On the control of secondary carbanion structure utilising ligand effects during directed metallation

Andrew E H Wheatley*, Jonathan Clayden, Ian H. Hillier, Alison Campbell Smith, Mark A. Vincent, Laurence J. Taylor, Joanna Haywood

*Corresponding author for this work

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

6 Citations (Scopus)

Abstract

N,N-Diisopropy1-2-propylbenzamide 6-H undergoes lateral deprotonation by t-BuLi in the presence of the Lewis base PMDTA (N,N,N',N",N"- pentamethyldiethylenetriamine) to give a benzyllithium 6-Li l· PMDTA that incorporates a trigonal planar secondary carbanion. In the solid state, the amide directing group and the PMDTA additive work together to abstract the metal ion from the deprotonated α-C of the propyl group (4.107(4) Å). A short distance of 1.376(3) Å is observed between the deprotonated carbon centre and a planar aromatic system that shows a pattern of bond lengths which contrasts with that reported for related tertiary carbanion systems. Analogous benzylic deprotonation is seen if 6-H is treated with t-BuLi in the presence of diglyme to give 6-Li l·DGME. X-ray crystallography now shows that the metal ion more closely approaches the tertiary carbanion (2.418(6) Å) but that the planarity of the deprotonated carbon centre and the bonding pattern in the organic anion seen in the PMDTA complex are retained. DFT analysis corroborates both the short distance between aromatic ring and carbanion centre and the unperturbed nature of aromaticity in 6-Li l·L (L = Lewis base). The observation of two structure-types for the carbanion in solution is explained theoretically and by NMR spectroscopy in terms of cis and trans isomerism imparted by partial double bond character in the arene-(α-C) bond.

Original languageEnglish
Pages (from-to)50-60
Number of pages11
JournalBeilstein Journal of Organic Chemistry
Volume8
DOIs
Publication statusPublished - 9 Jan 2012

Keywords

  • Directed metallation
  • Lewis base
  • Ligand effects
  • Lithium
  • Secondary carbanion

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