2426
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 12, December, 2001
Krayushkin et al.
Earlier, we have demonstrated9,10 that the fluores-
of 0.267 g (81%), m.p. 186187 °C. Found (%): C, 61.85;
H, 5.06; N, 8.12; S, 19.97. C17H16N2OS2. Calculated (%):
C, 62.17; H, 4.91; N, 8.53; S, 19.52. 1H NMR (DMSO-d6), δ:
1.98, 2.04, 2.30, and 2.39 (all s, 3 H each, Me); 6.46 and 6.61
(both s, 1 H each, CH); 7.50 (s, 2 H, NH2).
4-Amino-5,6-bis(2,5-dimethyl-3-thienyl)furo[2,3-d]pyrim-
idine (3). Onetwo drops of Ac2O were added to a solution of
compound 2 (0.130 g, 0.4 mmol) in HCONH2 (2 mL). The
reaction mixture was refluxed for 4 h and then cooled. Water
(5 mL) was added and the precipitate that formed was filtered
off. The product was recrystallized from dioxane and com-
cence properties of dithienylethene photochromes are
favored by the presence of benzooxazole- or oxadiazole-
containing substituents in the thiophene rings as well as
by the formation of thienothiophenes. Although the
structures of compounds 24 and 6 do not contain the
above-mentioned aromatic fragments, their open forms
fluoresce in acetonitrile solutions. It should be empha-
sized that this is the first example of fluorescence of
dithienylethenes resulting from the characteristic struc-
tural features of the ethene bridge. The quantum yield of
fluorescence is low (<0.1). Based on the measurements
of the lifetime of the excited state, the observed fluores-
cence can be assigned to emission from the ππ* state.
The cyclic forms of the photochromes under study do
not exhibit fluorescence.
To summarize, we have developed procedures
for the synthesis of substituted bis(2,5-dimethyl-3-
thienyl)ethenes in which the ethene fragment is involved
in the furan or furopyrimidine systems. It was found for
the first time that dihetarylethenes containing the
2,5-dimethylthienyl fragments fluoresce due to the char-
acteristic features of the heterocyclic bridging system.
pound
3 was obtained in a yield of 0.090 g (63%),
m.p. 211212 °C. Found (%): C, 60.43; H, 4.95; N, 12.20;
S, 18.59. C18H17N3OS2. Calculated (%): C, 60.82; H, 4.82;
N, 11.82; S, 18.04. 1H NMR (DMSO-d6), δ: 2.02, 2.21, 2.32,
and 2.44 (all s, 3 H each, Me); 6.53 (s, 1 H, CH); 6.62 (br.s,
2 H, NH2); 6.75 (s, 1 H, CH); 8.23 (s, 1 H, CH arom.).
5,6-Bis(2,5-dimethyl-3-thienyl)-3,4-dihydrofuro[2,3-d]pyr-
imidin-4-one (4). A mixture of compound 2 (0.164 g, 0.5 mmol)
and HCOOH (2 mL) was refluxed for 8 h and then cooled.
Water (5 mL) was added and the precipitate that formed was
filtered off. The product was recrystallized from dioxane and
compound 4 was obtained in a yield of 0.098 g (55%), m.p.
143144 °C. Found (%): C, 60.28; H, 4.47; N, 8.09; S, 18.32.
C
18H16N2O2S2. Calculated (%): C, 60.65; H, 4.52; N, 7.86;
S, 17.99. 1H NMR (DMSO-d6), δ: 1.96, 2.03, 2.34, and 2.38
(all s, 3 H each, Me); 6.59 and 6.62 (both s, 1 H each, CH);
8.14 (d, 1 H, CH arom., J = 2.1 Hz); 12.62 (br.s, 1 H, NH).
N-[3-Cyano-4,5-bis(2,5-dimethyl-3-thienyl)-2-furyl]-N,N´-
dimethylformimidamide (5). A mixture of compound 2 (0.450 g,
1.4 mmol) and (MeO)2CHNMe2 (0.245 g, 2.1 mmol) in
anhydrous MeOH (2 mL) was refluxed with stirring for 2 h and
then cooled. Water (3 mL) was added and the precipitate was
filtered off. The product was recrystallized from aqueous EtOH
and compound 5 was obtained in a yield of 0.455 g (85%),
m.p. 9394 °C. Found (%): C, 62.41; H, 5.64; N, 11.23;
S, 16.89. C20H21N3OS2. Calculated (%): C, 62.63; H, 5.52;
N, 10.96; S, 16.72. 1H NMR (DMSO-d6), δ: 2.01, 2.08, 2.31,
2.39, 3.04, and 3.15 (all s, 3 H each, Me); 6.54 and 6.63
(both s, 1 H each, CH); 8.32 (s, 1 H, N=CH).
