The use of N,N-dialkyltrimethylsilylamines in the synthesis of pentamethinium salts

Article abstract of DOI:10.1055/s-0028-1088036

N,N-Dialkyltrimethylsilylamines were used for the synthesis of a series of pentamethinium salts starting from 2,6-disubstituted or 2,4,6-trisubstituted pyrylium salts, N-substituted 5-aminopenta-2,4-dienals or N-substituted 5-aminopenta-2,4-dien-1-ones. The reactions proceed smoothly under mild conditions, furnishing the desired products in good yields. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1088036

PAPER
1291
The Use of N,N-Dialkyltrimethylsilylamines in the Synthesis of
Pentamethinium Salts
Pentamethinium Salts
a
N
,
N
-D
r
ialkyltrim
e
ethylsilyla
k
mines Kořínek, Markéta Rybáčková, Stanislav Böhm*
Department of Organic Chemistry, Institute of Chemical Technology, Prague, Technická 5, 166 28 Prague 6, Czech Republic
Fax +420(2)20443635; E-mail: Stanislav.Bohm@vscht.cz
Received 9 September 2008; revised 15 December 2008
Ph
Abstract: N,N-Dialkyltrimethylsilylamines were used for the syn-
O
O
O
–
thesis of a series of pentamethinium salts starting from 2,6-disubsti-
tuted or 2,4,6-trisubstituted pyrylium salts, N-substituted 5-
aminopenta-2,4-dienals or N-substituted 5-aminopenta-2,4-dien-1-
ones. The reactions proceed smoothly under mild conditions, fur-
nishing the desired products in good yields.
ClO4
+
Ar
Ar
Ph
Ar
Ar
O+
Ar
1a (Ar = Ph, 50%)
1b (Ar = biphenyl-4-yl, 50%)
1c (Ar = naphthalen-2-yl, 36%)
Key words: cyanines, pentamethinium salts, pyrylium salts, ring
opening, amino aldehydes
–
ClO4
+
CH(OEt)3
Ar
O+
Ar
1d (Ar = Ph, 60%)
With their characteristic, positively charged p-electron
system delocalized over an odd number of carbon atoms
and terminal nitrogen groups, streptocyanine dyes have
found numerous applications as advanced materials for
optical devices, laser dyes, nonlinear optics, etc.1 A num-
ber of synthetic procedures leading to pentamethinium
salts have been developed so far, including the action of
tris(dialkylamino)arsanes on pyrylium salts2 and reactions
of primary or secondary alkylamines with carboxonium
salts;3 however, simple and robust methods for facile
modification of the structure and properties of pentame-
thinium salts are still highly desirable. Herein, we de-
scribe an efficient synthesis of pentamethinium salts
employing several commercially available N,N-dialkyltri-
methylsilylamines.
The starting 2,4,6-trisubstituted pyrylium salts 1a,4 1b5
and 1c5 were obtained by the reaction between chalcones
and aryl methyl ketones. The 2,6-disubstituted pyrylium
salts 1d,6 1e and 1f were prepared by a perchloric acid cat-
alyzed reaction of aryl methyl ketones with triethyl ortho-
formate (Scheme 1).
Reaction of pyrylium salts7 1a–f, bearing two or three ar-
omatic substituents, with N,N-dialkyltrimethylsilyl-
amines in anhydrous acetonitrile afforded the
pentamethinium salts 2a–j in very good yield (Scheme 2,
Table 1). Hexamethyldisiloxane was formed as a byprod-
uct. All reactions were carried out under an inert atmo-
sphere at room temperature, except for the synthesis of 2c,
where refluxing of the reaction mixture was required. The
reactions usually proceeded quite rapidly (1–6 h) with al-
most complete conversion. The use of an excess of the
N,N-dialkyltrimethylsilylamine was required in order to
achieve a high yield.
1e (Ar = biphenyl-4-yl, 22%)
1f (Ar = naphthalen-2-yl, 42%)
Scheme 1
R1
R2
N
R2
N+
(R2)2NSiMe3
(3 equiv)
R2
R2
MeCN
r.t.
– (TMS)2O
R1
Ar
Ar
O+
Ar
ClO4
Ar
–
ClO4
–
2a–j
1a–f
Scheme 2
Thus, 3.0 equivalents of the silylated reagent seems opti-
mal. Reactions with 2.0 or 2.5 equivalents of this reagent
led to lower conversions even after prolonged reaction
times; on the other hand, the application of 4.0 or 5.0
equivalents did not significantly enhance the conversion
of the starting pyrylium salts.
In combination with trimethylsilyl chloride, N,N-dialkyl-
trimethylsilylamines could also be used for the conversion
of 5-aminopenta-2,4-dienal 3a8 or 5-(dimethylamino)pen-
ta-2,4-dien-1-ones 3b9 and 3c10 into pentamethinium
chlorides. These chlorides are known to be very hygro-
scopic and were therefore transformed in situ to the de-
sired hexafluorophosphates, in very good yield
(Scheme 3, Table 2). The trimethylsilyl chloride sca-
venges the silyl group originating from the reagent to pro-
duce hexamethyldisiloxane and supplies the chloride
anion. Notably, this reaction also enables the preparation
Me
N
1. Me2NSiMe3
TMSCl
Me
N
Me
N+
O
R1
R1
Me
2. NH4PF6
r.t.
