Stereochemistry of 1,2,4,5-tetraazanorbornanes and diaziridines: Exciting history and news

Article abstract of DOI:10.1070/MC2004v014n06ABEH002083

The enantiomers of asymmetric nitrogen compound 1 have been resolved; a chiral formal meso-form of diaziridine 2 has been obtained; a population of 1,2-cis-form 6a in solution has been found; and the crystal structures of 3-6 have been studied.

Full text of DOI:10.1070/MC2004v014n06ABEH002083

, 2004, 14(6), 315–318
Stereochemistry of 1,2,4,5-tetraazanorbornanes and diaziridines:
exciting history and news†
Remir G. Kostyanovsky,*a Oleg R. Malyshev,b Konstantin A. Lyssenko,c Yuri A. Strelenko,b
Gulnara K. Kadorkina,a Vasily R. Kostyanovsky,a Orudzh G. Nabieva and Ivan V. Fedyaninc
a N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 095 651 2191; e-mail: kost@chph.ras.ru
b N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 095 135 5328; e-mail: strel@ioc.ac.ru
c A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 095 135 5085; e-mail: kostya@xrlab.ineos.ac.ru
DOI: 10.1070/MC2004v014n06ABEH002083
The enantiomers of asymmetric nitrogen compound 1 have been resolved; a chiral formal meso-form of diaziridine 2 has been
obtained; a population of 1,2-cis-form 6a in solution has been found; and the crystal structures of 3–6 have been studied.
In 1885, Wellington and Tollens2 were the first to obtain the
( )-BusNH2 + CH2O + ButOCl
product (PhN2)2(CH2)3 (mp 183–184 °C) of the condensation
of PhNHNH2 with CH2O. Walker3 postulated its structure as
Et
Et
Me
S
H
H
H
Et
bis(diaziridine) A (Scheme 1). Schmitz4 [who has discovered
R
S
Me Me
diaziridines (see review5)] proposed its true isomeric structure
1
B, which was confirmed later by H NMR spectra6 and XRD
N
N
N
N
N
S
N
S
S
analysis7 (the crystal structure of 3,6,7-trimethyl-B was also
studied8). A similar long story of Tröger’s base is depicted (see
ref. 9). It was resolved by chiral chromatography, and its two
analogues were shown to undergo spontaneous resolution.9(b),(c)
Hitherto Tröger’s base is considered as the first known compound
with asymmetric nitrogen; however, the above data demonstrate
that it should be compound B.
Et
Me Et
Me
H
H
Et
Me
H
2
2a
2b
Scheme 2 Only one enantiomer is shown both for 2a and 2b.
symmetric atropenantiomers of such a formal meso-form have
been first obtained by Prelog et al.12 (Scheme 3). Enantiomeric
meso-forms of 3,4-disubstituted 1,3,4-oxadiazolidines have been
resolved by chiral chromatography.13 The chiral meso-form of a
Ar
N
N
N
N
Ar
R
R
S
S
7
A, Ar = Ph
4 N
N1
H+
ArNHNH2 + CH2O
Ar 5 N
N2 Ar
S
S
R
R
3
6
[a]D = +37°
(c 0.39, CHCl3)
[a]D = –36.2°
(c 0.42, CHCl3)
B, Ar = Ph
1, Ar = 4-O2NC6H4
R = (R)-(+)-Ph(Me)CHNHCO–
S = (S)-(–)-Ph(Me)CHNHCO–
Scheme 1
In this work, for the first time, compound 1 (B and Tröger’s
base analogue) was resolved into enantiomers using enantio-
selective chromatography‡ (Figure 1).
Scheme 3 Data by V. Prelog et al.12
molecular ‘hamburger’ containing a central tetrakis-methylene-
benzene spacer in the fixed chiral conformation of C2 symmetry
is described.14 Thus, the chirality of such meso-forms is a common
phenomenon in stereochemistry. For example, the enantiomers
of meso-forms can be prepared from diaziridine 3 (Scheme 4).
Amidation of 3 under the action of (S)-α-phenylethylamine was
found to occur at the trans-CO2Me group (in reference to MeN),
and the monoamide diastereomers are easily resolved by crys-
tallization.10(f) Subsequent amidation with (R)-amine gives the
desirable enantiomers.
