Synthesis and characterization of new long-wavelength-absorbing oxonol dyes from the 2,2-difluoro-1,3,2-dioxaborine type

Article abstract of DOI:10.1002/(SICI)1521-3765(19990903)5:9<2511::AID-CHEM2511>3.0.CO;2-6

By starting from methyl- and methylene-substituted 2,2-difluoro-1,3,2-dioxaborines 5, 20, and 21 and different types of reactive formyl derivatives (14-17) a series of anionic, cationic, and betainic methine dyes has been prepared. These dioxaborine dyes

Full text of DOI:10.1002/(SICI)1521-3765(19990903)5:9<2511::AID-CHEM2511>3.0.CO;2-6

FULL PAPER
Synthesis and Characterization of New Long-Wavelength-Absorbing Oxonol
Dyes from the 2,2-Difluoro-1,3,2-dioxaborine Type
Marcus Halik and Horst Hartmann*[a]
Dedicated to the memory of Professor Rudolf Gompper
Abstract: By starting from methyl- and methylene-substituted 2,2-difluoro-1,3,2-
dioxaborines 5, 20, and 21 and different types of reactive formyl derivatives (14 ± 17)
a series of anionic, cationic, and betainic methine dyes has been prepared. These
dioxaborine dyes 22 ± 28 represent a new type of oxonol dye that exhibits intense
long-wavelength absorptions in the near-infrared region. The positions of these
absorption bands are recorded between 730 and 1050 nm and are strongly influenced
by the length of the conjugated system, by the substitution pattern at their pendant
aryl groups, and by the different bridging groups attached to their chromophoric
systems.
Keywords: betaines ´ boron ´ dyes
´ fluorescence spectroscopy ´ infra-
red spectroscopy
substituted diketoboronates 5 (2,2-difluoro-1,3,2-dioxabor-
ines[b]) are formed. In contrast to compounds 1, which react
with simple formyl derivatives 3 to yield the condensation
products 4,[2] the dioxaborines 5 react with the same formyl
derivatives 3 to yield compounds of the general structure 7
(Scheme 1).[3] Whereas for the reaction of the formyl com-
Introduction
As observed with other enolizable 1,3-dicarbonyl compounds,
aroylacetones of the general structure 1 can be condensed
with boronic acid or its derivatives to give 1,3-diketoboron-
ates.[1] For example, with boron trifluoride the 2,2-difluoro-
Scheme 1.
[a] Prof. Dr. H. Hartmann, Dr. M. Halik
Fachbereich Chemie der Fachhochschule Merseburg
Geusaer Strasse, D-06217 Merseburg (Germany)
Fax : (‡ 49)3461-462192
pound 3 with the aroylacetones 1 the intermediate carbanions
2 have been assumed,[4] for the reaction of 3 with the 1,3,2-
dioxaborines 5 the anionic species 6 appear to be the
intermediates. Hence, in contrast to the parent carbonyl
compound 1, their boron-containing derivatives 5 possess a
reactive methyl group that is strongly activated (after its
deprotonation) for an electrophilic attack of a suitable
reagent (Scheme 1).
E-mail: horst.hartmann@cui.fh-merseburg.de
[b] Although the name 2,2-difluoro-1,3,2-dioxaborine was introduced by
Reynolds et al.,[3] it is not correct in respect to the IUPAC rules. Owing
to the betainic valence formulae given here, the correct name of the
boron-containing heterocycle is 2,2-difluoro-1,3,2-oxaoxoniaboratine.
However, for simplicity the former name is used in this paper.
Chem. Eur. J. 1999, 5, No. 9
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0509-2511 $ 17.50+.50/0
2511

H. Hartmann and M. Halik
FULL PAPER
Evidently, the reaction of methyl-substituted 1,3,2-dioxa-
borines 5 with the formyl compounds 3 has some parallels to
the reaction of methyl-substituted quaternary heterocyclic
compounds 8 that react, via intermediate methylene bases 9,
with the formyl compounds 3 to yield condensation products
of the general structure 10 (Scheme 2: Yˆ counterion, Z ˆ
heteroatom).[5]
dyes 12 exhibit long-wavelength absorptions; their maxima
shift by about 100 nm on increasing the number of vinyl
groups. Hence, the 1,3,2-dioxaborines 12 represent an inter-
esting class of long-wavelength-absorbing methine dyes that
attain absorptions in the near-infrared region with a relatively
low number of methine groups. Because the 1,3,2-dioxabor-
ine-based methine dyes do not easily crystallize, only a few
crystalline compounds of this structural type have been
prepared.
Scheme 2.
Results and Discussion
When the moiety X in compounds 10 is a nucleofugal group,
such as halogen, OR, SR, or NR2, the compounds are usually
able to react with a second equivalent of 8 (via 9) to yield
cationic methines of the general structure 11.[6] (Scheme 2)
Owing to the similarity in the reactivity of 5 and 8 towards
the electrophilic reagents 3, reaction of the condensation
products 7 with their precursors 5 (via 6) should also be
possible. The products that are formed in this case are,
however, species which are composed of an anionic chromo-
phore and a cationic counterion. Therefore, these compounds
have to be attributed to the oxonol type of methine dyes.[5, 7]
As a consequence, their isolation strongly depends, inter alia,
on the proper choice of the cationic counterion.
Usually, the protonated forms of the auxiliary base used in
this condensation reaction can act as the counterion. For
example, when 4-methyl-substituted naphtho[1,2-d]- or thio-
naphtho[3,2-d]-1,3,2-dioxaborines were condensed with trial-
kylorthoformates 13a or ethoxyacroleine diethylacetals 13b
in presence of triethylamine, anionic methine dyes of the
To overcome this difficulty and to synthesize a series of better
crystallizing, long-wavelength-absorbing methines with pend-
ant 2,2-difluoro-1,3,2-dioxaborine groups, we have developed
three different strategies. The first involves the synthesis of
methine dyes with a more rigid molecular framework,
especially within the methine chains, by starting from bridged
1,3,2-dioxaborine reactants and by using bridged methine
precursors. Whereas bridged methine precursors, for example,
the formyl compounds 14 and 15 or the iminium perchlorate
16, are easily available from cycloalkanones by a Vilsmeier±
Arnold reaction,[9] bridged 1,3,2-dioxaborines, for example,
compounds 20 and 21, are available by reaction of the BF3/
acetic acid complex with 2-aroyl-cycloalkenols 18 and 19.
