10.1002/chem.201800079
Chemistry - A European Journal
FULL PAPER
stirred at 50 °C for 1 h, water (10 mL) was added and the aqueous phase
was extracted with dichloromethane (3 × 5 mL/10 mL, monitored by TLC).
The combined organic layers were dried (anhydrous sodium sulfate) and
the solvents were removed in vacuo. The residue was absorbed onto
Celite® and purified by flash chromatography on silica gel (petroleum
ether (boiling range 40−60 °C)/ethyl acetate) to give 3-ethinylchinoxaline
4f (540 mg, 88%) as a yellow solid, Rf = 0.22 (petroleum ether/ethyl
acetate 25:1). Mp 93 °C. 1H NMR (CDCl3, 300 MHz): δ 0.37 (s, 9 H), 7.18
(dd, J = 5.1, 3.9 Hz, 1 H), 7.57 (dd, J = 5.1, 1.1 Hz, 1 H), 7.66-7.78 (m, 2
H), 7.99-8.09 (m, 2 H), 8.56 (dd, J = 3.9, J = 1.1 Hz, 1 H). 13C NMR
(CDCl3, 75 MHz): δ -0.44 (CH3), 102.8 (Cquat), 103.2 (Cquat), 127.9 (CH),
128.8 (CH), 128.9 (CH), 130.0 (CH), 130.3 (CH), 130.5 (CH), 131.1 (CH),
134.9 (Cquat), 140.4 (Cquat), 140.7 (Cquat), 141.9 (Cquat), 147.7 (Cquat). EI-
MS (m/z): 309 (15), 308 ([M]+, 58), 307 (33), 295 (10), 294 ([M-CH3]+, 24),
295 ([M-CH3]+, 100), 277 ([M-(CH3)2]+, 9), 263 ([M-(CH3)3]+, 19). IR [cm-1]
Conclusions
In conclusion we have successfully synthesized a series of
diversity-oriented
chromophores by applying the consecutive three-component
activation-alkynylation-cyclocondensation (AACC) protocol.
2-substituted
3-ethynyl
quinoxaline
Thereby 2-substituents at the quinoxaline core can be readily
introduced, which either very difficult or impossible to introduce
by our previous glyoxylation-alkynylation-cyclocondensation
(GACC) approach. A series of donor-acceptor conjugates
become accessible, in particular, by placing the p-dimethylamino
substituent as a strong donor. As a result the positive emission
solvochromicity is considerably enhanced in comparison to the
first generation of 3-ethynyl quinoxalines, causing a shift of the
emission color over a broad part of the spectral range by
variation of the solvent polarity. The highly polar nature of the
excited state is experimentally elucidated by linear structure-
property relationships of the Hammett σp+ parameters with the
absorption and emission bands and the Stokes shifts, as well as
by Lippert-Mataga determination of the change of dipole
moment upon photonic excitation. In addition the photophysical
behavior is supported by DFT and TD-DFT calculations,
employing the PBEh1PBE functional, which better reproduces
the charge transfer character of the longest wavelength
absorption bands. These emission solvatochromic dyes with
enhanced donor capacity are particularly well suited as highly
sensitive polarity sensors. Studies to increase the substituent
diversity on quinoxaline acceptors by multicomponent strategies
and the modulation of the polar excited states by variation of the
employed donor and acceptor moieties are currently underway.
ν̃: 3125 (w), 3102 (w), 3065 (w), 2957 (w), 2922 (w), 2901 (w), 2855 (w),
2164 (w), 1558 (w), 1526 (w), 1472 (w), 1460 (w), 1427 (m), 1393 (w),
1362 (w), 1335 (m), 1315 (w), 1300 (w), 1248 (m), 1231 (m), 1219 (m),
1179 (m), 1134 (m), 1119 (w), 1082 (w), 1059 (w), 947 (w), 912 (w), 843
(s), 810 (m), 799 (w), 760 (s), 739 (m), 708 (s), 662 (m), 631 (m), 617 (m).
Anal. calcd. for C17H16N2SSi (308.1): C 66.19, H 5.23, N 9.08, S 10.39;
Found: C 66.46, H 5.33, N 8.78, S 10.12. 314 mg (2.00 mmol) of 1c
Typical procedure for the three-component synthesis of 4-(6,7-
dichloro-3-((trimethylsilyl)ethynyl)quinoxalin-2-yl)-N,N-
dimethylaniline (4p). Glyoxylic acid 1j (386 mg, 2.00 mmol) was placed
in dry 1,4-dioxane (5 mL) under an argon atmosphere in a sintered
screw-cap Schlenk tube and the solution was then deaerated degassed
with argon (5 min). The mixture was stirred at room temp (water bath) for
5 min. Thereafter, the mixture was stirred at 50 °C (oil bath) for 4 h. Then
the mixture was cooled to room temp (water bath) for 5 min. Then, CuI
(20 mg, 0.10 mmol, 5.00 mol %), alkyne 2b (0.28 mL, 2.00 equiv), and
dry triethylamine (0.59 mL for 2.00 mmol, 4.20 mmol, 2.10 equiv) were
successively added to the reaction mixture, and stirring at room temp
was continued for 16 h. Then, methanol (2 mL), 1,2-diaminoarene 3b
(361 mg, 2.00 mmol) and of acetic acid (0.24 mL, 4.00 mmol) were
added successively to the reaction mixture. The mixture was stirred at
50 °C for 1 h, water (10 mL) was added and the aqueous phase was
extracted with dichloromethane (3 × 5 mL/10 mL, monitored by TLC).
