Xianchen
Hu
a,
Fengbo
Liu
a,
Xiongzhi
Zhang
ab,
Zhiyong
Zhao
ab and
Simin
Liu
*ab
aThe State Key Laboratory of Refractories and Metallurgy, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China. E-mail: liusimin@wust.edu.cn
bInstitute of Advanced Materials and Nanotechnology, Wuhan University of Science and Technology, Wuhan 430081, China
First published on 24th April 2020
By arranging substrates in a “reaction ready” state through noncovalent interactions, supramolecular nanoreactors/catalysts show high selectivity and/or rate acceleration features. Herein, we report the host–guest complexation of 9-(10-)substituted anthracene derivatives (G1–G3) with cucurbit[n]uril (CB[n], n = 8, 10), and the photoreactions of these derivatives in the presence of CB[n] hosts. Both CB[10] and CB[8] showed no obvious effects on the photoreaction of 9,10-disubstituted derivative G1. For G2 and G3, CB[10] operated as either a nanoreactor or catalyst (10%) for the photodimerization of two compounds with high selectivity and high yield. However, although CB[8] formed a 1:2 complex with G2, as also observed with CB[10], the photosolvolysis product (9-anthracenemethanol) was obtained quantitatively after photoirradiation of the CB[8]·2G2 complex. This unexpected photosolvolysis was rationalized by a plausible catalytic cycle in which anthracene acts as a photoremovable protecting group (PPG) and the carbonium ion intermediate is stabilized by CB[8].
Cucurbit[n]urils (CB[n]s, n = 5–8, 10) are a family of pumpkin-like macrocyclic hosts. CB[n] compounds bear a hydrophobic cavity and two identical portals surrounded by negative carbonyl groups as structural features, and have potential applications in many fields.26–29 Similar to other hosts, CB[n]s have also been employed to promote various organic reactions, especially photodimerization reactions,30–32 but have acted as supramolecular catalysts in only a few examples because the product is usually “trapped” in the CB[n] cavity, inhibiting the catalytic process.33,34
In this work, we investigated host–guest complexation between CB[n] (n = 8, 10) and 9-(10-)substituted anthracene derivatives (G1–G3) (Fig. 1) in aqueous solution, and the CB[n]-mediated (n = 8, 10) photoreaction of these guests. Notably, macrocyclic host-promoted photoreactions of 9-(10-)substituted anthracene derivatives have not been reported previously.35–39 Our results showed that CB[10] and CB[8] had no obvious effects on the photoreaction of G1, despite encapsulating G1. However, for G2 and G3, CB[10] operated as either a nanoreactor or catalyst (10%) for the photodimerization of two compounds with high selectivity and high yield. Although CB[8] formed a 1:2 complex with G2, as also observed with CB[10], photosolvolysis of G2 was unexpectedly observed instead of dimerization after photoirradiation of the CB[8]·2G2 complex (Scheme 1). A plausible reaction mechanism referring to anthracene as a PPG is provided.
Scheme 1 Schematic illustration of CB[n]-mediated photoreactions of 9-substituted anthracene derivative G2. |
UV/Vis and fluorescence titrations were used to examine the complexation of CB[10] with G1 in aqueous solution. With the addition of CB[10], the long-wavelength absorption of free G1 at λmax = 373 nm showed a bathochromic shift (Fig. S2†). In the absence of CB[10], G1 showed the anthracene emission with a maximum wavelength of 422 nm. The fluorescence intensity of G1 changed when CB[10] was added continuously, owing to host–guest complexation of G1 with the CB[10] host (Fig. 3).41 Furthermore, before the amount of CB[10] was increased to ∼0.5 equiv., the fluorescence intensity of G1 was observed to continuously decrease. However, at 1.0 equiv., the intensity increased about two-fold compared with that of G1 itself, with a slight redshift. Combined with the NMR data, we speculated that CB[10] initially increased π–π stacking of G1 by encapsulating two guest molecules in the cavity, causing fluorescence quenching through formation of the H-dimer of G1.42 The further formation of 1:1 inclusion complex CB[10]·G1 prevented G1 from stacking and exterior aqueous environment, resulting in a stronger G1 emission.41 Meanwhile, host–guest complexation between CB[8] and G1 was examined by 1H NMR (Fig. S3†) and ESI-MS (Fig. S4†). Broadening of all proton signals, including those of CB[8], was observed when the amount of CB[8] was 0.5 equiv. (Fig. S3c†), indicating that binding between CB[8] and G1 underwent intermediate exchange on the 1H NMR time scale.43 ESI-MS showed one ion peak at m/z 826.2, corresponding to 1:1 complex CB[8]·G1 ([CB[8] + G14+ − 2H+]2+ = 826.2).
