Pyridine N-Alkylation by Lithium, Magnesium, and Zinc Alkyl Reagents:  Synthetic, Structural, and Mechanistic Studies on the Bis(imino)pyridine System

• Published in: Journal of the American Chemical Society Vol. 127; no. 16; pp. 6012 - 6020
• Main Authors: Blackmore, Ian J, Gibson, Vernon C, Hitchcock, Peter B, Rees, Charles W, Williams, David J, White, Andrew J. P
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 27.04.2005; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Multidisciplinary; Exact sciences and technology; Kinetics and mechanisms; Organic chemistry; Physical Sciences; Reactivity and mechanisms; Science & Technology; COMPLEXES; NITROGEN; IRON; LIGAND; OLIGOMERIZATION; ETHYLENE POLYMERIZATION CATALYSTS; ELECTRON-TRANSFER REACTIONS; Zinc Organic compounds; Lithium Organic compounds; Nitrogen heterocycle; Six membered ring; EPR spectrometry; Reaction mechanism; Electron transfer; Magnesium Organic compounds; Reversible reaction; Experimental study; Alkylation; Pyridine derivatives
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja042657g

The 2,6-bis(α-iminoalkyl)pyridines 2,6-[ArNC(CR3)]2C5H3N [R = H, D; Ar = 2,6-i-Pr2C6H3 (DIPP), 2,6-Me2C6H3 (DMP)] react with MeLi in Et2O to give a binary mixture of products:  the pyridine N-methylated species 2,6-[ArNC(CR3)]2C5H3N(Me)Li(OEt2) and the deprotonated/dedeuterated species 2-[ArNC(CR3)],6-[ArNC(CR2)]C5H3NLi(OEt2). For R = D, the product ratio is 2:1 in favor of the N-methylated product, while, for R = H, the deprotonated product is favored by 5:1, increasing to 8:1 in toluene solvent. Warming solutions of the N-methylated species leads to clean conversion to the thermodynamically preferred deprotonated species. Crossover experiments show that MeLi is re-formed and dissociates from the terdentate ligand before deprotonating the ketimine methyl unit. For MgR2 (R = Et, i-Pr) and ZnR2 (R = Et) reagents, N-alkylation products are formed exclusively, but derivatives containing bulky aryl substituents are found to undergo further rearrangement to 2-alkylated species, arising by migration of the alkyl group of the N-alkyl moiety to the adjacent ring carbon atom. The reversibility of the N-alkylation process has been probed using deuterio-labeled Mg alkyl reagents and mixed alkyl zinc species. A cationic zinc derivative is shown to undergo “reverse” alkyl migration, from the heterocycle nitrogen atom to the zinc center. EPR spectroscopy reveals a paramagnetic intermediate in which the unpaired electron is delocalized over the heterocycle and di-imine moieties of the ligand, indicating that the N-alkylation reactions proceed via single electron-transfer processes.