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Cyclobutenediylium Dyes.

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Cyclobutenediylium Dyes
BY H.-E. SPRENGER AND W. ZIEGENBEIN I*l
In agreement with the aromaticity and electrophilic behavior of squaric acid, a cyclobuterrediyliumtetroxide ion is formulated as a limiting form of the squarate dianion. These
views are confirmed by the structure of a series of new compounds that are readily obtainable from squaric acid or its derivatives, and most of which are deeply colored. The compounds contain the cyclobutenediyliumdioxide grouping as the chromophore, and form a
new class of dyes, for which the name cyclobutenediylium dyes is proposed.
1. Introduction
Squaric acid ( I ) is a strong dibasic acid, which is
thermally very stable and fairly stable toward oxidizing
agents. The preparation and properties of its simple
derivatives such as esters, amides, and halides are
described in a recent review 111. Condensations of
squaric acid and its derivatives with nucleophilic
compounds have recently been reported c2-61, and
these reactions form the subject of the present article.
However, this formulation gives no indication of the
electrophilic behavior of squaric acid. The resonance
should therefore involve further limiting structures
including carbonium ions. This is supported by some
reactions of the esters and diamides of squaric acid [I].
Thus we have found that diethyl squarate can be
smoothly ethylated with triethyloxonium hexachloroantimonate to give a colorless, salt-like, extremely
hygroscopic product, to which the structure (3) can
be assigned [lo].
Diamides are converted in the same manner into saltlike, nonhygroscopic compounds (4).
2. Limiting Structures of the Squarate Dianion
The readiness to react with nucleophiles points to
limiting forms that would express the electrophilic
reactivity of the squarate ion. Limiting structures of
this nature have not previously been considered.
West et al. explained the aromatic character and the
equivalence of the four oxygen atoms in the squaric
acid molecule on the basis of resonance between (2a)
and (2b) [7-91:
[*I Dr. H.-E. Sprenger and Dr. W. Ziegenbein
Forschungslaboratorium der Chemische Werke Hiils AG.
437 Marl (Germany)
[l] G. Maahs and P. Hegenberg, Angew. Chem. 78, 927 (1966);
Angew. Chem. internat. Edit. 5, 888 (1966).
121 A . Treibs and K . Jacob, Angew. Chem. 77,680 (1965); Angew.
Chem. internat. Edit. 4, 694 (1965).
131 A . Treibs and K . Jacob, Liebigs Ann. Chem. 699, 153 (1966).
[41 W . Ziegenbein and H.-E. Sprenger, Angew. Chem. 78, 937
(1966); Angew. Chem. internat. Edit. 5, 893 (1966).
I51 H.-E. Sprenger and W . Ziegenbein, Angew. Chem. 78, 937
(1966); Angew. Chem. internat. Edit. 5, 894 (1966).
530
R2N
=
Morpholino, P i p e r i d i n o , P y r r o l i d i n o
Ethylation to give (3) obviously depends on the polarization of a carbonyl group of the cyclobutenedione
system in the diethyl squarate. Consideration of these
observations would lead to the limiting forms (2a)
and (5).
However, structure ( 5 ) is in conflict with the aromatic
properties of squaric acid, since only three of
the oxygen atoms would be equivalent. We therefore
propose a resonance between (2a) and the cyclo[6] H.-E. Sprenger and W. Ziegenbein, Angew. Chem. 79, 581
(1967); Angew. Chem. internat. Edit. 6, 553 (1967).
171 R. West, H . Y . Niu, D. L. Powell, and M . V. Evans, J. Amer.
chem. SOC.82, 6204 (1960).
[8] R . West and D . L. PoweIl, J. Amer. chem. SOC.85, 2571
(1963).
[9] M. If0 and R . West, J. Amer. chem. SOC.85, 2580 (1963).
[lo] H.-E. Sprenger, unpublished.
Angew. Chem. internat. Edit. 1 Vol. 7 (I968)
I No. 7
butenediyliumtetroxide limiting structures (6a) and
(6b),which may bethought ofasresultingfrom theinclusion of both carbonyl groups of the squariz acid in the
resonance system. These formulas express both the
aromaticity and the electrophilicity of squaric acid.
Similar bonding and charge distributions occur in the
anions of very stable inorganic acids. Thus the equivalence of the oxygen atoms in the phosphate, sulfate,
and perchlorate ions can be explained by the participation of limiting structures in which the central atom carries one or more positivecharges. These positive central
atoms correspond to the doubly positive cyclobutenediylium skeleton of squaric acid.