These starting materials were prepared from the literature
procedures[10] by acylating 1-morpholinocycloalkenes with
appropriate aroyl chlorides and subsequent hydrolysis of the
aroylated enamines primarily formed (Scheme 3).
Thus, by heating the bridged 1,3,2-dioxaborine derivatives
20 and 21 with triethylorthoformate 13a or ethoxyacrolein
diethyl acetal 13b in acetic anhydride in the presence of
triethylamine, deeply colored solutions were formed from
ˆ
general structure 12 with X ˆ CH CH or S and the hydro-
triethylammonium ion as the counterion are formed.[8] The
Scheme 3.
2512
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0509-2512 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 9

Oxonol Dyes
2511±2517
which the corresponding condensation products could be
obtained as crystalline solids in few special cases. For example,
by starting from the 4-nitro-substituted 1,3,2-dioxaborine
derivative 20d the corresponding nitro-substituted oxonol
dyes 22 (m ˆ 0) and 23 (m ˆ 1) containing the hydrotriethyl-
ammonium ion as cation were obtained in satisfactory yield.
A better result was obtained, however, when the same
bridged 1,3,2-dioxaborine derivatives 20 and 21 were heated
with the bridged formyl compounds 14 or 15. Crystalline
condensation products of the general structure 24 or 25 were
obtained when the chloro- or nitro-substituted 1,3,2-dioxa-
borine derivatives 20b and 20d, respectively, and the bridged
formyl compounds 14 and 15 are used. Crystalline condensa-
tion products of the same structural type 24 and 25 were
obtained also when the bridged formyl compounds 14 and 15
were condensed with the unbridged 1,3,2-dioxaborine deriv-
atives 5b and 5d (see Table 1).
A more complicated result was obtained if instead of the
formyl derivatives 14 and 15 the bridged iminium salt 16 as
methine precursor was used. Depending on the 1,3,2-dioxa-
borine reactants used and the conditions applied, mono-
condensation products as well as bis-condensation products of
the starting 1,3,2-dioxaborine derivative were obtained. Thus,
by starting from 16 and the methoxy-substituted 1,3,2-
dioxaborines 5c, 20c, and 21c the corresponding mono-
condensation products 28 with R ˆ CH3O could be isolated as
crystalline solids under the same conditions described above.
Although the products 28 are stable under the mentioned
preparation conditions, they can be transformed into their
corresponding symmetrically substituted methine dyes 24 by
applying more drastic conditions, for example, by refluxing
with the corresponding 1,3,2-dioxaborines 20 in acetic anhy-
dride in the presence of triethylamine for five hours. How-
ever, the methines 24 with R1/R2 ˆ (CH2)n so formed do not
crystallize from the reaction mixture after cooling. Therefore,
their formation could be detected only spectroscopically. For
example, their solutions exhibit intense long-wavelength
absorptions at about 1000 nm.
By starting from the iminium salt 16 and the chloro- and
nitro-substituted 1,3,2-dioxaborines 5b and 5d, respectively,
under the same conditions used as before, instead of mono-
Table 1. Preparative data for the methine dyes 22 ± 29.
R
R1
R2
n
Reactants
Yield [%] M.p. [8C]
Formula
(Mw)
N:calcd
N: found
22
23
NO2
NO2
Cl
NO2
Cl
NO2
Cl
NO2
Cl
±
±
H
H
±
±
H
H
2
2
2
2
2
2
3
3
3
3
2
2
2
±
±
±
±
±
±
±
±
±
±
±
±
±
±
20d, 13a
20d, 13b
5b, 14
56
23
7
280 (decomp)
C31H33B2F4N3O8
C33H35B2F4N3O8l
C33H25B2F4NO4Cl3
C33H25B2F4N3O8Cl
C37H29B2F4NO4Cl3
C37H29B2F4N3O8Cl
C34H27B2F4NO4Cl3
C34H27B2F4N3O8Cl
C38H31B2F4NO4Cl3
C38H31B2F4N3O8Cl
C38H36B2F4N2O4Cl4
C38H36B2F4N4O8Cl2
C33H36B2F4N2O8Cl2
C20H21BF2ClNO3
C22H23BF2ClNO3
C23H25BF2ClNO3
C34H30B2F4N2O4
C34H28B2F4N2O4Cl2
C36H34B2F4N2O6
C34H28B2F4N4O8
C38H34B2F4N2O4
C38H32B2F4N2O4Cl2
C40H38B2F4N2O6
C38H32B2F4N4O8
C40H36B2F4N2O4Cl2
C42H42B2F4N2O6
C40H36B2F4N4O8
(706.5)
(735.5)
(703.5)
(724.6)
(755.6)
(776.7)
(717.5)
(738.7)
(769.6)
(790.7)
(824.1)
(845.2)
(757.2)
(407.6)
(433.7)
(447.7)
(628.3)
(697.1)
(688.3)
(718.3)
(680.3)
(749.2)
(740.4)
(770.3)
(777.4)
(768.4)
(798.4)
5.95
5.74
1.99
5.80
1.85
5.41
1.95
5.69
1.82
5.32
3.40
6.63
3.70
3.44
3.23
3.13
4.46
4.02
4.07
7.80
4.12
3.74
3.78
7.27
3.60
3.65
7.02
6.09
5.33
1.92
5.37
1.68
5.74
1.76
5.36
1.44
4.83
3.27
6.11
3.26
3.37
3.19
3.03
4.32
3.89
4.11
7.67
4.01
3.35
3.89
7.78
3.62
3.34
6.91
198 (decomp)
238 (decomp)
263 (decomp)
240 (decomp)
235 (decomp)
215 ± 218
232 (decomp)
255 (decomp)
> 360
220 (decomp)
240 (decomp)
215 (decomp)
280 (decomp)
> 360
227 ± 229
300 (decomp)
280 (decomp)
330 (decomp)
360
290 (decomp)
312 ± 314
265 ± 268
> 360
220 (decomp)
189 (decomp)
275 (decomp)
24a
24b
24c
24d
25a
25b
25c
25d
26a
26b
27
28a
28b
28c
29a
29b
29c
29d
29e
29 f
29g
29h
29i
5d, 14
12
62
81
10
17
46
75
26
81
73
25
88
56
50
65
65
45
63
55
78
76
35
27
25
(CH2)2
(CH2)2
20b, 14
20d, 14
5b, 15
H
H
H
H
5d, 15
(CH2)2
(CH2)2
20b, 15
20d, 15
5b, 16
5d, 16
5e, 16
NO2
Cl
H
H
H
H
H
H
H
H
NO2
N(CH3)3
OCH3
OCH3
OCH3
H
5c, 16
(CH2)2
(CH2)3
20c, 16
21c, 16
5a, 17
5b, 17
5c, 17
H
H
H
H
H
H
H
H
Cl
OCH3
NO2
H
5d, 17
(CH2)2
(CH2)2
(CH2)2
(CH2)2
(CH2)3
(CH2)3
(CH2)3
20a, 17
20b, 17
20c, 17
20d, 17
21b, 17
21c, 17
21d, 17
Cl
OCH3
NO2
Cl
OCH3
NO2
29j
29k
Chem. Eur. J. 1999, 5, No. 9
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0509-2513 $ 17.50+.50/0
2513

H. Hartmann and M. Halik
FULL PAPER
condensation products 28, the corresponding bis-condensa-
tion products were formed. They precipitate from the hot
reaction mixture immediately. However, the products do not
contain, as expected, the hydrotriethylammonium ion as
cationic moiety, but as seen in the structural formula for 26,
the bridged iminium ion generated from the corresponding
starting material 16 acts as the cationic counterion.
