Tunable Push-Pull Interactions in 5-Nitrosopyrimidines

Article abstract of DOI:10.1021/acs.joc.6b00446

The effect of push-pull interactions in a series of variously substituted 5-nitrosopyrimidines on the strength of intramolecular hydrogen bonds, the height of rotational barriers around formally single bonds, UV-vis spectra and electrochemical behavior is explored. Intramolecular charge transfer (ICT) leads to a shift of electron density from electron-donating substituents, which is readily observable by NMR spectroscopy. The 5-nitroso group is able to form strong intramolecular hydrogen bonds with neighboring amino substituents. As a result, two rotamers with reversed orientation of the 5-nitroso group are observed for compounds with two different hydrogen-bond donors in neighboring positions. The barriers of interconversion between the two rotamers are strongly influenced by ICT, whereas the ratio of such rotamers depends primarily on the character of the hydrogen-bond donors. The ICT also significantly affects the position of UV-vis absorption maxima, which can be tuned in a broad range of 100 nm by the selection of appropriate substituents. Finally, ICT influences oxidation potential of the 5-nitrosopyrimidines and the stability of the resulting nitroso radical cations, the structures of which are determined by EPR spectroscopy.

Full text of DOI:10.1021/acs.joc.6b00446

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Article
Tunable push–pull interactions in 5-nitrosopyrimidines
Eliska Prochazkova, Lucie #echová, Ján Tarábek, Zlatko Janeba, and Martin Dracinsky
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b00446 • Publication Date (Web): 30 Mar 2016
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Tunable push–pull interactions in 5-nitrosopyrimidines
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Eliška Procházková, Lucie Čechová, Ján Tarábek, Zlatko Janeba, Martin Dračínský
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Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,
Flemingovo nam. 2, 166 10, Prague, Czech Republic
Eꢀmail: dracinsky@uochb.cas.cz, janeba@uochb.cas.cz
Abstract
The effect of push–pull interactions in a series of variously substituted 5ꢀnitrosopyrimidines
on the strength of intramolecular hydrogen bonds, the height of rotational barriers around
formally single bonds, UV/Vis spectra and electrochemical behavior is explored. Intramolecular
charge transfer (ICT) leads to a shift of electron density from electronꢀdonating substituents,
which is readily observable by NMR spectroscopy. The 5ꢀnitroso group is able to form strong
intramolecular hydrogen bonds with neighboring amino substituents. As a result, two rotamers
with reversed orientation of the 5ꢀnitroso group were observed for compounds with two different
hydrogenꢀbond donors in neighboring positions. The barriers of interconversion between the two
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rotamers are strongly influenced by ICT, whereas the ratio of such rotamers depends primarily on
the character of the hydrogenꢀbond donors. The ICT also significantly affects the position of
UV/Vis absorption maxima, which can be tuned in a broad range of 100 nm by the selection of
appropriate substituents. Finally, ICT influences oxidation potential of the 5ꢀnitrosopyrimidines
and the stability of the resulting nitroso radical cations, the structures of which are determined by
EPR spectroscopy.
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Introduction
1ꢀ4
Nitrosoarenes found many applications in synthetic organic chemistry. They, for
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example, play an important role as reactive metabolites, and are used as spin traps.
Recent studies of 5ꢀnitrosopyrimidines revealed that the nitroso group can form unusually
stable intramolecular hydrogen bonds (IMHBs) with amino substituents in neighboring C4
and C6 positions. In compounds with two different amino substituents in these positions,
two rotamers of the nitroso group stabilized by the IMHBs were observed by NMR
10ꢀ13
spectroscopy.
The ratio of such conformers depends significantly on the substitution
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of the hydrogen bond donors, and the barriers of the rotamer interconversion were
determined to be unusually high (> 22 kcal/mol). In several cases, slow rotamer
interconversion allowed the separation of pure rotamers by chromatographic techniques at
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room temperature. This phenomenon has been termed planamerism.
One of the reasons explaining the unusual stability of these IMHBs is resonance
stabilization of the hydrogen bond. High stability of intraꢀ and intermolecular hydrogen
bonds with a system of conjugated double bonds, such as in the enol form of acetylaceton,
15
has often been observed. The term resonanceꢀassisted hydrogen bonding (RAHB) has
been established for this type of hydrogen bonds, and the increased stability of these
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bonds was explained by a participation of distinct resonance structures differing in the
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position of the hydrogen atom. Planar aromatic nitrosoamines can be also stabilized by
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RAHB. We have recently investigated the resonanceꢀassisted stabilization of IMHBs in
polysubstituted 5ꢀnitrosopyrimidines by means of a combination of NMR experiments and
path integral molecular dynamics simulations that includes nuclear quantum effects
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(
NQEs). It was shown that resonance stabilization of hydrogen bonds is related with the
NQEs, particularly delocalization of the hydrogen atom. The formation of IMHB in 5ꢀ
nitrosopyrimidines leads to elongation of formally multiple bonds and the N–H bond, and
shortening of single bonds and the HꢁꢁꢁO contact.
Another principle that may contribute to the strength of the IMHBs in 5ꢀ
nitrosopyrimidines is the push–pull interaction of the substituents on the pyrimidine
moiety. Molecular πꢀsystems with an electron donating (D) and an electron accepting (A)
substituents are known as push–pull systems or systems with intramolecular charge
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transfer (ICT). Push–pull interaction of such substituents has been described to be
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crucial e.g. for nonꢀlinear optical properties,
high power conversion efficiency in dyeꢀ
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sensitized solar cells,
electroꢀoptic and piezochromic materials and supramolecular
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chemistry applications. The use of πꢀexcessive or πꢀdeficient heteroaromatics as the π
linker in the DꢀπꢀA systems has been shown to result in an increased ICT while the
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chemical and photochemical stability of these systems has been preserved.
Furthermore, the use of substituted heteroaromatic systems as the πꢀlinker allows finer
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tuning of the desired electronic and optical properties.
In all cases, where the high rotational barrier of the 5ꢀnitroso group has been observed,
three electronꢀdonating substituents were attached to the pyrimidine skeleton together
with the electronꢀwithdrawing nitroso group. This push–pull interaction may result in a
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shift of electron density from the electron donors to the 5ꢀnitroso group leading to an
increase of the bond order between the substituents and pyrimidine ring (Figure 1). Both
the partially double bond C5–NO and higher electron density at the nitroso group, making
it a better hydrogen bond acceptor, may contribute to the unusually high values of the
rotational barrier of the 5ꢀnitroso group.
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Figure 1. Push–pull interaction leading to intramolecular charge transfer (ICT) and higher
rotational barriers in polysubstituted 5ꢀnitrosopyrimidines.
Herein, we investigate the effect of the push–pull interactions on the extraordinary
stability of the intramolecular hydrogen bonds in polysubstituted 5ꢀnitrosopyrimidines,
which allowed, in several cases, the separation of two rotamers differing in the nitroso
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group orientation only. For a series of newly prepared compounds (Figure 2), we
determined the effects of ICT on the barriers of rotation around formally single bonds, on
their UV/Vis spectra, on electrochemical reactivity and stability of the generated nitroso
radical cations.
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Figure 2. The chemical structures of the studied compounds.
