Highly selective isomerization of N-allylamides and N-allylamines

Article abstract of DOI:10.1016/j.tetlet.2004.05.023

A highly selective rhodium and ruthenium catalyzed transformation of N-allylamines and N-allylamides to the corresponding 1-propenyl derivatives is described. Strong E-selectivity in the isomerization of allylamines was observed. The first catalytic syste

Full text of DOI:10.1016/j.tetlet.2004.05.023

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 5257–5261
Highly selective isomerization of N-allylamides and N-allylamines
Stanisław Krompiec,a Mariola Pigulla,a,* Michał Krompiec,a Stefan Baj,a
c
ꢀb
Julita Mrowiec-Białon and Janusz Kasperczyk
aFaculty of Chemistry, Silesian University of Technology, Strzody 7, 44-101 Gliwice, Poland
bInstitute of Chemical Engineering, Polish Academy of Sciences, Bałtycka 5, 44-100 Gliwice, Poland
cInstitute of Polymer Chemistry, Polish Academy of Sciences, M.C. Skłodowskiej 34, 41-800 Zabrze, Poland
Received 24 February2004; revised 30 April 2004; accepted 6 May2004
Abstract—A highlyselective rhodium and ruthenium catalyzed transformation of N-allylamines and N-allylamides to the corre-
sponding 1-propenyl derivatives is described. Strong E-selectivity in the isomerization of allylamines was observed. The first catalytic
system containing a transition metal complex for Z-selective isomerization of allylamides is presented. An application of siliceous
mesoporous cellular foams for effective removal of the catalyst from the post-reaction mixture is described.
Ó 2004 Elsevier Ltd. All rights reserved.
Enamides and enamines are interesting intermediates for
various transformations. N-Propenyl amides are
substrates for the synthesis of heterocyclic systems,1
Diels–Alder cycloaddition,2 reduction to enamines,3
enamide–olefin ring-closing metathesis4 and are
densation of a secondaryamine with a carbonyl com-
pound,17 hydroamination of alkynes,18 methylenation of
amides,19 vinylamination.20 An alternative method for
the synthesis of enamines is the isomerization of allyl-
amines using strong bases such as t-BuOK,21 n-BuLi22 or
ruthenium,23 rhodium24 and iron25 complexes. The ste-
reospecific isomerization of prochiral allylamines using
catalysts based on Rh, Ir and Ru having chiral ligands
gives the corresponding chiral enamines, which can then
be hydrolyzed to obtain chiral aldehydes.26 Isomeriza-
tion of N-allyl to N-propenyl amines is also the key step
in the deprotection of amino groups27 protected as their
N-allyl derivatives.
5
thoroughlyinvestigated monomers and co-monomers.
Several methods for the synthesis of enamides has been
reported. The most general method is N-acylation of
N-allylimines with an acyl halide or acid anhydride.3;6
Some enamides can also be synthesized via vinylation of
amides with vinyl halides in the presence of nickel
complexes7 or via titanium-mediated coupling of
ynamides with aldehydes and ketones.8 But the yield of
enamides is often modest and conditions used are in
manycases quite harsh. The most convenient method
for the synthesis of N-propenyl amides consists in the
isomerization of the appropriate N-allyl amides cata-
lyzed by LDA,9 n-BuLi10 and, particularly, by transition
metal complexes. Ruthenium,11 iron,11a;d;12 cobalt13 and
rhodium11a;12a;14 complexes have been applied. Isomeri-
zation of N-allylamides to enamides is very attractive
because N-allylamides are available via standard syn-
thetic methods. Enamines occupya prominent place as
intermediates in organic synthesis,15 in the biological
world and as monomers for polymerization.16 There are
manymethods for enamine formation such as: con-
In the present work, we describe a convenient and very
selective (frequently E or Z selective) method of syn-
thesis of N-(1-propenyl) enamines and enamides via
isomerization of appropriate N-allyl compounds.
The results of the isomerization of some N-allylamides
are summarized in Table 1. In all reactions the conver-
sion and selectivityto 1-propenyl derivatives was mainly
or exclusivelyquantitative. As expected, the isomeriza-
tion of the N-allylamides shown in Table 1 led to the
formation of a mixture of (E) and (Z) or (E,E) and
(E,Z)-enamides.
In our earlier papers we demonstrated that only N-aryl-
N-allyl-amides can be isomerized selectively to (E)-en-
amides in the presence of [RuClH(CO(PPh3)3)],
Keywords: Selective isomerization; (Z)-Enamides; (E)-Enamines.
* Corresponding author.
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.05.023