Evidence for this statement comes not only from the
elemental analytic data of the products 26 (see Table 1), but
also from their spectroscopic data (see Table 2). For example,
compound 26b exhibits two absorption maxima at about 480
and 940 nm in its VIS/NIR spectrum. With reference to the
spectral data of compound 16, the maximum in the visible
region can be attributed to its cationic moiety, whereas the
Table 2. Spectral properties of the methine dyes 22 ± 29 prepared; VIS/NIR data measured in DMF, 1H NMR data measured in [D6]DMSO.
R
R1
R2
n
lmax [nm] (log e)
1H NMR, d-values (assignement), J [Hz]
22
NO2
±
±
2
734 (4.92)
1.16 (t, 9H; CH3), 2.88 (m, 4H; CH2), 2.93 (m, 4H; CH2), 3.09 (q, 6H;
3
3
CH2), 7.76 (s, 1H; CH). 7.88 (d, J ˆ 8.8 Hz, 4H; CH) , 8.15 (d, J ˆ 8.8 Hz,
4H; CH), 8.82 (1H; NH)
23
NO2
Cl
±
±
2
2
2
2
2
3
830 (5.13)
892 (5.28)
941 (5.19)
981 (5.04)
1041 (4.99)
866 (5.22)
1.16 (t, 9H; CH3), 2.70 (m, 4H; CH2), 2.92 (m, 4H; CH2), 3.08 (q, 6H;
CH2), 6.03 (t, J ˆ 13.2 Hz, 1H; CH), 7.59 (d, J ˆ 13.3 Hz, 2H; CH),
3
3
3
3
7.59 (d, J ˆ 8.6 Hz, 4H; CH), 8.21 (d, J ˆ 8.6 Hz, 4H; CH), 8.83 (s, 1H; NH)
3
24a
24b
24c
24d
25a
H
H
H
H
1.16 (t, 3H; CH3), 2.76 (m, 4H; CH2), 3.1 (q, 6H; CH2), 5.88 (d, J ˆ 13.7 Hz,
3
3
2H; CH), 6.71 (s, 2H; CH), 7.54 (d, J ˆ 8.5 Hz, 4H), 7.91 (d, J ˆ 8.5 Hz, 4H;
3
CH), 8.10 (d, J ˆ 13.4 Hz, 2H; CH), 8.8 (s, 1H; NH)
3
NO2
Cl
1.16 (t, 9H; CH3), 2.78 (m, 4H; CH2), 3.1 (q, 6H; CH2), 5.88 (d, J ˆ 13.5 Hz,
3
3
2H; CH), 6.92 (s, 2H; CH), 7.71 (d, J ˆ 13.5 Hz, 2H; CH), 8.13 (d, J ˆ 8.8 Hz,
3
4H; CH)), 8.29 (d, J ˆ 8.9 Hz, 4H; CH), 8.8 (s, 1H; NH)
(CH2)2
(CH2)2
1.16 (t, 9H; CH3), 2.63 (m, 4H; CH2), 2.80 (m, 4H; CH2), 2.83 (m, 4H; CH2),
3
3.1 (q, 6H; CH2), 7.25 (d, J ˆ 8.9 Hz, 4H; CH), 7.28 (s, 2H; CH), 7.49 (d,
3J ˆ 8.5 Hz, 4H; CH), 8.8 (s, 1H; NH)
NO2
Cl
1.16 (t, 9H; CH3), 2.69 (m, 4H; CH2), 2.82 (m, 4H; CH2), 2.84 (m, 4H; CH2),
3
3.1 (q, 6H; CH2), 7.35 (s, 2H; CH), 7.57 (d, J ˆ 9.0 Hz, 4H; CH), 7.85 (d,
3J ˆ 9.0 Hz, 4H; CH), 8.8 (s, 1H; NH), 8.8 (s, 1H; NH)
H
H
H
H
1.16 (t, 9H; CH3), 1.78 (m, 2H; CH2), 2.53 (m, 4H; CH2), 3.1 (q, 6H; CH2),
3
3
5.96 (d, J ˆ 13.6 Hz, 2H; CH), 6.72 (s, 2H; CH), 7.56 (d, J ˆ 8.5 Hz,
3
3
4H; CH), 7.93 (d, J ˆ 8.5 Hz, 4H; CH), 8.13 (d, J ˆ 13.4 Hz, 2H; CH),
8.8 (s, 1H; NH)
25b
25c
25d
26a
26b
NO2
Cl
3
3
3
2
2
941 (5.19)
948 (4.92)
1000 (4.98)
892, 477[a]
941, 477[a]
1.16 (t, 9H; CH3), 1.79 (m, 2H; CH2), 2.57 (m, 4H; CH2), 3.1 (q, 6H; CH2),
3
3
6.07 (d, J ˆ 13.6 Hz, 2H; CH), 6.87 (s, 2H; CH), 8,12 (d, J ˆ 8.8 Hz, 4H; CH),
3
3
8.18 (d, J ˆ 13.4 Hz, 2H; CH), 8.26 (d, J ˆ 8.9 Hz, 4H; CH), 8.8 (s, 1H; NH)
(CH2)2
(CH2)2
1.16 (t, 9H; CH3), 1.73 (m, 2H; CH2), 2.75 (m, 4H; CH2), 2.88 (m, 8H; CH2),
3
3.1 (q, 6H; CH2), 7.43 (d, J ˆ 6.9 Hz, 4H; CH), 7.65 (s, 2H; CH), 7.74 (d,
3J ˆ 7.2 Hz, 4H; CH), 8.8 (s, 1H; NH)
NO2
Cl
1.16 (t, 9H; CH3), 1.74 (m, 2H; CH2), 2.77 (m, 4H; CH2), 2.90 (m, 8H;
3
3
CH2), 3.1 (q, 6H; CH2), 7.63 (d, J ˆ 8.5 Hz, 4H; CH), 7.79 (d, J ˆ 8.3 Hz,
