Lithiated Aza‐Heterocycles in Modern Synthesis

Abstract

Introduction Functional nitrogen-containing heteroaromatic compounds, especially sixmembered ones (pyridines, diazines, quinolines, and their fused derivatives), have attracted the attention of chemists worldwide because they play an important role in the development of new pharmaceuticals [1], agrochemicals [2], catalysts [3], or materials [4] (including polymers).Besides the classical reactions (i.e., electrophilic heteroaromatic substitutions on heterocycles or single electron-transfer (SET) mediated reactions with brominated derivatives) [5] and ex nihilo ring syntheses [6], metallation reactions providing organolithium compounds are of particular interest because they allow the direct functionalization starting from bare or sparsely substituted heterocycles intrinsically more acidic than similar benzene-ring systems. However, the strong bases used for metallation as well as the prepared organolithiums also exhibit a large propensity for nucleophilic addition. Further, though numerous methods have been described for the single derivatization, multiple functionalization often appears much more sensitive and difficult because of the complexation or aggregation phenomena observed either with primary substituents or with the nitrogen atom in the heterocycle itself. These complex phenomena, together with the variation in reaction conditions (temperature, solvent, ratio), produce various competitive reaction pathways. Thus, an arsenal of methodologies has then been developed to tune the regio-, chemio-, and sometimes stereoselectivities. The goal is generally to control these additional influences, providing variation of kinetic or thermodynamic acidity at various sites susceptible to lithiation.With a special focus on pyridines, quinolines, diazines, and fused pyridinic compounds, this chapter describes organolithium tools that allow (i) direct derivatization of bare N-containing heteroaromatics; (ii) metallation of dipolar adducts of pyridines and analogs; (iii) halogen/Li exchanges; (iv) ring metallation by using directing metallation groups; (v) lithiation invoking an halogen dance; and (vi) lateral and remote lithiations (Scheme 15.1). Next, the usefulness of these tools, often Lithium Compounds in Organic Synthesis: From Fundamentals to Applications, First Edition. Edited by Renzo Luisi and Vito Capriati.

Organometallic Reagents: Efficient Tools for the Synthesis of Fused Pyridinyl‐Lactones

“…To overcome this observed poor chemoselectivity, we planned to prepare the 2‐lithionicotinate derivative [1‐Li/LiDMAE] using the monometallic superbase [ n BuLi/LiDMAE] [1,9c,13] (DMAE: 2‐(dimethylamino)ethanol). Indeed, in an apolar solvent (hexanes) required to promote complexation, [ n BuLi/LiDMAE] is known to lead to a specific lithiated aggregate involving the pyridine nitrogen.…”