Alkyloximes and imines via silyl carbamates

Article abstract of DOI:10.1080/00397911003707121

The reactions between different N,O-bis-trimethylsilyl-carbamates and oxo compounds were studied. The N,O-bis-trimethylsilyl-N-methoxy-carbamate converts ketones to the corresponding O-methyl oximes. The product is usually a mixture of syn and anti isomers. If the carbonyl compound bears a hydroxyl group, the oxime formation and the O-silylation take place simultaneously. In the case of -haloketones, the undesired substitution of halogen was not observed. The reactions between N,O-bis-trimethylsilyl-N-methyl-carbamate and oxo compounds resulted in the corresponding imines, although the yield are moderate in some cases. Analoguos reaction of ferrocenylated oxo derivatives gave similar results.

Full text of DOI:10.1080/00397911003707121

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Alkyloximes and Imines via Silyl
Carbamates
Ferenc Kardon a , Mária Mörtl a , Antal Csámpai a , Kálmán Újszászy a
& Dezső Knausz a
a Laboratory of Organosilicon Chemistry , Institute of Chemistry,
Eötvös Loránd University , Budapest , Hungary
Published online: 25 Feb 2011.
To cite this article: Ferenc Kardon , Mária Mörtl , Antal Csámpai , Kálmán Újszászy & Dezső
Knausz (2011) Alkyloximes and Imines via Silyl Carbamates, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 41:6, 914-924, DOI:
10.1080/00397911003707121
To link to this article: http://dx.doi.org/10.1080/00397911003707121
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Synthetic Communications1, 41: 914–924, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003707121
ALKYLOXIMES AND IMINES VIA SILYL CARBAMATES
´
Ferenc Kardon, Maria Mortl, Antal Csampai,
Kalman Ujszaszy, and Dezso Knausz
´
¨
´
´
´
´
}
Laboratory of Organosilicon Chemistry, Institute of Chemistry, Eo¨tvo¨s
´
Lorand University, Budapest, Hungary
GRAPHICAL ABSTRACT
Abstract The reactions between different N,O-bis-trimethylsilyl-carbamates and oxo
compounds were studied. The N,O-bis-trimethylsilyl-N-methoxy-carbamate converts ketones
to the corresponding O-methyl oximes. The product is usually a mixture of syn and anti
isomers. If the carbonyl compound bears a hydroxyl group, the oxime formation and the O-sily-
lation take place simultaneously. In the case of a-haloketones, the undesired substitution of
halogen was not observed. The reactions between N,O-bis-trimethylsilyl-N-methyl-carbamate
and oxo compounds resulted in the corresponding imines, although the yield are moderate in
some cases. Analoguos reaction of ferrocenylated oxo derivatives gave similar results.
Keywords Carbamates; ferrocene derivatives; O-methyl oxime; imines; silicon and
carbonyl compounds
The silylcarbamates are very efficient silylating agents. As a part of our study on
silylcarbamates, we examined their reactions with several compounds.[1–3] We estab-
lished that the reaction between N,N-dialkyl-O-silyl carbamates and ketones gave
the corresponding enamines.[3] Some other silylating agent reacted similarly.[4]
However, the potassium salt of N,O-bis(trimethylsilyl)-hydroxylamine reacts with
ketones by formation of an oximate anion.[5] Earlier,[6] we used N,O-bis-trimethyl-
silyl-N-methoxy-carbamate for the silylation of steroids, and the oxo group turned
out to be also reactive. Therefore, we decided to carry out a systematic study on this
reaction.
Alkyl-oximes are generally prepared by alkylation of hydroximes. In this
reaction, nitrone is often an undesired by-product.[7] The reactions of haloketones
Received November 6, 2009.
Address correspondence to Maria Mo¨rtl, Plant Protection Institute of Hungarian Academy of
´
Science, Nagykovacsi str. 26-30, 1029 Budapest, Hungary (present address). E-mail: mortl@julia-nki.hu
´
914