4H; CH), 7.84(s, 2H; CH), 8.8 (s, 1H; NH)
H
H
H
H
2.76 (m, 4H; CH2), 3.09 (m, 4H; CH2), 3.31 (s, 12H; N(CH3)2), 5.88 (d,
3
3J ˆ 13.7 Hz, 2H; CH), 6.71 (s, 2H; CH), 7.54 (d, J ˆ 8.5 Hz, 4H; CH),
3
3
7.65 (s, 2H; CH), 7.91 (d, J ˆ 8.5 Hz, 4H; CH), 8.10 (d, J ˆ 13.4 Hz, 2H; CH)
NO2
2.78 (m, 4H; CH2), 3.1 (q, 4H; CH2), 3.31 (s, 12H; N(CH3)2), 5.88 (d,
3J ˆ 13.5 Hz, 2H; CH), 6.92 (s, 2H; CH), 7.64 (s, 2H; CH), 7.71 (d, J ˆ 13.5 Hz,
3
3
3
2H; CH), 8.13 (d, J ˆ 8.8 Hz, 4H; CH), 8.29 (d, J ˆ 8.9 Hz, 4H; CH),
8.8 (s, 1H; NH)
‡
3
27
N(CH3)3
OCH3
OCH3
OCH3
H
H
H
H
H
2
±
±
±
±
894 (5.13)
674 (5.10)
708 (5.07)
686 (5.01)
893 (5.28)
2.79 (m, 4H; CH2), 3.64 (s, 18H; N (CH3)3), 5.84 (d, J ˆ 13.6 Hz, 2H; CH),
3 3
6.87 (s, 2H; CH), 7.69 (d, J ˆ 13.5 Hz, 2H; CH), 8.06 (d, J ˆ 9.2 Hz, 4H; CH),
3
8.13 (d, J ˆ 9.2 Hz, 4H; CH)
28a
28b
28c
29a
2.72 (m, 4H; CH2), 3.26 (s, 6H; N(CH3)2), 3.86 (s, 3H; OCH3), 5.78 (d,
3J ˆ 13.9 Hz, 1H; CH), 6.67 (s, 1H; CH), 7.06 (d, J ˆ 8.8 Hz, 2H; CH), 7.31 (s,
3
1H; CH'), 7.84 (d, J ˆ 13.9 Hz, 1H; CH), 7.95 (d, J ˆ 8.9 HZ, 2H; CH)
3
3
(CH2)2
(CH2)3
2.94 (m, 4H; CH2), 3.08 (m, 4H; CH2), 3.26 (s, 6H; N(CH3)2), 3.85 (s,
3H; OCH3), 7.16 (d, J ˆ 8.9 Hz, 2H; CH), 7.35 (s, 1H; CH'), 7.98 (d,
3
3J ˆ 8.9 Hz, 2H; CH), 8.13 (s, 1H; CH)
1.61 (m, 2H; CH2), 2.56 (m, 2H; CH2), 2.76 (m, 2H; CH2), 3.07 (m, 4H;
3
CH2), 3.24 (s, 6H; N(CH3)2), 3.83 (s, 3H; OCH3), 7.03 (d, J ˆ 8.7 Hz, 2H;
CH), 7.32 (s, 1H; CH'), 7.63 (d, J ˆ 8.7 Hz, 2H; CH), 8.04 (s, 1H; CH)
3
3
H
H
H
H
2.98 (m, 4H; CH2), 3.34 (s, 6H; N(CH3)2), 5.88 (d, J ˆ 13.3 Hz, 2H; CH),
3
3
6.67 (s, 2H; CH), 6.96 (d, J ˆ 13.3 Hz, 2H; CH), 7.24 (d, J ˆ 7.4 Hz, 2H;
3
3
Py-H), 7.47 (m, 2H), 7.72 (m, 4H), 7.90 (d, J ˆ 7.6 Hz, 4H; CH), 8.28 (d, J ˆ
7.4 Hz, 2H; Py-H)
3
29b
Cl
±
903 (5.19)
2.89 (m, 4H; CH2), 3.34 (s, 6H; N(CH3)2), 5.88 (d, J ˆ 13.3 Hz, 2H; CH),
3
3
6.70 (s, 2H; CH), 6.97 (d, J ˆ 13.3 Hz, 2H; CH), 7.24 (d, J ˆ 7.3 Hz, 2H; Py-H)
3
3
3
7.54 (d, J ˆ 8.4 Hz, 4H; CH), 7.91 (d, J ˆ 7.9 Hz, 4H; CH), 8.26 (d, J ˆ
7.3 Hz, 2H; Py-H)
2514
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0509-2514 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 9

Oxonol Dyes
2511±2517
Table 2. (Continued)
R
R1
H
R2
H
n
lmax [nm] (log e)
1H NMR, d-values (assignement), J [Hz]
29c
OCH3
±
898 (5.28)
2.86 (m, 4H; CH2), 3.33 (s, 6H; N(CH3)2), 4.04 (s, 6H; OCH3), 5.80 (d,
3J ˆ 13.2 Hz, 2H; CH), 6.54 (s, 2H; CH), 6.89 (d, J ˆ 13.2 Hz, 2H; CH),
3
3
3
7.02 (d, J ˆ 8.7 Hz, 4H; CH), 7.22 (d, J ˆ 7.3 Hz, 2H; Py-H), 7.86 (d, 4H; J ˆ
3
8.6), 8.26 (d, J ˆ 7.3 Hz, 2H; Py-H)
3
29d
NO2
H
H
±
961 (5.16)
2.93 (m, 4H; CH2), 3.33 (s, 6H; N(CH3)2), 6.02 (d, J ˆ 14.0 Hz, 2H; CH),
3
3
6.81 (s, 2H; CH), 7.05 (d, J ˆ 14.0 Hz, 2H; CH), 7.26 (d, J ˆ 7.5 Hz, 2H; Py-H)
3
3
3
8.31 (d, J ˆ 8.4 Hz, 4H; CH), 8.30 (d, J ˆ 8.5 Hz, 4H; CH), 8.34 (d, J ˆ 7.5
Hz, 2H; Py-H)
29e
29 f
29g
29h
29i
H
(CH2)2
(CH2)2
(CH2)2
(CH2)2
(CH2)3
±
±
±
±
±
986