Experimental
The 1H NMR spectra were recorded on Bruker AM-300
(300.13 MHz) and Bruker WM-250 (250.13 MHz) instruments
in DMSO-d6 and CDCl3. The melting points were measured
on a Boetius heating stage and were not corrected. The mass
spectrum (EI) was obtained on a Kratos MS-30 instrument
with direct inlet of the sample into the ion source; the energy
of ionizing electrons was 70 eV. The course of the reactions
and the purities of the products were monitored by TLC on
Silica gel 60 F254 plates (Merck) using an AcOEthexane
solvent system.
The photochromic characteristics of compounds 24, 6,
and 8 were examined for solutions in MeCN. Acetonitrile of
special-purity grade was used in the measurements. Samples
were irradiated with a DRSh-500 mercury lamp using light
filters to separate lines of the mercury spectrum (λ = 313, 546,
and 578 nm). The intensity of radiation of the mercury lamp
was determined using an F4 photodetector calibrated using a
ferrioxalate actinometer11 for λ = 313 nm and using an
actinometer based on the Reinecke salt12 for λ = 546 and
578 nm. The electronic absorption spectra were recorded on a
Shimadzu UV-3100 spectrophotometer. The fluorescence spec-
tra were measured on an Elyumin-2M spectrofluorometer with
excitation in the maximum of the absorption spectrum of the
compound under study. Since the fluorescence excitation spectra
of the open forms corrected for the intensity of the exciting
light are identical with the electronic absorption spectra of
these forms, the observed fluorescence is attributable to emis-
sion of the open forms of the compounds under study.
2-Amino-4,5-bis(2,5-dimethyl-3-thienyl)furan-3-carbo-
nitrile (2). Diazabicycloundecene (DBU) (0.228 g, 1.5 mmol)
was added to a solution of thenoin 1 4 (0.280 g, 1 mmol) and
malononitrile (0.110 g, 1.66 mmol) in EtOH (3 mL). The
reaction mixture was heated with stirring at 6570 °C for 4 h
and then cooled. Water (3 mL) was added and the precipitate
that formed was filtered off. The product was recrystallized
from aqueous EtOH and compound 2 was obtained in a yield
4-Amino-5,6-bis(2,5-dimethyl-3-thienyl)furo[2,3-d]pyr-
imidine 3-oxide (6). A mixture of compound 5 (0.100 g,
0.3 mmol) and NH2OHHCl (0.070 g, 1 mmol) in EtOH
(1.5 mL) was refluxed for 56 h and then cooled. Water
(5 mL) was added and the precipitate was filtered off.
The product was recrystallized from EtOH and com-
pound 6 was obtained in a yield of 0.085 g (76%), m.p.
109110 °C. Found (%): C, 57.97; H, 4.54; N, 11.64; S, 16.88.
C18H17N3O2S2. Calculated (%): C, 58.20; H, 4.61; N, 11.31;
S, 17.26. 1H NMR (DMSO-d6), δ: 2.03, 2.23, 2.32, and 2.44
(all s, 3 H each, Me); 6.51 and 6.71 (both s, 1 H each, CH);
7.09 (br.s,
2 H, NH2); 8.69 (s, 1 H, CH arom.). MS,
m/z (Irel (%)): 371 [M+] (20).
2-[(5,5-Dimethyl-3-oxocyclohex-1-enyl)amino]-4,5-
bis(2,5-dimethyl-3-thienyl)furan-3-carbonitrile (7). A solution
of dimedone (0.140 g, 1 mmol), compound 2 (0.329 g, 1 mmol),
and p-toluenesulfonic acid (0.02 g, 0.0001 mol) in benzene
(5 mL) was refluxed with the use of a DeanStark trap for 5 h.
Then the reaction mixture was cooled and the precipitate of
the product that formed was filtered off and washed on a filter
with a small amount of benzene. Enaminoketone 7 was ob-
tained in a yield of 0.328 g (73%), m.p. 8889 °C. Found (%):
C, 66.78; H, 5.71; N, 6.51; S, 14.01. C25H26N2O2S2. Calcu-
lated (%): C, 66.64; H, 5.82; N, 6.22; S, 14.23. 1H NMR
(DMSO-d6), δ: 1.06 (s, 6 H, 2 Me); 2.01 and 2.11 (both s,
3 H each, Me); 2.15 (s, 2 H, CH2); 2.31 and 2.42 (both s,