Orthoamides and iminium salts, LXXV [1]. Contribution to the formation of 2-formyl-1,1,3,3-tetramethylguanidine and the isomeric 1,1-Dimethyl-3- dimethylaminomethylene-urea

Article abstract of DOI:10.1515/znb-2012-0406

2-Formyl-1,1,3,3-tetramethylguanidine (1) could be prepared from tris(dimethylamino)ethoxy-methane (3a) and formamide (4). Surprisingly, guanidine 1 does not result from the reaction of 1,1,3,3-tetramethylguanidine with formylating reagents such as dimeth

Full text of DOI:10.1515/znb-2012-0406

Orthoamides and Iminium Salts, LXXV [1].
Contribution to the Formation of 2-Formyl-1,1,3,3-tetramethylguanidine
and the Isomeric 1,1-Dimethyl-3-dimethylaminomethylene-urea
Willi Kantlehnera,b, Ivo C. Ivanovb, and Ioannis Tiritirisa,b
a Institut fu¨r Organische Chemie, Universita¨t Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart,
Germany
b Fakulta¨t Chemie/Organische Chemie, Hochschule Aalen, Beethovenstraße 1, 73430 Aalen,
Germany
Reprint requests to Prof. Dr. Willi Kantlehner. Fax: +49(0)7361-576-2250.
E-mail: willi.kantlehner@htw-aalen.de
Z. Naturforsch. 2012, 67b, 331 – 336; received January 13, 2012
Dedicated to Professor Ernst Anders on the occasion of his 70th birthday
2-Formyl-1,1,3,3-tetramethylguanidine (1) could be prepared from tris(dimethylamino)ethoxy-
methane (3a) and formamide (4). Surprisingly, guanidine 1 does not result from the reaction of
1,1,3,3-tetramethylguanidine with formylating reagents such as dimethylamino-methoxy-acetonitrile
(8) or the N,N-dimethylformamide-dimethylsulfate adduct (9), rather the isomeric 1,1-dimethyl-3-
dimethylaminomethylene-urea (2) is formed. The structure of 2 was confirmed by NMR spectroscopy
and crystal structure analysis.
Key words: Guanidines, Formamide, DMF-DMS Adduct, 1,3-Hydride Shift
Introduction
(Scheme 1) [2]. Compound 1 was reported to be ob-
tained as an oil, which solidified on purification to give
crystals with m. p. 54 – 56 ◦C; however, it was not fully
characterized, and no spectral data have been reported.
The second title compound, 1,1-dimethyl-3-dimeth-
ylaminomethylene-urea (2), was described as colorless
2-Formyl-1,1,3,3-tetramethylguanidine (1) and 1,1-
dimethyl-3-dimethylaminomethylene-urea(2) are con-
stitutional isomers, and their preparation has al-
ready been reported earlier in the literature. Guani-
dine 1 was prepared from formamide (4) by treatment
with tris(dimethylamino)isopropyloxymethane (3b)
◦
prisms with m. p. 50 C [3a]. It was prepared by re-
action of N,N-dimethylformamide (5) with cyanogen
chloride Cl-C≡N (6) (Scheme 1). This product has
been characterized by means of IR and 1H NMR
(60 MHz) spectroscopy, and its picrate (m. p. 148 –
◦
151 C). In the reaction of N,N-dimethylformamide-
diethylacetal with urea a complex product mixture has
been obtained, in which compound 2 could be detected
by means of mass spectrometry [3b].
While studying some new formylating agents
for aromatic compounds [4], our attention was at-
tracted by the N-formylguanidine 1, whose prepara-
tion from formamide and tris(dimethylamino)isoprop-
yloxymethane (3b) [2] seemed to be rather simple. In
order to produce 1 and to examine its formylating po-
tential, we reproduced the known procedure [2] which
uses the reaction of tris(dimethyamino)ethoxymethane
(3a) with formamide (4) (Scheme 2).
