Synthesis of fluorescent stilbene and tolan rotaxanes by Suzuki coupling

Abstract

Highly fluorescent stilbene and tolan cyclodextrin [2]rotaxanes have been synthesised in good yield using aqueous Suzuki coupling, and the crystal structure of one of these rotaxanes has been determined.A rotaxane is a supramolecular assembly of a dumbbell locked through the cavity of a macrocycle. 1 The formation of rotaxanes provides a means of stabilising dumbbell-shaped chromophores, by shielding them from the external environment. This type of encapsulation can also enhance the fluorescence efficiency. 2 Hydrophobic binding is a convenient way of directing rotaxane formation, provided the dumbbell can be synthesised in water. Recently we reported the synthesis of poly-p-phenylene rotaxanes and polyrotaxanes using aqueous Suzuki coupling, although in this case the [2]rotaxane (1a in Table 1) was only obtained in low yield (4%). 3 While exploring the scope of this route to rotaxanes, we discovered that the right combinations of aryl iodide stopper, diboronic acid core and macrocycle components give highly fluorescent cyclodextrin encapsulated stilbenes and tolans, such as 1g, in high yield. Previous stilbene rotaxanes have been prepared by aromatic nucleophilic substitution 4 and by slipping macrocycles over pre-formed dumbbells. 5 Six new rotaxanes 1b-g have been prepared by reacting bulky water-soluble aryl iodides 2a and 2b with diboronic acids 3a-c 6 in the presence of cyclodextrins (a-CD and b-CD; ca. 5 eq.), in aq. sodium carbonate containing palladium(II) acetate, as summarised in Scheme 1 and Table 1. † As expected, the 5-iodoisophthalic acid stopper 2b is too narrow to form rotaxanes with b-CD, and the biphenyl diboronic acid core 3a is too bulky to form rotaxanes with a-CD. Apart from these exceptions, all combinations of aryl iodides, diboronic acids and macrocycles yield rotaxanes. The lower yields obtained with the 1-iodonaphthalene-3,6-disulfonate stopper 2a can be attributed to an unfavourable interaction between the bound CD and the inwardly pointing H8 of the naphthalene. The tetracarboxylate Table 1 Yields for rotaxane synthesis, and fluorescence behaviour of rotaxanes and dumbbells a Scheme 1 This journal is

Isolierte molekulare Drähte

“…[68] Da die Reaktion gut in Wasser ausgeführt werden kann, haben wir sie zur Synthese von [2]Rotaxanen wie 17&a-CD und 18&a-CD herangezogen (Abbildung 7). [69,70] Die Festkörperstrukturen dieser [2]Rotaxane wurden durch Einkristall-Röntgenstrukturanalyse bestimmt. Ein Vergleich mit den Strukturen der einzigen weiteren kristallographisch charakterisierten Cyclodextrin-Rotaxane, 19&a-CD und 20&a-CD, [71,72] zeigt, dass die a-Cyclodextrine in allen vier Verbindungen eine Stilbeneinheit umschließen, ohne aber die Konformation des Stilben-p-Systems erkennbar zu beeinflussen.…”

“…[66,67] Abbildung 7. Ausgedehnte p-Stapel von [2]Rotaxan-Molekülen in den Kristallstrukturen von 17&a-CD (a), [69] 18&a-CD (b), [70] 19&a-CD (c) und 20&a-CD (d) [71] in zwei orthogonalen Ansichten. [73] Abbildung 8.…”

“…[69,70,73,74] Wie die Blauverschiebungen in den Emissionsspektren (Abschnitt 5.2) kann auch dies einer weniger flexiblen Konformation des angeregten Zustands zugeschrieben werden. Ein ähnlicher Effekt wird für CyaninRotaxane wie B-16&a-CD beobachtet.…”

Mechanostereochemistry and the mechanical bond

“…Linear molecules, such as the stilbene derivative 29, have been coupled to watersoluble stoppers, e.g. 28, in the presence of a-or b-cyclodextrin to form [2]rotaxanes such as 30, which was characterized by X-ray crystallography (Stanier et al 2001). This method is also well suited for the preparation of polyrotaxanes, which act as insulated molecular wires (Taylor et al 2000).…”

Host–Guest Complex of β-Cyclodextrin and Disulfide Form of 4-Aminothiophenol

“…Aromatic bifunctional aminothiophenols have been employed for this purpose by making a host-guest complex with β-CD [5]. It is known that guest molecules may behave differently inside the host molecule [9][10][11][12][13][14][15]. For example, highly conjugated cyanine dyes get a boost in fluorescence response when complexed with cyclodextrins [10,11].…”