Results and Discussion
Synthesis
In order to study the influence of the C2 substitution on the physicoꢀchemical properties
of polysubstituted 5ꢀnitrosopyrimidines, series of pyrimidines bearing a variety of substituents
(H, CH , NHCH , NH , OCH and SCH ) in C2 position has been prepared. A nucleophilic
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aromatic substitution (S Ar) represents the key reaction for the modifications of the pyrimidine
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moiety in C2, C4 and C6 positions. Chlorine atom is usually easily replaced by the corresponding
nucleophiles (e.g. amines), as well as a methoxy group that is activated towards S Ar by the
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presence of strongly electron withdrawing nitroso group in C5 position. The synthesis of all
pyrimidine derivatives under study is depicted in Scheme 1. Microwaveꢀassisted (MWꢀassisted)
synthesis is utilized where possible, as this methodology usually gives excellent yields of
products in short reaction times.
Compound 1 was prepared by microwave assisted (MWꢀassisted) treatment of
commercially available 4,6ꢀdimethoxyꢀ2ꢀ(methylsulfonyl)pyrimidine (6) with ethanolic
methylamine. Nitrosation of compound 1 with isoamyl nitrite (IAN) in DMSO provided 5ꢀnitroso
derivative 2 and subsequent replacement of one of the methoxy groups with ammonia gave
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derivative 4b. Compound 4a was prepared according to the published procedure. The treatment
of starting pyrimidine 7 with IAN in DMSO at room temperature yielded 5ꢀnitroso derivative 4c
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and subsequent MWꢀassisted treatment with ethanolic methylamine afforded both 2,6ꢀ
di(methylamino) and 6ꢀmethylamino substituted products 5b (60 %) and 5c (22 %), respectively.
Methanolysis of compound 8 gave derivative 9, that was further nitrosylated to the final product
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4d. Furthermore, the 2ꢀmethylsulfonyl group of compound 9 was oxidized with mCPBA in
dichloromethane to give 2ꢀmethylsulfinyl derivative 10. The good leaving 2ꢀmethylsulfinyl group
in derivative 10 was then replaced with methylamine to give product 3. MWꢀassisted aminolysis
of pyrimidine derivative 8 with ethanolic methylamine (to give intermediate 11) followed by the
treatment with IAN in DMSO led to 5ꢀnitrosoderivative 5d. Analogous MWꢀassisted
ammonolysis of 4,6ꢀdichloroꢀ2ꢀmethylpyrimidine (12) with ethanolic solution of ammonia gave
compound 13, that was further converted to final derivative 5e by MWꢀassisted treatment with
ethanolic methylamine (to give intermediate 14) and subsequent nitrosation using IAN in DMSO
at room temperature. Finally, 4ꢀaminoꢀ6ꢀchloropyrimidine (15) was converted to the 5ꢀ
nitrosopyrimidine derivative 5f by a replacement of the 6ꢀchloro group with ethanolic solution of
methylamine under microwave irradiation (to give intermediate 16) and subsequent nitrosation
using NaNO and aq. HCl (application of IAN in DMSO did not afford the desired product in this
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Scheme 1. Synthesis of the studied compounds. Conditions: (i) methylamine (33 % ethanolic
solution), MW; (ii) IAN, DMSO, rt; (iii) ammonia (25% aqueous solution), rt; (iv) mPCBA,
CH Cl , 0°C; (v) MeONa, MeOH, reflux; (vi) ammonia (2.5 M ethanolic solution), MW; (vii)
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NaNO , HCl, H O, 0°C.
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NMR spectroscopy
The presence of intramolecular charge transfer (ICT) can be inferred from chemical shift
changes upon nitrosation of compound 1 as shown in Figure 3. The presence of the nitroso group
in compound 2 leads to a significant deshielding of the NH hydrogen in position C2 (by 1.8
ppm), which can be explained by a partial positive charge on the nitrogen atom stabilized by the
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ICT. N NMR chemical shifts of the NHCH nitrogen (detected indirectly by H,NꢀHMBC
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experiment) also indicate large deshielding (by 25 ppm) of the methylamino group after
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nitrosation. The experimental changes of chemical shifts of compound 1 upon nitrosation to give
derivative 2 correlate well with DFT predictions, which further confirms the ICT (details in the
Supporting Information).
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Furthermore, the ICT increases the C5–NO and C2–NH bond order leading to higher
rotational barriers and hence to slower rotation around these bonds. The barriers of rotation
around partially double bonds determined by dynamic NMR spectroscopy (dNMR) have been
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used as an experimental measure of the push–pull interaction.
The OCH groups in
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compound 1 are equivalent, because the fast rotation of the NHCH group averages their signals.
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On the other hand, the rotations around C5–NO or C2–NH bond are slowed down in compound 2
leading to observation of two nonꢀequivalent signals of the methoxy groups at room temperature.
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When H NMR spectrum of compound 2 is acquired at –50 °C, rotations around both the C2–NH
and C5ꢀNO bonds become slow and four signals of OCH groups are observed. These four
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signals correspond to two nonꢀequivalent methoxy groups in two possible conformers of the
compound (syn and anti orientation of the NO and NHCH groups). The observation of two
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signals of OCH groups in compound 2 at room temperature can thus be caused by high
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rotational barrier either of C2–NH or C5–NO bond avoiding chemical shift averaging. By
applying methods of dNMR at a broad temperature range (–50 – +140 °C), we determined the
two rotational barriers to be 21 and 9 kcal/mol, respectively (Tables 1 and 2). The very good
correlation with calculated rotational barriers allowed us to assign unambiguously the higher
barrier to the C2–NH rotation.
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Figure 3. Part of H NMR spectrum of compounds 1 and 2 with signal assignment.
In compounds 4b, 4c and 4d, the nitrosoꢀgroup oxygen atom and one of the neighboring
amino NH hydrogens form an intramolecular hydrogen bond, whose presence was clearly
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confirmed by H NMR spectra, where one of the amino hydrogens was shifted by 1–2 ppm to
higher chemical shift values due to the deshielding caused by the hydrogen bonding. The
intramolecular hydrogen bond also prevents the rotation of the 5ꢀnitroso group as confirmed by
DFT calculations, where the energy of the conformer of compound 4b without Hꢀbond has higher
energy by 4.0 kcal/mol.
Two sets of signals in 7:3 ratio corresponding to two possible orientations of the NHCH3
group in compound 4b can be observed in proton and carbon NMR spectra and the barrier of
their interconversion was determined by dNMR to be 19.3 kcal/mol (Table 1). The signals were
assigned to individual conformers on the basis of longꢀrange heteronuclear constants (details in
SI). No signal splitting into two sets was observed in NMR spectra of compounds 4c and 4d; only
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minor line broadening of proton signals was observed at –50 °C, which indicates that the C2–R
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(
R = OCH or SCH ) rotational barriers are significantly lower in these compounds than in
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compound 4b, where larger ICT is promoted by strongly electronꢀdonating methylamino group in
C2 position.
A comparison of the rotational barriers in compounds 3 and 4b reveals similar difference
of ca 6.5 kcal/mol as in the case of compounds 1 and 2 but the barriers are slightly lower for the
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6ꢀamino substituted compounds 3 and 4b.
