Alternating Tripyridyl–Dipyrrole Molecular Strands
FULL PAPER
2H), 8.74 (d, J=7.9 Hz, 2H), 8.34 (t, J=7.9 Hz, 1H), 8.31 ppm (d, J=
9.2 Hz, 2H); 13C NMR (75.5 MHz, [D6]DMSO, 258C): d=160.8, 159.3,
151.6, 139.9, 131.4, 123.2, 122.7, 120.3 ppm; MS (70 eV): m/z (%): 531
(48) [M]+, 370 (51) [MꢁSO2CF3ꢁN2]+, 342 (100) [MꢁSO2CF3ꢁ2N2]+;
HRMS: m/z calcd for [M+Na]+ (C15H7F6N5NaO6S2): 553.9634; found:
553.9628.
ture was stirred at reflux for 48 h. Chromatography on silica gel (CH2Cl2/
Et2O, gradient from 100:0 to 50:50) of the crude product obtained by fol-
lowing the general work-up procedure yielded 306 mg (95%) of pure 2c
as a pale yellow solid. M.p.: 2698C; 1H NMR (300 MHz, CDCl3, 258C):
d=8.86 (d, J=7.8 Hz, 2H), 8.77 and 8.75 (AB system, J=8.9 Hz, 4H),
8.42 (s, 2H), 8.10 (t, J=7.8 Hz, 1H), 7.10 (s, 2H), 2.63 (s, 6H), 2.44 ppm
(s, 6H); 13C NMR (75.5 MHz, CDCl3, 258C): d=158.5, 158.1, 157.7,
153.2, 152.4, 148.6, 138.4, 125.4, 125.3, 124.9, 122.5, 119.9, 24.3, 21.1 ppm;
General procedure for synthesis of dipyridazines 2a, 2b and 2c by Ne-
gishi coupling reaction:
A solution of the requisite bromopyridine
UV/Vis
(CH2Cl2):
l(e)=318
(44600 molꢁ1 m3 cmꢁ1),
260 nm
(1.6 equiv) in THF (3 mLmmolꢁ1 of bromopyridine) at ꢁ788C was treat-
ed with nBuLi (2.5 molLꢁ1 in hexanes, 1.6 equiv). The mixture was
stirred for 30 min, and then a solution of zinc chloride (0.7 molLꢁ1 in
THF, 1.6 equiv) was added by canula at ꢁ788C. At the end of the addi-
tion, the mixture was allowed to warm to room temperature and stirred
for 30 min. Then a solution of tetrakis(triphenylphosphine)palladium
(0.05 equiv) and activated dipyridazine 14 (1.0 equiv) in THF
(6 mLmmolꢁ1 of bromopyridine) was added by a canula at this tempera-
ture. After 48 h at reflux, the mixture was quenched with saturated aque-
ous NaHCO3 (30 mL) and 25 molLꢁ1 aqueous NH4OH (50 mL) solutions,
and the resulting solution was stirred for 24 h. The heterogeneous mix-
ture was filtered through a pad of Celite, which was washed with CH2Cl2
then with 25 molLꢁ1 aqueous NH4OH. The filtrate was extracted with
CH2Cl2 (3ꢂ50 mL). The combined organic layers were dried over
Na2SO4 and the solvents were removed in vacuo to afford the respective
reaction product, which was purified by flash chromatography on silica
gel or crystallization.
(28600 molꢁ1 dm3 cmꢁ1); MS (70 eV): m/z: 445 (36) [M]+, 417 (33)
[MꢁN2]+; HRMS: m/z calcd for C27H23N7: 445.2015; found: 445.2016.
General procedure for the synthesis of dipyrroles 1b and 1c by electro-
chemical ring contraction: Dipyridazine 2b or 2c (70 to 200 mg) was dis-
solved in THF/acetate buffer/CH3CN (5:4:1, 90 mL), and the solution in-
troduced into the cathodic compartment of the electrochemical cell. The
same solvent was put in the anodic compartment. Prior to and during
electrolysis at Ew, the catholyte was deaerated with argon and stirred
magnetically. The course of the reaction was followed by cyclic voltam-
metry in the cathodic compartment, and the electrolysis was stopped
when the consumption of the starting substrate was complete (8 Fmolꢁ1).
Then the solvents were partially evaporated and the resulting precipitate
was collected by filtration. The yellow solid was dissolved in CH2Cl2 and
washed with saturated aqueous NaHCO3 solution and water. The organic
layer was dried (Na2SO4) and concentrated to dryness. After purification
by crystallisation (hexane/chloroform 4:1), dipyrroles 1b and 1c were ob-
tained as yellow powders.