R2
R2
R3
R2
R2
R3
–
PF6
3a–c
4a–c
SYNTHESIS 2009, No. 8, pp 1291–1296
Advanced online publication: 25.03.2009
x
x
.
x
x
.2
0
0
9
Scheme 3
DOI: 10.1055/s-0028-1088036; Art ID: T16008SS
© Georg Thieme Verlag Stuttgart · New York

1292
M. Kořínek et al.
PAPER
Table 1 Prepared Pentamethinium Salts 2a–j
Entry
Starting material Ar
R1
Ph
Ph
Ph
Ph
Ph
H
R2
Time (h)
Product (yielda)
2
1
2
1a
1a
1a
1b
1c
1d
1d
1d
1e
1f
Ph
Me2
3
6
2a (71%)
2b (69%)
2c (51%)
2d (68%)
2e (79%)
2f (82%)
2g (68%)
2h (82%)
2i (78%)
2j (86%)
Ph
(CH2)4
(CH2)2O(CH2)2
Me2
3
Ph
3b
20
24
1
4
biphenyl-4-yl
naphthalen-2-yl
Ph
5
Me2
6
Me2
7
Ph
H
(CH2)4
(CH2)2O(CH2)2
Me2
3
8
Ph
H
4
9
biphenyl-4-yl
naphthalen-2-yl
H
2
10
H
Me2
6
a Yield of isolated product.
b Under reflux.
Table 2 Prepared Pentamethinium Salts 4a–c
Table 3 Prepared Pentamethinium Salts 6a–f
Entry
Starting
material
R1
R2
R3
Product
Entry Starting R1
material
R2
R3
Time Product
2
(yielda)
(h)
0.5 6a (87%)
1 6b (75%)
(yielda)
1
2
3
3a
3b
3c
Ph
H
H
4a (83%)
4b (88%)
4c (69%)
1
2
3
4
5
6
5a
5a
5a
5b
H
H
H
H
H
H
H
Et
Me2
Me
Me
H
Ph
Ph
(CH2)4
Ph
(CH2)2O(CH2)2 0.5 6c (80%)
a Yield of isolated product.
Me2
1
1
2
6d (86%)
6e (79%)
6f (45%)
5c + 5c¢ H/Ph Ph/H Me2
5d Me Me Me2
of unsymmetrical pentamethinium salts bearing two dif-
ferent dialkylamino groups, e.g. compound 4a.
a Yield of isolated product.
Similarly, 5-(2,4-dinitrophenylamino)penta-2,4-dienals
could serve as starting materials in the reactions with N,N-
dialkyltrimethylsilylamines and trimethylsilyl chloride
(Scheme 4, Table 3). In this case, both substituents at the
ends of the conjugated carbon chain are displaced by the
dialkylamino groups. 5-(2,4-Dinitrophenylamino)penta-
2,4-dienals are easily accessible from N-(2,4-dinitrophe-
nyl)pyridinium salts by a nucleophilic ring-opening reac-
tion with aqueous sodium carbonate.11 It should be noted
that N-(2,4-dinitrophenyl)pyridinium salts can be trans-
formed to pentamethinium salts in a single step upon re-
action with secondary amines;12 however, this strategy is
not generally applicable. For example, attempted prepara-
tion of pentamethinium salt 6f from N-(2,4-dinitrophe-
nyl)-3,5-dimethylpyridinium chloride and dimethylamine
failed. Following the two-step protocol, salt 6f was
formed in a decent yield from aldehyde 5d and N,N-di-
methyltrimethylsilylamine. In the case of compound 6e,
an inseparable mixture of regioisomers 5c and 5c¢ was
used as starting material.
In conclusion, we have developed a novel and efficient
synthetic procedure leading to a number of pentamethini-
um salts using commercially available N,N-dialkyltri-
methylsilylamines.
2,6-Disubstituted
or
2,4,6-
trisubstituted pyrylium salts, N-substituted 5-aminopenta-
2,4-dienals and N-substituted 5-aminopenta-2,4-dien-1-
ones were used as starting materials. The advantages of
this method are the mild reaction conditions, simple work-
up and good yields. It is also noteworthy that our method
enables variation of the terminal amino groups, as well as
the preparation of unsymmetrical pentamethinium salts.
R1
R2
1. (R3)2NSiMe3
TMSCl
R3
N
R1
R2
R3
N+
H
N
O
R3
R3
2. NH4PF6
r.t.
–
O2N
NO2
PF6
5a–d
6a–f
Scheme 4
Synthesis 2009, No. 8, 1291–1296 © Thieme Stuttgart · New York

PAPER
Pentamethinium Salts from N,N-Dialkyltrimethylsilylamines
1293
5-(2,4-Dinitrophenylamino)-2-ethylpenta-2,4-dienal (5b) and 5-
(2,4-Dinitrophenylamino)-4-ethylpenta-2,4-dienal (5b¢)
All reactions were carried out in standard glassware under a nitro-
gen atmosphere. The N,N-dialkyltrimethylsilylamines are commer-
cially available (Aldrich Chemical Company) and were used
without further purification. Melting points were determined on a
Kofler apparatus and are uncorrected. NMR spectra were measured
A soln of N-(2,4-dinitrophenyl)-3-ethylpyridinium chloride14 (1.00
g, 3.23 mmol) and K2CO3 (1.79 g, 12.9 mmol) in H2O (20 mL) was
stirred at 50 °C for 2 h. The precipitated orange solid was collected
by filtration and washed with H2O (60 mL). The solid product was
dissolved in a small amount of CH2Cl2 and precipitated with hexane
to yield an orange powder, which was identified as the 2-ethyl re-
gioisomer 5b. The mother liquor was concentrated under reduced
pressure and precipitated with hexane to yield a red powder, which
was identified as the 4-ethyl regioisomer 5b¢.
on a Varian Gemini 300HC spectrometer (1H at 300 MHz and 13
C
at 75 MHz), a Bruker AMX-3 400 spectrometer (1H at 400 MHz
and 13C at 100 MHz) or a Bruker Avance DRX 500 spectrometer
(1H at 500 MHz and 13C at 125 MHz). Chemical shifts (in ppm, d
scale) are reported relative to residual solvent signals as internal
standard; coupling constants (J) are given in Hz. UV/Vis spectra
were recorded on a HP 8452A spectrophotometer at 25 °C using
CHCl3 as solvent.
