Pt-catalysed intermolecular hydroamination of non-activated olefins using a novel family of catalysts: Arbuzov-type phosphorus metal complexes

Article abstract of DOI:10.1016/j.molcata.2013.07.020

The catalytic system "PtBr₂(0.3 mol%)/2P(OR) ₃/10nBu₄PBr" (R = alkyl) discovered recently in our group, allows good to excellent catalytic activities for the intermolecular hydroamination of ethylene and higher α-olefins (1-hexene) with aniline type amines to give the expected N-ethyl- (1) and N,N-diethyl-anilines (2) along with 2-methyl-quinoline (3). A poisoning effect of alkyl and aromatic phosphines has been observed. This effect could be minimised using small amounts of molecular iodide which reacts with phosphorus ligands to form non-coordinating well-described ionic species. Interestingly, beneficial effects of added P(OR)₃ (R = alkyl) have been pointed out and further investigated. In this way, a new potential family of Arbuzov-type phosphorus-metal complexes has been suggested to be responsible for good catalytic activities found when using P(OR)₃ alkyl phosphites rather than PR₃ (R = alkyl, aromatic) and P(OPh)₃ co-catalysts.

Full text of DOI:10.1016/j.molcata.2013.07.020

Journal of Molecular Catalysis A: Chemical 379 (2013) 103–111
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Pt-catalysed intermolecular hydroamination of non-activated olefins
using a novel family of catalysts: Arbuzov-type phosphorus metal
complexes
Mireia Rodriguez-Zubiri∗,1, Stéphane Anguille2, Jean-Jacques Brunet, Jean-Claude Daran
CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic system “PtBr2(0.3 mol%)/2P(OR)3/10nBu4PBr” (R = alkyl) discovered recently in our group,
allows good to excellent catalytic activities for the intermolecular hydroamination of ethylene and higher
˛-olefins (1-hexene) with aniline type amines to give the expected N-ethyl- (1) and N,N-diethyl-anilines
(2) along with 2-methyl-quinoline (3). A poisoning effect of alkyl and aromatic phosphines has been
observed. This effect could be minimised using small amounts of molecular iodide which reacts with phos-
phorus ligands to form non-coordinating well-described ionic species. Interestingly, beneficial effects of
added P(OR)3 (R = alkyl) have been pointed out and further investigated. In this way, a new potential fam-
ily of Arbuzov-type phosphorus-metal complexes has been suggested to be responsible for good catalytic
activities found when using P(OR)3 alkyl phosphites rather than PR3 (R = alkyl, aromatic) and P(OPh)3 co-
catalysts.
Received 12 January 2013
Received in revised form 19 July 2013
Accepted 30 July 2013
Available online 12 August 2013
Keywords:
Platinum
Phosphorus ligands
Arbuzov
Hydroamination
Halides
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
improvements with the recently discovered Pt and Rh-based cat-
alytic systems [5].
Catalytic hydroamination of alkenes is a practical, economical
and sustainable alternative for the preparation of amines and its
derivatives. This atom-economic process allows the addition of an
The Pt-based catalysts have shown to be very efficient systems
for the intermolecular hydroamination of ethylene (Scheme 1) and
1-hexene (Scheme 2) affording the Markovnikov product with an
unprecedented regioselectivity (95%).
N
H bond over an unsaturated carbon carbon bond to yield a large
variety of alkylated amines, very important chemicals in industry
and everyday’s chemistry. Intense efforts have been devoted to the
study and optimisation of this reaction in the past 30 years [1].
Nowadays, the hydroamination reaction could be classified in
three categories: the intramolecular hydroamination of olefins [2],
the intermolecular hydroamination of activated olefins and alkynes
[3] and, the most challenging, the intermolecular hydroamination
of non-activated olefins [1–4]. The latter has known important
This Pt-based system results from the association of PtBr2 with
tetrabutylphosphonium salts.
It is also noteworthy that this system proved to be as efficient
when reactions were performed under aerobic conditions [5b,5c].
The
discovery
of
an
original
RhIII-based
system,
RhCl3·3H2O/2PR3/65nBu4PI/2I2, which consists in the associ-
ation of simple RhIII salts with phosphonium salts and PR3
ligands (PR3 = para substituted triarylphosphines, P(OMe)Ph2,
BINAP, dppe) (Scheme 3) was also described recently [5h].
The latter proved to allow the highest catalytic activity for
the intermolecular hydroamination of ethylene, but also 1-
butene and 1-hexene, with aniline type amines (0.3 mol%
catalytic precursor) to give the expected N-ethyl- (1) and
N,N-diethyl-anilines (2) along with 2-methyl-quinoline (3)
(Scheme 3). This system is particularly efficient in the presence of
2,2ꢀ-bis(diphenylphosphino)-1,1ꢀ-binaphthyl, BINAP (CE = 460,
∗
Corresponding author. Tel.: +33 2 5112 5697; fax: +33 2 5112 5402.
