Amphiphilic Viologen: Electrochemical Generation of Organic Reductant and Pd-Catalyzed Reductive Coupling of Aryl Halides in Water

Abstract

Electroreduction of 1,1′-bis(methoxyethoxyethoxyethyl)-4,4′-bipyridinium tosylate generated amphiphilic organic reductants, which promoted the Pd-catalyzed reductive coupling of aryl bromides in water to give the corresponding biaryls. The yields and selectivity of biaryls depended on the length of ethyleneoxy groups and substituents of the aryl bromides.

Organic reactions in water are of current interest in organic synthesis.[ ¹ ] Water is a cheap, nonflammable, and nontoxic solvent. In addition, the reactions are accelerated and unique selectivity is observed caused by hydrophobic effects. Water is also an ideal solvent for electroorganic synthesis[2] [3] since dielectric constant of water is high enough to pass electricity efficiently in the presence of a small amount of supporting electrolytes.

Viologen derivatives (1,1′-dialkyl-4,4′-bipyridinium salt, V²⁺) are easily prepared by N-alkylation of 4,4′-bipyridyl and are stable in the air. Reduction of V²⁺ gives the corresponding radical cation V^•+ and quinoid V⁰ (Scheme [1]), both of them work as organic reductants.[ ⁴ ]

The viologen V²⁺/quinoid V⁰ redox pair is usually used in organic–aqueous mixed solvent since V²⁺ dissolves in water, whereas the corresponding V⁰ is insoluble in water.[5] [6] We found that amphiphilic viologens V²⁺/quinoids V⁰ having poly(ethylene glycol) (PEG) groups dissolve in water, and the quinoids V⁰ work as reductants in an aqueous medium. In this paper, we describe that reductive coupling of aryl bromides proceeds in water by using the PEG-modified amphiphilic viologen 1a.

PEG-modified amphiphilic viologen, 1,1′-bis(methoxy­ethoxyethoxyethyl)-4,4′-bipyridinium tosylate (1a), was prepared by treatment of 4,4′-bipyridyl with two molar amount of Me(OCH₂CH₂)₃OTs at 120 °C in 96% yield as a brown viscous oil.[ ⁷ ] In a similar manner, viologen 1b–e [1b: R = MeOCH₂CH₂, 1c: R = Me(OCH₂CH₂)₂, 1d: R = Me, 1e: R = Bu] were prepared by treatment of 4,4′-bi­pyridyl with MeOCH₂CH₂OTs, Me(OCH₂CH₂)₂OTs, MeOTs, and BuOTs, respectively.[ ⁸ ] Viologen 1a was soluble in water and MeOH, whereas 1a was insoluble in usual organic solvents such as hexane, Et₂O, toluene, and EtOAc. A cyclic voltammogram of 1a in an aqueous Et₄NOTs (0.2 M) solution (vs. Ag/AgCl) exhibited two sets of reversible redox peaks at –0.57 V (1a/V^•+) and –1.00 V (V^•+/2a; Figure [1]). Yields and redox potential values of 1b–e are summarized in Table [1].

A typical procedure of electroreduction of 1a to 2a and subsequent reductive coupling of aryl bromide 3a in water is as follows. The electroreduction was carried out in a divided cell equipped with an Mg rod anode (ø 0.6 cm) and a Pt plate cathode (1.0 × 1.5 cm²). In the cathodic chamber was placed an aqueous NaClO₄ (0.2 M, 4 mL) solution of 1a (0.5 mmol), and in the anodic chamber was placed an aqueous NaClO₄ (0.2 M, 4 mL) solution. Electroreduction of 1a was carried out at room temperature under constant current conditions (30 mA) until 2 F/mol 1a of electricity was passed to afford a dark purple solution of 2a. To a mixture of 3a (0.25 mmol) and a catalytic amount of Pd(OAc)₂ (5 mol%) was added the catholyte by cannulation, and the whole mixture was stirred at 60 °C for four hours under argon atmosphere. After usual workup, the desired coupling product, 4,4′-dipropanoylbiphenyl (4a) and reduced byproduct propiophenone (5a) were obtained in 90% and 8% yield, respectively (Table [2], entry 1).

