Nucleophilic Aromatic Substitution by Organostannylsodiums. A Second-Order Reaction Displaying a Solvent Cage Effect

Results of a study of mechanistic aspects of the reaction of triorganostannylsodiums with halobenzenes are reported. The reaction in tetraglyme, which is first order in each reactant, is unusually fast for an unactivated nucleophilic aromatic substitution, and provides high yields of arylstannanes, R3SnAr. Minor amounts of R2SnAr2 and R4Sn are also formed by a mechanism which is presumed to involve initial dissociation of R3SnNa into (R2Sn) and RNa. Reduction product, ArH, is formed when effective proton donors are present in the reaction mixture. ArD is formed when 2-propanol-d is present, indicating that aryl anions are intermediates. The initial step in the proposed mechanism of the reaction between the aryl halide and organostannylsodium results in the formation of caged species, which may react in the cage to form substitution product, ArSnR3, or diffuse from the cage as ArNa and R3SnX, which may react with each other to form more substitution product or with other species present in the bulk of the solvent. This concept of the mechanism is based on the results of proton trapping, which shows a saturation effect, and on the effect of solvent viscosity on the fractions of ArH and ArSnR3 formed in the presence of tert-butyl alcohol. Study of the course of reactions of trimethylstannylsodium with aryl bromides bearing intramolecular traps such as ketone carbonyls and hydroxyl groups provides support for the mechanism proposed. The order of reactivities of aryl halides is ArI > ArBr > ArCl > ArF.