Table 1. Observed composition of conformational mixtures resulting from restricted rotation
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around the C2–R bond, and observed and DFTꢀcalculated barriers of rotation (kcal/mol) around
the bond.
2
5
a
‡
‡
Compound
R
R
%Cexp
ꢂG
ꢂG
calcd
exp
b
1
2
3
NHCH
NHCH
NHCH
NHCH
3
3
3
3
H
50
14.4±1.0
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12.8±1.0
19.3±1.0
15.5
20.4
14.8
20.1
8.3
NO
H
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30±2
4b
NO
NO
NO
NO
NO
NO
c
c
4c
OCH
SCH
NHCH
OCH
SCH
3
–
–
c
c
4d
3
–
–
9.2
5b
3
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19.6±1.0
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8.2
c
c
5c
3
–
–
c
c
5d
3
–
–
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a
The percentage of the rotamer with the methyl group of the C2 substituent heading towards C4
b
c
(
see Figure S2 in SI); The rotation around C2–NHCH bond leads to identical structure; No
3
2
signal splitting due to restricted rotation around C2–R bond observed at –50 °C.
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In the case of compounds 5a–5f, two IMHBs with different orientation of the 5ꢀnitroso
group are possible(Figure 1). The observed ratio of the two conformers with different orientation
of the nitroso group is almost independent on the nature of the substituent in C2 position (Table
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2
). This observation is in clear contrast to the influence of substituents in position C4 and C6,
which have been found to enable fine tuning of the NO group rotamer ratio in a very broad
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range. The electronic structure of compounds 5a–5f is also reflected in the observed chemical
shifts. The chemical shift of the 6ꢀmethylamino group hydrogen atom involved in the hydrogen
bonding in rotamer A is above 11 ppm, whereas the chemical shift of the unbound hydrogen in
rotamer B is in the range of 8.6–9.6 ppm (Table S2). Electronꢀdonating substituents in position
C2 increase the shielding of carbon atom C5, leading to lower chemical shift of C5. Chemical
shift of carbon atom C4 in rotamer A is almost unaffected by the nature of the C2 substituent,
while chemical shift of C6 increases with more electronꢀdonating character of the substituent in
the C2 position. This observation is somewhat unexpected, since electronꢀdonating substituents
normally increase shielding of carbon atoms in (hetero)aromatic systems. However, carbon C6 in
rotamer A of compounds 5a–5f is directly attached to the nitrogen atom involved in the IMHB
with the 5ꢀnitroso group, and electronꢀdonating substituents in the C2 position induce larger ICT.
This leads to larger negative charge of the NO oxygen making it a better hydrogenꢀbond
acceptor, which in turn leads to a deshielding of carbon C6. More detailed discussion of chemical
shifts of compounds 5a–5f is presented in SI.
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1
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2
2
2
2
2
2
2
2
2
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3
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3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2. Observed composition of conformational mixtures resulting from restricted rotation
around the C5–NO bond, and observed and DFTꢀcalculated barriers of rotation (kcal/mol) around
the bond.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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9
0
1
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8
9
0
2
a
‡
‡
Compound
R
%Aexp
ꢀ
ꢂG
ꢂG calcd(A→B)
exp
b
2
NHCH
NH
NHCH
OCH
SCH
3
~9
14.7
28.6
28.5
26.2
25.0
24.8
23.4
c
5a
2
64±2
63±2
66±2
66±2
66±2
66±2
23.0±0.2
d
5b
3
ꢀ
5c
3
22.0±1.0
18.2±1.0
5d
3
d
5e
CH
3
ꢀ
5f
H
16.1±1.0
a
b
The percentage of the rotamer with the NO group heading towards C6 (see Figure 1); The
1
height of the barrier is estimated from H NMR spectra acquired at two temperatures only due to
c
severe signal overlap at other temperatures; The rotational barrier was determined by following
the interconversion of the rotamers after dissolution of initially pure rotamer B (Figure S3 in SI);
d
Impossible to determine due to severe signal overlap.
The two rotamers of compound 5a were separated by column chromatography and
1
kinetics of the rotamer interconversion was observed by continuous measurement of H NMR
spectra of a solution of one initially pure rotamer. The barrier of rotamer interconversion was
determined to be 23 kcal/mol (see details in SI).
In compound 5b both rotational barriers around C2–NHCH and C5–NO bonds are high
3
enough to permit observation of four sets of signals corresponding to four possible conformers of
the compound in DMSO solution at room temperature. The assignment of all NMR signals was
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possible thanks to NMR measurements on an 850 MHz spectrometer, where the overcrowded
aliphatic region could be resolved (Figure S4), and was again based on the observation of longꢀ
range heteronuclear interactions. Analysis of variableꢀtemperature NMR spectra allowed
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estimating the rotational barrier around the C2–NHCH bond to be 19.6 kcal/mol.
3
In NMR spectra of compounds 5c–5f, only two sets of signals were observed, which
correspond to two rotamers defined by the orientation of the 5ꢀnitroso group. No signal splitting
2
2
due to slow rotation around the C2–R bonds (where R = OCH , SCH , CH , or H) was observed
3
3
3
down to –50 °C.
The influence of protonation on the rotational barriers and rotamer ratio was determined
by adding one equivalent of hydrochloric acid to the samples of compounds 4b and 5c. The
protonation disturbs the ICT by decreasing the electron density in the pyrimidine ring. The ratio
of conformers resulting from rotation of both the C2 and C5 substituents was not altered by the
protonation, while both rotational barriers decreased slightly: by 0.2 kcal/mol (C2–NHCH3
barrier in 4b) and 2.0 kcal/mol (C5–NO barrier in 5c).
2
The rotational barriers around the C2–R and C5–NO bonds were also calculated by DFT
method. A transition state (TS) with the C2 or C5 substituent in perpendicular orientation with
respect to the pyrimidine ring was found in all cases and the calculated barriers agree well with
experiments, although the C5–NO barriers are systematically overestimated by ca 5 kcal/mol.
This overestimation is probably due to an insufficient description of solvent effects by the
polarizable continuum model (PCM) used in the calculations. Hydrogen bonding between the
‘naked’ amino hydrogen and solvent molecules will stabilize the transition state, but these
3
7
specific interactions are not modelled well with the PCM approach. Generally, electronꢀ
donating substituents in position 2 increase both rotational barriers. The absence of the nitroso
2
group in C5 position leads to a drop of the C2–R barrier by 5 kcal/mol (compounds 1 and 2, and
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1
1
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3
and 4b, Table 1), and the change from strongly electronꢀdonating amino and methylamino
2
groups to less electronꢀdonating substituents leads to a significant decrease of the C2–R barrier.
The presence of the intramolecular hydrogen bond between NO and adjacent NH groups
stabilizes the planar arrangement of the compounds making the C5–NO rotational barrier
significantly higher and the barrier in compounds 5a–5f varies in a broad range depending on the
nature of the C2 substituent.
