Studies of the borderline between stepwise and concerted mechanisms of elimination: leaving group effects in elimination of (fluoren-9-ylmethyl)-ammonium ions

Abstract

By comparison with related substrates β-elimination of (fluoren-9-ylmethyl)trimethylammonium ion to form dibenzofulvene in methanolic sodium methoxide is unexpectedly rapid. The reaction has been investigated as a possible example of an E2 mechanism at the borderline between concerted and stepwise reaction paths. Evidence from buffer saturation in dimethylaminoacetonitrile buffers and comparisons with other amine leaving groups suggests, however, that the mechanism is probably ElcB, with ionization to a carbanion intermediate rate-determining. As measured by σ* for the trimethylammonium group the rate of elimination is 10⁵ times faster than predicted from a Taft correlation of rates of carbanion formation. Probably this reflects the sensitivity of substituent constants for monopoles to solvent and reaction, and a rate-enhancing effect from N-methylation of (fluoren-9-ylmethy1)amine. Comparison of the trimethylammonium leaving group with eliminations of neutral amines also confirms an earlier conclusion that intramolecular proton transfer is not an important reaction path in eliminations of amines activated by a β-fluorenyl group.