The PEG group of 1a played an important role:[ ⁹ ] Quinoids 2 derived from methyl viologen 1d and butyl viologen 1e gave 4a/5a in 17% and 25% and 31% and 6% yields, respectively, and half of the starting material 3a was recovered unchanged (Table [2], entries 4 and 5). The length of the PEG groups [n of Me(OCH₂CH₂)_n] also affected on the yields of 4a. Viologen having methoxyethyl group (1b, n = 1) gave a similar result (4a/5a/3a = 30%, 12%, and 45%, Table [2], entry 2) with butyl viologen 1e. Though homocoupling of 3a with methoxyethoxyethylated viologen 1c (n = 2) gave 4a as a major product (4a/5a/3a = 59%, 8%, and 19%, Table [2], entry 3), 1c was less effective than 1a. The ethyleneoxy group in viologen would increase the solubility of the reduced form 2 in water. In addition, the substrate 3a and Pd catalyst would interact with the ethyleneoxy group of 2a to make an aggregate (Figure [2]), and therefore, electron transfer and oxidative addition would proceed smoothly.

Among thus far examined catalysts, Pd(OAc)₂ gave the best result (Table [3], entry 1). PdCl₂ and Pd₂(dba)₃ gave 4a/5a/3a in 26%, 18%, and 53% yields and 62%, 9%, and 23% yields, respectively (Table [3], entries 2 and 3). Pd/C (10% Pd/C) gave the reduced product 5a as a major product (26% yield, Table [3], entry 4). Palladium catalyst was indispensable: the reductive coupling reaction did not occur in the presence of Ni catalysts in place of Pd catalysts (Table [3], entries 5 and 6) and in the absence of catalysts (Table [3], entry 7).

The reductive coupling was carried out with several aryl halides (Table [4]).

The coupling of aryl bromides having electron-withdrawing groups such as keto (3b) and cyano (3c,d) groups proceeded smoothly to give the corresponding biaryls 4b, 4c, and 4d in good yields (Table [4], entries 1–3). Heteroaromatic compound, 2-acetyl-5-bromothiophene (3e), gave dithiophene 4e in 82% yield (Table [4], entry 4). Unsubstituted bromobenzene (3f) gave biphenyl 4f in 75% yield (Table [4], entry 5). On the other hand, aryl bromides having electron-donating groups such as dimethylamino (3g), methyl (3h), and hydroxyl (3i) groups gave the corresponding biaryls 4g, 4h, and 4i in moderate yields (Table [4], entries 6–8). 4-Iodobenzonitrile (3j) and 4-choropropiophenone (3l) gave the corresponding biaryls in low yields, and homocoupling of 4-chlorobenzonitrile (3k) did not proceed at all (Table [4], entries 9–11).

Table 4 Reductive Coupling of Several Aryl Halides

Entry	ArX		Temp (°C)	Time (h)	Yield of 4 (%)a
1		3b	60	6	66
2		3c	100	4	74
3		3d	60	24	92
4		3e	100	4	82
5		3f	60	24	75
6		3g	80	24	45
7		3h	60	24	25
8		3i	60	22	34
9		3j	100	4	29
10		3k	100	24	0
11		3l	60	24	31

^a Isolated yield of biaryl 4.

A plausible mechanism of the reductive coupling is as follows (Scheme [2]). Pd(II) species would be reduced with V⁰ to give Pd(0) active species. Oxidative addition of ArBr 3 on the Pd(0) would give ArPd(II)Br. Reduction of ­ArPd(II)Br with V⁰ would afford [ArPd(0)]^–. The subsequent reaction of [ArPd(0)]^– with additional ArBr would produce ArPd(II)Ar. Reductive coupling from ArPd(II)Ar would give Ar–Ar 4 and Pd(0) species. Dissociation of [ArPd(0)]^– to Ar^– and Pd(0) followed by protonation would give ArH 5.

Substituents on the ArBr affect the reduction of ArPd(II)Br to [ArPd(0)]^–: Electron-withdrawing substituents would decrease the electron density of Pd in ArPd(II)Br to accelerate the reduction giving [ArPd(0)]^–, whereas electron-donating substituents would increase the electron density to prevent the reduction. In the case of ArCl, oxidative addition of ArCl on the Pd(0) and/or [ArPd(0)]^– would not occur efficiently.