0
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0
The ICT is also reflected in bond lengths and dipole moments of the molecules. In the TS
structures, where efficient ICT is blocked by the nonplanar arrangement of the molecules, the
C5–N and N=O bond lengths are almost constant for all molecules 5a–5f and the calculated
dipole moment varies only slightly. On the other hand, in the energyꢀminima structures, the C5–
N bond is by more than 0.1 Å shorter and the N=O bond by 0.05 Å longer than in the TS
structures. Furthermore, both distances and the dipole moments are significantly substituent
dependent (Table S3 in the SI); more electronꢀdonating substituents in C2 position make the C5–
N bond shorter, the N=O bond longer and the dipole moment larger. Similarly, the C2–NHCH3
bond is significantly shorter in compound 2 (with NO group in position 5) than in compound 1.
UV/Vis absorption spectra
The UV/Vis spectra of the 5ꢀnitrosopyrimidines solutions in DMSO exhibit one weak
electronic transition in the visible region (552–644 nm, Figure 4) and one strong transition in the
UV region (336–358 nm, Figure S7), which could be assigned with the help of timeꢀdependent
densityꢀfunctional theory (TDꢀDFT) calculations. The color of the solutions ranging from violet
to green (Figure S8) is caused by the weak electronic transition between lone pair and
antibonding π orbitals (n → π*, HOMO → LUMO), and the UV band is caused by π → π*
(HOMO–2 → LUMO) transition. The position of the maximum in the visible region depends
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significantly on the substituent in C2 position (Table 3, Figure 4) with a large hypsochromic shift
(
more than 100 nm) for electronꢀdonating substituents. On the other hand, the position of the UV
maximum is significantly less substituentꢀdependent. Electronꢀdonating substituents in C2
position increase the energy of both frontier orbitals. The effect is bigger, however, for LUMO,
which results in an increase of the energy gap between HOMO and LUMO for stronger electronꢀ
donors (Table 3).
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1
1
1
1
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1
1
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1
2
2
2
2
2
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9
0
Figure 4. Normalised absorption spectra of compounds 5a–5f in the visible region of light.
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5
5
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Table 3. Observed and calculated wavelengths of UV/Vis absorption maxima (nm) and energies
of frontier orbitals (eV) of compounds 5a–5f.
2
Compound
R
UVEXP VISEXP UVCALC VISCALC HOMO LUMO
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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9
0
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2
3
4
5
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8
9
0
1
2
3
4
5
6
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8
9
0
5
a
NH
2
336
340
342
354
354
358
552
552
596
618
630
644
321
326
330
347
348
350
570
567
597
617
627
642
–0.224 –0.089
–0.223 –0.088
–0.230 –0.097
–0.231 –0.102
–0.231 –0.103
–0.234 –0.107
5b
NHCH
OCH
SCH
3
5
c
3
5d
3
5e
CH
3
5
f
H
EPR spectroelectrochemistry
The push–pull effect leads to an increase of the electron density in the 5ꢀnitroso group and
can, therefore, lead to its easier oxidation. We performed a series of EPR spectroelectrochemical
experiments in order to corroborate the influence of the push–pull effect on the NO oxidation and
to confirm the structure of oxidized species.
Figure 5 shows cyclic voltammograms of compounds 5a and 5f. The most remarkable
feature, related to the ease of oxidation, is the potential difference of the first anodic wave
between compounds 5f and 5a (ꢂEp/2,ox = Ep/2,ox(5f) − Ep/2,ox(5a) = 0.32 V). While the cyclic
voltammogram of 5a is quasiꢀreversible, that of 5f is completely irreversible, probably due to a
followꢀup chemical reaction of the 5f radical cation (see the EPR analysis below). Therefore, the
halfꢀpeak potential values (E ) are compared instead of the more common halfꢀwave potentials
p/2
3
8 [21]
(E ).
Halfꢀwave potentials, which are directly related to standard redox potentials, can be
1
/2
38
estimated only from voltammograms showing a fully reversible electron transfer. As
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demonstrated by NMR spectroscopy, both 5ꢀnitrosoꢀgroup rotamers (A and B) of compounds 5a
and 5f exist in solution. DFT calculations revealed that the oxidation potential difference between
rotamer A and B is 0.013 V and 0.018 V for compounds 5a and 5f, respectively. These small
differences cannot be observed in the voltammograms, but may contribute to their rather broad
shape. The oxidation potential difference between compounds 5f and 5a obtained by DFT (0.290
V for rotamer A and 0.260 V for rotamer B) are reasonably close to the experimental ꢂEp/2,ox
values. Moreover, the potential difference between 5f and 5a in the moment of observation of the
first EPR spectrum during the EPR spectroelectrochemical experiment was 0.37 V, which also
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correlates well with the ꢂEp/2,ox
.
Figure 5. Cyclic voltammograms of compounds 5a and 5f in 0.25 M TBAPF /CH CN. Halfꢀpeak
6
3
potential (E ) of the first anodic wave is 0.787 V and 1.111 V for 5a and 5f, respectively. The
p/2
+
potential scale is referenced against that of the Fc/Fc redox couple. The scan rate was 5 mV/s.
The assumption that the anodic waves correspond to oneꢀelectron oxidation from neutral
to radical cationic species was proved by EPR spectroscopy measured simultaneously during the
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cyclovoltammetric scan. Figure 6 shows representative EPR spectra recorded in situ during the
electrochemical oxidation of compounds 5a and 5f. The spectra exhibit large hyperfine splitting
(a ≈ 3.1 mT) between three equally intensive lines. Large a values can be attributed to nitroso
iso
iso
0
1
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0
•+
•
39
radical cations (−N=O ) rather than neutral nitroxyl (>N−O ) radicals, which was also
confirmed by DFT calculations, where we found an excellent agreement of calculated EPR
parameters of the radical cations with experiment (see SI). Superhyperfine splitting (SHFS) can
be well recognized in the EPR spectrum of oxidized compound 5a. By comparison with spectra
of similar compounds and with DFT calculations (see SI), we conclude that the SHFS is caused
by interactions of the unpaired electron with the amino groups in the neighboring positions C4
and C6. The expected SHFS from the amino groups in the spectrum of oxidized compound 5f
was not observable because the radical cation of 5f was not persistent enough during the
experiment, which leads to broader EPR lines. After the oxidation of 5f the EPR signal appears
but it rapidly decreases during the reꢀreduction. This observation is consistent with the
irreversible voltammogram of 5f. On the other hand, the signal appearance and disappearance
was completely reversible during the oxidation and reꢀreduction process for compound 5a and
other 5ꢀnitrosopyrimidines with amino substituent in C2 position (shown in SI). Therefore, the
“
push–pull” effect may also play an important role in the stabilization of the 5ꢀnitrosopyrimidine
radical cations.
As shown by NMR spectroscopy, two stable conformers (rotamer A and B) are present in
solutions of compounds 5a–5f and therefore, the presented voltammograms and EPR spectra
correspond to the mixture of both as well. However, DFT calculations (shown in the SI) revealed
that hyperfine splitting/coupling as well as gꢀfactors of both rotamers of compounds 5a and 5f are
very close to each other. This complicates the analysis of SHFS by simulations of the EPR
spectra of these compounds. Therefore, we also performed lowꢀtemperature EPR
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spectroelectrochemical experiments with pure rotamer A of another 5ꢀnitrosopyrimidine
1
4
derivative (depicted in SI) that was shown recently to be easily isolable and sufficiently stable.
