Novel blended catalysts consisting of a TiO2 photocatalyst and an Al2O3 supported Pd–Au bimetallic catalyst for direct dehydrogenative cross-coupling between arenes and tetrahydrofuran

Dehydrogenative cross-coupling (DCC) is a clean methodology to make C–C bonds by using abundant C–H bonds. The blended catalyst, developed in this study, consists of a TiO2 photocatalyst and an Al2O3 supported Pd–Au bimetallic catalyst and shows superior activity to the conventional TiO2 photocatalyst loaded with the corresponding metal co-catalyst for the direct DCC between various arenes and tetrahydrofuran, with concomitant evolution of hydrogen gas. The reactions were done under mild conditions without consuming any oxidising agent or other additional chemicals. This new approach of separating the photocatalyst and the metal catalyst parts allows their independent modification to improve the overall catalytic performance.


Photocatalytic direct DCC between various arenes and THF
Various substituted arenes, with both electron withdrawing and electron releasing groups, were examined for the photocatalytic direct dehydrogenative cross-coupling (the photocatalytic direct DCC) with THF, as mentioned in the main text.All arenes successfully underwent the cross-coupling reaction to give the corresponding DCC products and hydrogen gas, as evidenced by the GC-TCD chromatogram in Fig. S1.Further, for all reactions, the blended catalyst containing the TiO2 photocatalyst and the Pd(2.0)Au(1.0)/Al2O3catalyst gave the highest activity and/or selectivity for the DCC products in these conditions.
Table S1 shows the results of the photocatalytic direct DCC between benzaldehyde (2) and THF with different catalysts.This reaction gave a mixture of ortho, meta and para substituted products, collectively shown as 2a, along with the oxidation and reduction products of benzaldehyde, benzoic acid (2b) and benzyl alcohol (2c) as by-products.Unfortunately, for all the catalysts tested for this reaction, the benzoic acid was the major product in these reaction conditions, which decreased the selectivity to the DCC product (Table S1, entries 1-6).This oxidation reaction is quite well known and the presence of the adsorbed water on the TiO2 surface might facilitate this reaction.S2 However, even in this condition, the blended catalyst containing the TiO2 photocatalyst and the Pd(2.0)Au(1.0)/Al2O3catalyst gave the highest activity and selectivity for the DCC products among the listed catalysts.The better reaction condition provided higher selectivity as shown in Fig. 7 in the main text.a General reaction conditions: 2 mL of arene, here benzaldehyde (2) (19.9 mmol), 2 mL (24 mmol) THF, and 50 mg of each catalyst was used for the reaction.The wavelength (λ) of the irradiated light was ≥ 350 nm, the light intensity was 40 mW/cm 2 (measured at 360 nm±60 nm).b The amount of all products was approximately determined from the calibration curve of 4-(tetrahydrofuran-2-yl)benzonitrile.c Selectivity for DCC products based on benzaldehyde, calculated as: %S=100×2a (μmol)/[2a+2b+2c (μmol)].
Table S2 shows the results of the photocatalytic DCC reaction between benzonitrile (3) and THF carried out with different catalysts.The reaction gave a mixture of three DCC products namely, 2-(tetrahydrofuran-2-yl)benzonitrile, 3-(tetrahydrofuran-2-yl)benzonitrile, and 4-(tetrahydrofuran-2yl)benzonitrile (shown collectively as 3a), and the oxidation product of 3, benzamide (3b) (Table S2, entries 1-6).The blended catalyst consisting of the TiO2 photocatalyst and the Pd(2.0)Au(1.0)/Al2O3catalyst gave the highest yield of the DCC product, albeit with low selectivity (Table S2, entry 5).However, under the improved reaction conditions, as shown in Fig. 7 in the main text, the oxidation of 3 could be suppressed to get complete selectivity to the DCC products was achieved.
Table S3 shows the results of the photocatalytic DCC between toluene (4) and THF carried out with different catalysts.This reaction gave a mixture of two cross-coupling products namely 2-(otolyl)tetrahydrofuran and 2-(p-tolyl)tetrahydrofuran (shown collectively as 4a), along with the oxidation products from toluene like benzaldehyde (4b) and benzyl alcohol (4b).The oxygen, for the formation of these oxidation products, would originate from the adsorbed water or surface hydroxyl groups on the catalysts.The partial oxidation of toluene to products like benzaldehyde is extensively reported in the literature.S3 Only the oxidation of toluene proceeded with the pristine TiO2 photocatalyst (Table S3, entry 1).However, the introduction of the Pd metal nanoparticles in the reaction mixture, supported on TiO2 or Al2O3, gave the DCC product (Table S3, entries 2-6).The blended catalyst consisting of the TiO2 photocatalyst and the Pd(2.0)Au(1.0)/Al2O3catalyst gave a moderate yield of the DCC product with the highest selectivity (Table S3, entry 5).The photocatalytic DCC reaction between aniline (5) and THF also gave a mixture of two products namely, 2-(tetrahydrofuran-2-yl)aniline and 4-(tetrahydrofuran-2-yl)aniline (shown collectively as 5a) and the homocoupling products from aniline, di(phenyl)diazine (5b).Table S4 shows the results of the reaction carried out with different catalysts.The NH2 group of aniline was very reactive in the photocatalytic reaction, which promoted its homocoupling to give 5b.This reaction mainly occurred for this system, which decreased the selectivity to DCC products (Table S4, entries 1-6).Even then, the blended catalyst consisting of the TiO2 photocatalyst and the Pd(2.0)Au(1.0)/Al2O3catalyst gave the highest yield of the DCC product with the highest selectivity (Table S4, entry 5). a 21.9 mmol of aniline ( 5), other reaction conditions were the same as those in Table S1.b The amount of all products was approximately determined from the calibration curve of 4-(tetrahydrofuran-2yl)benzonitrile.

Table S1
Photocatalytic direct DCC between benzaldehyde and THF with different catalysts a

Table S2
Photocatalytic direct DCC between benzonitrile and THF with different catalysts a 19.4 mmol of benzonitrile (3), other reaction conditions were same as those in TableS1.bTheamount of all products was approximately determined from the calibration curve of 4-(tetrahydrofuran-2-yl)benzonitrile. a

Table S3
Photocatalytic direct DCC between benzonitrile and THF with different catalysts a

Table S4
Photocatalytic direct DCC between aniline and THF with different catalysts a