title: Metal-assisted ring-closing/opening process of a chiral tetrahydroquinazoline.
abstract: 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 H(2)L(i) undergoes a selective ring-closing reaction upon complexation with Ni(2+). As a result, complexes of the type Ni(HL(a))(2) are obtained, whose chirality arises from the chiral ligand H(2)L(a) 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(HL(a))(2)·2HOAc and [Δ,Λ-C(S,R)N(R,S)]-Ni(HL(a))(2)·4MeOH. In contrast to the situation observed for Ni(2+), the cyclic tautomer of the ligand, H(2)L(a), undergoes a selective ring-opening reaction upon complex formation with Pd(2+), ultimately yielding Pd(HL(i))(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 (Ni(2+) and Pd(2+)) and to propose a mechanism for the metal-assisted opening/closing reaction of the tetrahydroquinazoline ring.
authors.affiliation: Departamento de Química Inorgánica, Facultad de Química, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain.
authors.forename: Ana M
authors.initials: AM
authors.lastname: García-Deibe
authors.forename: Jesús
authors.initials: J
authors.lastname: Sanmartín-Matalobos
authors.forename: Concepción
authors.initials: C
authors.lastname: González-Bello
authors.forename: Emilio
authors.initials: E
authors.lastname: Lence
authors.forename: Cristina
authors.initials: C
authors.lastname: Portela-García
authors.forename: Luis
authors.initials: L
authors.lastname: Martínez-Rodríguez
authors.forename: Matilde
authors.initials: M
authors.lastname: Fondo