E-mail address: mireia.rodriguez@univ-nantes.fr (M. Rodriguez-Zubiri).
Present address: CNRS, CEISAM, UMR 6230, UFR Sciences et Techniques, Uni-
1
versité de Nantes, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France.
2
Present address: UMR CNRS 7313 iSm2, Aix-Marseille Université, Centre Saint
CE = Catalytic
Efficiency = TON(1) + 2TON(2) + 2TON(3)
)
and
Jérôme, service 531, 13397 Marseille Cedex 20, France.
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.07.020

104
M. Rodriguez-Zubiri et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 103–111
Scheme 1. Intermolecular Pt-catalysed hydroamination of ethylene.
Scheme 2. Intermolecular Pt-catalysed hydroamination of 1-hexene.
tri-(p-tolyl)phosphine (CE = 528). Good to excellent activities
were also found combining the Wilkinson’s catalysts (RhI com-
plexes) with nBu4PI and I2. Interestingly, the simple association of
PPh3 and I2 was shown to be an extremely efficient “in situ gener-
ated” source of I− promoters able to substitute preformed nBu4PI.
It is important to note that the performance of this Rh-based
system depends upon the combination of both PR3/I− promoters
[5g,5h].
For many years now, the scientific community has brought spe-
cial attention to the use of phosphorus ligands in catalysis. This vast
family of ligands has extensively been reported in the literature as
efficient additives for a large amount of homogeneous catalytic pro-
cesses, including the hydroamination of olefins, offering a valuable
source of chirality [6–15].
In this area, the past decade has known an important growth
in the use of phosphorus ligands for the intermolecular hydroam-
ination of activated alkenes [6] such as styrenes [7], norbornenes
[8], dienes [9], allenes [10], and of alkynes [11], as well as for
the intramolecular hydroamination reaction [12–14]. Nevertheless,
studies on the association of a catalyst precursor with a phos-
phine ligand for the intermolecular hydroamination of non-activated
alkenes are very scarce [15]. Pioneering this area, Diamond and
co-workers explored, for the first time in 1979, the hydroamina-
tion of ethylene by aniline using RhCl3/2PPh3 and PdCl2/2PPh3
systems, although low activities were afforded [15d]. After a
long silence, in 2001 Keim et al. used a variety of neutral and
cationic phosphine and phosphite containing ruthenium com-
plexes to study the hydroamination of ethylene by piperidine
achieving good selectivities but low yields [15c]. Interestingly, the
hydroamination of 1-hexene combining a copper halide (5 mol%)
and a silver salt (5–10 mol%) in the presence of dppe, reported by
Michon and co-workers, allowed up to 90% yield although branched
isomers were formed in a ratio 4/6 [15b]. Recently, Widenhoe-
fer and co-workers successfully hydroaminated 1-alkenes with
very low basicity amines (cyclic ureas) in the presence of chiral
AuClPR3 (PR3 = P(tBu)2o-biphenyl and 2-di-tert-butylphosphino-
1,1ꢀ-binaphthyl) complexes affording high yields with up to 78%
ee. To the best of our knowledge, this is the only existing report on
transition-metal catalysed enantiomeric intermolecular hydroam-
ination of non-activated olefins, showing the urgency to further
develop this significant area of broad interest for the scientific
community [15a]. The development of stereoselective catalysts for
the synthesis of chiral alkylamines could be a useful and practical
tool for synthetic chemists and industry, and thus, an important
scientific and economic challenge, especially when targeting the
intermolecular hydroamination of non-activated olefins.
Herein we report the study on the Pt-catalysed intermolecu-
lar hydroamination of ethylene and 1-hexene by aniline in the
presence of a variety of phosphorus ligands and a potential expla-
nation on the recovery of the catalytic activity upon the addition
of I2. Unexpectedly, the inhibiting effect shown by aryl and alkyl
phosphines disappears in the presence of alkyl phosphites. A new
potential family of Arbuzov-type phosphorus-metal complexes
has been appointed to be responsible for good catalytic activ-
ities obtained when using P(OR)3 alkyl phosphites rather than
other phosphorus (III) ligands. This study could provide a signif-
icant insight on compatible ligands as potential chiral vectors.
Furthermore, we have found out an important difference in the
coordination behaviour of PtBr2 when reacting with aryl, aryloxy,
and alkoxy phosphines leading to a new X-ray structure of a phos-
phine containing PtII complex.
2. Experimental
2.1. Instrumentation
GC analyses were performed on a Hewlett-Packard HP 4890
(FID) chromatograph (HP 3395 integrator) equipped with a 30 m
HP1 capillary column. GC–MS analyses were performed on a
Hewlett-Packard HP 6890 apparatus equipped with a HP 5973M ion
detector. NMR analyses were performed on Bruker AM 250 and 400
machines. J values are given in Hz. Catalytic experiments were con-
ducted in a 100 mL stainless steel thermoregulated (electric oven)
autoclave with a glass liner and a magnetic stirring bar.
