High-pressure IR studies on the asymmetric hydroformylation of styrene catalyzed by Rh(I)-(R,S)-BINAPHOS

Article abstract of DOI:10.1021/om020245f

Rhodium-catalyzed asymmetric hydroformylation of styrene using (R,S)-BINAPHOS (1a) and its methoxy-substituted derivative 1b as chiral ligands was monitored by in situ high-pressure IR. The rate of aldehyde production is given by r_obs = k_obs[Rh]^1.0[styrene]^0.6[1b]^-0.1 P_CO^-0.9 (k_obs is a constant under isothermal conditions). The higher catalytic activity of the Rh-1b catalyst compared with that of Rh-1a is attributed to a higher concentration of the active species, which is generated by dissociation of CO from the major resting state RhH(CO)₂(1) (2). A rhodium carbonyl species 10, the structure of which is unknown, is suggested to be formed from the rhodium hydride RhH(CO)₂(1) (2), especially when the rhodium concentration is high. Complex 10 is less active for hydroformylation and is probably responsible for the loss of %ee observed at high rhodium concentrations.

Full text of DOI:10.1021/om020245f

594
Organometallics 2003, 22, 594-600
High -P r essu r e IR Stu d ies on th e Asym m etr ic
Hyd r ofor m yla tion of Styr en e Ca ta lyzed by
Rh (I)-(R,S)-BINAP HOS
Kyoko Nozaki,*,†,‡ Takeshi Matsuo,§ Fumitoshi Shibahara,‡ and
Tamejiro Hiyama§
Department of Chemistry and Biotechnology, Graduate School of Engineering,
University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656 J apan, Conversion and
Control by Advanced Chemistry, PRESTO, J apan Science and Technology, 2-4-8, Konan,
Minato-ku, Tokyo 108-0075 J apan, and Department of Material Chemistry, Graduate
School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, J apan
Received March 29, 2002
Rhodium-catalyzed asymmetric hydroformylation of styrene using (R,S)-BINAPHOS (1a )
and its methoxy-substituted derivative 1b as chiral ligands was monitored by in situ high-
-0.9
pressure IR. The rate of aldehyde production is given by robs ) kobs[Rh]1.0[styrene]0.6[1b]-0.1PCO
(kobs is a constant under isothermal conditions). The higher catalytic activity of the Rh-1b
catalyst compared with that of Rh-1a is attributed to a higher concentration of the active
species, which is generated by dissociation of CO from the major resting state RhH(CO)2(1)
(2). A rhodium carbonyl species 10, the structure of which is unknown, is suggested to be
formed from the rhodium hydride RhH(CO)2(1) (2), especially when the rhodium concentra-
tion is high. Complex 10 is less active for hydroformylation and is probably responsible for
the loss of %ee observed at high rhodium concentrations.
In tr od u ction
fied system that showed that hydrogenolysis is the rate-
determining step in that system for styrene hydroformy-
lation;4 in situ IR spectroscopy revealed that an acyl-
rhodium complex, Rh(acyl)(CO)4, was the resting state
of the catalyst.4,5,10,11 On the other hand, Bergounhou
reported that the hydroformylation of styrene catalyzed
by Rh/1,2,5-triphenyl-1H-phosphole is first order in both
styrene concentration and hydrogen pressure.12 Van
Leeuwen has reported that, using a rhodium complex
of a monodentate bulky phosphite, the rate-determining
step varies depending on the olefins employed, viz.,
hydrogenolysis for 1-octene and olefin coordination for
cyclohexene,13 whereas the styrene coordination step is
a candidate for the rate-determining step when the
hydroformylation is carried out at high pressure with
the bidentate ligand BDPP.14 Van Leeuwen has also
reported several steps may contribute to the reaction
rate in 1-octene hydroformylation catalyzed by Rh and
a monodentate phosphorus diamide as a ligand.8 Thus,
even with the same catalyst system, the rate-determin-
ing step can vary depending on the substrates or
reaction conditions.15 Previously, we reported mecha-
Optically active aldehydes are versatile intermediates
for the synthesis of many biologically active compounds,
and asymmetric hydroformylation of olefins is one of the
most straightforward synthetic strategies for such
compounds.1 Previously, we reported that the phos-
phine-phosphite (R,S)-BINAPHOS (1a ) is an efficient
chiral modified ligand for rhodium-catalyzed asym-
metric hydroformylation of various olefins.2 Recently,
we reported that substitution at the 3-position of the
phenyls in the phosphine part of 1a with a methoxy (1b)
or an isopropoxy (1c) group raised the regio- and
enantioselectivities in the hydroformylation of aryl-
ethenes and aliphatic alkenes.3 We thus became inter-
ested in how the substituents at the 3-position improved
the catalytic activity and initiated a high-pressure
infrared spectroscopic kinetic study of BINAPHOS/
Rh-catalyzed asymmetric hydroformylation.
High-pressure IR and NMR have been recognized as
powerful means to elucidate the intermediate complexes
under hydroformylation conditions, for both unmodified
Rh-catalysts4,5 and phosphine-modified Rh-catalysts.6-9
Thus, Garland reported kinetic studies on the unmodi-
(7) Die´guez, M.; Claver, C.; Masdeu-Bulto´, A. M.; Ruiz, A.; van
Leeuwen, P. W. N. M.; Schoemaker, G. C. Organometallics 1999, 18,
2107.
