Gold(I) Catalysis at Extreme Concentrations Inside Self‐Assembled Nanospheres

Abstract

Homogeneous transition-metal catalysis is a crucial technology for the sustainable preparation of valuable chemicals. The catalyst concentration is usually kept as low as possible, typically at mm or mm levels, and the effect of high catalyst concentration is hardly exploited because of solubility issues and the inherent unfavorable catalyst/substrate ratio. Herein, a self-assembly strategy is reported which leads to local catalyst concentrations ranging from 0.05 m to 1.1m, inside well-defined nanospheres, whilst the overall catalyst concentration in solution remains at the conventional mm levels. We disclose that only at this high concentration, the gold(I) chloride is reactive and shows high selectivity in intramolecular C À O and C À C bond-forming cyclization reactions.Transition-metal catalysis plays an extremely important role in the synthesis of chemicals relevant for pharmacology, biology, agrochemistry, materials science, and petrochemistry.[1] It allows the preparation of valuable molecules from readily available bulk chemicals in an atom-economic and sustainable manner.[2] It also enables the construction of sophisticated molecules in more efficient manners, thus simplifying synthetic routes towards the desired target molecules.[3] In the past decades several tools to control the activity and selectivity of transition-metal catalysts have been developed, and have mostly focused on the ligands which together with the metal form the active complex. [4,5] More recently, enzyme-inspired approaches have been explored, thus providing excellent tools to control the second coordination sphere around the transition-metal complex. [6] Various examples demonstrate that the use of a second coordination sphere can lead to activities and selectivities which are not accessible by traditional approaches. [7] The operational modes of these second spheres are many, including the precise orientation of the substrate at the metal center [8] and controlling the local pH value. [9] What all these catalysts have in common is that they are employed in a limited concentration window, typically between 10 À6 m and 10 À3 m. At these concentrations the catalysts generally dissolve well and a large excess of substrate can be added to achieve high turnover numbers without running into limitations of substrate solubility. From a reactivity point of view, however, it is interesting to also explore extremely high concentrations of catalyst as new reactivity patterns may evolve through, for example, metal cooperativity effects. We realized that if one would like to apply molar concentrations of catalyst, this can only be achieved by devising systems in which the "local catalyst concentration" is high, whereas the overall catalyst concentration in solution is still around the commonly used 10 À3 m (Figure 1), thus still allowing high turnover numbers to be achieved. Fujita et al. have developed appealing strategies to form nanosized molecular spheres (M 12 L 24 ) which form by self-assembly of 24 ditopic nitrogen ligands and...

Gold Functionalized Platinum M12L24‐Nanospheres and Their Application in Cyclization Reactions

“…It was synthesized from 2,6-dibromophenol in fours teps yielding complex A (depicted in Figure 1a (Figure 1b and c). [18] Fort he formation of the platinum analogue of these spheres, the assembly was formed by stirring A with as olution of [Pt(MeCN) 4 ](PF 6 ) 2 for 24 ha t7 0 8 8C. After the reaction, at ypical shift of pyridine signalsi nt he 1 HNMR spectrumi so bserved, confirming the formation of the sphere in solution.…”

“…To further investigate if we can control the reactivity of the systemb yc hanging only the local concentration of cationicg old(I) complexes,w ei nvestigatedt he effect of local dilution of the gold complex by using mixtures of functionalized/non-functionalized building blocks (A and B)i nd ifferent ratios during the sphere assembly.T his leads to statistical mixturesi nw hich the local concentration decreasesw ith increasing amount of B,astrategy that previously was demonstrated with the palladium-based spheres. [18] Fort he catalysis experiments,s olutions of spheres were prepared with building block ratios of 18:6 and6 :18 (A:B), while keeping the overall concentration of gold (building block A)t he same.H ence,w ed iluted the effectivel ocal concentration of the gold complex. Thec onversion of substrate 3 with these diluted gold complexes wasm onitored in time andi sp lotted in Figure 2.…”

“…We demonstrated that functionalization of the building block with ap hosphineg old(I) chloride moiety results in the encapsulation of 24 gold species inside these M 12 L 24 spheres.T he confinement of multiple gold species gives rise to ah igh local gold concentration [18] and leads to aurophilic d…”

Synthesis and Applications of Endohedrally Functionalized Metal‐Ligand Cage Complexes

“…The large internal spaces and robust nature of the M 12 L 24 nanospheres allows internalization of metal‐coordinating groups such as phosphines that can be used to perform catalytic reactions in the internal cavity . Reek showed that internalized Au(I) phosphine catalysts were fully functional towards hydro‐alkoxylation of a γ‐allenol 13 .…”

“… Organometallic catalysis in endohedrally functionalized nanospheres . Adapted with permission from reference 54, copyright 2016 Springer Nature.…”