Zwitterionic borane adducts of N-heterocyclic carbenes from mesomeric betaines of uracil

Article abstract of DOI:10.1039/c3ob42462f

We prepared a series of imidazolium-substituted uracil-anions which are members of the class of cross-conjugated heterocyclic mesomeric betaines. They are in tautomeric equilibrium with their N-heterocyclic carbenes, uracil-6-yl-imidazol-2-ylidenes. These carbenes can be trapped by reaction with sulfur, selenium, as well as by triethylborane and triphenylborane, respectively. The latter trapping reaction yielded the first representatives of a new heterocyclic zwitterionic ring system, imidazo[2′,1′:3,4][1,4, 2]diazaborolo[1,5-c]pyrimidinium-10-ide. Results of two single crystal X-ray structure analyses are presented. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ob42462f

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Zwitterionic borane adducts of N-heterocyclic
carbenes from mesomeric betaines of uracil†
Cite this: Org. Biomol. Chem., 2014,
12, 2737
Jiaxi Zhang,a Nazar Pidlypnyi,a Martin Nieger,b Jan C. Namysloa and
Andreas Schmidt*a
We prepared a series of imidazolium-substituted uracil-anions which are members of the class of cross-
conjugated heterocyclic mesomeric betaines. They are in tautomeric equilibrium with their N-hetero-
cyclic carbenes, uracil-6-yl-imidazol-2-ylidenes. These carbenes can be trapped by reaction with sulfur,
selenium, as well as by triethylborane and triphenylborane, respectively. The latter trapping reaction
yielded the first representatives of a new heterocyclic zwitterionic ring system, imidazo[2’,1’:3,4][1,4,2]diaza-
borolo[1,5-c]pyrimidinium-10-ide. Results of two single crystal X-ray structure analyses are presented.
Received 10th December 2013,
Accepted 24th February 2014
DOI: 10.1039/c3ob42462f
www.rsc.org/obc
the chemistry of heterocyclic mesomeric betaines (MB),9 we
aimed at mesomeric betaines of uracil-6-yl-imidazolium
Introduction
Uracil 1 is one of the four nucleobases of RNA and its deriva- betaines 2 which give rise to betaine 2–N-heterocyclic carbene
tives display a broad variety of biological activities.1 Due to 3 tautomerism. Borane adduct formation from N-heterocyclic
lactim–lactam tautomerism uracil can exist in six tautomeric carbenes is a rapidly growing field of chemistry10 so that we
forms, two of which are shown in Scheme 1. Spectroscopic intended to explore the behavior of our target compounds
examinations as well as calculations reveal that the diketo tau- toward boranes (Scheme 2).
tomer is the predominant form in the solid state and in solu-
Mesomeric betaines such as 2 are defined as neutral com-
tion.2 The two oxygen atoms of uracil were calculated to be the pounds which can exclusively be represented by dipolar reson-
most susceptible sites of attachment of complex formations,3 ance forms in which the positive and negative charges are
and numerous complexes through the O4 site4 or through the delocalized within a common π-electron system. Four distinct
N3/O45 and O4/N3/O26 sites have been described. Some uracil main classes are distinguished: (i) conjugated mesomeric
metal complexes through the deprotonated N1 site are known betaines (CMB), (ii) ylides, (iii) cross-conjugated (CCMB) meso-
as well.7 To our surprise, only very little information is avail- meric betaines, and (iv) pseudo-cross-conjugated mesomeric
able concerning the interaction of uracils with boranes.8
betaines (PCCMB).11,12 The relationships to normal (NHC),
In view of the current interest in the area of overlap abnormal (aNHC)13 [sometimes called mesoionic carbenes
between the chemistry of N-heterocyclic carbenes (NHC) and (MIC)14] and remote N-heterocyclic carbenes (rNHC)15 are
summarized in reviews on betaine–carbene interconversions,9
betaine and carbene chemistry of pyrazoles and indazoles,16 or
complex chemistry.17
Betaine–carbene interconversions can be achieved by decar-
boxylations, deprotonations, and tautomerisms starting from
suitable betaines. Members of the class of PCCMBs11,12 such
as imidazolium-2-carboxylates 318 and pyrazolium-3-carboxy-
lates19 are convenient precursors of normal N-heterocyclic
Scheme 1
aClausthal University of Technology, Institute of Organic Chemistry, Leibnizstrasse 6,
D-38678 Clausthal-Zellerfeld, Germany. E-mail: schmidt@ioc.tu-clausthal.de;
Fax: +49-5323-722858; Tel: +49-5323-723861
bLaboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki,
P.O. Box 55 (A.I. Virtasen aukio 1), FIN-00014, Finland
†Electronic supplementary information (ESI) available: Data of the single crystal
X-ray analyses as well as the cif-files. NMR spectra. CCDC 976125 (25b) and
976126 (25d). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3ob42462f
Scheme 2
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Scheme 5
Scheme 3
carbenes such as 4 (Scheme 3). On the other hand, PCCMBs
can be formed by trapping reactions of NHCs with hetero-
cumulenes.9,20 Decarboxylations of CCMBs such as imidazo-
lium-4-carboxylate 5 and pyrazolium-4-carboxylate 7 to yield
the aNHC 6 or the rNHC 8 require harsh conditions, and as a
consequence, only a few examples of successful conversions
exist.21 The latter molecule 8 has been described as a dipolar
structure as shown and as a cyclic bent allene, and has been
studied intensively.22
Scheme 6
Deprotonations of mesomeric betaines result in the for-
mation of anionic N-heterocyclic carbenes. Examples are the
anions of the CMB imidazolium-4-olate 9,23 the CCMB pyrimi-
dinium-olate 10,24 and the CMB sydnone 119,25 (Scheme 4).
