Electrochemical Synthesis of 1,1 (cid:48) -Binaphthalene-2,2 (cid:48) -Diamines via Transition-Metal-Free Oxidative Homocoupling

: The facile and green synthesis of 1,1 (cid:48) -binaphthalene-2,2 (cid:48) -diamine (BINAM) derivatives was established via the anodic dehydrogenative homo-coupling of 2-naphthylamines. The sustainable protocol provided a series of BINAMs in excellent yields of up to 98% with good current efﬁciency (66%) and H 2 as the sole coproduct without utilizing transition-metal

Biaryls synthesis through anodic oxidation utilizing electrochemical synthesis has emerged as a promising green and sustainable approach over the last few decades. The electrochemical dehydrogenative coupling of aryls utilizes electricity as an alternative to oxidants and produces H 2 as the sole coproduct without generating any toxic waste; thus, this approach exhibits great advantages in terms of high atom economy and environmentally benign synthesis protocols [25,26]. Although remarkable achievements have been made in the electrochemical dehydrogenative heterocoupling of anilines [27], few reports on the homocoupling of aniline derivatives [28,29], particularly 2-naphthylamines, are available. In the 1980s, Gossage [30] and Sereno [31] independently described the electrochemical dehydrogenative homocoupling of 2-naphthylamines, which afforded BINAMs, but in low yield. Thus, the development of efficient oxidative homocoupling protocols for 2-naphthylamines via electrochemical synthesis is of great importance.
Because the discharge of the electrolyte or solvent leads to a low yield and poor current efficiency, we reexamined the anodic dehydrogenative homocoupling conditions of 2-naphthylamines to save energy and chemical loading in the pursuit of developing environmentally benign chemical reactions. To our delight, under the newly established conditions, the corresponding homocoupling products were obtained in 98% yield with good current efficiency (66%; Scheme 1).
conditions, the corresponding homocoupling products were obtained in 98% yield with good current efficiency (66%; Scheme 1).

Materials
N-Phenyl-2-napthylamine (1a) was purchased from Tokyo Chemical Industries (TCI). All commercially available organic and inorganic compounds were used directly without further purification.

Spectroscopy and Spectrometry
1 H-and 13 C-NMR spectra were recorded at 25 °C using a JEOL JMN ECS400 FT NMR instrument ( 1 H-NMR 400 MHz; 13 C-NMR 100 MHz). The 1 H-NMR spectra are reported as follows: chemical shift in ppm downfield of tetramethylsilane and referenced to a residual solvent peak (CHCl3) at 7.26 ppm, integration, multiplicities (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), and coupling constants (Hz). The 13 C-NMR spectra are reported in ppm relative to the central line of the triplet for CDCl3 at 77.16 ppm. ESI-MS spectra were obtained using a JMS-T100LC instrument (JEOL). FT-IR spectra were recorded using a JASCO FT-IR system (FT/IR4100). Thin-layer chromatography (TLC) analysis of the reaction mixture was performed on Merck silica gel 60 F254 TLC plates and visualized under UV light. Column chromatography on SiO2 was performed using Kanto Silica Gel 60 (63-210 μm).

Optimization of the Reaction Conditions
Initially, we screened the electrodes, solvent systems, currents, and electrolytes to determine the optimal conditions (Table 1, also see supporting information). The electrodes were screened by employing 0.1 mmol N-phenyl-2-naphthylamine (1a, oxidative potential 1.05 eV; see Supporting Information) as the model substrate. The platinum electrode exhibited good reactivity, affording homocoupling product 2a in 98% yield (current efficiency, 66%) without the formation of any side product (entry 1). In contrast, the Scheme 1. Electrochemical dehydrogenative homocoupling of 2-naphthylamines.

Materials
N-Phenyl-2-napthylamine (1a) was purchased from Tokyo Chemical Industries (TCI). All commercially available organic and inorganic compounds were used directly without further purification.

Spectroscopy and Spectrometry
1 H-and 13 C-NMR spectra were recorded at 25 • C using a JEOL JMN ECS400 FT NMR instrument ( 1 H-NMR 400 MHz; 13 C-NMR 100 MHz). The 1 H-NMR spectra are reported as follows: chemical shift in ppm downfield of tetramethylsilane and referenced to a residual solvent peak (CHCl 3 ) at 7.26 ppm, integration, multiplicities (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), and coupling constants (Hz). The 13 C-NMR spectra are reported in ppm relative to the central line of the triplet for CDCl 3 at 77.16 ppm. ESI-MS spectra were obtained using a JMS-T100LC instrument (JEOL). FT-IR spectra were recorded using a JASCO FT-IR system (FT/IR4100). Thin-layer chromatography (TLC) analysis of the reaction mixture was performed on Merck silica gel 60 F254 TLC plates and visualized under UV light. Column chromatography on SiO 2 was performed using Kanto Silica Gel 60 (63-210 µm).

