Highly selective synthesis of (E)-N-aryl-N-(1-propenyl) ethanamides via isomerization of N-allyl ethanamides catalyzed by ruthenium complexes

Article abstract of DOI:10.1016/S0040-4039(01)01457-5

A convenient and highly selective method of synthesis of (E)-N-aryl-N-(1-propenyl)ethanamides via isomerization of respective N-allyl-N-arylethanamides catalyzed by [RuClH(CO)(PPh₃)₃] has been described. N-Allyl-N-arylethanamides hav

Full text of DOI:10.1016/S0040-4039(01)01457-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7095–7098
Highly selective synthesis of (E)-N-aryl-N-(1-propenyl)
ethanamides via isomerization of N-allyl ethanamides catalyzed
by ruthenium complexes
Stanislaw Krompiec,a,* Mariola Pigulla,a Wojciech Szczepankiewicz,a Tadeusz Bieg,a
Nikodem Kuznik,a Katarzyna Leszczynska-Sejda,a Maciej Kubickib and Teresa Borowiakb
aFaculty of Chemistry, Silesian University of Technology, Krzywoustego 6, 44-101 Gliwice, Poland
bFaculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan´, Poland
Received 6 June 2001; revised 2 August 2001; accepted 7 August 2001
Abstract—A convenient and highly selective method of synthesis of (E)-N-aryl-N-(1-propenyl)ethanamides via isomerization of
respective N-allyl-N-arylethanamides catalyzed by [RuClH(CO)(PPh3)3] has been described. N-Allyl-N-arylethanamides have been
obtained by allylation of respective N-arylethanamides under PTC conditions. It is proposed that the observed selectivity of the
double bond migration to (E)-enamides is due to the interaction of the arene ring with the Ru atom in the transition state. © 2001
Elsevier Science Ltd. All rights reserved.
N-Propenyl amides (in general: N-vinyl amides) are
interesting substrates mainly for synthesis of hetero-
cyclic systems,1–3 cycloaddition reactions,4,5 reduction
to enamines,6 and are thoroughly investigated
monomers and co-monomers.7,8 Isomerization of N-
allyl to N-propenyl amides is also a key step of protec-
tion and the following deprotection of amino groups.9
The most convenient method for the synthesis of N-
propenyl amides consists of isomerization of the appro-
priate N-allyl amides catalyzed by LDA10,11 and,
particularly, by transition metal complexes. Ruthe-
nium,12–15 iron,12,15,16 cobalt17 and rhodium12,15,18 com-
plexes have been used. Some enamides can also be
synthesized via vinylation of amides by vinyl halides in
the presence of nickel complexes19 and by N-acylation
of N-allyl imines.6 N-Allyl amides (RCON(Ar)Allyl)
were synthesized from amides (RCONHAr) by allyl
bromide allylation under PTC conditions20,21 in the
presence of NaH22 or NaOH in acetone.23 N-Acylation
of N-allylaniline12,24 has also been used. In our earlier
papers, we have described a series of examples of the
synthesis of O-, S- and N-(1-propenyl) systems via
isomerization of allylic compounds catalyzed by ruthe-
nium and rhodium complexes.25–31
In the present work, we describe a convenient and very
selective method of synthesis of various (E)-N-aryl-N-
(1-propenyl) amides from the appropriate N-allyl
amides via isomerization in the presence of
[RuClH(CO)(PPh3)3] as the catalyst.
N-Allylamides were obtained by allylation of the
respective amides by allyl chloride under PTC
conditions.32
The following N-allyl-N-arylethanamides were ob-
tained (isolated yield of MeCON(Y-C6H4)Allyl in
parentheses): Y=H (80%); o-Me (78%); p-Me (75%);
o-MeO (81%); m-MeO (75%); p-MeO (81%); o-Cl
(74%); p-Cl (76%); o-Br (80%); p-Br (82%). The purity
of the synthesized N-allylamides (determined by 1H
NMR and GC–MS) was higher than 99.5%. This
method of N-allylamide synthesis is simple and much
more effective than those already known. It can also be
applied to allylation of other N–H acids, such as carba-
zole and phthalimide. Pure (recrystalized) N-allyl-
carbazole and N-allylphthalimide have been prepared
under the same conditions in yields of 85 and 80%,
respectively. N-Allyl-N-(4-nitrophenyl)ethanamide was
Keywords: N-allylamides; selective isomerization; ruthenium com-
plexes; (E)-enamides.
* Corresponding author. Tel.: +48-32-237-1707; fax: +48-32-237-2277;
e-mail: krompiec@polsl.gliwice.pl
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01457-5