Me
O
Me
N
N
Me
Me
O
( )-C
Previously, various diaziridines have been obtained in opti-
cally active forms,10 in particular, symmetrically substituted
C;10(c) the population of 1,2-cis-form of diaziridine10(d) has been
detected, and the synthesis of 1-alkyldiaziridine-3,3-dicarboxylic
acids derivatives10(e) has been worked out for the subsequent
complete resolution of their diastereomers10(f) and enantiomers.10(g)
They are physiologically active as brain monoaminoxidase
inhibitors.10(h)
In this work, for the first time, a chiral meso-form of diaziri-
dine 2 and two diastereomers of d,l-form of 2a,b (Scheme 2,
Figure 2) have been obtained by a usual method;10(i) the latter
were separated by routine chromatography.‡ The resolution of
enantiomers 2, 2a, 2b by enantioselective chromatography, as
well as the determination of the activation parameters of their
enantiomerization, will be reported elsewhere.11 Note that C2-
1.10
140
100
60
(a)
(b)
1.05
1.00
0.95
0.90
0.85
20
–20
320 340 360 380 400
l/nm
–60
–100
0
2
4
6
t/min
†
Figure 1 (a) Chromatographic separation of racemic compound 1 and
(b) CD spectra of enantiomers in 1,2-dimethoxyethane.
Asymmetric Nitrogen. Part 92; Geminal Systems. Part 53; previous
communication see ref. 1.
Mendeleev Commun. 2004 315

, 2004, 14(6), 315–318
Diaziridines 3–610(e) have been studied in detail in this work
(Scheme 4).
O
NH2
O
N
NHMe
CO2Me
H
H
H
N
A curious peculiarity of asymmetric nitrogen compounds is
their optical enrichment during both crystallization from an
optically active solvent and heating in it followed by evapora-
tion.1,15(a) This is a test for conglomerate formation.1 Indeed,
1-methoxy-2,2-dicarbamoylaziridine forms a conglomerate (space
group P212121) and, therefore, undergoes efficient spontaneous
resolution.15(b) Upon heating in l-methyl lactate, diaziridine 6
is enriched with the (1S,2S)-(–)-enantiomer.15(a) However, the
examination of the crystal structures of 3–6 demonstrated that
all of them are true racemates.§ That is why the optical enrich-
ment of 6 in l-methyl lactate is explained by inversion with
the equilibrium shifting toward a more solvated enantiomer¶
(cf. ref. 1). Population of 1,2-cis-form revealed in 6 is a rare
case for diaziridines.10(d),15(c) Formation of 6a was detected by
1H and 13C NMR spectroscopy upon heating a sample of 6 in
[2H6]DMSO.‡ 1,2-cis-Structure of 6a was confirmed by a sig-
i
ii
N
O
N
N
O
N
CO2Me
3
Me
Me
Me
CO2Me
NH2
4
5
iii
NHMe
O
NHMe
H
N
iv
N
O
N
N
O
H
Me
Me
NHMe
NHMe
6a
6
CO2Me
H
N
15N
CO2Me
3a
v
(MeO2C)2C=15N–OTs
Me
‡
1, yield 43%, mp 270–271.5 °C (1,2-dimethoxyethane) [lit.,6 mp 269 °C
(toluene)]. 1H NMR (CDCl3) d: 3.55 (s, 2H, 7-CH2), 4.12 (d, 2H,
exo-3,6-CH, 2J 7.4 Hz), 4.50 (d, 2H, endo-3,6-CH, 2J –7.4 Hz), 7.00 (d,
4H, 2',6'-HC=, 3J 9.2 Hz), 8.20 (d, 4H, 3',5'-HC=, 3J 9.2 Hz).
Chromatographic separation of 1 was performed using a Laboratory
pristroje Praha chromatograph with an injector with a 20-µl sample loop.
Conditions: Chiralpak AD stationary phase (250×4.6 mm i.d.) available
from Diacel Chemical Industries (Japan); mobile phase, propan-2-ol (neat);
flow rate of 2 ml min–1; temperature, ambient; detection UV 365 nm. A
solution of 1 (5 mg in 100 µl of dimethylformamide) was injected into the
chromatograph in eight portions. Retention times of enantiomers are t1 =
= 4.29 min, t2 = 6.38 min; the void time is t0 = 1.56 min, as determined
by the injection of tri-tert-butylbenzene, and the separation factor a =
= (t2 – t0)/(t1 – t0) = 1.76.
