Multicomponent One-pot Reactions Towards the Synthesis of Stereoisomers of Dipicolylamine Complexes

Reported are multi-component one-pot syntheses of chiral complexes [M((LOR)-O-R')Cl-2] or [M((LSR)-S-R')Cl2] from the mixture of an N-substituted ethylenediamine, pyridine-2-carboxaldehyde, a primary alcohol or thiol and MCl2 utilizing in-situ formed cyclized Schiff bases where a C-O bond, two stereocenters, and three C-N bonds are formed (M=Zn, Cu, Ni, Cd; R=Et, Ph; R'=Me, Et, nPr, nBu). Tridentate ligands (LOR)-O-R' and (LSR)-S-R' comprise two chiral centers and a hemiaminal ether or hemiaminal thioether moiety on the dipicolylamine skeleton. Syn-[Zn((LOMe)-O-Ph)Cl-2] precipitates out readily from the reaction mixture as a major product whereas anti-[Zn((LOMe)-O-Ph)Cl-2] stays in solution as minor product. Both syn-[Zn((LOMe)-O-Ph)Cl-2] and anti-[Zn((LOMe)-O-Ph)Cl-2] were characterized using NMR spectroscopy and mass spectrometry. Solid-state structures revealed that syn-[Zn((LOMe)-O-Ph)Cl-2] adopted a square pyramidal geometry while anti-[Zn((LOMe)-O-Ph)Cl-2] possesses a trigonal bipyramidal geometry around the Zn centers. The scope of this method was shown to be wide by varying the components of the dynamic coordination assembly, and the structures of the complexes isolated were confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Syn complexes were isolated as major products with Zn-II and Cu-II, and anti complexes were found to be major products with Ni-II and Cd-II. Hemiaminals and hemiaminal ethers are known to be unstable and are seldom observed as part of cyclic organic compounds or as coordinated ligands assembled around metals. It is now shown, with the support of experimental results, that linear hemiaminal ethers or thioethers can be assembled without the assistance of Lewis acidic metals in the multi-component assembly, and a possible pathway of the formation of hemiaminal ethers has been proposed.