Mechanisms of Solvent- and Base-Promoted Imine-Forming Elimination Reactions

Abstract

Solvolysis of 9-(N-chloro-N-methylamino)fluorene (1-Cl) in 25 vol % acetonitrile in water gives the imine fluorenylidenemethylamine (3) as the sole product. The kinetic deuterium isotope effect was measured with the deuterated analogue (9-2H)-9-(N-chloro-N-methylamino)fluorene as kH/kD = 4.8 ± 0.2 (50 °C) without base and kH/kD = 6.7 ± 0.2 (25 °C) with hydroxide anion. The solvent- and base-promoted reactions of 1-Cl are concluded to be of E2 type. The corresponding substrates 9-(N-4-Y-benzenesulfonyl-N-methylamino)fluorene (2-Y, Y = OMe, Me, or Br), with very poor leaving groups, show reversible E1cB reactions with added bases. The strongly activated substrate 9-(N-4-nitrobenzenesulfonyl-N-methylamino)fluorene (2-NO2) does not give any elimination; it exclusively undergoes intramolecular nucleophilic aromatic substitution involving rate-limiting hydron transfer.