Scheme 3. Rh-catalysed hydroamination of ethylene.

M. Rodriguez-Zubiri et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 103–111
105
2.2. Methods and materials
441 mg, 1.3 mmol), closed and submitted to several argon-vacuum
cycles. Distilled and degassed aniline (4.1 mL, 45 mmol) was then
syringed into the autoclave. The ethylene pipe was connected to
the autoclave, purged, and the ethylene pressure was adjusted to
25 bars at room temperature (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 in a fume cupboard.
The reaction mixture was poured into 120 mL of diethylether,
stirred for 2 h, and then filtered. The external standard (N,N-di(n-
butyl)aniline, ca. 0.15 g) was added to the collected ethereal phases
and the solution analysed by GC and GC-MS.
Potassium bromide (Aldrich, +99%), 1-iodobutane (Alfa
Aesar, 99%), tri-n-butylphosphine (Acros Organics, 95%), triph-
enylphosphine (Aldrich, 99%), triphenylphosphite (Acros, 99%),
trimethylphosphite (Strem, 97%), methyldiphenylphosphinite
(Acros, 99%), tris((4-trifluoromethyl)phenyl)phosphine (Avocado,
98%), rac-2,2ꢀ-bis(diphenylphosphino)-1,1ꢀ-binaphthyl (Aldrich,
97%), 1,3-bis(diphenylphosphino)propane (Aldrich, 97%), 1,10-
phenantroline (Aldrich, +99%), potassium tetrachloroplatinate(II)
(Aldrich, 98%) and platinum(II) bromide (Strem Chemicals, 98%)
were used as received. Tetra-n-butylphosphonium bromide
(Aldrich, 98%) was stored in a desiccator under vacuum prior to
use. Molecular iodine was purchased from Acros Organics.
Aniline (Acros Organics, 99% for analysis ACS), N-ethylaniline
(Acros Organics, 98%), N,N-di-n-butylaniline (Aldrich, 97%),
1-hexene (Aldrich, 97%), tri-t-butylphosphine (Aldrich, 98%),
trimethylphosphine (Aldrich, 97%) tris(dimethylamino)phosphine
(Aldrich, 97%) and triethylphosphite (Aldrich, 98%) were distilled
before use and stored under argon. Ethylene (N25, +99.5%) was
purchased from Air Liquide.
2.5.2. Hydroamination of 1-hexene: typical procedure.
The autoclave was charged with PtBr2 (46.2 mg, 0.13 mmol),
and nBu4PBr (65 equiv./Pt, 2.88 g, 8.45 mmol), closed and sub-
mitted to several argon-vacuum cycles. Distilled and degassed
aniline (4.1 mL, 45 mmol) along with P(OMe)3 (1 equiv./Pt, 15.8 mg,
0.13 mmol) and 1-hexene (11.3 mL, 91 mmol) were then syringed
sequentially into the autoclave. The temperature was then raised
to 150 ◦C. After 10 h, the autoclave was allowed to cool down to
room temperature and slowly vented in a fume cupboard. The reac-
tion mixture was then poured into 120 mL of diethylether, and
stirred for 2 h, and then filtered. The external standard (N,N-di(n-
butyl)aniline, ca. 0.15 g) was added to the collected ethereal phases
and the solution analysed by GC and GC–MS.
Water was deionised and degassed. All other solvents were of
HPLC grade and were degassed prior to use.
cis-Dibromobis(trimethylphosphite)platinum(II) [16a] and cis-
Dibromobis(triphenylphosphine)platinum(II) [16b] were prepared
according to established methods.
Taking into account the liquid nature of both substrates (aniline
and 1-hexene), the hydroamination of 1-hexene by aniline could
analogously be carried out using adapted Schlenk glass-ware as fol-
lows: A 35 mL Ace Pressure Tube was charged with PtBr2 (46.2 mg,
0.13 mmol) and nBu4PBr (65 equiv./Pt, 2.88 g, 8.45 mmol). Dis-
tilled and degassed aniline (4.1 mL, 45 mmol) along with P(OMe)3
(1 equiv./Pt, 15.8 mg, 0.13 mmol) and 1-hexene (11.3 mL, 91 mmol)
were then syringed sequentially into the Ace Pressure Tube. The
temperature was then raised to 150 ◦C. After 10 h, the reaction
mixture was allowed to cool down to room temperature and
poured into 120 mL of diethylether. The external standard (N,N-
di(n-butyl)aniline, ca. 0.15 g) was added to the collected ethereal
phases and the solution analysed by GC and GC–MS.