5b
Orange powder; yield: 590 mg (63%); mp 147–149 °C (hexane).
The syntheses of pyrylium salts 1a,4 1b,5 1c5 and 1d,6 aminopenta-
dienals 3a,8 5a11 and 5d,13 and aminopentadienones 3b9 and 3c10
have been described previously.
1H NMR (500 MHz, CDCl3): d = 1.06 (t, J = 7.6 Hz, 3 H), 2.41 (q,
J = 7.6 Hz, 2 H), 6.54 (dd, J = 13.1, 11.6 Hz, 1 H), 6.89 (d, J = 11.6
Hz, 1 H), 7.30 (dd, J = 13.1, 11.5 Hz, 1 H), 7.36 (d, J = 9.4 Hz, 1
H), 8.42 (ddd, J = 9.4, 2.7, 0.7 Hz, 1 H), 9.20 (d, J = 2.7 Hz, 1 H),
9.44 (s, 1 H), 10.51 (br d, J = 11.5 Hz, 1 H).
13C NMR (125 MHz, CDCl3): d = 13.7 (CH3), 17.7 (CH2), 112.1
(CH), 115.0 (CH), 124.0 (CH), 130.5 (CH), 131.9 (CH), 132.2 (C),
139.1 (C), 142.0 (C), 143.0 (C), 145.1 (CH), 193.4 (CH).
CAUTION! Although no difficulties were encountered in our labo-
ratory, it is good to keep in mind that organic perchlorates are po-
tentially hazardous. Pyrylium perchlorates are known to be
explosive (impact sensitive). Proper safety precautions should be
taken during their synthesis, storage and handling. The dry perchlo-
rates should be handled with great care and should never be crushed,
rubbed or pushed through a narrow opening.
Anal. Calcd for C13H13N3O5: C, 53.61; H, 4.50; N, 14.43. Found: C,
53.39; H, 4.61; N, 14.24.
2,6-Dibiphenyl-4-ylpyrylium Perchlorate (1e)
To a soln of 4-acetylbiphenyl (6.48 g, 33.0 mmol) and CH(OEt)3
(20 mL) in Et2O (25 mL) was added 70% HClO4 (1.3 mL) in
CH(OEt)3 (8 mL). After 5 min at r.t., distilled H2O (2 mL) was add-
ed and the reaction mixture was stirred for an additional 30 min and
then left in a refrigerator overnight. The separated crystals were col-
lected by filtration, washed with Et2O, recrystallized twice from
MeCN–toluene (9:1) and dried under reduced pressure.
5b¢
Red powder; yield: 150 mg (16%); mp 115–118 °C (hexane).
1H NMR (500 MHz, CDCl3): d = 1.20 (t, J = 7.7 Hz, 3 H), 2.50 (q,
J = 7.7 Hz, 2 H), 6.27 (ddd, J = 15.5, 7.7, 0.5 Hz, 1 H), 7.14 (d,
J = 11.8 Hz, 1 H), 7.17 (d, J = 15.5 Hz, 1 H), 7.38 (d, J = 9.5 Hz, 1
H), 8.43 (ddd, J = 9.5, 2.6, 0.7 Hz, 1 H), 9.21 (d, J = 2.6 Hz, 1 H),
9.61 (d, J = 7.7 Hz, 1 H), 10.67 (br d, J = 11.8 Hz, 1 H).
13C NMR (125 MHz, CDCl3): d = 11.9 (CH3), 19.1 (CH2), 115.0
(CH), 124.0 (CH), 126.0 (C), 126.5 (CH), 129.7 (CH), 130.6 (CH),
132.3 (C), 139.2 (C), 141.9 (C), 152.3 (CH), 193.0 (CH).
Orange crystals; yield: 1.76 g (22%); mp 270–273 °C (MeCN–
toluene).
1H NMR (500 MHz, CD3CN): d = 7.50 (t, J = 7.2 Hz, 2 H), 7.56 (t,
J = 7.4 Hz, 4 H), 7.82 (d, J = 7.4 Hz, 4 H), 8.04 (d, J = 8.5 Hz, 4 H),
8.42 (d, J = 8.5 Hz, 4 H), 8.49 (d, J = 8.3 Hz, 2 H), 8.89 (t, J = 8.3
Hz, 1 H).
Anal. Calcd for C13H13N3O5: C, 53.61; H, 4.50; N, 14.43. Found: C,
53.89; H, 4.38; N, 14.16.
13C NMR (125 MHz, CD3CN): d = 120.1 (CH), 128.3 (C), 128.4
(CH), 129.5 (CH), 130.2 (CH), 130.3 (CH), 130.4 (CH), 139.5 (C),
148.9 (C), 157.3 (CH), 172.6 (C).