Scheme 4 Reagents and conditions: i, excess of NH3 in MeOH, 12 h at
20 °C; ii, MeNH2 in MeOH, 1 h at –20 °C and 12 h at 20 °C; iii, MeNH2 in
MeOH, 12 h at 20 °C; iv, in [2H6]DMSO (5% H2O), 0.5 h at 90 °C; v, excess
of MeNH2 in CH2Cl2, 3 h at –30 °C.
3
nificantly higher value of the spin coupling constant J
of
13CH
the fragment MeNNH‡ (Figure 3) as against those for 6 and
in comparison with the constant values found for 1,2-cis- and
1,2-trans-1,2-dimethyl-3-tert-butyldiaziridines (3Jtrans 3.3 Hz,
13CH
cis
3Jcis 5.7 Hz in 1,2-trans-form, and J
6.1 Hz in 1,2-cis-
13CH
3
13CH
form).15(c)
§
1
XRD data for 3–6 at 120 K.
2, 2a, 2b, yield 21%, bp 74 °C (40 Torr). H NMR (CDCl3) d: 0.90–
3
3
Crystals of 3 (C6H10N2O4) are triclinic, space group P1, a = 6.7669(5),
b = 7.7414(6) and c = 8.2660(6) Å, a = 77.957(2)°, b = 72.428(2)°, g =
0.92 (3t, 3H, MeCH2, J 7.5–8.1 Hz), 1.00–1.17 (4d, MeCH, J 6.0–
6.1 Hz), 1.35–1.55 (3m, CH2Me), 1.74–1.85 (3m, HCN), 2.21 (s, 2H,
NCH2N, 2a), 2.25 (m, 2H, NCH2N, AB spectrum, ∆n 19.0 Hz, 2J –9.0 Hz,
2), 2.32 (s, 2H, NCH2N, 2b), the ratio 2:2b:2a = 2.1:1.2:1.0 (Figure 2).
13C NMR (CDCl3) d: 9.10, 9.17, 9.99, 10.02, (4q, MeCH2, 1J 125.0 Hz),
15.34, 15.36, 18.73, 18.80, (4q, MeCH, 1J 125.0 Hz), 26.95, 27.54,
= 82.368(2)°, V = 402.59(5) Å3, Z = 2 (Z' = 1), M = 174.16, dcalc
= 1.437 g cm–3, m(MoKα) = 1.21 cm–1, F(000) = 184.
=
Crystals of 4 (C4H8N4O2) are triclinic, space group P1, a = 7.0775(6),
b = 9.4230(8) and c = 9.7375(8) Å, a = 82.725(2)°, b = 80.681(2)°, g =
1
= 85.462(2)°, V = 634.54(9) Å3, Z = 4 (Z' = 2), M = 144.14, dcalc
= 1.509 g cm–3, m(MoKα) = 1.23 cm–1, F(000) = 304.
=
27.68 (3t, CH2Me, J 126.0 Hz), 54.30 (2a), 55.30 (2), 56.30 (2b) (3t,
1
NCH2N, J 174.4 Hz, 3J 3.6 Hz), 65.77, 65.89, 65.93, 66.03 (4dm, CH,
Crystals of 5 (C6H11N3O3) are monoclinic, space group P21/c,
a = 6.607(1), b = 17.403(4) and c = 7.927(2) Å, b = 111.085(4)°, V =
= 850.4(3) Å3, Z = 2 (Z' = 1), M = 173.18, dcalc = 1.353 g cm–3, m(MoKα) =
= 1.09 cm–1, F(000) = 368.
Crystals of 6 (C6H12N4O2) are orthorhombic, space group Pbcn,
a = 17.386(2), b = 7.9419(9) and c = 12.116(1) Å, V = 1673.0(3) Å3,
Z = 8 (Z' = 1), M = 172.20, dcalc = 1.367 g cm–3, m(MoKα) = 1.05 cm–1,
F(000) = 736.
1J 135.0 Hz).
By chromatography on silica (eluent, Et2O/n-hexane, 1:1) the mix-
ture of 2 and 2b (1:1) was isolated. From the studies on kinetics of
epimerization 2b ® 2a by 1H NMR in [2H8]toluene at 102 °C there were
found k = 3.1×10–6 s–1, ∆G# = 31.3 kcal mol–1. It is in agreement with
the data in ref. 11.
