Hydroamination of ethylene by aniline: Catalysis in water

Article abstract of DOI:10.1039/c004727a

The platinum-catalyzed and halide-promoted hydroamination of ethylene with aniline is reported for the first time in the presence of simple sodium halides in water. Compounds K₂PtX₄ (X = Cl or Br), PtX₂ or PtX₄ (0.3% mol) in the presence of an aqueous solution of excess NaX and aniline under ethylene pressure (25 bar) affords N-ethylaniline with 60-85 turnovers after 10 h at 150°C. The best result (TON = 85) was obtained in the presence of excess NaBr, whereas a slightly lower activity was observed with NaCl (60 cycles) and practically no activity with NaF or NaI (2-4 cycles). The reaction also produces N,N-diethylaniline (up to 1 cycle) and 2-methylquinoline (up to 8 cycles) as by-products. The influence of added H ⁺ and different oxidizing agents was also examined.

Full text of DOI:10.1039/c004727a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/greenchem | Green Chemistry
Hydroamination of ethylene by aniline: catalysis in water
Pavel A. Dub,a,b Mireia Rodriguez-Zubiri,a Christine Baudequina and Rinaldo Poli*a,c
Received 29th March 2010, Accepted 27th May 2010
First published as an Advance Article on the web 23rd June 2010
DOI: 10.1039/c004727a
The platinum-catalyzed and halide-promoted hydroamination of ethylene with aniline is reported
for the first time in the presence of simple sodium halides in water. Compounds K2PtX4 (X = Cl
or Br), PtX2 or PtX4 (0.3% mol) in the presence of an aqueous solution of excess NaX and aniline
under ethylene pressure (25 bar) affords N-ethylaniline with 60–85 turnovers after 10 h at
150 ◦C. The best result (TON = 85) was obtained in the presence of excess NaBr, whereas a
slightly lower activity was observed with NaCl (60 cycles) and practically no activity with NaF or
NaI (2–4 cycles). The reaction also produces N,N-diethylaniline (up to 1 cycle) and
2-methylquinoline (up to 8 cycles) as by-products. The influence of added H+ and different
oxidizing agents was also examined.
15
The Brunet system, consisting of simple platinum halides (PtX2
Introduction
or PtX412) in the presence of a molten halide salt, nBu4PX, as
a promoter, appears the simplest and most effective among the
family of platinum catalysts. The initial report focused on the
properties of nBu4PX as an ionic solvent,7,15 but a more detailed
study of catalytic activity as a function of halide nature and
X/Pt ratio revealed that the salt rather functions as an activator
through the presence of the coordinating halide anion,12 with
bromide leading to the highest activity increase for the addition
of aniline to ethylene. Catalytic schemes based on the dominant
role of [PtBr4]2- or [PtBr3(C2H4)]- have then been proposed.13,16
The Brunet catalyst is already quite simple and robust,
because it is made up of commercially available ligandless
platinum halides plus an organic halide additive with no need to
use organic solvents, and was shown to be nearly as efficient
when taking no special precaution to remove air from the
system. However, we strove to further simplify it by changing
the salt additive to a more readily available and less expensive
alkali metal halide, dissolved in water. These are the “green-
est” possible conditions that one can imagine for this atom-
economical transformation. We were pleased with the results of
this investigation, which will be shown in this contribution.
Catalytic hydroamination, the direct formation of a new C–N
bond by addition of the N–H bond to an unsaturated C–C
function in the presence of a catalyst, is a subject of current
interest both for fundamental research and for the chemical
industry.1–3 It is an atom-economical reaction constituting an
interesting alternative entry to higher amines, which currently is
mostly achieved by the catalyzed condensation of ammonia or
amines with alcohols.4 Since most primary alcohols are indus-
trially produced from alkenes (hydroformylation–hydrogenation
sequence),5,6 the hydroamination process would suppress at
least one step and avoid the coproduction of water. The
intermolecular version of this reaction with non-activated olefins
is still a great challenge to chemists.2,3 The direct reaction of
ethene with compounds containing N–H bonds (amines, amides,
etc.) is of particular interest for the large-scale synthesis of ethyl-
substituted nitrogen compounds.