5-(2,4-Dinitrophenylamino)-2-phenylpenta-2,4-dienal (5c) and
5-(2,4-Dinitrophenylamino)-4-phenylpenta-2,4-dienal (5c¢)
A soln of N-(2,4-dinitrophenyl)-3-phenylpyridinium chloride15
(250 mg, 0.70 mmol) and K2CO3 (1.00 g, 7.24 mmol) in H2O (12
mL) was heated to 50 °C and then stirred at this temperature for 1
h. After cooling, the precipitated product was collected by filtration,
washed with H2O (50 mL) and dried under reduced pressure. A mix-
ture of regioisomers 5c and 5c¢ (7:3) was obtained.
Anal. Calcd for C29H21ClO5: C, 71.83; H, 4.36. Found: C, 71.60; H,
4.42.
2,6-Dinaphthalen-2-ylpyrylium Perchlorate (1f)
To a soln of 2-acetylnaphthalene (5.62 g, 33.0 mmol) in CH(OEt)3
(17 mL) was added 70% HClO4 (1.3 mL). The reaction mixture was
stirred at r.t. for 3 h. Then, it was concentrated to dryness on a rota-
tory evaporator, and the residue was crystallized from MeCN–
toluene (2:1) and dried under reduced pressure.
Yield: 140 mg (59%); mp 110–114 °C (H2O).
1H NMR (300 MHz, CDCl3): d = 5.83 (dd, J = 15.4, 7.7 Hz, 1 H,
5c¢), 6.41 (dd, J = 12.6, 12.1 Hz, 1 H, 5c), 7.10–7.53 (m, 16 H, 5c +
5c¢), 8.34 (d, J = 9.3 Hz, 2 H, 5c + 5c¢), 9.02 (d, J = 2.8 Hz, 1 H, 5c¢),
9.06 (d, J = 2.7 Hz, 1 H, 5c), 9.55 (d, J = 7.7 Hz, 1 H, 5c¢), 9.58 (s,
1 H, 5c), 10.33 (br s, 2 H, 5c + 5c¢).
Red crystals; yield: 2.98 g (42%); mp 262–265 °C (MeCN–
toluene).
1H NMR (400 MHz, CD3CN): d = 7.74 (t, J = 7.0 Hz, 2 H), 7.80 (t,
J = 6.9 Hz, 2 H), 8.08 (d, J = 7.8 Hz, 2 H), 8.22–8.29 (m, 4 H), 8.35
(dd, J = 8.8, 1.9 Hz, 2 H), 8.60 (d, J = 8.3 Hz, 2 H), 8.94 (t, J = 8.3
Hz, 1 H), 9.02 (s, 2 H).
13C NMR (100 MHz, CD3CN): d = 120.5 (CH), 123.9 (CH), 126.9
(C), 129.1 (CH), 129.2 (CH), 131.1 (CH), 131.3 (CH), 131.7 (CH),
132.7 (CH), 133.8 (C), 137.4 (C), 157.3 (CH), 173.2 (C).
Anal. Calcd for C17H13N3O5·0.5 H2O: C, 58.62; H, 4.05; N, 12.06.
Found: C, 58.88; H, 4.08; N, 12.01.
Pentamethinium Salts 2a–j; General Procedure
To a stirred soln of a pyrylium perchlorate 1a–f (300 mg, 1.0 equiv)
in anhyd MeCN (15 mL) was added a N,N-dialkyltrimethylsilyl-
amine (3.0 equiv) under inert atmosphere. The reaction mixture was
stirred at r.t. for 1–24 h (see Table 1), then concentrated to dryness
on a rotatory evaporator. The residue was crystallized from the ap-
propriate solvent.
Anal. Calcd for C25H17ClO5: C, 69.37; H, 3.96. Found: C, 69.45; H,
3.92.
Synthesis 2009, No. 8, 1291–1296 © Thieme Stuttgart · New York

1294
M. Kořínek et al.
PAPER
1,5-Bis(dimethylamino)-1,3,5-triphenylpentamethinium
1H NMR (300 MHz, CDCl3): d = 2.96 (s, 6 H), 3.39 (s, 6 H), 6.05
(dd, J = 13.8, 11.8 Hz, 1 H), 6.31 (d, J = 12.8 Hz, 2 H), 6.96 (dd,
J = 7.6, 1.5 Hz, 4 H), 7.19–7.32 (m, 6 H).
Perchlorate (2a)2
Red crystals; yield: 250 mg (71%); mp 191–193 °C (i-PrOH) (Lit.2
195 °C).
13C NMR (75 MHz, CDCl3): d = 41.1 (CH3), 43.5 (CH3), 106.6
(CH), 128.1 (CH), 128.4 (CH), 129.7 (CH), 132.5 (C), 161.7 (CH),
169.9 (C).
1H NMR (300 MHz, CDCl3): d = 2.85 (br s, 12 H), 5.32 (s, 2 H),
7.13–7.39 (m, 15 H).
13C NMR (75 MHz, CDCl3): d = 42.5 (CH3), 109.3 (CH), 128.1
(CH), 128.8 (2 × CH), 129.4 (CH), 130.0 (CH), 130.4 (CH), 134.4
(C), 141.1 (C), 168.8 (C), 169.0 (C).
UV/Vis (CHCl3): lmax (e) = 446 nm (95700).
1,5-Diphenyl-1,5-dipyrrolidin-1-ylpentamethiniumPerchlorate
(2g)
Orange crystals; yield: 280 mg (68%); mp 246–248 °C (EtOH).
UV/Vis (CHCl3): lmax (e) = 498 nm (32200).
1,3,5-Triphenyl-1,5-dipyrrolidin-1-ylpentamethinium Perchlo-
rate (2b)2
Red crystals; yield: 270 mg (69%); mp 170–172 °C (i-PrOH).