Scheme 1. Synthetic methods for the preparation of 2-
formyl-1,1,3,3-tetramethylguanidine (1) and 1,1-dimethyl-3-
dimethylaminomethylene-urea (2).
c
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332
W. Kantlehner et al. · Orthoamides and Iminium Salts
Scheme 2. Preparation of 2-formyl-1,1,-
3,3-tetramethylguanidine (1) and 1,1-di-
methyl-3-dimethylaminomethylene-urea
(2).
Scheme 3. Reaction products 2 and 10
from 1,1,3,3-tetramethylguanidine (7)
and N,N-dimethylformamide-dimethyl-
sulfate adduct (9) (Method B).
ingly, however, the low-melting solid thus obtained
was not identical with the oily one prepared from
formamide as mentioned above. Its structure was
proved to be 1,1-dimethyl-3-dimethylaminomethyl-
ene-urea (2) (Scheme 2, Method A) on the basis of its
1H NMR and 13C NMR spectra and an X-ray crystal-
lographic analysis.
According to Method B (Scheme 3) the result of
the aforementioned reaction was fully confirmed by
the reaction of TMG (7) with the N,N-dimethylform-
amide-dimethylsulfate adduct (9) where the first step
in Scheme 3 was expected to be the same as in the
reaction of TMG (7) with dimethylaminomethoxyace-
tonitrile (8) described in Scheme 2.
In this case, compound 2 was only isolated as a
by-product (yield 18 %) since the main product was
found to be a viscous liquid, probably N,Nꢀ-bis(di-
aminomethylene)formamidinium methylsulfate (10)
(Scheme 3) which features a strongly resonance-
stabilized cation. However, the latter compound could
not be completely purified to be fully characterized.
In Method C, 1,1-dimethylurea (11) is reacted
Fig. 1. Resonance structure of N-formylguanidine 1.
An oily product with no tendency to crystallization
was isolated, and its structure (1) could unambigu-
1
ously be verified by its spectral data. In the H NMR
spectrum all four N-methyl groups are magnetically
equivalent and display a sharp singlet for 12 H at δ =
2.97 ppm. This is an indication of the existence of free
rotation around the C–N bonds in the partially zwitter-
ionic structure of 1 (Fig. 1) at ambient temperature.
The second 1H singlet at δ = 8.54 ppm is ascribed to
the formyl proton of compound 1.
Results and Discussion
We were looking for an alternative synthetic ap-
proach to formylguanidine 1, namely, by formyla-
tion of 1,1,3,3-tetramethylguanidine (TMG) (7) with
dimethylaminomethoxyacetonitrile (8) [5]. Surpris- with N,N-dimethylformamide-dimethylacetal (DMF-
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W. Kantlehner et al. · Orthoamides and Iminium Salts
333
Scheme 4. 1,1-Dimethyl-3-dimethyl-
aminomethylene-urea (2) from 1,1-
dimethylurea (11) and DMF-DMA
(12) (Method C).
Table 1. Crystal structure data for 2.
Formula
Mr
C6H13N3O
143.19
Crystal size, mm3
Temperature, K
Crystal system
Space group
0.32×0.25×0.18
100(2)
triclinic
¯
P1 (no. 2)
˚
a, A
7.0182(3)
10.5055(4)
11.9481(5)
66.943(2)
76.914(2)
82.617(3)
788.73(6)
4
˚
b, A
˚
c, A
α, deg
β, deg
γ, deg
3
˚
V, A
Z
Dcalcd, g cm−3
1.21
0.1
312
µ(MoKα ), mm−1
F(000), e
θ range, deg
3.21 – 27.85
9, 13, 15
24360
Fig. 2. Projection of the unit cell of 1,1-dimethyl-3-di-
methylaminomethylene-urea (2) along the crystallographic
a axis.
hkl range
Refl. measured
unique
3709
Rint
Rσ
0.0255
0.0875
285
0.0837 / 0.1847
0.0650 / 0.1674
1.121
0.348 / −0.297
DMA (12) [6]. This reaction can be considered as a
chemical evidence for the structure of 2.