An interesting catalytic system (in terms of both activity
and operating conditions) for the platinum-catalyzed addition
of aniline to ethylene was introduced in 2004 by Brunet
and co-workers.7 Other reports of Pt-based intermolecular
hydroamination reactions (mostly of ethylene or norbornene)
have subsequently appeared.8–14 All these catalytic systems
appear to be effective for weakly basic amines such as aryl-
substituted sulfonamides, carboxamides, and electron-deficient
anilines, while they did not lead to a satisfactory conversion
for more basic amines. For example, no reaction was observed
for the ethylene hydroamination with secondary amines in
Experimental
Methods and materials
PtCl2 (STREM), PtBr2 (Alfa Aesar), PtCl4 (STREM), PtBr4
(STREM), K2PtCl4 (Alfa Aesar), K2PtBr4 (Aldrich), PdBr2
(Aldrich), Pt black (STREM), NaF (Alfa Aesar), NaX (X =
Cl, Br, I Acros Organics), CuCl2 (STREM), and nBu4PBr
(Aldrich) were used as received. Benzoquinone (Acros) was
resublimed before use. Aniline and 2-methylquinoline (Alfa
Aesar), N-ethylaniline and N,N-diethylaniline (Acros), and
N,N-dibutylaniline (Acros) were distilled under vacuum and
kept under argon in the dark. Distilled water was degassed by
an argon flow before each experiment. DMSO, DMF (VWR
Prolabo) and EtOH (Carlo Erba) were used as received and
degassed by an argon flow before experiments. Ethylene (N25)
8
the presence of [PtCl2(C2H4)]2/PPh3 or for the norbornene
hydroamination with aniline in the presence of (COD)Pt(OTf)2.9
aCNRS; LCC (Laboratoire de Chimie de Coordination); Universite´ de
Toulouse; UPS, INPT; 205, route de Narbonne, F-31077, Toulouse,
France. E-mail: rinaldo.poli@lcc-toulouse.fr; Fax: +33 561553003;
Tel: +33 561333173
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian
Academy of Sciences, Vavilov Street 26, 119991, Moscow, Russia
cInstitut Universitaire de France, 103, bd Saint-Michel, 75005, Paris,
France
1392 | Green Chem., 2010, 12, 1392–1396
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Table 1 Pt-catalyzed aniline addition to ethylene in the presence of
nBu4Bra
was purchased from Air Liquide. Unless otherwise stated, all
the manipulations were performed under argon.
Glass PhNHEt PhNEt2 Quinaldine
Instrumentation
Run Salt/Pt liner TON
TON
TON
Conv. (%) Ref.
TheGC analyses wereperformed on aHewlett-Packard HP4890
chromatograph equipped with an FID, an HP 3395 integrator
and a 30 m HP1 capillary column. Under the selected operati◦ng
conditions [helium as carrier gas at p = 50 kPa, Tstart = 65 C
10
150
10
Yes
Yes
No
No
130
80
140
100
1
1
3
1
10
10
15
15
40
26
45
33
13
7
b
1
2
b
150
◦
(2 min), DT = 6 C min-1, Tend = 200 ◦C (20 min)] the
a Conditions: PtBr2 (0.13 mmol), aniline (4.15 mL), ethylene (25 bar at
RT), nBu4Br, T = 150 ◦C, t = 10 h. b This work.
retention times were: PhNH2, 7 min; PhNHEt, 11 min; PhNEt2,
12 min; quinaldine, 15 min; PhN(nBu)2 (external standard),
21 min. Small amounts of by-products (maximum intensity of
50% relative to the peak of PhNEt2) were eluted after 13, 15,
17 and 18 min. Calibration curves for each compound were
obtained using pure samples at several different concentrations.
All subsequent experiments were therefore run in the absence of
glass liner.