Tautomerism of a mesomeric betaine to an N-heterocyclic
carbene has surprisingly been observed on examination of the
mesoionic compound 12 (Busch’s reagent, nitron) which is in
equilibrium with 13 (Scheme 5).26 Correspondingly imidazo-
lium-4-aminide 14 is in tautomeric equilibrium with carbene
15.27
We investigated tautomeric equilibria of the ylides imidazo-
lium-indolates 16 with their N-heterocyclic carbenes 17
(Scheme 6). We also investigated conversions into the anionic
N-heterocyclic carbenes 18 and reported on borane adduct for-
mations to 19.28
Results and discussion
In continuation of our earlier work on uracils,29 we reacted
6-chlorouracil 20 (6-chloropyrimidine-2,4(1H,3H)-dione) with
the imidazoles 21a–e and obtained the imidazolium salts
22a–e in moderate to excellent yields. The anion exchange
resin Amberlite IRA-400 in its hydroxy form deprotonated the
salts 22a–e to the target mesomeric betaines 2a–e in good
yields (Scheme 7).
The resonance frequency of 5-H of the uracil ring is diag-
nostic of the success of the deprotonation, as it shifts upfield
by 0.15–0.54 ppm in the 1H NMR spectra, depending on the
substitution pattern. The betaines 2a–e are soluble in water
and other highly polar solvents such as DMSO and methanol,
but almost insoluble in acetonitrile or chloroform. In the
spectra of NMR measurements taken in DMSO-d6 only
the betaine tautomer of 2a–e can be seen. No traces of the
N-heterocyclic carbenes 3a–e are visible under these con-
ditions. At least in part this can be explained by the solubility
of 2a–e which compelled us to use highly polar solvents for the
characterization. The betaines 2a–e belong to the class of
cross-conjugated heterocyclic mesomeric betaines (CCMB).
Characteristically, the charges are restricted to separate parts
of the common π-electron system in the resonance forms (I).
The anionic part of the molecule is joined to the cationic part
through a union bond which starts from an unstarred position
of the isoconjugated equivalent of the π-electron system which
delocalizes the negative charge (II). This is a nodal (inactive)
We describe here the first equilibria of imidazolium-substi-
tuted uracils with their corresponding N-heterocyclic carbenes.
The carbenes can be trapped with boranes to yield first repre-
sentatives of a new heterocyclic ring system, a zwitterionic
imidazo[2′,1′:3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide.
Scheme 4
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Scheme 9
reactions was performed with triphenylborane so that 2a–d
were converted into 26a–d which were also obtained as crystal-
line compounds. Betaine 2e, however, did not react under
these conditions. The resonance frequency of 5-H of the uracil
moiety was detected between 5.96 ppm and 6.01 ppm (25) and
between 6.20 ppm and 6.32 ppm (26), and the 11B NMR
signals appear between −1.6 ppm and 1.4 ppm depending on
the solvent and the substitution pattern of the borane adduct,
respectively. Thermogravimetric analyses show that decompo-
sition of 25a, 25c, and 25d occurs above 270 °C in the solid
state. The borane adducts proved to be stable towards acids.
Thus, after treatment of 25a with trifluoroacetic acid in metha-
nol at reflux temperature we were able to recover the material
unchanged.
Single crystals of the boron adducts 25b and 25d were
obtained by slow evaporation of concentrated solutions in
ethanol. Results of single crystal X-ray analyses are as follows.
The zwitterions 25b (Fig. 2) and 25d (Fig. 3 and 4) crystallized
monoclinic and orthorhombic, respectively. In 25b the ethyl-
hydroxy group is disordered. The B–Ccarbene bond lengths
[B–C11; crystallographic numbering] were determined to be
161.8(4) pm (25b) and 160.7(5) pm (25d), and the B–Nuracil
bonds 162.3(3) pm and 162.8(4) pm, respectively. The corres-
ponding bond angles N1–B1–C11 were determined to be
92.37(19)° (25b) and 92.2(3)° (25d). The three rings uracil, dia-
zaborole, and imidazole are almost planar and dihedral angles
Scheme 7
position of the highest occupied molecular orbital (HOMO)
(III) which induces the charge separation in the ground state
in cross-conjugated mesomeric betaines (Fig. 1).11,12,30
In the presence of trapping reagents, the equilibrium can
be shifted toward the N-heterocyclic carbene tautomer 3a.
