Insights into the Self‐Filling Effects of Branched Isopropyl Groups on the Conformational and Supramolecular Properties of Isopropoxyprism[6]arene

Abstract

Here is reported the direct macrocyclization of a prism[6]arene macrocycle bearing branched alkyl chains on the rims. Isopropoxyprism[6]arene adopts in solution and in the solid state a cuboid D2‐conformation in which four isopropyl groups are folded inside the cavity, to give C‒H···p interactions and filling the internal void. The conformational features of isopropoxyprism[6]arene have been studied by dynamic 1H NMR experiments. The presence of branched isopropyl chains on the prism[6]arene rims, stabilizes the cuboid D2‐conformation to a greater extent than ethyl or propyl groups in PrS[6]Et and PrS[6]nPr. The higher resistence of PrS[6]iPr to open its cuboid D2 conformation, with respect to PrS[6]Et and PrS[6]nPr, also affected its binding abilities. In fact, alkylammonium‐based endo‐cavity complexes of PrS[6]iPr show lower binding constant values than the analogous propoxy/ethoxy‐prism[6]arene complexes.

Synthesis, conformational properties, and molecular recognition abilities of novel prism[5]arenes with branched and bulky alkyl groups

“…As previously reported by us and others, 11–26 prismarenes form endo -cavity supramolecular complexes stabilized by secondary interactions such as cation⋯π, C–H⋯π, van der Waals and the hydrophobic effect. Specifically, the PrS[5] R (R = Et, n Pr) 15 show a high binding affinity toward 1,4-dihexyl-DABCO 2 2+ and N , N , N ′, N ’-tetramethylpiperazonium 3 2+ as barfate salts (Table 3 and Fig.…”

“…Subsequently, a series of experiments were conducted to investigate the effects of branched alkyl groups on the efficiency of the macrocyclization outlined in Scheme 1 (Table 1, entries 6–15). Starting with 2,6-diisopropoxynaphthalene 1f , the 2,6-bis(isopropoxy)prism[5]arene, PrS[5] i Pr , was obtained in 48% yield after 24 h. In this case, the formation of the hexamer 2,6-bis(isopropoxy)prism[6]arene, PrS[6] i Pr , 17 was significant, at a 36% yield even in presence of the template 1,4-dihexyl-DABCO 2 2+ cation. In fact, the PrS[6] i Pr was formed as major product (60%) after 120 h. 17 As recently reported by us, 17 the isopropyl groups can stabilize the cuboid D 2 -conformation of the 2,6-bis(isopropoxy)prism[6]arene, PrS[6] i Pr , through a self-filling effect.…”

“…Starting with 2,6-diisopropoxynaphthalene 1f , the 2,6-bis(isopropoxy)prism[5]arene, PrS[5] i Pr , was obtained in 48% yield after 24 h. In this case, the formation of the hexamer 2,6-bis(isopropoxy)prism[6]arene, PrS[6] i Pr , 17 was significant, at a 36% yield even in presence of the template 1,4-dihexyl-DABCO 2 2+ cation. In fact, the PrS[6] i Pr was formed as major product (60%) after 120 h. 17 As recently reported by us, 17 the isopropyl groups can stabilize the cuboid D 2 -conformation of the 2,6-bis(isopropoxy)prism[6]arene, PrS[6] i Pr , through a self-filling effect. 17 Starting with 2,6-diisobutoxynaphthalene 1g , a PrS[5] i Bu / PrS[6] i Bu ratio of 3/1 was observed with an overall lower yield.…”

“…Similar results were observed for PrS[5] i Bu and PrS[5] i Pe , which show coalescence temperatures of 373 K and energy barriers of 18.9 and 18.3 kcal mol −1 , respectively, higher than that observed for PrS[5] n Bu and PrS[5] n Pe (see Table 2). We have previously shown 17 that 2,6-bis(isopropoxy)prism[6]arene PrS[6] i Pr exhibits an energy barrier to the racemization process significantly higher than that measured for the isomer 2,6-bis(propoxy)prism[6]arene PrS[6] n Pr . 15 These results clearly indicate that the presence of branched groups at the rims of prismarene macrocycles increase the energy barrier to the racemization process to a greater extent than the respective isomeric linear groups.…”

“…1). Prismarenes exhibit a deep π-electron rich aromatic cavity [14][15][16][17][18][19] and can form endo-cavity complexes with ammonium guests, both in organic [14][15][16][17] and aqueous media. These complexes are stabilized by cation⋯π and C-H⋯π interactions.…”