Preparation of cyan dyes from 6-diethylaminobenzo[b]furan-2-carboxaldehyde

Article abstract of DOI:10.1080/00397910903351568

6-Diethylaminobenzo[b]furan-2-carboxaldehyde was synthesized in four steps and 48% overall yield starting from 4-diethylaminosalicylaldehyde. Five cyan dyes were made from this aldehyde using malononitrile dimer and 2-dicyanomethylene-3-cyano-4,5,5-trimet

Full text of DOI:10.1080/00397910903351568

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Preparation of Cyan Dyes from 6-
Diethylaminobenzo[b]furan-2-
carboxaldehyde
Matthew C. Davis a , Thomas J. Groshens a & Damon A. Parrish b
a Chemistry and Materials Division, Michelson Laboratory, Naval Air
Warfare Center, China Lake, California, USA
b Laboratory for the Structure of Matter, Naval Research Laboratory,
Washington, DC, USA
Version of record first published: 13 Sep 2010.
To cite this article: Matthew C. Davis, Thomas J. Groshens & Damon A. Parrish (2010): Preparation
of Cyan Dyes from 6-Diethylaminobenzo[b]furan-2-carboxaldehyde, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:20, 3008-3020
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Synthetic Communications1, 40: 3008–3020, 2010
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903351568
PREPARATION OF CYAN DYES FROM
6-DIETHYLAMINOBENZO[b]FURAN-
2-CARBOXALDEHYDE
Matthew C. Davis,1 Thomas J. Groshens,1 and
Damon A. Parrish2
1Chemistry and Materials Division, Michelson Laboratory, Naval Air
Warfare Center, China Lake, California, USA
2Laboratory for the Structure of Matter, Naval Research Laboratory,
Washington, DC, USA
6-Diethylaminobenzo[b]furan-2-carboxaldehyde was synthesized in four steps and 48%
overall yield starting from 4-diethylaminosalicylaldehyde. Five cyan dyes were made from
this aldehyde using malononitrile dimer and 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-
2,5-dihydrofuran. The crystal structures of two dyes are included.
Keywords: Benzofuran; dyes; malononitrile dimer; polyene
As part of a program to discover new organic nonlinear optical dyes for electro-optic
devices, several ‘‘push–pull’’ dyes[1] based on the benzofuran ring system were
synthesized (Fig. 1). These dyes are composed of various lengths of olefin linkage
between the electron donor diethylamino group and the electron acceptor tricyano-
ligand at each end of the molecule. The electron acceptors used to complete the
dyes were malononitrile dimer or 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-
dihydrofuran (TCF).
The benzofuran aldehydes, 11[2–6] and 14, were prepared as shown in Scheme 1.
Deprotonation of commercially available 4-diethylaminosalicylaldehyde (7)[7]
with potassium tert-butoxide and alkylation with diethyl bromomalonate in
dimethylformamide gave the cyclized alcohol 8.[8,9] Dehydration and decarboxyla-
tion of 8 was carried out by brief treatment in hot polyphosphoric acid to give
the benzofuran ester 9.[10] In working up this reaction, it was essential to keep the
aqueous reaction mixture cold while slowly neutralizing with cold aqueous sodium
hydroxide to prevent hydrolysis of the product. (The amount of aqueous
sodium hydroxide necessary to neutralize a known quantity of polyphosphoric acid
was previously determined.)
Received June 29, 2009.
This article is not subject to U.S. copyright laws.
Address correspondence to Matthew C. Davis, Chemistry and Materials Division, Michelson
Laboratory, Naval Air Warfare Center, China Lake, CA 93555, USA. E-mail: matthew.davis@navy.mil
3008

PREPARATION OF CYAN DYES
3009
Figure 1. Chemical structures of the dyes.
The ester 9 was reduced with lithium tetrahydroaluminate to the alcohol 10
and then oxidized to aldehyde 11 with 2,3,4,5-tetrachlorobenzoquinone (p-chloranil),
a sequence previously reported by Chen and Fox.[4]
Continuing onward, Knoevenagel condensation of aldehyde 10 with isophor-
one gave the ketone 12.[11] Lithioacetonitrile[12–14] was best prepared by adding
Scheme 1. Synthesis of the aldehydes.