(8) van der Slot, S. C.; Kamer, P. C. J .; van Leeuwen, P. W. N. M.;
Iggo, J . A.; Heaton, B. T. Organometallics 2001, 20, 430.
(9) Bianchini, C.; Lee, H. M.; Meli, A.; Vizza, F. Organometallics
2000, 19, 849.
(10) Garland, M.; Pino, P. Organometallics 1991, 10, 1693.
(11) Fyhr, C.; Garland, M. Organometallics 1993, 12, 1753.
(12) Bergounhou, C.; Neibecker, D.; Re´au, R. Bull. Soc. Chim. Fr.
1995, 132, 815.
† University of Tokyo.
‡ PRESTO.
§ Kyoto University.
(1) Botteghi, C.; Schionato, A.; Marchetti, M. Chirality 1991, 3, 355.
(2) (a) Nozaki, K.; Sakai, N.; Nanno, T.; Higashijima, T.; Mano, S.;
Horiuchi, T.; Takaya, H. J . Am. Chem. Soc. 1997, 119, 4413. (b) Nozaki,
K.; Takaya, H.; Hiyama, T. Top. Catal. 1997, 4, 175.
(3) Nozaki, K.; Matsuo, T.; Shibahara, F.; Hiyama, T. Adv. Synth.
Catal. 2001, 343, 61.
(4) Feng, J .; Garland, M. Organometallics 1999, 18, 417.
(5) Liu, G.; Volken, R.; Garland, M. Organometallics 1999, 18, 3429.
(6) Castellanos-Pa´ez, A.; Castillo´n S.; Claver, C.; van Leeuwen, P.
W. N. M.; de Lange, W. G. J . Organometallics1998, 17, 2543.
(13) van Rooy, A.; Orij, E. N.; Kamer, P. C. J .; van Leeuwen, P. W.
N. M. Organometallics 1995, 14, 34.
(14) del R´ıo, I.; Pa`mies, O.; van Leeuwen, P. W. N. M.; Claver, C. J .
Organomet. Chem. 2000, 608, 115.
10.1021/om020245f CCC: $25.00 © 2003 American Chemical Society
Publication on Web 01/07/2003

Asymmetric Hydroformylation of Styrene
Organometallics, Vol. 22, No. 3, 2003 595
Ta ble 1. Asym m etr ic Hyd r ofor m yla tion of Olefin s Ca ta lyzed by Rh (a ca c)(CO)2/1 (1:4)a
run
olefin
ligand
P
H2/PCO (atm)
temp (°C)
time (h)
conv (%)
isoselect. (%)
%ee of iso-
1
2
3
4
5
6
7
8
styreneb
1a
1a
1a
1b
1b
1c
1c
1a
1b
1c
1a
1b
1a
1a
1b
1c
1a
1b
1a
1b
1a f
1cf
10/10
50/50
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
50/50
10/10
10/10
10/10
10/10
16/16
16/16
10/10
10/10
60
60
23
60
23
60
23
60
60
60
30
30
60
60
60
60
60
60
60
60
80
80
20
43
22
20
22
20
22
4
>99
>99
21
>99
34
>99
25
>99
>99
>99
34
66
68
>99
96
92
34
49
88.0
88
93.2 (R)j
94 (R)i
90.6
92.5
95.0
91.1
94.0
83.4
89.4
87.8
24.3
29.8
87.2
86
95.2 (R)j
95.0 (R)j
97.5 (R)j
95.7 (R)j
98.3 (R)j
81.9 (R)
82.3 (R)
93.2 (R)
80.2 (R)j
90.0 (R)j
86.1 (S)
92 (S)i
2-vinylnaphthalenec
1-hexeneb
9
4
4
10
11
12
13
14
15
16
17
18
19
20
21
22
40
40
15
36
15
15
20
20
8
vinyl acetatec
d
c
c
93.7
83.7
89.6
91.3
86.6 (S)
90.2 (S)
83.3 (R)j
88.9 (R)j
82.0 (R)j
89.9 (R)j
54.6 (R)
56.5 (R)
indenee
(Z)-2-butene
(23)g
(7.6)g
57
9
40
40
methyl N-acetamidoacrylate
>99h
>99h
83
a
A solution of styrene (1.04 g, 10.0 mmol), Rh(acac)(CO)2 (1.3 mg, 5.0 µmol), and 1 (15.4 mg, 0.020 mmol) in benzene (0.5 mL) was
treated with carbon monoxide and hydrogen. Substrate/catalyst ) 2000 ([Rh] ) 3.0 mM). c Substrate/catalyst ) 1000 ([Rh] ) 5.0 mM
b
for 2-vinylnaphthalene, 7.0 mM for vinyl acetate). Substrate/catalyst ) 400. e Substrate/catalyst ) 250 ([Rh] ) 7.7 mM). f RhH(CO)(PPh3)3
d
g
was used as a precursor instead of Rh(acac)(CO)2. Substrate/catalyst ) 50 ([Rh] ) 6.7 mM). Turnover frequency ([Rh] ) 0.83 mM).
h
Hydrogenation was observed as a side reaction. Hydroformylation product/hydrogenation product ) 90:10 (run 21) and 92:8 (run 22).
i
j
Data taken from ref 2a. Reported in our preliminary communication, ref 3.
nistic studies on hydroformylation catalyzed by Rh-
(R,S)-BINAPHOS, in which the following two features
were disclosed: (1) the olefin insertion into the rhodium-
hydride bond is irreversible, and (2) the reaction rate
is independent of hydrogen pressure.16 In this paper,
with the aid of high-pressure IR, we further elucidate
mechanistic aspects of the reaction, especially the
formation and decomposition of the active species.
