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.