The resulting EPR spectrum corresponding to pure rotamer A of the radical cation was then
readily analyzed by spectral simulations and all spectral parameters including SFHS values were
determined (see details in SI).
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Figure 6. Representative EPR spectra recorded during the EPR spectroelectrochemical
experiments in 0.25M TBAPF /ACN solution of compounds 5a and 5f. The starting
6
concentration of individual compounds in electrolyte solution was c = 2 mM. Corresponding
DFT calculated spin densities are shown in SI.
Conclusions
We have demonstrated that the “push–pull” effect, based on cooperative interactions of
the electronꢀaccepting 5ꢀnitroso group and electronꢀdonating substituents in other positions of the
pyrimidine moiety, has a large impact on electronic structure in polysubstituted 5ꢀ
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nitrosopyrimidines. This effect is detectable in NMR spectra by significant chemical shift
changes and by a dramatic increase of rotational barriers around formally single bonds. The
presence of a single electronꢀaccepting nitroso group increased the rotational barrier of NHCH3
group in C2 position by 6.5 kcal/mol leading to observation of two sets of signals in NMR
spectra at room temperature corresponding to two possible rotamers of the methylamino group.
Similarly, the electronꢀdonating character of the C2 substituent has an influence on the C5–NO
bond order leading to large changes of the rotational barriers around this bond.
0
1
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1
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9
0
The intramolecular charge transfer (ICT) also contributes significantly to the stabilization
of intramolecular hydrogen bonds in 5ꢀnitrosopyrimidines. In compounds with two competing
intramolecular hydrogen bonds, ICT significantly affects the interconversion barrier between the
two structures but not the composition of the equilibrium mixture. Both the partially double C5–
NO bond and higher electron density at the nitroso oxygen, making it a better hydrogen bond
acceptor, may contribute to the height of the rotational barrier of the 5ꢀnitroso group. As shown
previously, the composition of the equilibrium mixture (the rotamer ratio) is, on the other hand,
strongly influenced by the nature of the hydrogen bond donors. A clever design of 5ꢀ
nitrosopyrimidines can thus lead to development of versatile “molecular switches” with tunable
equilibrium composition and interconversion (“switching”) barrier.
The ICT also influences the reactivity of 5ꢀnitrosopyrimidines, namely the oxidation of
the nitroso group to nitroso radical cation. Electron donors attached to the pyrimidine ring
increase the electron density of the nitroso group, which is then oxidized more easily.
Polysubstituted 5ꢀnitrosopyrimidines may find various interesting applications. For
example, 5ꢀnitrosopyrimidines with intramolecular hydrogen bond forming a pseudo ring have
been proposed to behave as purine mimics with potential use in drug development. Furthermore,
these colorful compounds also absorb strongly in the UV region of light, which makes them
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promising candidates for (bio)sensing. For that purpose, the position of absorption maxima can
be finely tuned by choice of appropriate substituents.
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Experimental Section
General. Instrumentation and calculations.
Starting compounds 6, 7, 8, 12, and 15 (Scheme 1) and other chemicals were purchased
from commercial suppliers. Solvents were dried by standard procedures. Unless otherwise stated,
solvents were evaporated at 40 °C/2 kPa, and the compounds were dried over P O at 2 kPa.
2
5
Analytical TLC was performed on silica gel preꢀcoated aluminum plates with fluorescent
indicator. Spots were visualized with UV light (254 nm). Column chromatography was carried
out on silica gel (40ꢀ63 ꢀm). Mass spectra were measured on a QꢀTof micro and HR MS were
taken on a LTQ Orbitrap XL spectrometer. Melting points were determined on a Stuart SMP3
Melting Point Apparatus and are uncorrected.
The microwaveꢀassisted reactions were carried out in CEM Discover (Explorer)
microwave apparatus, 24ꢀposition system for 10ꢀmL vessels sealed with Teflon septum. It was
operated at a frequency of 2.45 GHz with continuous irradiation power from 0 to 300 W. The
solutions were steadily stirred during the reaction. The temperature was measured with an IR
sensor on the outer of the process vessel. The vials were cooled to ambient temperature with gas
jet cooling system. The pressure was measured with an inboard CEM Explorer pressure control
system (0ꢀ21 bar).
The NMR spectra for structure determination were recorded at temperature.t. on a
1
13
1
spectrometer operating at 850.3 MHz for H and 213.8 MHz for C, or at 500.0 MHz for H and
13
1
13
1
25.7 MHz for C, or at 401.0 MHz for H and 100.8 MHz for C in DMSOꢀd solutions
6
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6
(
referenced to the solvent signal δ = 2.50 and 39.70 ppm, respectively). 1D and 2D correlation
NMR experiments (COSY, HSQC, HMBC) were combined for the signal assignment. The
variableꢀtemperature NMR spectra were recorded on a spectrometer operating at 499.9 MHz for
0
1
2
3
4
5
6
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9
0
1
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0
1
H in DMSOꢀd or DMFꢀd solutions.
6
6
2
The barriers of rotation around C5–N and C2–R bonds were determined by complete
1
lineꢀshape analysis of variableꢀtemperature H NMR spectra (dNMR approach). Where possible,
the analysis was done for singlet signals of nonꢀexchangeable protons (OCH , CH groups),
3
3
because chemical exchange of NH hydrogen atoms with traces of water present in the solutions
complicates the analysis of the lineꢀshapes of signals of these protons and of the protons coupled
with these protons (NHCH groups). Note, however, that the signals were sometimes overlapped
3
with water or solvent residual signals leading to a limited number of available temperature points
for the analysis. Therefore, the error bars were estimated to be higher than usually found in
dNMR analysis. The influence of protonation on the rotational barriers was studied on DMSOꢀd6
solutions of compounds 4b and 5c with one equivalent of hydrochloride acid.
plus
EPR spectra were measured on EMX −10/12 CW (continuous wave) EPR spectrometer
equipped with the Premium Xꢀband microwave bridge. Flat EPR quartz cell was used during the
EPR spectroelectrochemical experiments at room temperature. A homeꢀmade threeꢀelectrode
system consisting of a Ptꢀwire contacted with Ptꢀµꢀmesh, serving as a working electrode, a
quasireference Agꢀwire electrode and Ptꢀwire, serving as a counter electrode, was inserted into
the flat cell. The nonꢀactive electrode parts were isolated either with a Teflon tape or sealed in a
glass capillary. After each EPR spectroelectrochemical measurement, stability of the Agꢀ
+
quasireference electrode was checked against the ferrocene/ferrocenium (Fc/Fc ) couple. All
presented potentials are calculated against this redox couple. The solution of the studied
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compound in 0.25M TBAPF /ACN was intensively flushed with N gas before every experiment,
6
2
in order to minimize the influence of the air oxygen on the electrochemical experiments and on
the EPR line broadening. Simultaneous voltammetric/potentiostatic experiments were controlled
by the Potentiostat/Galvanostat Autolab PGSTAT302N. Low voltammetric scan rate (ν =
10
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−
1
5
mVs ) allowed the determination of every potential corresponding to the middle of the EPR
spectrum (dIEPR/ dB = 0) with the precision of ±4mV. This enabled assignment of the EPR
spectra to paramagnetic species formed at defined potential. The spectroelectrochemical
experiments at lower temperatures (258 K) were performed with a compatible EPR flat cell with
3
ꢀelectrode system for the variable temperature setꢀup, and temperature was controlled by the
liquid/gas nitrogen unit ER 4141VTꢀU and the WinꢀEPR acquisition software. The gꢀfactor of the
radical ionic species was determined using a builtꢀin spectrometer frequency counter and an ER
0
36TM NMRꢀTeslameter.