1H NMR (300 MHz, CDCl3): d = 1.83 (br s, 4 H), 2.00 (br s, 4 H),
3.08 (br s, 4 H), 3.26 (br s, 4 H), 5.26 (s, 2 H), 7.00–7.40 (m, 15 H).
13C NMR (75 MHz, CDCl3): d = 24.9 (CH2), 25.2 (CH2), 50.8
(CH2), 52.5 (CH2), 108.4 (CH), 127.7 (CH), 128.5 (CH), 128.8
(CH), 129.1 (CH), 129.5 (CH), 129.6 (CH), 135.4 (C), 141.5 (C),
164.9 (C), 168.5 (C).
1H NMR (500 MHz, CDCl3): d = 1.90 (quin, J = 7.0 Hz, 4 H), 2.15
(quin, J = 7.0 Hz, 4 H), 3.24 (t, J = 7.0 Hz, 4 H), 3.75 (t, J = 7.0 Hz,
4 H), 6.15–6.25 (m, 3 H), 7.02 (dd, J = 7.4, 1.9 Hz, 4 H), 7.18–7.30
(m, 6 H).
13C NMR (75 MHz, CDCl3): d = 24.8 (CH2), 25.0 (CH2), 49.9
(CH2), 52.5 (CH2), 106.6 (CH), 127.6 (CH), 128.4 (CH), 129.6
(CH), 133.2 (C), 159.6 (CH), 166.7 (C).
UV/Vis (CHCl3): lmax (e) = 450 nm (104300).
Anal. Calcd for C25H29ClN2O4: C, 65.71; H, 6.40; N, 6.13. Found:
C, 65.57; H, 6.10; N, 6.20.
UV/Vis (CHCl3): lmax (e) = 496 nm (43200).
1,5-Dimorpholin-4-yl-1,3,5-triphenylpentamethinium
Perchlorate (2c)2
Red crystals; yield: 210 mg (51%); mp 215–218 °C (i-PrOH).
1,5-Dimorpholin-4-yl-1,5-diphenylpentamethinium Perchlor-
ate (2h)
Orange crystals; yield: 360 mg (82%); mp 234–235 °C (EtOH).
1H NMR (300 MHz, CDCl3): d = 3.23 (br s, 8 H), 3.69 (br s, 8 H),
1H NMR (300 MHz, CDCl3): d = 3.27 (br s, 4 H), 3.60 (br s, 4 H),
3.90 (br s, 4 H), 3.93 (br s, 4 H), 6.19 (t, J = 12.7 Hz, 1 H), 6.66 (d,
J = 12.7 Hz, 2 H), 6.99 (d, J = 7.2 Hz, 4 H), 7.20–7.36 (m, 6 H).
13C NMR (125 MHz, CDCl3): d = 48.8 (CH2), 51.3 (CH2), 66.2
(CH2), 66.8 (CH2), 107.8 (CH), 128.4 (CH), 128.6 (CH), 130.1
(CH), 131.9 (C), 163.3 (CH), 169.1 (C).
5.65 (s, 2 H), 7.15–7.45 (m, 15 H).
13C NMR (100 MHz, CDCl3): d = 51.0 (CH2), 66.1 (CH2), 111.4
(CH), 128.3 (CH), 129.2 (CH), 129.7 (CH), 130.1 (CH), 130.6
(CH), 130.8 (CH), 133.9 (C), 140.4 (C), 168.7 (C), 170.9 (C).
UV/Vis (CHCl3): lmax (e) = 512 nm (27100).
UV/Vis (CHCl3): lmax (e) = 458 nm (100700).
1,5-Dibiphenyl-4-yl-1,5-bis(dimethylamino)-3-phenylpenta-
methinium Perchlorate (2d)
Red crystals; yield: 230 mg (68%); mp 183–185 °C (i-PrOH).
Anal. Calcd for C25H29ClN2O6: C, 61.41; H, 5.98; N, 5.73. Found:
C, 61.24; H, 5.81; N, 5.57.
1H NMR (300 MHz, CDCl3): d = 2.90 (br s, 12 H), 5.43 (s, 2 H),
7.17–7.61 (m, 23 H).
1,5-Dibiphenyl-4-yl-1,5-bis(dimethylamino)pentamethinium
Perchlorate (2i)
Yellow crystals; yield: 270 mg (78%); mp 242–245 °C (EtOH).
13C NMR (75 MHz, CDCl3): d = 42.5 (CH3), 109.9 (CH), 127.0
(CH), 127.4 (CH), 128.1 (CH), 128.2 (CH), 129.0 (CH), 129.6
(CH), 130.0 (CH), 130.2 (CH), 133.1 (C), 139.5 (C), 140.9 (C),
143.3 (C), 168.9 (C), 169.0 (C).
1H NMR (300 MHz, CDCl3): d = 3.02 (br s, 6 H), 3.42 (br s, 6 H),
6.26–6.44 (m, 3 H), 7.09 (d, J = 8.5 Hz, 4 H), 7.19–7.35 (m, 10 H),
7.45 (d, J = 8.5 Hz, 4 H).
UV/Vis (CHCl3): lmax (e) = 502 nm (29100).
13C NMR (100 MHz, CDCl3): d = 41.2 (CH3), 43.5 (CH3), 107.3
(CH), 126.9 (CH), 127.0 (CH), 127.9 (CH), 128.9 (CH), 129.0
(CH), 131.4 (C), 139.3 (C), 142.9 (C), 161.8 (CH), 169.9 (C).
Anal. Calcd for C39H37ClN2O4: C, 73.98; H, 5.89; N, 4.42. Found:
C, 73.62; H, 5.71; N, 4.12.