Thus, in the presence of sulfur and selenium, respectively, the
thione 23 and the selenone 24 were formed in high yields
(Scheme 8).
Similarly, triethylborane in dioxane converted the betaine-
NHC equilibrium into the new ring system of imidazo[2′,1′]-
[1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ides 25a–d as fluo-
rescent crystalline solids (Scheme 9). A similar series of
Fig. 1 Characteristic features of cross-conjugated mesomeric betaines.
Fig. 2 Molecular structure of 25b (minor disordered part is omitted for
Scheme 8
clarity; displacement parameters are drawn at the 50% probability level).
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Experimental
General considerations
Flash-chromatography was performed using silica gel
60 (0.040–0.063 mm). Nuclear magnetic resonance (NMR)
spectra were obtained using Bruker Avance 400 and Bruker
Avance III 600 MHz instruments. 1H NMR spectra were
recorded at 400 MHz or 600 MHz. 13C NMR spectra were
recorded at 100 MHz or 150 MHz, with the solvent peak or
tetramethylsilane used as the internal reference. 11B NMR
spectra were recorded at 128 MHz or 192 MHz. Multiplicities
are described by using the following abbreviations: s = singlet,
d = doublet, t = triplet, q = quartet, and m = multiplet. FT-IR
spectra were recorded on a Bruker Vector 22 in the range of
400 to 4000 cm−1. ATR-IR spectra were recorded on a Bruker
Alpha in the range of 400 to 4000 cm−1. The mass spectra were
measured with a Varian 320 MS Triple Quad GC/MS/MS with a
Varian 450-GC. The electrospray ionization mass spectra
(ESIMS) were measured with an Agilent LCMSD series HP 1100
with APIES. Melting points are uncorrected and were deter-
mined in an apparatus according to Dr Tottoli (Büchi). Yields
are not optimized. The compounds 22a and 2a have been
reported in one of our earlier publications.29
Fig. 3 Molecular structure of 25d (one of the crystallographically inde-
pendent molecules is shown; displacement parameters are drawn at the
50% probability level).
3-(2,6-Dioxo-1,2,3,6-tetrahydropyrimidin-4-yl)-1-methyl-1H-
imidazolium chloride 22a. A solution of 147 mg (1.0 mmol) of
20 and 164 mg (2.0 mmol) of 21a in 15 mL of chlorobenzene
was heated under reflux for 8 h. After cooling, a fine, nearly
white precipitate was deposited, which was filtered off and
thoroughly washed with diethyl ether. Recrystallization from
water yielded 224 mg (98%) of a white solid. All spectroscopic
data are in agreement with those reported earlier.29
Fig. 4 Asymmetric unit of 25d showing the dimer.
3-(2,6-Dioxo-1,2,3,6-tetrahydropyrimidin-4-yl)-1-(2-hydroxyethyl)-
1H-imidazolium chloride 22b. A solution of 147 mg (1.0 mmol)
of 20 and 224 mg (2.0 mmol) of 21b in 15 mL of chloroben-
zene was heated under reflux for 8 h. After cooling, a fine, gray
green precipitate was deposited, which was filtered off and
thoroughly washed with diethyl ether. Recrystallization from
water yielded 197 mg (76%) of a gray green solid: dec > 243 °C;
1H NMR (400 MHz, DMSO-d6): δ = 11.54 (s, 1H), 9.95 (s, 1H),
8.03 (m, 1H), 8.22 (m, 1H), 6.15 (s, 1H), 4.33 (t, J = 5.1 Hz, 2H),
3.79 (t, J = 5.1 Hz, 2H) ppm; 13C NMR (100 MHz, DMSO-d6):
δ = 163.4, 150.7, 144.3, 136.9, 123.7, 120.6, 93.6, 58.9,
52.5 ppm; IR (ATR): 3348, 3133, 3064, 3026, 2833, 1718, 1675,
N1–C2–N3–C4 = 1.8(5)°/0.0(4)°, C11–B1–N1–C6 = −1.0(3)°/
−1.2(3)° and N7–C8–C9–N10
=
−0.3(4)°/−0.2(3)° were
measured in 25b and 25d, respectively. In 25b hydrogen bonds
were found between O2 and the ethyl-hydroxy group with N3–
H and O4 of a neighbouring molecule, respectively.
The zwitterion 25d dimerizes through hydrogen bonds
between O4 and C8–H (264 pm, 164°; crystallographic
numbering).
1415, 1161, 1057, 832, 583, 533 cm−1
C9H11N4O3 required 223.0831. Found 223.0833.
