Imines are one of the most frequently used substrates in synthetic chemistry where they feature strongly in a variety of organic transformations, including cyclization reactions and reactions involving the addition of nucleophiles to the carbon atom of the imine bond. The oxidation of cyclic amines to the corresponding cyclic imines is an important synthetic methodology, which generally requires the addition of a stoichiometric oxidant [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11], such as iodine, sulfur, tert-butylhydroperoxide or 3,3-dimethyl-1-butene, which can lead to the formation of harmful waste products. Furthermore, reactions involving the oxidation of amines with trichloroisocyanuric acid [12] or tert-butyl hypochlorite [13] always proceed via a two-step process of N-chlorination and dehydrochlorination to give the corresponding imines (Scheme 1) The transition-metal catalyzed dehydrogenation of organic compounds represents a powerful, atom-economical and environmentally benign approach for the introduction of unsaturated double bonds, such as C=C [14, 15, 16, 17, 18, 19], C=N [20, 21, 22, 23, 24, 25] and C=O [26, 27, 28, 29, 30, 31] bonds, whilst avoiding the use of stoichiometric amounts of harmful oxidants. The dehydrogenation of N-heterocycles has attracted considerable interest from both academic and industrial research groups during the course of the past two decades. This method is generally used to prepare N-heteroaromatic compounds, which are common structural motifs in pharmaceutical and material chemistry [32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47], because it provides rapid access to stable dehydroaromatization products.
Mechanistic studies have shown that the dehydrogenation of N-heterocyclic compounds occurs via a reactive cyclic imine intermediate, followed by further dehydroaromatization [48, 49]. In theory, cyclic imines could be formed by the controllable dehydrogenation of N-heterocyclic compounds. However, reports pertaining to the development of partial dehydrogenative processes with cyclic imines as products are scarce [50, 51, 52]. Stahl’s group [50] recently described a Zn/quinone complex catalyzed reaction for the aerobic oxidation of amines to imines with good to excellent yields. Turner’s group [51] creatively applied the monoamine oxidase MAO-N D11C as a catalyst for the enantioselective oxidation of amines. It is easy to understand why the dehydrogenation of N-heterocycles is prone to the formation of the final dehydroaromatization products because the resulting aromatic products are much more stable than the corresponding partially oxidized imine intermediates, which are formed as transient species during the dehydrogenative process. The development of new processes capable of achieving high levels of selectivity for the partial dehydrogenation of N-heterocyclic compounds remains a challenging subject in this field of research. A critical issue that needs to be addressed by any new methodology is the suppression of further aromatization, which would lead to significant improvements in the chemoselectivity of dehydrogenation. Given that the different dehydrogenative products of N-heterocyclic compounds, including aromatic compounds and imines, are valuable organic building blocks, the development of an efficient and controllable process for the dehydrogenation of N-heterocyclic compounds is highly desirable. Herein, we report a new Pd/C-promoted process for the partial dehydrogenation of 1,2,3,4-tetrahydroisoquinolines to 3,4- dihydroisoquinolines exclusively with high levels of activity and chemoselectivity.
Commercially available reagents and solvents were used without further purification. The Pd/C (5% Pd on carbon) catalyst used in the current study was purchased from J&K. 1H, 13C and 19F NMR spectra were recorded at room temperature in CDCl3 on a 400 MHz instrument (Brucker) with tetramethylsilane (TMS) as an internal standard. Flash column chromatography was performed on silica gel (200-300 mesh). All of the reactions were monitored by TLC analysis. The 1- substituted-1,2,3,4-tetrahydroisoquinolines were prepared according to the literature methods [53].
Pd/C (254 mg, 0.12 mmol) and K3PO4×3H2O (16 mg, 0.06 mmol) were placed in a Schlenk tube followed by acetonitrile (1 mL), and the resulting mixture was stirred at room temperature for 10 min. A solution of 1-substituted-1,2,3,4- tetrahydroisoquinoline (0.30 mmol) in acetonitrile (4 mL) was then added to the reaction mixture, and the Schlenk tube was carefully and quickly vacuum purged before being filled with oxygen using an oxygen balloon. The reaction mixture was then stirred at 60 °C until the 1-substituted-1,2,3,4- tetrahydroisoquinoline had been completely consumed (as determined by TLC analysis). Upon completion of the reaction, the mixture was slowly cooled to room temperature and filtered through diatomite to remove the Pd/C catalyst. The filtrate was then concentrated in vacuo to give the crude product as a residue, which was purified by flash chromatography over silica gel eluting with petroleum ether and ethyl acetate to give the imine product 2.