Resu lts a n d Discu ssion s
F igu r e 1. Concentrations of styrene and aldehydes with
Rh(acac)(1b) and RhH(CO)2(1b) () 2b). Conditions: [sty-
rene]0 (initial concentration of styrene) ) 1.0 M, [Rh] )
1.0 mM, [1b] ) 4.0 mM, PH2 ) PCO ) 5 atm (constant), 60
°C. With Rh(acac)(1b): O aldehyde, 4 styrene (plot a). With
RhH(CO)2(1b ) () 2b ) prepared from Rh(acac)(1b ) and
H2/CO at 60 °C for 2 h: b aldehyde and 2 styrene (plot b).
Hyd r ofor m yla tion of Va r iou s Olefin s w ith Rh
Com p lexes of MeO- or i-P r O-Su bstitu ted BINA-
P HOS. Asymmetric hydroformylation of styrene, 2-vi-
nylnaphthalene, 1-hexene, vinyl acetate, indene, (Z)-2-
butene, and methyl N-acetamidoacrylate was examined
using Rh(acac) complexes of BINAPHOS (1a ), 3-MeO-
BINAPHOS (1b), and 3-i-PrO-BINAPHOS (1c) as cata-
lyst precursors. The results are summarized in Table
1. For all of the olefins, the isoselectivity and the %ee
values were higher using 1b and 1c compared to those
obtained using 1a .
Ca ta lyst P r ecu r sor s a n d In d u ction P er iod . Hy-
droformylation of styrene was carried out under H2 (5
atm) and CO (5 atm) at 60 °C, and the reaction was
followed by the decay of the IR absorption at 911 cm-1
(alkene C-H of styrene, out-of-plane deformation vibra-
tion) and by the appearance of the aldehyde, CdO
stretching vibration at 1727 cm-1 (Figure 1, plot a). An
induction period of ca. 20 min was observed. This agrees
with the previous studies that incubation of the catalyst
under a H2/CO atmosphere is necessary in order to form
the key intermediate RhH(CO)2(ligand).8 We next ex-
amined the hydroformylation reaction using preformed
RhH(CO)2(1b) (2b). After treatment of Rh(acac)(1b)
with H2 (5 atm) and CO (5 atm) at 60 °C for 2 h, the
pressure was released, styrene introduced, and the mix-
ture repressurized with H2 (5 atm) and CO (5 atm). The
decrease of the IR absorption of styrene and the increase
of the aldehyde absorptions started immediately after
the reintroduction of H2/CO (Figure 1, plot b).
The formation of the rhodium hydride 2 from
Rh(acac)(1) was confirmed by IR as follows. When a
Rh(acac)(1a ) solution in benzene (50 mM) was treated
with H2 (20 atm) and CO (20 atm) at 40 °C, we observed
the formation of three peaks at 2049, 2014, and 1969
cm-1. The wavenumbers of these peaks are identical
with those of RhH(CO)2(1a ) (2a ) recently reported by
van Leeuwen.17 Similarly, the formation of 2b from Rh-
(acac)(1b) was confirmed by the appearance of absorp-
tion peaks at 2043 (Rh-H), 2013 (Rh-CO), and 1968
(Rh-CO) cm-1 (Figure 2). Under conditions of H2 (20
(15) van der Slot, S. C.; Kamer, P. C. J .; van Leeuwen, P. W. N. M.
Organometallics 2001, 20, 1079.
(16) Horiuchi, T.; Shirakawa, E.; Nozaki, K.; Takaya, H. Organo-
metallics 1997, 16, 2981.
(17) Deerenberg, S.; Kamer, P. C. J .; van Leeuwen, P. W. N. M.
Organometallics 2000, 19, 2065.

596 Organometallics, Vol. 22, No. 3, 2003
Nozaki et al.
aldehyde production rate robs has been studied, and the
results are summarized in Table 3. The hydrogen partial
pressure does not influence the rate (runs 1 and 2, see
also ref 16), while higher carbon monoxide partial
pressures retard the reaction (runs 1 and 3-6). The
reaction rate is calculated to be -0.9 order in CO
pressure. Higher concentrations of the ligand also slow
the reaction (runs 1 and 7-10). The reaction is -0.1
order in ligand concentration. The reaction is first order
in the Rh concentration up to 1.0 mM (runs 1 and 11-
15); however, the reaction rate became constant regard-
less of the Rh concentration above 1.0 mM (runs 16-
18). The isoselectivity and enantioselectivity are constant
in runs 1 and 11-15, whereas, at Rh concentrations
above 1.0 mM, both the isoselectivity and %ee of
isoaldehyde decline. Thus, at higher Rh concentrations,
another, possibly multinuclei,6 species may be formed
that is less active and less selective. A rhodium carbonyl
species, complex 10, has been observed in the decom-
position of rhodium hydride 2 (vide infra). The effect of
styrene concentration has also been investigated. The
H2 and CO pressures were kept constant, and the
reaction was followed by IR, Figure 4a. A plot of
[styrene](1-x)/(1-x) against time is linear when x ) 0.6
(Figure 4b). The selectivity is not affected by the initial
styrene concentration; thus, under otherwise the same
conditions as run 1, an isoselectivity of 91.1% and %ee
of the isoaldehyde of 94.1 and 94.9%, respectively, were
observed when the reaction was started with either 0.10
or 0.010 M of styrene. The fact that the selectivites are
independent of the initial styrene concentration sug-
gests that the active species is the same regardless of
the styrene concentration. In other words, during the
hydroformylation reaction, the active species are not
affected by the styrene consumption, as long as the Rh
concentration is kept below 1.0 mM. Summarizing all
the kinetic data, the hydroformylation rate with 2b at
F igu r e 2. HP IR spectra for the preparation of 2b from
Rh(acac)(1b). Conditions: [Rh(acac)(1b)] ) 50 mM, [free
1b ] ) 50 mM, PH2 ) 20 atm, PCO ) 20 atm, 40 °C. (a)
Rh(acac)(1b) without H2/CO; (b) 0 min with H2/CO; (c) 12
min; (d) 24 min; (e) 36 min; (f) 48 min; (g) 60-96 min
(constant).
