Synthetic and theoretical MO calculational studies of lithiotriazine intermediates produced during alkyllithium-induced cyclotrimerisation reactions of organic nitriles, and comparison of their structures with that of a methylmagnesiotriazine derivative

Abstract

Benzonitrile can be readily cyclotrimerised by treatment with a suitable alkyllithium to give a simple triazine or to a solvated lithiodihydrotriazine derivative. Which cyclic product dominates depends mainly on the source of the active Li + cation ( n-butyllithium, t-butyllithium and tetramethylguanidinolithium are considered here), and on the solvent employed. X-ray crystallographic studies on a representative compound show that the lithio species exists as a mononuclear, contact ion pair structure, with the triazine ring in a 1,4-dihydro state. On reaction with the Grignard reagent, MeMgCl, this compound gives a methylmagnesiodihydrotriazine, which has also been crystallographically characterised and found to closely resemble its lithio precursor. Ab initio MO calculations on model systems reveal that the 1,4-dihydrotriazine arrangement is energetically preferred to the 1,2-dihydro alternative irrespective of the counterion (Li + or H +) present. A theoretical investigation of the methanolysis of the lithio species indicates that the formation of an intermediate MeOH complex, with the alcohol attached to a ring N atom and not to Li +, directs the reaction towards ultimate formation of a 1,2-dihydrotriazine.