3a, yield 48%, mp 68–69 °C (PriOH/n-pentane, 1:5). 1H NMR (C6D6)
3
4
d: 2.53 (dd, 3H, MeN, JHCN15 2.9 Hz, JHCNNH 0.7 Hz), 3.38 (dq, 1H,
N
Intensities of 2890 (3), 7592 (4), 9292 (5) or 11748 (6) reflections
were measured with a Smart 1000 CCD diffractometer at 120 K
[l(MoKα) = 0.71072 Å, 2q < 58] and 2084 (3), 3368 (4), 2250 (5) or
2179 (6) independent reflections [Rint = 0.0193 (3), 0.0181 (4), 0.0210
(5), 0.0432 (6)] were used in the further refinement. The structures were
solved by a direct method and refined by the full-matrix least-squares
technique against F2 in the anisotropic-isotropic approximation. The
hydrogen atoms were located from the Fourier density synthesis. The
refinement converged to: wR2 = 0.1042 and GOF = 1.055 [R1 = 0.0465
for 1677 observed reflections with I > 2s(I)] for 3; wR2 = 0.1118 and
GOF = 0.984 [R1 = 0.0457 for 2873 observed reflections with I > 2s(I)]
for 4; wR2 = 0.0921 and GOF = 0.985 [R1 = 0.0361 for 2030 observed
reflections with I > 2s(I)] for 5; wR2 = 0.1151 and GOF = 0.995 [R1 =
= 0.0511 for 1438 observed reflections with I > 2s(I)] for 6. All calcu-
lations were performed using SHELXTL PLUS 5.0.
Atomic coordinates, bond lengths, bond angles and thermal param-
eters have been deposited at the Cambridge Crystallographic Data Centre
(CCDC). These data can be obtained free of charge via www.ccdc.cam.uk/
conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336 033; or deposit@ccdc.cam.ac.uk).
Any request to the CCDC for data should quote the full literature citation
and CCDC reference numbers 257663–257666. For details, see ‘Notice to
Authors’, Mendeleev Commun., Issue 1, 2004.
HN, 1JH15N 59.4 Hz, 4J 0.7 Hz), 3.29 (s, 3H, b-MeO), 3.44 (s, 3H, a-MeO).
13C NMR (C6D6) d: 44.2 (qdd, MeN, 1J 137.3 Hz, JCN15 3.6 Hz,
2
3JCNNH 5.8 Hz), 52.2 (q, a-MeO, 1J 148.2 Hz), 53.1 (q, bN-MeO, 1J
148.2 Hz), 61.1 (dd, NCN, 1JC15 5.6 Hz, JCNH 4.4 Hz), 164.7 (dq,
2
a-CO, JCC15 4.5 Hz, JCH 4.0 HNz), 167.6 (dq, b-CO, JCCNH 4.3 Hz,
2
3
3
3JCH 4.0 Hz,N JCC15N < 0.5 Hz), cf. the spectra of aziridines 15N, refs.
2
10(i)–(k). 15N NMR (CDCl3) d: –289.06.
4, yield 56%, mp 162–164 °C (MeOH/Et2O). 1H NMR (CDCl3) d:
2.60 (s, 3H, MeN), 3.70 (s, 1H, HN), 5.80 (br. d, 2H, 2HNCO), 6.75 and
8.04 (2br. s, 2H, 2HNCO).
5, yield 94%, mp 105–106 °C (C6H6). 1H NMR (CDCl3) d: 2.34 (s,
3
3H, MeN), 2.69 (d, 3H, MeNH, J 5.1 Hz), 3.15 (s, 1H, HNN), 3.73
(s, 3H, MeO), 8.00 (br. s, 1H, HNMe).
6, yield 60%, after sublimation (120 °C at 1 Torr), mp 160–161 °C.