atm), CO (20 atm), 40 °C, HPIR revealed that 1 h was
required before the concentration of RhH(CO)2(1b) (2b)
reached a constant value; at lower pressures of H2 (10
atm) and CO (10 atm), 4.6 h was required. The peak at
around 2000 cm-1 has not yet been characterized.
It should be noted that the H2/CO pressure during
the incubation period significantly affects the %ee of the
isoaldehyde, Table 2. Without incubation, hydroformy-
lation under a total pressure of 20 atm (H2/CO ) 1:1)
afforded the isoaldehyde with 94.9% ee (run 1). Expo-
sure to air does not significantly affect the selectivity
(run 2). Similarly, catalysts prepared by incubation of
Rh(acac)(1b) at 40 °C under a total pressure of 20 atm
for 2 h to form the hydride 2b gave selectivities that
are not affected by the conditions of styrene introduc-
tion (compare runs 1 and 2 with runs 3 and 4). On the
other hand, when 2b was prepared under a total
pressure of 70 atm for 2 h, followed by pressure release
to 1 atm prior to styrene introduction, resulted in a
significant loss of %ee, 82.5% (run 5). This drop in
%ee can be avoided by shortening the incubation
time to 0.5 h (run 6), by the introduction of styrene
without pressure release (runs 7 and 8), or by perform-
ing the hydroformylation under higher pressure, 70 atm
(run 9).
Im p r oved R ea ct ion R a t e w it h 3-MeO-BINA-
P H OS (1b ) a n d 3-i-P r O-BINAP H OS (1c) Com -
p lexes. The reaction rates using 2b and RhH(CO)2(1c)
(2c) have been compared with that obtained using 2a
as a catalyst. Before styrene introduction, Rh(acac)(1)
was converted into RhH(CO)2(1) (2) and the resulting
2 used in the hydroformylation reaction under the con-
ditions of run 6 in Table 2. The reactions were monitored
by IR, and the increase in aldehyde concentration was
plotted against time (Figure 3). The initial rates,
expressed as increase of aldehyde concentration per
second, were calculated over the region of conversion
0-40% and found to be robs ) 6.3 × 10-5 (2a ), 14.9 ×
10-5 (2b), and 14.7 × 10-5 M‚s-1 (2c). The observed
aldehyde production rate () styrene decay rate) robs
using 2b and 2c is about twice that using 2a . Thus,
substitution at the 3-position of phenyls in BINAPHOS
improves not only the selectivity but also the catalytic
activity.
60 °C can be described by robs ) kobs[Rh]1.0[styrene]0.6
-
[1b]-0.1PCO-0.9, where kobs is a constant, in good agree-
ment with the equation proposed by van Leeuwen, rate
) A[Rh][alkene]/{B[CO] + C[alkene] + [ligand]}.8 This
equation is appropriate for a reaction in which the
overall rate is determined by the alkene coordination/
hydride migration step. Thus, in the Wilkinson’s dis-
sociative mechanism described in Scheme 2, the reaction
rate is mostly controlled by (1) the equilibrium between
the resting state 2 and its decarbonylated counterpart
3 and (2) the styrene coordination/hydride migration
steps, i.e., 3 to 4 or 5.
Activa tion En er gy for th e Hyd r ofor m yla tion . As
mentioned above, a consideration of the various order
of the reaction allows us to ascribe the rate-determining
step to the equilibrium between 2 and 3 and bimolecular
reaction of 3 with styrene. The temperature dependency
of robs was measured for catalysts 2a and 2b, and the
results are summarized in Table 4. If the temperature
dependency of H2 and CO solubilities in benzene and
the temperature dependency of the equilibrium between
2 and 3 are neglected, the apparent activation energy
Ea from 3 to 4 or to 5 can be estimated from the
Arrhenius plot shown in Figure 5 and is found to be
20.0 kcal/mol for the reaction using 2b and 18.5 kcal/
mol for 2a . These values are comparable to those
previously reported for hydroformylation with other
Kin etic Stu d ies w ith 2b. For the reaction with 2b,
the effect of the concentration of each component on the

Asymmetric Hydroformylation of Styrene
Organometallics, Vol. 22, No. 3, 2003 597
Ta ble 2. Effect of H2/CO Tota l P r essu r e on P r ep a r a tion of 2b, Styr en e In tr od u ction , a n d
Hyd r ofor m yla tion a
H2/CO (1:1) total pressure (atm)
selectivities
isoselectivity %b
run
preparation of 2b
addition of styrene
hydroformylation
%ee of iso
1c
2d
3
20
20
20
20
20
20
20
70
70
93.5
93.3
92.6
92.2
91.5
91.5
93.0
93.3
93.7
94.9
95.3
95.3
95.5
82.5
94.8
95.6
95.0
95.3
20
20
70
70
70
70
70
1
20
1
4
5
6e
7
1
20
70
1
8
9
a
[Rh(acac)(CO)2] ) 1.0 mM, [1b] ) 4.0 mM, [styrene]0 ) 0.010 M, 60 °C in 10 mL of benzene. The reaction was carried out either with
a mixture of Rh(acac)(CO)2 and 1b (runs 1 and 2) or with 2b preformed prior to the introduction of styrene (runs 3-9). For the preparation
of 2b, a mixture of Rh(acac)(CO)2 and 1b was treated with H2/CO under a total pressure of 20 or 70 atm for 2 h at 40 °C unless otherwise
stated. Determined by GC, not by 1H NMR. c Styrene was added to a Rh(acac)(1b) solution under argon. Styrene was added to a
b
d
Rh(acac)(1b) solution under air. e Preparation of 2b for 0.5 h instead of 2 h.
