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. Experimental part. General. All chemicals were used as received from the supplier; solvents were p.a. quality and used without further purification. If necessary, the solvents were dried by standard literature procedures, toluene with Na/benzophenone, CH₂Cl₂ over CaH, ethanol using magnesium turnings. If not mentioned otherwise, all reactions were carried out under an atmosphere of nitrogen or argon. ¹H-NMR and ¹³C-NMR spectra were recorded on a Bruker Ultra Shield 300 MHz, the J values are given in Hz. MALDI-TOF MS spectra were recorded on a time-of-flight mass spectrometer (PerSeptive Biosystems Voyager -DE PRO). EI-MS were recorded on a Finnigan MAT 95Q mass spectrometer. Melting points were measured with a Büchi Melting Point B-540 apparatus. TLC was performed on Merck silica gel 60 F254 plates and column chromatography using Merck silica gel 60 (0.040-0.063 mm). The R_f values are quoted for the solvents used for column chromatography. Elemental analyses were recorded using a ThermoQuest FlashEA 1112 N/Protein Analyzer.

. 4-Nitro-biphenyl (15.936 g, 80.00 mmol) was dissolved in methoxyethanol (400 ml) and a solution of NH₄Cl (6.793 g, 127.00 mmol) in H₂O (100 ml) was added. The reaction mixture was warmed to 30°C and finely powdered zinc (20.384 g, 311.72 mmol) was added in portions over a period of 30 min maintaining the temperature between 30-35°C while stirring vigorously using mechanical stirring. The mixture was allowed to react at 35°C for 2 h and then filtered and washed with methoxyethanol. The filtrate was added to a solution of FeCl₃ (25.200 g, 155.37 mmol) in H₂O (300 ml) and ethanol (120 ml) over the course of 90 min while the temperature was kept at -5°C. After 1 h the mixture was poured into water (ca. 800 ml) and the yellow precipitate collected. It was dried and then dissolved in CH₂Cl₂ and filtered over silica gel. After evaporation of the solvents a dark orange solid was obtained that was purified by recrystallisation from ethanol affording 2 as an orange solid (9.266 g, 50.58 mmol; 63%). M.p.: 73.0-74.0°C (Lit.[1]: 74°C). ¹H-NMR (300 MHz, CDCl₃): d = 7.45-7.54 (m, 3 H), 7.68 (td, ³J(H,H) = 8 Hz, ⁴J(H,H) = 2 Hz, 2 H), 7.83 (dd, ³J(H,H) = 9 Hz, ⁴J(H,H) = 1 Hz, 2 H), 7.98 (d, ³J(H,H) = 9 Hz, 2 H). ¹³C-NMR (75 MHz, CDCl₃): d = 121.70 (C3), 127.51 (C2'), 127.89 (C3'), 128.67 (C4'), 129.16 (C2), 148.13 (C1), 165.00 (C4).

Sodium methanethiolate (4.110 g, 0.059 mol) was dissolved in dry DMI (310 ml) and 4,4'-dibromo-biphenyl (13.104 g, 0.042 mol) was added at once. The mixture was heated to 150°C for 41 h. It was allowed to cool to room temperature and poured into sat. NaCl-solution (ca. 1,200 ml). The aqueous phase was extracted with diethylether, and the combined ethereal layers were washed with water and dried over Na₂SO₄. Evaporation of the solvents under reduced pressure followed by CC (silica gel, starting with hexane/toluene 15/1 increasing to hexane/toluene 4/1) yielded 3 as a white solid (7.038 g, 0.025 mol; 60%). M.p.: 148.0-149.5°C (Lit.:[2] 148.0-150.0). R_f = 0.16 (hexane/toluene 15/1). ¹H-NMR (300 MHz, CDCl₃): d = 2.52 (s, 3 H), 7.32 (td, ³J(H,H) = 9 Hz, ⁴J(H,H) = 2 Hz, 2 H), 7.43 (td, ³J(H,H) = 9 Hz, ⁴J(H,H) = 2 Hz, 2 H), 7.48 (td, ³J(H,H) = 9 Hz, ⁴J(H,H) = 2 Hz, 2 H), 7.55 (td, ³J(H,H) = 9 Hz, ⁴J(H,H) = 2 Hz, 2 H). ¹³C-NMR (75 MHz, CDCl₃): d = 16.19 (CH₃S), 121.80 (C4'), 127.30 (C3), 127.65 (C2), 128.80 (C2'), 132.31 (C3'), 137.11 (C1), 138.62 (C4), 139.85 (C1'). MS (EI): m/z (%) = 279.9, 277.9 (100) [M⁺], 264.9, 262.9 (30) [M⁺-CH₃], 184.0 (15) [M⁺-Br, -CH₃], 152.1 [M⁺-Br, -SCH₃]. Elemental analysis calculated (%) for C₁₃H₁₁BrS (279.20): C 55.92, H 3.97; found C 55.96, H 3.92.

