Asymmetric hydroformylation of styrene catalyzed by platinum complexes of chiral diphosphites with atropisomeric terminal moieties

Article abstract of DOI:10.1139/v01-085

To study the nature of steric and electronic effects of diphosphites, a novel diphosphite was synthesized and tested with regard of its catalytic performance in the hydroformylation of styrene catalyzed by platinum complexes. The novel ligand 3 was prepar

Full text of DOI:10.1139/v01-085

725
Asymmetric hydroformylation of styrene catalyzed
by platinum complexes of chiral diphosphites with
atropisomeric terminal moieties
J. Bakos, S. Cserépi-Szács, Á. Gömöry, C. Hegedüs, L. Markó, and Á. SzöllÅsy
Abstract: To study the nature of steric and electronic effects of diphosphites, a novel diphosphite was synthesized and
tested with regard of its catalytic performance in the hydroformylation of styrene catalyzed by platinum complexes.
The novel ligand 3 was prepared by the reaction of enantiomerically pure pentane-2,4-diol with (S)-2-chloro-
5,5′,6,6′,7,7′,8,8′-octahydro-dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepine based on H8-BINOL (H8-BINOL =
5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-bi-2-naphthol). For a comparative study, the BINOL (1,1′-bi-2-naphthol) based analogue
4 of 3 has also been synthesized. The highest chemoselectivity to aldehyde (71%) and regioselectivity to branched al-
dehyde (85%) with an enantiomer excess (ee) of 86% was obtained with the platinum(II)–SnCl2 catalytic system asso-
ciated with (2S,4S)-bis(S)-3.
Key words: asymmetric hydroformylation, chirality, homogeneous catalysis, P ligands, platinum.
Résumé : Dans le but d’étudier la nature des effets stériques et électroniques des diphosphites, on a synthétisé un nou-
veau diphosphite et on a évalué sa performance catalytique dans l’hydroformylation du styrène catalysée par des com-
plexes de platine. On a préparé le nouveau ligand 3 par réaction du pentane-2,4-diol énantiomériquement pur avec de
la (S)-2-chloro-5,5′,6,6′,7,7′,8,8′-octahydrodinaphto[2,1-d:1′,2′-f][1,3,2]dioxaphosphépine préparé à partir de H8-BINOL
(H8-BINOL = 5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binapht-2-ol). Pour fins d’études comparatives, on a aussi synthétisé
l’analogue 4 du composé 3, préparé à partir du BINOL (1,1′-binapht-2-ol). Les niveaux les plus élevés de chimiosélec-
tivité pour conduire à l’aldéhyde (71%) et de régiosélectivité pour conduire à l’aldéhyde ramifié (85%) avec un ee de
86% ont été obtenus avec un système platine(II)-SnCl2 associé au (2S,4S)-bis(S)-3.
Mots clés : hydroformylation asymétrique, chiralité, catalyse homogène, ligands phosphorés, platine.
[Traduit par la Rédaction] Bakos et al. 730
Introduction
importance of applying phosphite (5) and phosphine-
phosphite (6) modified systems is increasing. For example,
rhodum complexes of the chiral phosphinephosphite ligand,
(R)-2-(diphenylphosphino)-1,1′-binaphthalene-2′-yl-(S)-1,1′-
binaphthalene-2,2′-diyl phosphite ((R,S)-BINAPHOS) cata-
lyze asymmetric hydroformylation of styrene with an out-
standing level of regio- and enantio-control (94% ee,
iso:normal = 88:12). The catalyst system was also effective
for a variety of other alkenes.
Surprisingly, however, there have been only a few reports
on the application of bisphosphite-Pt(II)–SnCl2 complexes in
asymmetric hydroformylation (7). We have already reported
that in the asymmetric hydroformylation of styrene by Pt-
complexes containing chiral diphosphites with terminal
atropisomeric substituents, the handiness of the product is
dictated by the configuration of the stereogenic carbons of
Homogeneously catalyzed asymmetric hydroformylation
represents a rapidly growing field because of its industrial
potential and academic challenge. Applying this synthetic
method aldehydes of great value can be obtained, which are
attractive building blocks in organic synthesis (1). High ac-
tivity and stability of the catalyst, high enantiomeric excess
(ee), and an almost exclusive chemoselectivity to aldehydes
and regioselectivity to branched aldehydes are required for
an efficient catalytic system. These challenges prompted us
to continue our research in the field of asymmetric
hydroformylation by matching the structure of the ligand
(for recent reviews, see ref. 2).
Although the investigation of the phosphine-modified rho-
dium (3) and platinum (4) systems is still overwhelming, the
Received September 5, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on June 30, 2001.
Dedicated to Professor Brian James on the occasion of his 65th birthday.
J. Bakos1 and C. Hegedüs. Department of Organic Chemistry, University of Veszprém, H-8201 Veszprém, P.O. Box 158,
Hungary.
S. Cserépi-Szács and L. Markó. Research Group for Petrochemistry, Hungarian Academy of Science, H-8201 Veszprém, P.O. Box
158, Hungary.
Á. Gömöry. Chemical Research Center, H-1027 Budapest, P.O. Box 5967, Hungary.
Á. SzöllÅsy. