1372 Zhang et al.
Asian J. Chem.
n-C6H13
Rh(acac)(CO)2, CO/H2
Ligand
CHO
O
O
NEt3
n-C6H13
+
n-C6H13
(COCl)
+
CHO
2
2
rt
N
H
N
H
Liner aldehyde
Branched aldehyde
NH2
Fig. 1. Hydroformylation of 1-octene
t-BuOK
220-300 °C
H
N
NEt3
N
2
1
N
P
N
N
P
N
N
P
N
N
P
Cl +
rt
N
H
N
N
N
N
P
N
N
Fig. 2. Synthesis of 2,2'-biindolyl and ligand 1
2
1
little or no product. Then reaction of chlorodipyrrolylphosphine
with 2,2'-biindolyl in the presence of triethylamine afforded
the desired bisphosphane ligand 1 in 56 % unoptimized yield.
The structure of the new ligand 1 was unambiguously proven
by means of 1H, 13C and 31P NMR spectroscopy, together with
mass spectrometry.
Scheme-I: Bisphosphane ligand 1 vs. 2
amine (1 mL) and a solution of 2,2'-biindolyl (2 mmol, 0.46 g)
in THF (5 mL) at room temperature. The triethylamine HCl
salts were formed immediately after the addition. The reaction
mixture was stirred for 10 h at room temperature. The triethyl-
amine HCl salts were then filtered off and the solvent was
removed under vacuum. The crude product was purified by
crystallized from toluene to afford the pure ligand 1 (0.83 g,
56 %), as an air-stable white solid. 31P NMR (166 MHz,
Hydroformylation experiments: Firstly, the effect of
ligand 1/Rh ratio on the activity and regioselectivity of 1-octene
hydroformylation was investigated and the results are summa-
rized in Table-1. With the ligand/rhodium ratio increasing from
5 to 20, the L/B ratio of aldehyde rose from 3.4 to 4.2. The
data suggested that the increase of the L/B ratio could be
attributed to the high ligand concentration and the increase of
ligand/rhodium ratio in a suitable concentration range could
stabilize the catalytic active species16,17. Leeuwen et al.9 con-
cluded that the ligand/Rh ratio determined the concentration
of the active species HRh(P^P)(CO) (Scheme-II) in solution.
Under the appropriate ligand/Rh ratio, high concentration of
species II that could transform into active species IV gave
excellent regioselectivity. However, more excess ligand resulted
in low reaction rate, as shown in Table-1 (entry 4), when the
ratio was 20, the TOF value decreased to 256 h-1. It could be
explained that more excess ligand would form the species III
which block the coordination site and the coordination of 1-
octene with rhodiun active species IV would become difficult,
1
CDCl3): δ = 73.66. H NMR (400 MHz, CDCl3): δ = 7.39-
7.41 (d, J = 7.7 Hz, 2H), 7.34-7.35 (d, J = 7.7 Hz, 2H), 7.15-
7.19 (t, J = 12.0 Hz, 2H), 7.04-7.07 (t, J = 15.4 Hz, 2H), 6.36-
6.42 (d, J = 21.8 Hz, 8H), 6.12-6.25 (d, J = 52.6 Hz, 8H), 5.21
(s, 2H). 13C NMR (100 MHz, CDCl3): 139.77-139.97 (d, JPC =
20.1 Hz), 130.15, 124.58, 123.39, 122.65-122.80 (d, JPC = 15.4
Hz), 122.32, 121.77, 119.96, 113.15-113.30 (d, JPC = 15.4 Hz),
111.66-111.81 (d, JPC = 15.4 Hz). -HRMS (ES+) calcd for
C32H26NaN6P2 [M + Na+] 579.1592, found 579.1618.
Hydroformylation of 1-octene: The hydroformylation
of olefin was carried out in a 60 mL stainless steel autoclave
equipped with a magnetic stirrer. The aqueous solution of
Rh(acac)(CO)2, a certain amount of ligand and olefin were
introduced into the autoclave. Then it was evacuated and
purged five times by syngas. The autoclave was pressurized
to each desired pressure with syngas. The reaction mixture
was stirred at a constant speed of 10 rps when the temperature
reached the desired value. After the reaction was completed,
the autoclave was quickly cooled in an ice-water bath and
carefully depressurized. The reaction mixture was immediately
analyzed by GC to determine the activity and regioselectivity.
which would cause its activity to drop18,19
.
TABLE-1
EFFECT OF MOLAR RATIO OF LIGAND (L) TO
RHODIUM ON 1-OCTENE HYDROFORMYLATION
Entry
L/Rh
5
8
10
20
Aldehyde yield (%)
L/Ba
3.4
3.6
3.9
4.2
TOFb
358.5
359.0
399.5
256.0
1
2
3
4
71.7
71.8
79.9
51.2
RESULTS AND DISCUSSION
Synthesis of ligand: The synthesis of backbone and ligand
1 was realized as shown in Fig. 2. Following known literature
procedures14,15, oxalyl chloride was condensed with o-tolyl-
amine according to the procedure of Wallace to form N,N'-
bis(o-tolyl)oxamide, in quantitative yields. The Madelung
cyclization was then accomplished by heating N,N'-bis(o-
tolyl)oxamide with freshly prepared potassium tert-butoxide
to 220 °C and then slowly to 300 °C. Much decomposition
occurred and lots of gas released under these extreme
conditions and yield of 2,2'-biindolyl was typically on the order
of 50 %, though reactions at lower temperatures resulted in
Reaction conditions: S/C-Molar ratio of substrate/catalyst = 1000. 100
°C, p = 2 MPa (CO/H2 = 1), [Rh] = 1.3 × 10–3 mmol, 2 h, toluene 2
mL; aL/B-Molar ratio of linear to branched aldehyde; bTOF: Aldehydes
(mole)/(Rh (mole) × time (h))
The effect of temperature on the behaviours of the catalyst
system was also investigated and the results were listed in
Table-2. The data showed that the reaction temperature also
plays a key role in hydroformylation. High temperatures
generally led to high reaction rates, for example, the TOF value
at 60 °C was 285.0 h-1, whereas this number increased to 399.0
h-1 at 100 °C. But the sharp change of reaction rate occurred at