Abstract
Ethyl- and propyl-prism[6]arenes are obtained in high yields and in short reaction times, independent of the nature and size of the solvent, in the cyclization of 2,6-dialkoxynaphthalene with paraformaldehyde. PrS[6]Et or PrS[6]nPr adopt, both in solution and in the solid state, a folded cuboid-shaped conformation, in which four inward oriented alkyl chains fill the cavity of the macrocycle. On these bases, we proposed that the cyclization of PrS[6]Et or PrS[6]nPr occurs through an intramolecular thermodynamic self-templating effect. In other words, the self-filling of the internal cavity of PrS[6]Et or PrS[6]nPr stabilizes their cuboid structure, driving the equilibrium toward their formation. Molecular recognition studies, both in solution and in the solid state, show that the introduction of guests into the macrocycle cavity forces the cuboid scaffold to open, through an induced-fit mechanism. An analogous conformational change from a closed to an open state occurs during the endo-cavity complexation process of the pentamer, PrS[5]. These results represent a rare example of a thermodynamically controlled cyclization process driven through an intramolecular self-template effect, which could be exploited in the synthesis of novel macrocycles.
Highlights
We showed that under these conditions, c-PrS[5]Me was the thermodynamic product, while PrS[5]Me was the kinetic one and its formation was templated by the 1,2-DCE solvent
When 2,6dipropoxynaphthalene 4c was used as the precursor, the PrS [6]nPr hexamer was again the favored product (65% yield) a er 90 min (Table 1, entry 5). These results were unexpected because, according to the data previously reported by Ogoshi22 and by us,19 the 1,2-DCE solvent does not play any role as a template for the larger pillar[6]arene and prism[6]arene methoxy-derivatives
We investigated the role of 6+ in the synthesis of PrS[6]Et starting from 4b in 1,2-DCE (Scheme 1)
Summary
Since the birth of supramolecular chemistry, the eld of macrocyclic hosts and related host–guest complexes has grown considerably.1,2 The internal cavity of macrocyclic hosts is reminiscent of the binding sites of natural bioreceptors and for this reason macrocycles have been the focus of biomimetic chemistry research.3 The equilibrium of inclusion of a guest inside the host cavity is thermodynamically in uenced by many factors, such as host–guest desolvation processes, ion pair rupture, conformational control of the host and guest, and host/ guest weak interactions.4,5 The strength of such non-covalent interactions in guest@host complexes plays a crucial role in overcoming the other unfavorable thermodynamic factors.4–6 In general, an extended contact area4,5 between the host and guest and a high level of preorganization of the host can lead to stronger complexation. We previously showed that the cation 52+ acted as a template for the synthesis of the methylated pentamer PrS[5]Me (47%, Table 1, entry 2).19 Differently, starting from 4b, under the conditions in Scheme 1 and in the presence of 52+$2IÀ, the hexamer PrS[6]Et was preferentially formed in 35% yield a er 22 h (thermodynamic conditions), while the pentamer PrS[5]Et was obtained in 10%
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