β-Elimination of 9-(dimethylaminomethyl)fluorene; buffer catalysis and pH dependence indicating a zwitterion intermediate

Abstract

In aqueous sodium hydroxide and tertiary amine buffers at 25° 9-(dimethylaminomethyl)fluorene eliminates dimethylamine to form dibenzofulvene. Between 0.02 and 0.2M-OH^– the reaction is first order in hydroxide with a rate constant expected of ionisation to a fluorenyl anion. Above 0.2M the order in hydroxide falls, consistent with a change in rate-determining step to loss of the leaving group. Below 0.02M the order also falls and in buffer solutions the reaction shows general acid catalysis, as expected of rate-determining attack of hydroxide and buffer base on protonated substrate. Buffer saturation is observed with a limiting rate constant close to that at high hydroxide concentrations, again indicating a change in rate-determining step. In weakly basic buffers the saturation rate becomes pH dependent and a pK_a of 8.54 is calculated for the substrate. A stepwise mechanism is proposed with formation of a zwitterion intermediate preceded respectively at high pH by a fluoren-9-yl anion and at low pH by a dimethylammonium cation. The behaviour is compared with that of Mannich bases, β-arylsulphonylalkylamines and carbonyl forming eliminations of amines. Relative to carbonyl reactions the rate of expulsion of the amine leaving group appears slow, and there is no evidence of a ‘proton switch’ between β-carbon atom and leaving group.