2,6-Bis[6-(pyridin-2-yl)pyridazin-3-yl]pyridine (2a): Following the general
2,6-Bis[5-(6-methylpyridin-2-yl)pyrrol-2ACTHUNTGRENNG(U 1H)-yl]pyridine (1b): A solution
procedure,
a solution of activated dipyridazine–pyridine 14 (1.06 g,
of dipyridazine 2b (150 mg, 0.36 mmol) in THF/acetate buffer/CH3CN
(5:4:1, (100 mL) was reduced at Ew =ꢁ1.25 V/SCE. Then crystallization
(hexane/chloroform 4:1) of the crude product, obtained by following the
general work-up procedure, yielded pure 1b (101 mg, 71%) as a yellow
powder. M.p. 1038C; 1H NMR (300 MHz, CDCl3, 258C): d=11.50 (brs,
2H), 7.60 (t, J=7.8 Hz, 2H), 7.56 (t, J=7.5 Hz, 2H), 7.49 (d, J=7.5 Hz,
2H), 7.35 (d, J=7.8 Hz, 1H), 6.94 (d, J=7.5 Hz, 2H), 6.82–6.76 (m, 4H),
2.66 ppm (s, 6H); 13C NMR (75 MHz, CDCl3, 258C): d=157.4, 150.3,
149.6, 137.0, 136.9, 134.2, 133.0, 120.4, 116.7, 115.5, 110.2, 108.8,
2.0 mmol) and tetrakis(triphenylphosphine)palladium (0.46 g, 0.32 mmol)
in THF (40 mL) was added to the organozinc reagent [prepared from 2-
bromopyridine (1.01 g, 6.4 mmol), nBuli (2.4m in hexanes, 3.0 mL,
7.0 mmol) in THF (20 mL)] and zinc chloride (0.7 molLꢁ1 in THF,
10.0 mL, 7.0 mmol). The reaction mixture was stirred at reflux for 48 h.
Crystallization (AcOEt) of the crude product obtained by following the
general work-up procedure yielded 210 mg (27%) of pure 2a as a pale
yellow solid. M.p. 2588C; 1H NMR (300 MHz, CDCl3, 258C): d=8.87–
8.71 (m, 10H), 8.10 (t, J=8.1 Hz, 1H), 7.89 (dt, J=7.8, 1.8 Hz, 2H),
7.41 ppm (ddd, J=7.5, 4.8, 1.2 Hz, 2H); 13C NMR (75 MHz, CDCl3,
258C): d=158.3, 157.8, 153.3, 153.1, 149.4, 138.5, 137.2, 128.5, 125.1,
124.8, 122.6, 121.7 ppm; UV/Vis (CH2Cl2): l(e)=315 (38200), 290
(33600), 256 nm (29250 molꢁ1 dm3 cmꢁ1); MS (70 eV): m/z (%): 389 (46)
[M]+, 361 (64) [MꢁN2]+, 128 (100) [PyrC4H2]+; HRMS: m/z calcd for
[M+H]+ (C23H16N7): 390.1473; found 390.1467.
24.7 ppm;
UV/Vis
(CH2Cl2):
l(e)=380
(25900),
343 nm
(56300 molꢁ1 dm3 cmꢁ1); UV/Vis (MeOH): l(e)=378 (29500), 341 nm
(62100 molꢁ1 dm3 cmꢁ1); MS (70 eV): m/z (%): 392.1 (100) [M+H]+;
HRMS: m/z calcd for [M+H]+ (C25H22N5): 392.1875; found 392.1869.
