Journal of Organic Chemistry p. 4687 - 4695 (1984)
Update date:2022-08-04
Topics:
Effenberger, Franz
Krebs, Andreas
The isomeric 3-, 4-, and 5-(trimethylsilyl)- as well as the 3,4-, 3,5-, and 4,5-bis(trimethylsilyl)-1-methylpyrazoles (2, 7, 3, 5, 9, and 10, respectively) are obtained by methylation of the corresponding (trimethylsilyl)-1H-pyrazoles or by silylation of Grignard or lithio derivatives of appropriate 1-methylpyrazoles with chlorotrimethylsilane. 5 and 10 are halodesilylated regioselectively by Br2 or ICl in the 4-position, yielding 13 and 15.With addditional bromine, these monobromo compounds suffer exclusively bromodesilylation to give 3,4- and 4,5-dibromo-1-methylpyrazole (14 and 16, respectively).These findings are in accord with the electrophilic substitution reactivity indices for 1-methylpyrazole (8) and with ipso-directing influence of the Me3Si group.The reaction of 5 with I2, unexpectedly, attacks preferentially at the 3-position.Regioselective carbodesilylation in the 5-position is observed in the fluoride-catalyzed reactions of 3, 9, and 10 with carbon electrophiles.The high regiospecificity of this reaction is rationalized in terms of carbanion stabilization at the individual pyrazole positions.
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