Tetra-n-butylphosphonium iodide was prepared from nBu3P
and nBuI (see below), stored under argon, in a freezer, and protected
from light.
Tetrabutylphosphonium cis-dibromo(dimethylphosphonato)
(trimethylphosphite)platinum(II), 6, was prepared from
cis-dibromobis(trimethylphosphite)platinum(II)
and
tetra-n-
butylphosphonium bromide (see below).
2.3. Preparation of tetra(n-butyl)phosphonium iodide
Tri(n-butyl)phosphine (38 mL, 0.15 mol) was slowly added to 1-
iodobutane (40 mL, 0.35 mol) under argon. The mixture was stirred
for 1 h at RT and then at 100 ◦C for 20 h. After cooling, the resulting
precipitate was washed with diethylether (4 × 50 mL) until a white
solid was obtained. Evaporation of residual solvent under vacuum
overnight afforded pure nBu4PI as a white powder (55.6 g, 96%),
mp = 95–96 ◦C. Anal. Found: C, 49.75; H, 9.57. Calc. for C16H36IP: C,
49.74; H, 9.39. ıH (250 MHz, acetone-d6) 0.96 (3H, t, 3J 3), 1.55 (2H,
m), 1.70 (2H, m), 2.54 (2H, m); ıP (250 MHz, acetone-d6) 33.52 (s).
2.6. Study on the coordination chemistry of the
PtBr2/nBu4PBr/PR3 catalytic system
Typical procedure: PtBr2 (0.13 mmol, 46.2 mg), nBu4PBr
(65 equiv./Pt, 45 mmol, 2.87 g) and PPh3 (2 equiv./Pt, 0.26 mmol,
68.0 mg) were weighted and placed in to a 25 mL round-bottom
flask. o-Dichlorobenzene (5 mL) was added and a reflux condenser
adapted to the system. The stirred reaction mixture was heated
to 150 ◦C in an oil bath. After 10 h, the reaction mixture was
allowed to cool down to room temperature, then the solvent was
removed under reduced pressure and the crude analysed by 31P
NMR (250 MHz, CDCl3, see Supplementary Information, SI). When
PPh3 and P(OPh)3 were used, crystals suitable for X-ray diffrac-
tion were grown while cooling down the reaction mixture to room
temperature (see SI).
2.4. Preparation of cis-[PtBr2(P(OCH3)3)(OP(OCH3)2)](nBu4P), 6
To a vigorously stirred solution of cis-PtBr2(P(OMe)3)2 (243 mg,
0.404 mmol) in acetonitrile (2 mL) placed in to a round-bottom
flask at 70 ◦C (oil bath), was dropwise added another solution of
nBu4PBr (137 mg, 0.404 mmol) in acetonitrile (2 mL). The reaction
mixture was stirred for further 2 h until completion. Solvent
removal afforded cis-[PtBr2(PA(OCH3)3)(OPB(OCH3)2)](nBu4P)
quantitatively (340 mg). Anal. Found: C, 29.78; H, 6.25. Calc. for
C
21H51O6P3Br2Pt: C, 29.76; H, 6.07. ıH (400 MHz, acetone-d6) 0.97
(3H, t, 3J 3, CH3 from nBu4P) 1.55 (2H, m, CH2 from nBu4P) 1.7
(2H, m, CH2 from nBu4P) 2.54 (2H, m, CH2 from nBu4P) 3.5 (6H, d,
2.7. X-ray structure determination
3JPH 4.9, PB-OCH3); 3.77 (9H, d, JPH 4.9, PA-OCH3); ıP (400 MHz,
3
Single crystals of 7, 8 and 9 were mounted under inert perflu-
oropolyether at the tip of glass fibre and cooled in the cryostream
of the Oxford-Diffraction XCALIBUR CCD diffractometer. Data were
collected using the monochromatic MoK␣ radiation (ꢀ = 0.71073).
The structures were solved by direct methods (SIR97) [17] and
refined by least-squares procedures on F2 using SHELXL-97 [18].
All H atoms attached to carbon were introduced in calculation in
idealised positions and treated as riding models. The drawing of the
acetone-d6) 31.06 (m, 1JPB-Pt 4962, PB) 33.34 (m, +PBu4) 82.40 (m,
1JPA-Pt 6631, PA).
2.5. Catalytic reactions
2.5.1. Hydroamination of ethylene: typical procedure.
The autoclave was charged with PtBr2 (46.2 mg, 0.13 mmol),
PPh3 (2 equiv/Pt, 68.0 mg, 0.26 mmol) and nBu4PBr (10 equiv/Pt,

106
M. Rodriguez-Zubiri et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 103–111
Table 1
(entry 1, Table 1) for the formation of the hydroamination product
1 (Scheme 4).