ALKYLOXIMES AND IMINES VIA SILYL CARBAMATES
915
with an excess of hydroxylamine result in substitution of halogen and formation of
hydroxylamino-oximes.[8,9] In the case of aromatic aldehydes and a-haloketones, the
reaction with methanol solvent takes place and gives the corresponding acetale.[10,11]
Some other reactions with organosilicon reagents can afford either oximes or
nitrones.[12–14]
We next turned our attention to the application of the present reaction for the
preparation of Schiff bases using the corresponding N-methyl carbamate. There are
a lot of methods for the preparation of imines.[15] It is usually more difficult to obtain
them from ketones than from aldehydes, and their synthesis becomes increasingly
difficult with the starting materials bearing more aromatic groups.[16]
The direct formation of imines and related derivatives from carbonyl
compounds with amines proceeds in two steps, the formation of carbinolamine inter-
mediate and its dehydration, to give the product.[7] The rate of reaction is greatest
between pH 4.5 and 5.5. The reaction between N,O-bis-trimethylsilyl-N-methoxy-
carbamate with ketones is also acid catalyzed.[6] Therefore, in all cases, a small
amount of trifluoroacetic acid catalyst (usually 2%) was applied in this study. As acid
consumes the reagent, the application of a large amount of catalyst requires the
addition of reagent in excess.
First, we examined the reactions of N,O-bis-trimethylsilyl-N-methoxy-
carbamate with ketones, which led to the formation of the corresponding O-methyl
oximes (see Scheme 1) as a mixture of syn and anti isomers.[17] Data concerning
compounds prepared by this method are summarized in Table 1.
The oxime formation with ethyl acetoacetate takes place on the ketone moiety.
The situation with acetylacetone is more complicated. The reaction between
acetyl-acetone and N,O-bis-trimethylsilyl-N-methoxy-carbamate (molar ratio 1:1)
gives a mixture of O-methyl monoxime (3a) and dioxime (3b) (see Scheme 2A),
but when using carbamate in excess (molar ratio 1:2.5), silyl enol ether (3c), its O-
methyl oxime derivative (3d), and pentane-2,4-dione bis(O-methyl oxime) (3b) were
also formed (see Scheme 2B). On treating this latter mixture with ethanol, only 3a
and 3b remained, which could be isolated by distillation. The isomers of these com-
pounds were observed and identified by gas chromatography–mass spectrometry
(GC-MS) measurements.
The silyl enol ether (3c) was prepared independently from acetyl acetone with
hexamethyldisilazane. This does not react with N,O-bis-trimethylsilyl-O-methoxy-
carbamate, so the methoxime formation must precede the formation of silyl enol
ether in the case of a mixed derivative (3d).
For compounds 5 and 6, where the reactant contains a phenol-type hydrogen,
the oxime formation and the silylation take place simultaneously, although the
silylation is much faster. The oxime formation from aromatic aldehyde took place
Scheme 1. Formation of syn and anti isomers of O-alkyloximes.

916

917

918
F. KARDON ET AL.
Scheme 2. Reaction between acetyl-acetone and N,O-bis-trimethylsilyl-N-methoxy-carbamate. (A) molar
ratio 1:1, and (B) molar ratio 1:2.5.
readily, while harsher conditions are necessary to obtain the corresponding oxime
from aromatic ketone. The optimum molar ratio of silylcarbamates and oxo
compound in these cases is 2.2:1.
Haloketones react readily without formation of any undesired by-product
(7 and 8).
No reaction was observed with 1-tetralone (10); however, the reaction with
a-ketoglutaric acid (11) gave a mixture of corresponding silylated and=or oximated
derivatives. The reactions with aliphatic aldehydes such as propionaldehyde (12)
gave a complex mixture, probably because of the undesired self-condensations as
observed earlier with trimethylsilyl-N,N-dialkylcarbamates.[3]
Similarly, by reaction of N,O-bis-trimethylsilyl-N-trimethylsiloxy-carbamate
and ethyl acetoacetate, silyloxime (9) formation takes place. The silyl enol ether of
acetyl acetone (3c) does not react even with N,O-bis-trimethylsilyl-N-trimethylsiloxy-
carbamate.
For the preparation of imines, the N,O-bis-trimethylsilyl-N-methyl-carbamate
and N,O-bis-trimethylsilyl-N-phenyl-carbamate were synthesized, and their reactions
(Scheme 3) were studied with different ketones.
In the cases of N-methyl derivatives, the yields are moderate (see Table 2). For
sterically hindered ketones (i.e., 4-methyl-2-pentanone) and aromatic ketones (i.e.,
acetophenone), the yields are poor, 19% and 12%, respectively, even if the reaction
is carried out in the presence of excess silyl carbamate reagent at elevated temperature
(entries 14 and 15). The reaction with propionaldehyde gave a complex mixture. On
the basis of NMR spectra, compounds 16 and 17 rearranged to the more stable
enamine forms. Ferrocenecarboxaldehyde and 1,10-ferrocenedicarboxaldehyde
transform to the desired N-methyl imines (18 and 19), although it required threefold
excess of reagent. However, acetylferrocene does not react, and in the case of
10-acetylferrocene-1-carboxaldehyde only the aldehyde moiety was converted selec-
tively to the corresponding imine (20).
Scheme 3. Formation of imines.

