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Structural Studies of (rac)-BIPHEN Organomagnesiates and Intermediates in the Halogen-Metal Exchange of 2-Bromopyridine

Abstract : Four lithium magnesiate complexes (2-5) containing the dianionic (rac)-BIPHEN ligand have been prepared and characterized using X-ray crystallography and NMR spectroscopy. (THF)(3)center dot Li2Mg\(rac)-BIPHEN\Bu-n(2), 2, (THF)(3)center dot Li2Mg\(rac)-BIPHEN\(CH2SiMe3)(2), 3, and (THF)(2)center dot Li2Mg\(rac)-BIPHEN\(neo)Pe(2), 4, have been prepared by complexation of the appropriate dialkylmagnesium compound with in situ prepared Li(rac)-BIPHEN in a mixture of hydrocarbon/THF. For all structures, the Mg centers are four-coordinate (and retain the alkyl groups); however, in 2 and 3 the two Li centers have different coordination spheres (one binding to one THF molecule, the other to two). The solid-state structures of 2 and 3 are essentially isostructural with that of 4 except that both Li atoms in this molecule have equivalent coordination spheres. The solution behaviors of these three molecules have been studied by H-1, C-13, and POSY NMR spectroscopy. During the synthesis of 2, it was discovered that a (rac)-BIPHEN-rich (or n-butyl-free) lithium magnesiate, (THF)(4)Li2Mg\(rac)-BIPHEN\fo(2), 2b, could be isolated. The lithium precursor to 2-5, (THF)(4)center dot Li-4\(rac)-BIPHEN)\(2), 1, has also been isolated. Within the molecular structure of this tetranuclear complex, there are three different Li coordination environments. Finally, 2 has already shown promise as a reagent in a halogen metal exchange reaction with 2-bromopyridine: The structural chemistry at play in this reaction was probed by X-ray crystallography and NMR spectroscopy. The organometallic intermediate pyridyl-magnesiated 5, (THF)(2)center dot Li2Mg\(rac)-BIPHEN\(2-pyridyl)(2), was isolated in high yield.
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Javier Francos, Philippe C. Gros, Alan R. Kennedy, Charles T. O'Hara. Structural Studies of (rac)-BIPHEN Organomagnesiates and Intermediates in the Halogen-Metal Exchange of 2-Bromopyridine. Organometallics, American Chemical Society, 2015, 34 (11), pp.2550-2557. ⟨10.1021/om501225g⟩. ⟨hal-01495973⟩



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