1-Phenyl-3,4-dihydroisoquinoline (2a): 86% yield, known compound [54], yellow oil, Rf = 0.75 (ethyl acetate). 1H NMR (400 MHz, CDCl3) δ = 7.60-7.56 (m, 2H), 7.44-7.35 (m, 4H), 7.26-7.21 (m, 3H), 3.85-3.82 (m, 2H), 2.80-2.77 (m, 2H); 13C NMR (100 MHz, CDCl3) δ = 167.3, 139.0, 138.9, 130.7, 129.3, 128.9, 128.8, 128.1, 127.9, 127.4, 126.6, 47.7, 26.3.
1-Phenylisoquinoline (3a): known compound [55], white solid, Rf = 0.93 (ethyl acetate), mp = 73-74 °C.1H NMR (400 MHz, CDCl3) δ = 8.61 (d, J = 5.7 Hz, 1H), 8.10 (d, J = 8.5 Hz, 1H), 7.87 (d, J = 8.2 Hz, 1H), 7.71-7.63 (m, 4H), 7.55-7.47 (m, 4H); 13C NMR (100 MHz, CDCl3) δ = 160.9, 142.4, 139.8, 137.0, 130.2, 130.1, 128.8, 128.5, 127.8, 127.3, 127.2, 126.9, 120.1.
1-(4-Chlorophenyl)-3,4-dihydroisoquinoline (2b): 84% yield, known compound [54], colorless oil, Rf = 0.50 (ethyl acetate). 1H NMR (400 MHz, CDCl3) δ = 7.56-7.40 (m, 2H), 7.48-7.37 (m, 3H), 7.27-7.21 (m, 3H), 3.85-3.82 (m, 2H), 2.81-2.77 (m, 2H); 13C NMR (100 MHz, CDCl3) δ = 166.2, 138.9, 137.5, 135.4, 130.9, 130.2, 128.5, 128.4, 127.6, 127.5, 126.7, 47.7, 26.3.
1-(4-Methoxyphenyl)-3,4-dihydroisoquinoline (2c): 89% yield, known compound [54], colorless oil, Rf = 0.60 (ethyl acetate). 1H NMR (400 MHz, CDCl3) δ = 7.57-7.55 (m, 2H), 7.36-7.23 (m, 4H), 6.95-6.93 (m, 2H), 3.84 (s, 3H), 3.81-3.78 (m, 2H), 2.78-2.75 (m, 2H); 13C NMR (100 MHz, CDCl3) δ = 166.6, 160.6, 139.1, 131.5, 130.5, 130.3, 128.9, 127.9, 127.4, 126.5, 113.5, 55.3, 47.5, 26.4.
1-m-Tolyl-3,4-dihydroisoquinoline (2d): 82% yield, known compound [54], colorless oil, Rf = 0.40 (petroleum ether/ethyl acetate = 2/1). 1H NMR (400 MHz, CDCl3) δ = 7.43 (d, J = 9.1 Hz, 1H), 7.41-7.32 (m, 2H), 7.32-7.20 (m, 5H), 3.84 (dd, J = 8.2, 6.4 Hz, 2H), 2.83-2.74 (m, 2H), 2.39 (s, 3H); 13C NMR (100 MHz, CDCl3) δ = 167.4, 139.0, 138.8, 137.9, 130.6, 130.0, 129.3, 128.9, 128.0, 127.9, 127.4, 126.6, 126.0, 47.7, 26.4, 21.4.
1-p-Tolyl-3,4-dihydroisoquinoline (2e): 84% yield, known compound [54], white solid, Rf = 0.45 (petroleum ether/ethyl acetate = 2/1), mp = 73-74 °C. 1H NMR (400 MHz, CDCl3) δ = 7.49 (d, J = 8.1 Hz, 2H), 7.35 (td, J = 7.4, 1.4 Hz, 1H), 7.29-7.21 (m, 5H), 3.84-3.76 (m, 2H), 2.80-2.76 (m, 2H), 2.40 (s, 3H); 13C NMR (100 MHz, CDCl3) δ = 167.1, 139.2, 138.9, 136.2, 130.5, 128.9, 128.8, 128.8, 128.0, 127.4, 126.5, 47.6, 26.4, 21.4.
