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Unexpected Anion Effect in the Alkoxylation of Alkynes Catalyzed by N‐Heterocyclic Carbene (NHC) Cationic Gold Complexes
Author(s) -
Biasiolo Luca,
Trinchillo Marina,
Belanzoni Paola,
Belpassi Leonardo,
Busico Vincenzo,
Ciancaleoni Gianluca,
D'Amora Angela,
Macchioni Alceo,
Tarantelli Francesco,
Zuccaccia Daniele
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201404539
Subject(s) - catalysis , chemistry , cationic polymerization , carbene , nucleophile , alkyne , medicinal chemistry , methoxide , combinatorial chemistry , photochemistry , polymer chemistry , organic chemistry
The intermolecular alkoxylation of alkynes is the oldest application of cationic gold(I) catalysts; however, no systematic experimental data about the role of the anion are available. In this contribution, the role of the anion in this catalytic reaction as promoted by a N‐heterocyclic carbene‐based gold catalyst, [(NHC)AuX] (X=BARF − , BF 4 − , OTf − , OTs − , TFA − , or OAc − ) is analyzed, through a combined experimental (NMR spectroscopy) and theoretical (DFT calculation) approach. The most important factor seems to be the ability to abstract the proton from the methanol during the nucleophilic attack, and such ability is related to the anion basicity. On the other hand, too high coordination power or basicity of the anion worsens the catalytic performance by preventing alkyne coordination or by forming too much free methoxide in solution, which poisons the catalyst. The intermediate coordinating power and basicity of the OTs − anion provides the best compromise to achieve efficient catalysis.