Three-component synthesis of substituted eta(5)-cyclopentadienyltricarbonylrhenium complexes: Scope, limitations, and mechanistic interpretations

We have investigated the scope and mechanism of the "three-component" synth esis of substituted CpRe(CO)(3) complexes which involves the reaction of nu cleophiles with diazocyclopentadiene (C5H4N2) and a fac-Re(CO)(3)(+) specie s. We found that only moderately strong nucleophiles (halogen, carboxylates , boronic acids) are suitable for this transformation and that it shows a g reat sensitivity to the steric and electronic features of the nucleophile. A Hammett-type po analysis of the effect of para-substituents on the relati ve rate of this reaction with several benzoates showed that the reaction is accelerated by electron-donating substituents. A mechanistic analysis, bas ed on structure/reactivity relationships and NMR experiments, indicated tha t the nucleophile initially reacts with the rhenium precursor. Then, in the rate-determining step, the resulting preassociated rhenium-nucleophile int ermediate reacts with C5H4N2 via a concerted S(N)2-like transition state. T he same general mechanistic pathway seems to be followed by two very differ ent classes of nucleophiles, carboxylates and boronic acids, in the synthes is 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 h ydroxy group of the nucleophile, which is possible only with free boronic a cids.