A solution of 4'-bromo-4-methylsulfanyl-biphenyl 3 (5.026 g, 18.00 mmol) in dry CH₂Cl₂ was cooled to 0°C using an ice/water bath. To the cooled solution 3-chloroperoxybenzoic acid (mCPBA) (4.038 g, 23.40 mmol) was added in three portions and the mixture stirred at 0°C for 1 h. Then Ca(OH)₂ (2.222 g, 30.00 mmol) was added and the reaction mixture was stirred for additional 30 min at room temperature. The reaction mixture was filtered, washed with CH₂Cl₂ and the solvents removed in vacuo. The crude product was purified by column chromatography (silica gel, ethyl acetate/toluene 9/1) giving 4 as a white solid (4.732 g, 16.03 mmol; 89%). M.p.: 144.0-145.0°C. R_f = 0.20. ¹H-NMR (300 MHz, CDCl₃: d = 2.77 (s, 3 H), 7.47 (td, ³J(H,H) = 9 Hz, ⁴J(H,H) = 2 Hz, 2 H), 7.60 (td, ³J(H,H) = 9 Hz, ⁴J(H,H) = 2 Hz, 2 H), 7.71 (s, 4 H). ¹³C-NMR (75 MHz, CDCl₃): d = 44.02 (CH₃SO), 122.58 (C4'), 124.22 (C3), 127.92 (C2), 128.86 (C2'), 132.17 (C3'), 138.70 (C1'), 142.94 (C1), 144.89 (C4). MS (EI): m/z (%) = 295.9, 293.9 (55) [M⁺], 280.8, 278.8 (100) [M⁺-CH₃], 246.9 (21) [M⁺-SCH₃], 152 (21) [M⁺-Br, -SOCH₃]. Elemental analysis calculated (%) for C₁₃H₁₁BrOS (295.20): C 52.89, H 3.76; found: C 52.79, H 3.73.

A flame-dried, two-necked flask was successively charged with [Pd₂(dba)₃]×CHCl₃ (0.083 g, 0.08 mmol), BINAP (0.149 g, 0.24 mmol), 4'-Bromo-4-methanesulfinyl-biphenyl 4 (2.362 g, 8.00 mmol), benzophenone imine (1.740 g, 9.60 mmol), NaOtBu (1.076 g, 11.20 mmol) and dry toluene. The reaction mixture was heated to 80°C for 12 h. After cooling down to room temperature the reaction mixture was poured into Et₂O and filtered through a silica short plug with EtOAc as eluent. Evaporation of the solvents under reduced pressure and recrystallization from methanol afforded the crude diphenyl ketimine 5 as yellow solid (2.444 g, 6.18 mmol; 77%). The ketimine 5 was suspended in methanol and sodium acetate and hydroxylamine hydrochloride were added. After stirring the reaction for 2 h at r. t., it was poured into 0.1 M NaOH-solution. The aqueous phase was extracted with CH₂Cl₂ and the collected organic phases were dried over Na₂SO₄. The crude product was obtained by evaporation of the solvents and purification by CC (silica gel, EtOAc/toluene 9/1) afforded 6 as a yellowish solid (0.657 g, 2.84 mmol; 95%). M.p.: 191.0-192.0°C. R_f = 0.12. ¹H-NMR (300 MHz, CDCl₃): d = 2.75 (s, 3 H), 3.82 (br, 2 H), 6.82 (td, ³J(H,H) = 9 Hz, ⁴J(H,H) = 2 Hz, 2 H), 7.43 (td, ³J(H,H) = 9 Hz, ⁴J(H,H) = 2 Hz, 2 H), 7.67 (s, 4 H). ¹³C-NMR (75 MHz, CDCl₃): d = 44.03 (CH₃SO), 115.41 (C3), 124.03 (C3'), 127.14, 128.21 (C2', C2), 129.83 (C1), 143.05 (C1'), 144.18 (C4'), 146.71 (C4). MS (EI): m/z (%) = 231 (60) [M⁺], 216 (100) [M⁺-CH₃], 184.0 (10) [M⁺-SCH₃]. Elemental analysis calculated (%) for C₁₃H₁₃NOS (231.31): C 67.50, H 5.66, N 6.06; found: C 67.58, H 5.88, N 5.92.