4
0ꢀ41
Simulation of EPR spectra was done within the EasySpin 5.0.16 package
and treated
by the Origin data analysis software. The modulation amplitude, central field, sweep width,
microwave frequency and the spectral resolution (number of points) were included in the
simulation.
The geometry of all of the studied structures was optimized at the DFT level of theory,
42ꢀ43
using B3LYP functional
and a standard 6ꢀ31+G(d,p) basis set. The NMR parameters were
calculated using the GIAO method with polarizable continuum model used for implicit DMSO
44ꢀ45
46
solvation.
The Gaussian09 program package was used throughout this study. The QST3
47ꢀ48
optimization method
was applied in the search for the transition state structures of the
rotamer interconversion, that is the structures of the reactant, product, and estimated transition
state were used as input for the TS search. The vibrational frequencies and free energies were
calculated for all of the optimized structures, and the stationaryꢀpoint character (a minimum or a
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firstꢀorder saddle point) was thus confirmed. Five lowestꢀenergy electronic transitions for the
4
9
compounds 5a–5f were calculated with the timeꢀdependent DFT method with PCM method of
DMSO solvation.
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The EPR hyperfine coupling(A)/splitting(a) constants (HFCCs/HFSCs) as well as the gꢀ
factors and the spin densities were calculated by B3LYP using the doubleꢀ and tripleꢀζ basis sets:
5
0
EPR–II for C and H atoms, EPR–III for N and O atoms. The oxidation potential of the studied
51
nitroso pyrimidines was estimated by Born-Haber cycle, evaluating the Gibbs free energies of
the neutral and radical cationic species in the gasꢀphase and in the solution environment, with
5
2
53
M06L local density functional and augmented ccꢀpVTZ Dunning’s basis set (see details in the
Supporting Information). All computations for the openꢀshell systems (radical cations) were
performed in unrestricted fashion and the resulting spin contamination was always small with the
2
ɸ
eigenvalue of ͍ not exceeding 0.77. Solvent (acetonitrile) effects in the EPR calculations were
incorporated by the selfꢀconsistent reaction field theory at the level of conductorꢀlike polarizable
4
4ꢀ45
continuum model CꢀPCM.
All geometries and spin densities were visualized by VMD
54ꢀ55
software package.
General procedure A.
Isoamyl nitrite (IAN, 1.1 equivalent) was added dropwise to the solution of starting
compound (1, 7, 9, 11 and 14) in DMSO (20–40 mL). The reaction mixture was stirred at room
temperature overnight and water (20–40 mL) was added. Compound 4c precipitated from the
solution and product was filtrated of. The reaction mixtures with products 2, 4d, 5d and 5e were
extracted with EtOAc (3 x 30 mL) and the products were isolated by silica gel column
chromatography (10 % MeOH in CHCl3).
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4
,6-Dimethoxy-N-methylpyrimidin-2-amine (1). A mixture of 6 (400 mg, 1.8 mmol)
and methylamine (61.4 mg, 2 mmol, 33 % ethanolic solution) in iPrOH (20 mL) was treated
under MW conditions (130 °C, 30 min). The reaction mixture was diluted with water (20 mL)
10
11
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and extracted with EtOAc (3 x 30 mL). The combined organic layers were dried over MgSO and
4
filtered. The resulting mixture was evaporated under vacuum and product was isolated by silica
gel flash chromatography (5–10 % MeOH in CHCl ). Compound 1 (268 mg, 89 %) was obtained
3
1
as a white solid with mp = 123–125 °C. H NMR (500.0 MHz, DMSOꢀd ): 6.98 (1H, q, J
=
NHꢀCH3
6
13
4.7, NH), 5.33 (1H, s, H5), 3.78 (6H, bs, OꢀCH ), 2.77 (3H, d, JCH3ꢀNH = 4.7, NHꢀCH ); C NMR
3
3
(
125.7 MHz, DMSOꢀd ): 171.6 (C4 and C6), 162.2 (C2), 77.6 (C5), 53.2 (OꢀCH ), 27.8 (NHꢀ
6
3
+
+
CH ); ESI MS, m/z (%): 170.2 [M + H] ; HRMS (ESI) calcd for C H N O [M + H] 170.0924,
3
7
12
3
2
found 170.0922.
4
,6-Dimethoxy-N-methyl-5-nitrosopyrimidin-2-amine (2). Treatment of 1 (200 mg, 1.2
mmol) with IAN (154 mg, 1.3 mmol) by general procedure A gave 2 (88 mg, 37 %) as a green
1
solid with mp = 185–188 °C. H NMR (500.0 MHz, DMSOꢀd ): 8.78 (1H, q, J
= 4.7, NH),
6
NHꢀCH3
13
4
.01 and 3.94 (2 x 3H, 2 x s, OꢀCH ), 2.93 (3H, d, J
= 4.7, NHꢀCH ); C NMR (125.7
3
CH3ꢀNH 3
MHz, DMSOꢀd ): 161.2 (C2), 141.6 (C5), 54.3 (OꢀCH ), 28.3 (NHꢀCH ), C4 and C6 were not
6
3
3
+
+
detected at r.t.; ESI MS, m/z (%): 199.2 [M + H] ; HRMS (ESI) calcd for C H N O [M + H]
7
11
4
3
199.0825, found 199.0824.
6-Methoxy-2-(methylthio)pyrimidin-4-amine (9). A mixture of sodium methoxide (308
mg, 5.7 mmol) in MeOH (10 mL) was slowly added to the solution of 8 (300 mg, 1.7 mmol) in
MeOH (30 mL). The reaction mixture was refluxed for 72 h. The solution was cooled down,
diluted with water (30 mL) and extracted with EtOAc (3 x 30 mL). The organic layers were
collected, dried over MgSO and filtered. The solvent was evaporated under vacuum. Compound
4
1
9
(264 mg, 91 %) was obtained as a white solid with mp = 125–126 °C. H NMR (500.0 MHz,
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
13
DMSOꢀd ): 6.66 (2H, bs, 4ꢀNH ), 5.42 (1H, s, H5), 3.77 (3H, s, OꢀCH ), 2.40 (3H, s, SꢀCH ); C
6
2
3
3
NMR (125.7 MHz, DMSOꢀd ): 169.8 (Cꢀ2), 169.2 (Cꢀ6), 165.3 (Cꢀ4), 81.3 (Cꢀ5), 53.2 (OꢀCH ),
6
3
+
2.40 (3H, s, SꢀCH ); ESI MS, m/z (%): 172.1 [M + H] ; HRMS (ESI) calcd for C H N OS [M +
3
6
10
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
H] 172.0539, found 172.0536.
