J. Tijani, B. El Ali / Journal of Organometallic Chemistry 692 (2007) 3492–3497
3493
Table 1
In the present work, a highly regioselective rhodium-cat-
alyzed hydroformylation of 1-alkenes was achieved in the
presence of bis(2,4-di-tert-butyl)pentaerythritol diphosph-
ite (alkanox P-24) (I) as a ligand.
Hydroformylation of 1-octene by Rh(CO)2(acac)/alkanox (I). Effect of the
temperature and the type of solventa
Entry T °C Solvent
Conv.b, Products distributionb, %
%
Aldehydesb B1/
Octene
%
Lc, % isomersb,d
,
(CH3)3C
C(CH3)3
%
O
O
O
O
1
2
3
4
5
6
7e
8
9
80
100
120
130
120
120
120
120
120
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
DMSO
18
73
98
99
96
75
98
77
66
94
93
82
80
91
82
77
86
89
16/84
13/87
5/95 16
7/93 18
5
7
P
O
C(CH3)3
(CH3)3C
O P
Bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite (I)
12/88
8
7/93 17
21/49 19
10/90 12
THF
n-Heptane
Propylene
carbonate
8/92
9
2. Results and discussion
a
Reaction conditions: Rh(CO)2(acac) (0.005 mmol), ligand I (0.03
2.1. Hydroformylation of 1-octene by Rh(CO)2(acac)-I:
effect of the temperature and the type of solvent
mmol), 1-octene (5.0 mmol), solvent (5 ml), 300 psi (CO/H2 = 1/2), 1 h.
b
Determined by GC.
Determined by GC and 1H NMR.
Octene isomers include cis- and trans-2-octene, cis- and trans-3-octene,
c
Hydroformylation of 1-octene, chosen as a model sub-
strate, into 1-nonanal (L) and 2-methyl octanal (B1)occurred
smoothly at relatively mild conditions (120 °C, 300 psi (CO/
H2 = 1:2), and 1 h), with Rh(CO)2(acac) used as a catalyst
and the diphosphite alkanox (I) used as a ligand
d
and cis- and trans-4-octene and were determined by GC–MS.
e
Other branched aldehydes B2 and B3 (B2 = 22, B3 = 8) were formed.
CHO
The study of the effect of the type of solvent on the hyd-
roformylation of 1-octene by the catalytic system Rh(CO)2-
(acac)/alkanox (I) is also shown in Table 1 (entries 5–9).
Among the tested solvents, toluene, DMSO, n-heptane,
and propylene carbonate led to total aldehydes yield of
91%, 82%, 86%, and 89%, with 88%, 93%, 90%, and 92%
linear aldehydes, respectively (Table 1, entries 5–9). For
THF, the selectivity to the linear isomer is 70%, and that
of branched isomers 2-methyl octanaldehyde (IV) and
2-ethyl heptanaldehyde (V) (Eq. (2)) are 22% and 8%,
respectively (Table 1, entry 7).
The results obtained showed clearly that the highest
ratio (L/B = 19) of linear to branched aldehyde was
obtained at 120 °C in CH2Cl2 as a solvent. DMSO is the
other polar solvent that gave almost similar results to
CH2Cl2 compared to THF and propylene carbonate that
gave either lower selectivity in linear aldehyde or low con-
version of 1-octene. The stability of the active rhodium
intermediates Rhx(CO)yIz depends strongly on the type of
solvent and on the temperature of the reaction.
Rh(CO)2(acac) / I
+
CHO
CH2Cl2, ToC
CO/H2
II
L (III)
B1 (IV)
ð1Þ
A systematic study on the influence of the temperature
on the regioselectivity and the catalytic activity in the hyd-
roformylation of 1-octene in dichlomethane as a solvent
was carried out at a variety of temperatures ranging from
80 °C to 130 °C. The results are shown in Table 1. The for-
mation of the linear aldehyde as a major product prevailed
in all cases at all temperatures.
The results of the reaction after an hour showed a very
low conversion (18%) at 80 °C (Table 1, entry 1) with 84%
linear aldehyde. The conversion increased to 73% and 99%
at 100 °C and 120 °C, respectively. The selectivity for the lin-
ear isomer increases from 87% at 100 °C to 95% at 120 °C
(Table 1, entries 2,3). It was also observed that the isomeri-
zation of 1-octene increases with the temperature. For
instance, only 5% of isomerized octenes were detected in
the products of the reaction at 80 °C, while the percentage
of these isomers increased to 18% at 130 °C. In fact, it was
reported that branched Rh-alkyl intermediate forms faster
than the linear isomer intermediate. However, the first iso-
mer was shown to be reversible through b-elimination to
form 2-octene, while the second is irreversible and can only
evolve to the linear aldehyde. The b-elimination of the
branched Rh-alkyl intermediate significantly increases with
temperature, thus forming more linear aldehyde and also
more products of isomerization [14].
2.2. Hydroformylation of 1-octene by Rh(CO)2(acac)-I:
effect of CO/H2 pressure and reaction time
The improvement of the results of the reaction requi-
red further optimization of the reaction conditions. For
instance, the effect of the ratio of CO/H2 and the reaction
time were studied with the catalytic system Rh(CO)2(acac)/
alkanox (I) (Table 2). At a total pressure of 300 psi of CO/
H2, the selectivity for the linear aldehydes decreases from
96% at CO/H2 = 50/250, to 95% at CO/H2 = 100/200
and finally to 84% at CO/H2 = 200/100. It was observed
that the percentage of the isomerized olefins decreases with
In addition, it is important to note that the percentages
of octane, which is the hydrogenated product, were gener-
ally low (1–2%).