ALKYLOXIMES AND IMINES VIA SILYL CARBAMATES
919
Table 2. Reaction conditions and yields of the prepared imines
Oxo
compound
Reaction
conditions
Yield
(%)
No.
Stoichiometrya TFAb
Product
13
1:2.5
1:2.5
1:2.5
1:10
1:10
1:10
rt, 8 h
43c
19c
12
14
15
50 ꢀC, 8 h
110 ꢀC, 40 h
16
17
1:1
1:50
1:50
55 ꢀC, 4 h
50 ꢀC, 6 h
50
43
1:1.2
18
19
1:3
1:6
1:50
1:50
rt, 4 h
86
76
rt, 6 h
20
1:6
1:50
rt, 8 h
79
21
22
1:3
1:1
1:50
1:50
rt, 8 h
rt, 2 h
Reaction failed
—
—
Complex mixture
aMolar ratio of oxo compound and N,O-bis-trimethylsilyl-N-methyl-carbamate.
bMolar ratio of trifluoroacetic acid catalyst and oxo compound.
cNot isolated; even repeated distillation resulted in a mixture of the product and the starting ketone.

920
F. KARDON ET AL.
The N-phenyl imines of acetoacetate, acetylacetone, and benzophenone could
be detected by GC-MS; however, the conversions are poor.
In conclusion, we have a one-pot procedure, which resulted in O-alkyl or
O-silyl oximes under mild conditions. This synthetic route would be especially useful
if the direct reaction between oxo compounds and amines produced undesired
by-products, as was the case for example with halogenated ketones. For the prep-
aration of N-methyl imines, our method is limited in use.
EXPERIMENTAL
1H and 13C NMR spectra were recorded on a Bruker Avance spectrometer at
250 MHz in CDCl3 solution. Infrared (IR; KBr) spectra were obtained using a
Bruker Fourier transform FT–IR IFS 55 spectrometer. The spectra for compounds,
not mentioned here, are identical with literature data. All preparations were carried
out under moisture-free conditions.
General Procedure for the Preparation of Oximes and Imines
The reactions were carried under moisture-free conditions without use of any
solvents. Equimolar amounts of a ketone and N,O-bis-trimethylsilyl-N-methoxy-
carbamate were stirred, usually at ambient temperature. Trifluoroacetic acid (TFA)
as catalyst was applied (molar ratio TFA–ketone 1:50). In the cases of sterically
hindered or aromatic ketones, the reaction requires higher temperature and=or more
TFA. The proceeding of reaction is indicated by evolution of carbon dioxide. The
products were distilled in vacuo. The reactions with N,O-bis-trimethylsilyl-N-methyl-
carbamate were carried out in the same way.
Ferrocene derivatives 18–20 were obtained as red oils after chromatographic
purification on silica with ether–petrolether 1:1 as eluent.
The NMR data for 1 and 8 are identical with those previously reported.[17] IR
for 1 (KBr) n (cmꢂ1) 1640.
Some further spectral data for prepared compounds and experimental detail
are given next.
Ethyl-3-methoxyiminobutanoate (2)
Bp 52–54 (1–2 mmHg).
Anti isomer. 1H NMR: d 1.11 (br, 3 H), 1.79 (s, 3 H), 3.16 (s, 2 H), 3.68 (s, 3 H),
3.99 (br, 2 H). 13C NMR d 14.4, 20.5, 41.5, 61.1, 61.5, 151.4, 169.7.
Syn isomer. 1H NMR: d 1.11 (br, 3 H), 1.76 (s, 3 H), 3.06 (s, 2 H), 3.65 (s, 3 H),
3.99 (br, 2 H). 13C NMR: d 14.3, 20.5, 35.3, 61.1, 61.4, 150.4, 168.9. Anal. calcd. for
C7H13O3N (159.18): C, 52.82; H, 8.23; N, 8.80. Found: C, 52.7; H, 8.2; N, 8.6. IR
(KBr) n (cmꢂ1) 1633, 1739.
A reaction mixture containing 3a and 3b isomers (see 3A in Table 1) was
distilled in vacuo, but the isomers could not be separated from each other. The ratio
of 3a to 3b is equal to 40:60 on the basis of GC measurements (FID).