F igu r e 3. Increase of aldehyde concentration in the
asymmetric hydroformylation with 2a -c: b 2a , ] 2b, 2
2c. Conditions: [2] ) 1.0 mM, free 1 ) 3.0 mM, [styrene]0
) 1.0 M, PH2-0 (the initial H2 pressure) ) PCO-0 (the initial
CO pressure) ) 10 atm, 60 °C, benzene solution (total 20
mL). Aldehyde production rates robs ) 6.3 × 10-5 M‚s-1 (2a ),
14.9 × 10-5 M‚s-1 (2b), and 14.7 × 10-5 M‚s-1 (2c) were
calculated in the region of conversion 0-40%.
Ta ble 3. Effect of H2 a n d CO P r essu r es a n d
Rh od iu m a n d Liga n d 1b Con cen tr a tion s on th e
F igu r e 4. (a) Effect of styrene concentration. Conditions:
[2b] ) 1.0 mM, free 1b ) 3.0 mM, [styrene]0 ) 1.0 M, PH2
) PCO ) 10 atm (constant), 60 °C, benzene solution (total
20 mL). For the product, isoselectivity was 91.6% with
94.9% ee. (b) Integration of the equation “-d[styrene]/dt
Rea ction Ra tea
P
H2-0/PCO-0 [Rh]0 [1b]0 isoselect. %ee of robs × 105
run
(atm)
(mM) (mM)
(%)
iso-
(M/s)
1
2
3
4
5
6
7
8
10/10
80/10
10/15
10/20
10/40
10/80
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
10/10
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
4.0
4.0
4.0
4.0
4.0
4.0
6.0
8.0
12.0
16.0
89.2
89.1
91.4
91.4
92.0
91.8
90.2
90.3
89.9
90.3
90.2
90.9
91.6
90.5
89.8
87.4
85.0
77.9
94.8
94.5
95.1
94.7
95.5
95.5
95.2
95.1
94.8
94.8
93.4
95.1
95.3
95.2
94.7
93.4
92.3
87.8
14.9
15.3
6.98
5.39
3.29
1.60
13.8
12.9
12.7
12.3
1.26
2.81
5.42
9.79
15.2
15.4
15.9
13.4
) k[styrene]” provides us with the equation “[styrene]1-x
/
(1 - x) ) -kt +A (A: a constant) (x * 1)”. Shown here is
a linear plot of [styrene]1-x/(1 - x) vs time when x ) 0.6.
Thus, the reaction is regarded as 0.6th order to styrene
concentration.
Obser va tion of th e Rh od iu m Ca r bon yl Sp ecies
10. When a concentrated solution of the rhodium
hydride 2b was treated with a high CO pressure in the
absence of styrene, the IR absorption characteristic of
a Rh-H disappeared. Thus, 2b was prepared by treat-
ment of Rh(acac)(CO)2/1b (1:2, [Rh] ) 50.0 mM in
benzene) with H2/CO (10 atm/10 atm) at 40 °C for 4.6
h. The pressure was then released to 1 atm, and H2
9
1.0
1.0
10
11
12
13
14
15
16
17
18
0.10
0.25
0.38
0.50
0.75
1.5
0.40
1.00
1.50
2.0
3.0
6.0
2.0
4.0
8.0
16.0
(18) Below a CO pressure of 0.5 atm, %ee decreased in our previous
study.16
(19) Bhanage, B. M.; Divekar, S. S.; Deshpande, R. M.; Chaudhari,
R. V. J . Mol. Catal. A 1997, 115, 247.
a
Conditions: [styrene]0 ) 1.0 M, 60 °C, benzene solution (total
20 mL). A reaction rate was measured in conversion 0-40%.
(20) Nair, V. S.; Mathew, S. P.; Chaudhari, R. V. J . Mol. Catal. A
1999, 143, 99.
(21) Kiss, G.; Mozeleski, E. J .; Nadler, K. C.; VanDriessche, E.;
DeRoover, C. J . Mol. Catal. A 1999, 138, 155.
systems, which are in the range 13-22 kcal/mol for
either rate-determining hydrogenolysis or alkene coor-
dination/hydride migration.12,19-21

598 Organometallics, Vol. 22, No. 3, 2003
Nozaki et al.
Sch em e 1
Sch em e 2
Ta ble 4. Rea ction Ra te Con sta n t r obs of th e
Asym m etr ic Hyd r ofor m yla tion of Styr en e u n d er
Va r iou s Tem p er a tu r esa
temp
(°C)
isoselect.