3010
M. C. DAVIS, T. J. GROSHENS, AND D. A. PARRISH
the hexyllithium into the solution of acetonitrile in tetrahydrofuran rather than
the reverse. Addition of lithioacetonitrile to 12 at ꢀ78 ꢁC gave an intermediate b-
hydroxynitrile, which was difficult to characterize. Thus, the crude product from
this reaction was refluxed in glacial acetic acid to give the unsaturated nitrile
13 as an almost 1:1 mixture of geometric isomers.[15] Finally, 13 was reduced
with diisobutylaluminum hydride to give aldehyde 14. Both aldehydes 11 and 14 were
obtained as oils that were difficult to purify completely for accurate combustion
analyses; nevertheless, they reacted readily with the nucleophilic acceptor groups.
The final reactions to prepare the dyes are shown in Scheme 2. Dyes 4 and 6
were made by condensation of 11 and 14 with the TCF acceptor in the presence
of piperidinium acetate. Dyes 1 and 5 were made by refluxing ethanol solutions of
the aldehydes 11 and 14 with malononitrile dimer and piperidinium acetate. In dyes
5 and 6, the cis=trans ratio changed from nearly 50%, established at the preparation
of 13, to an 85% trans=15% cis mixture. This isomerization phenomena has been
observed previously with other related dyes.[15,16] The amino group of 1 could be
further functionalization by reaction with ethyl chloroformate. When using triethy-
lamine, the monoethoxycarbonyl derivative 2 was obtained. Reaction of 1 with
excess ethyl chloroformate and pyridine yielded the diethoxycarbonyl derivative 3.
The curious behavior of malononitrile dimer derivatives to acylation was described
previously by Vanmaele.[17,18] Similar attempts to prepare the corresponding ethox-
ycarbonyl derivatives of 5 appeared to be successful by thin-layer chromatographic
(TLC) analysis of the reaction mixtures. However, attempts to isolate and purify the
products failed, possibly because of hydrolytic instability.[19]
The kmax for the dyes dissolved in methylene chloride are collected in Table 1.
In the series of dyes 1–3, there is a progressive bathochromic shift caused by
acylation of the nitrogen atom of 1.[17,18] The longer length conjugation is probably
responsible for the blue appearance of 5 as compared to the red color of 1.[1] There
is a bathochromic shift when the conjugation of 4 is extended to 6. The malononitrile
Scheme 2. Preparations of the dyes.

PREPARATION OF CYAN DYES
3011
Table 1. kmax of 1–6 in methylene chloride solution
Compound
kmax (nm)
1
2
3
4
5
6
550
599
655
662
624
697
dimer is a weaker electron acceptor group compared to TCF (compare 1 to 4 and 5 to
6). Only when the amino group is acylated twice does the malononitrile dimer deriva-
tive approach the electron-accepting strength of the TCF acceptor (compare 3 to 4).
The crystal structure of malononitrile[19,20] dye 5, Fig. 2, is composed of one
molecule with a nearly planar conformation and the second with a slight twist from
planarity at the trinitrile end. The planar structure has the transoid conformation
(Fig. 3, and the nonplanar is cisoid. The crystal structure of the TCF dye[21–25]
6
(Fig. 4) shows a planar conformation along the polyene backbone with the benzo-
furan ring rotated 180ꢁ as compared to 5. The TCF group resides in the transoid
conformation.
Figure 2. The molecular structure of the asymmetric unit of 5. Hydrogen atoms are not shown, and only
the heteroatoms are labeled. Displacement ellipsoids are drawn at the 50% probability level. The toluene
solvent molecule shows rotational disorder and occupies a special position.
Figure 3. Conformations of the acceptor end of 5 in the solid state.

3012
M. C. DAVIS, T. J. GROSHENS, AND D. A. PARRISH
Figure 4. The molecular structure of the symmetric unit of 6 in ORTEP format showing the atom
numbering scheme. The hydrogen atoms are drawn as small spheres of arbitrary radii. Only one major
conformation of the disordered diethylamino group is shown.
EXPERIMENTAL
The melting points were collected on an electrothermal capillary melting-point
apparatus and are not corrected. All NMR data were collected on a Bruker Avance
II 300-MHz spectrometer (1H at 300 MHz, 13C at 75 MHz). Nuclear magnetic reson-
ance data (free-induction decays) were processed using NUTS software from Acorn
NMR (Livermore, CA). All spectra are referenced to solvent or tetramethylsilane.
The transmission ultraviolet—visible (UV-vis) spectra for compounds dissolved in
CH2Cl2 were obtained with a Cary 5 near infrared (NIR)–vis-UV spectrophotometer
and quartz cuvettes. Elemental analyses were performed by Atlantic Microlab, Inc.
(Norcross, GA).
Malononitrile dimer was prepared according to the procedures of Taylor
and Hartke[26] or Mittelbach.[27] Piperidinium acetate and the TCF acceptor were
prepared according to literature procedures.[28,29] The 2,3,5,6-tetrachloro-1,4-benzo-
quinone (p-chloranil, 99%) was purchased from Sigma-Aldrich (Milwuakee) and
recrystallized from toluene before use. The 4-(diethylamino)salicylaldehyde (7,
98%) was purchased from Sigma-Aldrich and purified before use: 50 g was dissolved
in 500 mL Et2O and magnetically stirred with 5 g Darco G-60. After 1 h, the mixture
was filtered through diatomaceous earth, and the solvent was rotary evaporated
to give the product as a tan amorphous crystal. All other reagents were obtained
commercially and used as received.
Racemic Diethyl 3-Hydroxy-6-diethylamino-2,3-dihydrobenzo[b]
furan-2,2-dicarboxylate (8)
A 1-L round-bottomed flask equipped with mechanical stirrer and internal ther-
mometer was charged with 400 mL anhydrous dimethylformamide (DMF) and 50 g 7
(0.26 mol) were added. An endotherm of 7 ꢁC occurred while the solids dissolved. The
solution was then cooled to 5 ꢁC with an ice bath. In one portion, 32 g KOtBu
(0.286 mol, 1.1 equiv) was added, and the temperature climbed to 20 ꢁC then slowly
fell back to 10 ꢁC. After 5 min, the potassium salt of the 4-diethylaminosalicylalde-
hyde precipitated. In a gentle stream, 74.57 g diethyl bromomalonate (0.312 mol,
1.2 equiv) was added using an addition funnel over 15 min. After ꢂ10 g of the bro-
mide had been added, the solids broke up and stirring resumed. The temperature
peaked at 29 ꢁC, and the cooling bath was removed. The mixture was stirred at rt