The same cyclobutenediyliumtetroxide limiting structure of the squarate dianon follows from a comparison with cyanine dyes, which leads to contradictions,
since the resonance in the cyanine dye molecule is best
described by an alternating charge distribution along
the C - C chain [ I l l .
Finally, according to Hiickei, a cyclobutene ring carrying two positive charges should be a homolog of
ethylene, and should possess aromatic properties [1*1.
Cyclobutenediylium compounds have recently been
prepared by Freedman and Frantzjr. 1131. Thus, the deep
red tetraphenylcyclobutenediylium hexachlorostannate (7) is surprisingly stable in dry air and is relatively
stable in general.
pounds, most of which are deeply colored. These compounds were prepared in our research laboratories 14-61,
and independently by Treibs and Jacob [2,31, from
squaric acid and nucleophilic compounds. The previously unknown chromophores in these dyes are the
cyclobutenediyliumdioxide groups (8a) or (86) (61.
Organic dyes are generally characterized by a polymethine structure with alternating charges [ I l l . This is
not so in the compounds prepared by Treibs and by
our team. On the contrary, instead of a single positive
carbon atom in the cyanine chain, these dyes contain
the cyclobutenediyliumdioxide group as a doubly positive charge center and are therefore to be considered
as a new class of dyes.
3.1. Condensations w i t h E s t e r s of Squaric Acid
We have found that squarates condense smoothly with
compounds having acidic hydrogen atoms on carbon
in alcoholic solution in the presence of sodium alkoxide. Pyridinium and benzothiazolium iodides bearing a methyl group in the 2- or 4-position, and an
ethyl group on the N atom, were used as model compounds. These salts readily lose hydrogen iodide to
form reactive bases, which react with esters of squaric
acid to give colored products (9) that have been found
to be derivatives of the unknown cyclobutenedione,
the o-quinone of cyclobutadiene 161 (see Table 1).
C o m p o u n d s ( 9 a ) - ( 9 e ) a r e not very soluble in alcohols or
ethers, b u t dissolve m o r e readily in high-boiling polar solvents such as dirnethylformarnide a n d benzonitrile. T h e deep
color of all t h e new compounds c a n be explained by t h e
contribution of a cyclobutenediylium limiting form t o t h e
resonance hybrid:
3. Condensations of Squaric Acid and its
Derivatives
According to the views presented in Section 2, squaric
acid and its esters should undergo condensations with
nucleophilic compounds. This prediction has been
confirmed by the synthesis of a number of new com[ll] S . Dahne and D . Leupold, Angew. Chem. 78, 1029 (1966);
Angew. Chem. internat. Edit. 5, 984 (1966).
1121 M . P. Cava and M . J . Mifchell in: Cyclobutadiene and
Related Compounds. Academic Press, New York, London 1967,
p. 122.
1131 H . H . Freedman and A . M. Frantz j r . , J . Amer. chern. SOC. 84,
4165 (1962).
Angew. Chem. infernat. Edir. / Yol. 7 (196%)/ No. 7
53 1
Table 1 . Colored products (9)obtained from esters of squaric acid and
N-ethyl nitrogen bases.
R=cK
(9)
R=C
M.p. ("C)
I R
N-Ethyl-1,4-dihydro-4-pyridylidene
N-Ethyl-1 ,2-dihydro-2-pyridylidene
N-Ethyl- 1,4-dihydro-4-quinolylidene
N-Ethyl- 1,2-dihydro-2-quinolylidene
N-Ethyl-2,3-dihydro-2benzo thiazolylidene
328 (decomp.)
218 (decornp.)
257 (decomp.)
320
274 (decomp.)
Yield
( %)
3s
16
68
55
28
3.3. Condensations with Squaric Acid
The reaction of squaric acid with excess secondary
amines does not lead to diamides of squaric acid. Instead, colorless to pale yellow stable compounds (12)
of high melting point are formed, presumably by way
of hydrosquarates [10,141. These products form a new
class of intramolecular salts, and are derived from 1,3bisaminocyclobutenediyliumdioxides(see Table 2).
Table 2.
Intramolecular salts (I2a)-(12d) from squaric acid and
secondary amines, and (I3a)-(13g) f r o m squaric acid and aromatic
Esters of squaric acid react with compounds such as
malononitrile in the presence of alkali metal alkoxide
to form deep yellow salts (10) [61, which are soluble in
water and remain unchanged on heating to 360°C.