1-(4-(Trifluoromethyl)phenyl)-3,4-dihydroisoquinoline (2f): 82% yield, known compound [54], pale yellow solid, Rf = 0.60 (petroleum ether/ethyl acetate = 2/1), mp = 72-74 °C. 1H NMR (400 MHz, CDCl3) δ = 7.70 (q, J = 8.4 Hz, 4H), 7.40 (td, J = 7.4, 1.2 Hz, 1H), 7.27 (dd, J = 16.0, 7.4 Hz, 2H), 7.20 (d, J = 7.4 Hz, 1H), 3.90-3.86 (m, 2H), 2.84-2.80 (m, 2H); 13C NMR (100 MHz, CDCl3) δ = 166.3, 142.5, 138.7, 131.8, 131.4, 131.1, 129.2, 128.4, 127.6, 127.5, 125.2 (q, J = 3.7 Hz), 124.1 (q, J = 271 Hz), 47.8, 26.2; 19F NMR (376 MHz, CDCl3) δ -62.66.
1-(Furan-2-yl)-3,4-dihydroisoquinoline (2g): 88% yield, known compound [54], yellow oil, Rf = 0.60 (ethyl acetate). 1H NMR (400 MHz, CDCl3) δ = 7.71 (d, J = 7.6 Hz, 1H), 7.58 (d, J = 1.1 Hz, 1H), 7.40 (td, J = 7.4, 1.0 Hz, 1H), 7.32 (t, J = 7.2 Hz, 1H), 7.26-7.25 (m, 1H), 6.86 (d, J = 3.4 Hz, 1H), 6.52 (dd, J = 3.3, 1.7 Hz, 1H), 3.83 (dd, J = 8.1, 6.2 Hz, 2H), 2.76-2.72 (m, 2H); 13C NMR (100 MHz, CDCl3) δ = 157.0, 151.7, 144.0, 138.9, 130.8, 127.6, 127.5, 126.9, 126.8, 113.1, 111.2, 47.0, 26.2.
7-Methyl-1-phenyl-3,4-dihydroisoquinoline (2h): 79% yield, known compound [56], yellow oil, Rf = 0.50 (petroleum ether/ethyl acetate = 2/1). 1H NMR (400 MHz, CDCl3) δ = 7.59 (dt, J = 8.7, 3.7 Hz, 2H), 7.44-7.42 (m, 3H), 7.20-7.15 (m, 2H), 7.07 (s, 1H), 3.85-3.81 (m, 2H), 2.77-2.74 (m, 2H), 2.29 (s, 3H); 13C NMR (100 MHz, CDCl3) δ = 167.6, 139.4, 136.3, 136.0, 131.5, 129.4, 129.0, 128.9, 128.6, 128.3, 127.4, 48.1, 26.2, 21.4.
7-Chloro-1-phenyl-3,4-dihydroisoquinoline (2i): 76% yield, known compound [57], white solid, Rf = 0.25 (petroleum ether/ethyl acetate = 2/1), mp = 75-76 °C. 1H NMR (400 MHz, CDCl3) δ = 7.59-7.57 (m, 2H), 7.46-7.42 (m, 3H), 7.35 (dd, J = 7.9, 2.1 Hz, 1H), 7.25 (d, J = 2.1 Hz, 1H), 7.20 (d, J = 8.1 Hz, 1H), 3.84 (dd, J = 8.1, 6.4 Hz, 2H), 2.77-2.74 (m, 2H); 13C NMR (100 MHz, CDCl3) δ = 166.2, 138.4, 137.1, 132.3, 130.5, 130.1, 129.6, 128.7, 128.7, 128.4, 127.8, 47.6, 25.7.
7-Methoxy-1-phenyl-3,4-dihydroisoquinoline (2j): 74% yield, known compound [58], colorless oil, Rf = 0.74 (ethyl acetate). 1H NMR (400 MHz, CDCl3) δ = 7.60 (dd, J = 6.5, 3.2 Hz, 2H), 7.43-7.41 (m, 3H), 7.18 (d, J = 8.2 Hz, 1H), 6.93 (dd, J = 8.2, 2.7 Hz, 1H), 6.82 (d, J = 2.6 Hz, 1H), 3.84-3.81 (m, 2H), 3.71 (s, 3H), 2.74-2.70 (m, 2H); 13C NMR (100 MHz, CDCl3) δ = 167.2, 158.2, 138.9, 130.9, 129.5, 129.3, 128.8, 128.2, 128.2, 116.1, 113.8, 55.5, 48.1, 25.5.
