2-Dihydromethylpiperazinediium-M-II (M-II = Cu-II, Fe-II, Co-II, Zn-II) double sulfates and their catalytic activity in diastereoselective nitroaldol (Henry) reaction

A series of double dihydromethylpiperazinediium metallic sulfates 1-7 [H(2)mpz]M-II(SO4)(2)center dot nH(2)O (mpz = 2-methylpiperazine, C5H12N2) are prepared by slow evaporation using a racemic (R/S)-mpz (for 1, 2) or enantiopure R-mpz (for 3), S-mpz (for 4-7) and sulfates of Cu-II (for 5), Fe-II (for 1, 4), Co-II (for 7) and Zn-II (for 2, 3, 6), and characterized by infrared spectroscopy, elemental analysis and single crystal X-ray diffraction. The [M-II(H2O)(6)](2+), [(R/S)-H(2)mpz](2+), [(R)-H(2)mpz](2+) or [(S)-H(2)mpz](2+) cations and 2SO(4)(2-) anions are linked together via two types of hydrogen bonds, Ow-Hw center dot center dot center dot O and N-H center dot center dot center dot O, leading to supramolecular arrangements. The use of the racemic 2-mpz provides alternatives in crystallization: a centrosymmetric structure where the enantiomers are related by an appropriate crystallographic symmetry operation or one where the enantiomers occupy the same crystallographic position, generating disorder. Compounds 1-7 act as diastereoselective catalysts for the nitroaldol (Henry) reaction. The diastereoselectivity can be regulated from exclusive threo to exclusive erythro isomer preparation with typical yields of 50-99%, depending on the catalyst and the substrate used.