1
NMR (1H and 13C{ H}) investigations were carried out on a
Bruker DPX300 spectrometer operating at 300.1 MHz (1H) and
1
75.47 MHz (13C{ H}). D2O (traces) and CD2Cl2 (traces) were
Scheme 1
added for locking purposes for the analysis of the water-rich and
organic-rich phases, respectively.
The process was then investigated with aqueous NaBr in place
of nBu4PBr. This salt was initially selected because the bromide
anion was previously shown to yield the best activities for the
tetra-n-butylammonium salt series (see Introduction). All runs
were carried out over 10 h, the same lapse of time used in the
previous work with nBu4PBr.7 The same three products shown in
Scheme 1 were again obtained in roughly the same proportions,
with only a marginally lower catalytic activity, see Table 2,
run 3. Some other organic compounds were also detected
in trace amounts (<0.5%), among which 1,2,3,4-tetrahydro-2-
methylquinoline could be identified on the basis of a GC-MS
analysis. Note that exactly the same product composition was
previously observed when using nBu4PBr, underlying the same
mechanism.7 Note also that no catalysis was observed in the
absence of PtBr2 (run 4), and a very low activity was recorded in
the presence of PtBr2 but in the absence of NaBr (run 5). Thus,
the activating role of the bromide salt is confirmed for the new
catalytic conditions reported here.
One question that needs to be addressed is the effect of the
reaction time and of possible catalyst deactivation. The Brunet
catalyst was already shown to deposit inactive metallic Pt with
activities reaching a plateau after ca. 10 h: the TON for the
N-ethylaniline product was 37 after 1 h, 64 after 2.5 h, 72 after
5 h and 80 after 10 h.7 We have therefore tested our system at
longer reaction time (runs 6 and 7). Activities were essentially
the same as after 10 h, confirming that the catalyst is totally
Hydroamination of ethylene by aniline
Catalytic experiments were conducted in a 100 ml stainless steel
autoclave without glass liner under magnetic stirring. It was
experimentally observed (see Results and discussion) that the
use of glass liners results in inhomogeneous conditions with
loss of reproducibility. In a typical procedure, the autoclave
was charged with the Pt compound (PtX2, or PtX4, or K2PtX4;
0.13 mmol) and the desired excess amount of NaX (see Results
and discussion section), closed, and submitted to vacuum/argon
cycles. Degassed water (15 or 5 ml) and then aniline (4.15 ml,
45 mmol) were added to the autoclave by syringe through a
valve equipped with septum and the ethylene pressure adjusted
to 25 bar (ca. 100 mmol). The temperature was then raised to
◦
150 C. After 10 h, the autoclave was allowed to cool to room
temperature and slowly vented. The entire reaction mixture
was treated with the external standard (N,N-dibutylaniline) and
extracted with dichloromethane (ca. 60 ml). The organic layer
was analyzed by GC and GC-MS. Each run presented in this
work was performed at least twice (up to five times for the most
relevant results). A statistical analysis of reproducibility gives
the following errors for the measured turnover number (TON)
values: PhNHEt, 10; PhNEt2 and quinaldine, 1.
Results and discussion
Table 2 Pt-catalyzed aniline addition to ethylene in the presence of
aqueous NaBra
Our study started with a verification of the previously pub-
lished experimental work.7 We found the same results at the
qualitative level, with the reaction producing N-ethylaniline
(major), N,N-diethylaniline (traces) and 2-methylquinoline or
quinaldine (minor), see Scheme 1. However, the occasional lack
of reproducibility encouraged us to test the reaction in the
absence of glass liner, yielding reproducibly higher TON, see
Table 1. This phenomenon is attributed to the accumulation of
condensed substrate vapours in the small cavity between the
glass liner and the autoclave walls, as indeed experimentally
observed. As shown in Table 1, the previously reported decrease
of the activity upon increasing the salt amount was confirmed.
