Cristina Portela-García

Metal-assisted ring-closing/opening process of a chiral tetrahydroquinazoline.

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.

Serendipitous formation of 3-tosyl-1,2,3,4-tetrahydroquinazoline

Both experimental and computational studies were undertaken to elucidate the formation process of 3-tosyl-1,2,3,4-tetrahydroquinazoline from methanolic mother liquors of Pd(LBS)·3H2O, where LBS is the dianionic form of the imine ligand N-{2-[(8-hydroxyquinolin-2-yl)methyleneamino]benzyl}-4-methylbenzenesulfonamide. Experimental studies have shown that the tetrahydroquinazoline is obtained by condensation of 2-tosylaminomethylaniline and formaldehyde, which come from the acid-catalyzed hydrolysis of the imine ligand LBS and metal-mediated aerobic oxidation of methanol, respectively. Computational studies have revealed relevant intermediates and key steps in the reaction pathway.

A simple route to dinuclear complexes containing unusual μ-Nsulfonamido bridges

Abstract We report here the straightforward synthesis of some dimeric transition metal complexes of three Schiff base ligands, which were obtained by condensation of 2-tosylaminomethylaniline with 2,3-dihydroxybenzaldehyde (H2L1), 2,4-dihydroxybenzaldehyde (H2L2), and 4-formyl-3-hydroxybenzoic acid (H2L3), respectively. Both ligands and complexes were characterized by a combination of 1H NMR, infrared and mass spectrometries, and elemental analyses. Furthermore, the crystal structures of H2L2 and Pd2L22·2MeOH were elucidated by X-ray diffraction. The crystal structure of Pd2L22·2MeOH has instead of typical μ-Ophenoxo bridges, repeating units held together by unusual μ-Nsulfonamido bridges. This complex is chiral, although it crystallizes as a racemate, with C2v symmetry, where the two bridging nitrogens are asymmetric centers with identical handedness.

Controlling ring-chain tautomerism through steric hindrance

We have explored the use of steric hindrance for favouring/hindering the tautomerisation of Schiff bases (SB) into tetrahydroquinazolines (TQ) in two systems that derive from the condensation of 2-tosylaminobenzylamine with two different aldehydes: 2,3-dihydroxybenzaldehyde (H2L1SB/H2L1TQ) and N-(3-formylpyridin-2-yl)pivalamide (H2L2SB/H2L2TQ). The four possible ring-chain tautomers were unequivocally characterised by a combination of 1H NMR spectroscopy, infrared spectroscopy, mass spectrometry and elemental analysis. Furthermore, two of the tautomers, H2L1SB and H2L2TQ, have been characterised by X-ray crystallography. Crystal data of E-H2L1SB have revealed the existence of a prototropic ketoenamine–enolimine equilibrium at room temperature that is the cause of the thermochromism of H2L1SB. A firm intramolecular interaction Ohydroxyl–H⋯Nimine hinders the conversion of the chain tautomer H2L1SB into the ring tautomer H2L1TQ. Crystals of H2L2TQ and H2L2TQ·HCCl3 consist of racemic mixtures of their enantiomers, C(R),N(R)-H2L2TQ and C(S),N(S)-H2L2TQ. A terminal pivalamide group prevents the existence of the intramolecular interaction Npivalamide–H⋯Nimine in the chain tautomer H2L2SB, favouring its conversion into the ring tautomer H2L2TQ.

Different Bridging Preferences of Zinc and Palladium in Dimeric Complexes of a Ligand Based on 2-Tosylaminomethylaniline

We report here the formation of dimeric palladium(II) and zinc(II) complexes of a ligand (H2L) derived from the condensation of 2 tosylaminomethylaniline and 2,3-dihydroxybenzaldehyde.The structures in solution of the corresponding palladium(II) and zinc(II) complexes could be deduced from NMR studies. With the aim of establishing an energy-structure relationship between the two possible linkage isomers (μ-O bridged and μ-N bridged), some theoretical energy calculations have been performed.

Computational studies on the palladium-mediated oxidation of methanol to formaldehyde

We have carried out computational studies on the metal-mediated oxidation of methanol to formaldehyde using Pd(LSB)•3H2O, where LSB is the dianionic form of the Schiff base ligand N-{2-[(8-hydroxyquinolin-2-yl)methyleneamino]benzyl}-4-methylbenzenesulfonamide. With the aim of reducing the overall Gibbs energy of the global process, we have studied the replacement of hydroxymethoxymethyl by hydroxymethyl and aminomethyl groups at 2-position of the quinoline ring.

Mechanistic insights into a Zn-assisted ring-chain tautomerism process involving a 1,2,3,4-tetrahydroquinazoline and a Schiff base

Density functional theory calculations and spectroscopic monitoring of the reactions of the chain tautomer N-{2-[(8-hydroxyquinolin-2-yl)methyleneamino]benzyl}-4-methylbenzene-sulfonamide (H2Lchain) and the ring tautomer 2-(3-tosyl-1,2,3,4-tetrahydroquinazolin-2-yl)quinolin-8-ol (H2Lring) against Zn(OAc)2•2H2O have been performed in order to clarify the role of the metal ion in the ring-opening/closing reaction under investigation. The results obtained show that, among the processes leading to zinc complexes that could occur, the path leading to Zn(HLring)2 is the energetically more favorable one. Thus, this path would be preferred in the presence of sufficient free ligand in solution, regardless of its tautomeric form. The latter is due to the formation of energetically similar key intermediate complexes Zn(OAc)(HLchain) and Zn(OAc)(HLring). The process to afford Zn2(Lchain)2 by dimerization of the intermediate complex Zn(OAc)(HLchain) is energetically unfavourable and is only obtained in good yield by decreasing the stoichiometry of the ligand and at high reaction temperature.