7-Methoxy-1-phenylisoquinoline (3j): 10% yield, known compound [55], yellow oil, Rf = 0.88 (ethyl acetate). 1H NMR (400 MHz, CDCl3) δ = 8.51 (d, J = 5.6 Hz, 1H), 7.79 (d, J = 8.9 Hz, 1H), 7.73-7.71 (m, 2H), 7.54 (m, 4H), 7.39-7.34 (m, 2H), 3.81 (s, 3H); 13C NMR (100 MHz, CDCl3) δ = 159.2, 158.6, 140.6, 140.5, 139.9, 132.5, 129.6, 128.6, 128.5, 128.4, 127.8, 122.9, 119.7, 105.3, 55.4.
6,7-Dimethoxy-1-phenyl-3,4-dihydroisoquinoline (2k): 84% yield, known compound [54], pale yellow solid, Rf = 0.60 (dichloromethane/methanol = 15/1), mp = 110-112 °C. 1H NMR (400 MHz, CDCl3) δ = 7.60 (dd, J = 6.3, 3.0 Hz, 2H), 7.47-7.36 (m, 3H), 6.80-6.77 (m, 2H), 3.94 (d, J = 3.1 Hz, 3H), 3.83-3.78 (m, 2H), 3.72 (d, J = 2.7 Hz, 3H), 2.72 (dd, J = 10.3, 4.3 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ = 166.7, 151.0, 147.1, 139.2, 132.6, 129.3, 128.8, 128.1, 121.6, 111.7, 110.3, 56.2, 56.0, 47.7, 26.0.
6,7-Dimethoxy-1-phenylisoquinoline (3k): 7% yield, known compound [55], colorless oil, Rf = 0.82 (dichloromethane/ methanol = 15/1). 1H NMR (400 MHz, CDCl3) δ = 8.48 (d, J = 5.6 Hz, 1H), 7.71 (dd, J = 8.1, 1.3 Hz, 2H), 7.55-7.47 (m, 4H), 7.37 (s, 1H), 7.12 (s, 1H), 4.04 (s, 3H), 3.86 (s, 3H); 13C NMR (100 MHz, CDCl3) δ = 158.3, 152.7, 150.1, 150.1, 141.4, 140.1, 133.8, 129.6, 128.4, 122.6, 118.7, 105.7, 105.0, 56.1, 55.9.
1-Cyclohexyl-3,4-dihydroisoquinoline (2l): 51% yield, known compound [54], yellow oil, Rf = 0.70 (ethyl acetate). 1H NMR (400 MHz, CDCl3) δ = 7.53-7.50 (m, 1H), 7.33-7.20 (m, 2H), 7.29-7.18 (m, 1H), 3.68-3.64 (m, 2H), 2.90 (dd, J = 15.1, 6.7 Hz, 1H), 2.67-2.63 (m, 2H), 1.91-1.85 (m, 4H), 1.83 (d, J = 12.3 Hz, 1H), 1.76-1.24 (m, 5H); 13C NMR (100 MHz, CDCl3) δ = 170.8, 138.3, 130.1, 128.9, 127.6, 126.8, 124.6, 46.8, 42.1, 31.3, 26.6, 26.4, 26.3.
1-Cyclohexylisoquinoline (3l): 10% yield, known compound [39], yellow oil, Rf = 0.82 (ethyl acetate). 1H NMR (400 MHz, CDCl3) δ = 8.48 (d, J = 5.7 Hz, 1H), 8.23 (d, J = 8.4 Hz, 1H), 7.81 (d, J = 8.0 Hz, 1H), 7.67-7.56 (m, 2H), 7.48 (d, J = 5.6 Hz, 1H), 3.57 (tt, J = 11.7, 3.2 Hz, 1H), 2.01-1.82 (m, 7H), 1.58-1.50 (m, 2H), 1.45-1.33 (m, 1H); 13C NMR (100 MHz, CDCl3) δ = 165.9, 142.1, 136.6, 129.7, 127.7, 127.0, 126.5, 124.9, 119.0, 41.7, 32.8, 27.1, 26.4.
