Does phenothiazine give a dianion on addition of two equivalents of strong base?
Jacques Mortier,* Thi-Huu Nguyen, David Tilly, and Anne-Sophie Castanet
Université du Maine and CNRS, Unité de Chimie Organique Moléculaire and Macromoléculaire (UMR 6011), Faculté des Sciences, avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France
E-mail:
[email protected]
Dedicated to Jan Epsztajn on the occasion of his 75th Anniversary
Abstract
A careful analysis of results obtained by different authors allowed us to unravel the pathway of the metalation reaction of phenothiazine (1) by n-BuLi (2 equiv). The reaction is shown not to yield a dilithio species as believed. Electrophiles such as RCONMe2 (R = H, Ph, 4-MeOC6H4), RCO2H (R = Ph and Me) and CO2 assist a second deprotonation.
Keywords: ortho-Lithiation reaction, phenothiazine, organolithiums, mechanism
Introduction
Much is known about the structures of lithium alkyls and amides,1 but far less about how they operate, the origin of their selectivity with aromatic substrates, and the nature of the lithiated organic compounds produced prior to workup. All crystalline organolithium salts for which an X-ray structure analysis has been successfully performed so far are dimeric, tetrameric, or hexameric aggregates, the degree of aggregation depending less upon the particular alkyllithium structure than upon solvent and added complexing or chelating agents. Structural data for ortho- lithiated aromatics in which the Li atom is intramolecularly coordinated to an Me2N or MeO group have been reported.2 Crystal structures of N,N-diisopropyl-2-lithiobenzamide-diethyl ether complex and N,N-diisopropyl-2-lithionaphthamide-THF complex are dimers.3 However, by NMR spectroscopy, these dianions when dissolved in [D8]THF give at least five observable aggregates!
The fact that ortho-lithiated aromatics are found to be more or less aggregated in solution and crystals does not, of course, tell us anything about the actual reactive species and the transition state when the electrophile is added.
