A series of BiVO4 were efficiently prepared from the solution of Bi(NO3)3•5H2O and NH4VO3 via hydrothermal methods at different temperatures (140, 160, 180 oC). Then the as-prepared BiVO4 powder was annealed at 400 oC for 2 h in air to obtain the final BiVO4 (denoted as BiVO4-140, BiVO4-160, and BiVO4-180) (Scheme 1i, see the Supporting Information for details). The synthesized BiVO4 semiconductors were further characterized by X-ray diffraction (XRD). It was found that all three crystals are monoclinic scheelite bismuth vanadate compared with the standard pattern (Fig. 1i-iv).
In addition, the morphology was studied by scanning electron microscopy (SEM). As shown in Fig. 2 the BiVO4 prepared at different temperatures showed similar elliptical rod nanoparticles shapes.
Then, we are surprised to find that the prepared BiVO4 had good absorption in the ultraviolet and visible light regions, and the band gap is calculated to be 2.4 eV from the UV/Vis absorbance spectrum (Fig. 3i, Fig. S2). The transient photocurrent response analysis was conducted to evaluate the photoelectrochemical reversibility of these materials (Fig. 3ii). It was found that the light irradiation could induce the separation of holes and electrons to cause a sharp increase in current, which returned to the original state under dark. These results show that the three types of prepared BiVO4 semiconductors have good reversibility and optical stability. Among them, BiVO4-180 gives the highest photocurrent, which implies BiVO4-180 may be an active catalyst for photocatalytic reactions.
Next, the photoluminescence (PL) spectrum was studied to reflect the recombination process of photogenerated electron-hole pairs in semiconductor materials. As shown in Fig. 4i, the PL intensity of BiVO4-180 is lower than BiVO4-160 and BiVO4-140 (λex = 320 nm). Generally, the weaker the emission intensity, the lower the photocarrier recombination efficiency, and the better the separation rate of electron-hole pairs.64,65 Therefore, these results suggest that the photogenerated electron-hole pairs of BiVO4-180 are easier to separate, which will be advantageous to the photocatalytic activity. Moreover, the electron paramagnetic resonance (EPR) spectrum shows that the prepared BiVO4-180 has a characteristic oxygen vacancies (OVs) signal after 0.5 h of visible light exposure, indicating that OVs have been successfully generated on the BiVO4 semiconductor (Fig. 4ii).66 Based on the above-mentioned results, we envisioned that BiVO4-180 could be a privileged catalyst for heterogeneous photocatalytic organic transformations.
In order to explore the photocatalytic performance of the prepared BiVO4 catalysts, we started our research on the construction of tetrahydroquinoline derivatives through visible-light-induced C–H functionalization of N,N-dimethylanilines. First, N,N-dimethylaniline (1a) and N-phenylmaleimide (2a) were employed as the model substrates for the synthesis of pyrrolotetrahydroquinoline (4a). The optimization study was promoted by BiVO4 catalysts under irradiation of blue LED (details see Supporting Information, Table S1). After extensive experiments, the optimal conditions A were established as follows: 1a (0.6 mmol), 2a (0.2 mmol), BiVO4-180 (5 mol%), 2-methyltetrahydrofuran (2-Me-THF, 2 mL) as a green solvent at room temperature for 3 h in an air atmosphere under the irradiation of 5 W blue LED (460 nm). Under the optimized conditions A, the target product 4a was obtained with 80% isolated yield. Subsequently, we explored the scope of the synthesized pyrrolotetrahydroquinolines (4) under optimal conditions (Table 1). Firstly, N,N-dimethylanilines 1 bearing different substituents (–CH3, –OCH3, –Br) on benzene rings were studied. When electron-donating groups (–CH3, –OCH3) bearing N,N-dimethylanilines were applied as substrates, the target products (4b-c) were obtained in moderate yields. The N,N-dimethylaniline with an electron-withdrawing halide (–Br) at the p-position rendered the desired product 4d in good yield. As can be seen, N,N-dimethylanilines with −CH3 at the m-position gave the product in 88% yield (4e:4e’=17:1). Surprisingly, 3,5-dimethyl substituted 1 gave the target product 4f with 97% yield. Meanwhile, 1a reacted with different maleimide 2 bearing benzyl and alkyl groups (−Bn, −CH3, −tBu, –Cy) delivered the target products 4g-j in 35-71% yields. When N,N-dimethylnaphthalen-1-amine 1k was used as a substrate, the target product 4k was still obtained in a moderate yield. In addition, substituted phenylmaleimides 2 could also be converted into the corresponding products in good yield when R4 was –Et, –Br, and –Ac groups. Importantly, the reaction of hormone drug mifepristone and 2a successfully provides the target product 4o with 55% yield. Meanwhile, the model reaction in gram-scale generated product 4a with the isolated yield of 65%, indicating that the catalytic method has practical value (Scheme S2).
