Tricarbonylrhenium(I) halide complexes of chiral non-racemic 2-(dioxolanyl)-6-(dioxanyl)pyridine ligands: Synthesis, NMR and DFT calculations

Abstract

The chiral non-racemic O/N/O donor ligands 2-[(4R,5R)-4,5-dimethyl-1,3- dioxolan-2-yl]-6-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine and 2-[(4R,5R)-4,5-dimethyl-1,3-dioxolan-2-deuteryl]-6-[(4R,6R)-4,6-dimethyl-1, 3-dioxan-2-yl]pyridine were prepared in a stepwise fashion form 2,6-dibromopyridine. Reaction with the pentacarbonylhalogenorhenium(I) compounds yields the complexes [ReX(CO)3L], in which the ligands act in a N/O bidentate chelate fashion. There are eight possible diastereoisomers, three of which are observable in solution. DFT calculations indicate that the relative stability of the diastereoisomers is SR5>RR 5>SS5≈RS5>RS6>SS 6>RR6>SR6. Above ambient temperature, a dynamic process leads to the exchange of 2 of the 3 diastereoisomers: the free energy of activation is ca. 79 kJmol-1. The results of the DFT calculations and the magnitude of ΔG‡ suggest the dynamic process to be the flip of the co-ordinated acetal ring. © 2003 Elsevier Ltd. All rights reserved.