PhNHEt PhNEt2 Quinaldine
Run Salt/Pt Time/h TON
TON
TON
Conv. (%)
3
4
5
6
7
150
10
10
10
24
48
85
~0.6
5
90
90
1
0
~0.1
1
1
8
1
1
8
8
27
b
0.5
1.7
28
0
150
150
28
a Conditions: PtBr2 (0.13 mmol), aniline (4.15 ml), ethylene (25 bar at
RT), NaBr (150 equiv.) in water (15 mL), T = 150 ◦C. b Same amount
of NaBr as in run 3, in the absence of PtBr2.
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 1392–1396 | 1393
©

View Article Online
Table 3 Pt-catalyzed aniline addition to ethylene. Influence of the NaBr
Table 4 Pt-catalyzed aniline addition to ethylene. Influence of added
and water amountsa
acids.a
Water/ PhNHEt PhNEt2 Quinaldine
PhNHEt PhNEt2 Quinaldine
Run Salt/Pt mL
TON
TON
TON
Conv. (%)
Run Acid (eq.)
TON
TON
TON
Conv. (%)
8
9
10
15
15
15
15
5
5
5
24
64
45
27
62
74
61
31
~0.3
3
6
5
3
7
8
8
7
8
16
17
18
19
20
21
HBr (1)
HBr (3)
HBr (5)
HBr (20)
CF3COOH (1) 79
Et2O·HBF4 (3) 51
88
78
41
26
2
3
4
9
2
4
8
7
5
3
8
6
28
25
14
11
25
17
100
200
500
10
50
100
150
1
1
6
2
3
3
2
20
15
10
20
24
21
11
10
11
12
13
14
15
a Conditions as in Table 2, run 3, except for the acid addition.
5
a Conditions as in Table 2, except for the amount of NaBr and/or water.
Table 5 Pt-catalyzed aniline addition to ethylene. Influence of
oxidantsa
deactivated. Visual inspection of the reaction mixture confirms
the total conversion of the catalyst into the metallic state. The
discharged reaction mixture consists of a two phase system with
a colorless aqueous phase and a black stable suspension in the
organic layer.
PhNHEt PhNEt2 Quinaldine
Run Additive (eq.)
TON
TON
TON
Conv. (%)
b
22
—
43
2
2
2
1
2
1
0.4
1
5
2
6
1
3
7
6
7
14
8
13
5
7
22
6
23 CuCl2 (5), HCl (5) b 24
24 HCl (5) b
25 CuCl2 (5)
26 CuCl2 (5)b
39
15
20
We next proceeded to investigate the influence of the NaBr
amount. Runs 3 and 4 involved ~2 g of NaBr, whereas >10 g can
be dissolved in the same amount of water at room temperature.
It should be noted that ◦aniline is also partially soluble in water
(3.6 g per 100 mL at 20 C)17 and this certainly has an influence
on the NaBr solubility. At the reaction temperature of 150 ◦C,
the water/aniline miscibility is expected to significantly increase
and the NaBr solubility may also increase, but a miscibility study
of the water/aniline/NaBr ternary system does not appear to
be available. We were only able to find a phase diagram of the
water-aniline mixture at 1 atm, showing equilibrium between
two phases containing respectively 16% and 78% aniline at
150 ◦C. The presence of NaBr in large quantities is expected
to render the two liquids less miscible, thus a biphasic system
is certainly present under catalytic conditions. In addition, the
system is further complicated by the presence of ethylene. A
phase diagram investigation for this complex mixture is beyond
the scope of the present study. The catalytic results obtained in
the presence of variable NaBr amounts are presented in Table 3.
It is clearly seen that upon increasing the NaBr amount
beyond 150 equivalents the total activity decreases (runs 10
and 11), which is in agreement with the results obtained in the
presence of nBu4PBr (Table 1), while the selectivity tends to
shift in favour of the N,N-diethylamine product. Reducing the
amount of salt also resulted in loss of activity (runs 8 and 9).