The yields of (IOU)-(IOc) are 93, 89, and 97% respectively.
0
R1
0
(a), R e = OC(CN)2
f b ) , R e = eC(COOCzH5)CN
(c), RQ=@N-CN
The smooth, ready formation of compounds (10) can
be formulated as an addition of the nucleophile to the
cyclobutenediyliumdioxide form of the squarate. The
final product is then formed by elimination of alcohol.
The reaction thus corresponds to an ester condensation.
R1 R 2
1 - 2 ROH
fa):
f b):
3.2. Condensations with Squaryl Diacetate
Treibs and Jacob 131 also obtained 1,Zderivatives of
cyclobutenedione ( I l a ) and ( I l b ) (m.p.'s 245 "C and
205 O C ; yields 45 % and 19 %) from squaryl diacetate
and 2-methyl- or 2,4-dimethylpyrrole in acetic anhydride. 1,3-Derivatives of cyclobutenediyliumdioxide
were also isolated as by-products.
a-Naphthyl
6-Naphthyl
I-Anthraquinonyl
2-Antbraquinonyl
I
M.P. ( "C)
Yield
R2
281-283
> 340
287-293
233-236
192
> 360
360 (decornp.)
340
313
H
H
60
72
60
17
48
81
66
85
63
H
H
Similar condensation products (13) have been prepared
from squaric acid and aromatic amines [14,151 (see
Table 2).
Compounds (13) were prepared by azeotropic removal of the water of reaction in butanol/benzene mixtures. These stable, high-melting substances are pale
yellow to deep brown, depending on the nature of the
aromatic substituent. Although not involved in the
resonance, the aromatic group has a color-deepening
effect. This leads one to assume that reaction occurs
ortho or para to the amino group, as is the case with
other reactions of anilines. However, condensation
on the phenyl substituent of the amino group can be
definitely ruled out on the basis of IH-NMR studies.
In the case of N,N-dialkyl aromatic amines or nucleophilic tertiary amines having an enamine character,
on the other hand, condensation should occur on the
substituents of the amino group. The products should
be phenylogs or vinylogs of the 1,3-diaminocyclobutenediyliumdioxides described above. Treibs and
Jacob actually obtained deep blue, high-melting
1,3-dipyrrolylcyclobutenediyliumdioxides (14) from
squaxic acid and reactive pyrroles; as expected, these
products are vinylogs of I ,3-diaminocyclobutenediyliumdioxides I2,31 (see Table 3).
[14]G. Manecke and J. Gauger, Tetrahedron Letters 1967, 3509;
1968, 1339.
[15] H.-E. Sprenger, German Pat. Appl. C 41650 IVb/l20
(March 1, 1967), Chem. Werke Hiils.
532
Angew. Chem. internat. Edit.
I
Vol. 7 (1968) I No. 7
half-ester 1161, which probably reacts with the nucleophilic compound RIH, subsequent elimination of
alcohol leading to the formation of the 1:l reaction
Table 3. 1.3-Dipyrrolylcyclobutenediyliumdioxides(14)
R'
ir'
HO
M.P. ("C)
CH3
CH3
CH3
CHI
CH3
CzH5
H
H
CO~CZHS
COCH,
265
240- 250
242
296
299
0
Yield
90
75
19
60
product (16) containing one electrophilic C atom 1171.
A further molecule of nucleophile R1H can add to this
remaining electrophilic center; dehydration of ( 1 7)
then yields the final product (18).
76
The condensation is carried out in ethanol or acetic acid.
Treibs and Jacob assigned the I R band at 1640cm-1 to a
vinylogous carboxylate grouping. It appears t o us that the
spectroscopic findings agree more closely with the presence
of a cyclobutenediylium moiety.
In a similar manner, these authors readily obtained compounds of the same type by reaction of nucleophilic phenols,
e.g. phloroglucinol. The wine-red. very stable compound
(f5) melts at 340-345 OC, and is obtained in yields of 67%.
0
To obtain phenylogs of the 1,3-diaminocyclobutenediyliumdioxides, we aIIowed squaric acid to react with
N,N-dialkylanilines. Stable 1,3-bis(dialkylaminopheny1)cyclobutenediyliumdioxides (18a) -(18e) were formed in good yields151 (see Table 4). In agreement with
Table 4. Dyes with cyclobutenediyliumdioxide structure (18a)-(f&j.