4-Nitroso-biphenyl 2 (0.572 g, 3.12 mmol) and 4'-methanesulfinyl-4-amino-biphenyl 6 (0.657 g, 2.84 mmol) were dissolved in CH₂Cl₂ (25 ml) and dry AcOH (75 ml) and the reaction mixture was heated to 65°C for 67 h. The solvents were removed by rotary evaporation leaving a dark orange solid, which was recrystallized from toluene affording 7 as a red-brown solid (0.935 g, 2.36 mmol; 83%). M.p.: 270.0-272.0°C. ¹H-NMR (300 MHz, CDCl₃, T = 48°C): d = 2.78 (s, 3 H), 7.39 (t, ³J(H,H) = 8 Hz, 1 H), 7.48 (t, ³J(H,H) = 8 Hz, 2 H), 7.68 (d, ³J(H,H) = 8 Hz, 2 H), 7.75 - 7.79 (m, 6 H), 7.83 (d, ³J(H,H) = 8 Hz, 2 H), 8.02-8.06 (m, 4 H). ¹³C-NMR (75 MHz, CDCl₃, T = 48°C): d = 44.03 (CH₃SO), 123.45, 123.52, 124.16, 127.18, 127.79, 127.91, 127.94, 128.09, 128.88, 140.25, 142.13, 143.22, 144.03, 145.55, 151.98, 152.55 (Ar). MALDI-ToF-MS: 396.77 [M⁺]. Elemental analysis calculated (%) for C₂₅H₂₀N₂OS (396.51): C 75.73, H 5.08, N 7.07; found: C 75.57, H 5.26, N 6.80.

Biphenyl-4-yl-(4'-methanesulfinyl-biphenyl-4-yl)-diazene 7 (0.119 g, 0.30 mmol) was dissolved in dry, argon saturated (Ar.-satd.) toluene (170 ml) and the mixture was heated to 40°C. Trifluoroacetic anhydride (0.752 g, 0.5 ml, 3.5 mmol) was added drop-wise and the reaction was kept at 40°C for 2.5 h. The volatile solvents were removed by rotary evaporation and the residue was coevaporated with dry toluene (2 ´ 20 ml) to remove residual trifluoroacetic acid and anhydride. The obtained dark red solid was dried at HV for 1 h. Dry Ar.-satd. toluene (200 ml) and dry, Ar.-satd. EtOH (60 ml) was added followed by triethylamine (3.5 ml). The mixture was stirred at r. t. for 88 h. The solvents were removed in vacuo, the residue dissolved in CH₂Cl₂, and washed twice with Ar.-satd. sat. NH₄Cl-solution, once with Ar.-satd. H₂O, dried over Na₂SO₄ and then filtered over a small column (silica gel, CH₂Cl₂). After rotary evaporation an orange solid was obtained which was recrystallized from toluene/EtOH 3/1 to afford 1 as an orange solid (0.030 g, 0.08 mmol; 27%). M.p.: 290.0°C (decomp.). ¹H-NMR (300 MHz, CDCl₃, T = 48°C): d = 3.53 (s, 1 H), 7.37 - 7.39 (m, 3 H), 7.48 (t, ³J(H,H) = 7 Hz, 2 H), 7.56 (t, ³J(H,H) = 8 Hz, 2 H), 7.67-7.78 (m, 6 H), 8.00-8.04 (m, 4 H). MALDI-TOF-MS: 366.75 [M⁺]. Elemental analysis calculated (%) for C₂₄H₁₈N₂S (366.48): C 78.66, H 4.95, N 7.64; found: C 78.36, H 5.18, N 7.26.

Of particular interest were structural information of the azobiphenyl backbone and attempts to growth single crystals have been made with both compounds comprising the structural motive 1 and 7. While the target structure 1 always provided too-thin needles, single crystals suitable for x-ray analysis have been obtained by slow cooling a hot solution of 7 in toluene. 7 crystallizes acentric in the orthorhombic space group Pna2₁. As displayed below in figure S1, both biphenyl subunits are essentially planar, in the biphenyl subunit bearing the sulfur-group a torsion angle of 2.2(3)° and in the other biphenyl an angle of 2.5(3)° was determined between the two phenyl-rings. The bond length between the two phenyl rings is 150.3(3) pm (C(5)-C(8)) and 151.0(3) pm (C(17)-C(20)), respectively, which is slightly longer than the length of the C(sp2)-C(sp2) bond in unsubstituted biphenyls (148 pm). Due to the planer n = N-group both biphenyl subunits reside in the same plane. Apart from the methyl group of the methylsulfoxy substituent, 7 shows an almost completely planar arrangement. The length of the molecule estimated by the S(1)-C(23) distance amounts to 1.9724(4) nm.

1. Haack T, Erdinger L, Boche G (2001) Mutat Res 491:183-194.

2. Janczewski M, Charmas W (1967) Chem Abstr 66: 37570z.