Supramolecular Encapsulation of Neutral Diazoacetate Esters and Catalyzed 1,3‐Dipolar Cycloaddition Reaction by a Self‐Assembled Hexameric Capsule

Abstract

Diazoacetate esters proved to be suitable neutral guests for\ud the self-assembled resorcin[4]arene hexameric capsule. The hydrogen- bonded supramolecular host catalyzed the 1,3-dipolar cycloaddition reaction between diazoacetate esters and electron- poor alkenes such as acrolein, acrylonitrile, crotonaldehyde, trans-2-hexenal, methyl, and butyl acrylate,\ud which led to the corresponding 4,5-dihydro-1H-pyrazole\ud derivatives. The cycloaddition reaction occurred within the\ud cavity of the capsule. In fact, substantial inhibition of the catalytic activity was observed by employing tetraethylammonium tetrafluoroborate characterized by greater affinity for the hexameric capsule as a competitive guest; its presence inhibited access of the substrates. The 1,3-dipolar cycloaddition reaction between diazoacetate esters and acrylate esters of different lengths showed a significant degree of substrate selectivity owing to the encapsulation of the reagents before the cycloaddition reaction

Catalysis Within the Self-Assembled Resorcin[4]arene Hexamer

“…Their carbene-like character prompted their employment as potential substrates for the hexamer capsule. Scarso and coworkers investigated their encapsulation in the resorcin [4] arene assembly and found that the efficient capsule catalyzed 1,3-dipolar cycloaddition between diazoacetate esters and electron-poor alkenes, leading to 4,5-dihydro-1H-pyrazole derivatives [57]. It is worth noting that Diels-Alder reactions are among the most investigated chemical transformations, displaying the effects of supramolecular unimolecular, or self-assembled capsules in water [58,59], while examples in organic solvents are rare apart from the seminal work of Rebek [15].…”

To catalyze or not to catalyze: elucidation of the subtle differences between the hexameric capsules of pyrogallolarene and resorcinarene

“…For the design of new artificial enzyme-like catalysts, it is of fundamental importance to understand the prerequisites for catalytic activity. We 2 and others 3 have shown that the hexameric resorcinarene capsule I , 4 which self-assembles from six units of resorcinarene 1 and eight water molecules (Fig. 1), is an efficient catalyst for a variety of cationic reactions.…”

The Hexameric Resorcinarene Capsule at Work: Supramolecular Catalysis in Confined Spaces

“…[35] If ethyl diazoacetate (44)a nd acrolein (45)w ere reacted inside the cavity of CR 6 ,p roduct 46 wasf ormed in 47 %y ield, whereas, in the bulk solvent, in the absence of CR 6 ,o nly a 12 %y ield was obtained. [35] The presence of neutral guest 44 inside CR 6 was confirmed by means of NMR spectroscopy.I f44 was added to aw ater-saturated solution of CR 6 in CDCl 3 ,t hen new resonancesa tn egative values appeared in the upfield region of the 1 HNMR spectrum.A nalogous results were observedw ith tert-butyl diazoacetate and benzyld iazoacetate. Also, in this case, the encapsulationo fd iazoacetatei nside CR 6 was rationalized on the basis of stabilization imparted by the electron-rich internal surfaceo ft he capsule on the carbenelike guest.…”