Effect of the amount and nature of the phosphorus ligands used on the PtBr2-
catalysed hydroamination of ethylene by aniline.a
The influence of the nature of the phosphorus ligands was stud-
ied using several types of phosphines containing alkyl, aryl, aryloxy
or alkoxy substituents and the results are listed in Table 1.
These results showed that to the exception of P(OMe)3 (entries
6–7, Table 1), phosphorus ligands seem to be inhibitors. It seems
that this effect is enhanced when using 2 equiv. of ligand/Pt mainly
due to a lower production of 1.
In the case of PPh3 (entries 2–3, Table 1) the observed cat-
alytic performance is in contrast with our previous results on the
RhIII-catalysed reactions of aniline with ethylene [5g,5h], where
aromatic phosphorus ligands such as PPh3 seemed to allow excel-
lent catalytic activities. In the same way, aryloxy-substituted
ligands (triarylphosphites) and trialkylphosphines allow the reac-
tion but are inefficient co-catalysts (entries 4–5, 8–9).
Entry
Ligand (equiv./Pt)
TON 1b
TON 2b
TON 3b
1
2
3
4
5
6
7
8
9
–
102
18
11
31
5
98
112
24
<1
5
–
–
–
–
4
6
–
–
15
23
19
6
4
10
8
PPh3 (1)
PPh3 (2)
nBu3P (1)
nBu3P (2)
P(OMe)3 (1)
P(OMe)3 (2)
P(OPh)3 (1)
P(OPh)3 (2)
–
–
a
Conditions: PtBr2 (46.1 mg, 0.13 mmol), PR3, nBu4PBr (0.4412 g, 1.30 mmol),
PhNH2 (4.1 mL, 45.5 mmol), C2H4 (25 bar, ca.100 mmol), 150 ◦C, 10 h.
b
N-ethylaniline (1), N,N-diethylaniline (2), 2-methyl-quinoline (3).
Unexpectedly, catalytic activity is fully recovered upon the
introduction of alkoxy groups as substituents on the phosphorus
atoms. Indeed, results obtained with 2 equiv./Pt of P(OMe)3 (entries
6–7, Table 1) showed that this ligand does afford higher selectivity
(less production of 2 and 3 side-products) and catalytic activity for
the formation of the desired product, N-ethylaniline 1, than when
the reaction is performed in the absence of ligand. Thus, the pres-
ence of alkoxy groups in the structure of the phosphorus ligands
seems to be a key factor for achieving good catalytic activities.
According to results stated in Table 1, alkyl phosphites may rep-
resent the only type of phosphorus ligand without an inhibitor
effect and thus suitable for the studied reaction.
molecules was realised with the help of ORTEP32 [19]. Crystal data
and refinement parameters are shown in Tables 3–5 from the SI.
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as sup-
plementary publication no. CCDC 878731–878733. Copies of the
data can be obtained free of charge on application to the Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336
033; deposit@ccdc.cam.ac.uk).
3. Results and discussion
3.1. Hydroamination of ethylene by aniline in the presence of
phosphorus ligands
3.2. Hydroamination of higher ˛-olefins in the presence of
phosphorus ligands
Taken the urgency to develop new catalytic systems for the
intermolecular hydroamination of non-activated olefins and the
consolidated reputation of phosphorus ligands to accomplish this
type of task for a large range of catalytic reactions (see Section
1), we were prompted to investigate the effect of several classical
and simple phosphorus ligands in the intermolecular hydroamina-
tion of ethylene by aniline as model reaction (Scheme 4). Aiming to
improve the already efficient stable low-loading (0.3 mol%) “one-
pot” ready-to-use PtBr2/nBu4PBr system recently discovered in our
team [5b,5c], we carried some studies to identify a compatible class
of ligand able to afford good catalytic activities and capable of play-
ing the role of a potential chiral vector.
In order to potentially widen the scope of this catalytic process,
we studied the addition of aniline to higher ˛-olefins such as 1-
hexene in the presence of a variety of phosphorus ligands.
The products resulting from these reactions are of great rel-
evance in industry and the transformation of higher ˛-olefins is
itself an important scientific challenge [21,15d]. Literature on the
intermolecular hydroamination of higher olefins by simple amines
is very scarce and quite recent [2a,5c,15b,22–25]. Marks and co-
workers [2a,22] studied the hydroamination of 1-pentene with
n-propylamine and reported to deal with a challenging reaction, the
intermolecular hydroamination of non-activated alkenes, in com-
parison to intramolecular hydroamination, 350× faster, and alkyne
hydroamination, 1400× faster. To the best of our knowledge, only
four reports have appointed the use of phosphorus ligands for
the intermolecular hydroamination of higher non-activated ˛-olefins.