2,6-Bis[5-(4,6-dimethylpyridin-2-yl)pyrrol-2ACTHNUTRGNE(UNG 1H)-yl]pyridine (1c): A solu-
tion of dipyridazine 2c (71 mg, 0.16 mmol) in THF/acetate buffer/CH3CN
(5:4:1, 100 mL) was reduced at Ew =ꢁ1.20 V/SCE. Then crystallisation
(hexane/chloroform 4:1) of the crude product, obtained by following the
general work-up procedure, yielded pure 1c (62 mg, 93%) as a yellow
powder. M.p. 2198C; 1H NMR (300 MHz, CDCl3, 258C): d=11.50 (brs,
2H), 7.58 (t, J=7.8 Hz, 1H), 7.35 (d, J=7.8 Hz, 2H), 7.32 (s, 2H), 6.78–
6.75 (m, 6H), 2.61 (s, 6H), 2.33 ppm (s, 6H); 13C NMR (75.5 MHz,
CDCl3, 258C): d=157.0, 150.1, 149.6, 147.9, 136.8, 134.0, 133.0, 121.6,
117.4, 115.3, 109.9, 108.7, 24.4, 21.1 ppm; UV/Vis (CH2Cl2): l(e)=380 nm
(26900 molꢁ1 m3 cmꢁ1), 341 (58600 molꢁ1 m3 cmꢁ1); UV/Vis (MeOH):
l(e)=378 nm (29400 molꢁ1 m3 cmꢁ1), 340 (64200 molꢁ1 m3 cmꢁ1); MS
(70 eV): m/z (%): 419 (100) [M]+, 106 (7) [MePyr]+; HRMS: m/z calcd
for C27H25N5: 419.2110; found: 419.2108; elemental analysis calcd (%) for
2,6-Bis[6-(6-methylpyridin-2-yl)pyridazin-3-yl]pyridine (2b): Following
the general procedure, a solution of activated dipyridazine–pyridine 14
(532 mg, 1.00 mmol) and tetrakis(triphenylphosphine)palladium (231 mg,
0.20 mmol) in THF (20 mL) was added to the organozinc reagent [pre-
pared from 2-bromo-6-methylpyridine (550 mg, 3.20 mmol) and nBuli
(2.5 molLꢁ1 in hexanes, 1.4 mL, 3.50 mmol) in THF (10 mL)] and zinc
chloride (0.7m in THF, 5.0 mL, 3.50 mmol). The reaction mixture was
stirred at reflux for 48 h. Chromatography on silica gel (CH2Cl2/Et2O,
gradient from 100:0 to 50:50) of the crude product obtained by following
the general work-up procedure yielded 284 mg (68%) of pure 2b as a
white solid. M.p. 2598C; 1H NMR (300 MHz, CDCl3, 258C): d=8.88 (d,
J=7.9 Hz, 2H), 8.81 and 8.78 (AB Syst, J=8.9 Hz, 4H), 8.57 (d, J=
7.8 Hz, 2H), 8.12 (t, J=7.9 Hz, 1H), 7.80 (t, J=7.8 Hz, 2H), 7.28 (d, J=
7.8 Hz, 2H), 2.68 ppm (s, 6H); 13C NMR (75 MHz, CDCl3, 258C): d=
158.6, 158.3, 157.8, 153.2, 152.7, 138.4, 137.3, 125.1, 124.9, 124.3, 122.5,
118.7, 24.6 ppm; UV/Vis (CH2Cl2): l(e)=316 (38000), 257 nm
(23800 molꢁ1 dm3 cmꢁ1); MS (70 eV): m/z (%): 417 (100) [M]+, 389 (94)
[MꢁN2]+; HRMS: m/z calcd for [M+Na]+ (C25H19N7Na): 440.1600,
found 440.1599.
.
C27H25N5 1.2H20: C 73.51, H 6.26, N 15.87; found C 73.31, H 6.21, N
15.96.
Synthesis of complex 1b-CuI2II: Cu
ACTHNUTRGNEN(UG OAc)2 (23 mg, 0.12 mmol) was added
to a solution of 2b (10 mg, 0.024 mmol) in THF (5 mL). The mixture was
stirred for 1 h at 608C and the solvent was removed in vacuo. Then the
solid was dissolved in CHCl3 (10 mL). The resulting solution was filtered
through cotton and evaporated to dryness. The powder was again dis-
solved in CHCl3 (1 mL) and dry MeOH was added slowly to form two
distinct layers. After one week at room temperature, the microcrystals
that had formed were collected by filtration and dried under vacuo.
2,6-Bis[6-(4,6-dimethylpyridin-2-yl)pyridazin-3-yl]pyridine (2c): Follow-
ing the general procedure, a solution of activated dipyridazine–pyridine
14 (385 mg, 0.72 mmol) and tetrakis(triphenylphosphine)palladium
(167 mg, 0.14 mmol) in THF (20 mL) was added to the organozinc re-
agent [prepared from 2-bromo-4,6-dimethylpyridine (431 mg, 2.32 mmol)
and nBuli (2.5 molLꢁ1 in hexanes, 1.0 mL, 2.54 mmol) in THF (10 mL)]
and zinc chloride (0.7m in THF, 3.6 mL, 2.54 mmol). The reaction mix-
Chem. Eur. J. 2010, 16, 11876 – 11889
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11887