Pd-H from Pd/C and triethylamine: Implications in palladium catalysed reactions involving amines

Article abstract of DOI:10.1016/j.jorganchem.2007.05.053

The palladium hydride-iminium complex generated from Pd/C and triethylamine catalyses the isomerisation of allylic alcohols into carbonyl compounds, and Pd/C catalyses the conjugate reduction of activated double bonds using triethylamine as the source of the two newly incorporated hydrogen atoms via the same complex.

Full text of DOI:10.1016/j.jorganchem.2007.05.053

Journal of Organometallic Chemistry 692 (2007) 4805–4808
www.elsevier.com/locate/jorganchem
Communication
Pd–H from Pd/C and triethylamine: Implications in palladium
catalysed reactions involving amines
*
*
´
Yoann Coquerel , Paul Bremond, Jean Rodriguez
Universite´ Paul Ce´zanne (Aix-Marseille III), UMR CNRS 6178, Centre Universitaire de St Je´roˆme, Service 531, 13397 Marseille Cedex 20, France
Received 10 April 2007; received in revised form 16 May 2007; accepted 16 May 2007
Available online 3 August 2007
Abstract
The palladium hydride–iminium complex generated from Pd/C and triethylamine catalyses the isomerisation of allylic alcohols into
carbonyl compounds, and Pd/C catalyses the conjugate reduction of activated double bonds using triethylamine as the source of the two
newly incorporated hydrogen atoms via the same complex.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Catalysis; Palladium hydride; Triethylamine; Isomerisation; Conjugate reduction
1. Introduction
moiety of the complex generated from Pd/C and triethyl-
amine for the catalytic isomerisation of allylic alcohols into
carbonyl compounds, and the conjugate reduction of acti-
vated double bonds using triethylamine as the source of the
two newly incorporated hydrogen atoms.
Palladium based catalysts are involved in a number of
highly useful synthetic transformations. In the cases where
an amine bearing a and b hydrogen atoms is used as a co-
reagent or co-catalyst in the reaction mixture, unexpected
isomerisations and/or reductions are sometimes observed
[1]. A rationale for this behaviour has been proposed which
relies on the formation of a palladium hydride–iminium (or
imine) complex, in equilibrium with the corresponding
enamine complex. This palladium hydride–iminium com-
plex, which has been otherwise characterized [2], has been
found to be involved in the catalytic substituent exchange
of trialkyl amines [3], and in the presence of water, the
hydrolysis of the complex produces carbonyl compounds
[4]. These reactions were also applied to primary and sec-
ondary amines, via an imine–palladium hydride complex
[5], and a microwave-assisted version appeared recently
[6]. So far, reactions of the complexes have concentrate
on the imine or the iminium. Herein, we report a series
of preliminary results on the implication of the hydride
2. Discussion
2.1. Isomerisation of allylic alcohols
Examples of palladium catalysed isomerisation of allylic
alcohols into carbonyl compounds are quite rare and are
limited to Pd(II) catalysts [7], except for one example with
hydrogen pre-activated Pd/C which was early recognized to
be able of isomerising 4-penten-3-ol into 3-pentanone in
the gas phase [8]. However, all attempts to isomerise allyl
alcohols in the liquid-phase with Pd/C failed. We specu-
lated that the palladium hydride generated from Pd/C
and triethylamine could effect the isomerisation of allylic
alcohols, and indeed 1-phenyl-2-propen-1-ol was partially
converted to ethyl phenyl ketone on heating at 70 °C in
the presence of 5 mol% of Pd/C and 10 mol% of triethyl-
amine for 22 h (Table 1, entry 1). In that case, 8% of the
saturated product 1-phenyl-propanol were also formed.
Nearly complete conversion and better selectivity was
obtained on heating at 150 °C for 3 h (entry 2), still with
*
Corresponding authors. Tel.: +33 (0) 491 28 90 88/89 33; fax: +33 (0)
491 28 91 27.
E-mail addresses: yoann.coquerel@univ-cezanne.fr (Y. Coquerel),
jean.rodriguez@univ-cezanne.fr (J. Rodriguez).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.05.053