3-(2,6-Dioxo-1,2,3,6-tetrahydropyrimidin-4-yl)-1-phenyl-1H-
; HR-ESI-MS for
Conclusions
We present first examples of cross-conjugated heterocyclic
mesomeric betaines which are in tautomeric equilibrium with imidazolium chloride 22c. A solution of 147 mg (1.0 mmol) of
their N-heterocyclic carbenes. Thus imidazolium-uracilates 20 and 288 mg (2.0 mmol) of 21c in 15 mL of chlorobenzene
undergo tautomerism to uracil-6-yl-imidazol-2-ylidenes. Trap- was heated under reflux for 8 h. After cooling, a fine, light
ping reactions were performed with sulfur and selenium to yellow precipitate formed, which was filtered off and
give an imidazole-thione and -selenone, respectively. Triethyl- thoroughly washed with diethyl ether. Recrystallization from
borane and triphenylborane yield the first representatives of a new water yielded 242 mg (83%) of a light yellow solid: dec >
1
heterocyclic zwitterionic ring system, imidazo[2′,1′:3,4][1,4,2]- 243 °C; H NMR (400 MHz, DMSO-d6): δ = 11.47 (s, 1H), 10.56
diazaborolo[1,5-c]pyrimidinium-10-ide. These results supple- (s, 1H), 8.60 (m, 1H), 8.49 (m, 1H), 7.94–7.92 (m, 2H),
ment our knowledge about the interesting area of overlap 7.73–7.69 (m, 2H), 7.66–7.63 (m, 1H), 6.26 (s, 1H) ppm; 13C
between mesomeric betaines and N-heterocyclic carbenes.
NMR (100 MHz, DMSO-d6): δ = 163.5, 151.1, 144.8, 135.9,
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134.3, 130.4, 130.2, 122.1, 121.7, 121.5, 93.1 ppm; IR (ATR): 2H) ppm; 13C NMR (100 MHz, D2O): δ = 169.5, 160.5, 156.0,
3451, 3305, 2940, 2732, 1733, 1668, 1635, 1412, 1391, 1320, 132.4, 119.5, 87.0, 59.6, 52.3 ppm; IR (ATR): 3381, 3079, 3044,
1235, 823, 754, 685, 511, 430 cm−1; HR-ESI-MS for C13H11N4O2 2955, 1608, 1535, 1476, 1439, 1373, 1101, 1071, 788, 585,
required 255.0882. Found 255.0884.
1-Benzyl-3-(2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-yl)-1H-imi- 223.0831.
dazolium chloride 22d. A solution of 147 mg (1.0 mmol) of 20 6-(3-Phenylimidazolio)-2,4(1H,3H)-pyrimidinedionate
546 cm−1; HR-ESI-MS for C9H11N4O3 required 223.0831. Found
2c.
and 316 mg (2.0 mmol) of 21d in 15 mL of chlorobenzene was 582 mg (2 mmol) of 22c was used. The general procedure
1
heated under reflux for 8 h. After cooling, a fine, light yellow yielded 406 mg (80%) of a light yellow solid: dec > 267 °C; H
precipitate formed, which was filtered off and thoroughly NMR (400 MHz, DMSO-d6): δ = 10.28 (s, 1H), 10.20 (s, 1H),
washed with diethyl ether. Recrystallization from water yielded 8.48 (m, 1H), 8.46 (m, 1H), 7.92–7.90 (m, 2H), 7.69–7.65 (m,
149 mg (49%) of a light yellow solid: dec > 258 °C; 1H NMR 2H), 7.63–7.59 (m, 1H), 5.90 (s, 1H) ppm; 13C NMR (100 MHz,
(400 MHz, DMSO-d6): δ = 11.58 (s, 1H), 10.19 (s, 1H), 8.24 (m, DMSO-d6): δ = 166.2, 157.1, 152.7, 134.6, 133.9, 130.0, 122.2,
1H), 8.09 (m, 1H), 7.55–7.53 (m, 2H), 7.47–7.71 (m, 3H), 6.16 121.8, 119.9, 85.5 ppm; IR (ATR): 3394, 3116, 3050, 2970, 1726,
(s, 1H), 5.56 (s, 2H) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 1676, 1618, 1545, 1217, 1109, 1081, 972, 764, 601, 528,
163.3, 150.6, 144.1, 137.0, 133.9, 128.9, 128.7, 128.2, 123.0, 421 cm−1; HR-ESI-MS for C13H11N4O2 required 255.0882.
121.5, 93.9, 52.6 ppm; IR (ATR): 3028, 2973, 2844, 1639, 1548, Found 255.0884.
1528, 1356, 1206, 974, 699, 438 cm−1
C14H13N4O2 required 269.1039. Found 269.1039.
;
HR-ESI-MS for
6-(3-Benzylimidazolio)-2,4(1H,3H)-pyrimidinedionate 2d.