1H NMR (D2O) d: 2.38 (s, 3H, MeNN), 2.70 (s, 3H, MeNCO), 2.78 (s,
1
3H, MeNCO). H NMR (CDCl3) d: 2.49 (s, 3H, MeNN), 2.87 (d, 3H,
MeNH, 3J 5.0 Hz), 2.89 (d, 3H, MeNH, 3J 5.0 Hz), 3.68 (s, 1H, HNN),
7.26 (br. s, HNCO), 8.23 (br. s, 1H, HNCO). 13C NMR (CDCl3) d: 25.71
(q, MeNH, 1J 129.0 Hz), 26.77 (q, MeNH, 1J 128.5 Hz), 41.33 (qd,
MeNN, 1J 128.1 Hz, 3J 5.0 Hz), 59.95 (dq, NCN, 2J = 3J = 4.0 Hz),
165.70 (m, CO), 166.33 (m, CO). 1H NMR ([2H6]DMSO) d: 2.26 (s,
3H, MeNN), 2.58 (d, 3H, MeNH, 3J 4.7 Hz), 2.61 (d, 3H, MeNH,
3J 4.7 Hz), 4.00 (s, 1H, HNN), 7.86 (br. q, 1H, HNCO, 3J 4.7 Hz), 8.08
(1H, HNCO). After heating 6 (1 h at 100 °C) for 6a 1H NMR ([2H6]DMSO)
¶
In 1893, Van’t Hoff wrote: ‘it should be possible to find a difference in
3
d: 2.28 (s, 3H, MeNN), 2.62 (d, 3H, MeNH, J 4.8 Hz), 2.67 (d, 3H,
the solubility of enantiomers in an optically active solvent’, and later
‘...Le Bel might already have carried out experiments in this direction,
and previously Pasteur also considered possibility’.16
3
MeNH, J 4.8 Hz), 4.05 (s, 1H, HNN), 7.80 (br. q, 1H, HNCO), 8.35
(br. q, 1H, HNCO).
316 Mendeleev Commun. 2004

, 2004, 14(6), 315–318
(a)
H(5NA)
C(6)
O(5)
C(6)
N(5)
(c)
(a)
C(6)
N(5)
C(4)
H(5NB)
H(5N)
(b)
O(1)
O(1)
N(5)
C(7)
O(1)
C(4)
O(1)
C(4)
H(5N)
H(2N)
C(4)
H(2N)
C(3)
H(2N)
N(2)
H(2N)
N(2)
C(3)
C(3)
N(1)
N(2)
C(3)
O(8)
N(1)
N(1)
O(2)
N(2)
N(1)
H(8N)
C(1N)
C(7)
C(1N)
C(7)
H(8NA)
C(7)
N(8)
H(8NB)
N(8)
O(2)
C(1N)
C(1N)
C(9)
C(9)
2.3
2.2
O(2)
O(2)
C(9)
d/ppm
3
4
5
6
Figure 2 1H NMR spectrum of NCH2N protons in a mixture of 2, 2a and
2b: (a) minor diastereomer 2a; (b) AB-spectrum of meso-2; (c) predomi-
nant diastereomer 2b.
(b)
C(6)
H(5N)
C(6)
C(6)
H(5N)
N(5)
N(5)
N(5)
C(4)
O(1)
H(5N)
20
O(1)
Starting from (S,S)-(–)-3,10(g) (S,S)-(–)-6, [a]
–76.0°
O(1)
C(3)
N(1)
546
C(4)
C(4)
C(3)
H(2N)
(c 0.8, MeOH), was obtained. The kinetics of racemization
C(3)
N(1)
O(2)
N(2)
H(2N)
H(8N)
N(2)
C(1N)
was studied for (–)-3 in C6H6 at 70 °C (k = 1.7×10–5 s–1,
H(8N)
N(1)
O(2)
N(2)
∆G = 27.7 kcal mol–1) and for (–)-6 in MeOH at 70 °C (k =
¹
H(2N)
C(7)
C(7)
N(8)
= 1.2×10–5 s–1, ∆Gi#nv = 27.9 kcal mol–1). The data are very close
to those for 5.10(f) Noticeably faster isomerization of 6 into 6a
can be explained by proton exchange in HN under the action of
H2O in the solvent (Scheme 4).
C(7)
N(8)
C(1N)
N(8)
H(8N)
C(1N)
O(2)
C(9)
C(9)
C(9)
6a
TS1
TS2
By XRD study§ of 3–6, the regiospecificity of the low-tem-
perature amidation 3 ® 5 and the structures of all of the products
have been unambiguously proved (Figure 4). The basic geomet-
rical parameters of 3–6 are practically constant: limits of bond
lengths changing are 1.501(2)–1.507(2) for N(1)–N(2), 1.442(2)–
1.453(2) for C–N (ring), and 1.471(2)–1.474(2) Å for N(1)–
C(1N).