(%)
%ee
of iso
robs × 105
catalyst
(M/s)a
2a
20
30
45
60
20
30
45
60
91.1
89.8
88.4
87.2
95.8
94.3
93.2
89.3
94.6
94.9
93.7
92.5
97.5
96.8
95.7
95.2
0.149
0.413
2.00
6.29
2b
0.254
0.675
3.64
14.9
a
Conditions: [styrene]0 ) 1.0 M, [2b] ) 1.0 mM, [free 1b] )
3.0 mM, PH2 ) PCO ) 10 atm (constant), benzene solution (total
20 mL). A reaction rate was measured in conversion 0-40%.
F igu r e 6. Formation of rhodium carbonyl 10b from 2b.
To a benzene solution of 2b ([2b] ) 50.0 mM, [free 1b] )
50.0 mM) first H2 was introduced to its partial pressure of
10 atm, and to this, CO was added stepwise. The IR
absorption was recorded at the CO partial pressures of 5,
10, 20, and 35 atm. The peaks at 1968 and 2013 cm-1
increased, while that at 2043 cm-1 decreased.
carbonyl-bridged Rh dimer is a possible candidate for
this new species, the low signal/noise ratio did not allow
us to characterize any peaks below 1800 cm-1, i.e., in
the bridging carbonyl region.22,23 We therefore ascribe
the species to the rhodium carbonyl complex 10b of
unknown structure. The conversion of 2b to 10b was
reversible. When a solution of 2b prepared as above
under PH2 ) PCO ) 10 atm was further pressurized with
H2 to PH2 ) 80 atm and PCO ) 10 atm, a slight increase
in the Rh-H absorption was detected. Thus, solutions
of 2b prepared under 20 atm are likely to contain 10b
to some extent, at least at a Rh concentration of 50 mM.
We cannot exclude the possibility that a solution of 2b
might contain some 10b, even at Rh concentrations
below 1.0 mM. However, the fact that the selectivities
are independent of [Rh] below 1.0 mM allows us to
F igu r e 5. Arrhenius plot for the reactions with (- - -) 2a
and (s) 2b. When the equation ln k ) (-Ea/R)(1/T) + ln A
(A: frequency factor, Ea: activation energy, R ) 1.987 cal/
mol‚K) is applied, with 2a : ln k ) -9.32(1/T) + 25.3, Ea )
18.5 kcal/mol; and with 2b: ln k ) -10.0(1/T) + 28.2, Ea
) 20.0 kcal/mol.
introduced to a pressure of 10 atm. To this, CO was
added stepwise. The IR spectrum was recorded at CO
partial pressures of 5, 10, 20, and 35 atm, Figure 6. The
peak at 2043 cm-1 (Rh-H) decreased with increasing
P
CO, while the peaks at 1968 (Rh-CO) and 2013
(22) J ames, B. R.; Mahajan, D.; Rettig, S. J .; Williams, G. M.
Organometallics 1983, 2, 1452.
(23) Moasser, B.; Gladfelter, W. L. Organometallics 1995, 14, 3832.
(Rh-CO) cm-1 increased. The absorptions changed
immediately after each pressure change. Although a

Asymmetric Hydroformylation of Styrene
Organometallics, Vol. 22, No. 3, 2003 599
F igu r e 7. Time dependency of absorbance of (b) aldehydes
ν(CO) 1726 cm-1 (plot a), (2) ν(Rh-H) at 2044 cm-1 (plot
b), (4) ν(CO) at 2013 cm-1 (plot c), and (O) ν(CO) at 1969
cm-1 (plot d) for the real time observation of asymmetric
hydroformylation of styrene with Rh(acac)(1a ). Condi-
tions: [Rh(acac)(1a )] ) 34.2 mM, [free 1a ] ) 34.2 mM,
[styrene]0 ) 2.74 M, PH2 ) PCO ) 20 atm (constant), in
benzene (total 14.6 mL), at 40 °C. For the y axis, the scale
on the left side corresponds to aldehyde ν(CO) and the right
side corresponds to either ν(Rh-H) or ν(Rh-CO).
F igu r e 8. Time dependency of absorbance of aldehydes
(ν(CO): 1726 cm-1) with (O) 2a (plot aa) and (b) 2b (plot
bb); and absorbance of 2(ν(Rh-H)) of (2) 2a [ν(Rh-H]] at
2046 cm-1, plot a] and (4) 2b [ν(Rh-H]] at 2044 cm-1, plot
b]. For the y axis, the scale on the left side corresponds to
aldehyde ν(CO) and the right side corresponds to ν(Rh-
H). Applied conditions are [“8”] ) 40.7 mM, [free 1] ) 40.7
mM, [styrene]0 ) 1.63 M, PH2 ) PCO ) 20 atm (constant),
in benzene (total 12.3 mL), at 40 °C.
estimate that, even if it is present, the contribution of
10b to the reaction is not significant at Rh concentra-
tions below this level.
however, the low signal/noise ratio of the spectra did
not allow us to measure the region below 1800 cm-1 to
confirm the presence of acyl carbonyl absorptions.
Although the acyl complex 8b has not been fully
characterized, this mixture was employed as a catalyst
precursor and was pressurized with H2 to initiate the
reaction. The production and decay of the resting state
2b were observed as described above, Figure 8, plot b.