Parameters refined
R1 / wR2 (all reflections)
R1 / wR2 for 2924 refl. with I ≥ 2σ(I)
GoF (F2)
At r. t. all four N-methyl groups of 2, both in CDCl3
and in [D6]DMSO, show four different chemical shifts
in the range δ = 2.92 – 3.07 ppm (CDCl3) and 3.00 –
3.14 ppm ([D6]DMSO) in its highly resolved 1H NMR
spectrum (s. Experimental Section). Hence, rotation
around the C–N bonds is hindered in both solvents
due to conjugation with the corresponding polarized
double bonds. The signal of the aldehyde-like methine
proton is observed as a singlet at δ = 8.29 (CDCl3)
and 8.38 ppm ([D6]DMSO). These values agree with
those reported in lit. [2a]: two singlets at δ = 2.98 (6H)
and 3.02 (6H) for the N-methyl groups, and δ = 8.3 (s,
1H) for the methine proton (60 MHz spectra).
−3
˚
∆ρfin (max / min), e A
˚
Table 2. Selected bond lengths (A) and angles (deg) for 2
with estimated standard deviations in parentheses.
O1–C3 1.240(2)
O2–C9 1.241(2)
N1–C3 1.362(2)
N1–C1 1.450(2)
N1–C2 1.455(2)
N2–C4 1.308(2)
N2–C3 1.397(2)
N3–C4 1.333(2)
N3–C6 1.452(3)
Hydrogen bondsa
D–H···A
N3–C5 1.456(2)
N4–C9 1.362(2)
N4–C7 1.451(3)
N4–C8 1.456(3)
N5–C10 1.309(2)
N5–C9 1.397(2)
N6–C10 1.327(2)
N6–C12 1.453(2)
N6–C11 1.457(2)
O2–C9–N4 120.2(2)
O2–C9–N5 125.9(2)
O1–C3–N1 120.7(2)
O1–C3–N2 125.6(2)
N1–C3–N2 113.7(2)
N2–C4–N3 122.2(2)
N4–C9–N5 113.9(2)
N5–C10–N6 122.2(2)
The structure and configuration of compound 2 was
finally confirmed by means of X-ray crystallographic
¯
d(D–H) d(H···A) d(D···A) ꢀ(D–H···A) analysis. It crystallizes in the triclinic space group P1
C4–H4···O2
0.98(2)
0.94(2)
0.99(2)
0.97(2)
2.420
2.455
2.464
2.514
3.357(2)
3.308(2)
3.399(2)
3.441(2)
159.3(2)
150.9(2)
156.7(2)
159.7(2)
(Table 1) with Z = 4. It thus contains two crystallo-
graphically non-equivalent molecules in the unit cell.
Fig. 2 shows a view of the unit cell along the a axis.
Hydrogen bonds between the carbonyl-oxygen atoms
of the urea moiety and the methine hydrogen atoms H4
C10–H10···O1
C12–H12A···O1
C5–H5A···O2
a
D = hydrogen bond donor, A = hydrogen bond acceptor.
DMA) (12) to afford the product 2 as well (Scheme 4). and H10 as well as the hydrogen atoms H5A and H12A
This is a further example of the well-known N-form- of the methyl groups have been observed [d(O–H) =
˚
ylation of primary carboxamides by means of DMF- 2.420 – 2.514 A; Table 2 and Fig. 3]. Taking this hydro-
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334
W. Kantlehner et al. · Orthoamides and Iminium Salts
Scheme 5. Explanation of the formation
of compound 2.
Scheme 6. Picrates 15 and 16 obtained from N-formylguan-
idine 1 and dimethylaminomethylene-urea 2.