General Protocol for the Anodic Homocoupling of 2-Naphthylamines
ElectraSyn 2.0 and platinum were utilized as the reaction device and electrode, respectively. A suspension of 2-naphthylamines (0.1 mmol) and n Bu 4 NPF 6 (0.1 M) in 1,1,1,3,3,3hexafluoro-2-propanol (HFIP) (5 mL) was added to an undivided vessel and stirred under a constant current of 4 mA for 2 h. The electrolyte was removed using short silica gel column chromatography ( n hexane/ethyl acetate = 1/1). The fraction was dried, and the crude product was purified by silica-gel column chromatography ( n hexane/ethyl acetate = 20/1) to afford the pure homocoupling product.

Optimization of the Reaction Conditions
Initially, we screened the electrodes, solvent systems, currents, and electrolytes to determine the optimal conditions (Table 1, also see supporting information). The electrodes were screened by employing 0.1 mmol N-phenyl-2-naphthylamine (1a, oxidative potential 1.05 eV; see Supporting Information) as the model substrate. The platinum electrode exhibited good reactivity, affording homocoupling product 2a in 98% yield (current efficiency, 66%) without the formation of any side product (entry 1). In contrast, the carbon-platinum [32] and fluorine-doped tin oxide(FTO) [33] electrodes gave 2a in 43% and 29% yields, respectively, with low current efficiencies (entries 2 and 3). Other alcoholic reaction solvents, such as methanol, ethanol, and trifluoroethanol, reduced the yield of 2a to 6-12% with current efficiencies of 4-8% (entries 4-6), along with a 5% yield of aza [5] helicene. HFIP, an appropriate solvent for the reaction, serves as an excellent hydrogen bond donor and provides highly persistent radical cations [34,35]. The effect of a constant current was also investigated. As shown in entry 7, when a current of 2 mA was employed for the electrosynthesis process, the current efficiency increased to 80%, but the yield of 2a decreased to 60%. In contrast, employing a current of 6 mA for the electrosynthesis process led to the formation of 2a in 49% yield, with a current efficiency of 22% (entry 8). Suppressing the discharge of the electrolyte or solvent resulted in higher yield and current efficiency. Among the electrolytes we screened, n Bu 4 NPF 6 proved to be superior to LiClO 4 (2a, 57% yield due to low solubility in HFIP) and n Bu 4 NClO 4 (2a, 29% yield) (entries 9 and 10). No reaction occurred in the absence of electricity (entry 11). carbon-platinum [32] and fluorine-doped tin oxide(FTO) [33] electrodes gave 2a in 43% and 29% yields, respectively, with low current efficiencies (entries 2 and 3). Other alcoholic reaction solvents, such as methanol, ethanol, and trifluoroethanol, reduced the yield of 2a to 6-12% with current efficiencies of 4-8% (entries 4-6), along with a 5% yield of aza [5]helicene. HFIP, an appropriate solvent for the reaction, serves as an excellent hydrogen bond donor and provides highly persistent radical cations [34,35]. The effect of a constant current was also investigated. As shown in entry 7, when a current of 2 mA was employed for the electrosynthesis process, the current efficiency increased to 80%, but the yield of 2a decreased to 60%. In contrast, employing a current of 6 mA for the electrosynthesis process led to the formation of 2a in 49% yield, with a current efficiency of 22% (entry 8). Suppressing the discharge of the electrolyte or solvent resulted in higher yield and current efficiency. Among the electrolytes we screened, n Bu4NPF6 proved to be superior to LiClO4 (2a, 57% yield due to low solubility in HFIP) and n Bu4NClO4 (2a, 29% yield) (entries 9 and 10). No reaction occurred in the absence of electricity (entry 11).

