Structurally Defined Potassium-Mediated Zincation of Pyridine and 4-R-Substituted Pyridines (R=Et, iPr, tBu, Ph, and Me ₂N) by Using Dialkyl–TMP–Zincate Bases

Abstract

Two potassium–dialkyl–TMP–zincate bases [(pmdeta)K(μ-Et)(μ-tmp)Zn(Et)] ( 1) (PMDETA= N, N, N′, N′′, N′′-pentamethyldiethylenetriamine, TMP=2,2,6,6-tetramethylpiperidide), and [(pmdeta)K(μ- nBu)(μ-tmp)Zn( nBu)] ( 2), have been synthesized by a simple co-complexation procedure. Treatment of 1 with a series of substituted 4-R-pyridines (R=Me ₂N, H, Et, iPr, tBu, and Ph) gave 2-zincated products of the general formula [{2-Zn(Et) ₂-μ-4-R-C ₅H ₃N} ₂ ⋅2{K(pmdeta)}] ( 3– 8, respectively) in isolated crystalline yields of 53, 16, 7, 23, 67, and 51%, respectively; the treatment of 2 with 4- tBu-pyridine gave [{2-Zn( nBu) ₂-μ-4- tBu-C ₅H ₃N} ₂ ⋅2{K(pmdeta)}] ( 9) in an isolated crystalline yield of 58%. Single-crystal X-ray crystallographic and NMR spectroscopic characterization of 3– 9 revealed a novel structural motif consisting of a dianionic dihydroanthracene-like tricyclic ring system with a central diazadicarbadizinca (ZnCN) ₂ ring, face-capped on either side by PMDETA-wrapped K ⁺ cations. All the new metalated pyridine complexes share this dimeric arrangement. As determined by NMR spectroscopic investigations of the reaction filtrates, those solutions producing 3, 7, 8, and 9 appear to be essentially clean reactions, in contrast to those producing 4, 5, and 6, which also contain laterally zincated coproducts. In all of these metalation reactions, the potassium–zincate base acts as an amido transfer agent with a subsequent ligand-exchange mechanism (amido replacing alkyl) inhibited by the coordinative saturation, and thus, low Lewis acidity of the 4-coordinate Zn centers in these dimeric molecules. Studies on analogous trialkyl–zincate reagents in the absence and presence of stoichiometric or substoichiometric amounts of TMP(H) established the importance of Zn–N bonds for efficient zincation.