A similar behavior is observed for 2a starting from 8a ,
Figure 8, plot a. As a reference, aldehyde formation is
also plotted in the same graph, plot aa for the reaction
using 8a and 8b (bb). It should be noted that the highest
concentration of 2b is 2.5 times greater than that of 2a ,
indicating that the concentration of the active species
is 2.5 times higher with 1b than with 1a . This difference
in the position of the equilibrium between 2 and 10 is a
possible explanation for the faster reaction with 2b than
with 2a , even under low Rh-concentration conditions.
Rea l Tim e Obser va tion of Styr en e Hyd r ofor m y-
la tion . The asymmetric hydroformylation of styrene
with Rh(acac)(CO)2/1a was followed by HPIR. In the
presence of Rh(acac)(CO)2/1a (1:2), a benzene solution
of styrene was treated with H2 and CO at 40 °C. The
intensities of the absorptions due to aldehydes ν(CO)
at 1726 cm-1, ν(Rh-H) at 2044 cm-1, ν(Rh-CO) at 2013
cm-1, and ν(Rh-CO) at 1969 cm-1 are plotted in Figure
7 against reaction time. Because of the limited signal/
noise ratio, this experiment was carried out at a high
concentration of Rh (34.2 mM) compared to the standard
hydroformylation reaction (cf. Table 3, runs 1 and 11-
18). The absorption at 2044 cm-1 is due to ν(Rh-H) of
2a , and those at 2013 and 1969 cm-1 are attributable
to the sum of 2a and 10a (vide supra). Here we remark
on the following two features: (1) When the aldehyde
production rate is greatest, the concentration of 2a
reaches its highest value. Thus, apparently, the con-
centration of the hydride complex 2a reflects the concen-
tration of the active species. (2) When styrene is com-
pletely consumed, the absorptions of the hydride com-
plex 2a become almost invisible and the rhodium
carbonyl species 10a becomes the only detectable Rh
complex. The following two explanations are possible:
(1) As the olefin concentration decreases, the concentra-
tion of coordinatively unsaturated 3a increases. 3a
might then react with 2a to give 10a . (2) Slow sponta-
neous formation of 10a occurs. In this a case, 2a is
presumably first formed and then converted to 10a
during the reaction.
Con clu sion
Introduction of MeO and i-PrO groups into (R,S)-
BINAPHOS results in the acceleration of the reaction
as well as an improvement in the regio- and enantiose-
lectivities. Kinetic studies have clarified the detailed
mechanism; in particular, the equilibrium between 2
and 3 and styrene coordination/hydride migration con-
trol the reaction rate. The faster reaction using 1b is
attributed to the higher concentration of 3 (reflected by
2) under the catalytic conditions. The steric repulsion
between the MeO group and the carbonyl might be
responsible for either the acceleration of CO dissociation
from 2 or the deceleration of CO re-coordination to 3;
however, the possibility of an electronic effect cannot
be excluded. More importantly, the existence of a
rhodium carbonyl species 10 at Rh concentrations above
1.0 mM is associated with a decline in both activity and
enantioselectivity, compared with 2, in the asymmetric
hydroformylation of styrene. The concentration of 10
was observed to increase as the olefin is consumed by
real-time in situ IR spectroscopy, especially when high
concentrations of rhodium are used.
As another species to start the hydroformylation, we
attempted to prepare the acyl complex 8b. The infrared
absorption of 2b at 2043 cm-1 (Rh-H stretching)
disappears immediately when styrene (1.0 M) and CO
(10 atm) were added to a solution of rhodium hydride
2b (40.7 mM) in that absence of H2, suggesting the
styrene insertion to the Rh-H bond occurred. Three new
absorptions appeared at 2016, 1972, and 1960 cm-1
.
Similarly, peaks were observed at 2011, 1971, and 1961
cm-1 for the analogous reaction with 2a . We attribute
these absorptions to the acyl complexes 8b and 8a ;

600 Organometallics, Vol. 22, No. 3, 2003
Nozaki et al.
benzene (0.3 mL) was treated with carbon monoxide (10 atm)
and hydrogen (10 atm) at 60 °C for 20 h, conversion of styrene
was 33%, suggesting the slower reaction in the absence of 1a .
The regioselectivity was 2-phenylpropanal/3-phenylpropanal
) 68.6:31.4. At 25 °C for 22 h in the absence of 1a , the
conversion of styrene was 0.3%.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations involving air- and
moisture-sensitive compounds were carried out using standard
Schlenk techniques under argon purified by passing through
a hot column packed with BASF-Catalyst R3-11. The in situ
infrared spectra were recorded on an ASI, REACT IR 1000
equipped with a silicon probe. All NMR spectra were measured
using a Varian Mercury 200 (1H 200 MHz; 31P 81 MHz)
spectrometer. Tetramethylsilane (1H) and H3PO4 (31P) were
employed as internal and external standards, respectively. Gas
chromatographic (GLC) analyses were conducted on a Shi-
madzu GC-2010 equipped with a flame ionization detector.
Optical rotation was measured on a J ASCO DIP-360. Melting
points were measured on a Yanagimoto-Seisakusho micro
melting point apparatus, MD-500D. Elemental analyses were
performed at the Microanalytical Center, Kyoto University.
Most reagents were purchased from Wako Pure Chemical
Industries Ltd., or Aldrich Chemical Co., Inc., and benzene
was purified by distillation over sodium benzophenone ketyl
under argon. Styrene, vinyl acetate, 1-hexene, and indene were
purified by distillation in vacuo. Carbon monoxide, hydrogen,
and carbon monoxide/hydrogen gas mixture (50.1%/49.9%)
were obtained from Teisan CO.
