Regresar

Acylation of Pyridine Rings Mediated by Group 4 Metal Aryl Oxide Compounds: High Yield Formation of α,α'-Disubstituted-2,6-pyridinedimethoxide Ligands

Abstract:

The room-temperature carbonylation (typically 1000 psi) of solutions of the dialkyl compounds Zr(OAr')2(R)2 (OAr') = 2,6-di-tert-butylphenoxide; R = CH3, CH2Ph) in the presence of 1 equiv of a suitable pyridine leads to the formation of a series of α,α'-disubstituted-2,6-pyridinedimethoxide compounds (Ar'O)2Zr[OCH(R)-pyr-CH(R)O] [R = CH3 (1), CH2Ph (2); pyr = NC5H3, a; NC5H2-4-Ph, b; NC5H2-4-Me, c]. Spectroscopic studies, 1H and 13C NMR, of the compounds show in all cases an approximately 50/50 mixture of the threo and meso diastereomers to be present in the initial reaction product mixture. Partial recrystallization of the zirconium benzyl product (2b) obtained by using 4-phenylpyridine yielded crystals of the pure threo diastereoisomer. A single-crystal X-ray diffraction of this product confirmed the stereochemistry and showed the five-coordinate zirconium metal center to be chelated by the tridentate pyridinedimethoxide ligand was well as the two 2,6-di-tert-butylphenoxide groups. Recrystallization of all the other derivatives yielded crystals containing both the threo and meso forms. In the presence of excess pyridine carbonylation of the compounds M(OAr')2(CH3)2 (M = Zr, Hf) yields the six-coordinate adducts M(OAr')2[OCH(R)NC5H3CH(R)O](C5H5N) [M = Zr (3), Hf (4)]. The stoichiometry of the adducts was confirmed by a single-crystal X-ray diffraction analysis of the zirconium compound Zr(OAr')2[OCH(CH3)C5H3CH-(CH3)O](C5H5N) (3). In this case, the crystals were found to contain a 50/50 mixture of the threo and meso diastereomers, leading to a disorder involving the α-methyl groups. Hydrolysis of these compounds allows isolation of the corresponding α.α'-disubstituted pyridinedimethanol ligands in good yield. A mechanistic study of the carbonylation reaction employing a competition reaction between pyridine and pyridine-d5 showed the absence of a significant kinetic isotope effect. This and other studies are interpreted in terms of the reaction involving nucleophilic attack of intermediate metal-acyl groups at the 2-and 6-positions of the pyridine rings. Crystal data for ZrO4NC55H65 (2b) at-135 °C: a = 12.933 (5), b = 14.088 (5), c = 14.336 (5) Å; α = 67.44 (3), β = 80.02 (3), γ = 81.61 (3)°; Z = 2;dcalod = 1.256 g cm-3 in space group PI for ZrO4N2C42H58(3) at 22 °C: a= 10.599 (5), b= 18.153 (5), c = 11.155 (4)Å;β= 105.73 (4)°; Z = 2; dcalcod = 1.199 g cm-3 in space group P21. © 1990, American Chemical Society. All rights reserved.