6
-Methoxy-2-(methylsulfinyl)pyrimidin-4-amine (10). Compound 9 (200 mg, 1.2
mmol) was dissolved in DCM (40 mL) and obtained solution was cooled to 0 °C. Mixture of
mCPBA (309 mg, 1.8 mmol) in DCM (20 mL) was then added dropwise during 30 min. After
full conversion (TLC), solvent was evaporated under vacuum. Silica gel column chromatography
1
(
10 % MeOH in CHCl ) gave 10 (159 mg, 71 %) as a white solid with mp = 187–190 °C. H
3
NMR (401.0 MHz, DMSOꢀd ): 7.18 (2H, bs, 4ꢀNH ), 5.71 (1H, s, H5), 3.82 (3H, s, 6ꢀOCH ),
6
2
3
13
2.77 (3H, s, SOꢀCH ); C NMR (100.8 MHz, DMSOꢀd ): 172.5 (C2), 170.0 (C6), 166.2 (C4),
3
6
+
8
5.2 (C5), 54.0 (6ꢀOCH ), 39.4 (SOꢀCH ); ESI MS, m/z (%): 210.1 [M + Na] ; HRMS (ESI)
3 3
+
calcd for C H N O S [M + H] 188.0488, found 188.0487.
6
10
3
2
2
4-Methoxy-N -methylpyrimidin-2,6-diamine (3). A mixture of 10 (100 mg, 0.5 mmol)
in iPrOH (10 mL) was treated under MW conditions (150 °C, 60 min). Water (50 mL) was added
and obtained solution was extracted with EtOAc (3 x 30 mL). Organic layers were collected,
dried over MgSO and filtered. Solvent was evaporated under vacuum. Silica gel column
4
1
chromatography gave product 3 (30 mg, 39 %) as a white solid with mp = 148–150 °C. H NMR
(500.0 MHz, DMSOꢀd ): 6.29 (1H, bs, NH), 6.06 (2H, s, 6ꢀNH ), 5.03 (1H, s, H5), 3.69 (3H, s,
6
2
1
3
OꢀCH ), 2.70 (3H, d, J
= 4.8, NHꢀCH ); C NMR (125.7 MHz, DMSOꢀd ): 170.3 (C4),
3 6
3
CH3ꢀNH
1
65.9 (C6), 162.7 (C2), 75.6 (C5), 52.4 (OꢀCH ), 28.0 (NHꢀCH ); ESI MS, m/z (%): 155.2 [M +
3
3
+
+
H] ; HRMS (ESI) calcd for C H N O [M + H] 155.0927, found 155.0925.
6
11
4
2
4
-Methoxy-N -methyl-5-nitrosopyrimidin-2,6-diamine (4b). Compound 2 (100 mg, 0.5
mmol) was treated with ammonia (25% aqueous solution, 15 mL) for 1 h. Water (20 mL) was
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added and the obtained solution was extracted with EtOAc (3 x 15 mL). Organic layers were
collected, dried over the MgSO , filtered and evaporated under vacuum. Silica gel column
4
chromatography (10 % MeOH in CHCl ) gave 4b (35 mg, 38 %) as a pink solid with mp = 229–
3
10
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13
14
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16
17
18
19
20
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22
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47
48
49
50
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53
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59
60
1
2
31 °C (decomp.). H NMR (401.0 MHz, DMSOꢀd ): Rotamer C (see Figure S2 for the structure
6
A
of the rotamers): 9.90 (1H, d, JGEM = 3.5, 6ꢀNH ), 8.17 (1H, q, JNHꢀCH3=4.8, 2ꢀNH), 7.89 (1H, d,
B
JGEM = 3.5, 6ꢀNH ), 4.11 (3H, s, OꢀCH ), 2.93 (3H, d, J
= 4.8, NHꢀCH ), Rotamer D: 10.29
3
CH3ꢀNH
3
A
B
(
1H, d, JGEM = 3.8, 6ꢀNH ), 8.34 (1H, q, JNHꢀCH3 = 4.9, 2ꢀNH), 8.25 (1H, d, JGEM = 3.8, 6ꢀNH ),
13
4.03 (3H, s, OꢀCH ), 2.84 (3H, d, JCH3ꢀNH = 4.9, NHꢀCH ); C NMR (100.8 MHz, DMSOꢀd ):
3
3
6
Rotamer C: 171.1 (C4), 162.3 (C2), 150.7 (C6), 139.8 (C5), 54.3 (C4ꢀOCH ), 28.2 (NHꢀCH ),
3
3
Rotamer D: 170.2 (C4), 161.8 (C2), 150.7 (C6), 139.7 (C5), 54.2 (C4ꢀOCH ), 28.2 (NHꢀCH );
3
3
+
+
ESI MS, m/z (%): 184.2 [M + H] ; HRMS (ESI) calcd for C H N O Na [M + Na] 206.0648,
6
9
5
2
found 206.0646.
,4-Dimethoxy-5-nitrosopyrimidin-6-amine (4c). Treatment of 7 (300 mg, 2.0 mmol)
2
with IAN (257 mg, 2.2 mmol) by general method A gave 4c (220 mg, 60 %) as a blue solid with
1
mp = 192–194 °C. H NMR (500.0 MHz, DMSOꢀd ): 10.08 and 8.80 (2 x 1H, 2 x bs, NH ), 4.15
6
2
1
3
(
3H, s, 4ꢀOꢀCH ), 3.93 (3H, s, 2ꢀOꢀCH ); C NMR (125.7 MHz, DMSOꢀd ): 173.3 (C4), 166.1
3 3 6
(
C2), 149.3 (C6), 140.8 (C5), 55.6 (2ꢀOꢀCH ), 55.2 (4ꢀOꢀCH ); ESI MS, m/z (%): 185.1 [M +
3
3
+
+
H] ; HRMS (ESI) calcd for C H N O [M + H] 185.0669, found 185.0670.
6
9
4
3
4-Methoxy-2-(methylthio)-5-nitrosopyrimidin-6-amine (4d). Treatment of 9 (180 mg,
.0 mmol) with IAN (129 mg, 1.1 mmol) by general method A gave 4d (80 mg, 40 %) as a green
1
1
A
solid with mp = 179–182 °C. H NMR (500.0 MHz, DMSOꢀd ): 9.95 (1H, s, NH ), 8.72 (1H, s,
6
B
13
NH ), 4.17 (3H, s, OꢀCH ), 2.55 (3H, s, SꢀCH ); C NMR (125.7 MHz, DMSOꢀd ): 177.9 (C2),
3
3
6
1
68.3 (C4), 140.4 (C5), 55.2 (C4ꢀOꢀCH ), 14.1 (C2ꢀSꢀCH ), C6 was not detected at r.t.; ESI MS,
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1
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1
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1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
+
+
m/z (%): 201.1 [M + H] ; HRMS (ESI) calcd for C H N O S [M + H] 201.0440, found
6
9
4
2
201.0441.
Synthesis of compounds 5b and 5c.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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0
Compound 4c (100 mg, 0.5 mmol) was dissolved in iPrOH (15 mL) and 3.0 equivalents
of methylamine (47 mg, 1.5 mmol, 33% ethanolic solution) were added. The reaction mixture
was heated under MW conditions (160 °C, 1 h). Water (30 mL) was added and the mixture was
extracted with EtOAc (4 x 15 mL). Organic layers were collected, dried over MgSO and filtered.