1-Cyclohexylidene-1,2,3,4-tetrahydroisoquinoline (4l): 27% yield, unknown compound, yellow oil, Rf = 0.95 (ethyl acetate). 1H NMR (400 MHz, CDCl3) δ = 7.86 (d, J = 7.8 Hz, 1H), 7.37-7.28 (m, 2H), 7.24 (d, J = 7.3 Hz, 1H), 5.70 (brs, 1H), 3.72-3.68 (m, 2H), 2.70-2.66 (m, 2H), 2.14 (td, J = 13.1, 4.2 Hz, 2H), 1.93-1.79 (m, 3H), 1.69-1.66 (m, 2H), 1.56 (d, J = 13.1 Hz, 2H), 1.45-1.30 (m, 1H); 13C NMR (100 MHz, CDCl3) δ = 171.0, 139.4, 130.4, 128.1, 126.9, 126.6, 126.4, 73.9, 45.8, 36.7, 27.1, 25.6, 22.2. HRMS: m/z [M+H]+ calcd for C15H20N = 214.1590; found = 214.1586.
Pd/C (254 mg, 0.12 mmol) and K3PO4×3H2O (16.0 mg, 0.06 mmol) were placed in a Schlenk tube followed by acetonitrile (1 mL), and the resulting mixture was stirred at room temperature for 10 min. 1-Phenyl-1,2,3,4-tetrahydroisoquinoline (63 mg, 0.30 mmol) and acetonitrile (3 mL) were then added to the Schlenk tube, and the resulting mixture was carefully and quickly vacuum purged before being filled with oxygen using a balloon. The resulting mixture was then stirred at 60 °C for 18 h. TLC analysis revealed the complete consumption of 1- phenyl-1,2,3,4-tetrahydroisoquinoline and the mixture was slowly cooled to room temperature before being filtered through a membrane filter to remove the catalyst. The filtrate was then concentrated in vacuo to give the crude product. 1H NMR analysis of the crude material revealed that it consisted of a 35/1 mixture of 2a/3a. The crude product was purified by flash chromatography over silica gel eluting with a 5/1 (V/V) mixture of petroleum ether/ethyl acetate to give the corresponding product imine 2a in 79% yield.
The Pd/C catalyst recovered from the experiment described above was placed in a Schlenk tube containing K3PO4×3H2O (16.0 mg, 0.06 mmol) and acetonitrile (1 mL), and the resulting mixture was stirred for 10 min. 1-Phenyl-1,2,3,4- tetrahydroisoquinoline (63 mg, 0.30 mmol) and acetonitrile (3 mL) were then added to the reaction, and the resulting mixture was stirred at 60 °C for 42 h to allow for the complete consumption of the starting materials (as determined by TLC). The reaction mixture was then worked up according to the procedure described above to give the crude product, which was found to consist of a 44/1 mixture of 2a/3a by 1H NMR analysis. The crude product was then purified by flash chromatography over silica gel eluting with a 5/1 mixture of petroleum ether/ethyl acetate to give the corresponding product imine 2a in 80% yield.
The Pd/C catalyst used above was recovered and used for a third time according to the same procedure, except the reaction required 96 h at 60 °C to reach completion. 1H NMR analysis of the crude product revealed that it consisted of a mixture of 2a/3a = 42/1. The crude product was then purified by flash chromatography over silica gel eluting with a mixture of petroleum ether/ethyl acetate (V/V) = 5/1 to give the corresponding product imine 2a in 87% yield.