In conclusion, amphiphilic organic reductant V⁰, electrochemically generated from 1,1′-bis(methoxyethoxy­ethoxyethyl)-4,4′-bipyridinium (V²⁺) tosylate (1a), promoted the Pd-catalyzed reductive coupling of aryl bromides in water to give the corresponding biaryls. The yields and selectivity of biaryls were affected by the length of ethyleneoxy groups and substituents of the aryl bromides.[ ¹⁰ ]

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number 22550102 and Shorai Foundation for Science and Technology. The authors thank Mrs. Takamuru Atsuko, Okayama University, for elemental analyses.

• References and Notes

• 1a Grieco PA. Organic Synthesis in Water . Blackie Academic; London: 1998
  CrossrefSearch in Google Scholar
• 1b Baldwin JE, Herchen SR, Schulz G. J. Am. Chem. Soc. 1980; 102: 7816
  CrossrefSearch in Google Scholar
• 1c Razler TM, Hsiao Y, Qian F, Fu R, Khan RK, Doubleday W. J. Org. Chem. 2009; 74: 1381
  CrossrefSearch in Google Scholar
• 1d Wu W.-Y, Chen S.-N, Tsai F.-Y. Tetrahedron Lett. 2006; 47: 9267
  CrossrefSearch in Google Scholar
• 1e Alacid E, Nájera C. J. Org. Chem. 2008; 73: 2315
  CrossrefPubMedSearch in Google Scholar
• 1f Srimani D, Sawoo S, Sarkar A. Org. Lett. 2007; 9: 3639
  CrossrefSearch in Google Scholar
• 1g Lipshutz BH, Chung DW, Rich B. Org. Lett. 2008; 10: 3793
  CrossrefSearch in Google Scholar
• 1h Uozumi Y, Nakai Y. Org. Lett. 2002; 4: 2997
  CrossrefPubMedSearch in Google Scholar
• 1i Wu W.-Y, Chen S.-N, Tsai F.-Y. Tetrahedron Lett. 2006; 47: 9267
  CrossrefSearch in Google Scholar
• 2 Tanaka H, Kuroboshi M. Yuki Denkai Gosei no Shintenkai . Fuchigami T. CMC; Tokyo: 2004: 216-228
  Search in Google Scholar
• 3a Kuroboshi M, Goto K, Tanaka H. Synthesis 2009; 903
  Thieme Connect
• 3b Tanaka H, Kubota J, Miyahara S, Kuroboshi M. Bull. Chem. Soc. Jpn. 2005; 78: 1677
  CrossrefSearch in Google Scholar
• 3c Kubota J, Shimizu Y, Mitsudo K, Tanaka H. Tetrahedron Lett. 2005; 46: 8975
  CrossrefSearch in Google Scholar
• 3d Mitsudo K, Kumagai H, Takabatake F, Kubota J, Tanaka H. Tetrahedron Lett. 2007; 48: 8994
  CrossrefSearch in Google Scholar
• 3e Kuroboshi M, Yoshida T, Oshitani J, Goto K, Tanaka H. Tetrahedron 2009; 65: 7177
  CrossrefSearch in Google Scholar
• 4a Endo T, Saotome Y, Okawara M. Tetrahedron Lett. 1985; 26: 4525
  CrossrefSearch in Google Scholar
• 4b Park KK, Lee CW, Choi SY. J. Chem. Soc., Perkin Trans. 1 1992; 601
