Activation of exocyclic α-positions of azole N-oxides by O-silylation

Abstract

Exocyclic α-positions at immonium ring carbon atoms of pyrazole and 1,2,3-triazole 1-oxides are selectively attacked in a one-pot sequence comprising silylation with iodotrimethylsilane, deprotonation with 1,2,2,6,6-pentamethylpiperidine and allylic substitution of the trimethylsilyloxy group with the iodide ions liberated by the silylation. In this way 3- and 5-iodomethylpyrazoles as well as 4-iodo-methyl-1,2,3-triazoles are obtained in good yields. These may be useful for the preparation of heteroarylmethyl derivatives since the iodine atom can be displaced by even weak nucleophiles such as acetate ions. The side chain iodination is complementary to O-alkylation of the N-oxides followed by nucleophilic attack which preferentially takes place at ring carbon atoms.