UV/Vis (CHCl3): lmax (e) = 450 nm (82500).
1,5-Bis(dimethylamino)-1,5-dinaphthalen-2-yl-3-phenylpenta-
methinium Perchlorate (2e)
Red crystals; yield: 270 mg (79%); mp 174–176 °C (i-PrOH).
Anal. Calcd for C33H33ClN2O4: C, 71.15; H, 5.97; N, 5.03. Found:
C, 70.84; H, 6.15; N, 4.96.
1H NMR (300 MHz, CDCl3): d = 2.78 (br s, 12 H), 5.42 (s, 2 H),
6.96 (s, 2 H), 7.15–8.05 (m, 17 H).
1,5-Bis(dimethylamino)-1,5-dinaphthalen-2-ylpentamethinium
Perchlorate (2j)
Yellow crystals; yield: 300 mg (86%); mp 213–216 °C (acetone–
Et2O).
1H NMR (300 MHz, CDCl3): d = 2.97 (s, 6 H), 3.43 (s, 6 H), 5.92–
6.18 (m, 1 H), 6.38 (t, J = 12.5 Hz, 2 H), 6.84–7.70 (m, 14 H).
13C NMR (75 MHz, CDCl3): d = 42.4 (CH3), 109.8 (CH), 125.9
(CH), 127.0 (CH), 127.5 (CH), 127.7 (CH), 127.8 (CH), 128.5
(CH), 128.6 (CH), 129.2 (CH), 129.5 (CH), 129.9 (CH), 131.6 (C),
132.5 (C), 133.5 (C), 140.9 (C), 168.8 (C), 169.2 (C).
UV/Vis (CHCl3): lmax (e) = 502 nm (34300).
13C NMR (75 MHz, CDCl3): d = 40.9 (CH3), 43.5 (CH3), 106.5
(CH), 124.5 (CH), 126.9 (CH), 127.1 (CH), 127.3 (CH), 127.7
(CH), 127.9 (CH), 128.2 (CH), 129.8 (C), 131.7 (C), 132.7 (C),
162.2 (CH), 170.2 (C).
Anal. Calcd for C35H33ClN2O4: C, 72.34; H, 5.72; N, 4.82. Found:
C, 71.94; H, 5.86; N, 4.59.
1,5-Bis(dimethylamino)-1,5-diphenylpentamethinium
Perchlorate (2f)2
UV/Vis (CHCl3): lmax (e) = 448 nm (87100).
Orange crystals; yield: 300 mg (82%); mp 235–238 °C (i-PrOH).
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Pentamethinium Salts from N,N-Dialkyltrimethylsilylamines
1295
Anal. Calcd for C29H29ClN2O4: C, 68.97; H, 5.79; N, 5.55. Found:
C, 69.27; H, 5.93; N, 5.50.
tion of Et2O (20 mL) yielded crude product, which was purified by
crystallization from the appropriate solvent (6a–e) or by column
chromatography (6f; silica gel 40, CH2Cl2–EtOH, 1:0 → 4:1).
Pentamethinium Salts 4a–c; General Procedure
To a stirred soln of compound 3a,8 3b9 or 3c10 (300 mg, 1.0 equiv)
in anhyd MeCN (10 mL), N,N-dimethyltrimethylsilylamine (1.2
equiv) and TMSCl (1.2 equiv) were added successively under inert
atmosphere. After 2 h at r.t., NH4PF6 (1.0 equiv) in MeCN (20 mL)
was added to the reaction mixture. The resulting precipitate was re-
moved by filtration, and the solution was concentrated on a rotary
evaporator and Et2O (30 mL) was added. The precipitated product
was collected by filtration and recrystallized from the appropriate
solvent.
1,5-Bis(dimethylamino)pentamethinium Hexafluorophosphate
(6a)16
Yellow crystals; yield: 295 mg (87%); mp 215–217 °C (i-PrOH)
(Lit.16 198 °C).
1H NMR (300 MHz, DMSO-d6): d = 3.04 (s, 6 H), 3.25 (s, 6 H),
5.79 (t, J = 11.8 Hz, 2 H), 7.37 (t, J = 12.7 Hz, 1 H), 7.72 (d,
J = 11.6 Hz, 2 H).
13C NMR (100 MHz, DMSO-d6): d = 37.6 (CH3), 45.3 (CH3), 102.7
(CH), 161.6 (CH), 161.8 (CH).
1-(Dimethylamino)-5-(N-methyl-N-phenylamino)pentamethin-
ium Hexafluorophosphate (4a)
Orange crystals; yield: 480 mg (83%); mp 120–121 °C (EtOH).
1H NMR (300 MHz, DMSO-d6, 80 °C): d = 3.18 (s, 3 H), 3.37 (s, 3
H), 3.49 (s, 3 H), 5.99 (t, J = 12.1 Hz, 1 H), 6.13 (t, J = 11.8 Hz, 1
H), 7.30 (t, J = 7.2 Hz, 1 H), 7.38–7.50 (m, 4 H), 7.64 (t, J = 12.6
Hz, 1 H), 7.92 (d, J = 11.3 Hz, 1 H), 8.02 (d, J = 12.1 Hz, 1 H).
UV/Vis (CHCl3): lmax (e) = 412 nm (108100).
Anal. Calcd for C9H17F6N2P: C, 36.25; H, 5.75; N, 9.39. Found: C,
36.03; H, 5.80; N, 9.29.
1,5-Dipyrrolidin-1-ylpentamethinium Hexafluorophosphate
(6b)
Yellow crystals; yield: 300 mg (75%); mp 208–210 °C (EtOH).