• 4c Tomioka H, Ueda K, Ohi H, Izawa Y. Chem. Lett. 1986; 1359
• 4d Maidan R, Goren Z, Becker JY, Willner I. J. Am. Chem. Soc. 1984; 106: 6217
  CrossrefSearch in Google Scholar
• 4e Park KK, Lee CW, Oh SY. J. Chem. Soc., Perkin Trans. 1 1990; 2356
• 4f Shosenji H, Nakano Y, Yamada K. Chem. Lett. 1988; 1033
• 4g Mandler D, Willner I. J. Phys. Chem. 1987; 91: 3600
  CrossrefSearch in Google Scholar
• 4h Coche L, Moutet JC. J. Am. Chem. Soc. 1987; 109: 6887
  CrossrefSearch in Google Scholar
• 4i Kashiwagi Y, Shibayama N, Anzai J, Osa T. Electrochemistry 2000; 68: 42
  Search in Google Scholar
• 4j Yuan R, Watanabe S, Kuwabata S, Yoneyama H. J. Org. Chem. 1997; 62: 2494
  CrossrefSearch in Google Scholar
• 4k Chen X, Fenton JM, Fisher RJ, Peattie RA. J. Electrochem. Soc. 2004; 2: E56
  Search in Google Scholar
• 5 Octyl viologen bis(triflimide) and the corresponding quinoid dissolve in organic solvent to promote the reductive homocoupling of aryl halide. See: Kuroboshi M, Kobayashi R, Nakagawa T, Tanaka H. Synlett 2009; 85
  Thieme Connect
• 6 Viologen also promotes the reductive homocoupling of aryl halides in ionic liquid. See: Kuroboshi M, Kuwano A, Tanaka H. Electrochemistry 2008; 862
• 7 Preparation of 1a In a round-bottomed flask were placed 4,4′-bipyridyl (3.531 g, 23 mmol) and 3,6,9-trioxadecyl p-tosylate (14.39 g, 45 mmol). The mixture was stirred at 120 °C for 24 h under argon atmosphere to form a viscous layer. The supernatant was decanted off, and the viscous layer was washed with Et₂O and toluene (2 × 10 mL), successively. The viscous liquid layer was concentrated under reduced pressure to give 1,1′-bis(3,6,9-trioxadecyl)-4,4′-bipyridinium bis(p-toluenesulfonate) (1a, 17.14 g, 22 mmol, 96%) as a brown liquid. ¹H NMR (200 MHz, CDCl₃): δ = 2.28 (s, 6 H), 3.28 (s, 6 H), 3.42–3.53 (m, 16 H), 3.85 (br, 4 H), 4.93 (br, 4 H), 7.11 (d, J = 8.1 Hz, 4 H), 7.69 (d, J = 8.1 Hz, 4 H), 8.71 (d, J = 6.8 Hz, 4 H), 9.24 (d, J = 6.8 Hz, 4 H). ¹³C NMR (150 MHz, CDCl₃): δ = 21.04, 58.69, 61.18, 68.89, 69.91, 70.08, 70.14, 71.56, 125.66, 126.64, 128.71, 1139.42, 143.25, 146.60, 148.87; IR (neat): 3464, 3127, 3059, 2921, 2876, 1638, 1451, 1191, 1122, 822 cm^–1.
