Spectroscopic and theoretical investigations of electrophilic bromination reactions of alkynes: The first evidence for π complexes as reaction intermediates

A bromine-alkyne π complex (λ_max = 294 nm) of 1:1 stoichiometry has been observed for the first time in the course of the bromination of 1-phenylpropyne by means of a diode-array stopped-flow technique. The formation enthalpy and entropy (ΔH^θ = -2.95 kcal mol^-1, ΔS^θ₂₅ = -15.4 eu) of this species are similar to those of charge-transfer complexes observed between bromine and alkenes. A negative apparent activation energy is found in the reaction of Br₂ with 1-phenylpropyne (ΔH^≠ = -0.61 kcal mol^-1); this demonstrates that the complex is actually an essential intermediate on the reaction coordinate. The bromination of a series of nine alkynes has been studied. Bromination reactions with negative apparent activation parameters lead to mixtures of E and Z vinyl dibromides, whereas reactions with positive activation energy yield the E isomers exclusively. The reason for the difference in reactivity of these alkynes compared with structurally similar alkenes most likely lies in the stability of these 1:1 charge-transfer complexes. Usually open arylvinyl cations correspond to the energetically favored product-determining intermediates; bridged bromirenium ions are formed from deactivated alkynes and react to give E isomers. The kinetic effect of alkyl groups and of p-OCH₃, p-CN, and p-NO₂ substituents at the aryl group on the bromination of arylalkylacetylenes is discussed. Density functional calculations provide insight into the geometries, energies, and bonding of the intermediate 1:1 and 2:1 Br₂-acetylene complexes involved. These theoretical investigations demonstrate that the most stable trimolecular Br₂-Br₂-acetylene adduct possesses a structure very similar to a crystallographically characterized Br₂-Br₂-alkene species, which can directly yield the ionic products, Br^-3 and vinyl cation, driven by the heterolytic action of a solvent.