2.96 (m, 4H; CH2), 3.05 (m, 8H; CH2), 3.35 (s, 6H; N(CH3)2), 6.52 (s, 2H; CH),
3
3
7.24 (d, J ˆ 7.4 Hz, 2H; Py-H), 7.49 (m, 6H; CH), 7.81 (d, J ˆ 7.9 Hz, 4H; CH)
3
8.23 (d, J ˆ 7.4 Hz, 2H; Py-H)
Cl
998
2.94 (m, 4H; CH2), 3.10 (m, 8H; CH2), 3.35 (s, 6H; N(CH3)2), 6.53 (s, 2H; CH),
3
3
3
7.25 (d, J ˆ 7.5 Hz, 2H; Py-H), 7.54 (d, J ˆ 8.6 Hz, 4H; CH), 7.81 (d, J ˆ 8.6
3
Hz, 4H; CH), 8.23 (d, J ˆ 7.5 Hz, 2H; Py-H)
OCH3
NO2
Cl
992
2.90 (m, 4H; CH2), 2.95 (m, 8H; CH2), 3.33 (s, 6H; N(CH3)2), 3.83 (s, ˆ
3
3
6H; OCH3), 6.45 (s, 2H; CH), 7.04 (d, J ˆ 9.0 Hz, 4H; CH), 7.22 (d, J ˆ 7.3
3
3
Hz, 2H; Py-H), 7.79 (d, J ˆ 8.4 Hz, 4H; CH), 8.19 (d, J ˆ 7.3 Hz, 2H; Py-H)
1052
928 (4.92)
2.85 (m, 8H; CH2), 2.92 (m, 4H; CH2), 3.34 (s, 6H; N(CH3)2), 6.49 (s, 2H; CH),
3
3
3
7.23 (d, J ˆ 8.0 Hz, 2H; Py-H), 8.03 (d, J ˆ 9.0 Hz, 4H; CH), 8.17 (d, J ˆ 8.8
3
Hz, 4H; CH), 8.30 (d, J ˆ 8.0 Hz, 2H; Py-H)
1.58 (m, 4H; CH2), 2.45 (m, 4H; CH2), 2.76 (m, 4H; CH2), 3.17 (m, 4H;
3
CH2), 3.32 (s, 6H; N(CH3)2), 6.99 (s, 2H; CH), 7.25 (d, J ˆ 8.0 Hz, 2H; Py-H),
3
3
3
7.49 (d, J ˆ 8.7 Hz, 4H; CH), 7.57 (d, J ˆ 8.7 Hz, 4H; CH), 8.26 (d, J ˆ 8.0
Hz, 2H; Py-H)
29j
OCH3
NO2
(CH2)3
(CH2)3
±
±
929 (4.98)
965 (4.82)
1.57 (m, 4H; CH2), 2.45 (m, 4H; CH2), 2.77 (m, 4H; CH2), 3.18 (m, 4H;
CH2), 3.33 (s, 6H; N(CH3)2), 3.81 (s, 6H; OCH3), 7.0 (s, 2H; CH), 7.01
3
3
3
(d, J ˆ 8.8 Hz, 4H; CH), 7.25 (d, J ˆ 8.1 Hz, 2H; Py-H), 7.56 (d, J ˆ 8.4 Hz,
3
4H; CH), 8.28(d, J ˆ 8.1 Hz, 2H; Py-H)
29k
1.62 (m, 4H; CH2), 2.47 (m, 4H; CH2), 2.80 (m, 4H; CH2), 3.21 (m, 4H; CH2),
3
3.34 (s, 6H; N(CH3)2), 7.06 (s, 2H; CH), 7.29 (d, J ˆ 7.8 Hz, 2H; Py-H), 7.83
3
3
3
(d, J ˆ 8.7 Hz, 4H; CH), 8.27 (d, J ˆ 9.0 Hz, 4H; CH), 8.31 (d, J ˆ 7.8 Hz,
2H; Py-H)
[a] Owing to the insolubility of the compound, its absorption data were estimated qualitatively only.
second maximum in the near infrared region can be
attributed to the anionic 1,3,2-dioxaborine chromophore
unambiguously.
presence of triethylamine, deeply colored methines 29 were
formed. As a result of their zwitterionic nature, these salts 29
exhibit a high tendency to crystallize and can, therefore, easily
be isolated in most satisfactory yields (see Table 1) from the
reaction mixture.
A second strategy for preparing crystalline 1,3,2-dioxabor-
ine methines involves the use of 1,3,2-dioxaborine reactants
containing suitable cationic groups, for example, at their
pendant aryl groups. The cationic 1,3,2-dioxaborine derivative
5e is an example of such a compound. This salt can be
prepared, as recorded elsewhere,[11] by reaction of 4-acetyl-
phenyltrimethylammonium perchlorate with acetic anhydride
in presence of boron trifluoride. It condenses with compound
16 under the same conditions mentioned above to give a
deeply colored crystalline methine dye 27. This dye is
composed of a cationic 1,3,2-dioxaborine moiety and a
perchlorate anion and could be obtained, as seen from
Table 1, in satisfactory yield.
