Mechanistic studies in strongly basic media. Part V. Kinetics and mechanism of bifluorenylidene formation from 2-substituted 9-halogenofluorenes in t-butyl alcohol

Abstract

The rate of conversion of a variety of 2-substituted 9-bromofluorenes into the related bifluorenylidene by benzyltrimethylammonium hydroxide in t-butyl alcohol is proportional to the square of the 9-bromofluorene concentration. With sodium or potassium t-butoxide as the base, a first-order dependence of the rate of elimination on the concentration of 2-substituted 9-halogenofluorenes is sometimes observed. Factors which promote the change in kinetic form are the presence of powerful electron-attracting 2-substituents, reaction media containing dipolar aprotic additives (e.g., dimethyl sulphoxide), and bromine rather than chlorine as the leaving group. By a detailed examination of the kinetics of these reactions, particularly the influence of an added electrolyte, sodium tetraphenylboron, it has been shown that, for systems obeying the first-order kinetic law, the rate-limiting process is the dissociation of a rapidly-formed ion pair consisting of a 9-halogenofluorenyl anion and an alkali-metal cation. The paired and unpaired carbanions displace halide ion from the starting material to give, after a further elimination step, the bifluorenylidene. When these concurrent bimolecular displacements are rate-determining, the second-order kinetic form is observed. Carbene formation does not appear to play any part in these reactions.