PREPARATION OF CYAN DYES
3013
for 1 h. The mixture was poured into 2 L distilled H2O containing 16.42 mL glacial
HOAc (0.286 mol, 1.1 equiv). A brown oil separated initially and after stirring for
several hours became a tan solid. The crude product was collected on a coarse-
porosity glass frit. The title compound was obtained as white microcrystals from
hexanes=EtOAc (75 g, 83%). Mp 87–89 ꢁC. dH (CDCl3): 7.19 (d, J ¼ 9.1 Hz, 1H),
6.33–6.27 (m, 2H), 5.79 (d, J ¼ 8.3 Hz, 1H), 4.43–4.14 (m, 4H), 3.32 (q, J ¼ 7.1 Hz,
4H), 2.22 (d, J ¼ 8.4 Hz, OH), 1.32 (t, J ¼ 7.1 Hz, 3H), 1.28 (t, J ¼ 7.2 Hz, 3H), 1.14
(t, J ¼ 7.1 Hz, 6H); dC (CDCl3): 166.77, 165.73, 160.83, 150.93, 126.15, 112.23,
106.29, 93.89, 93.38, 62.64, 62.56, 44.84, 14.23, 14.07, 12.59. Elemental analysis cal-
culated for C18H25NO6: C, 61.52; H, 7.17; N, 3.99. Found: C, 61.62; H, 7.22; N, 3.97.
Ethyl 6-Diethylaminobenzo[b]furan-2-oate (9)
A 1-L beaker was charged with 410 g polyphosphoric acid (PPA) followed by
81.9 g 8 (0.23 mol). The mixture was heated on a hotplate to 100 ꢁC and mechanically
stirred with a steel spatula by hand. The color became bright yellow, and gas that
evolved caused foaming. After 30 min, the yellow color noticeably changed to tan,
1
and H NMR of an aliquot of the reaction mixture dissolved in DMSO-d6 showed
that all the starting material was consumed and the reaction was complete. Water
(1 L) was added, and the mixture was dissolved to give a clear solution. The mixture
was transferred to a 3-L beaker, magnetically stirred, and cooled in an ice bath while
a previously prepared, cold solution of 205 g NaOH dissolved in 1 L H2O was added
in a gentle stream. Care was taken to keep the internal temperature below 20 ꢁC to
prevent ester hydrolysis. The resulting pH was ꢂ5, and a brown oil separated. The
mixture was extracted twice with 500-mL portions of Et2O. After drying over anhy-
drous MgSO4, the solution was evaporated to half the volume and diluted with an
equal volume of hexanes. The mixture was filtered through SiO2 to remove some
polar impurities. Evaporation of the filtrate gave the title compound as a pale yellow
oil (51.6 g, 86%). No further purification was performed. dH (CDCl3): 7.43 (dd,
J ¼ 8.9 and 0.6 Hz, 1H), 7.40 (d, J ¼ 1.0 Hz, 1H), 6.77 (m, 1H), 6.74 (dd, J ¼ 8.7
and 2.3 Hz, 1H), 4.40 (q, J ¼ 7.2 Hz, 2H), 3.41 (q, J ¼ 7.0 Hz, 4H), 1.40 (t, J ¼ 7.2 Hz,
3H), 1.19 (t, J ¼ 6.9 Hz, 6H); dC (CDCl3): 159.62, 158.48, 148.46, 142.79, 122.80,
115.84, 114.38, 110.55, 92.93, 60.62, 44.72, 14.25, 12.26. Elemental analysis calcu-
lated for C15H19NO3: C, 68.94; H, 7.33; N, 5.36. Found: C, 70.01; H, 7.40, 5.33.
2-Hydroxymethyl-6-diethylaminobenzo[b]furan (10)
A 1-L round-bottomed flask equipped with magnetic stirbar was charged with
400 mL anhydrous tetrahydrofuran (THF). The flask was stirred and purged with N2
while 10.56 g lithium aluminum hydride (0.278 mol, 1.75 equiv) was added over
10 min. A reflux condenser was attached to the flask, and an addition funnel with
N2 bubbler was connected on top of the condenser. The funnel was charged with
a solution of 41.58 g 9 (0.159 mol) in 100 mL anhydrous THF. The ester was added
over 20 min. The mixture was then refluxed until TLC showed complete consump-
tion of the ester (30 min). The mixture was cooled in an ice bath and carefully
quenched with 10.56 mL H2O followed by 10.56 mL 15% NaOH and finally
31.68 mL H2O. After stirring 1 h, the mixture was filtered through diatomaceous