The combined organic layers were dried (anhydrous sodium sulfate) and
the solvents were removed in vacuo. The residue was absorbed onto
Celite® and purified by flash chromatography on silica gel (petroleum
ether (boiling range 40−60 °C)/ethyl acetate) to give 3-ethinylchinoxaline
4p (241 mg, 35%) as an orange solid, Rf = 0.10 (petroleum ether/ethyl
acetate 6:1). Mp 186 °C. 1H NMR (CDCl3, 300 MHz): δ 0.22 (s, 9 H),
3.00 (s, 6 H), 6.68-6.74 (m, 2 H), 8.04-8.11 (m, 4 H). 13C NMR (CDCl3,
75 MHz): δ -0.4 (CH3), 40.4 (CH3), 102.9 (Cquat), 103.4 (Cquat), 111.4 (CH),
129.2 (Cquat), 129.6 (Cquat), 131.3 (Cquat), 133.6 (CH), 135.1 (CH), 138.2
(CH), 138.9 (CH), 140.0 (CH), 151.8 (CH), 155.2 (CH). EI-MS (m/z): 415
([M]+(37Cl), 21), 414 (13), 413 ([M]+(35Cl), 28), 322 (14), 141 (13), 129
(18), 111 ((37Cl), 14), 109 ((35Cl), 5), 97 ([M(37Cl)-Si(CH3)3]+,17), 95
([M(35Cl)-Si(CH3)3]+,7), 85 ((37Cl), 26), 83 ((35Cl), 15), 71 ((35Cl), 31),
69((37Cl), 17), 57 ((37Cl), 47), 55 ((35Cl), 22) 43((37Cl), 36), 41 ((35Cl), 17).
Supporting information for this article is available on the
on glyoxylation-alkynylation-cyclocondensation sequences with
anilines and other π-nucleophiles, experimental details and full
characterization of compounds 4, 1H and 13C NMR spectra of
compounds 4, absorption and emission spectra of compounds 4,
Hammett correlations, solvatochromism, X-ray and DFT
computed XYZ-coordinates and energies of structures 4o-r.
Experimental Section
Typical procedure for the three-component synthesis of 2-
(thiophen-2-yl)-3-((trimethylsilyl)ethynyl)quinoxaline (4f). Glyoxylic
acid 1c (314 mg, 2.00 mmol) was placed in dry 1,4-dioxane (5 mL) under
an argon atmosphere in a sintered screw-cap Schlenk tube and the
solution was then deaerated degassed with argon (5 min). The mixture
was stirred at room temp (water bath) for 5 min. Thereafter, the mixture
was stirred at room temp (water bath) for 4 h. Then the mixture was
cooled to room temp (water bath) for 5 min. Then, CuI (20 mg, 0.10 mmol,
5.00 mol %), alkyne 2b (0.28 mL, 2.00 equiv), and dry triethylamine (0.59
mL, 4.20 mmol) were successively added to the reaction mixture, and
stirring at room temp was continued for 16 h. Then, methanol (2 mL), 1,2-
diaminoarene 3a (216 mg, 2.00 mmol) and of acetic acid (0.24 mL, 4.00
mmol) were added successively to the reaction mixture. The mixture was
IR [cm-1] ν
̃: 3107 (w), 3086 (w), 2955 (w), 2897 (w), 2866 (w), 2824 (w),
2803 (w), 2783 (w), 2745 (w), 2475 (w), 2158 (w), 1603 (m), 1530 (m),
1512 (m), 1479 (w), 1443 (m), 1408 (m), 1393 (m), 1373 (m), 1314 (m),
1246 (m), 1233 (m), 1190 (m), 1165 (s), 1099 8m), 1065 (m), 1020 (w),
976 (m), 949 (m), 866 (m), 839 (s), 820 (s), 791 (m), 760 (s), 739 (m),
704 (m), 687 (m), 664 (m), 650 (m), 629 (m). Anal. calcd. for
C21H21Cl2N3Si (415.0): C 54.11, H 3.74, N 7.42, S 8.50; Found: C 54.40,
H 3.95, N 7.20, S 8.20.
This article is protected by copyright. All rights reserved.
Merkt, Franziska K.