7096
S. Krompiec et al. / Tetrahedron Letters 42 (2001) 7095–7098
We claim that the observed stereoselectivity cannot be
explained by the higher thermodynamical stability of
the E-isomer. Quantum calculations (AM1 method)40
show that the E- and Z-isomers differ in their heat of
formation by only 10.52 kJ/mol. An analysis of the
shapes of the HOMO and LUMO of the (E)-N-phenyl-
N-(1-propenyl)ethanamide (in the optimal conforma-
tion, determined by the AM1 method) proves the
possibility of interaction between the Ru atom and the
arene ring during the reaction. According to the calcu-
lations, the HOMO is essentially different from zero
both in the aliphatic fragment and in the aromatic one,
whereas the LUMO is different in the arene fragment
only. It means that a back-bonding can form between
the Ru atom and the arene ring in the course of the
reaction (see Fig. 2), which leads to the formation of
(E)-isomers. We have assumed that the double bond
migration catalyzed by [RuClH(CO)(PPh3)3] occurs
according to the hydride mechanism, as in the case of
alkenes and allyl ethers.25
obtained by allylation of the corresponding amide by
allyl bromide in the presence of KOH in acetone.23
(E)-N-Aryl-N-(1-propenyl)ethanamides were obtained
by isomerization of the respective N-aryl-N-allyl
amides.33
The
following
pure
(E)-N-aryl-N-(1-propenyl)-
ethanamides were obtained (% isolated yield of
MeCON(Y-C6H4)(1-propenyl) and % (E) selectivity in
parentheses): Y=H (90, 99.0); Y=o-Me (95, 99.8);
p-Me (93, 98.9); o-MeO (95, 99.3); m-MeO (92, 99.0);
p-MeO (93, 98.9); o-Cl (95, 99.8); p-Cl (89, 98.9); o-Br
(87, 99.5); p-Br (90, 98.8); p-O2N (95, 99.3). The con-
version of N-allyl amides to 1-propenyl derivatives was
1
always quantitative (determined by H NMR and GC–
MS) and no by-products were observed. Therefore, the
yields given are the yields of product separation. More-
over, in this reaction only (or almost only) (E)-enam-
ides were formed. The configuration (E) of enamides
synthesized was proven by X-ray crystallography,34 e.g.
the structure of (E)-N-(p-methylphenyl)-N-(1-pro-
penyl)ethanamide (Fig. 1).
We have found that isomerization of Me-CONRAllyl
(where R=H, allyl or cyclohexyl) catalyzed by
[RuClH(CO)(PPh3)3] leads to the formation of a mix-
ture of (Z)-, (E)-, or (Z,Z)-, (E,E)-, (Z,E)-enamides
(isomerization conditions as before; quantitative con-
version). The result of MeCONRAllyl isomerization is
particularly important for R=cyclohexyl. Although the
cyclohexyl substituent is very large (its steric effect is
bigger than Ph) an (E)- and (Z)-enamides mixture is
formed. This result indicates that the observed high
selectivity of the Me-CONArAllyl isomerization steams
from the coordination and not steric effects. Moreover,
in the isomerization of RCONHAllyl of RCONAllyl2
(R=Me or Me3C) a mixture of isomers is also formed.
This means that the increase in steric effect of the acyl
group has no effect on the selectivity. It is clear now
that the isomerization of RCONR1 (allyl) is selective
(i.e. (E)-enamides form) if and only if R1 is an aryl.
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S. Krompiec et al. / Tetrahedron Letters 42 (2001) 7095–7098
7097
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33. Synthesis of (E)-N-aryl-N-(1-propenyl)ethanamides (gen-
eral routine). N-Aryl-N-allyl amide (0.1 mol) and
[RuClH(CO)(PPh3)3] (0.5% mol) were heated at 120°C
for 2 h (o-Cl and o-MeO 16 h) under an argon atmo-
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J=14.1 Hz, -HCꢀCH-CH3), 7.26 (AA%XX%, 2H, -C6H4-),
7.05 (AA%XX%, 2H, -C6H4-), 4.42 (dq, 1H, J=14.1, 6.8
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6
-CH3), 3.30 (s, 3H, p-C6H4-CH3
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6
6
6
6
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21.1 (Ar-CH3), 15.0 (-HCꢀCH-C6