• 8 1,1′-Bis(2-methoxyethyl)-4,4′-bipyridinium Bis(p-toluenesulfonate) (1b) Colorless solids. ¹H NMR (200 MHz, D₂O): δ = 2.13 (s, 6 H), 3.19 (s, 6 H), 3.79 (t, J = 5.0 Hz, 4 H), 4.68 (t, J = 5.0 Hz, 4 H), 7.08 (d, J = 8.3 Hz, 4 H), 7.40 (d, J = 8.3 Hz, 4 H), 8.26 (d, J = 7.0 Hz, 4 H), 8.85 (d, J = 7.0 Hz, 4 H). ¹³C NMR (50 MHz, CD₃OD): δ = 21.3, 59.2, 62.7, 71.3, 126.8, 127.8, 129.8, 141.6, 143.5, 147.5, 151.3. IR (KBr): 3532, 3451, 3134, 3062, 2992, 2864, 1639, 1510, 1448, 1191, 1120, 1035, 568 cm^–1. 1,1′-Bis(3,6-dioxaheptyl)-4,4′-bipyridinium Bis(p-toluenesulfonate) (1c) Colorless solids. ¹H NMR (200 MHz, D₂O): δ = 2.15 (s, 6 H), 3.09 (s, 6 H), 3.32–3.36 (m, 4 H), 3.44–3.49 (m, 4 H), 3.86 (t, J = 4.4 Hz, 4 H), 4.63–4.76 (m, 4 H), 7.11 (d, J = 7.7 Hz, 4 H), 7.42 (d, J = 7.7 Hz, 4 H), 8.30 (d, J = 6.2 Hz, 4 H), 8.89 (d, J = 6.2 Hz, 4 H). ¹³C NMR (150 MHz, CD₃OD): δ = 21.3, 59.1, 62.7, 70.0, 71.2, 72.7, 126.9, 127.7, 129.9, 141.6, 143.7, 147.6, 151.2. IR (KBr): 3509, 3131, 3055, 2879, 1640, 1442, 1192, 1123, 1033, 685 cm^–1. 1,1′-Dimethyl-4,4′-bipyridinium Bis(p-toluenesulfonate) (1d) Yellow solids. ¹H NMR (200 MHz, D₂O): δ = 2.18 (s, 6 H), 4.31 (s, 6 H), 7.13 (d, J = 8.2 Hz, 4 H), 7.45 (d, J = 8.2 Hz, 4 H), 8.25 (d, J = 6.9 Hz, 4 H), 8.81 (d, J = 6.9 Hz, 4 H). ¹³C NMR (150 MHz, CD₃OD): δ = 21.3, 49.0, 126.8, 127.6, 130.0, 141.6, 143.7, 147.8, 150.4. IR (KBr): 3444, 3053, 3020, 2994, 1642, 1509, 1440, 1358, 1208, 1119, 818, 683 cm^–1. 1,1′-Dibutyl-4,4′-bipyridinium Bis(p-toluenesulfonate) (1e)Colorless solids. ¹H NMR (200 MHz, D₂O): δ = 0.71 (t, J = 7.4 Hz, 6 H), 1.13 (sext, J = 7.4 Hz, 4 H), 1.76 (quin, J = 7.4 Hz, 4 H), 2.07 (s, 6 H), 4.40 (t, J = 7.4 Hz, 4 H), 7.01 (d, J = 8.1 Hz, 4 H), 7.32 (d, J = 8.1 Hz, 4 H), 8.16 (d, J = 6.7 Hz, 4 H), 8.77 (d, J = 6.7 Hz, 4 H). ¹³C NMR (150 MHz, CD₃OD): δ = 13.8, 20.4, 21.3, 34.3, 62.9, 126.9, 128.2, 129.9, 141.6, 143.7, 147.0, 151.0. IR (KBr): 3434, 3127, 3054, 2957, 2870, 1643, 1450, 1169, 1120, 1035, 818 cm^–1.
• 9 Viologens V²⁺ were easily soluble in H₂O, whereas the reduced form V⁰ were soluble in organic solvents. Therefore, viologens were used in a mixed solution of H₂O–MeOH or H₂O–MeCN. The amphiphilic PEG group would increase the solubility of V²⁺/V⁰ as well as the substrates.
• 10 Recycle use of aqueous viologen/Pd solution was also examined. In the second run, the recovered aqueous solution was placed in the cathodic chamber. The electroreduction was carried out under constant current conditions to generate V⁰. Though the reaction conditions have not been optimized yet, 4a was obtained in 40% yield.

