Zinc(ii) ortho-hydroxyphenylhydrazo-β-diketonate complexes and their catalytic ability towards diastereoselective nitroaldol (Henry) reaction

Abstract

The zinc(II) complexes with ortho-hydroxy substituted arylhydrazo-β-diketonates [Zn₂(CH₃OH)₂(μ-L¹)₂] (5), [Zn{(CH₃)₂SO}(H₂O)(L²)] (6), [Zn₂(H₂O)₂(μ-L³)₂] (7) and [Zn(H₂O)₂(L⁴)]·H₂O (8) were synthesized by reaction of a zinc(II) salt with the appropriate hydrazo-β-diketone, HO-2-C₆H₄-NHNC{C(O)CH₃}₂ (H₂L¹, 1), HO-2-O₂N-4-C₆H₃-NHNC{C(O)CH₃}₂ (H₂L², 2), HO-2-C₆H₄-NHNCC(O)CH₂C(CH₃)₂CH₂C(O) (H₂L³, 3) or HO-2-O₂N-4-C₆H₃-NHNCC(O)CH₂C(CH₃)₂CH₂C(O) (H₂L⁴, 4). They were fully characterized, namely by X-ray diffraction analysis that disclosed the formation of extensive H-bonds leading to 1D chains (5 and 6), 2D layers (7) or 3D networks (8). The thermodynamic parameters of the Zn(II) reaction with H₂L² in solution, as well as of the thermal decomposition of 1–8 were determined. Complexes 5–8 act as diastereoselective catalysts for the nitroaldol (Henry) reaction. The threo/erythro diastereoselectivity of the β-nitroalkanol products ranges from 8 : 1 to 1 : 10 with typical yields of 80–99%, depending on the catalyst and substrate used.