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methylphenyl)-N-(1-propenyl)ethanamide. All measure-
ments of diffraction intensities were performed on a
KUMA KM4 four-circle diffractometer, Zr foil filtered
Mo Ka radiation, ꢀ/2[ scan mode.35 The structures
were solved by direct methods using the program
SHELXS-9736 and refined by full-matrix least-squares
with the aid of the program SHELXL-97.37 All non-
hydrogen atoms were refined anisotropically. The hydro-
gen positions were calculated according to the standard
geometry, and refined as a riding model with isotropic
thermal parameters.37 Crystal data for (E)-N-(p-
methylphenyl)-N-(1-propenyl)ethanamide). The crystal
chosen for X-ray analysis was a clear colorless plate with
approximate dimensions 0.2×0.4×0.4 mm. C12H15NO
(189.25 g mol−1) crystallizes in the monoclinic system,
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32. Synthesis of N-allyl-N-arylethanamides (general method):
The amide (0.2 mol), 50% aq NaOH (50 cm3), Bu4N+
−
HSO4 (0.002 mol) and an excess of allyl chloride (50
cm3) was intensively stirred and refluxed in a water bath
for 4 h. After cooling, 100 cm3 of water was added and
an excess of allyl chloride was removed by distillation
from the water bath. The residue was extracted twice with
100 cm3 of hexane (or pentane). Then the extract was
dried with anhydrous magnesium sulfate and decolorized
by active coal. After distilling off all volatiles with a
vacuum evaporator, the residue was distilled under
reduced pressure (0.5–1 mmHg). N-Allyl-N-(p-
methylphenyl)ethanamide, bp=125–126°C/1 mmHg; 1H
NMR (C6D6): l=6.97 (AA%XX%, 2H, -C6H4-), 6.86
(AA%XX%, 2H, -C6H4-), 5.89 (ddt, 1H, J=16.5, 10.1, 6.3
space group P21/m, with a=10.632(2), b=6.854(1), c=
3
,
,
16.068(3) A, i=106.35(3)°, V=1123.6(3) A , Z=4,
v(Mo Ka)=0.07 mm−1, and Dcalcd=1.119 g cm−3. The
e.s.d. unit cell parameters were determined by least-
squares refinement using 60 centered reflections within
2°<[<16°. A total of 1695 reflections were collected to
2[max=47.92° (h: 10“10, k: 0“7, l: 0“11), of which
1493 were unique. The intensity decay of the reference
reflections was 34%. In refinements, weights were used
according to the scheme w=1/[|2(Fo )+(0.0753P)2+
0.50P], where P=(Fo +2Fc )/3. The refinement of 158
parameters (data-to-parameter ratio being 9.45) has led
to the final agreement factors R=0.0529, Rw=0.1573,
2
2
2
Hz, -H2CꢀCH
6
-CH2), 4.97 (dd, 1H, J=10.1, 1.4 Hz,
-cis), 4.96 (dd, 1H, J=16.5, 1.4 Hz,
H2CꢀCH-CH2
6

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S. Krompiec et al. / Tetrahedron Letters 42 (2001) 7095–7098
and S=1.002 for 897 observed reflections with F>4|(Fo).
The electron density of the largest difference peak was
39. A complete listing of the atomic coordinates of (E)-N-(p-
methylphenyl)-N-(1-propenyl)ethanamide can be obtained
free of charge, on request, from the Director, Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: (+int) 44-1223 336 033; e-mail:
deposit@ccdc.cam.ac.uk], on quoting the depository num-
bers, the names of the authors, and the journal citation.39
40. The calculation has been carried out with the AM1
method41 of MOPAC 2000.42 The stop criterion was to
achieve a gradient less than 0.1 by the EF following vector
optimisation method.43
found to be 0.24 e A−3, while that of the largest difference
,
hole was 0.20 e A−3. Tables of bond distances and angles,
,
atomic coordinates, and anisotropic thermal parameters of
(E) - N - (p - methylphenyl) - N - (1 - propenyl)ethanamide
(CCDC 162490) have been deposited with the Cambridge
Crystallographic Data Centre.
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of Crystal Structures. University of Gottingen, Germany,
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P. P. J. Am. Chem. Soc. 1985, 107, 3902.
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