Ligand effects in the formation of tertiary carbanions from substituted tertiary aromatic amides

Alison Campbell Smith, Morgan Donnard, Joanna Haywood, Mary McPartlin, Mark A. Vincent, Ian H. Hillier, Jonathan Clayden*, Andrew E H Wheatley

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

13 Citations (Scopus)

Abstract

Reaction of 2-isopropyl-(N,N-diisopropyl)-benzamide 5 with tBuLi in ether results in ortho deprotonation and the formation of a hemisolvate based on a tetranuclear dimer of (5-Lio)2̇Et2O. The solid-state structure exhibits a dimer core in which the amide oxygen atoms fail to stabilize the metal ions but are instead available for interaction with two metalated monomers that reside peripheral to the core. Reaction of 5 with tBuLi in the presence of the tridentate Lewis base PMDTA (N,N,N,N',N- pentamethyldiethylenetriamine) takes a different course. In spite of the tertiary aliphatic group at the 2-position in 5, X-ray crystallography revealed that a remarkable benzylic (lateral) deprotonation had occurred, giving the tertiary benzyllithium 5-Lil̇PMDTA. The solid-state structure reveals that amide coordination and solvation by PMDTA combine to distance the Li+ ion from the deprotonated α-C of the 2-iPr group (3.859(4) Å), thus giving an essentially flat tertiary carbanion and a highly distorted aromatic system. DFT analysis suggests that the metal ion resides closer to the carbanion center in solution. In line with this, the same (benzylic) deprotonation is noted if the reaction is attempted in the presence of tridentate diglyme, with X-ray crystallography revealing that the metal is now closer to the tertiary carbanion (2.497(4) Å). Electrophilic quenches of lithiated 5 have allowed, for the first time, the formation of quaternary benzylic substituents by lateral lithiation.

Original languageEnglish
Pages (from-to)8078-8084
Number of pages7
JournalChemistry - A European Journal
Volume17
Issue number29
Early online date3 Jun 2011
DOIs
Publication statusPublished - 11 Jul 2011

Keywords

  • directed metalation
  • Lewis bases
  • ligand effects
  • lithium
  • tertiary carbanions

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