Piperidine and piperazine analogs in action: zinc(ii)-mediated formation of amidines

Abstract

The catalytic activity of zinc(II) compounds was exploited in the nucleophilic addition of amines to nitriles to form amidines. The reaction systems comprised a Zn(II) starting material [Zn(quin)₂(H₂O)] (quin⁻ = quinaldinate, an anion of quinaldinic acid), secondary cyclic amine, acetonitrile, and in some cases, methanol. Amines with additional heteroatoms in the ring (thiomorpholine, piperazine) or ring substituents (piperidine derivatives, piperazine derivatives) with different steric and electronic effects were used. The aim of the study was to determine how the nature of the amine affects the formation of amidines. Different types of Zn(II) products were obtained: mono- or diamine complexes, amidine complexes, and also an ionic compound with a protonated amine. In one case, the amidine itself crystallized. A Zn(II) complex with acetamidine was also obtained. Acetamidine was formed from acetonitrile and ammonia, which most likely originated from the hydrolysis of acetonitrile under harsh reaction conditions. The hydrolysis could terminate at the acetamide step, which was confirmed by the isolation of a cocrystal containing acetamide. In the case of piperazine (pz), a polymeric compound with the composition [Zn(quin)₂(pz)]_n was isolated regardless of the reaction conditions. The same coordination polymer was also observed to form in 1-methylpiperazine, 1-ethylpiperazine, and 1-acetylpiperazine reaction systems, containing piperazine as an inherent impurity. This was unambiguously confirmed by the crystal structure of a cocrystal, [Zn(quin)₂(1-Acpz)₂]·[Zn(quin)₂(pz)]_n·4CH₃CN, which formed in the 1-acetylpiperazine (1-Acpz) reaction system.