First, He et al. worked on the addition of TosNH2 to 1-octene
with 5 mol% of Ph3PAuCl/AgOTf in toluene at 85 ◦C [23]. Later on,
Loh and co-workers studied the same reaction in the presence of
InBr3 (20 mol%) in toluene at 120 ◦C [24]. Both groups obtained the
desired Markovnikov hydroamination product along with another
Markovnikov isomer resulting from the hydroamination of an
internal alkene formed by isomerisation of the initial terminal
double bond alkene. Next, Agbossou-Niedercorn and Michon (see
Section 1), used dppe for the hydroamination of 1-hexene by
TosNH2 [15b]. Using AuClPR3 (PR3 = P(tBu)2o-biphenyl and 2-di-
tert-butylphosphino-1,1ꢀ-binaphthyl) complexes Widenhoefer and
In this work, we were led to consider the TON (mol of product
formed by mol of catalyst) in order to evaluate the efficiency of the
catalyst.
In a previous report, it was briefly pointed out that the catalytic
efficiency for the hydroamination of ethylene (Scheme 4) could be
dramatically affected by the presence of phosphorus ligands [20].
In order to confirm and generalise our observations, it was decided
to investigate the influence of the PR3/PtBr2 ratio and the nature
of the associated ligand for the hydroamination of ethylene with
aniline. For this study, experiments were conducted without acid
co-catalyst in order to better observe the role of the phosphorus
additives. According to previous studies, the selected reaction time
was 10 h and the chosen reaction temperature was 150 ◦C.
At first, it must be recalled that, in the absence of PR3 ligands,
the PtBr2-based systems exhibit catalytic activity up to TON = 102
Scheme 4. Pt-catalysed intermolecular hydroamination of ethylene by aniline in the presence of PR3.

M. Rodriguez-Zubiri et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 103–111
107
Scheme 5. Pt-catalysed intermolecular hydroamination of 1-hexene by aniline in the presence of PR3.
the presence of phosphorus ligands and/or I2 is available in the
Supporting Information (SI).
Scheme 6. Formation of [R3PX]+X− salts from PR3 and I2.
It is premature to speculate about the exact role of these
phosphorus-containing ligands and mechanistic studies are
required to gain full insight into these phenomena. A full mech-
anistic study on the title reaction is out of the scope of this article
and will be addressed in future publications. Although the NMR
monitoring of the catalytic process could have provided some
important insights on the active catalytic species and on possi-
ble intermediates of the catalytic cycle, this study could not be
done due to technical difficulties (high temperature, 150 ◦C, dur-
ing 10 h, and high pressure, 25 bar, in the case of ethylene as
starting olefin). As a preliminary alternative we have tried to
recover the catalyst at the end of the reaction in order to evalu-
ate, by 195Pt NMR, the absence/presence of phosphorus containing
platinum complexes that could explain the observed catalytic
results. Decomposition of the active platinum species to black
Pt0 (black precipitate at the end of the catalytic process) did not
allow us to observe any platinum containing species by the NMR
technique.
co-workers successfully hydroaminated 1-alkenes with very low
basicity amines [25].
As may be seen from literature, only group 11 (Au, Cu, Ag)
transition metals have been associated with phosphorus ligands
to investigate the title reaction. Thus, the addition of aniline to 1-
hexene (Scheme 5) was studied using Pt/P-based catalytic systems.
Catalytic results have revealed that, to the exception of alkyl
phosphites (P(OEt)3 and P(OMe)3, entries 7–10, Table 1 from SI),
the catalytic activity of this reaction seems, once more, inhibited
in the presence of phosphorus ligands. As in the case of ethylene,
this is true for aromatic phosphines, alkyl phosphines and aromatic
phosphites (see Table 1 from SI for further details), confirming our
hypothesis on the need of a P
O C bond in the structure of the
ligand to obtain a good catalytic performance.
These results seem to confirm a strong coordination mode
between the metal centre and the ligands, completely deactivating
the catalytic system by blocking its active sites.
This is somehow confirmed by the results obtained (TON = 1.5)
for the hydroamination of 1-hexene (1.6 mL, 12.73 mmol, 150 ◦C,
144 h) by aniline (4.1 mL, 45 mmol) in the presence of pre-
formed cis-PtBr2(PPh3)2 (0.114 g, 0.13 mmol) and nBu4PBr (6.62 g,
19.5 mmol).
On the basis of previous results on the platinum-
catalysed hydroamination of alkenes [5c], the possible role
of molecular iodine was also considered. The association of
the reported inhibitor systems, nBu4PBr/dppp (dppp = 1,2-
bis(diphenylphosphino)propane) and nBu4PI/PPh3, with molecular
iodine allows to recover the catalytic activity in the presence of
PtBr2 (see Table 2 from SI for further details). These results may
be understood as I2 being an antidote for the poisoning effect of
aromatic phosphines by forming [R3PX]+X− salts (Scheme 6) as
described in the literature [26].