Gen er a l P r oced u r e for th e Asym m etr ic Hyd r ofor m y-
la tion of Olefin s Ca ta lyzed by Rh od iu m (I) Com p lexes
of BINAP HOS Der iva tives. A procedure similar to our
previous report was employed.2 A solution of styrene (1.04 g,
10.0 mmol), Rh(acac)(CO)2 (1.3 mg, 5.0 µmol), and 1a (15.4
mg, 0.020 mmol) in benzene (0.5 mL) was treated with carbon
monoxide (10 atm) and hydrogen (10 atm) at 60 °C for 20 h.
Conversion to aldehydes (>99%) and the regioselectivity of the
reaction (2-phenylpropanal/3-phenylpropanal, 88.0:12.0) were
determined by 1H NMR spectroscopy of the crude reaction
mixture without evaporation of the solvent. The enantiomeric
excess of the 2-phenylpropanal was determined to be 93.2%
by oxidation of 2-phenylpropanal to the corresponding car-
boxylic acid. An aliquot of the aldehydes mixture was dissolved
in acetone, and to this, J ones reagent (CrO3/H2SO4, [Cr] ) 2.67
M) was added until the color does not change from orange to
green. After extraction, the ee of carboxylic acid was deter-
mined by GLC analysis using a chiral capillary column
(Chirasil-DEX CB, 0.25 mm × 25 m, 150 °C, He 1.4 kg‚cm-2).
Other olefins were subjected to hydroformylation by the same
procedure. The ee’s of 2-methylheptanal, 2,3-dihydro-1H-
indene-1-carboxaldehyde, and 2-methylbutyraldehyde were
determined by J ones oxidation followed by GLC analysis using
the same capillary column (110, 150, and 90 °C, respectively).
The ee of 3-acetoxy-2-methylpropanal was determined by GLC
analysis using the same capillary column without oxidation
(50 °C). The ee of R-methyl-2-naphthaleneacetaldehyde and
N-acetyl-2-methyl-3-oxo-D-alanine methyl ester was deter-
mined by 1H NMR using Eu(hfc)3. As a reference, blank
experiments were performed as follows. When a solution of
styrene (6 mmol) and Rh(acac)(CO)2 (0.8 mg, 3 µmol) in
Gen er a l P r oced u r e for Kin etics Stu d ies. A solution of
Rh(acac)(CO)2 (5.4 mg, 0.020 mmol) and 1b (66.3 mg, 0.080
mmol) in benzene (10 mL) was degassed by freeze-pump-
thaw cycles and transferred into a 100 mL stainless steel
autoclave equipped with a silicon IR probe and a magnetic
stirrer. Carbon monoxide (35 atm) and hydrogen (35 atm) were
charged, and the solution was stirred at 60 °C for 30 min with
an agitation speed of 800 rpm. After depressurization, a
degassed mixture of styrene (2.08 g, 20 mmol) and benzene
(total 10 mL) was added. The autoclave was put under a
pressure of carbon monoxide (10 atm) and hydrogen (10 atm)
at 40 °C stirring with 800 rpm, and IR spectra were recorded.
Recording was started 30 s after the gas was added. To
calculate the reaction rate, the intensity of the absorption peak
at 1726 cm-1 was converted to the aldehyde concentration on
the basis that the saturated absorption corresponds to 1.0 M
(100% conversion to aldehyde).
Gen er a l P r oced u r e for th e Obser va tion of Rh Com -
p lexes. In a 100 mL stainless steel autoclave equipped with
a silicon probe and a magnetic stirrer, a solution of Rh(acac)-
(CO)2 (129 mg, 0.50 mmol) and 1b (829 mg, 1.0 mmol) in
benzene (10 mL) was pressurized with carbon monoxide (20
atm) and hydrogen (20 atm), and the solution was stirred with
an agitation speed of 800 rpm at 40 °C. Recording of IR spectra
was started when gases were added.
P r ep a r a tion of P h osp h in e-P h osp h ite Liga n d s 1b a n d
1c. Ligands are readily accessible from commercially available
aryl bromides, 3-methoxybromobenzene, and 3-(2-propoxy)-
bromobenzene for 1b and 1c, respectively, according to the
reported procedures.2 1b: white solid, mp 165.1-166.4 °C
(benzene). 31P NMR (CDCl3): δ -11.8 (phosphine, J P-P ) 38.1
Hz), 145.0 (phosphite); [R]25D 163 (c 0.48, CH2Cl2). Anal. Calcd
for C54H38O5P2: C, 78.25; H, 4.62. Found: C, 77.95; H, 4.73.
1c: white solid, mp 174.5-176.0 °C (benzene). 31P NMR
(CDCl3) δ -12.0 (phosphine, J P-P ) 45.8 Hz), 145.2 (phosphite);
[R]25 133 (c 0.52, CH2Cl2). Anal. Calcd for C58H46O5P2: C,
D
78.72; H, 5.24. Found: C, 78.47; H, 5.42.
Ack n ow led gm en t. We are grateful to Professor J .
A. Iggo (University of Liverpool) for helpful suggestions.
Also, we thank Professor K. Oshima (Kyoto University)
for allowing us to use the React IR machine.
Su p p or tin g In for m a tion Ava ila ble: Original plots of
reaction rate varying H2 and CO pressures, Rh and ligand
concentrations, and temperature. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM020245F