• References and Notes

• 1a Grieco PA. Organic Synthesis in Water . Blackie Academic; London: 1998
  CrossrefSearch in Google Scholar
• 1b Baldwin JE, Herchen SR, Schulz G. J. Am. Chem. Soc. 1980; 102: 7816
  CrossrefSearch in Google Scholar
• 1c Razler TM, Hsiao Y, Qian F, Fu R, Khan RK, Doubleday W. J. Org. Chem. 2009; 74: 1381
  CrossrefSearch in Google Scholar
• 1d Wu W.-Y, Chen S.-N, Tsai F.-Y. Tetrahedron Lett. 2006; 47: 9267
  CrossrefSearch in Google Scholar
• 1e Alacid E, Nájera C. J. Org. Chem. 2008; 73: 2315
  CrossrefPubMedSearch in Google Scholar
• 1f Srimani D, Sawoo S, Sarkar A. Org. Lett. 2007; 9: 3639
  CrossrefSearch in Google Scholar
• 1g Lipshutz BH, Chung DW, Rich B. Org. Lett. 2008; 10: 3793
  CrossrefSearch in Google Scholar
• 1h Uozumi Y, Nakai Y. Org. Lett. 2002; 4: 2997
  CrossrefPubMedSearch in Google Scholar
• 1i Wu W.-Y, Chen S.-N, Tsai F.-Y. Tetrahedron Lett. 2006; 47: 9267
  CrossrefSearch in Google Scholar
• 2 Tanaka H, Kuroboshi M. Yuki Denkai Gosei no Shintenkai . Fuchigami T. CMC; Tokyo: 2004: 216-228
  Search in Google Scholar
• 3a Kuroboshi M, Goto K, Tanaka H. Synthesis 2009; 903
  Thieme Connect
• 3b Tanaka H, Kubota J, Miyahara S, Kuroboshi M. Bull. Chem. Soc. Jpn. 2005; 78: 1677
  CrossrefSearch in Google Scholar
• 3c Kubota J, Shimizu Y, Mitsudo K, Tanaka H. Tetrahedron Lett. 2005; 46: 8975
  CrossrefSearch in Google Scholar
• 3d Mitsudo K, Kumagai H, Takabatake F, Kubota J, Tanaka H. Tetrahedron Lett. 2007; 48: 8994
  CrossrefSearch in Google Scholar
• 3e Kuroboshi M, Yoshida T, Oshitani J, Goto K, Tanaka H. Tetrahedron 2009; 65: 7177
  CrossrefSearch in Google Scholar
• 4a Endo T, Saotome Y, Okawara M. Tetrahedron Lett. 1985; 26: 4525
  CrossrefSearch in Google Scholar
• 4b Park KK, Lee CW, Choi SY. J. Chem. Soc., Perkin Trans. 1 1992; 601
• 4c Tomioka H, Ueda K, Ohi H, Izawa Y. Chem. Lett. 1986; 1359
• 4d Maidan R, Goren Z, Becker JY, Willner I. J. Am. Chem. Soc. 1984; 106: 6217
  CrossrefSearch in Google Scholar
• 4e Park KK, Lee CW, Oh SY. J. Chem. Soc., Perkin Trans. 1 1990; 2356
• 4f Shosenji H, Nakano Y, Yamada K. Chem. Lett. 1988; 1033
• 4g Mandler D, Willner I. J. Phys. Chem. 1987; 91: 3600
  CrossrefSearch in Google Scholar
• 4h Coche L, Moutet JC. J. Am. Chem. Soc. 1987; 109: 6887
  CrossrefSearch in Google Scholar
• 4i Kashiwagi Y, Shibayama N, Anzai J, Osa T. Electrochemistry 2000; 68: 42
  Search in Google Scholar
• 4j Yuan R, Watanabe S, Kuwabata S, Yoneyama H. J. Org. Chem. 1997; 62: 2494
  CrossrefSearch in Google Scholar
• 4k Chen X, Fenton JM, Fisher RJ, Peattie RA. J. Electrochem. Soc. 2004; 2: E56
  Search in Google Scholar
• 5 Octyl viologen bis(triflimide) and the corresponding quinoid dissolve in organic solvent to promote the reductive homocoupling of aryl halide. See: Kuroboshi M, Kobayashi R, Nakagawa T, Tanaka H. Synlett 2009; 85
  Thieme Connect
• 6 Viologen also promotes the reductive homocoupling of aryl halides in ionic liquid. See: Kuroboshi M, Kuwano A, Tanaka H. Electrochemistry 2008; 862