This behaviour is different from that of the nBu4PBr promoter,
for which a higher activity was observed with a Br/Pt ratio of
10 (Table 1). The best activity remains that observed in run 3
(Table 2) with 150 equivalents of NaBr per Pt. Note also that
the entire NaBr amount was found dissolved (not observed as
a solid) in all the recovered mixtures. Next, we have tested the
influence of the amount of water in the medium. Reducing the
amount of water from 15 to 5 mL for either a tenfold (run 12) or
a fifty-fold (run 13) excess of NaBr gave a higher activity, close
to that of run 3 in Table 2. However, upon further increasing the
NaBr amount the overall activity decreases (runs 14, 15). In this
case the best activity was found with a fifty-fold excess.
27 benzoquinone (10) 69
28 DMSO b(10)
29 DMSO (10)
13
23
9
a Conditions as in Table 2, run 3, except for the additive. b Experiment
run without removing air from the aniline and from the autoclave head
space.
catalytic amounts of strong acids (3 equiv. per Pt), which would
be levelled in strength to anilinium in the reaction medium.7
The same phenomenon might be expected under the present
catalytic conditions, since aniline is a stronger base than water.
The catalytic results, see Table 4, show that acids do not have a
promoting effect under the present conditions. Rather, a small
but significant decrease of activity was revealed in the presence
of excess acid. One equivalent of acid (either HBr or CF3COOH)
has essentially no effect on the activity (cf. runs 16 and 20 with
run 3), but further acid addition (runs 17–19, 21) has a negative
effect on the activity. Note also that the reaction selectivity
changes, the TON decreasing for ethylaniline and quinaldine
and increasing for diethylaniline as the amount of HBr increases
(runs 16–19).
We have also investigated the effect of specific oxidants as
additives, in an attempt to avoid the deposition of Pt0 or oxidize
it back to an active form of the catalyst. Pt0 is not active
for the ethylene hydroamination: a test run in the presence
of the same catalytic amount of platinum black gave only a
0.6% conversion with ca. 1 TON for ethylaniline and 1 TON
for quinaldine (essentially the same results as in the control
without Pt compound, run 4). Agents such as benzoquinone,18–20
O2/CuCl2,21–23 or O2/DMSO24 were shown to oxidize Pd0 for
related catalytic systems. The results are shown in Table 5. We
first carried out the reaction in the presence of air (run 22),
yielding approximately half the activity recorded under argon
(run 3). Further addition of CuCl2 and HCl to the system
(conditions of the Wacker process) further decreases the activity.
The negative effect of strong acids has been pointed out above
(runs 16–19), but note that the presence of CuCl2 also seems
The influence of H+ on the reaction was also examined. This
study was justified by the previous report of a positive effect of
1394 | Green Chem., 2010, 12, 1392–1396
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Table 6 Pt-catalyzed aniline addition to ethylene. Influence of added
Table 8 Catalyzed aniline addition to ethylene. Influence of the pre-
catalyst naturea
solventsa
PhNHEt PhNEt2 Quinaldine
PhNHEt
TON
PhNEt2
TON
Quinaldine
TON
Run Additive/mL TON
TON
TON
Conv. (%)
Run
[Pt]
Conv. (%)
30
31
32
DMSO (7)
DMF (7)
EtOH (2)
0
3
72
0
0
2
0
1
8
0
1
23
36
37
38
39
40
41
42
K2PtCl4
K2PtBr4
PtCl2
PtI2
PtCl4
PtBr4
PdBr2
86
81
70
19
80
76
1
1
1
1
1
2
3
0
8
7
8
2
8
6
~0.3
27
25
23
7
26
24
0.4
a Conditions as in Table 2, run 3, except for the additive.
a Conditions as in Table 2, run 3, except for the nature of the pre-catalyst.
to have a negative effect in this case, because the activity in the
presence of CuCl2 alone is lower than that in the presence of HCl
alone (run 24), and this does not greatly depend upon whether
the catalysis is run under argon (run 25) or in the presence of
air (run 26). The presence of benzoquinone or DMSO/air also
does not improve the results (runs 27 and 28). The activity in the
presence of DMSO under argon (run 29) is slightly better than
in air (run 28), but still lower than that of run 3 in the absence
of DMSO. The formation of a Pt0 deposit was still observed in
all these catalytic runs.
