Three-Component Synthesis of Substituted eta5-Cyclopentadienyltricarbonylrhenium Complexes: Scope, Limitations and Mechanistic Interpretations

We have investigated the scope and mechanism of the "three-component" synthesis of substituted CpRe(CO)3 complexes which involves the reaction of nucleophiles with diazocyclopentadiene (C5H4N2) and a fac-Re(CO)3(+) species. We found that only moderately strong nucleophiles (halogen, carboxylates, boronic acids) are suitable for this transformation and that it shows a great sensitivity to the steric and electronic features of the nucleophile. A Hammett-type po analysis of the effect of para-substituents on the relative rate of this reaction with several benzoates showed that the reaction is accelerated by electron-donating substituents. A mechanistic analysis, based on structure/reactivity relationships and NMR experiments, indicated that the nucleophile initially reacts with the rhenium precursor. Then, in the rate-determining step, the resulting preassociated rhenium-nucleophile intermediate reacts with C5H4N2 via a concerted S(N)2-like transition state. The same general mechanistic pathway seems to be followed by two very different classes of nucleophiles, carboxylates and boronic acids, in the synthesis of acyloxy- and carbon-substituted CpRe(CO)3 complexes, respectively. In particular, the lack of reactivity of boronic esters can be explained by the necessary preassociation step between the rhenium and a deprotonated hydroxy group of the nucleophile, which is possible only with free boronic acids.