3014
M. C. DAVIS, T. J. GROSHENS, AND D. A. PARRISH
earth. The filtrate was washed with 100 mL brine, and the organic phase was sepa-
rated and dried over anhydrous MgSO4. The solvent was stripped at the rotovap
to give 30.3 g of the title compound as a pale yellow oil (87%). No further purifi-
cation was performed. dH (CDCl3): 7.28 (d, J ¼ 8.5 Hz, 1H), 6.75 (d, J ¼ 1.8 Hz,
1H), 6.65 (dd, J ¼ 8.8 and 2.1 Hz, 1H), 6.42 (s, 1H), 4.60 (s, 2H), 3.31 (q, J ¼ 7.4 Hz,
Hz, 4H), 3.2 (bs, OH), 1.12 (t, J ¼ 7.1 Hz, 6H); dC (CDCl3): 157.38, 154.09, 146.54,
121.14, 117.84, 110.16, 104.08, 95.09, 57.89, 45.13, 12.47. Elemental analysis calcu-
lated for C13H17NO2: C, 71.21; H, 7.81; N, 6.39. Found: C, 71.06; H, 7.92; N, 6.34.
6-Diethylaminobenzo[b]furan-2-carboxaldehyde (11)
A 2-L round-bottomed flask equipped with magnetic stirbar was charged with
36.53 g 2,3,5,6-tetrachloro-1,4-benzoquinone (0.149 mmol, 1.1 equiv) and 900 mL
CHCl3. An addition funnel was attached, and 29.48 g 10 (0.135 mol) dissolved in
100 mL CHCl3 were added over 20 min. The mixture heated to 40 ꢁC for 8 h. After
this time, TLC showed the alcohol was completely consumed. The dark mixture
was cooled to rt, and the precipitated 2,3,5,6-tetrachloro-1,4-hydroquinone
[32.44 g (97%), mp 235–238 ꢁC (HOAc), dH (DMSO): 10.21 (bs, 2 OH); dC (DMSO):
144.11, 120.74] was filtered. The solvent was stripped at the rotovap, and 500 mL
50% hexanes=Et2O were added. The supernatant was decanted away from some dark
residues on the flask wall. The solution was then chromatographed on silica gel to
give the title compound as a yellow-brown oil (22.56 g, 77%). dH (CDCl3): 9.63 (s,
1H), 7.50 (d, J ¼ 9.2, 1H), 7.41 (d, J ¼ 1.0, 1H), 6.77 (dd, J ¼ 8.9 and 2.3 Hz, 1H),
6.71 (d, J ¼ 2.0 Hz, 1H), 3.45 (q, J ¼ 7.2 Hz, 4H), 1.23 (t, J ¼ 7.1 Hz, 6H); dC
(CDCl3): 177.79, 159.92, 151.33, 150.14, 124.18, 116.08, 111.49, 92.73, 45.22, 12.66.
(E)-3-(2-(6-(Diethylamino)benzo[b]furan-2-yl)vinyl)-
5,5-dimethylcyclohex-2-enone (12)
A 250-mL round-bottomed flask equipped with magnetic stirbar and reflux
condenser was charged with 8.4 g 11 (39 mmol), 5.34 g isophorone (39 mmol, 1
equiv), and 50 mL EtOH. Over 5 min, a solution of 1.55 g NaOH (39 mmol, 1 equiv)
in 5 mL H2O was added dropwise. The mixture was heated to reflux for 15 min and
then allowed to cool to rt. The mixture was poured into 200 mL H2O and neutralized
with 2.2 mL glacial HOAc (39 mmol, 1 equiv). The mixture was extracted three times
with 100-mL portions of Et2O. The organic phases were collected and washed
once with 100 mL H2O followed by 100 mL brine. The organic phase was dried over
anhydrous MgSO4 and rotary evaporated to a crude orange solid. Recrystallization
from heptane gave 12.07 g of the title compound as orange needles (93%). Mp
102–104 ꢁC. dH (CDCl3): 7.33 (d, J ¼ 9.0 Hz, 1H), 6.93 (d, Jab ¼ 15.7 Hz, 1H), 6.81
(d, Jab ¼ 16.2 Hz, 1H), 6.71 (d, J ¼ 1.8 Hz, 1H), 6.66 (dd, J ¼ 8.7 and 2.3 Hz, 1H),
6.66 (s, 1H), 6.08 (s, 1H), 3.41 (q, J ¼ 7.1 Hz, 4H), 2.42 (s, 2H), 2.29 (s, 2H), 1.20
(t, J ¼ 6.9 Hz, 6H), 1.09 (s, 6H); dC (CDCl3): 200.12, 158.26, 154.77, 151.71,
147.76, 127.41, 126.54, 122.59, 121.77, 118.15, 109.99, 109.55, 93.35, 51.55, 45.09,
38.99, 33.41, 28.66, 12.72. Elemental analysis calculated for C22H27NO2: C, 78.23;
H, 8.00; N, 4.15. Found: C, 78.21; H, 8.06; N, 4.13.