610 mg (2 mmol) of 22d was used. The general procedure
1
3-(2,6-Dioxo-1,2,3,6-tetrahydropyrimidin-4-yl)-1-vinyl-1H-imi- yielded 413 mg (77%) of a light yellow solid: dec > 278 °C; H
dazolium chloride 22e. A solution of 147 mg (1.0 mmol) of 20 NMR (400 MHz, DMSO-d6): δ = 9.97 (s, 1H), 9.74 (s, 1H), 8.28
and 188 mg (2.0 mmol) of 21e in 15 mL of chlorobenzene was (m, 1H), 7.88 (m, 1H), 7.49–7.47 (m, 2H), 7.44–7.41 (m, 2H),
heated under reflux for 8 h. After cooling, a fine, light yellow 7.40–7.38 (m, 1H), 5.62 (s, 1H), 5.45 (s, 2H) ppm; 13C NMR
precipitate formed which was filtered off and thoroughly (100 MHz, DMSO-d6): δ = 167.4, 159.0, 155.3, 135.1, 129.5,
washed with diethyl ether. Recrystallization from water yielded 129.3, 128.9, 123.5, 119.7, 83.3, 52.8 ppm; IR (ATR): 3152,
135 mg (56%) of a light yellow solid: dec > 194 °C; 1H NMR 3061, 2948, 2775, 1610, 1544, 1475, 1395, 1376, 1185, 1110,
(400 MHz, D2O): δ = 9.79 (s, 1H), 8.08 (m, 1H), 8.02 (m, 1H), 908, 851, 768, 716, 701, 543, 447 cm−1; HR-ESI-MS for
7.25 (dd, J = 15.5/8.6 Hz, 1H), 6.11 (s, 1H), 5.98 (dd, J = 15.5/3.1 C14H13N4O2 required 269.1039. Found 269.1039.
Hz, 1H), 5.60 (dd, J = 8.6/3.1 Hz, 1H) ppm; 13C NMR (100 MHz,
6-(3-Vinylimidazolio)-2,4(1H,3H)-pyrimidinedionate
2e.
D2O): δ = 165.8, 152.0, 144.5, 135.0, 127.9, 121.6, 121.2, 112.5, 482 mg (2 mmol) of 22e was used. The general procedure
1
95.5 ppm; IR (ATR): 3101, 3061, 2923, 2832, 1727, 1681, 1571, yielded 318 mg (78%) of a light yellow solid: dec > 226 °C; H
1547, 1160, 1096, 1041, 579, 526 cm−1
C9H9N4O2 required 205.0726. Found 205.0726.
;
HR-ESI-MS for NMR (400 MHz, D2O): δ = 8.08 (d, J = 2.3 Hz, 1H), 7.94 (d, J =
2.3 Hz, 1H), 7.23 (dd, J = 15.5/8.6 Hz, 1H), 5.96 (s, 1H), 5.95
(dd, J = 15.5/3.0 Hz, 1H), 5.56 (dd, J = 8.6/3.0 Hz, 1H) ppm; 13
C
General procedure for the synthesis of the cross-conjugated
mesomeric betaines 2a–2e
A 150 mL portion of the anion-exchange resin Amberlite 1490, 1085, 975, 781, 612, 448 cm−1; HR-ESI-MS for C9H9N4O2
IRA-400 was filled into a column (height: 16 cm, diameter: required 205.0726. Found 205.0727.
NMR (100 MHz, D2O): δ = 169.4, 160.5, 155.8, 128.1, 120.2,
119.8, 111.1, 87.0 ppm; IR (ATR): 3372, 3110, 2966, 1676, 1623,
3 cm) and washed with 2 L of water. Then 100 mL of a 5%
6-(3-Methyl-2-thioxo-2,3-dihydro-1H-imidazol-1-yl)pyrimidine-
hydrochloric acid solution was added and remained in the 2,4(1H,3H)-dione 23. 326 mg (1 mmol) of caesium carbonate
column for 2 h. The hydrochloric acid was then rinsed out was added to a suspension of 96 mg (0.5 mmol) of 22a and
with water until pH 7 was reached. 100 mL of a 5% aqueous 38 mg (0.6 mmol) of sulfur in 15 mL of dry acetonitrile. The
solution of sodium hydroxide was added and remained in the mixture was heated under a nitrogen atmosphere under reflux
column for 2 h. Then the base was rinsed out with water until for 7 h. After cooling, the solvent was evaporated in vacuo. The
pH = 7 was reached. Then, samples of 2 mmol of uracilylhetare- product was separated with column chromatography (pure
nium salts 22a–22e in 100 mL of water, respectively, were ethyl acetate). Evaporating to dryness in vacuo formed a light
added on the resin. A flow rate of one drop per second was yellow solid: yield 82 mg (73%); dec > 320 °C; 1H NMR
adjusted. The eluates were evaporated to dryness in vacuo.
6-(3-Methylimidazolio)-2,4(1H,3H)-pyrimidinedionate
(400 MHz, DMSO-d6): δ = 9.77 (s, 1H), 7.62 (d, J = 2.6 Hz, 1H),
2a. 7.22 (d, J = 2.6 Hz, 1H), 6.78 (s, 1H), 3.48 (s, 3H) ppm; 13C
458 mg (2 mmol) of 22a was used. The general procedure NMR (100 MHz, DMSO-d6): δ = 167.0, 160.9, 158.7, 157.0,
yielded 323 mg (84%) of a white solid. All spectroscopic data 118.4, 115.5, 88.0, 34.4 ppm; IR (ATR): 3114, 2915, 2812, 1683,
are in agreement with those reported earlier.29
1617, 1564, 1399, 1366, 1268, 1233, 811, 541, 531, 431 cm−1
;
6-(3-(2-Hydroxyethyl)-imidazolio)-2,4(1H,3H)-pyrimidinedio- HR-ESI-MS for C8H7N4O2S required 223.0290. Found 223.0285.