As mentioned above, the cataly◦tic reaction most certainly
occurs in a biphasic system at 150 C. Thus, we have explored
the effect of the addition of compatibilizing solvents, in an
attempt to homogenise the system. The addition of bipolar
aprotic solvents such as DMF or DMSO (ca. 7 ml) completely
homogenise the system aniline-water-NaBr (150 eq.) at room
temperature. The catalytic results, see Table 6, show complete
or nearly complete deactivation by DMSO (run 30) or DMF
(run 31), respectively. It should be noted that DMSO and DMF
are also strong ligands. In a previous study, we have shown that
isolated Pt-aniline complexes lose the aniline and/or ethylene
ligand upon dissolution in DMSO.16 On the other hand, heating
the same complexes in DMF leads to decomposition. Thus,
although DMSO and DMF are able to homogenise the system,
they probably lead to other stable and catalytically inactive
Pt complexes. Conversely, the addition of EtOH leads to a
marginally lower activity (cf. run 32 and run 3).
Finally, wehavetestedothersourcesofPtdifferentfromPtBr2,
namely K2PtX4, PtX4 (X = Cl or Br) or PtX2 (X = Cl, I),
and also PdBr2. According to the proposed mechanism,13,16 all
the Pt precatalysts should yield [PtBr4]2- dominant species in
the presence of excess NaBr, or [PtBr3(C2H4)]- in the presence
of C2H4.16 Furthermore, PtX4 could be easily reduced to the
same PtII complexes under the catalytic conditions, for example
by ethylene to form CH2BrCH2Br. Indeed, compounds PtBr2,
PtBr4 and PtCl4 were previously shown to have essentially the
same catalytic activity in the presence of the same excess amount
of nBu4PBr.12 As can be seen from the results in Table 8, the
different Pt compounds also give essentially identical results
under the new conditions reported in this contribution. Only
marginally lower activities were obtained with PtCl2 (run 38),
perhaps because it slowly decomposes upon standing in air and
a partially impure sample may have been used. Much poorer
results were also obtained with PtI2 (run 39), possibly for similar
reasons.
A last catalytic run was performed with PdBr2 (run 42). Both
platinum(II) and palladium(II) complexes activate olefins toward
nucleophilic attack, but in contrast to their Pd(II) counterparts,
Pt(II) alkyl complexes are less prone to b-hydride elimina-
tion, whereas they maintain reactivity toward protonolysis.
Thus, Pd systems resulting from amine nucleophilic addition
to coordinated ethylene usually prefer to undergo oxidative
amination.1,20,25 However, run 42 led to essentially no reaction.
Neither the expected hydroamination products nor the oxidative
amination products were detected by either gas chromatography
or 1H NMR in both the organic and the aqueous phases (a few
small and unassigned resonances corresponded to <0.5% of
aniline consumption).
An interesting question concerns the possible nucleophilic
addition of water to coordinated ethylene. Such reactivity would
result in parallel catalytic processes (formation of ethanol by
protonolysis with regeneration of PtII, although the reverse
process has rather been shown to be favored26) or stoichiometric
processes (formation of acetaldehyde by b-H elimination with
formation of Pt0 as in the Pd-catalyzed Wacker process),
see Scheme 2. The latter events might in fact rationalize
the slightly lower TON (faster catalytic deactivation) of this
aqueous catalytic system relative to the Brunet system. Note
that, although no direct nucleophilic attack of PtII-coordinated
olefins by water or alcohols has been reported under neutral
conditions,27 both ethanol and acetaldehyde were previously
observed from the reaction of PtII-ethylene derivatives upon
Another important point to investigate was the influence of
the halide nature. We have tested the entire series of sodium
halides NaX, including the fluoride not previously investigated
along the nBu4PX series.7,10,12 The results, reported in Table 7,
show that practically no reaction takes place when X = F or
I. The highest activities were observed in the middle of the group
for X = Br and Cl, with Br being slightly better. These results
perfectly parallel those previously established for the nBu4PX
system.