PREPARATION OF CYAN DYES
3015
(E and Z)-2-(3-((E)-2-(6-(Diethylamino)benzo[b]furan-2-yl)vinyl)-
5,5-dimethylcyclohex-2-enylidene)acetonitrile (13)
A 1-L round-bottomed flask equipped with magnetic stirbar and 200-mL
addition funnel was charged with 200 mL anhydrous THF and 13.3 g anhydrous
MeCN (320mmol, 9 equiv). The apparatus was protected by N2 bubbler and cooled
in a dry ice=acetone bath. The addition funnel was charged with 126 mL of 2.3 M hex-
yllithium in hexanes (290mmol, 8 equiv), which were added over 30min. During the
addition, the lithioacetonitrile precipitated as a white solid. After 20min, a new
addition funnel was equipped and charged with a previously prepared solution of
12.07 g 12 (36 mmol) in 30mL anhydrous THF. The ketone solution was run in over
20 min. The cooling bath was then removed, and the mixture was stirred at rt overnight.
A saturated aqueous solution of NH4Cl (100mL) and 250 mL Et2O was added, and the
mixture was vigorously stirred. The organic layer was separated and washed with
100 mL H2O followed by 100 mL brine. The organic layer was dried over anhydrous
MgSO4 and rotary evaporated to an orange oil, which was the intermediate b-hydro-
xynitrile. The oil was dissolved in 200 mL glacial HOAc in a 500-mL round-bottomed
flask and refluxed for 5 h. After this time, the reaction was complete by TLC. The gla-
cial HOAc and other volatiles were rotary evaporated, leaving a crude orange solid.
Recrystallization from MeCN gave the title compound as orange needles (7.75 g,
60%). Mp 138–140 ꢁC. dH (CDCl3): 7.32 (d, J ¼ 8.7 Hz, 1H), 7.03–6.25 (m, 6H), 5.12
(s, 0.54 H, trans), 4.95 (s, 0.44 H, cis), 3.42 (q, J ¼ 7.0 Hz, 4H, cis), 3.41 (q, J ¼ 7.0 Hz,
4H, trans), 2.49–2.21 (m, 4H), 1.22 (t, J ¼ 7.0 Hz, 3H, cis), 1.21 (t, J ¼ 7.0 Hz, 3H,
trans), 1.06–0.98 (m, 6H); dC (CDCl3): 158.19, 157.69, 152.46, 152.41, 147.56, 145.01,
144.67, 128.69, 128.36, 126.91, 124.70, 121.55, 121.49, 119.94, 119.77, 118.64, 118.56,
110.00, 107.96, 107.87, 93.83, 93.69, 93.32, 91.89, 45.21, 44.87, 42.28, 39.03, 38.93,
31.42, 31.26, 28.37, 28.32, 12.82. Elemental analysis calculated for C24H28N2O: C,
79.96; H, 7.83; N, 7.77. Found: C, 79.84; H, 7.91; N, 7.90.
(E and Z)-2-(3-((E)-2-(6-(Diethylamino)benzo[b]furan-2-yl)vinyl)-
5,5-dimethylcyclohex-2-enylidene)acetaldehyde (14)
A 500-mL round-bottomed flask equipped with a magnetic stirbar was charged
with 7.75 g 13 (22 mmol) and 250 mL toluene. The mixture was cooled in a dry ice =
acetone bath. A 100-mL addition funnel and N2 bubbler were equipped, and the
funnel was charged with 32 mL 1 M diisobutylaluminum hydride in hexanes (32 mmol,
1.5 equiv), which was added over 1 h. The mixture was stirred at rt for 12 h. After this
time, a paste of 30 g SiO2 and 10 mL H2O was added, followed by 1 g oxalic acid dihy-
drate. The mixture was stirred another 12 h to allow complete hydrolysis of the imine
intermediate. The mixture was filtered through celite to remove insolubles. The organic
phase was separated and washed with 100 mL H2O followed by 100 mL brine. After
drying over anhydrous MgSO4, the solvent was rotary evaporated, leaving an orange
oil. The crude product was chromatographed on SiO2 (hexanes=EtOAc) to obain
6.92 g of the title compound as an orange oil (88%). dH (CDCl3): 10.22 (d, J ¼ 8.2 Hz,
0.37 Hz, cis), 10.01 (d, J ¼ 8.4 Hz, 0.63 H, trans), 7.32 (d, J ¼ 8.6 Hz, 1H), 7.02–6.89 (m,
1H), 6.75–6.33 (m, 4H), 5.94 (d, J ¼ 8.4 Hz, 0.66 H, trans), 5.74 (d, J ¼ 8.4 Hz, 0.52 H,
cis), 3.41 (q, J ¼ 7.0 Hz, 4H), 2.71–2.26 (m, 4H), 1.21 (t, J ¼ 7.0 Hz, 6H), 1.08–0.99