nate 2b. 518 mg (2 mmol) of 22b was used. The general pro- 6-(3-Methyl-2-selenoxo-2,3-dihydro-1H-imidazol-1-yl)pyrimi-
1
cedure yielded 320 mg (72%) of a gray solid: dec > 230 °C; H dine-2,4(1H,3H)-dione 24. 326 mg (1 mmol) of caesium car-
NMR (400 MHz, D2O): δ = 8.05 (d, J = 2.2 Hz, 1H), 7.71 (d, J = bonate was added to a suspension of 96 mg (0.5 mmol) of 22a
2.2 Hz, 1H), 5.97 (s, 1H), 4.45 (t, J = 4.8, 2H), 4.01 (t, J = 4.8, and 47 mg (0.6 mmol) of selenium in 15 mL of dry
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acetonitrile. The mixture was heated under a nitrogen atmos- column chromatography (ethyl acetate–ethanol = 2 : 1). After
phere under reflux for 7 h. After cooling, the solvent was evap- evaporating to dryness in vacuo, recrystallization from ethanol
orated in vacuo. The product was separated with column formed a yellow solid: yield 40 mg (25%); dec > 204 °C; 1H
chromatography (pure ethyl acetate). Evaporating to dryness in NMR (400 MHz, DMSO-d6): δ = 10.70 (s, 1H), 8.49 (d, J = 2.1
vacuo formed a yellow solid: yield 125 mg (92%); dec > 312 °C; Hz, 1H), 8.20 (d, J = 2.1 Hz, 1H), 7.69–7.64 (m, 2H), 7.62–7.58
1H NMR (400 MHz, DMSO-d6): δ = 9.80 (s, 1H), 7.69 (d, J = (m, 3H), 6.13 (s, 1H), 0.91–0.82 (m, 2H), 0.37 (t, J = 7.5 Hz, 6H),
2.4 Hz, 1H), 7.41 (d, J = 2.4 Hz, 1H), 6.58 (s, 1H), 3.58 (s, 3H) 0.33–0.24 (m, 2H) ppm; 13C NMR (100 MHz, DMSO-d6): δ =
ppm; 13C NMR (100 MHz, DMSO-d6): δ = 166.7, 158.4, 157.2, 165.2, 151.3, 151.0, 126.7, 114.0, 81.9, 59.6, 50.8, 11.7,
153.7, 120.5, 118.4, 89.6, 36.5 ppm; IR (ATR): 3115, 2916, 1612, 10.5 ppm; 11B NMR (128 MHz, DMSO-d6): δ = 0.0 ppm; IR
1563, 1486, 1446, 1365, 1227, 862, 783, 459 cm−1; HR-ESI-MS (ATR): 3135, 2917, 2858, 1694, 1637, 1593, 1471, 1451, 1395,
for C8H7N4O2Se required 270.9734. Found 270.9736.
10,10-Diethyl-1-methyl-6,8-dioxo-6,7,8,10-tetrahydro-1H-imidazo- C17H19N4O2BNa required 345.1499. Found 345.1500.
[2′,1′:3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide 25a. A 1-Benzyl-10,10-diethyl-6,8-dioxo-6,7,8,10-tetrahydro-1H-
1258, 793, 763, 750, 735, 690 cm−1
; HR-ESI-MS for
solution of 460 mg (5 mmol) of BEt3 in 2 mL of dioxane was imidazo[2′,1′:3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide
added to a suspension of 96 mg (0.5 mmol) of 2a in 5 mL of 25d. A solution of 460 mg (5 mmol) of BEt3 in 2 mL of
dioxane. The mixture was then stirred at 200 °C under a nitro- dioxane was added to a suspension of 134 mg (0.5 mmol) of
gen atmosphere in a Schlenk-tube for 24 h. After cooling, the 2d in 5 mL of dioxane. The mixture was stirred at 200 °C
solvent was evaporated in vacuo. The product was separated by under a nitrogen atmosphere in a Schlenk-tube for 24 h. After
column chromatography (ethyl acetate–ethanol = 2 : 1). After cooling, the solvent was evaporated in vacuo. The product was
evaporating to dryness in vacuo, recrystallization from ethanol separated by column chromatography (ethyl acetate–ethanol =
1
formed a colorless solid: yield 61 mg (47%); dec > 251 °C; H 2 : 1). After evaporating to dryness in vacuo, recrystallization
NMR (400 MHz, CD3OD): δ = 8.00 (d, J = 2.0 Hz, 1H), 7.56 (d, from ethanol formed a yellow solid: yield 56 mg (33%); dec >
1
J = 2.0 Hz, 1H), 5.96 (s, 1H), 3.89 (s, 1H), 1.07–0.98 (m, 2H), 262 °C; H NMR (400 MHz, DMSO-d6): δ = 10.62 (s, 1H), 8.27
0.74–0.65 (m, 2H), 0.51 (t, J = 7.7 Hz, 6H) ppm; 13C NMR (d, J = 1.4 Hz, 1H), 7.81 (d, J = 1.4 Hz, 1H), 7.43–7.40 (m, 2H),
(100 MHz, D2O): δ = 168.5, 153.8, 153.5, 128.4, 115.2, 83.4, 7.38–7.35 (m, 1H), 7.32–7.31 (m, 2H), 6.01 (s, 1H), 5.34 (s, 2H),
35.8, 12.4, 10.9 ppm; 11B NMR (192 MHz, CD3CN): δ = 0.89–0.83 (m, 2H), 0.50–0.43 (m, 2H), 0.33 (t, J = 5.1 Hz, 6H)
1.3 ppm; IR (ATR): 3118, 2942, 2864, 1704, 1634, 1482, 1398, ppm; 13C NMR (100 MHz, DMSO-d6): δ = 165.6, 151.8, 151.4,
1317, 796, 769, 599, 451 cm−1; HR-ESI-MS for C12H18N4O2B 135.6, 129.3, 128.9, 128.1, 127.0, 115.3, 82.6, 52.1, 12.1,
required 261.1523. Found 261.1525.