Table 7 Pt-catalyzed aniline addition to ethylene. Influence of the
halogen in NaXa
PhNHEt
TON
PhNEt2
TON
Quinaldine
TON
Run
Salt
Conv. (%)
33
34
35
NaF
NaCl
NaI
2
60
4
0
1
1
5
2
1
19
2
~ 0.3
a Conditions as in Table 2, run 3, except for the nature of NaX.
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 1392–1396 | 1395
©

View Article Online
and from the RFBR through the bilateral grant no 08-03-
92506. P.A.D. is recipient of a Ph.D. fellowship from the
´
French MENESR (Ministe`re de l’Education Nationale, de
l’Enseignement Supe´rieur et de la Recherche).
Notes and References
1 T. E. Mu¨ller and M. Beller, Chem. Rev., 1998, 98, 675.
2 J.-J. Brunet and D. Neibecker, in Catalytic Heterofunctionalization,
ed. A. Togni and H. Gru¨tzmacher, Weinheim, 2001.
3 T. E. Mu¨ller, K. C. Hultzsch, M. Yus, F. Foubelo and M. Tada, Chem.
Rev., 2008, 108, 3795.
Scheme 2
standing in water and ethylene glycol may also be formed in the
presence of oxidants.28–32 In an aqueous aniline environment, it is
also envisageable that acetaldehyde may be present as the hydrate
4 K. Visek, Kirk-Othmer Encyclopedia of Chemistry and Technology,
Wiley & Sons, 2003.
=
5 S. A. Lawrence, Amines, Cambridge University Press, 2004.
6 J. Falbe, New Syntheses with Carbon Monoxide, Springer Verlag,
1980.
7 J. J. Brunet, M. Cadena, N. C. Chu, O. Diallo, K. Jacob and E.
Mothes, Organometallics, 2004, 23, 1264.
8 X. Wang and R. A. Widenhoefer, Organometallics, 2004, 23,
1649.
9 D. Karshtedt, A. T. Bell and T. D. Tilley, J. Am. Chem. Soc., 2005,
127, 12640.
10 J. J. Brunet, N. C. Chu and O. Diallo, Organometallics, 2005, 24,
3104.
11 S. Anguille, J.-J. Brunet, N. C. Chu, O. Diallo, C. Pages and S.
Vincendeau, Organometallics, 2006, 25, 2943.
12 M. Rodriguez-Zubiri, S. Anguille and J.-J. Brunet, J. Mol. Catal. A:
Chem., 2007, 271, 145.
13 J.-J. Brunet, N.-C. Chu and M. Rodriguez-Zubiri, Eur. J. Inorg.
Chem., 2007, 4711.
14 J. L. McBee, A. T. Bell and T. D. Tilley, J. Am. Chem. Soc., 2008,
130, 16562.
CH3CH(OH)2 or transformed to the imine CH3CH NPh. In
order to verify whether any of these products are formed, an
NMR (1H and 13C{ H}) analysis of both the aqueous and
1
organic phases was performed for runs 6 and 7. In addition
to the recognizable resonances of aniline and the three observed
products (Scheme 1), other small resonances were indeed visible
in the methyl proton region (d ca. 1.8, 1.4 and 1.3), and only
in the organic phase. Integration of these resonances, however,
indicated that they correspond to <1 TON. Thus, even assuming
that one of these resonances belong to ethanol, we can conclude
that the catalyzed ethylene hydration is much less efficient than
the hydroamination by aniline. The possible formation of a small
=
stoichiometric amount of CH3CHO, CH3CH NPh, or other
products of further reactions cannot be excluded and it may
indeed contribute to faster catalyst deactivation.
15 J. J. Brunet, N. C. Chu, O. Diallo and E. Mothes, J. Mol. Catal. A:
Chem., 2003, 198, 107.