13C NMR (100 MHz, DMSO-d6, 80 °C): d = 38.5 (CH3), 40.3
(CH3), 46.1 (CH3), 105.1 (CH), 106.9 (CH), 121.4 (CH), 126.4
(CH), 129.5 (CH), 144.6 (C), 156.6 (CH), 163.0 (CH), 164.1 (CH).
1H NMR (300 MHz, DMSO-d6): d = 1.86–2.00 (m, 8 H), 3.41 (t,
J = 6.6 Hz, 4 H), 3.67 (t, J = 6.6 Hz, 4 H), 5.70 (t, J = 12.1 Hz, 2 H),
7.38 (t, J = 12.7 Hz, 1 H), 7.90 (d, J = 11.6 Hz, 2 H).
13C NMR (100 MHz, DMSO-d6): d = 24.3 (2 × CH2), 47.7 (CH2),
UV/Vis (CHCl3): lmax (e) = 430 nm (93000).
53.1 (CH2), 103.9 (CH), 157.2 (CH), 160.9 (CH).
Anal. Calcd for C14H19F6N2P: C, 46.67; H, 5.32; N, 7.78. Found: C,
46.55; H, 5.17; N, 7.68.
UV/Vis (CHCl3): lmax (e) = 424 nm (116200).
Anal. Calcd for C13H21F6N2P: C, 44.58; H, 6.04; N, 8.00. Found: C,
44.41; H, 6.21; N, 7.87.
1,5-Bis(dimethylamino)-1-phenylpentamethinium Hexafluoro-
phosphate (4b)
Light orange crystals; yield: 490 mg (88%); mp 206–207 °C
(EtOH).
1H NMR (300 MHz, CDCl3): d = 2.99 (s, 3 H), 3.09 (s, 3 H), 3.22
(s, 3 H), 3.37 (s, 3 H), 5.88 (t, J = 12.1 Hz, 1 H), 6.17 (t, J = 12.6
Hz, 1 H), 6.72 (t, J = 12.6 Hz, 1 H), 7.07 (d, J = 11.5 Hz, 1 H), 7.20–
7.26 (m, 2 H), 7.51–7.60 (m, 3 H).
13C NMR (75 MHz, DMSO-d6): d = 37.8 (CH3), 40.4 (CH3), 42.7
(CH3), 45.3 (CH3), 104.0 (CH), 104.6 (CH), 128.5 (CH), 129.0
(CH), 130.1 (CH), 132.8 (C), 160.1 (CH), 162.2 (CH), 168.7 (C).
1,5-Dimorpholin-4-ylpentamethinium Hexafluorophosphate
(6c)
Yellow crystals; yield: 350 mg (80%); mp 195–197 °C (EtOH).
1H NMR (300 MHz, DMSO-d6): d = 3.60 (m, 8 H), 3.69 (m, 8 H),
5.95 (t, J = 12.2 Hz, 2 H), 7.46 (t, J = 12.7 Hz, 1 H), 7.79 (d,
J = 11.8 Hz, 2 H).
13C NMR (125 MHz, DMSO-d6): d = 46.4 (CH2), 53.8 (CH2), 65.3
(CH2), 66.3 (CH2), 102.6 (CH), 160.2 (CH), 163.0 (CH).
UV/Vis (CHCl3): lmax (e) = 418 nm (95600).
UV/Vis (CHCl3): lmax (e) = 432 nm (96000).
Anal. Calcd for C13H21F6N2O2P: C, 40.84; H, 5.54; N, 7.33. Found:
C, 41.10; H, 5.45; N, 7.25.
Anal. Calcd for C15H21F6N2P: C, 48.13; H, 5.66; N, 7.49. Found: C,
48.28; H, 5.39; N, 7.51.
1,5-Bis(dimethylamino)-2-ethylpentamethinium Hexafluoro-
phosphate (6d)
Orange crystals; yield: 290 mg (86%); mp 126–127 °C (i-PrOH).
1,5-Bis(dimethylamino)-1,3,5-triphenylpentamethinium
Hexafluorophosphate (4c)
Red crystals; yield: 310 mg (69%); mp 115–120 °C (EtOH).
1H NMR (300 MHz, CDCl3): d = 2.85 (br s, 12 H), 5.31 (s, 2 H),
7.15–7.28 (m, 9 H), 7.36–7.39 (m, 6 H).
13C NMR (75 MHz, CDCl3): d = 42.4 (CH3), 109.3 (CH), 128.2
(CH), 128.7 (CH), 129.0 (CH), 129.5 (CH), 130.1 (CH), 130.5
(CH), 134.4 (C), 141.2 (C), 169.1 (C), 169.8 (C).
1H NMR (300 MHz, CDCl3): d = 1.11 (t, J = 7.4 Hz, 3 H), 2.45 (q,
J = 7.4 Hz, 2 H), 3.10 (s, 3 H), 3.31 (s, 3 H), 3.33 (s, 6 H), 5.55 (t,
J = 12.2 Hz, 1 H), 7.19 (s, 1 H), 7.37 (d, J = 12.7 Hz, 1 H), 7.52 (d,
J = 11.6 Hz, 1 H).
13C NMR (75 MHz, DMSO-d6): d = 14.5 (CH3), 18.2 (CH2), 37.9
(CH3), 45.6 (CH3), 46.0 (CH3), 99.1 (CH), 116.0 (C), 161.7 (CH),
161.9 (CH), 164.5 (CH).
UV/Vis (CHCl3): lmax (e) = 498 nm (31000).