3.3. Hydroamination in the presence of Arbuzov-type phosphorus
containing Pt-complexes
On the basis of the good catalytic results obtained with the asso-
ciation of PtBr2 with nBu4PI and alkylphosphites, our next efforts
were aimed at investigating the particularly beneficial effect of
this unique type of phosphorus ligands in the intermolecular Pt-
catalysed hydroamination of ethylene by aniline.
For this study we addressed an important question concerning
the fact that alkylphosphites allow high catalytic activities whereas
alkyl- and arylphosphines, and even arylphosphites, behave as
inhibitors.
Our first hypothesis was based on chemical structural differ-
ences between phosphorus promoters and inhibitors: alkylphos-
phites are the only ones to have O-alkyl substituents on the
phosphorus atom. Thus we were prompted to consider an
“in situ” transformation, under the catalytic conditions, of metal-
coordinated alkylphosphites into somehow more active species
according to a Michaelis-Arbuzov type reaction. As described by
Odinets et al. [27], the Michaelis-Arbuzov reaction allows the for-
mation of a P O bond by reaction of an alkylphosphite with an
alkylhalide (Scheme 7).
To confirm this hypothesis, we have studied the 31P NMR (under
inert atmosphere) of the reaction mixture of a typical catalytic run
in the presence of triphenylphosphine. Two signals were identified:
the expected signal corresponding to the phosphonium salt, nBu4PI,
at ıP = 33.1 ppm, along with another signal (ıP = 26.9 ppm, major
product) that could be attributed to iodotriphenylphosphonium
iodide, [R3PI]+I−, by correlation with the 31P NMR data obtained
from the in situ NMR reaction (at room temperature in acetone-d6)
between PPh3 and I2.
Taking a look into literature data we could find several reports
on a special type of Michaelis-Arbuzov reaction where, similarly, a
metallic complex containing one or more halogen ligands allows
the reaction (Scheme 8) [28]. Interestingly, authors report this
A more detailed discussion about the obtained catalytic results
along with literature data for the hydroamination of 1-hexene in
Scheme 7. Michaelis-Arbuzov reaction.

108
M. Rodriguez-Zubiri et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 103–111
Scheme 8. Michaelis-Arbuzov reaction in metallic complexes.
Scheme 9. Transition-metal Michaelis-Arbuzov reaction described by Roundhill
and co-workers [29].
Fig. 1. Molecular view of compound 7 trans-[PtBr2(PPh3)]2 with the atom-labelling
scheme.
A mechanistic study on the Pt-catalysed hydroamination reac-
tion in the presence of P containing ligands will be addressed in
future works.
Scheme 10. Preparation of compound cis-[PtBr2(P(OCH3)3)(OP(OCH3)2)](nBu4P)
(6).
3.4. Coordination chemistry of the PtBr2/nBu4PBr/PR3 catalytic
systems
reaction to take place at moderate to high temperatures when reac-
tion times are prolonged, as in our case.
As stated above, one of the advantages of the catalytic strategy
used in the present study was the possibility to add in a “one-pot”
manner the components of the catalytic system. Nevertheless, in
order to better understand the coordination abilities of the metal
centre towards the added phosphine ligands, we decided to explore
a few more experiments on the reactivity of PtBr2 with some of
the phosphine and phosphites used for the hydroamination reac-
tion. Surprisingly, literature data on this subject are still scarce
[30].
For this purpose, PtBr2 was reacted with 1 and 2 equiv./Pt of
three different phosphorus ligands (PPh3, P(OEt)3 and P(OPh)3), in
the presence of nBu4PBr.
Surprisingly, the reactivity of PtBr2 towards aromatic phos-
phines, aromatic phosphites and alkyl phosphites was found to be
quite different under the same reaction conditions.
As expected, the reaction of PtBr2 with 1 equiv./Pt of PPh3
gave a new binuclear complex trans-[PtBr2(PPh3)]2 7 (Fig. 1) while
2 equiv./Pt of PPh3 gave the already known mononuclear complex
cis-[PtBr2(PPh3)2] 8 (Fig. 2).
Interestingly, the same reaction under the same conditions but
using 1 or 2 equiv./Pt of P(OPh)3 gave only the mononuclear com-
plex cis-[PtBr2(P(OPh)3)2 ] 9 (Fig. 3) as stated by a very clean 31P
NMR spectra.
Concerning P(OEt)3 ligand, its coordination chemistry with
PtBr2 seems less straight forward and we tentatively propose the
formation of cis-PtBr2(P(OEt)3)2 along with, as expected, platinum
complexes containing several Arbuzov-type phosphorus ligand
according to related literature data.
Single crystals suitable for X-ray diffraction were obtained for
compounds 7–9 while cooling down the reaction mixture to room
temperature (see Section 2 and SI for further details).
