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)-(-)-alpha-methylbenzylamine] using palladium-catalysed amination. Protonolysis reactions of these proligands with ZrX(4)(X = NMe(2), CH(2)Ph, CH(2)Bu(t)) yield zirconium aminopyridinates, usually of the type [ML(2)X(2)], 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 [small alpha]-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 (DeltaH 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 (DeltaH 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.

Expression of chirality in salicyloxazolinate complexes of zirconium

“…On increasing the temperature, the signals for H A and H B broaden but do not coalesce, then finally resharpen at new chemical shifts in the fast exchange regime above 353 K. Notably N-dissociation in this system does not increase the distance of the stereogenic C centres from the inner coordination sphere. A similar comparison is possible between the present system and our recently reported chiral aminopyridinates [10] where the portion of the ligand remaining bonded to the metal contains the chiral information.…”

“…We set out to investigate the relative energies of the sixcoordinate diastereomers of ½L 1 2 ZrðCH 2 PhÞ 2 via DFT [21]; predicted diastereoselectivities from this approach consis- Table 1 Selected bond lengths (Å ) and angles (°) for ½L 1 2 ZrðCH 2 PhÞ 2 (1) 143.2 (7) tently agree well with experimental observations in related systems [10][11][12].…”

“…It is not easy to quantify the effect of the presence of the asymmetry on the magnitude of the apparent chemical shift difference between H A and H B , i.e., how well the chirality of the system is expressed in this physical property [22]. Experience dictates however that in a given chiral-atmetal system with diastereotopic co-ligands, the magnitude of the splitting (Dd ppm) is affected strongly by the degree of fluxionality; as temperature is increased and exchange processes become more rapid, Dd tends to fall [9,10,12]. Fig.…”

“…group 4 metals, this points us toward octahedral complexes of the type [M(AB) 2 X 2 ], where AB is a chiral bidentate monoanionic ligand. While this class of complex can in principle form eight diastereomers we have shown that it is quite possible to design highly diastereoselective systems for zirconium using the bidentate N-N ligands aminopyridinate [9,10] and aminooxazolinate [11]. Surprisingly high kinetic stability and excellent diastereoselection was found for a pyridine/alcoholate N-O system [12].…”

Diazaallyls of group 4 metals based on trans-1,2-diaminocyclohexane

“…We have recently reported work on the synthesis, structural and dynamic properties of group 4 metal complexes of bidentate aminopyridinato ligands I [1][2][3]. This has been concerned with control of stoichiometry and structure for both achiral [1] and chiral [2] systems.…”