3016
M. C. DAVIS, T. J. GROSHENS, AND D. A. PARRISH
(m, 6H); dC (CDCl3): 190.76, 189.73, 158.12, 158.10, 156.45, 156.15, 152.51, 152.48,
147.58, 145.66, 145.54, 129.99, 129.09, 128.79, 127.15, 125.21, 122.63, 121.54, 119.86,
119.68, 118.57, 110.00, 108.07, 108.00, 93.72, 46.37, 45.21, 39.47, 39.18, 39.07, 31.37,
31.27, 29.23, 28.58, 28.45, 22.89, 14.31, 12.82.
2-Amino-4-(6-(diethylamino)benzo[b]furan-2-yl)buts-1,3-diene-
1,1,3-tricarbonitrile (1)
A 250-mL round-bottomed flask equipped with magnetic stirbar and reflux
condenser was charged with 2.14 g 11 (10 mmol), 1.30 g malononitrile dimer (1
equiv), and 50 mL EtOH. The mixture was heated briefly to dissolve all the solids.
In one portion, 1.4 g piperidinium acetate (1 equiv) was added, and the mixture
was refluxed for 1 h. After cooling to rt, the mixture was poured into 500 mL
H2O, causing a dark solid to precipitate. The solid was collected on a medium-
porosity glass frit. After air-drying on the frit, 2.32 g of the title compound were
obtained as a dark green powder, which was shown to be a monohydrate by combus-
tion analysis (66%). Mp 180–182 ꢁC. kmax ¼ 550 nm. dH (DMSO): 8.85 (bs, NH2),
7.78 (s, 1H), 7.64 (s, 1H), 7.59 (d, J ¼ 8.4 Hz, 1H), 6.87 (dd, J ¼ 9.3 and 1.9 Hz,
1H), 6.70 (d, J ¼ 1.1 Hz, 1H), 3.48 (q, J ¼ 7.0 Hz, 4H), 1.16 (t, J ¼ 7.0 Hz, 6H); dH
(DMSO): 165.95, 159.71, 149.92, 146.67, 137.11, 124.05, 121.56, 116.66, 115.58,
111.69, 93.35, 91.24, 48.72, 44.37, 12.35. Elemental analysis calculated for
C19H17N5O ꢃ H2O: C, 65.32; H, 5.48; N, 20.04. Found: C, 65.82; H, 4.86; N, 20.47.
Ethyl 1,1,3-Tricyano-4-(6-(diethylamino)benzo[b]furan-2-yl)buta-
1,3-dien-2-ylcarbamate (2)
A 100-mL round-bottomed flask equipped with magnetic stirbar was charged
with 1 g 1 (3 mmol), 300 mg triethylamine (TEA; 6 mmol, 2 equiv), and 20 mL
CH2Cl2. The mixture was cooled in an ice bath while 327 mg ethyl chloroformate
(1 equiv) were added dropwise. After stirring 30 min, the mixture was poured into
100 mL H2O. The organic phase was separated and washed with 50 mL saturated
aqueous NaHCO3 and then 50 mL brine. The organic phase was dried over anhy-
drous MgSO4 and rotary evaporated to 1.43 g of a dark residue. Recrystallization
of the crude product from a mixture of MeOH and H2O gave 370 mg of the title
compound as copper-colored microcrystals (31%). Mp 172–175 ꢁC. kmax ¼ 599 nm.
dH (CDCl3): 7.62–7.44 (m, 4H), 6.79 (dd, J ¼ 9.1 and 2.3 Hz, 1H), 6.67 (d, J ¼ 1.9 Hz,
1H), 4.34 (q, J ¼ 7.1 Hz, 2H), 3.49 (q, J ¼ 7.4 Hz, 4H), 1.37 (t, J ¼ 7.1 Hz, 3H), 1.26
(t, J ¼ 7.1 Hz, 6H); dC (CDCl3): 161.86, 161.13, 151.52, 150.89, 147.89, 136.52,
124.50, 123.81, 118.54, 115.54, 113.23, 112.87, 112.75, 92.46, 92.18, 65.79, 64.26,
45.58, 14.38, 12.79. Elemental analysis calculated for C22H21N5O3: C, 65.50; H,
5.25; N, 17.36. Found: C, 65.29; H, 5.41; N, 17.47.
Diethyl 1,1,3-Tricyano-4-(6-(diethylamino)benzo[b]furan-2-yl)buta-
1,3-dien-2-yldicarbamate (3)
A 50-mL round-bottomed flask equipped with magnetic stirbar was charged
with 500 mg 1 (1.5 mmol), 20 mL CH2Cl2, and 2.37 g pyridine (30 mmol, 20 equiv).