Finally, a third route to prepare crystalline methine dyes
from the 1,3,2-dioxaborine precursors 5 or 20 and 21 has been
developed. It involves the use of methine precursors with
suitable cationic moieties attached to their molecular frame-
work. The dicationic compound 17 was used as methine
precursor for this strategy. This diperchlorate 17, which has
been applied as methine precursor by other authors, can be
easily prepared by reaction of the iminium salt 16 with
4-dimethylaminopyridine.[12]
The structures of all the crystalline methine dyes described
here were confirmed by their analytical and spectroscopic
data unambiguously. Some of the spectroscopic data are
1
depicted in the Table 2. Thus, in the H NMR spectra of the
methine compounds 22 ± 29 characteristic proton signals were
detected. They could be attributed, according to their
intensities and coupling parameters, unambiguously to the
corresponding CHn fragments (n ˆ 1, 2, or 3) in these
compounds.
All the 1,3,2-dioxaborine methine dyes prepared are deeply
colored compounds that exhibit intense long-wavelength
absorption bands. Their positions depend, in a characteristic
manner, on the length of their methine chain, on their
By heating of compound 17 with one of the neutral 1,3,2-
dioxaborine reactants 5, 20, or 21 in acetic anhydride in
Chem. Eur. J. 1999, 5, No. 9
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0509-2515 $ 17.50+.50/0
2515

H. Hartmann and M. Halik
FULL PAPER
substitution pattern, and on the size of their bridging groups
(see Table 2).
Experimental Section
Melting points were determined by means of a Boetius heating-table
microscope and are uncorrected. The visible and near-infrared spectra were
recorded with a Carl ± Zeiss Spectrometer M40 and a Shimadzu spectrom-
eter UV3101, respectively, and the NMR spectra with a Varian 300 MHz
spectrometer Gemini 300. The elemental analytical data were estimated by
means of a LECO analyzer CHNS 932. (Owing to the formation of boron
carbide during the combustion of the compounds the estimated C and H
values are incorrect to some extent and, therefore, were not documented).
Thus, the trimethine dye 22 (m ˆ 0) absorbs at about
730 nm and the pentamethine dye 23 (m ˆ 1) at 830 nm. For
the heptamethine dyes 24b, 25b, and 29d, bearing the same
nitrophenyl substituent as in 22 and 23 and no bridging group
at their 1,3,2-dioxaborine moieties, longest wavelength ab-
sorption maxima at about 940 nm were recorded. Hence, a
vinylene shift of about 100 nm is observed on going from a
trimethine to a pentamethine to a heptamethine system
indicating a pronounced polymethinic character of these
novel boron-containing oxonol dyes.
Concerning the aryl substituents, a significant influence on
the position of the long-wavelength absorptions is also
observed. Thus, extremely long-wavelength absorp-
tion maxima were found for all nitro-substituted dyes.
They absorb, as exemplified in the series 29, at wavelengths
nearly 70 nm longer than their H-substituted parent com-
pounds.
Preparation of the alkylene-bridged 2,2-difluoro-1,3,2-dioxaborines 20 and
21 (general procedure): A solution of an aroyl chloride (1.0 mol) in
benzene (100 mL) was added over a period of 1 h at 458C under stirring to
a mixture of a N-(cyloalkenyl)morpholine (1.1 mol), triethylamine (111 g,
1.0 mol), and benzene (400 mL). After stirring overnight the mixture was
neutralized by addition of aqueous hydrochloric acid and was subsequently
refluxed for 15 min. After cooling the organic layer was separated, washed
with water (3 Â 100 mL), dried with Na2SO4, and evaporated under
reduced pressure after filtration. The oily residue was mixed with a
equivalent of BF3/acetic acid adduct and refluxed for 1 h. After cooling the
solid formed was isolated by filtration and recrystallized. The following
alkylene-bridged 2,2-difluoro-1,3,2-dioxaborines 20 and 21 were prepared.
2,2-Difluoro-4-phenyl-5,6,7-trihydrocyclopenta[1,2-b]-1,3,2-dioxaborine
(20a): Compound 20a was prepared from benzoyl chloride (1 mol, 14.1 g)
and N-(1-cyclopentenyl)morpholine (1.1 mol, 168.3 g). Yield: 141.6 g
(60%); m.p. 165 ± 1688C (acetic acid); 1H NMR (CDCl3): d ˆ 2.13 (m,
2H; CH2), 2.79 (t, 2H; CH2), 3.03 (t, 2H; CH2), 7.50 (t, 3J ˆ 7.3, 7.6 Hz, 2H;
CH), 7.62 (t, 3J ˆ 7.3 Hz, 1H; CH), 8.00 (d, 3J ˆ 7.6 Hz, 2H; CH);
C12H11BF2O2 (236.0): calcd C 61.06, H, 4.70; found C 61.82, H, 4.58.
A significant influence on the position of the longest
wavelength absorptions of the boron-containing methine dyes
is also observed for the bridging groups attached at the central
methine chains. Whereas trimethylene bridged dyes absorb
nearly at the same wavelength than their unbridged analogues
(compare, for example, the absorption data of the dyes 28a,
29c, or 29d with the one of their trimethylene bridged
analogues 28c, 29j, or 29k, respectively), dimethylene
bridged dyes, for example, the compounds 28b, 29g, or 29h,
absorb at considerably longer wavelength than their unbridg-
ed analogues 28a, 29c, or 29d, respectively (see Figure 1).
Both electronic and steric effects should be taken into
consideration when accounting for these observations.
2,2-Difluoro-4-(4-chlorophenyl)-5,6,7-trihydrocyclopenta[1,2-b]-1,3,2-di-
oxaborine (20b): Compound 20b was prepared from 4-chlorobenzoyl
chloride (1 mol, 17.5 g) and N-(1-cyclopentenyl)morpholine (1.1 mol,
168.3 g). Yield: 170.4 g (270.5%); m.p. 195 ± 1978C (acetic acid); 1H NMR
(CDCl3): d ˆ 2.15 (m, 2H; CH2), 2.82 (t, 2H; CH2), 3.02 (t, 2H; CH2), 7.48
(d, 3J ˆ 8.6 Hz, 2H; CH), 7.93 (d, 3J ˆ 8.6 Hz, 2H; CH); C12H10BF2O2Cl
(270.5): calcd C 53.29, H 3.73, Cl 13.11; found: C 53.89, H 3.80, Cl 13.25.
