Reactions of silylcarbamates with ketones

Article abstract of DOI:10.1016/S0040-4039(00)01521-5

The reaction of N,N-dialkyl-O-trimethylsilyl carbamates with different carbonyl compounds gives enamines under mild conditions in good yields. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01521-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8937–8939
Reactions of silylcarbamates with ketones
F. Kardon, M. M o¨ rtl* and D. Knausz
Department of General and Inorganic Chemistry, E o¨ tv o¨ s Lor a´ nd University, 1518 Budapest 112,
PO Box 32, Hungary
Received 28 June 2000; revised 22 August 2000; accepted 7 September 2000
Abstract
The reaction of N,N-dialkyl-O-trimethylsilyl carbamates with different carbonyl compounds gives
enamines under mild conditions in good yields. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: enamines; carbamates; silicon; carbonyl compounds.
Although a number of methods are available for the synthesis of enamines, difficulties are
often encountered if the amine is volatile, e.g. dimethylamine (bp 7°C). The preparation of
1
–11
dimethylamino enamines requires either a long reaction time or expensive reagents.
of our study on silylcarbamates
compounds.
As part
1
2,13
we have examined their reactions with several carbonyl
The reactions of N,N-dialkyl-O-trimethylsilyl carbamates with different carbonyl compounds
were carried out without any solvent, generally at room temperature with exclusion of moisture.
The optimum molar ratio of silylcarbamates and oxo compound was 2.5:1. The reaction usually
gives enamines (see Scheme 1).
Scheme 1.
At least two equivalents of silyl carbamates are needed in the reaction since they take part,
not only in the formation of the enamine, but also as dehydrating agents.
In the case of acetylacetone, only one of the two carbonyl groups is converted into an
enamine, even with an excess of silylcarbamate. The reaction with ethyl acetoacetate takes place
on the oxo group. Data concerning compounds prepared by the above method are summarized
*
Corresponding author. Tel: +0036-1-209-0555/1607; fax: +0036-1-209-0602; e-mail: mortl@para.chem.elte.hu
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01521-5

8938
1
13
in Table 1. All compounds were identified by IR, H and C NMR spectroscopy. This reaction
proceeded to completeness also with morpholine-N-carboxylic acid trimethylsilyl ester (product
4
), but the amine and carbon dioxide, which is evolved in the reactions, gave the ammonium
carbamate salt as an undesired byproduct. Using a nonpolar solvent or applying higher
temperature (50–60°C), the amount of this salt can be decreased. At the same time the yield of
enamine increases.
Table 1
Reaction conditions and yields of the prepared enamines 1–4
We examined some further reactions by GC-MS and IR spectroscopy. The N,N-dialkyl-O-
trimethylsilyl carbamates (R=Et, Pr, Bu) reacted with 2-butanone and 2-pentanone by the
formation of 2-dialkyl(but-2-en-2-yl)amine and 2-dialkyl(pent-2-en-2-yl)amine. However if R=
Me, the reaction gave the self-condensation products of the ketones. In all cases cyclic ketones
(
cyclopentanone, cyclohexanone) were converted to enamines. The reaction of N,N-dialkyl-O-
trimethylsilyl carbamates with acetone and aldehydes always led to self-condensation. Some of
the above products were isolated; related data are shown in Table 2.
Table 2
Reaction conditions and yields of the prepared enamines 5–10
Ketone
R NC(O)OSiMe R=
Bp °C/mmHg
Yield%
Conditions
2
3
5
6
7
8
9
0
3-Heptanone
3-Pentanone
Cyclopentanone
Cyclopentanone
Cyclopentanone
Cyclohexanone
Me
Me
Pr
Bu
Morpholino
Bu
165–170/760
125–130/760
213–215/760
216–220/760
250–254/760
168–171/20
84
85
80
68
76
73
Rt, 4 h
Rt, 1 h
100°C, 1 h
100°C, 1.5 h
100°C, 2 h
100°C, 2 h
1

8939
The reaction between N,N-dimethyl-O-trimethylsilyl carbamate and propionaldehyde, 4%-
hydroxy-acetophenone, acetonylacetone, 1-hydroxy-2-butanone, a-ketoglutaric acid and 1-tetra-
lone did not lead to the desired enamine product. Since the silylamines react with 1-tetralone or
10,11
cyclohexanone by the formation of silyl enol ethers,
therefore we also examined the reaction
of silyl enol ethers with N,N-dimethyl-O-trimethylsilyl carbamate. No reaction was observed.
On the basis of this result we conclude that the formation of enamines does not take place via
silyl enol ethers.
In the above-mentioned reactions products are formed under mild conditions. They require
only a small excess of reagents. The reactions are fast and the yields are fairly good. These
advantages make this reaction attractive, especially for the preparation of enamines from
volatile amines.
Acknowledgements
This work was supported by Hungarian National Science Foundation (OTKA) grant num-
bers: F030821 and T025830.
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