Abstract
The development of gold catalysis has allowed significant levels of activity and complexity in organic synthesis. Recently, the use of very active small gold subnanoclusters (Aun, n < 10) has been reported. The stabilization of such nanocatalysts to prevent self-aggregation represents a true challenge that has been partially remediated, for instance, by their immobilization in polymer matrices. Here, we describe the transient stabilization of very small gold subnanoclusters (Aun, n < 5) by alkyl chains or aromatic groups appended to the reactive π bond of simple alkynes. The superior performance toward Brønsted acid-free hydration of medium to long aliphatic alkynes (1-hexyne and 1-docecyne) and benzylacetylene with respect to phenylacetylene is demonstrated experimentally and investigated computationally. A cooperative network of dispersive Au···C–H and/or Au···π interactions, supported by quantum mechanical calculations and time-resolved luminescence experiments, is proposed to be at the origin of this stabilization.
Highlights
The development of gold catalysis has allowed significant levels of activity and complexity in organic synthesis
This key finding, which was not discussed in the manuscript, suggests that the catalytic activity of Aun subnanoclusters might decrease due to competitive aggregation processes
We tested the possibility of enhancing catalysis through substrate-Au(0) dispersive interactions in the Brønsted acid-free Aun-catalyzed hydration of 1-hexyne [1], 1-dodecyne [2] and benzylacetylene [3], using phenylacetylene [4] as a reference (Fig. 1a)
Summary
The development of gold catalysis has allowed significant levels of activity and complexity in organic synthesis. Adding to the great number of ground-breaking applications described by the groups of Hashmi, Echavarren, Toste and others, Corma and co-workers have recently reported on a new type of an extremely efficient gold-catalyst consisting on ultra-small gnoalkdedcluAsutenrsclourstgeorlsd(snu=bn3a–n1o0clautsotemrss1)4, –a1l7s.oTchailsletdypseuobfnsamnoamll egtorlidc clusters can be prepared from either Au NPs or Au(I) or Au(III) species, which made the authors wonder about the identity of the real active catalysts in several of these homogeneous transformations In one of these reports, the authors described the catalytic performance of solely Au3–Au5 subnanoclusters in the ester-assisted hydration of alkynes at very low catalyst loadings (parts per billion), or Au5–Au9 subnanoclusters in the bromination of arenes. We show through mechanistic, fluorescence, and computational studies that Au···H–C and Au···π interactions between the catalyst and alkynes bearing flexible, medium-sized alkyl and aryl side chains preclude the aggregation and enhance the catalytic properties of small gold subnanoclusters
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