10.9 ppm; 11B NMR (192 MHz, CD3CN): δ = −0.2 ppm; IR
10,10-Diethyl-1-(hydroxymethyl)-6,8-dioxo-6,7,8,10-tetrahydro- (ATR): 3173, 2928, 2896, 2855, 2814, 1706, 1661, 1560, 1475,
1H-imidazo[2′,1′:3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide 1367, 1238, 794, 758, 706, 694, 446, 419 cm−1; HR-ESI-MS for
25b. A solution of 460 mg (5 mmol) of BEt3 in 2 mL of C18H21N4O2BNa required 359.1655. Found 359.1658.
dioxane was added to a suspension of 111 mg (0.5 mmol) of
1-Methyl-6,8-dioxo-10,10-diphenyl-6,7,8,10-tetrahydro-1H-
2b in 5 mL of dioxane. The mixture was stirred at 200 °C under imidazo[2′,1′:3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide
a nitrogen atmosphere in a Schlenk-tube for 24 h. After 26a. A solution of 242 mg (1 mmol) of BPh3 in 2 mL of
cooling, the solvent was evaporated in vacuo. The product was dichloromethane was added to a suspension of 96 mg
separated by column chromatography (ethyl acetate–ethanol = (0.5 mmol) of 2a in 5 mL of dioxane. The mixture was stirred
2 : 1). After evaporating to dryness in vacuo, recrystallization at 200 °C under a nitrogen atmosphere in a Schlenk-tube for
from ethanol formed a colorless solid: yield 30 mg (16%); dec 24 h. After cooling, the solvent was evaporated in vacuo. The
> 185 °C; 1H NMR (400 MHz, DMSO-d6): δ = 10.60 (s, 1H), 8.22 product was separated by column chromatography (ethyl
(d, J = 2.0 Hz, 1H), 7.77 (d, J = 2.0 Hz, 1H), 6.00 (s, 1H), 4.12 (t, acetate–ethanol = 2 : 1). After evaporating to dryness in vacuo,
J = 5.3 Hz, 2H), 3.75 (t, J = 5.3 Hz, 2H), 0.92–0.83 (m, 2H), recrystallization from ethanol formed a white solid: yield
1
0.54–0.45 (m, 2H), 0.40 (t, J = 7.6 Hz, 6H) ppm; 13C NMR 91 mg (51%); dec > 352 °C; H NMR (400 MHz, DMSO-d6): δ =
(100 MHz, DMSO-d6): δ = 165.0, 151.2, 150.7, 136.0, 129.9, 10.73 (s, 1H), 8.32 (d, J = 2.0 Hz, 1H), 7.76 (d, J = 2.0 Hz, 1H),
129.8, 127.0, 123.7, 115.0, 82.5, 12.0, 10.4 ppm; 11B NMR 7.23–7.12 (m, 10H), 6.20 (s, 1H), 3.29 (s, 3H) ppm; 13C NMR
(192 MHz, CD3CN): δ = −0.4 ppm; IR (ATR): 3386, 3158, 3136, (100 MHz, DMSO-d6): δ = 164.9, 150.9, 150.3, 144.3, 133.5,
3022, 2935, 2900, 2659, 1668, 1641, 1563, 1452, 1402, 1079, 128.0, 127.1, 126.0, 114.6, 83.4, 35.4 ppm; 11B NMR (192 MHz,
787, 724, 595, 446 cm−1; HR-ESI-MS for C13H19N4O3BNa CD3CN): δ = −1.3 ppm; IR (ATR): 3154, 3115, 3041, 1683, 1661,
required 313.1448. Found 313.1448.
1460, 1398, 1240, 989, 876, 714, 700, 592, 582, 435 cm−1
.
10,10-Diethyl-6,8-dioxo-1-phenyl-6,7,8,10-tetrahydro-1H- HR-ESI-MS for C20H17N4O2BNa required 379.1342. Found
imidazo[2′,1′:3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide 379.1343.