PREPARATION OF CYAN DYES
3017
The burgundy-colored mixture was cooled in an ice bath, and 1.3 g ethyl chlorofor-
mate (12 mmol, 8 equiv) was added dropwise over 30 min. The color of the reaction
became brilliant blue. After stirring 30 min further, the mixture was poured into
100 mL H2O. The organic layer was separated and washed with 50 mL saturated
aqueous Na2CO3 and finally 50 mL brine. The organic phase was dried over anhy-
drous MgSO4 and rotary evaporated to a 620 mg of a purple residue. Recrystalliza-
tion from EtOH gave the title compound as metallic green crystals. Mp 80–82 ꢁC.
kmax ¼ 655 nm. dH (CDCl3): 7.65–7.40 (m, 3H), 6.82 (dd, J ¼ 9.0 and 2.0 Hz, 1H),
6.64 (s, 1H), 4.38 (q, J ¼ 7.2 Hz, 4H), 3.52 (q, J ¼ 7.2 Hz, 4H), 1.35 (t, J ¼ 7.2 Hz,
6H), 1.28 (t, J ¼ 7.2 Hz, 6H); dC (CDCl3): 162.74, 159.28, 152.51, 149.94, 148.46,
133.87, 125.19, 119.66, 114.94, 113.69, 112.49, 112.35, 95.59, 91.92, 65.15, 45.78,
14.28, 12.89. Elemental analysis calculated for C25H25N5O5: C, 63.15; H, 5.30; N,
14.73. Found: C, 62.79; H, 5.43; N, 14.49.
(E)-2-(3-Cyano-4-(2-(6-(diethylamino)benzo[b]furan-2-yl)vinyl)-
5,5-dimethylfuran-2(5H)-ylidene)malononitrile (4)
A 50-mL round-bottomed flask equipped with a magnetic stirbar and reflux
condenser was charged with 1.3 g 11 (6 mmol), 1.19 g TCF (1 equiv), and 25 mL
EtOH. The mixture was heated briefly to dissolve all the solids. In one portion,
867 mg piperdidinium acetate (1 equiv) was added, and the mixture was refluxed
for 1 h. After cooling to rt, the green microcrystalline powder was filtered on a
medium-porosity glass frit (2.1 g, 88%). No further purification was necessary. Mp
275–277 ꢁC. kmax ¼ 662 nm. dH (CDCl3): 7.53 (d, J ¼ 15.5 Hz, 1H), 7.45 (d,
J ¼ 9.0 Hz, 1H), 7.17 (s, 1H), 6.81 (d, J ¼ 15.5 Hz, 1H), 6.76 (dd, J ¼ 9.0 and
2.3 Hz, 1H), 6.67 (d, J ¼ 1.7 Hz, 1H), 3.49 (q, J ¼ 7.1 Hz, 4H), 1.75 (s, 6H), 1.28 (t,
J ¼ 7.4 Hz, 6H). Elemental analysis calculated for C24H22N4O2: C, 72.34; H, 5.57;
N, 14.06. Found: C, 72.50: H, 5.46; N, 14.07.
(E and Z)-2-Amino-5-(3-((E)-2-(6-(diethylamino)benzo[b]furan-
2-yl)vinyl)-5,5-dimethylcyclohex-2-enylidene)penta-1,3-diene-
1,1,3-tricarbonitrile (5)
A 50-mL round-bottomed flask equipped with magnetic stirbar and reflux con-
denser was charged with 1 g 14 (2.8 mmol), 363 mg malononitrile dimer (1 equiv),
and 15 mL EtOH. The mixture was heated briefly to dissolve all the solids. In one
portion, 400 mg piperidinium acetate (1 equiv) were added, and the mixture was
heated to reflux. After 1 h, the mixture was cooled to rt, and the precipitate was fil-
tered on a medium-porosity glass frit. After vacuum drying, 1.17 g of the title com-
pound was obtained as a green microcrystalline powder (90%). The compound could
be recrystallized from MeCN or toluene to give the title compound as copper-
colored microcrystals. Mp 163–165 ꢁC. kmax ¼ 624 nm. dH (CDCl3): 8.65 (d,
J ¼ 11.7 Hz, 0.15 H, cis), 8.42 (d, J ¼ 12.7 Hz, 0.85 H, trans), 7.33 (d, J ¼ 8.5 Hz,
1H), 7.02–6.91 (m, 1H), 6.80–6.45 (m, 6H), 5.88 (bs, NH2), 3.44 (q, J ¼ 7.4 Hz,
4H), 2.50 (s, 2H), 2.39 (s, 2H), 1.22 (t, J ¼ 7.0 Hz, 6H), 1.06 (s, 6H). Elemental analy-
sis calculated for C30H31N5O: C, 75.44; H, 6.54; N, 14.66. Found: C, 75.51; H, 6.48;
N, 14.62.

