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

A bromine-alkyne pi complex (lambda(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 (Delta H?= -2.95 kcalmol(-1), Delta S-25?=-15.4eu) 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 Br, with 1-phenylpropyne (Delta H-not equal=-0.61 kcalmol(-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 Br-2-acetylene complexes involved. These theoretical investigations demonstrate that the most stable trimolecular Br-2-Br-2-acetylene adduct possesses a structure very similar to a crystallographically characterized Br-2-Br-2-alkene species, which can directly yield the ionic products, Br-3(-) and vinyl cation, driven by the heterolytic action of a solvent