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S. Krompiec et al. / Tetrahedron Letters 45 (2004) 5257–5261
Table 1. Isomerization of N-allylamides (QCH2CH@CH2) to enamides (QCH@CHCH3) catalyzed by [RuClH(CO)(PPh3)3]a
N-Allylamide
S/C
So
Enamide
T (°C), s (h)
yb (%)
E/Zc
O
N
H
80, 2
80, 3
96
59
C6H6, 0.50
C6H6, 0.50
100, 85d
59/41
1
Ph
O
100, 84d
67/33
42/58
N
H
2
H N
2
O
80, 3
143
THF, 1.00
100
N
H
3
H
N
O
80, 3
70, 3
53
57
THF, 2.50
C6H6, 0.84
––
100
79/21e
77/23
85/15
44/66
N
O
H
4
S
100
N
H
5
O
O
120, 2
120, 2
200
98
100, 93f
100, 70f
N
6
––
N
7
a Experimental procedure as in our previous work;11e S/C––substrate/catalyst; So––solvent, cm3 per 1 mmol of substrate; y––yield of the double bond
migration product, determined by 1H NMR.
b Conversion was always quantitative (determined by 1H NMR and GC–MS).
c Determined by 1H NMR and GC–MS.
d Isolated yield after reduced pressure distillation.
e EZ/ZZ.
f Isolated yield after column chromatography.
[RuCl2(PPh3)3] or [RhH(CO(PPh3)3)].11e;f This (E)-
selectivityis a result of a specific coordination of the
O
O
[Ru] + L + CaH2
R
N
H
11e;f
R
N
H
metal atom bythe arly substituent.
However, the
THF, (80 oC; 2 h)1)
application of a new catalytic system, containing a
precursor––{[RuCl2(1,5-COD)]x}, a phosphine ligand
and a hydride ligand donor affords a quantitative (Z)-
selective isomerization of some N-allylamides (see
Fig. 1).
R = CH3, Ph, NH2; [Ru] = {[RuCl2(1,5-COD)]x};
L = tris(2,4-di-t-butylphenyl) phosphite
amide : [Ru] : L : CaH2 = 100 : 1 : 1 : 10
1) N-allylbenzamide: 90 oC/4 h, N-allylurea: 100o C/1 h
To the best of our knowledge, this is the first example of
a selective isomerization of allyl systems exclusively to
(Z)-1-propenyl derivatives in the presence of transition
metal complexes. The formation of (Z) isomers in this
reaction is, in our opinion, a result of steric interactions
in the transition state (see Fig. 2A).
Figure 1. Stereoselective isomerization of N-allylamides to (Z)-en-
amides.28
the [Ru]–H elimination plane than groups Q and CH3.
In such a conformation, steric interactions between Q,
CH3 and other Ru ligands are minimized. We have
observed a similar effect in the isomerization of R–S–
CH2CH@CH2 (R ¼ Me3C or Ph3C).29 It is noteworthy
In the transition state, due to the aforementioned steric
effects, the bulkyphosphite ligand is on the other side of

S. Krompiec et al. / Tetrahedron Letters 45 (2004) 5257–5261
5259
of a specific coordination of the metal atom bythe
Q
H
substrates and products of double bond migration.
Quantum chemical calculations33 suggest that the sub-
strate and the product coordinate the ruthenium atom
mostlythrough the double bond (from the allly or
propenyl fragment) and, in the case of amines, the
nitrogen atom (Fig. 2B). In particular, the coordination
of the product has, in our opinion, a profound impact
on (E)-stereoselectivity. The decrease in (E)-selectivityof
the reaction in the case of isomerization of amine 13 is
due to a steric influence of Me3Si groups, which hinder
the participation of the nitrogen atom in the coordina-
tion of the metal atom. On the other hand, amine 12 can
coordinate through both nitrogen atom and the double
bond from the allyl (and 1-propenyl) fragments in so
manyvarious ways that it is difficult to predict stereo-
selectivityusing this simple rationale.
R1
R2
H3C
H
N
H
H
Cl
PP h3
H3C
Ru
CO
Ru
P(OR)3
X
A
B
Figure 2. (A) The postulated transition structure for the (Z)-selective
isomerization of N-allylamides (R ¼ 2,4-di-t-butylphenyl; X ¼ H or
Cl). (B) Coordination of the ruthenium atom by N-(1-propenyl)amines
(this suggestion is based on the results of quantum chemical calcula-
tions).
that the application of such phosphine ligands as tri-
phenylphosphine, tris(o-tolyl)phosphine or triphenyl
phosphite leads to a mixture of (E) and (Z)-enamides. In
order to remove [Ru] from the post-reaction mixture,
siliceous mesoporous cellular foams (MCFs) were used,
which constitute a new class of materials with well-
defined uniform ultralarge mesopores.30 We have found
that [Ru] separation efficiencyusing a column with
MCFs31;32 was significantlyhigher compared with a
typical silica gel (200–400 mesh, ALDRICH) (Table 2).
Acknowledgements
This work was supported bya Grant from the State
Committee for Scientific Research for the years 2003–
2004 (No. 4T09A 131 25) and a subsidyto J.M.-B. from
the State Committee for Scientific Research (No. 4T09C
023 23).
Isomerization of N-allylamines, in turn, generally
showed high (E) selectivity. In our opinion, it is a result
Table 2. Isomerization of N-allylamines (QCH2CH@CH2) to enamines (QCH@CHCH3) catalyzed by [M]: [RhH(CO)(PPh3)3] and
[RuClH(CO)(PPh3)3]a
N-Allylamine
T (°C), s (h)
60, 2
S/C, [M]
108, [Rh]
73, [Rh]
So
Enamine
yb (%)
E/Zc
N
C6D6, 1.1
C6D6, 0.63
>98
100/0
8
N
80, 2
>98
100/0
9
N
80, 2
60, 2
65, [Rh]
67, [Ru]
C6D6, 0.71
C6D6, 0.71
>99
>99
100/0
100/0
10
Ph
N
80, 2
60, 2
41, [Rh]
41, [Rh]
C6D6, 1.1
C6D6, 1.1
>99
>98
99/1
Ph
11
N
N
83/10/7/0/0d
12
Si
N
Si
100, 3
120, [Ru]
C6H6, 1.1
>98
89/11
13
a Experimental procedure as described earlier for N-allylamides;11e S/C––substrate/catalyst; So––solvent, cm3 per 1 mmol of substrate; y––yield of the
double bond migration product, determined by 1H NMR.
b Conversion was always quantitative (determined by 1H NMR and GC–MS).
c Determined by 1H NMR and GC–MS.
d EEEE/EEEZ/EEZZ/EZZZ/ZZZZ.

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S. Krompiec et al. / Tetrahedron Letters 45 (2004) 5257–5261
4200; (b) Barluenga, J.; Fernandez, M. A.; Aznar, F.;
Valdes, C. Chem. Commun. 2002, 2362–2363.
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