To elucidate the cause of unusual population of cis-form in
6a, a theoretical investigation of 6 and 6a in correlation with the
experimental structure of 6 in a crystal (Figure 4) was carried
out. The geometry of 6 as individual species (Figure 4) simulates
perfectly the experimental one; in particular, the N(1)–N(2)
distance is equal to 1.501 Å (1.507 Å in a crystal), the torsion
angle H(2N)N(2)N(1)C(1N) is 149.6° (149.5° in a crystal).
Like in a crystal, the intramolecular bond N(5)H(5N)···N(1)
is revealed in individual 6. The shortening of the distance
N(5)···N(1) to 2.706 Å as against crystal (2.773 Å) indicates a
strengthening of this bond in the individual state (Figure 3). The
bond length N(1)–N(2) in 6a is slightly reduced to 1.493 Å, the
torsion angle H(2N)N(2)N(1)C(1N) is 12.3°. As distinct from 6,
in addition to the bond N(5)H(5N)···N(1) [N(5)···N(1) 2.682 Å],
there is the intramolecular bond N(8)–H(8N)···O(1) [N(8)···O(1)
2.733 Å] in 6a, which leads to the formation of a six-membered
H-bonded ring.
Figure 4 (a) Molecular structures of 3–6; (b) calculated structures of 6
and 6a.
References
1
R. G. Kostyanovsky, V. Schurig, K. A. Lyssenko, O. Trapp, G. K.
Kadorkina, V. R. Kostyanovsky and B. B. Averkiev, Mendeleev Commun.,
2004, 306.
Ch. Wellington and B. Tollens, Ber., 1885, 18 (II), 3298.
J. W. Walker, J. Chem. Soc. (London), 1896, 69, 1280.
E. Schmitz and R. Ohme, Liebigs Ann. Chem., 1960, 635, 82.
R. G. Kostyanovsky, R. Murugun and M. Sutharchanadevi, in Compre-
hensive Heterocyclic Chemistry II, ed. A. Padwa, Pergamon, Oxford,
1996, vol. 1A, pp. 347–364.
2
3
4
5
6
S. R. Johns, J. A. Lamberton and E. R. Nelson, Aust. J. Chem., 1971,
24, 1859.
7
8
G. A. Sim, Acta Crystallogr., 1987, C43, 2178.
N. G. Bokii, T. A. Babushkina, A. M. Vasil'ev, T. A. Volodina, T. A.
Kozik, Yu. T. Struchkov and N. N. Suvorov, Zh. Org. Khim., 1975, 11,
996 [J. Org. Chem. USSR (Engl. Transl.), 1975, 11, 984].
(a) D. A. Lenev, K. A. Lyssenko and R. G. Kostyanovsky, Izv. Akad.
Nauk, Ser. Khim., 2000, 1244 (Russ. Chem. Bull., Int. Ed., 2000, 49,
1241); (b) R. G. Kostyanovsky, V. R. Kostyanovsky, G. K. Kadorkina
and K. A. Lyssenko, Mendeleev Commun., 2003, 111; (c) D. A. Lenev,
K. A. Lyssenko, O. R. Malyshev, D. G. Golovanov, P. A. Levkin, V. Buss
and R. G. Kostyanovsky, unpublished results.
9
As an individual species, 6a is more stable than 6 by
1.07 kcal mol–1 (Figure 4). The isomerization 6 ® 6a can proceed
by means of the nitrogen inversion of either N–Me (TS1) or NH
(TS2) (Figure 4). Seeking TS showed that the energy of TS2
with taking into account ZPE is lower by only 0.54 kcal mol–1
than those of TS1, and inversion barriers 6 ® 6a via TS1 and
TS2 are 33.74 and 34.28 kcal mol–1, respectively.
Presumably, due to the presence of an additional NH···O
bond, as well as to shortening H(5N)···N(1) distance in 6a, the
intramolecular H-bonds provide its stability in individual state.
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This work was supported by the Russian Academy of Sciences,
the Russian Foundation for Basic Research (grant nos. 03-03-32019,
03-03-04010 and 03-03-32214) and the Presidential Program for
Support of Leading Russian Scientific (grant nos. RF 1060.2003.3
and YS-1209.2003.03).
41.0
d/ppm
43.0
42.0
38.0
37.0
Figure 3 13C NMR ([2H6]DMSO) spectrum of carbons MeNNH of 6a (in
a high field) and 6 (in a low field), signals of the partly deuterated solvent
are ommited for clarity.
Mendeleev Commun. 2004 317

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Received: 1st November 2004; Com. 04/2408
318 Mendeleev Commun. 2004