M. Rodriguez-Zubiri et al.
less-steel thermoregulated (electric oven) autoclave with a glass
liner and a magnetic stirring bar.
As expected, Wilkinson’s catalyst allows very high catalytic
activities when performing the hydroamination of ethylene by
aniline under optimised experimental conditions, using nBu4PI
and I2 promoters. Under similar conditions the RhCl3.3H2O/
nBu4PI/PPh3/I2 catalytic system afforded slightly higher TON
(TON1 =28, TON2 =225, TON3 =3, CE=484) mainly because it
seems to better promote the formation of 2 from 1, and thus
the hydroamination by more basic amines.
Methods and materials
Tetra-n-butylphosphonium iodide was prepared from nBu3P and
nBuI (see below), stored under argon, in a freezer, and protected
from light. Tetra-n-butylphosphonium bromide (Aldrich, 98%), tet-
raphenylphosphonium iodide (Alfa Aesar,>98%), tetra-n-butyla-
monium iodide (Fluka,>99%), 1-iodobutane (Alfa Aesar, 99%), tri-
n-butylphosphine (Acros Organics, 95%), triphenylphosphine (Al-
drich, 99%), triphenylphosphite (Acros, 99%), trimethylphosphite
(Strem, 97%), methyldiphenylphosphinite (Acros, 99%), tri(4-me-
thoxyphenyl)phosphine (Alfa Aesar, 98%), tri((4-trifluoromethyl)-
phenyl)phosphine (Avocado, 98%), tri(p-tolyl)phosphine (Fluka,>
97%), rac-2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (Aldrich,
97%), 1,2-bis(diphenylphosphino)ethane (Strem, 99%), and rhodi-
um trichloride trihydrate (Johnson Matthey, 41.92%) were used as
received. Molecular iodine was purchased from Acros. Aniline
(Acros Organics, 99% for analysis ACS), N-ethylaniline (Acros Or-
ganics, 98%), morpholine (Fluka, 99%), piperidine (Acros Organics,
99%), benzylamine (Aldrich,>99.5%), N,N-di-n-butylaniline (Al-
drich, 97%), 4-n-octylaniline (TCI,>95%), 1-hexene (Aldrich, 97%)
and triethylphosphite (Aldrich, 98%) were distilled before use and
stored under argon. Ethylene (N25) and 1-butene were purchased
from Air Liquide. Tricyclohexylphosphine was recrystallised from
toluene/methanol. [RhCl(PPh3)3], [{RhCl(PPh3)2}2] were prepared ac-
cording to established methods.[34]
In the same way, Wilkinson’s dimer appeared to be a good
hydroamination catalyst although the selectivity of the reac-
tion is different because less N,N-diethylaniline is formed either
because the formation of 2 is less favoured than in the case of
the corresponding monomer, or simply because the reaction is
slower. It is important to note that no quinoline was observed
when using either of the Wilkinson’s complexes.
Our study on the performance of rhodium(I)-phosphine pre-
cursors, in the presence of phosphonium salts, could provide
important access to a new and large family of asymmetric cat-
alytic systems for the enantioselective intermolecular hydroa-
mination of non-activated olefins by simple amines.
Conclusion
In summary, the association of RhCl3·3H2O/2PPh3/nBu4PI/I2 has
been shown to afford excellent catalytic activities for the hy-
droamination of ethylene and higher non-activated a-olefins
with aniline. Even more important is to note the very high re-
gioselectivity of this rhodium-based catalytic system towards
Markovnikov addition products (up to 100% for the reaction
of 1-hexene with aniline). It is also an excellent catalytic system
for the addition of more basic anilines, such as N-ethylaniline,
to non-activated olefins.
Preparation of tetra-n-butylphosphonium iodide
Tri-n-butylphosphine (38 mL, 0.15 mol) was slowly added to 1-io-
dobutane (40 mL, 0.35 mol) under argon. The mixture was stirred
for 1 h at RT and then at 1008C for 20 h. The solution was heated
with a heat gun for approximately 15 min. After cooling, the result-
ing precipitate was washed with diethyl ether (4ꢂ50 mL) until
a white solid was obtained. Evaporation of residual solvent was
carried out under vacuum overnight to afford pure nBu4PI as
a white powder (96% yield). m.p. 95–968C; 1H NMR (250 MHz,
[D6]acetone): d=0.96 (t, 3H, J=3 Hz), 1.55 (m, 2H), 1.70 (m, 2H),
2.54 ppm (m, 2H); 31P NMR (250 MHz, [D6]acetone): d=33.52 ppm
(s); elemental analysis calcd (%) for C16H36IP: C 49.74, H 9.39;
found: C 49.75, H 9.57.
From a practical point of view, this system offers several ad-
vantages because it involves simple starting materials and tol-
erates the presence of common aromatic phosphines as po-
tential chiral vectors for the enantioselective version of this re-
action. Furthermore, good to excellent activities were achieved
by “one-pot” addition of PPh3 and I2, a source of an “in-situ”
generated halide promoter, in the absence of nBu4PI.
Research is now in progress to elucidate the nature and the
energy of the intermediates of the catalytic cycle of this reac-
tion and to take advantage of these promoting effects to ex-
plore the enantioselective intermolecular hydroamination of
non-activated alkenes by simple amines in the presence of rac-
emic and chiral rhodium(I)-based N-heterocyclic carbene–phos-
phine precursors.
Typical hydroamination reactions
Hydroamination of ethylene: The autoclave was charged with
RhCl3·3H2O (34.0 mg, 0.13 mmol), I2 (2 equiv with respect to Rh,
66.0 mg, 0.26 mmol), PPh3 (2 equiv with respect to Rh, 68.0 mg,
0.26 mmol), and nBu4PI (3.3 g, 8.45 mmol). The autoclave was
closed and submitted to several argon-vacuum cycles. Distilled and
degassed aniline (4.1 mL, 45 mmol) was then syringed into the au-
toclave. The ethylene pipe was connected to the autoclave,
purged, and the ethylene pressure was adjusted to 25 bar at RT
(ca. 100 mmol). The temperature was then raised to 1508C. After
96 h, the autoclave was cooled to RT and slowly vented in a fume
cupboard. The reaction mixture was poured into diethyl ether
(120 mL), stirred for 2 h, and then filtered. The external standard
(N,N-di-n-butylaniline, ca. 0.15 g) was added to the collected ethe-
real phases and the solution was analysed by GC and GCMS.
Experimental Section
Instrumentation
The GC analyses were performed on a Hewlett–Packard HP 4890
(FID) chromatograph (HP 3395 integrator) equipped with a 30 m
HP1 capillary column. The GC-MS analyses were performed on
a Hewlett–Packard HP 6890 apparatus equipped with a HP 5973 M
ion detector. The NMR analyses were performed on Bruker AM 250
machine. Catalytic experiments were conducted in a 100 mL stain-
Hydroamination of 1-butene: The autoclave was charged with
RhCl3·3H2O (34.0 mg, 0.13 mmol), I2 (2 equiv with respect to Rh,
452
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ChemPlusChem 2012, 77, 445 – 454