2,2-Difluoro-4-(4-methoxyphenyl)-5,6,7-trihydrocyclopenta[1,2-b]-1,3,2-
dioxaborine (20c): Compound 20c was prepared from 4-methoxybenzoyl
chloride (1 mol, 17.1 g) and N-(1-cyclopentenyl)morpholine (1.1 mol,
168.3 g). Yield: 207.5 g (78%); m.p. 212 ± 2148C (acetic acid); 1H NMR
(CDCl3): d ˆ 2.12 (m, 2H; CH2), 2.77 (t, 2H; CH2), 3.02 (t, 2H; CH2), 3.88
(s, 3H; OCH3), 6.97 (d, 3J ˆ 9.0 Hz, 2H; CH), 8.05 (d, 3J ˆ 9.0 Hz, 2H;
CH); C13H13BF2O3 (266.0): calcd C 58.69, H 4.92; found C 59.28, H 4.78.
2,2-Difluoro-4-(4-nitrophenyl)-5,6,7-trihydrocyclopenta[1,2-b]-1,3,2-dioxa-
borine (20d): Compound 20d was prepared from 4-nitrobenzoyl chloride
(1 mol, 18.5 g) and N-(1-cyclopentenyl)morpholine (1.1 mol, 168.3 g).
Yield: 233.2 g (83%); m.p. 157 ± 1588C (acetic acid); 1H NMR (CD3NO2):
3
d ˆ 2.20 (m, 2H; CH2), 2.91 (t, 2H; CH2), 3.10 (t, 2H; CH2), 8.22 (d, J ˆ
8.8 Hz, 2H; CH), 8.36 (d, 3J ˆ 8.8 Hz, 2H; CH); C12H10BF2NO4 (281.0):
calcd C 51.28, H 3.56, NO2 4.99; found C 51.17, H 3.42, NO2 4.90.
2,2-Difluoro-4-phenyl-5,6,7,8-tetrahydrocyclohexa-[1,2-b]-1,3,2-dioxabor-
ine (21a): Compound 21a was prepared from benzoyl chloride (1 mol,
14.1 g) and N-(1-cyclohexenyl)morpholine (1.1 mol, 183.7 g): Yield: 112.5 g
(45%); m.p. 113 ± 1158C (ethyl acetate); 1H NMR (CDCl3): d ˆ 1.67 (m,
2H; CH2) 1.82 (m, 2H; CH2), 2.58 (t, 2H; CH2), 2.65 (t, 2H; CH2), 7.46 (t,
3J ˆ 7.2, 7.4 Hz, 2H; CH), 7.56 (t, 3J ˆ 7.2 Hz, 1H; CH), 7.72 (d, 3J ˆ 7.4 Hz,
2H; CH); C13H13BF2O2 (250.0): calcd C 62.44, H 5.24; found C 62.72, H,
5.11.
Figure 1. Absorption spectra of some betainic methine dyes 29; wave-
length given in nm.
2,2-Difluoro-4-(4-chloro-phenyl)-5,6,7,8-tetrahydrocyclohexa-[1,2-b]-1,3,2-
dioxaborine (21b): Compound 21b was prepared from 4-chlorobenzoyl
chloride (1 mol, 17.5 g) and N-(1-cyclohexenyl)morpholine (1.1 mol,
1
183.7 g). Yield: 122.3 g (43%); m.p. 165 ± 1698C (ethyl acetate); H NMR
([D6]DMSO): d ˆ 1.61 (m, 2H; CH2) 1.76 (m, 2H; CH2), 2.51 (t, 2H; CH2),
2.74 (t, 2H; CH2), 7.63 (t, 3J ˆ 8.6 Hz, 2H; CH), 7.81 (d, 3J ˆ 8.6 Hz, 2H;
CH); C13H12BF2O2Cl (284.5): calcd C 54.88, H 4.25, Cl 12.46; found C 55.21,
H 4.11, Cl 12.65.
Most of the methine dyes prepared exhibit a pronounced
fluorescence in the NIR region. The wavelengths and
intensities of this fluorescence are also influenced by the
structural pattern of the corresponding methine dyes. Details
on this subject will be recorded and discussed in a subsequent
paper more in detail.
2,2-Difluoro-4-(4-methoxyphenyl)-5,6,7,8-tetrahydrocyclohexa-[1,2-b]-1,3,2-
dioxaborine (21c): Compound 21c was prepared from 4-methoxybenzoyl
chloride (1 mol, 17.1 g) and N-(1-cyclohexenyl)morpholine (1.1 mol,
2516
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0509-2516 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 9

Oxonol Dyes
2511±2517
183.7 g). Yield: 207.2 g (74%); m.p. 190 ± 1938C (acetic acid); 1H NMR
(CDCl3): d ˆ 1.68 (m, 2H; CH2) 1.82 (m, 2H; CH2), 2.62 (t, 2H; CH2), 2.65
(t, 2H; CH2), 3.87 (s, 3H; OCH3), 6.95 (t, 3J ˆ 9.0 Hz, 2H; CH), 7.82 (d,
3J ˆ 9.0 Hz, 2H; CH); C14H15BF2O3 (280.1): calcd C 60.03, H 5.41; found C
60.45, H 5.40.
nitromethane. The preparative data of the compounds so prepared are
depicted in Table 1.
2,2-Difluoro-4-(4-nitrophenyl)-5,6,7,8-tetrahydrocyclohexa-[1,2-b]-1,3,2-
dioxaborine (21d): Compound 21d was prepared from 4-nitrobenzoyl
chloride (1 mol, 18.5 g) and N-(1-cyclohexenyl)morpholine (1.1 mol,
183.7 g). Yield: 128.9 g (44%); m.p. 136 ± 138 (ethyl acetate); 1H NMR
(CDCl3): d ˆ 1.73 (m, 2H; CH2) 1.85 (m, 2H; CH2), 2.52 (t, 2H; CH2), 2.73
Acknowledgment
The authors thanks the Deutsche Forschungsgemeinschaft for generous
financial support.
3
3
(t, 2H; CH2), 7.87 (d, J ˆ 8.8 Hz, 2H; CH), 8.30 (t, J ˆ 8.8 Hz, 2H; CH);
C13H12BF2NO4 (293.1): calcd C 52.92, H 4.10, NO2 4.75; found C 53.13, H
4.45, NO2 4.70.
[1] a) B. M. Mikhailov, Pure Appl. Chem. 1977, 49, 749 ± 764; b) J.