• 7 Preparation of 1a In a round-bottomed flask were placed 4,4′-bipyridyl (3.531 g, 23 mmol) and 3,6,9-trioxadecyl p-tosylate (14.39 g, 45 mmol). The mixture was stirred at 120 °C for 24 h under argon atmosphere to form a viscous layer. The supernatant was decanted off, and the viscous layer was washed with Et₂O and toluene (2 × 10 mL), successively. The viscous liquid layer was concentrated under reduced pressure to give 1,1′-bis(3,6,9-trioxadecyl)-4,4′-bipyridinium bis(p-toluenesulfonate) (1a, 17.14 g, 22 mmol, 96%) as a brown liquid. ¹H NMR (200 MHz, CDCl₃): δ = 2.28 (s, 6 H), 3.28 (s, 6 H), 3.42–3.53 (m, 16 H), 3.85 (br, 4 H), 4.93 (br, 4 H), 7.11 (d, J = 8.1 Hz, 4 H), 7.69 (d, J = 8.1 Hz, 4 H), 8.71 (d, J = 6.8 Hz, 4 H), 9.24 (d, J = 6.8 Hz, 4 H). ¹³C NMR (150 MHz, CDCl₃): δ = 21.04, 58.69, 61.18, 68.89, 69.91, 70.08, 70.14, 71.56, 125.66, 126.64, 128.71, 1139.42, 143.25, 146.60, 148.87; IR (neat): 3464, 3127, 3059, 2921, 2876, 1638, 1451, 1191, 1122, 822 cm^–1.
• 8 1,1′-Bis(2-methoxyethyl)-4,4′-bipyridinium Bis(p-toluenesulfonate) (1b) Colorless solids. ¹H NMR (200 MHz, D₂O): δ = 2.13 (s, 6 H), 3.19 (s, 6 H), 3.79 (t, J = 5.0 Hz, 4 H), 4.68 (t, J = 5.0 Hz, 4 H), 7.08 (d, J = 8.3 Hz, 4 H), 7.40 (d, J = 8.3 Hz, 4 H), 8.26 (d, J = 7.0 Hz, 4 H), 8.85 (d, J = 7.0 Hz, 4 H). ¹³C NMR (50 MHz, CD₃OD): δ = 21.3, 59.2, 62.7, 71.3, 126.8, 127.8, 129.8, 141.6, 143.5, 147.5, 151.3. IR (KBr): 3532, 3451, 3134, 3062, 2992, 2864, 1639, 1510, 1448, 1191, 1120, 1035, 568 cm^–1. 1,1′-Bis(3,6-dioxaheptyl)-4,4′-bipyridinium Bis(p-toluenesulfonate) (1c) Colorless solids. ¹H NMR (200 MHz, D₂O): δ = 2.15 (s, 6 H), 3.09 (s, 6 H), 3.32–3.36 (m, 4 H), 3.44–3.49 (m, 4 H), 3.86 (t, J = 4.4 Hz, 4 H), 4.63–4.76 (m, 4 H), 7.11 (d, J = 7.7 Hz, 4 H), 7.42 (d, J = 7.7 Hz, 4 H), 8.30 (d, J = 6.2 Hz, 4 H), 8.89 (d, J = 6.2 Hz, 4 H). ¹³C NMR (150 MHz, CD₃OD): δ = 21.3, 59.1, 62.7, 70.0, 71.2, 72.7, 126.9, 127.7, 129.9, 141.6, 143.7, 147.6, 151.2. IR (KBr): 3509, 3131, 3055, 2879, 1640, 1442, 1192, 1123, 1033, 685 cm^–1. 1,1′-Dimethyl-4,4′-bipyridinium Bis(p-toluenesulfonate) (1d) Yellow solids. ¹H NMR (200 MHz, D₂O): δ = 2.18 (s, 6 H), 4.31 (s, 6 H), 7.13 (d, J = 8.2 Hz, 4 H), 7.45 (d, J = 8.2 Hz, 4 H), 8.25 (d, J = 6.9 Hz, 4 H), 8.81 (d, J = 6.9 Hz, 4 H). ¹³C NMR (150 MHz, CD₃OD): δ = 21.3, 49.0, 126.8, 127.6, 130.0, 141.6, 143.7, 147.8, 150.4. IR (KBr): 3444, 3053, 3020, 2994, 1642, 1509, 1440, 1358, 1208, 1119, 818, 683 cm^–1. 1,1′-Dibutyl-4,4′-bipyridinium Bis(p-toluenesulfonate) (1e)Colorless solids. ¹H NMR (200 MHz, D₂O): δ = 0.71 (t, J = 7.4 Hz, 6 H), 1.13 (sext, J = 7.4 Hz, 4 H), 1.76 (quin, J = 7.4 Hz, 4 H), 2.07 (s, 6 H), 4.40 (t, J = 7.4 Hz, 4 H), 7.01 (d, J = 8.1 Hz, 4 H), 7.32 (d, J = 8.1 Hz, 4 H), 8.16 (d, J = 6.7 Hz, 4 H), 8.77 (d, J = 6.7 Hz, 4 H). ¹³C NMR (150 MHz, CD₃OD): δ = 13.8, 20.4, 21.3, 34.3, 62.9, 126.9, 128.2, 129.9, 141.6, 143.7, 147.0, 151.0. IR (KBr): 3434, 3127, 3054, 2957, 2870, 1643, 1450, 1169, 1120, 1035, 818 cm^–1.
• 9 Viologens V²⁺ were easily soluble in H₂O, whereas the reduced form V⁰ were soluble in organic solvents. Therefore, viologens were used in a mixed solution of H₂O–MeOH or H₂O–MeCN. The amphiphilic PEG group would increase the solubility of V²⁺/V⁰ as well as the substrates.
• 10 Recycle use of aqueous viologen/Pd solution was also examined. In the second run, the recovered aqueous solution was placed in the cathodic chamber. The electroreduction was carried out under constant current conditions to generate V⁰. Though the reaction conditions have not been optimized yet, 4a was obtained in 40% yield.