Alkylations of N(4)-(4-pyridyl)-3,5-di(2-pyridyl)-1,2,4-triazole: first observation of room-temperature rearrangement of an N(4)-substituted triazole to the N(1) analogue.

Attempts to use alkylation to introduce a positive charge at the nitrogen atom of the 4-pyridyl ring in the bis(bidentate) triazole ligand N(4)-(4-pyridyl)-3,5-di(2-pyridyl)-1,2,4-triazole (pydpt) were made to ascertain what effect a strongly electron-withdrawing group would have on the magnetic properties of any subsequent iron(II) complexes. Alkylation of pydpt under relatively mild conditions led in some cases to unexpected rearrangement products. Specifically, when benzyl bromide is used as the alkylating agent, and the reaction is carried out in refluxing acetonitrile, the N(4) substituent moves to the N(1) position. However, when the same reaction is performed in dichloromethane at room temperature, the rearrangement does not occur and the desired product containing an alkylated N(4) substituent is obtained. Heating a pure sample of N(4)-BzpydptBr to reflux in MeCN resulted in clean conversion to N(1)-BzpydptBr. This is consistent with N(4)-BzpydptBr being the kinetic product whereas N(1)-BzpydptBr is the thermodynamic product. When methyl iodide is used as the alkylating agent, the N(4) to N(1) rearrangement occurs even at room temperature, and at reflux pydpt is doubly alkylated. The observation of the lowest reported temperatures for an N(4) to N(1) rearrangement is due to this particular rearrangement involving nucleophilic aromatic substitution: a possible mechanism for this transformation is suggested.