Simpson, G. B. Porter, J. Chem. Soc. Perkin Trans 1, 1973, 1796 ± 1798;
c) H. Hartmann, J. Prakt. Chem. 1986, 328, 755 ± 762.
[2] H. Henecka, in Houben-Weyl, Methoden der Organischen Chemie,
Vol. 8, Thieme, Stuttgart, 1952, pp. 610 ± 612.
[3] a) J. A. VanAllen, G. A. Reynolds, J. Hetocyclic Chem. 1969, 6, 29 ±
35; J. A. VanAllen, G. A. Reynolds, J. Hetocyclic Chem. 1969, 6, 375 ±
377; b) G. A. Reynolds, J. A. VanAllen, A. K. Seidel, J. Heterocyclic
Chem. 1969, 16, 369 ± 370; c) G. A. Reynolds, C. H. Chen, J. Hetero-
cycic Chem. 1985, 22, 657 ± 659; d) W. Jenny, Helv. Chim. Acta 1951,
34, 539 ± 554; e) H. Hartmann, T. Schumann, R. Dusi, U. Bartsch,
H. D. Ilge, Z. Chem. 1986, 26, 330 ± 331.
[4] A. T. Nielsen, W. J. Houlihan, Org. React. 1968, 16, 1 ± 438.
[5] L. Berlin, O. Riester, in Houben-Weyl, Methoden der Organischen
Chemie, Vol. V,1d, Thieme, Stuttgart, 1972, pp. 227 ± 299.
[6] a) F. M. Hamer, in The Chemistry of Heterocyclic Compounds, Vol. 18
(Ed.: A. Weissberger), Wiley, New York, 1964, pp. 1 ± 790; b) N.
Tyutyulkov, J. Fabian, A. Mehlhorn, F. Dietz, A. Tadjer, Polymethine
Dyes, St. Kliment Ohridski, University Press, Sofia, 1991.
[7] a) S. Hünig, H. Quast, in Optische Anregung Organischer Systeme, 2.
Internat. Farbensymposium, Interlaken, VCH, Weinheim, 1966,
pp. 184 ± 262; b) S. Dähne, Z. Chem. 1981, 21, 58 ± 67.
[8] V. S. Markin, P. I. Abramenko, I. I. Boiko, Zh. Vses. Khim. Ova.
im. D. I. Mendeleeva 1984, 29, 457 ± 459.
[9] a) J. Zemlicka, Z. Arnold, Collect. Czech. Chem. Commun. 1961, 26,
2852 ± 2864; b) G. A. Reynolds, K. H. Drexhage, J. Org. Chem. 1955,
42, 885 ± 888.
Preparation of the dimethylene-bridged methine dyes 22 and 23 (general
procedure): A mixture of 20d (2.93 g, 10 mmol), triethylorthoformate (13a,
2.13 g, 10 mmol) or 3-ethoxyacrolein diethylacetal (13b, 1.80 g, 10 mmol),
and triethylamine (2.00 g, 20 mmol) in acetic anhydride (5 mL) and
acetonitrile (50 mL) was heated at 608C for 30 min. After standing
overnight the product had precipitated and was isolated by filtration. The
preparative data of the compounds 22 and 23 so prepared are depicted in
Table 1.
Preparation of the alkylene-bridged heptamethines 24 and 25 (general
procedure): A mixture of compounds 5 or 20 (20 mmol), a bridged formyl
compound 14 or 15 (10 mmol), and acetic anhydride (5 mL) in acetonitrile
(80 mL) was heated at 608C for 10 min. After addition of triethylamine
(40 mmol, 4 g) the resulting mixture was refluxed for 10 min and then
cooled at room temperature. After standing overnight, the crystalline solid
formed was isolated by filtration, washed with cooled acetonitrile, and
recrystallized from acetonitrile. The preparative data of the compounds
prepared by this procedure are depicted in Table 1.
Preparation of the alkylene-bridged heptamethines 26, 27, and 28 (general
procedure): Compounds 5 or 20 (10 mmol) and the bridged iminium
perchlorate 16 (10 mmol for preparing the dyes 26 and 28 and 5 mmol for
preparing the dyes 27) was added to a mixture of acetic anhydride (5 mL)
and acetonitrile (80 mL). After heating the mixture at 608C for
10 min, triethylamine (20 mmol, 2 g) was added, and the resulting
mixture was refluxed for further 10 min, cooled at room tempera-
ture, and kept overnight. The crystalline solid formed was isolated by
filtration, washed with cooled acetonitrile, and recrystallized from aceto-
nitrile. The preparative data of the compounds so prepared are depicted in
Table 1.
[10] a) R. D. Campell, H. M. Gilow, J. Am. Chem. Soc. 1960, 82, 2389 ±
2392; J. Am. Chem. Soc. 1962, 84, 1140 ± 142; b) R. D. Campell, W. L.
Harmer, J. Org. Chem. 1963, 28, 379 ± 380; c) G. Stork, A. Brizzolara,
H. Landesmann, J. Szmuszkovicz, R. Terrell, J. Am. Chem. Soc. 1963,
85, 207 ± 222.
[11] a) G. Görlitz, H. Hartmann, Heteroat. Chem. 1997, 8, 147 ± 155; b) G.
Görlitz, H. Hartmann, B. Nuber, J. J. Wolff, J. Prakt. Chem. 1999, 341,
167 ± 172; c) G. Görlitz, PhD Thesis, 1997, University of Halle.
[12] C. Harfmann, PhD Thesis, 1996, University of Munich.
Preparation of the betainic 2,2-difluoro-1,3,2-dioxaborine dyes 29 (general
procedure): Compounds 5 or 20 (20 mmol) and the iminium diperchlorate
17 (10 mmol, 5 g) was added to a mixture of acetic anhydride (5 mL),
acetonitrile (100 mL), and triethylamine (20 mmol, 2.0 g). After refluxing
for 10 min, this mixture was cooled to room temperature. The solid formed
was isolated by filtration and washed, subsequently, with water, acetic
anhydride, ethyl acetate, and ether. For purification the products were
recrystallized from dimethylformamide (compound 19a and 19c) or
Received: December 21, 1998 [F1500]
Chem. Eur. J. 1999, 5, No. 9
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0509-2517 $ 17.50+.50/0
2517