Fig. 3 Fluorescence emission spectra recorded for G1 (31.25 μM) upon titration with CB[10]. Inset: emission of G1vs. CB[10] equivalent (λex = 250 nm, λem = 422 nm). |
With the binding results in hand, we checked the photoreactions of G1 in the absence and presence of CB[n] (n = 8, 10). Prior to UV irradiation at room temperature, all samples (in D2O or pure water) were degassed with nitrogen for 15 min ([G] = 2.5 mM), and the reaction was monitored by 1H NMR and UV/Vis spectroscopy. As shown in Fig. S5a,† the photoproducts of G1 were complicated and unidentified. In the presence of 0.25 or 0.5 equiv. of CB[10], ratios which benefitted the dimerization reaction, the dimerization product was not observed as expected (Fig. S5b and c†). Molecular modeling (MMFF) suggested two G1 molecules could adopt an “X” shape in the CB[10]·2G1 complex owing to electrostatic repulsion between the 9,10-substituted positive groups (Fig. 4),43 which was not a “reaction ready” state for dimerization. Similar results were obtained for the photoreaction of G1 with various contents of CB[8]. Therefore, guest G2 containing one substituent at the 9-position was designed.
Under the same conditions as for G1, photoreactions of G2 were conducted in the absence and presence of CB[n] (n = 8, 10). As shown in Fig. S12,† after UV irradiation for 6 h, h–t dimer product DG2 (purified and identified by irradiating complex CB[10]·2G2) was observed with unidentified side products and unreacted G2. In comparison, in the presence of 0.5 equiv. of CB[10], almost 100% of G2 was converted into a single product within 25 min. As shown in Fig. 5b, under UV irradiation for 10 min, G2 was partially converted into presumed new photoproduct(s). G2 was consumed after 25 min, with only one set of CB[10] host and bound product(s) peaks remaining (Fig. 5c). After the appropriate amount of 3,5-dimethylamantadine hydrochloride (3,5-DMADA) was added to the irradiated solution of CB[10]·2G2 to displace the product(s) from the CB[10] cavity (CB[10]·2(3,5-DMADA) is insoluble in water44), only one exclusive product, characterized as h–t dimer DG2, was obtained in almost quantitative yield (characterization data for DG2 is shown in Fig. S13–S15 in ESI†). By monitoring absorbance changes of G2 at λ = 370 nm, we calculated the yields of DG2 with various irradiation times. As “the intra-complex photodimerization is unimolecular in nature”,18 an apparent rate constant (k1) of 0.1158 ± 0.0079 min−1 for the dimerization reaction of G2 with 50% CB[10] was calculated with first-order kinetics (Fig. S16†). The half-conversion time (t1/2, time for conversion of half the substrate) was used because the rate constant could not be accurately measured in some cases. Clearly, in the presence of 50% CB[10], the dimerization reaction of G2 was accelerated tremendously (Table 1). The high conversion, high selectivity, and rate acceleration of this photoreaction of G2 within CB[10] suggested that two G2 molecules were preorganized by CB[10] to adopt a “reaction ready” h–t orientation, which was in good agreement with the MMFF-minimized model of complex CB[10]·2G2 in Fig. 4.
Fig. 5 1H NMR spectra (600 MHz, D2O, 298 K) of CB[10]·2G2 ([G2] = 2.5 mM) solution with various UV irradiation times: (a) 0, (b) 10, (c) 25 min, and (d) free dimerization product DG2. |
Host | t 1/2 (G2) | t 1/2 (G3) |
---|---|---|
a Estimated from NMR integrals (Fig. S12 and S32). b Calculated by fitting equation (Fig. S16, S18 and S39). c Estimated from diagrams of product yields (Fig. S26). d Not determined. | ||
None | 1.7 ha | 7.5 ha |
CB[10] (50%) | 6 minb | 37 minb |
CB[10] (10%) | 44 minb | 110 minb |
CB[8] (50%) | 8 minc | —d |
CB[8] (5%) | 35 minc | —d |
Inspired by a previous sample,33 we investigated whether CB[10] could act as a supramolecular catalyst to convert G2 into DG2 exclusively. In the presence of 0.1 equiv. of CB[10] (10%), almost 100% of G2 was converted into photoproduct DG2 with trace amounts of side products within 120 min (Fig. S17†). The t1/2 of 44 min was calculated from the first-order kinetics equation (Fig. S18†). In comparison, the t1/2 of G2 without CB[10] was estimated to be 1.7 hours (Table 1). All data suggested that CB[10] operated as the supramolecular catalyst in this case, although the photoreaction took longer to achieve than that with 0.5 equiv. of CB[10] (Scheme 1). As mentioned above, effective replacement of the product (DG2) with the starting material (G2) was necessary for host (CB[10]) to operate as a catalyst. To verify this, 1H NMR competition experiments between DG2 and G2 with CB[10] were performed.
As predicted, the mixture of G2, DG2, and CB[10] with a 2:1:1 ratio resulted in most CB[10] being occupied by G2 (Fig. S19c†). An approximate equilibrium constant (Keq) was calculated as 8.8 × 105 M−1 (Fig. S20 and S21†), suggesting that CB[10] operated as a supramolecular catalyst owing to the spontaneous replacement of product DG2 with starting material G2. The results showed that, as either the supramolecular nanoreactor or catalyst, CB[10] not only shortened the photoreaction time, but also tremendously improved the reaction selectivity.