4806
Y. Coquerel et al. / Journal of Organometallic Chemistry 692 (2007) 4805–4808
Table 1
Isomerisation of allylic alcohols with Pd/C–NEt3
Entry
1
Substrate
Conditionsa
Products
Yield (conversion)b (%)
67 + 8 (84)
70 °C, 22 h
O
OH
OH
OH
Ph
O
Ph
Ph
Ph
+
+
OH
2
3
4
5
6
150 °C, 3 h
91 + 5 (97)
Ph
Ph
OH
Cy
O
OH
OH
150 °C, 3 h
72 + 17 (100)
3 + 1 (5)
Cy
Cy
Ph
+
O
OH
Ph
150 °C, 3 h
Ph
+
+
OH
O
OH
Ph
200 °C, 12 h
69 + 9 (91)
Ph
Ph
O
OH
OH
OH
OH
Ph
150 °C, 3 h
19 + 30 + 9 (76)
63 + 9 + 19 (98)
Ph
Ph
Ph
Ph
Ph
+
+
+
+
O
OH
OH
Ph
7
200 °C, 12 h, 20 mol% NEt3
Ph
a
Unless stated otherwise, all reactions were conducted with 10% Pd/C (5 mol% Pd) and 10 mol% NEt3 in 0.8–1.0 M dry toluene (sealed tube).
Yields and conversions are obtained by NMR analysis of the crude reaction mixture.
b
production of a minor amount of 1-phenyl-propanol (5%).
and were pleased to find that reduction was achieved effi-
ciently in the presence of 10 mol% Pd/C and 1.2 equiv. of
triethylamine at 140 °C (Table 2, entry 1). Benzylideneac-
etone and cinnamonitrile are also reduced under these
conditions (entries 2 and 3, no attempt were made to opti-
mize the yields). Some interesting observations emerged
from the reactions with a-chlorocyclopentenones and its
acetal (entries 4 and 5, respectively). Indeed, in the case
of entry 4, the conjugated reduction is followed by
in situ dehydrohalogenation to restore the thermodynam-
ically favored enone system. The reaction thus results in
the net reduction of the a-chlorocyclopentenone into a
cyclopentenone, and a small amount of over-reduction
product is observed. The corresponding acetal is inert
under the reaction conditions (entry 5), thus demonstrat-
ing that Pd/C neither catalyses the deshydrogenation of
triethylamine in the absence of an hydride acceptor, nor
the reduction of the vinylic chlorine atom [12]. Since the
report of Sodeoka’s group [11] we verified that Pd/C
was not a catalyst for the reduction of electro-deficient
alkenes using ethanol as the hydride source under the con-
ditions of the study (entry 6). Several other catalysts were
tested in the reaction. No reduction of tert-butyl acrylate
occurred with Pd(PPh3)4, PdCl2, Pd(OAc)2 and Pd(al-
lyl)2Cl2 when the reaction was conducted in degazed dry
toluene in the presence of triethylamine. If the same reac-
tions were conducted in a dry but not degazed medium,
palladium black precipitated and the reaction proceeded.
Rhodium on alumina was also tested for the same reac-
tion and resulted in very low conversion (ca. 5%) under
the normalized conditions of the study.
Similarly 1-cyclohexyl-2-propen-1-ol was isomerised to the
corresponding ketone (entry 3) with production of 17% of
saturated product. Under the same conditions, 1-phenyl-2-
methyl-2-propen-1-ol proved almost inert, and 69% of the
expected ketone were formed after 12 h at 200 °C (entries 4
and 5, respectively). With 1-phenyl-3-buten-1-ol (homoallyl
alcohol), the expected ketone was formed in only 19%
yield, together with the substituted allylic alcohol raising
from the mono-isomerisation as the major product (30%)
after 3 hours at 150 °C (entry 6). At higher temperature,
63% of the isomeric ketone were obtained together with
9% of the corresponding allylic alcohol (entry 7) [9]. The
decrease in the efficiency of the isomerisation with substi-
tuted allyl alcohols is also observed with other metal
hydrides [7a].
2.2. Conjugate reduction of electro- deficient alkenes
The ability of the double bond of allyl alcohols to
undergo hydro-palladation with the palladium hydride
generated from triethylamine and Pd/C logically promp-
ted us to examine the system for the reduction of elec-
tro-deficient alkenes. Hydrido-palladium compounds
obtained by transmetallation of a metal hydride were
reported to catalyse some conjugate reductions [10], and
recently, Sodeoka’s group reported a palladium(II) cataly-
sed enantioselective conjugate reduction of enones using
ethanol as hydride source [11]. To test the feasibility of
the conjugate reduction with the Pd/C–NEt3 system, we
first examined the model reduction of tert-butyl acrylate

