González-Lainez M, Gallegos M, Munarriz J, Azpiroz R, Passarelli V, Jiménez MV, Pérez-Torrente JJ. Copper-Catalyzed Azide–Alkyne Cycloaddition (CuAAC) by Functionalized NHC-Based Polynuclear Catalysts: Scope and Mechanistic Insights.
Organometallics 2022;
41:2154-2169. [PMID:
35971402 PMCID:
PMC9374069 DOI:
10.1021/acs.organomet.2c00246]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2022] [Indexed: 12/02/2022]
Abstract
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Copper(I) [Cu2(μ-Br)2(tBuImCH2pyCH2L)]n (L = OMe,
NEt2, NHtBu) compounds supported by flexible
functionalized NHC-based polydentate ligands have been prepared in
a one-pot procedure by reacting the corresponding imidazolium salt
with an excess of copper powder and Ag2O. An X-ray diffraction
analysis has revealed that
[Cu2(μ-Br)2(tBuImCH2pyCH2NEt2)]n is
a linear coordination polymer formed by bimetallic [Cu(μ-Br)]2 units linked by the lutidine-based NHC-py-NEt2 ligand, which acts as a heteroditopic ligand with a 1κC-2κ2N,N′ coordination
mode. We propose that the polymeric compounds break down in the solution
into more compact tetranuclear [Cu2(μ-Br)2(tBuImCH2pyCH2L)]2 compounds
with a coordination mode identical to the functionalized NHC ligands.
These compounds have been found to exhibit high catalytic activity
in the Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction.
In particular, [Cu2(μ-Br)2(tBuImCH2pyCH2NEt2)]2 efficiently
catalyzes the click reaction of a range of azides and alkynes, under
an inert atmosphere at room temperature in neat conditions at a very
low catalyst loading, to quantitatively afford the corresponding 1,4-disubstituted
1,2,3-triazole derivatives in a few minutes. The cycloaddition reaction
of benzyl azide to phenylacetylene can be performed at 25–50
ppm catalyst loading by increasing the reaction time and/or temperature.
Reactivity studies have shown that the activation of the polynuclear
catalyst precursor involves the alkyne deprotonation by the NHC moiety
of the polydentate ligand to afford a copper(I)-alkynyl species bearing
a functionalized imidazolium ligand. DFT calculations support the
participation of the dinuclear species [(CuBr)2(μ-tBuImCH2pyCH2NEt2)], resulting
from the fragmentation of the tetranuclear compound, as the catalytically
active species. The proposed reaction pathway proceeds through zwitterionic
dinuclear intermediates and entails the active participation of both
copper atoms, as well as the NHC moiety as an internal base, which
activates the reacting alkyne via deprotonation.
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