Fig. 3. View of two 1,1-dimethyl-3-dimethylaminomethyl-
ene-urea (2) molecules connected via C–H···O hydrogen
bonds (dashed lines) into a dimer.
sonably explained by initial addition of the guanidine
to a delocalized iminium ion of type 13, followed
by 1,3 hydride shift in the adduct 14 thus formed
(Scheme 5). Simultaneous attack of the cyanide ion on
the methyl group should promote this step. After sub-
sequent elimination of dimethylamine, the final stable
product 2 is produced.
gen bonding situation into account, dimeric molecules
connected via (weak) C–H···O hydrogen bonds are
formed (Fig. 3), which are the main structural motif in
the crystal structure of 2. The experimental N–C bond
lengths are in the range as expected for N–C single or
By treatment of N-formylguanidine 1 with picric
acid in ethanol the guanidinium picrate 15 (m. p. 173 –
˚
double bonds (d(N–C)double = 1.308(2)– 1.309(2) A;
◦
˚
d(N–C)single = 1.450(2)– 1.457(2) A; Table 2). Fur-
176 C) could be prepared (Scheme 6). In contrast,
thermore the C–O and N–C distances of the urea moi-
ety are well comparable with the crystal structure data
of solid N,N,Nꢀ,Nꢀ-tetramethylurea at 200 K (d(C–
from the analogous reaction with dimethylaminometh-
ylene-urea 2, dimethylammonium picrate (16) was ob-
tained. Obviously the amidine function of 2 is cleaved
with release of dimethylamine under the conditions ap-
˚
˚
O) = 1.226 A; d(N–C) = 1.370; 1.451 – 1.460 A [8]).
The formation of the substituted urea 2 under the plied. The dimethylamine thus formed reacts with pi-
conditions given for Methods A and B could be rea- cric acid to give 16 (Scheme 6).
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W. Kantlehner et al. · Orthoamides and Iminium Salts
335
Conclusion
N 22.57; found C 38.80, H 4.47, N 22.45. The picrate 15 is
stable at r. t.
2-Formyl-1,1,3,3-tetramethylguanidine (1) could
only be synthesized from tris(dimethylamino)ethoxy-
methane and formamide. We found that the very stable
urea derivative 2 can be prepared from N,N,Nꢀ,Nꢀ-
tetramethylguanidine and formylating agents as 8
and 9, most likely by an intermediate hydride shift.
1,1-Dimethyl-3-dimethylaminomethylene-urea (2)
Method A: From dimethylaminomethoxyacetonitrile (8)
To a solution of 30.0 g (0.26 mol) of 8 in 100 mL of cy-
clohexane 29.4 g (0.25 mol) of tetramethylguanidine (7) was
added dropwise, and the reaction mixture was distilled with
stirring over a 15 cm Vigreux column (KOH drying tube). An
azeotropic mixture of cyclohexane-methanol (b. p. ca. 56 ◦C)
was collected, and evolution of dimethylamine was observed.
Experimental Section
General information.
◦
The heating temperature was increased to 110 C until the
Melting points were determined on a Bu¨chi 450 aparatus.
TLC: Macherey & Nagel pre-coated plastic sheets SIL254
with silica gel, layer 0.2 mm; eluted with methylene chloride-
acetone-ethanol 6 : 3 : 1 (v/v).
