Synthesis, Structure, and DFT Analysis of the THF Solvate of 2‐Picolyllithium: A 2‐Picolyllithium Solvate with Significant Carbanionic Character

Abstract

Previous studies of different solvates of 2‐methylpyridyllithium (2‐picolyllithium) have uncovered electronic structures corresponding to aza‐allyl and enamido resonance forms of the metallated pyridine‐based compounds. Here, we report the synthesis and characterization of [2‐CH2Li(THF)2C5H4N], a new THF solvate. X‐ray crystallographic studies reveal a dimeric arrangement featuring a non‐planar eight‐membered [NCCLi]2 ring, in which the primary cation‐anion interaction is between the central Li atom and the C atom of the deprotonated methyl group [length, 2.285(2) Å], suggesting a new carbanionic resonance structure for this 2‐picolyllithium series. The significant carbanionic character of [2‐CH2Li(THF)2C5H4N] was confirmed by gas‐phase DFT calculations [B3LYP/6‐311+G(d)] with the calculated electron density interrogated by means of quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. For comparison these computational analyses were also performed on the literature structures of [2‐CH2Li(2‐Picoline)C5H4N] and [2‐CH2Li(PMDETA)C5H4N]. In a reactivity study, [2‐CH2Li(THF)2C5H4N] was found to undergo nucleophilic addition to pyridine to generate dipyridylmethane in a good yield.

Lateral Deprotometallation‐Trapping Reactions on Methylated Pyridines, Quinolines and Quinoxalines Using Lithium Diethylamide

“…Under these conditions, inspired by the work of Evans and co‐workers on 2,4‐lutidine, [3a] the alcohol 1 b was formed from 2‐picoline ( 1 a ) in a moderate 26 % yield whereas the more acidic 4‐picoline ( 2 a ) afforded the alcohol 2 b in 34 % yield (as well as the ketone 2 c , which presumably results from its oxidation, in 13 % yield; Scheme 1). It is in fact likely that it is not the trapping step that limits the yield obtained from 2‐picoline ( 1 a ); indeed, the teams of Mulvey and Robertson demonstrated a significant carbanionic character for the α‐lithiated 2‐picoline [35] which makes it a good nucleophile.…”

Crystal structure and Hirshfeld surface analysis of 2-picolyllithium·3thf

“…The enamido motif shows a divergent C para -N-Li angle of about 146 � indicating sp 3hybridization of the nitrogen center (Kennedy et al, 2014). Due to the usage of different solvents, a follow-up dimeric structure [2-PicLi•(thf) 2 ] 2 (5) could be obtained by Brouillet et al (2020) (Fig. 2).…”

“…Unlike the previous dimeric structure 2 of Stalke et al, NBO calculations determined negative charges at N (-0.68 e), O (-0.65 e) and C2 (-0.80 e) suggesting a carbanionic structural motif. Thus, all three possible structural motifs have been detected and characterized in the solid state (Brouillet et al, 2020).…”

“…A search of the Cambridge Crystallographic Database (WebCSD, November 2023; Groom et al, 2016) for lithiated 2-picoline or lithiated 2-methylpyridine leads to the previously discussed structures 2 (Ott et al, 2009), 2 and 5 (Kennedy et al, 2014;Brouillet et al, 2020). A few other lithiated solid state structures of 2-picoline were published, for example bis(�2dimesitylborinato)bis(2-methylpyridine)dilithium (ROLRIU; Saravana et al (2009).…”