Regioselective halo- and carbodesilylation of (trimethylsilyl)-1-methylpyrazoles

The isomeric 3-, 4-, and 5-(trimethylsily1)- as well as the 3,4-, 3,5-, and 4,5-bis(trimethylsilyl)-l-methylpyrazoles (2,7,3,5,9,a nd 10, respectively) are obtained by methylation of the corresponding (trimethylsily1)-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 additional bromine, these monobromo compounds suffer exclusively bromodesilylation to give 3,4- and 4,5-dibromo-lmethylpyrazole (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 MeBSi group. The reaction of 5 with Iz, 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.