cs501681f_si_002.zip (47.21 kB)
Counterion Effect in the Reaction Mechanism of NHC Gold(I)-Catalyzed Alkoxylation of Alkynes: Computational Insight into Experiment
dataset
posted on 2015-02-06, 00:00 authored by Gianluca Ciancaleoni, Leonardo Belpassi, Daniele Zuccaccia, Francesco Tarantelli, Paola BelanzoniExperimental data suggest that anions
that provide a compromise
between the hydrogen-bond acceptor and the coordinating powers rather
than poor coordinating anions unexpectedly increase the efficiency
of L–Au–X (L = ligand, X = anion) catalyzed alkyne alkoxylation
reactions, where the nucleophilic attack is the rate-determining step.
No systematic computational studies about the role of the anion in
the different steps of the catalytic cycle are available yet. In this
paper, the remarkable anion influence on the catalytic efficiency
of [NHCAuX] (X = BF4–, OTf–, OTs–, TFA–, and OAc–) complexes in the intermolecular addition of methanol to 2-butyne
process has been analyzed through a density functional theory (DFT)
approach. The role of the anion has been considered in all the steps
of the reaction mechanism: pre-equilibrium, nucleophilic addition,
and protodeauration. In the nucleophilic attack step, the anion acts
(i) as a template, holding the methanol in the right position for
the outer-sphere attack; (ii) as a hydrogen-bond acceptor, enhancing
the nucleophilicity of the attacking methanol; (iii) as catalyst deactivator,
by either its strong coordinating and/or basicity power, preventing
the alkyne coordination or forming free alkoxide, respectively. In
the protodeauration step, the anion acts as a proton shuttle, lowering
the activation barrier. DFT calculations support intermediate coordinating
and basicity power anions as the most efficient catalysts.