Compound 1a was selected as a model substrate to explore the partial dehydrogenation of 1-substitued-1,2,3,4- tetrahydroisoquinolines using Pd/C as the catalyst. Several solvents were screened in the reaction, including DCM, MeOH, THF, toluene and acetonitrile (Table 1, entries 1-5). Acetonitrile was found to be the best solvent for the reaction, giving a 62% conversion of the starting material. The reaction was also found to be sensitive to temperature (Table 1, entries 5-8). Increasing the temperature led to an increase in the rate of the reaction, but also led to an increase in the amount of the aromatization product. When the reaction was conducted at 80 °C, the ratio of imine to isoquinoline decreased significantly to 6/1. This result therefore confirmed that a temperature of 60 °C was optimum for this transformation in terms of the reactivity and chemoselectivity. It is well known that the catalytic property of heterogeneous catalysts can be modified by the introduction of an additive, such as the modification of Lindlar’s catalyst with a P2-nickel catalyst additive. Li’s group [59] recently developed an alkaline salt-modified supported Pd catalyst for the selective racemization and dynamic kinetic resolution of primary amines. Inspired by this work, we investigated the effect of adding different bases to the Pd/C catalyst used in the current transformation (Table 1, entries 9-13). As expected, the addition of a base led to an increase in the chemoselectivity of the reaction. The use of weakly basic CH3COONa as an additive led to an increase in the chemoselectivity of 2a/3a from 8/1 to 10/1. Pleasingly, the use of stronger inorganic bases such as K2CO3 and Cs2CO3 led to greater increases in the chemoselectivity of 2a/3a = 13/1 and 31/1, respectively (Table 1, entries 10-11). Interestingly, the addition of K3PO4×3H2O to the Pd/C catalyst gave excellent activity and good selectivity (Table 1, entry 12). Furthermore, the use of K3PO4×3H2O under an oxygen atmosphere led to a significant increase in the chemoselectivity from 16/1 to 32/1 (Table 1, entry 13). However, the effect of oxygen on the selectivity of the dehydrogenation reaction remains unclear. Taken together, these experiments revealed that the optimum conditions for the reaction were Pd/C (0.12 mmol) and K3PO4×3H2O (0.06 mmol) in acetonitrile at 60 °C under an atmosphere of O2 (balloon).
With the optimal conditions in hand, we proceeded to explore the scope of this transformation using a range of 1- substituted-1,2,3,4-tetrahydroisoquinolines 1, and the results are summarized in Table 2. The results showed that almost all of the 1-aryl substituted substrates tested in the current study reacted smoothly to afford the desired products in good to excellent yields, regardless of the electronic properties of the C1 substituent of the aromatic ring (Table 2, entries 1-11). Although the corresponding isoquinoline products were also formed in each of these reactions, the ratio of 2/3 was greater than 20/1 in most cases. It is noteworthy that 6,7- dimethoxy-1- phenyl-1,2,3,4-tetrahydroisoquinoline (1k) gave the corresponding product in 84% yield with a lower catalyst loading of 0.06 mmol Pd/C (Table 2, entry 11).
Furthermore, 1-cyclohexyl-1,2,3,4-tetrahydroisoquinoline (1l) was quantitatively converted to three different dehydrogenative products (2l, 3l and 4l) under the optimized conditions, with the desired partial dehydrogenative product 2l being formed in 51% yield. The unwanted dehydrogenative products 1-cyclohexylisoquinoline (3l) and 1-cyclohexylidene- 1,2,3,4-tetrahydroisoquinoline (4l) were formed in 10% and 27% yields, respectively (Scheme 2).
The recyclability of this K3PO4-modified Pd/C catalyst was also explored (Table 3). The dehydrogenation of 1-phenyl- 1,2,3,4-tetrahydroisoquinoline (1a) was performed under the standard reaction conditions. Upon completion of the reaction (as determined by TLC analysis), the solvent was removed by membrane-filtration and the catalyst was recovered and reused in the next reaction. Although there was a small decrease in the activity of the recovered catalyst, the reaction could be pushed to completion by extending the reaction time. Pleasingly, the recovered catalyst afforded excellent chemoselectivity after the third cycle. This result therefore demonstrates the potential of this newly developed K3PO4-modified Pd/C catalyst as a highly practical system for the dehydrogenation of tetrahydroisoquinolines.
We have described the development of a highly selective process for the partial dehydrogenation of 1-substituted- 1,2,3,4-tetrahydroisoquinolines using modified Pd/C. This new process provides an atom-economical and environmentally friendly method for the preparation of 1-substituted-3,4- dihydroisoquinolines without using stoichiometric amounts of harmful oxidants. The key feature of this reaction is the addition of K3PO4×3H2O to modify the Pd/C catalyst, which dramatically improves the chemoselectivity by suppressing the reaction of the desired product to give aromatic products. Further research into the use of this catalytic system to prepare simple acyclic imines and its application in cascade reactions is currently underway in our laboratory.
2015, Vol. 36 Issue (1): 33-39
PDF (6750 KB)
2015, Vol. 36 Issue (1): 33-39
PDF (6750 KB)