Table 1. Synthesis of functionalized tetrahydroquinolinesa
aReaction conditions A: 1 (0.6 mmol), 2 (0.2 mmol), BiVO4-180 (5 mol%), 2-Me-THF (2 mL) with the irradiation of blue LED (5 W) under air at room temperature for 3-4 h. Isolated yields were given.
In addition to maleimide, various 1,1-dicyanostyrenes 3 could also be applied as substrates for the coupling with N,N-dimethylanilines catalyzed by BiVO4-180. After slight modification of the reaction conditions, the model reaction of 1b and 3a in green solvent dimethyl carbonate (DMC) gave the target product 5a with a yield of 70% (Table 2). Further scope expansion showed good applicability, yielding the desired products 5b−j in 45−85% yields. Surprisingly, the hormone therapy drug ulipristal acetate was also suitable in this procedure, and the corresponding product 5k was obtained in a moderate yield (45%).
Table 2. Synthesis of 5a
aReaction conditions B: 1 (0.3 mmol), 3 (0.2 mmol), BiVO4 (5 mol%), DMC (2 mL) with the irradiation of blue LED (10 W) under air at room temperature for 19-48 h.
To explore the versatility of the catalytic activity of BiVO4-180, the 3-arylmethyl indoles 7 were constructed by the C-H functionalization reaction of N,N-dimethylanilines 1 and indoles 6 using BiVO4-180 as a photocatalyst (Table 3). After conditions optimization (details see Supporting Information, Table S2), the scope of various substituted 1 and 6 was further investigated. The products 7a−s were afforded in 48−85% isolated yields under visible light irradiation.
As a component of peptides and proteins, amino acids have attracted much attention, and their modification, especially through organic synthesis, is of great significance in the field of pharmacy and biological research.67-70 By using the BiVO4-180 catalysis strategy, we also realized the modification of amino acid derivatives and peptides through semiconductor photocatalysis. Firstly, the synthesis of aromatic/heterocyclic α-amino acid esters 10 was achieved through the reaction of aromatic heterocycles 8 and N-arylglycine esters 9 (Table 3). A range of aromatic heterocycles reacted with N-arylglycine esters smoothly to give the desired products 10a−i in moderate to excellent yields (45-92%). Furthermore, more challenging transformations to construct functionalized peptides are investigated, which are usually prone to oxidative cleavage and therefore difficult to control. Surprisingly, the reaction of different substituted peptides with 2-phenylindole delivered the desired product 10j-o in moderate yields (43-50%).
In addition, we also investigated the C-H functionalization of aromatic heterocycles with thioethers, selenoether, and NH4SCN (Table 3). A series of vulcanization, selenation, and thiocyanation products 12a-q were obtained in moderate to excellent yields (40-95%).
[Table 3 is available in the supplementary files section.]
In order to verify the stability and reusability of the catalytic system, we carried out the recovery experiment of BiVO4-180 (Scheme 2). The target product 4a was obtained in 80% yield by model reaction of 1a and 2a under the reaction conditions A. Then the catalyst was recovered by centrifugation, washed with water and ethanol, and then dried at 80 °C. Subsequently, the recovered catalyst could be used for the next run. The results showed that the recovered catalyst still maintain excellent activity in the eighth run. In addition, powder X-ray diffraction (XRD) confirmed that the structure of the recovered catalyst BiVO4-180 did not change significantly (Fig. 1v).
To further explore the mechanism of this catalytic strategy, we conducted some control experiments (Scheme 3i). First, the two radical scavengers 2,2,6,6-tetramethylpiperidin-1-yl-oxidanyl (TEMPO) and 2,6-di-tert-butyl-4-methylphenol (BHT) were added to the model reaction of 1a and 2a. It was found that both reactions were completely inhibited, indicating that this transformation may be a radical process. To support this hypothesis, the reaction solution with BHT was analyzed by high-resolution mass spectrometry (HRMS), the signal of m/z 340.2635 indicated that the radical adduct 4a’ was formed by BHT and the in situ produced aminomethyl radical (Fig. S3). Moreover, when ammonium oxalate and K2S2O8 as the scavengers for holes (h+) and electrons, respectively, were added to the template reaction, the reactions were inhibited, showing that the BiVO4-180 semiconductor participates in the reaction. The reaction under a nitrogen atmosphere didn’t occur, suggesting that the oxygen in the air should participate in the reactions.
Based on the above control experiments and previous reports, we proposed the tentative mechanism of this transformation (Scheme 3ii). First, under visible light irradiation, BiVO4-180 semiconductor was excited to generate holes on the valence band (VB) and electrons on the conduction band (CB). Then, the holes on VB obtain electrons from 1a by single electron transfer (SET) to form intermediate 1aa. Subsequently, 1aa loosed a proton to form radical 1ab. Then, radical 1ab experienced an addition reaction with 2a to form intermediate 4aa, which was followed by an intramolecular cyclization to generate intermediate 4ab. Finally, 4ab was converted into the target product 4a through the SET and deprotonation process. In addition, the control experiments and the plausible mechanism for the synthesis of 7 are shown in Scheme S3-4.