Reaction of chloroauric acid with histidine in microdroplets yields a catalytic Au–(His)₂complex

Abstract

Microdroplets form the Au–(His)2 complex for catalyzing the reaction of phenylacetylene and indoline, which cannot occur in bulk.

Accelerated microdroplet synthesis of benzimidazoles by nucleophilic addition to protonated carboxylic acids

“…These results are consistent with a balance between reagent solubility, methanol evaporation and formation of a superacid surface layer of water in the almost fully evaporated microdroplets. This clearly aligns with literature data [47][48][49] on the high acidity of the surfaces of aqueous droplets while extending the concept of superacid formation in microdroplets to organic/ aqueous mixed solvents. We imagine that water, being a stronger base than methanol, takes the charge and occupies the surface of a small methanol droplet thereby protonating neutral formic acid and initiating the benzimidazole synthesis.…”

“…It is known that the proton concentration at the surface of positively charged electrosprayed aqueous droplets is much higher than in the bulk. [47][48][49] So we propose that a very high proton concentration at the surface of a droplet containing water will protonate neutral formic acid to create the electrophilic carbon (2a) which is subject to nucleophilic attack by the lone electron pair of the amine. (We observed protonated formic and triuoracetic acid in the mass spectra ( Fig.…”

Accelerated reactions of amines with carbon dioxide driven by superacid at the microdroplet interface

“…The protonated carbon dioxide and deprotonated amine reactions are of special interest because these forms of the reagents could arise from superacid/superbase reactions at the interface as discussed in the Introduction. 28 Previous reports 28,[30][31][32][33][34] have suggested that superacid (extreme pH) at the interface of organic droplets (and dielectric layers at the interface of water microdroplets) accelerate reactions. These results indicated the importance of water, even traces of water 28 in interfacial microdroplet chemistry.…”

“… 35 In these circumstances, the hydronium ion appears to act as a superacid. 32 Significantly, even for organic solvents, trace amounts of water have been implicated in reaction acceleration, e.g. in benzimidazole formation from aromatic diamines and simple carboxylic acids.…”

“…[26][27][28] Most studies of reaction acceleration have been done in organic solvents but aqueous droplet reactions are also well represented. [29][30][31][32][33] Active species generated in the dielectric layer at the surface of water microdroplets include free radicals and hydrogen peroxide. 29,30,34 In very signicant work, the strong electric eld at the surface of aqueous droplets was recognized as playing a crucial role in reaction acceleration due to hydronium and hydroxide ion formation by ready autoionization of water at the surface.…”