distillation ceased. All volatile components were removed
◦
at 40 C in vacuo, and the residue was fractionally distilled
in a◦n oil pump vacuum. A pale-yellow liquid with b. p. 89 –
97 C crystallized in the condenser to give 13.2 g (36 %)
1,1-dimethyl-3-dimethylaminomethylene-urea (2), TLC ho-
mogeneous. Repetitions: yields 31 – 52 %. An analytically
pure sample was recrystallized twice from pentane, colorless
prisms with m. p. 54 – 55 ◦C. – 1H NMR (300 MHz, CDCl3):
δ = 3.07 (br. s, 3H, NCH3), 3.01 (s, 3H, NCH3), 2.96 (s,
3H, NCH3), 2.92 (br. s, 3H, NCH3), 8.29 (s, 1H, N=CH-);
(500 MHz, [D6]DMSO): 3.00 (br. s, 3H, NCH3), 3.03 (s, 3H,
NCH3), 3.07 (s, 3H, NCH3), 3.14 (br. s, 3H, NCH3), 8.38 (s,
1H, N=CH-). – 13C NMR (75.47 MHz, CDCl3): δ = 34.64,
35.65, 36.79, 40.77 (4× NCH3), 159.86 (CH=N), 164.37
(C=O). – C6H13N3O (143.19): calcd. C 50.34, H 9.15,
N 29.34; found C 50.25, H 9.06, N 29.30.
2-Formyl-1,1,3,3-tetramethyl-urea (1)
From tris-(dimethyamino)ethoxymethane (3a) and form-
amide (4). A mixture of 18.9 g (0.1 mol) of tris(dimethyl-
amino)ethoxymethane (3a) and 4.5 g (0.1 mol) of formamide
(4) in 100 mL cyclohexane was heated in a flask provided
with a 15 cm Vigreux column (KOH-d◦rying tube). The com-
pounds boiling in the range 73 – 80 C were distilled off.
The residue was then distilled in an oil pump vacuum (about
0.1 Torr) to give a first fraction of 5.5 g with b. p. 27 – 28 ◦C,
20
n
= 1.4554 (tetramethylurea), a second fraction of 1.4 g
D
◦
20
Picrate: In an attempt to characterize this product as a
picrate we isolated yellow crystals with m. p. 159 – 161 C
with b. p. 52 – 53 C, n = 1.4552 (tetramethylurea), and a
D
◦
third fraction of 2.3 g (yield 16 %) of pure 1 with b. p. 91 –
(ethanol) which correspond to dimethylammonium picrate
(lit. [7]: m. p. 161 ◦C). – C8H10N4O7 (274.19): calcd.
C 35.04, H 3.68, N 20.43; found C 35.01, H 3.82, N 20.41.
Its constitution was confirmed also by means of 1H NMR
and 13C NMR spectroscopy (in [D6]DMSO). It can be in-
ferred that under these conditions compound 2 decomposes
by elimination of dimethylamine. For the alleged picrate
of 2 ◦described in the literature [3a] a melting point of 148 –
151 C was reported, and we suppose that the authors have
also obtained the dimethylammonium picrate.
94 ◦C, n = 1.5056, with no tendency to crystallization even
20
D
after staying for 4 d at −20 ◦C. This product is not identical
(TLC, m. p., IR) with the crystalline compound 2 obtained by
1
Methods A–C (see below). – H NMR (300 MHz, CDCl3):
δ = 2.97 (s, 12H, 4× N-Me), 8.54 (s, 1H, CHO). – 13C NMR
(75.48 MHz, CDCl3): δ = 40.36 (4× N-Me), 163.61 [N-
C(=N)-N], 170.36 (CHO). – IR (ATR): ν = 3321, 3172,
1688 (C=O), 1610, 1387 cm−1. – GC-MS: m/z (%) = 143
(7) [M]+, 142 (9) [M–1]+, 126, 99, 85, 71, 44 (100), 42. –
C6H13N3O (143.19): for elemental analysis see picrate 15.
Picrate 15: Prepared from picric acid (453 mg; 2 mmol)
in 5 mL of ethanol and a solution of guanidine 1 (289 mg;
2 mmol) in 2 mL of ethanol. On standing for 2 d at 6 –
Method B: From N,N-dimethylformamide-dimethylsulfate
adduct (9) and 1,1,3,3-tetramethylguanidine (7) (Scheme 3).