4
The solvents were evaporated under vacuum. Silica gel column chromatography (10 % MeOH in
CHCl ) gave 5c. Water layer was evaporated under vacuum and crystallization (from MeOH)
3
gave 5b.
2
6
N ,N -Dimethyl-5-nitrosopyrimidin-2,4,6-triamine (5b). Compound 5b (60 mg, 65 %)
1
was obtained as a pink solid with mp > 300 °C. H NMR (850.3 MHz, DMSOꢀd ): Rotamer CA
6
(
see Figure S4 for the structure of the rotamers): 11.13 (1H, q, JNHꢀCH3 = 4.8, 6ꢀNH), 8.23 (1H, s,
A
B
4ꢀNH ), 7.74 (1H, q, J
= 4.8, 2ꢀNH), 7.54 (1H, s, 4ꢀNH ), 2.84 (3H, d, J
= 4.8, 2ꢀNHꢀ
NHꢀCH3
CH3ꢀNH
A
CH ), 2.82 (3H, d, J
= 4.8, 6ꢀNHꢀCH ), Rotamer CB: 10.26 (1H, d, J
= 4.8, 4ꢀNH ),
3
CH3ꢀNH
3
GEM
B
8
4
4
.81 (1H, q, JNHꢀCH3 = 4.7, 6ꢀNH), 7.64ꢀ7.60 (2H, m, 4ꢀNH and 2ꢀNH), 2.98 (3H, d, J
=
CH3ꢀNH
.7, 6ꢀNHꢀCH ), 2.88 (3H, d, J
= 4.8, 2ꢀNHꢀCH ), Rotamer DA: 11.47 (1H, q, J
=
3
CH3ꢀNH
3
NHꢀCH3
B
A
.9, 6ꢀNH), 7.97 (1H, s, 4ꢀNH ), 7.66 (1H, q, J
= 4.8, 2ꢀNH), 7.23 (1H, s, 4ꢀNH ), 2.89
NHꢀCH3
(3H, d, JCH3ꢀNH = 4.9, 6ꢀNHꢀCH ), 2.84 (3H, d, JCH3ꢀNH= 4.8, 2ꢀNHꢀCH ), Rotamer DB: 10.64
3
3
A
B
(
1H, d, JGEM= 4.6, 4ꢀNH ), 8.58 (1H, q, JNHꢀCH3 = 4.6, 6ꢀNH), 8.01 (1H, d, JGEM = 4.6, 4ꢀNH ),
1
3
7
.79 (1H, q, JNHꢀCH3 = 4.6, 2ꢀNH), 2.91 (3H, d, JCH3ꢀNH = 4.6, 6ꢀNHꢀCH ); C NMR (213.8 MHz,
3
DMSOꢀd ): Rotamer CA: 165.9 (C4), 163.4 (C2), 151.1 (C6), 136.9 (C5), 28.1 (C2ꢀNHꢀCH ),
6
3
2
5.9 (C6ꢀNHꢀCH ), Rotamer CB: 163.3 (C6), 163.1 (C2), 150.9 (C4), 137.6 (C5), 27.9 (C2ꢀNHꢀ
3
CH ), 27.3 (C6ꢀNHꢀCH ), Rotamer DA: 165.3 (C4), 163.1 (C2), 150.8 (C6), 137.0 (C5), 27.9
3
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(
C2ꢀNHꢀCH ), 25.7 (C6ꢀNHꢀCH ), Rotamer DB: 163.2 (C2), 162.9 (C6), 150.8 (C4), 137.6
3
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
+
(
C5), 28.1 (C2ꢀNHꢀCH ), 27.4 (C6ꢀNHꢀCH ); ESI MS, m/z (%): 183.1 [M + H] ; HRMS (ESI)
3 3
+
calcd for C H N O [M + H] 183.0988, found 183.0990.
6
11
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6
2
-Methoxy-N -methyl-5-nitrosopyrimidin-4,6-diamine (5c). Compound 5c (20 mg, 22
1
%
) was obtained as a violet solid with mp = 215–217 °C. H NMR (500.0 MHz, DMSOꢀd ):
6
Rotamer A: 11.25 (1H, q, JNHꢀCH3 = 4.9, 6ꢀNH), 8.88 and 8.05 (2 x 1H, 2 x s, 4ꢀNH ), 3.87 (3H, s,
2
A
OꢀCH ), 2.89 (3H, d, J
= 4.9, NHꢀCH ), Rotamer B: 10.40 (1H, s, 4ꢀNH ), 9.40 (1H, q,
3
CH3ꢀNH
3
B
JNHꢀCH3 = 4.7, 6ꢀNH), 8.51 (1H, s, 4ꢀNH ), 3.87 (3H, s, OꢀCH ), 3.00 (3H, d, J
= 4.7, NHꢀ
3
CH3ꢀNH
1
3
CH ); C NMR (125.7 MHz, DMSOꢀd ): Rotamer A: 167.9 (C2), 165.1 (C4), 150.2 (C6), 138.4
3
6
(
C5), 54.8 (OꢀCH ), 26.4 (NHꢀCH ), Rotamer B: 167.6 and 167.5 (C2 and C6), 150.1 (C4), 139.1
3 3
+
(C5), 54.8 (OꢀCH ), 27.8 (NHꢀCH ); ESI MS, m/z (%): 184.2 [M + H] ; HRMS (ESI) calcd for
3
3
+
C H N O [M + H] 184.0829, found 184.0829.
6
10
5
2
6
N -Methyl-2-(methylthio)pyrimidine-4,6-diamine (11). Compound 8 (800 mg, 4.6
mmol) was treated with methylamine (157 mg, 5.1 mmol, 33% ethanolic solution) in iPrOH (30
mL) under MW conditions (180 °C, 1 h). Water (50 mL) was added and the obtained solution
was extracted with EtOAc (3 x 30 mL). Organic layers were collected, dried over MgSO and
4
filtered. Solvents were evaporated under vacuum. Silica gel column chromatography (10 %
1
MeOH in CHCl ) gave 11 (390 mg, 50 %) as a white solid with mp = 144–146 °C. H NMR
3
(401.0 MHz, DMSOꢀd ): 6.50 (1H, q, JNHꢀCH3 = 4.6, NH), 6.09 (2H, s, NH ), 5.07 (1H, s, H5),
6
2
1
3
2
.66 (3H, d, JCH3ꢀNH = 4.6, NHꢀCH ), 2.34 (3H, s, SꢀCH ); C NMR (100.8 MHz, DMSOꢀd ):
3 3 6
1
68.7 (C2), 163.4 (C6), 163.2 (C4), 77.6 (C5), 27.6 (NHꢀCH ), 13.1 (SꢀCH ); ESI MS, m/z (%):
3
3
+
+
171.1 [M + H] ; HRMS (ESI) calcd for C H N S [M + H] 171.0704, found 171.0705.
6
11
4
6
N -Methyl-2-(methylthio)-5-nitrosopyrimidin-4,6-diamine (5d). Treatment of 11 (100
mg, 0.6 mmol) with IAN (77 mg, 0.66 mmol) by general method A gave 5d (110 mg, 92 %) as a
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