25c. A solution of 460 mg (5 mmol) of BEt3 in 2 mL of dioxane
1-(2-Hydroxyethyl)-6,8-dioxo-10,10-diphenyl-6,7,8,10-tetra-
was added to a suspension of 127 mg (0.5 mmol) of 2c in 5 mL hydro-1H-imidazo[2′,1′:3,4][1,4,2]diazaborolo[1,5-c]pyrimidi-
of dioxane. The mixture was stirred at 200 °C under a nitrogen nium-10-ide 26b. A solution of 242 mg (1 mmol) of BPh3 in
atmosphere in a Schlenk-tube for 24 h. After cooling, the 2 mL of dichloromethane was added to a suspension of
solvent was evaporated in vacuo. The product was separated by 111 mg (0.5 mmol) of 2b in 5 mL of dioxane. The mixture was
2742 | Org. Biomol. Chem., 2014, 12, 2737–2744
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stirred at 200 °C under a nitrogen atmosphere in a Schlenk-
tube for 24 h. After cooling, the solvent was evaporated
Acknowledgements
in vacuo. The product was separated with column chromato- Dr Gerald Dräger, University of Hannover (Germany) is grate-
graphy (ethyl acetate–ethanol = 2 : 1). After evaporating to fully acknowledged for measuring the high resolution mass
dryness in vacuo, recrystallization from ethanol formed a light spectra.
brown solid: yield 37 mg (19%); dec > 255 °C; 1H NMR
(400 MHz, DMSO-d6): δ = 10.71 (s, 1H), 8.34 (d, J = 2.0 Hz, 1H),
7.74 (d, J = 2.0 Hz, 1H), 7.23–7.15 (m, 10H), 6.21 (s, 1H), 5.01
(t, J = 5.0 Hz, 1H), 3.95 (t, J = 5.0 Hz, 2H), 3.23 (q, J = 5.0 Hz,
Notes and references
2H) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 164.9, 150.7,
150.3, 144.3, 133.5, 127.7, 127.1, 126.0, 114.3, 83.4, 58.8,
50.8 ppm; 11B NMR (128 MHz, DMSO-d6): δ = −1.2 ppm; IR
(ATR): 3156, 3091, 3004, 1699, 1645, 1478, 1079, 996, 810, 752,
717, 701, 590, 433 cm−1; HR-ESI-MS for C21H19N4O3BNa
required 409.1448. Found 409.1447.
1 M. S. Novikov and A. N. Geisman, Chem. Heterocycl.
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6,8-Dioxo-1,10,10-triphenyl-6,7,8,10-tetrahydro-1H-imidazo-
[2′,1′:3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide 26c. A
solution of 242 mg (1 mmol) of BPh3 in 2 mL of dichloro-
methane was added to a suspension of 127 mg (0.5 mmol) of
2c in 5 mL of dioxane. The mixture was stirred at 200 °C under
a nitrogen atmosphere in a Schlenk-tube for 24 h. After
cooling, the solvent was evaporated in vacuo. The product was
separated by column chromatography (ethyl acetate–ethanol =
2 : 1). After evaporating to dryness in vacuo, recrystallization
from ethanol formed a light yellow solid: yield 65 mg (31%);
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dec >339 °C; H NMR (400 MHz, DMSO-d6): δ = 10.79 (s, 1H),
8.62 (d, J = 2.0 Hz, 1H), 8.22 (d, J = 2.0 Hz, 1H), 7.47–7.42 (m,
1H), 7.36–7.32 (m, 2H), 7.20–7.17 (m, 2H), 7.10 (s, 10H), 6.32
(s, 1H) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 164.9, 150.7,
150.0, 144.4, 135.7, 133.5, 129.7, 129.4, 127.4, 126.9, 125.9,
123.9, 115.5, 83.9 ppm; 11B NMR (128 MHz, DMSO-d6): δ =
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1428, 1392, 1251, 804, 762, 720, 689, 598, 546, 443 cm−1
;
HR-ESI-MS for C25H19N4O2BNa required 441.1499. Found
441.1501.
1-Benzyl-6,8-dioxo-10,10-diphenyl-6,7,8,10-tetrahydro-1H-
imidazo[2′,1′:3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide 10 D. P. Curran, A. Solovyev, M. M. Brahmi, L. Fensterbank,
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10294.
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1
yield 90 mg (42%); dec > 346 °C; H NMR (400 MHz, DMSO-
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13C NMR (100 MHz, DMSO-d6): δ = 164.9, 150.8, 150.2, 15 O. Schuster, L. Yang, H. G. Raubenheimer and M. Albrecht,
144.4, 134.0, 133.5, 128.5, 128.4, 128.1, 127.3, 126.2,
126.1, 115.6, 83.6, 51.6 ppm; 11B NMR (128 MHz, DMSO-d6):
δ = −1.5 ppm; IR (ATR): 3169, 3148, 3024, 3002, 1712, 1662,
Chem. Rev., 2009, 109, 3445; H. G. Raubenheimer and
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.
HR-ESI-MS for C26H21N4O2BNa required 455.1655. Found 16 A. Schmidt and Z. Guan, Synthesis, 2012, 3251; A. Schmidt
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2744 | Org. Biomol. Chem., 2014, 12, 2737–2744
This journal is © The Royal Society of Chemistry 2014