Anal. Calcd for C27H29F6N2P: C, 61.59; H, 5.55; N, 5.32. Found: C,
61.75; H, 5.78; N, 5.10.
UV/Vis (CHCl3): lmax (e) = 416 nm (100200).
Anal. Calcd for C11H21F6N2P: C, 40.49; H, 6.49; N, 8.59. Found: C,
40.47; H, 6.69; N, 8.48.
Pentamethinium Salts 6a–f; General Procedure
An aminopentadienal 5a–d (300 mg, 1.0 equiv) was dissolved or
suspended in anhyd MeCN (10 mL). A N,N-dialkyltrimethylsilyl-
amine (2.1 equiv) and, after 5 min, TMSCl (2.1 equiv) were added.
The reaction mixture was maintained at r.t. for 0.5–2 h (see Table 3)
and then NH4PF6 (1.0 equiv) dissolved in MeCN (10 mL) was add-
ed. The resulting inorganic precipitate was removed by filtration
and the filtrate was concentrated on a rotary evaporator. The addi-
1,5-Bis(dimethylamino)-2-phenylpentamethinium Hexafluoro-
phosphate (6e)
Yellow crystals; yield: 260 mg (79%); mp 152–154 °C (EtOH).
1H NMR (300 MHz, CDCl3): d = 2.57 (br s, 3 H), 2.86 (br s, 3 H),
3.28 (br s, 3 H), 3.31 (br s, 3 H), 5.10 (t, J = 11.5 Hz, 1 H), 7.15–
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M. Kořínek et al.
PAPER
7.20 (m, 2 H), 7.36–7.44 (m, 3 H), 7.53 (s, 1 H), 7.61 (d, J = 11.5
Hz, 1 H), 7.75 (d, J = 12.6 Hz, 1 H).
13C NMR (100 MHz, DMSO-d6): d = 37.4 (CH3), 38.9 (CH3), 45.4
(CH3), 100.1 (CH), 115.8 (C), 127.9 (CH), 128.8 (CH), 130.4 (CH),
135.2 (C), 160.3 (CH), 161.7 (CH), 163.9 (CH).
(2) Madaule, Y.; Ramarohetra, M.; Wolf, J.-G.; Declercq, J.-P.;
Dubourg, A. Angew. Chem. Int. Ed. 1991, 30, 994.
(3) Obaya, N.; Payrastre, C.; Madaule, Y. Tetrahedron 2001,
57, 9137.
(4) VanAllan, J. A.; Reynolds, G. A. J. Org. Chem. 1968, 33,
1102.
(5) Simalty-Siemiatycki, M. Bull. Soc. Chim. Fr. 1965, 1944.
(6) Farcasiu, D.; Sharma, S. J. Org. Chem. 1991, 56, 126.
(7) Balaban, T. S.; Balaban, A. T. In Science of Synthesis,
Houben–Weyl Methods of Molecular Transformations, Vol.
14; Thomas, E. J., Ed.; Georg Thieme: Stuttgart, 2003, 11.
(8) Grewe, R.; von Bonin, W. Chem. Ber. 1961, 94, 234.
(9) Jutz, C.; Wagner, R.-M.; Kraatz, A.; Löbering, H.-G. Justus
Liebigs Ann. Chem. 1975, 874.
(10) Balaban, A. T.; Silhan, W. Tetrahedron 1970, 26, 743.
(11) Becher, J. Synthesis 1980, 589.
(12) (a) Parikh, I.; Hilpert, H.; Hermann, K.; Dreiding, A. S.
Helv. Chim. Acta 1986, 69, 1588. (b) Wypych, J.-C.;
Nguyen, T. M.; Bénéchie, M.; Marazano, C. J. Org. Chem.
2008, 73, 1169.
UV/Vis (CHCl3): lmax (e) = 416 nm (105000).
Anal. Calcd for C15H21F6N2P: C, 48.12; H, 5.66; N, 7.49. Found: C,
47.74; H, 5.50; N, 7.45.
1,5-Bis(dimethylamino)-2,4-dimethylpentamethinium
Hexafluorophosphate (6f)
Orange crystals; yield: 150 mg (45%); mp 113–116 °C (CH2Cl2–
EtOH).
1H NMR (300 MHz, DMSO-d6): d = 2.12 (s, 6 H), 3.27 (s, 12 H),
7.04 (s, 1 H), 7.33 (br s, 2 H).
13C NMR (75 MHz, DMSO-d6): d = 15.8 (CH3), 44.0 (CH3), 109.3
(C), 121.0 (CH), 127.4 (CH).
UV/Vis (CHCl3): lmax (e) = 432 nm (59000).
(13) Lukeš, R.; Jizba, J. Collect. Czech. Chem. Commun. 1958,
Anal. Calcd for C11H21F6N2P: C, 40.49; H, 6.49; N, 8.59. Found: C,
40.08; H, 6.03; N, 8.37.
23, 1095.
(14) Wong, Y.-S.; Marazano, C.; Gnecco, D.; Génisson, Y.;
Chiaroni, A.; Das, B. C. J. Org. Chem. 1997, 62, 729.
(15) Steinhardt, S. E.; Silverston, J. S.; Vanderwal, C. D. J. Am.
Chem. Soc. 2008, 130, 7560.
References
(16) Dale, J.; Lichtenthaler, R. G.; Teien, G. Acta Chem. Scand.,
Ser. B 1979, 33, 141.
(1) Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.;
Behera, G. B. Chem. Rev. 2000, 100, 1973.
Synthesis 2009, No. 8, 1291–1296 © Thieme Stuttgart · New York