Independently of Treibs, we found that squaric acid
reacts under surprisingly mild conditions with a
number of classes of nucleophilic compounds in the
absence of catalysts. Condensation takes place in a
butanol/benzene mixture, so that the water formed
can be removed azeotropically as the reaction proceeds.
The resulting high-melting, extremely stable dyes,
which are 1,3-derivatives of cyclobutenediyliumdioxide, are obtained as crystalline solids, usually in
an analytically pure state, toward the end of the reaction or after cooling.
The dyes (18) are not very soluble in alcohols or ethers, but
dissolve more readily in high-boiling polar solvents such as
DMF, benzonitrile, or nitrobenzene. They dissolve in cold
concentrated sulfuric acid, with lightening of the color, probably because of salt formation; however, they are reprecipitated on addition of water. The compounds are not very
stable toward bases, and are generally decomposed with
formation of brown products. Strong reducing agents, such
a s rongalite solutions, decolorize the dye solutions. A strong
IR absorption band between 1500 and 1630 cm-1, which we
regard as evidence of a cyclobutenediyliumdioxide structure,
is common t o all compounds.
The formation of these dyes can be explained as
follows. It is known from investigations by Coherz et al.
that when squaric acid is heated in alcohols it forms a
Angew. Chem. internat. Edit.
Yol. 7 (1968) / No. 7
H
H
H
H
OH
216
60
230 (decomp.)
6
250
38
214-276 (decomp.) 27
303 (decomp.)
98
our concepts these products are deep blue. Furthermore, we condensed nucleophilic quinaldine, benzothiazole, and benzoselenazole bases with squaric acid
to obtain vinylogous 1,3-diaminocyclobutenediyliumdioxides ( l S f ) , (18g), and (18h) 161 (m.p.'s 320 "C,
300 "C, and 286 " C ; yields 31 %, 80 %, and 42 %, resp.).
The deeply colored, thermally stable products can be
distinguished from the corresponding 1,2 compounds,
which can be obtained from squarate esters, by thin
layer chromatography.
1161 S . Cohen and S . G. Cohen, J. Amer. chem. SOC.88, 1533
(1966).
[17] A . Treibs, private communication; see also A . Treibs and
K . Jacob, Liebigs Ann. Chem. 712, 123 (1968).
533
blue-green in solution, and separate out as green
crystals with a metallic luster[41. It is interesting to
note that in this case a hydrocarbon is sufficiently
nucleophilic to react.
0
n
An analogous product was obtained from barbituric
acid and squaric acid. The deep red dye (180) is insolubIe in all common solvents [lo] (m.p.330 OC;yield
72 %).
0
9
0
A similar vinylogous 1,3-diaminocyclobutenedioxide
derivative ( I & ) was obtained from squaric acid and
Fischer base161 (m.p. 301.5 OC; yield 92%). The deep
c: H,
I
d H3
8
(I8i)
bluestable dye wasobtainedindependentlyby Treibsand
Jacob, though by a different route[31. The identity of
the two products was verified by thin layer chromatography.
We also condensed 3-methyl-1-phenyl-5-pyrazolone
with squaric acid. The product was a yellow-red dye
(18k), which is soluble in alcohols and crystallizes
from the solutions as long needles [lo1 (m.p. 170 O C ;
yield 54 %).
0
(18p), R' = R2 = H
(l89), R' = C1, R 2 = CH,
(18q) are slightly soluble in dimethylformamide, but
are otherwise insoluble in the usual solvents, even in
concentrated sulfuric acid.
Azulenes, which are nucleophilic in position 1 or 3,
react similarly with squaric acid to give high yields
of thermally stable, 1,3-cyclobutenediyliumdioxides
(181)-(18n) (see Table 5). These compounds are deep
Table 5.
No dye is formed in the reaction of indoxyl with
squaric acid. The investigation of the product obtained
has not yet been completed1101. On the other hand,
3-hydroxybenzothiophene gives deep violet, bronzecolored crystals (18p) (m.p. 290 OC; yield 12%). From
6-chloro-3-hydroxy-4-methylbenzothiophene
the compound (18q) is obtained as bronze colored crystals
[m.p. 326 "C (decornp.); yield 35 %f 1101. (18p) and
Mention should also be made of some other products
obtained by reaction of squaric acid with nucleophiles.