Roundhill and co-workers reported an interesting study on suc-
cessive Michaelis-Arbuzov reactions on various alkoxy groups from
the phosphorus ligand. They succeeded in the formation of tran-
sition metal complexes containing different type of phosphorus
ligands in equilibrium, when several equivalents of alkylphosphite
reacted with a chosen metallic complex (Scheme 9) [29].
Taking into account the state of the art, and in order to confirm
our hypothesis on the Michaelis-Arbuzov reaction to be responsible
of the particular catalytic effect of alkylphosphites, further catalytic
experiments were conducted in the presence of a preformed Pt-
complex containing a Michaelis-Arbuzov type phosphorus ligand.
Thus, the complex cis-[PtBr2(P(OCH3)3)(OP(OCH3)2)](nBu4P) (6)
(Scheme 10) was synthesised (see Section 2) and used along with
nBu4PBr for the catalytic hydroamination of ethylene by aniline
(Scheme 11).
Results in this area were particularly interesting. Indeed,
TON up to 127 towards the production of
1 (TON1 = 127,
TON2 = 5, TON3 = 4) were obtained using the association of cis-
[PtBr2(P(OCH3)3)(OP(OCH3)2)](nBu4P) (6) (0.3 mol%, 0.13 mmol,
0.110 g) with nBu4PBr (10 equiv./Pt, 0.4412 g, 1.30 mmol), a partic-
ularly efficient system for the hydroamination of ethylene (25 bar,
ca. 100 mmol, 150 ◦C and 10 h) by aniline (4.1 mL, 45.5 mmol)
and even more active and selective towards the formation
of 1, under comparable conditions, than the platinum system
PtBr2/10nBu4PBr/2P(OMe)3 (entry 7, Table 1). It is important to
note that no black Pt(0) has been observed at the end of the
reaction when performing the catalytic tests in the presence of cis-
[PtBr2(P(OCH3)3)(OP(OCH3)2)](nBu4P). This seems to point out an
important feature of this system: a high stability of the catalytically
active species even at high temperature and in a reductive media.
Scheme 11. Catalytic hydroamination of ethylene by aniline in the presence of compound (6).

M. Rodriguez-Zubiri et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 103–111
109
Fig. 2. Molecular view of compound cis-[PtBr2(PPh3)2], 8, with the atom-labelling scheme.
A more detailed discussion about the obtained results along
4. Conclusions
with experimental, spectroscopic and crystallographic data for 7–9
complexes and related compounds is available in the Supporting
Information (SI).
An important effect of phosphorus co-catalysts on the
intermolecular Pt-catalysed hydroamination of ethylene and
1-hexene by aniline has been evidenced. Indeed, although
alkyl- and arylphosphines, as well as arylphosphites, inhibit
the reaction, alkylphosphites showed to be a unique class of
ligands suitable for the title reaction. We have investigated
this particular effect of alkylphosphites and found that the
association PtBr2(P(OMe)3)(OP(OMe)2)/10nBu4PBr is
a
very
efficient catalytic system allowing even higher TON than the
PtBr2/10nBu4PBr/2P(OMe)3 catalyst studied in this work and the
previously reported PtBr2/10nBu4PBr system [5b].
These results give access to a new broad family of efficient
Arbuzov-type ligands for homogeneous catalytic processes. Mod-
ulation of their structural properties could provide a large range
of new Arbuzov-like phosphoranes. Even more important, these
phosphorus containing co-catalysts may represent the first family
of ligands suitable for the intermolecular Pt-catalysed hydroamina-
tion of ethylene and higher ␣-olefins and thus become an important
option when considering the enantioselective version of this reac-
tion; an important scientific challenge (see Section 1). Indeed,
a new series of chiral catalysts could be considered combining,
in the coordination sphere of a transition metal centre, different
phosphonites P(OR1)2R2 and/or phosphinites P(OR1)R2R3 bearing
a chiral R2 and/or R3 group or, alternatively, 2 different R2 and R3
groups.
Furthermore, we have been able to evidence a preventing effect
of I2 towards the poisoning effect of phosphines and arylphos-
phites on the studied reactions by the formation of [R3PI]+I− type
salts.
Investigations on the coordination chemistry of PtBr2 with PPh3,
P(OPh)3 and P(OEt)3 have revealed an interesting behaviour when
using phosphites (only monomers are formed) and provide a new
X-ray structure for the dimer complex trans-[PtBr2(PPh3)]2.
Acknowledgments
We are grateful to The Centre National de la Recherche Scien-
tifique (CNRS, France) and the FSE (post-doctoral grant to SA) for
funding. Dr. J.-J. Bonnet and Dr. D. Neibecker are acknowledged
for academic support and Prof. F.-X. Felpin and Dr. F. Malbosc
(Solvionic) for fruitful discussions.
Fig. 3. Molecular view of compound cis-[PtBr2(P(OPh)3)2], 9, with the atom-
labelling scheme.

110
M. Rodriguez-Zubiri et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 103–111
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