3018
M. C. DAVIS, T. J. GROSHENS, AND D. A. PARRISH
(E and Z)-2-(3-Cyano-4-(3-(3-((E)-2-(6-(diethylamino)benzo[b]furan-
2-yl)vinyl)-5,5-dimethylcyclohex-2-enylidene)prop-1-enyl)-
5,5-dimethylfuran-2(5H)-ylidene)malononitrile (6)
A 50-mL round-bottomed flask equipped with magnetic stirbar was charged
with 1 g 14 (2.8 mmol), 547 mg TCF (1 equiv), and 15 mL CH2Cl2. The mixture
was heated briefly to dissolve all the solids. One portion of 400 mg piperidinium acet-
ate (1 equiv) was added, and the mixture was heated to 40 ꢁC for 2 h. During this
time, a dark purple solid precipitated. After cooling to rt, the solid was collected
on a medium-porosity glass frit. After washing with EtOH, the product was air-dried
on the frit (1 g, 67%). The product was recrystallized from MeCN to give the title
compound as microscopic purple needles. Mp 243–245 ꢁC. kmax ¼ 697 nm. dH
(CDCl3): 8.26 (dd, J ¼ 15.1 and 13.7 Hz, 0.15 H, cis), 7.99 (dd, J ¼ 15.1 and
12.5 Hz, 0.85 H, trans), 7.33 (d, J ¼ 8.6 Hz, 1H), 7.05–6.59 (m, 5H), 6.48–6.09 (m,
3H), 3.43 (q, J ¼ 6.7 Hz, 4H), 2.43 (s, 2H), 2.37 (s, 2H), 1.72–1.67 (m, 6H), 1.22 (t,
J ¼ 7.0 Hz, 6H), 1.08–1.03 (m, 6H). Elemental analysis calculated for C33H34N4O2:
C, 76.42; H, 6.61; N, 10.80. Found: C, 76.71; H, 6.66, N, 10.57.
X-Ray Structure Determination of 5 and 6
Some selected data and refinement details are collected in Table 2. CCDC
736600 (5) and CCDC 737420 (6) contain the supplementary crystallographic data
for this article. These data can be obtained free of charge from the Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif, by e-mailing
Table 2. Crystal structure data and refinement details for compounds 5 and 6
Compound
5
6
Empirical formula
Formula weight
Temperature (K)
Crystallization solvent
Crystal system
C30H31N5O
477.60
296
C33H34N4O2
544.68
223(2)
Toluene
Triclinic
P1
Acetonitrile
Monoclinic
P21=n
Space group
Unit cell dimensions (A, ꢁ) a
˚
10.0455(5)
14.2645(7)
20.6577(11)
89.0130(10)
79.9670(10)
79.2660(10)
2863.5(3)
2
8.8392(2)
14.8729(3)
23.5183(5)
90
B
C
a
b
c
95.2460(10)
90
3
˚
Volume (A )
3078.87(11)
4
1.175
Z
Density (g=cm3)
1.153
0.06 ꢄ 0.18 ꢄ 0.35 mm3
Crystal size (mm)
Reflections collected
Independent reflections
Completeness (%)
Data=restraints=parameters
Goodness of fit on F2
Final R [I > 2sigma(I)]
0.21 ꢄ 0.04 ꢄ 0.03 mm3
33504
12902
5111
5161
98.1
12902 = 0 = 697
0.797
91.7
5161 = 0 = 398
1.049
R1 ¼ 0.0486, wR2 ¼ 0.1509
R1 ¼ 0.0407, wR2 ¼ 0.0988

PREPARATION OF CYAN DYES
3019
data_request@ccdc.cam.ac.uk, or by contacting the CCDC at 12 Union Road,
Cambridge CB2 1EZ, UK; fax: þ44 1223 336033.
ACKNOWLEDGMENTS
The authors thank several colleagues from Naval Air Warfare Center:
Dr. Andrew P. Chafin for molecular modeling of structures related to dyes 3
and 6; Brad Douglas for preparing the piperidinium acetate; Dr. Richard A. Hollins
for preparing the TCF acceptor; Dr. Geoffrey A. Lindsay for advice and encourage-
ment throughout the course of this work; Ann M. Moorehead and Linda L.
Robinett of the Technical Library for obtaining Refs. 1 and 25; and Dr. Zachary
A. Sechrist for assistance with UV=vis spectroscopy.
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