Chiral metal architectures in aminopyridinato complexes of zirconium

Abstract

Optically pure 2-alkylaminopyridines (HL) are synthesised readily from bromopyridines and chiral amines [(S)-1,2,3,4-tetrahydro-1-naphthylamine and (S)-(−)-α-methylbenzylamine] using palladium-catalysed amination. Protonolysis reactions of these proligands with ZrX₄ (X = NMe₂, CH₂Ph, CH₂Bu^t) yield zirconium aminopyridinates, usually of the type [ML₂X₂], some of which have been characterised by X-ray crystallography. Control of absolute configuration at the metal centre is pursued by investigation of the effects of chiral amine substituent, substitution at the pyridine rings and the identity of co-ligands. Surprisingly the conformationally flexible α-methylbenzyl based aminopyridinato ligands promote much better control of chirality-at-zirconium than do the cyclic tetrahydronaphthyl analogues. One complex of the former class displays complete control of stereochemistry at 193 K; only one diastereomer out of eight possible structures is observed. It is found that there is an excellent correlation between observed selectivities and calculated diastereomer energy differences from DFT. All the complexes studied are in dynamic exchange between diastereomers. The rate of these processes (ΔH^‡ca. 40 kJ mol⁻¹) as studied by Selective Polarisation Transfer–Selective Inversion Recovery experiments (SPT-SIR) and lineshape analyses are significantly faster than those for aminopyridines containing bulkier amido substituents (ΔH^‡ca. 70 kJ mol⁻¹). This type of dependence on steric effects, and the impact of the trans effect, is consistent with an N-dissociative mechanism, i.e. conversion from six- to five-coordinate structure followed by rapid intramolecular scrambling.