Discussion
In our study, the homocoupling reaction of 2-naphthylamines proceeded smoothly with good current efficiency. To further understand the relevant mechanism, we conducted the heterocoupling reactions of 1b with N-4-tolyl (an electron-donating group) and

Discussion
In our study, the homocoupling reaction of 2-naphthylamines proceeded smoothly with good current efficiency. To further understand the relevant mechanism, we conducted the heterocoupling reactions of 1b with N-4-tolyl (an electron-donating group) and 1e with N-4-bromo-phenyl (an electron-withdrawing group) under the optimal conditions (Scheme 2).
Homocoupling product 2b was obtained in 90% yield as the major product, along with homocoupling product 2e in 10% yield and trace amounts of heterocoupling product 2be. The relative proportions of the products arising from the radical-radical coupling reactions are aligned with their relative reactivities [36][37][38]. In principle, 1e is less oxidizable than 1b, which should result in the formation of homocoupling 2b as the major product. The results support our hypothesis that the present coupling reaction of 1 proceeds through radical-radical coupling, as shown in Scheme 3. Triggering by single-electron transfer (SET) of 1a on the anode made the formation of intermediate I. The generated I species could be in equilibrium into Ia and Ib by electron transfer. Then a radical-radical coupling of Ib proceeded to afford the coupling product 2b through the oxidation of intermediate II. On the cathode, the generated H + was reduced to give H 2 as a sole coproduct.
Homocoupling product 2b was obtained in 90% yield as the major product, along with homocoupling product 2e in 10% yield and trace amounts of heterocoupling product 2be. The relative proportions of the products arising from the radical-radical coupling reactions are aligned with their relative reactivities [36][37][38]. In principle, 1e is less oxidizable than 1b, which should result in the formation of homocoupling 2b as the major product. The results support our hypothesis that the present coupling reaction of 1 proceeds through radical-radical coupling, as shown in Scheme 3. Triggering by single-electron transfer (SET) of 1a on the anode made the formation of intermediate I. The generated I species could be in equilibrium into Ia and Ib by electron transfer. Then a radical-radical coupling of Ib proceeded to afford the coupling product 2b through the oxidation of intermediate II. On the cathode, the generated H + was reduced to give H2 as a sole coproduct.

Conclusions
We developed a facile and sustainable protocol for the homocoupling of various 2naphthylamines with up to 98% yield and good current efficiency (66%). This new protocol not only saves energy and chemical loading but also significantly improves the product yield, thus representing a significant improvement to the previous electrosynthesis approach. Investigations of the further applications of homocoupling products are ongoing in our laboratory.
Supplementary Materials: The supporting information can be downloaded at: www.mdpi.com/xxx/s1; Figure S1: IKA device ElectraSyn 2.0 standard setup; Table S1. Screening solvent of constant current for optimizing reaction conditions; Figure   Homocoupling product 2b was obtained in 90% yield as the major product, along with homocoupling product 2e in 10% yield and trace amounts of heterocoupling product 2be. The relative proportions of the products arising from the radical-radical coupling reactions are aligned with their relative reactivities [36][37][38]. In principle, 1e is less oxidizable than 1b, which should result in the formation of homocoupling 2b as the major product. The results support our hypothesis that the present coupling reaction of 1 proceeds through radical-radical coupling, as shown in Scheme 3. Triggering by single-electron transfer (SET) of 1a on the anode made the formation of intermediate I. The generated I species could be in equilibrium into Ia and Ib by electron transfer. Then a radical-radical coupling of Ib proceeded to afford the coupling product 2b through the oxidation of intermediate II. On the cathode, the generated H + was reduced to give H2 as a sole coproduct.

Conclusions
We developed a facile and sustainable protocol for the homocoupling of various 2naphthylamines with up to 98% yield and good current efficiency (66%). This new protocol not only saves energy and chemical loading but also significantly improves the product yield, thus representing a significant improvement to the previous electrosynthesis approach. Investigations of the further applications of homocoupling products are ongoing in our laboratory.
Supplementary Materials: The supporting information can be downloaded at: www.mdpi.com/xxx/s1; Figure S1: IKA device ElectraSyn 2.0 standard setup; Table S1. Screening solvent of constant current for optimizing reaction conditions; Figure

Conclusions
We developed a facile and sustainable protocol for the homocoupling of various 2-naphthylamines with up to 98% yield and good current efficiency (66%). This new protocol not only saves energy and chemical loading but also significantly improves the product yield, thus representing a significant improvement to the previous electrosynthesis approach. Investigations of the further applications of homocoupling products are ongoing in our laboratory.