When the host molecule was switched to CB[8], an unexpected photoreaction of G2 occurred, although CB[8] bound G2 with the same binding ratio as CB[10]. In the presence of 0.5 equiv. of CB[8], precipitate formation was observed when the G2 solution was irradiated with UV light. With extended irradiation time, additional yellow precipitate was generated. Therefore, the solutions after centrifugation were carefully monitored by 1H NMR. By analyzing the stacking NMR spectra (Fig. 6) and the precipitate, the bound peaks of G2 were found to fully disappear in 60 min, while a singlet peak at 3.1 ppm appeared and remained, and the yellow precipitate contained no CB[8] and was soluble in DMSO and CDCl3. First, we suspected that the precipitate was anthraquinone, because anthracene derivatives can easily be oxidized,45,46 despite the photoreaction being conducted under N2 atmosphere. However, the NMR signals for the precipitate did not match those of anthraquinone. Further hypotheses included the photosolvolysis of G2. Eventually, the yellow precipitate was verified to be 9-anthracenemethanol by EI-MS and 1H NMR (Fig. S22a and S23†). When irradiated in air, G2 first generated 9-anthracenemethanol as a precipitate (suspended), which was then converted to anthraquinone (Fig. S22b†), further confirming the occurrence of photosolvolysis prior to photooxidation. The singlet peak in the aqueous phase represented the ethylene proton signal of ethylenediamine (EDA). And the binding of protonated EDA in CB[8] helped solubilize CB[8] in water at high concentrations (maximum solubility of CB[8] in water is about 0.2 mM). Indeed, NMR titration experiments proved that small EDA molecules could be encapsulated by CB[8] (Fig. S24†). Furthermore, CB[8] was tested to determine whether it could act as a supramolecular catalyst in the photosolvolysis of G2. In the presence of 0.05 equiv. of CB[8] (5%), G2 was almost all converted to 9-anthracenemethanol within 4 h (Fig. S25†). In comparison, no precipitate was observed without CB[8]. The t1/2 of G2 in the presence of CB[8] was estimated because the data could not be ideally fitted with first-order kinetics (Table 1 and Fig. S26†). CB[8] doubtless operated as a supramolecular catalyst for the photosolvolysis of G2. Therefore, why photosolvolysis occurred when G2 itself did not show the same reactivity required further investigation.
Fig. 6 1H NMR spectra (600 MHz, D2O, 298 K) of 2:1 mixture of G2 (2.5 mM) and CB[8] with various UV irradiation times. |
After a broad search of the photosolvolysis literature, we noted that a “photoremovable protecting group (PPG)” could be involved in this reaction.12,47 To date, only one example of an anthracene unit acting as a PPG has been reported, with the plausible mechanism involving heterolysis and/or homolysis of the CH2–X bond in PPG–CH2–X (X, leaving group) to produce a carbonium ion intermediate. CB[n] hosts are known to be capable of stabilizing active carbocations.34,48,49 Therefore, a plausible mechanism for the photosolvolysis of G2 with CB[8] as the supramolecular catalyst in aqueous solution was proposed. As shown in Scheme 2, the initial step is light-induced heterolytic cleavage of the C–N bond of encapsulated G2, and/or homolytic cleavage of the C–N bond followed by rapid electron transfer (ET), to give the carbonium ion intermediate and EDA. The CB[8]-stabilized carbonium ion is then attacked by solvent molecule H2O to give product 9-anthracenemethanol. Owing to its weak binding with CB[8] and insolubility, 9-anthracenemethanol is displaced by G2 or EDA and precipitated out of solution, allowing the catalytic process to continue. Compared with the observation of radical side products (caused by homolysis) in most reported examples,12 the generation of a single product with quantitative yield in this case suggested that, with the assistance of CB[8], either C–N bond heterolysis was favored over homolysis or the electron transfer step after homolysis was accelerated.
Scheme 2 Plausible reaction mechanism for the photosolvolysis reaction of G2 in the presence of CB[8] host (5%). |
The high conversion and high selectivity of this photosolvolysis of G2 suggested that the orientation of the two G2 molecules in CB[8] was not only unfavorable for dimerization, but also favorable for stabilization of the carbonium ion intermediate by CB[8]. In addition to acting as the supramolecular catalyst, the most interesting role of CB[8] in this case was efficiently switching the reaction pathway of G2 from familiar photodimerization to unusual photosolvolysis.
As the first example of CB[10] operating as a supramolecular nanoreactor/catalyst, we anticipate that more reactions, involving large-sized or more than two substrate molecules, can be promoted by the large cavity of CB[10]. The selectivity for photosolvolysis of aromatic substrates with a PPG is also expected to be improved by the ability of CB[n] to stabilize cations. These finding will further benefit CB[n] chemistry and supramolecular catalysis.
Footnote |
† Electronic supplementary information (ESI) available: Experimental details, binding 1H NMR spectra, UV/Vis spectra of guests with CB[n], and relevant COSY, ESI-MS, 13C NMR spectra, kinetics fitting. See DOI: 10.1039/d0sc00409j |
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