Nanocatalysts for High Selectivity Enyne Cyclization: Oxidative Surface Reorganization of Gold Sub-2-nm Nanoparticle Networks

Abstract

Ultrasmall gold nanoparticles (NPs) stabilized in networks by polymantane ligands (diamondoids) were successfully used as precatalysts for highly selective heterogeneous gold-catalyzed dimethyl allyl(propargyl)malonate cyclization to 5-membered conjugated diene. Such reaction usually suffers from selectivity issues with homogeneous catalysts. This control over selectivity further opened the way to one-pot cascade reaction, as illustrated by the 1,6-enyne cycloisomerization–Diels–Alder reaction of dimethyl allyl propargyl malonate with maleic anhydride. The ability to assemble nanoparticles with controllable sizes and shapes within networks concerns research in sensors, medical diagnostics, information storage, and catalysis applications. Herein, the control of the synthesis of sub-2-nm gold NPs is achieved by the formation of dense networks, which are assembled in a single step reaction by employing ditopic polymantanethiols. By using 1,1′-bisadamantane-3,3′-dithiol (BAd-SH) and diamantane-4,9-dithiol (DAd-SH), serving both as bulky surface stabilizers and short-sized linkers, we provide a simple method to form uniformly small gold NPs (1.3 ± 0.2 nm to 1.6 ± 0.3 nm) embedded in rigid frameworks. These NP arrays are organized alongside short interparticular distances ranging from 1.9 to 2.7 nm. The analysis of gold NP surfaces and their modification were achieved in joint experimental and theoretical studies, using notably XPS, NMR, and DFT modeling. Our experimental studies and DFT analyses highlighted the necessary oxidative surface reorganization of individual nanoparticles for an effective enyne cycloisomerization. The modifications at bulky stabilizing ligands allow surface steric decongestion for the alkyne moiety activation but also result in network alteration by overoxidation of sulfurs. Thus, sub-2-nm nanoparticles originating from networks building create convenient conditions for generating reactive Au(I) surface single-sites—in the absence of silver additives—useful for heterogeneous gold-catalyzed enyne cyclization. These nanocatalysts, which as such ease organic products separation, also provide a convenient access for building further polycyclic complexity, owing to their high reactivity and selectivity.

Tetranuclear Dicationic Aurophilic Gold(I) Catalysts in Enyne Cycloisomerization: Cooperativity for a Dramatic Shift in Selectivity

“…From dimethyl allyl(propargyl)malonate the five‐membered 22 b was isolated in 41 % yield (79 % selectivity, 99 % enyne conversion). The selective cycloisomerization of dimethyl allyl(propargyl)malonate is usually challenging, and gold homogeneous catalysts often achieved mixtures of six and five‐membered cyclic dienes [22,26,45] . Notably, very high selectivity from this malonate precursor to the five‐membered 22 b has been recently achieved at the surface of ultra‐small gold nanoparticles (NPs, sizing 1.3 nm, >99 % selectivity in 22 b ) [45] .…”

“…The selective cycloisomerization of dimethyl allyl(propargyl)malonate is usually challenging, and gold homogeneous catalysts often achieved mixtures of six and fivemembered cyclic dienes. [22,26,45] Notably, very high selectivity from this malonate precursor to the five-membered 22 b has been recently achieved at the surface of ultra-small gold nanoparticles (NPs, sizing 1.3 nm, > 99 % selectivity in 22 b). [45] It is tempting to hypothesize some structural relationship between the reactivity at the surface of ultra-small gold NPs and the present gold polynuclear species in this substrate case.…”

“…To the best of our knowledge, the competence of molecularly‐defined higher rank polynuclear complexes, and clusters has never been reported in cycloisomerization of 1,6‐enynes, despite the conceivable benefit of pre‐organizing gold centers in a constrained environment regarding selectivity outcome in such reactions. This issue is all the more pertinent that, surprisingly, very high but divergent selectivity is also achieved at the surface of heterogeneous gold catalysts and from nanoparticle catalysts [43–45] . Herein, we report the use of a molecularly‐defined tetranuclear dicationic aurophilic gold(I) catalyst for the selectivity‐challenging cycloisomerization of low‐substituted 1,6‐enynes (Figure 1, bottom).…”

“…This issue is all the more pertinent that, surprisingly, very high but divergent selectivity is also achieved at the surface of heterogeneous gold catalysts and from nanoparticle catalysts. [43][44][45] Herein, we report the use of a molecularly-defined tetranuclear dicationic aurophilic gold(I) catalyst for the selectivity-challenging cycloisomerization of low-substituted 1,6-enynes (Figure 1, bottom). We show that such tetranuclear Au(I) species specifically favors the formation of strained azabicyclo[4.1.0]hept-4-enes with high selectivityeven in the complete absence of activating/orienting substituents on alkyne and olefin reactive functions -which is usually hardly achieved by phosphine-and carbene-stabilized monoand dinuclear cationic gold(I) complexes or heterogeneous gold catalysts.…”

MOF-Triggered Synthesis of Subnanometer Ag02 Clusters and Fe3+ Single Atoms: Heterogenization Led to Efficient and Synergetic One-Pot Catalytic Reactions

“…Isolated single-metal atoms (SMAs) and subnanometer metal clusters (SNMCs) − have been revealed as extraordinary materials with exciting physicochemical properties due to their unique intrinsic chemical nature and electronic features. − SMAs and SNMCs are hailed to trigger a paradigmatic shift in diverse technological applications. , However, prior to that, certain issues related to the total control of their synthesis, especially the scale-up, as well as stabilization and atomically precise characterization, need to be solved. ,− During the last decade, intense research has been performed in order to gain control over the synthesis of both SNMCs and SMAs and, indeed, commendable advances have been obtained. − Despite such advances, challenging issues remain to be further studied, and others have not even been explored, which inherently represent new avenues of research full of possibilities waiting to be explored. This is the case, for example, of the preparation and stabilization of different metallic species, with distinct chemical nature and atomicitythat is, SMAs and SNMCscoexisting and having the potential to be ready to be used concomitantly …”