ALKYLOXIMES AND IMINES VIA SILYL CARBAMATES
921
A reaction mixture containing 3c, 3d, and 3b isomers (see 3B in Table 1) was
distilled in vacuo, but the isomers could not be separated from each other. The ratio
of 3b=3c=3d 45:35:25 on the basis of GC measurements (FID). A further reaction
with ethanol resulted in 3a and 3b. On the basis of GC-MS, NMR, and IR spectra
of mixtures, the products could be identified, but these reactions are not suitable
for preparative purposes. ‘‘Isolated’’ yields cannot be given, but conversion of acetyl-
acetone was complete in this latter case.
4-Methoxyiminopentane-2-one (3a)
Bp 42–44 (1–2 mmHg). The NMR data are in accordance with Refs. 18 and 19.
IR (KBr) n (cmꢂ1) 1635, 1732. m=z (EI) 129 (Mþ, 1), 98 (6), 87 (18), 42 (100).
N2,N4-Dimethoxypentane-2,4-diimine (3b)
The NMR data are in accordance with Ref. 19. IR (KBr) n (cmꢂ1) 1627. m=z
(EI) 158 (Mþ, 1), 127 (100), 100 (20).
N-Methoxy-4-methyl-pentane-2-imine (4)
Bp 130–132 (760 mmHg).
Anti isomer. 1H NMR: d ¼ 0.81 (d, J 6.9 Hz, 6 H), 1.70 (s, 3 H), 1.78 (m, J
7.0 Hz, 1 H), 1.93 (d, J 7.3 Hz, 2 H), 3.72 (s, 3 H). 13C NMR: d 13.6, 22.1, 26.7,
44.5, 60.8, 156.6.
Syn isomer. 1H NMR: d 0.81 (d, J 6.9 Hz, 6 H), 1.73 (s, 3 H), 1.84 (m, J
7.0 Hz, 1 H), 2.11 (d, J 7.6 Hz, 2 H), 3.67 (s, 3 H). 13C NMR: d ¼ 20.2, 22.4, 26.6,
37.9, 60.6, 157.1. Anal. calcd. for C7H15ON (129.21): C, 65.07; H, 11.70; N, 10.84.
Found: C, 65.0; H, 11.6; N, 10.8. IR (KBr) n (cmꢂ1) 1633.
N-Methoxy-1-(4-trimethylsiloxyphenyl)ethanimine (5)
1
Bp 84–86 (1–2 mmHg). H NMR: d 0.29 (s, 9 H), 2.21 (s, 3H), 3.97 (s, 3H),
6.8–6.9 (m, 2H), 7.0–7.2 (m, 2H). 13C NMR: d 0,2, 62.1, 13.1, 120.1, 128.7, 129.8,
133.7, 149.4, 152.5. Anal. calcd. for C12H19O2NSi (237.37): C, 60.72; H, 8.07; N,
5.90. Found: C, 60.6; H, 8.0; N, 8.8. IR (KBr) n (cmꢂ1) 1601.
N-Methoxy-1-(3-trimethylsiloxyphenyl)methanimine (6)
1
Bp 78–80 (1–2 mmHg). H NMR: d 0.08 (s, 9 H), 3.95 (s, 3H), 6.85 (d, 1H),
7.0–7.2 (m, 3H), 7.98 (s, 1H). 13C NMR: d 2.0, 62.0, 118.3, 120.6, 121.7, 129.0,
133.7, 148.4, 155.6. Anal. calcd. for C11H17O2NSi (223.35): C, 59.15; H, 7.67; N,
6.27. Found: C, 59.1; H, 7.6; N, 6.2. IR (KBr) n (cmꢂ1) 1605.

