Structurally Defined Potassium-Mediated Zincation of Pyridine and 4-R-Substituted Pyridines (R = Et, iPr, tBu, Ph, and Me2N) by Using Dialkyl-TMP-Zincate Bases

Two potassium-dialkyl-TMP-zincate bases [(pmdeta)K(mu-Et)(mu-tmp)Zn(Et)] (1) (PMDETA = N,N,N',N '',N ''-pentamethyldiethylene-triamine, TMP=2,2,6,6-tetramethylpiperidide), and [(pmdeta)K(mu-nBu)(mu-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=Me2N, H, Et, iPr, tBu, and Ph) gave 2-zincated products of the general formula [{2Zn(Et)(2)-mu-4-R-C5H3N}(2)center dot 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)-mu-4-tBu-C5H3N}(2)center dot 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 trialkylzincate reagents in the absence and presence of stoichiometric or substoichiometric amounts of TMP(H) established the importance of Zn-N bonds for efficient zincation.