Metal-Assisted Ring-Closing/Opening Process of a Chiral Tetrahydroquinazoline

• Published in: Inorganic chemistry 2012-02, Vol.51 (3), p.1278-1293
• Main Authors: García-Deibe, Ana M, Sanmartín-Matalobos, Jesús, González-Bello, Concepción, Lence, Emilio, Portela-García, Cristina, Martínez-Rodríguez, Luis, Fondo, Matilde
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic2017038

The ring–chain tautomerism of a 2-aryl-1,2,3,4-tetrahydroquinazoline has been exploited to induce reversible changes in the aminal–imine equilibrium, as desired, by coordination of a suitable metal ion. This process was studied by NMR and UV–vis spectroscopies, X-ray crystallography, and molecular modeling approach. The results obtained show that the imine H2Li undergoes a selective ring-closing reaction upon complexation with Ni2+. As a result, complexes of the type Ni­(HLa)2 are obtained, whose chirality arises from the chiral ligand H2La and the helicity of the structure. Hence, helical enantiomers form the following racemates: [Δ-C­(R,R)­N­(S,S),Λ-C­(S,S)­N­(R,R)]-Ni­(HLa)2·2HOAc and [Δ,Λ-C­(S,R)­N­(R,S)]-Ni­(HLa)2·4MeOH. In contrast to the situation observed for Ni2+, the cyclic tautomer of the ligand, H2La, undergoes a selective ring-opening reaction upon complex formation with Pd2+, ultimately yielding Pd­(HLi)2·MeOH, in which the open-chain imine ligand is bidentate through the N,O donor set of the quinoline residue. Density functional theory calculations were conducted to provide insight into the different behavior of both coordinated metals (Ni2+ and Pd2+) and to propose a mechanism for the metal-assisted opening/closing reaction of the tetrahydroquinazoline ring.