◦
9 C (refrigerator) the fine yellow precipitate was filtered
To a solution of 23.0 g (0.2 mol) of 1,1,3,3-tetrameth-
and washed with ethanol and ether. Yield of air-dried pi- ylguanidine (7) in 50 mL of anhydrous acetonitrile 20.0 g
crate 15 376 mg (50 %), m. p. 173 – 176 ◦C (dec.) (ethanol). – (0.1 mol) of DMF-DMS adduct 9 was added dropwise under
1H NMR (250 MHz, [D6]DMSO): δ = 3.07 (s, 12H, 4× N- stirring and ice-cooling at 0 – 5 ◦C for 2 h. Thereafter the re-
CH3), 8.46 (s, 1H, N=CH-O), 8.61 (s, 2Harom., 3ꢀ-H, 5ꢀ-H), action mixture was heated under reflux for 1 h. The solvent
11.16 (br. s, 1H, OH). – 13C NMR (62.9 MHz, [D6]DMSO): and some dimethylamine were removed in vacuo. The yel-
δ = 40.8 (4 C, 4× N-Me), 124.2 (C-4), 125.2 (C-3, C-5), low viscous residue was extracted with anhydrous ether (5×
141.8 (C-2, C-6), 155.7 (br., 1C), 160.8 (C-1), 163.2 (br., 1C, 50 mL). The solvent was removed from this extract by distil-
N=C-O). – C12H16N6O8 (372.29): calcd. C 38.71, H 4.33, lation, and the oily residue was dried in vacuo. It solidified on
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336
W. Kantlehner et al. · Orthoamides and Iminium Salts
cooling to give pure 2 which is identical (TLC, IR, m. p.) to Crystal structure determination of 2
the products obtained by Method A or C. Yield 2.57 g (18 %),
X-Ray single-crystal diffraction data were collected on a
almost colorless solid. The ether-insoluble layer was also
dried in vacuo to afford a pale-yellow viscous fluid (25.7 g),
probably corresponding to a rather impure compound with
the structure 10 (Scheme 3). The crude compound 10 pos-
sesses properties of an ionic liquid.
Bruker-Nonius Kappa CCD diffractometer, using graphite-
˚
monochromatized MoKα radiation (λ = 0.71073 A). A sin-
gle crystal, coated with perfluorinated oil, was mounted on
the tip of a glass fiber. Because the crystal liquefies imme-
diately in the air, the data collection was performed under a
stream of nitrogen at 100 K. Unit cell parameters were ob-
tained by indexing of the peaks in the first 10 frames and
refined by employing the whole data set. All frames were in-
tegrated and corrected for Lorentz and polarization effects.
The structure was solved by Direct Method using SHELXS-
97 [9]. All non-hydrogen atoms were located and refined
anisotropically by full-matrix least-squares using SHELXL-
97 [10]. All hydrogen atoms were found in the final differ-
ence Fourier maps and allowed to refine freely with isotropic
displacement parameters. The results of the crystal structure
analysis are presented in the Tables 1 and 2. For the prepa-
ration of the structural images the program ORTEP-III was
used [11].
Method C: From 1,1-dimethylurea (11) and N,N-dimethyl-
formamide-dimethylacetal (12)
A suspension of 1.8 g (0.02 mol) of 1,1-dimethylurea (11)
(m. p. 160 – 170 ◦C; prepared from N,N-dimethylcarbamoyl
chloride and ammonia) and 4.5 g (0.038 mol) of 12 (DMF-
DMA) was heated with stirring and continuous d◦istillation
of methanol over a 20 cm Vigreux column at 100 C (KOH
drying tube). The urea dissolved slowly, and about 2 mL
of a methanol/DMF-DMA mixture distilled off within 2 h.
◦
Heating was continued at 110 C for further 4 h. The reac-
tion mixture was evaporated in vacuo. The yellowish oily
residue was then extracted with boiling pentane. After cool-
ing, from the pentane extract the almost colorless prod-
uct 2 crystallized, which was identical with the product ob-
tained by application of Methods A and B (IR, TLC and
m. p.).
CCDC 767479 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
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