Enhancement of benzylic basicity by a fluorine substituent at the para-position: a case of lone pair/lone pair repulsion

The introduction of a halogen atom at any arom. position of toluene considerably accelerates the base-promoted deprotonation of the Me group. P-Fluorotoluene is the only exception; proton abstraction from its benzylic site occurs approx. at one tenth of the rate found with toluene (at -75 Deg). 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 soln. of lithium 2,2,6,6-tetramethylpiperidide (LITMP) in di-Et ether in the presence of potassium tert-butoxide and N,N,N',N\",N\"-pentamethyldiethylenetriamine at -100 Deg. Due to extensive side reactions (\"aryne\" formation as a consequence of concomitant deprotonation of arom. sites adjacent to the halogen atom), products can be isolated only in moderate yield (10-35%), but they are regioisomerically pure. [on SciFinder (R)]


Published in:
Chemistry - A European Journal, 6, 5, 771-777
Year:
2000
Keywords:
Note:
CAN 132:293476 22-12 Physical Organic Chemistry Dpt. of Org. Chem. Technology,Technical University,Budapest,Hung. Journal 0947-6539 written in English. 95-46-5 (2-Bromotoluene); 95-49-8 (2-Chlorotoluene); 95-52-3 (2-Fluorotoluene); 106-38-7 (4-Bromotoluene); 106-43-4 (4-Chlorotoluene); 108-41-8 (3-Chlorotoluene); 108-88-3 (Toluene); 352-32-9 (4-Fluorotoluene); 352-70-5 (3-Fluorotoluene); 591-17-3 (3-Bromotoluene) Role: PEP (Physical, engineering or chemical process), PRP (Properties), PROC (Process) (enhancement of benzylic basicity by a fluorine substituent at the para-position: a case of lone pair/lone pair repulsion)
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 Record created 2006-03-03, last modified 2018-12-03


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