Y. Coquerel et al. / Journal of Organometallic Chemistry 692 (2007) 4805–4808
4807
Table 2
Conjugate reduction of activated double bonds with Pd/C–NEt3
Entry
1
Substrate
Product(s)
Conditionsa
Yield (conversion)b (%)
100 (100)
O
O
NEt3 (1.2 equiv.)
O
t
Bu
OtBu
O
O
2
3
4
5
NEt3 (2.2 equiv.)
NEt3 (2.2 equiv.)
NEt3 (2.2 equiv.)
NEt3 (2.5 equiv.)
EtOH (10 equiv.)
69 (69)
37 (37)
61 + 6 (71)
0 (0)
Ph
Ph
Ph
Ph
CN
CN
H
H
O
+
O
O
Cl
H
O
–
–
O
Cl
O
6
0 (0)
OtBu
a
All reactions were conducted with 10% Pd/C (10 mol% Pd) at 140 °C for 16 h in 0.6–1.0 M dry toluene (sealed tube).
Yields and conversions are unoptimized and were obtained by NMR analysis of the crude reaction mixture.
b
Scheme 1. Conjugate reduction with Pd/C–NEt3.
On a mechanistic point of view, the simplistic mecha-
activated double bonds using triethylamine as the source of
hydrogen atoms. It should be stressed here that the forma-
tion and the catalytic activity of the palladium hydride
complex occurred at quite low temperature (70 °C), but
only with unliganted metallic palladium under the condi-
tions of our study. Although the Pd/C–NEt3 system is
clearly not the reagent of choice for both the isomerisation
and reduction reactions presented above, it might be a use-
ful and cheap satisfactory alternative in some cases. In light
of these results, unexpected isomerisations and/or reduc-
tions occurring in Pd catalysed reactions involving a ter-
tiary amine can now be rationalized. Further studies on
the synthetic implications of the formation of the palla-
dium hydride–iminium complex of triethylamine are cur-
rently underway.
nism depicted on Scheme 1 accounts for the observed reac-
tion. The reaction would proceed via initial insertion of
palladium into the carbon–hydrogen bond a to the
nitrogen atom and subsequent formation of a trialkylimin-
ium–palladium hydride complex, which would then
undergo conjugate addition and enamine formation.
Although, at this time, we cannot provide a direct evidence
for the formation of the vinyl(diethyl)amine as a product of
the Pd/C–NEt3 mediated reduction of electro-deficient
alkenes [13], triethylamine is clearly the source of the two
hydrogen atoms incorporated onto the double bond (com-
pare entries 1 and 6 in Table 2).
3. Conclusion
In summary, we have demonstrated that the palladium
hydride generated from Pd/C and triethylamine can cata-
lyse the isomerisation of allylic alcohols into carbonyl com-
pounds, and that Pd/C catalyses the conjugate reduction of
Acknowledgements
´
´
We thank Alexis Roulet (Universite Paul Cezanne, Mar-
seille) for early experiments during his master thesis, our

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Y. Coquerel et al. / Journal of Organometallic Chemistry 692 (2007) 4805–4808
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´
´
friend Professor Jean-Pierre Depres (Universite Joseph
Fourier, Grenoble) for generous gift of a-chlorocyclopente-
´
´
none, Professor Laurence Charles (Universite Paul Ce-
zanne, Marseille) for helpful assistance with mass
spectroscopy, and the french research ministry, the CNRS
´
´
[7] (a) R. Uma, C. Crevisy, R. Gree, Chem. Rev. 103 (2003) 27–51;
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and the Universite Paul Cezanne (UMR 6178) for financial
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