om5b00925_si_004.xyz (89.13 kB)
Download fileExtensive Experimental and Computational Study of Counterion Effect in the Reaction Mechanism of NHC-Gold(I)-Catalyzed Alkoxylation of Alkynes
dataset
posted on 2016-03-02, 00:00 authored by Marina Trinchillo, Paola Belanzoni, Leonardo Belpassi, Luca Biasiolo, Vincenzo Busico, Angela D’Amora, Lorenzo D’Amore, Alessandro Del Zotto, Francesco Tarantelli, Angela Tuzi, Daniele ZuccacciaHerein,
we synthesized and characterized through NMR and X-ray
techniques a new set of [(NHC)-Au-X] complexes (NHC = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene),
differing in the counterion X– (X– = OMs–, NO3–, ClO4–, 2,2,3,3,4,4,5,5,6,6,7,7,7-tridecafluoroheptanoate
(PFHp–)). All of these complexes, together with
those already known having NTf2– and
phthalimide (ptm–) as counterions, were tested as
catalysts in the methoxylation of 3-hexyne. The results were analyzed
together with those obtained previously. The values of activation
parameters (ΔH⧧ and ΔS⧧) for different anions are also reported.
The overall catalytic and kinetic evidence, together with an extensive
computational work, confirm the general mechanistic picture given
recently in which the anion plays an active role in all steps of the
reaction mechanism: pre-equilibrium, nucleophilic attack, and protodeauration.
Medium-coordinating anions (OMs–, OTs–) containing a highly symmetric anchoring group give the best catalytic
performances. This is due to the following reasons: (a) the pre-equilibrium
is shifted toward the outer sphere ion pair, (b) their characteristic
basicity promotes the nucleophilic attack, and (c) the possible paths
leading to the deactivation of the catalyst are inhibited. These highly
symmetric tridentate anions destabilize the unreactive tricoordinated
gold species, which instead may be formed by anions with a “planar”
anchoring group, such as PFHp– and TFA–. A general trend between coordinating ability and catalytic performances
in the alkoxylation of alkynes may be established only when the geometric
features of the anion are taken into account. The role of the anion
has been also investigated in connection with the nature of the nucleophile.
In particular, when the alcohol is a poor nucleophile, a large difference
in reactivity is observed, while the use of suitably functionalized
alcohols, which may contribute to polarizing the −OH bond through
intramolecular interactions, flattens the anion effect.