922
F. KARDON ET AL.
2-Chloro-N-methoxy-cyclopentanimine (7)
IR (KBr) n (cmꢂ1) 1636. m=z (EI) 149 (Mþ, 37Cl, 9), 147 (Mþ, 35Cl, 30), 116
(100), 112 (69), 80 (95), 53 (94), 39 (78).
2-Chloro-N-methoxy-cyclohexanimine (8)
Bp 52–54 (1–2 mmHg). m=z (EI) 161 (Mþ, 35Cl, 46), 163 (Mþ, 37Cl, 15), 112
(69), 80 (95), 53 (94), 39 (78).
Ethyl-3-trimethylsilyloxyiminobutanoate (9)
Bp 58–60 (1–2 mmHg). IR (KBr) n (cmꢂ1) 1627, 1741.
Anti isomer. 1H NMR: d 0.11 (s, 9H), 1.18 (t, J 7.1 Hz, 3H), 1.90 (s, 3H), 3.15
(s, 2H), 4.09 (q, J 7.1 Hz, 2H). 13C NMR: d 1.7, 14.0, 20.3, 38.4, 60.7, 155.8, 169.5.
Syn isomer. 1H NMR: d 0.11 (s, 9H), 1.18 (t, J 7.1 Hz, 3H), 1.86 (s, 3H), 3.27
(s, 2H), 4.09 (q, J 7.1 Hz, 2H). 13C NMR: d ¼ 1.7, 14.1, 21.6, 41.3, 60.6, 154.7, 168.9.
Anal. calcd. for C9H19O3NSi (217.34): C, 49.74; H, 8.81; N, 6.44. Found: C, 49.6; H,
8.7; N, 6.4.
Imine 13 could not be separated from starting ketone by distillation. The
product was identified by GC-MS. We found the same situation with N-methyl imine
14 formed from 4-methyl-2-pentanone. IR (KBr) n (cmꢂ1) 1652.
4-Methylamino-pent-3-en-2-one (16)
Bp 42–44 (1–2 mmHg).1H NMR: d 1.91 (s, 3H), 1.99 (s, 3H), 2.92 (d, J 5.3 Hz,
3H), 4.98 (s, 1H), 10.7 (br, 1H). 13C NMR: d 28.7, 29.5, 95.1, 164.1, 194.7. Anal.
calcd. for C6H11ON (113.16): C, 63.69; H, 9.80; N, 12.38. Found: C, 63.6; H, 9.7;
N, 12.3. IR (KBr) n (cmꢂ1) 1575, 1612. m=z (EI) 113 (Mþ, 100), 99 (58), 56 (94).
3-Methylamino-but-2-eonic Acid Ethyl Ester (17)
Bp 52–53 (1–2 mmHg). 1H NMR: d 1.21 (t, J 7.1 Hz, 3H), 1.90 (s, 3H), 2.92 (d, J
5.2 Hz, 3H), 4.07 (q, J 7.1 Hz, 2H), 4.43 (s, 1H), 8.5 (br, 1H). 13C NMR: d 14.7, 19.1,
29.5, 58.2, 82.0, 162.8, 169.1. Anal. calcd. for C7H13O2N (143.18): C, 58.72; H, 9.15;
N, 9.78. Found: C, 58.6; H, 9.1; N, 9.7. IR (KBr) (cmꢂ1) 1608, 1650. m=z (EI) 143
(Mþ, 42), 98 (54), 70 (60), 56 (100).
N-Methyl-iminomethyl-ferrocene (18)
1H NMR: d 3.39 (s, 3H), 4.22 (s, 5H), 4.39 (s, 2H), 8.16 (s, 1H). 13C NMR: d
48.2, 68.0, 69.0, 70.1, 80.6, 161.9. Anal. calcd. for C12H13FeN (227.09): C, 63.47; H,
5.77; N, 6.17. Found: C, 63.4; H, 5.7; N, 6.1. IR (KBr) n (cmꢂ1) 1060, 1252, 1651,
1706 2956. m=z (EI) 227 (Mþ, 100), 186 (48), 121 (56), 56 (47).

ALKYLOXIMES AND IMINES VIA SILYL CARBAMATES
923
1,1’-Bis(N-methyl-iminomethyl)ferrocene (19)
1H NMR: d 3.28 (s, 3H), 7.95 (s, 1H), 4.32 (s, 2H), 4.61 (s, 2H). 13C NMR: d 48.6,
69.0, 71.1, 81.2, 161.6. Anal. calcd. for C14H16FeN2 (268.14): C, 62.71; H, 6.01; N, 10.4.
Found: C, 62.6; H, 5.9; N, 10.4. IR (KBr) n (cmꢂ1) 2962, 1656, 1258, 1082, 1012, 791.
m=z (EI) 268 (Mþ, 77), 253 (100), 162 (30), 147 (23), 121 (18), 106 (20), 56 (33).
1-Acetyl-1’-N-methyl-iminomethyl-ferrocene (20)
1H NMR: d 2.35 (s, 3H), 3.34 (s, 3H), 4.33 (s, 2H), 4.45 (s, 2H), 4.53 (s, 2H),
4.65 (s, 2H), 8.02 (s, 1H). 13C NMR: d 27.5, 48.1, 69.5, 70.3, 71.6, 73.1, 80.1, 80.6,
162.2, 172.6. Anal. calcd. for C14H15FeNO (269.13): C, 62.48; H, 5.62; N, 5.21.
Found: C, 62.4; H, 5.5; N, 5.2. IR (KBr) n (cmꢂ1) 2962, 1645, 1259, 1088, 1019,
797. m=z (EI) 269 (Mþ, 4), 228 (40), 213 (25), 185 (100), 163 (28), 120 (56), 56 (53).
ACKNOWLEDGMENT
This work was supported by the Hungarian National Science Foundation
(OTKA) Grant Nos. F030821 and T025830.
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