16 P. A. Dub, M. Rodriguez-Zubiri, J.-C. Daran, J.-J. Brunet and R.
Poli, Organometallics, 2009, 28, 4764–4777.
17 W. Arlt, M. E. A. Macedo, P. Rasmussen and J. M. Sorensen,
Liquid–Liquid Equilibrium Data Collection, Dechema Chemistry
Data Series, 1997.
18 L. S. Hegedus, G. F. Allen and E. L. Waterman, J. Am. Chem. Soc.,
1976, 98, 2674.
19 L. S. Hegedus, G. F. Allen, J. J. Bozell and E. L. Waterman, J. Am.
Chem. Soc., 1978, 100, 5800.
Conclusions
The efficiency and potential of the Pt-catalysed ethylene hy-
droamination by aniline under green operating conditions
(NaBr/water as additive) has been demonstrated. The optimized
conditions (run 3 in Table 2) lead to about 85 catalytic turnovers
in 10 h. Although the catalytic activity, in terms of TON,
is ca. 44% lower than the system operating with nBu4PBr
and without water under identical conditions, this new system
represents a valuable gain in terms of operational simplicity and
environmental impact, because the nBu4PBr salt can be replaced
by the much more accessible, less toxic,33,34 and water-soluble
NaBr. New mechanistic information is also given by the lack of a
major inhibiting effect of water or ethanol, whereas the presence
of large amounts of DMF and DMSO results in a dramatic
decrease of catalytic activity, as was already shown earlier
for phosphine ligands.7 The absence of significant amounts of
ethanol in the reaction products demonstrates that hydration is
not competitive relative to the addition of the aniline N–H bond,
whereas possible stoichiometric Wacker-type processes leading
to Pt reduction may rationalize the slightly lower performance of
the aqueous system relative to PtBr2/nBu4PBr. Higher turnovers
cannot be achieved because of complete catalyst deactivation.
A future challenge will be to engineer a catalyst for which this
deactivation process is retarded or eliminated.
20 L. S. Hegedus, Angew. Chem., Int. Ed. Engl., 1988, 27,
1113.
21 R. F. Heck, Palladium Reagents in Organic Synthesis, Academic Press,
1985.
22 L. S. Hegedus, in Comprehensive Organic Synthesis, ed. B. M. Trost
and I. Fleming, 1991, 4, 551.
23 J. Tsuji, Palladium Reagents and Catalysts, Innovations in Organic
Synthesis, Wiley & Sons, 1995.
24 See ref. 1, and references cited therein.
25 V. I. Timokhin, N. R. Anastasi and S. S. Stahl, J. Am. Chem. Soc.,
2003, 125, 12996.
26 F. R. Hartley, Inorg. Chim. Acta, 1971, 5, 197.
27 M. Benedetti, F. P. Fanizzi, L. Maresca and G. Natile, Chem.
Commun., 2006, 1118.
28 G. A. Luinstra, L. Wang, S. S. Stahl, J. A. Labinger and J. E. Bercaw,
J. Organomet. Chem., 1995, 504, 75.
29 A. C. Hutson, M. R. Lin, N. Basickes and A. Sen, J. Organomet.
Chem., 1995, 504, 69.
30 N. Basickes, A. C. Hutson, A. Sen, G. P. A. Yap and A. L. Rheingold,
Organometallics, 1996, 15, 4116.
31 D. S. Helfer and J. D. Atwood, Organometallics, 2004, 23,
2412.
32 R. S. Pryadun and J. D. Atwood, Organometallics, 2007, 26,
4830.
Acknowledgements
33 P. J. Scammells, J. L. Scott and R. D. Singer, Aust. J. Chem., 2005,
58, 155.
34 J. S. Torrecilla, J. Garcia, E. Rojo and F. Rodriguez, J. Hazard. Mater.,
2009, 164, 182.
We acknowledged financial support from the CNRS through the
GDRE “Homogeneous Catalysis for Sustainable Development”
1396 | Green Chem., 2010, 12, 1392–1396
This journal is
The Royal Society of Chemistry 2010
©