An attempt to condense antipyrine (which is nucleophilic in position 4) with squaric acid did not yield the
expected 1,3-cyclobutenediyliumdioxide, since only
one antipyrine molecule combined with squaric acid
to form (19) 1181 (m.p. 238 OC;yield 56 %). The bright
yellow crystalline product is formed on azeotropic
removal of the water formed in the reaction.
n
Dyes with cyclobutenediyliurn structure (181)-(18n).
R3
3-Aminopyridine reacts likewise with squaric acid to
give a deep yellow, sparingly soluble compound, for
which we propose the structure (20) 1101 [m.p. 324 "C
(decornp.); yield 91 741. The deep yellow color points
to ring condensation of the 3-aminopyridine with the
squaric acid. Owing to the low solubility of the compound, its N M R spectrum could not be recorded.
(decamp.)
CH3
534
245-250
(decomp.)
> 350
80
90
1181 H.-E. Sprenger, German Pat. Appl. C 39587 IVd/lZp (July
12, 1966), Chem. Werke Hiils.
Angew. Chem. internat. Edit. 1 VoL 7 (1968) No. 7
$8
H,
0
(20)
0
On condensation of squaric acid with ethyl 2,4-dimethyl-3-pyrrolecarboxylate Treibs and Jacob obtained equal quantities of the cyclobutenediyliumdioxide
dye and a bright yellow compound to which they
assigned the structure of a 3: 1-condensation product
(21a) [31 (see Table 6).
obtained a 3: 1 condensation product from resorcinol
and squaric acid under more vigorous conditions. As
in the case of the pyrrole, however, the resulting compound (22) (m.p. 273-214 ' C ; yield 24 %) is unstable,
and readily loses resorcinol to form the f,3-derivative 133.
Table 6. Condensation products (ZZu)-(Zlc) f r o m squaric acid and
pyrrole derivatives.
OH
It should in principle also be possible to prepare 1,2,3,4-tetrasubstituted cyclobutenediylium derivatives having the formula (23) from squaric acid and nucleophilic compounds.
r--7
(23) R
@
R
R
M.p. ("C)
130-135
185-188
(decornp.)
266-268
(decornp.)
11%)
IS
93 [a]
79
[a] Crude product
However, the compounds are not very stable, since the
third pyrrole ring is readily split off by electrophilic
reagents, with regeneration of the 1,3-bispyrrolylcyclobutenediyliumdioxide. Treibs and Jacob also
2 X@
R
Attempts to do so have been made both by Treibs [I71 and by
our own group, but so far without success.
We are very grateful to Prof. Becher of the Universitat
Minster and to Dr. Bohm-God, Dr. Frenzel, and
G. Peitscher of Chemische Werke Hiils AG for the
recording and discussion of the spectra.
We thank the Management of the Company, andparticuiarly Prof. H. Hellmann, for their support of this work
and for permission to publish it.
Received: January 2, 1968
[A 639 IE]
German version: Angew. Chern. 80. 541 (1968)
Translated by Express Translation Service, London
C 0MMUNICATION S
A New Pteridine Synthesis[**I
By W. Pfleiderer and H.-U. Blank[*]
Traube
reported that 7-benzyl-8-phenylpurinesare obtained by the action of2 molecular equivalents of benzaldehyde
on 4.5-diaminopyrimidines. However, his 7-benzyl-8-phenyltheophylline was not identical with the substance obtained
by us by cyclization of 4-benzylamino-5-benzylideneamino1.3-dimethyluracil 121 or by benzylation of b-phenyltheophylline.
We have found that purines are not formed on treatment of
the 4,5-diaminopyrimidine ( I ) or (2) with 2 molecular equivalents of benzaldehyde or of the readily accessible 4-amino5-benzylideneaminopyrimidines(3 ) -(6) with 1 molecular
equivalent of benzaldehyde, in each case at 180 O C ; instead,
the 6,7-diphenylpteridine derivatives (7)-(10) are obtained.
The reaction course is not completely homogeneous, but
pteridines always form the main product and can be isolated
by fractional crystallization.
2,4,5-Triamino-6-pyrimidinol(11) or its 5-benzylidene
derivative (15) similarly affords the 6,7-diphenylpterin (13)
in 53 % yield, but (12) or (16)undergoes an obscure reaction
to afford a difficultly separable mixture of many products, in
Angew. Chem. internat. Edit. f Vol. 7 (1968) f No. 7
Product
CH3
CH3
CH3
CH3
H
H
33
60
47
65
62
51
227-232
270-272
230-232
276
304-305
316-323
231-232
278-280
231-232
278-280
307
315-322
535
131
[41
[31
[4]
151
161
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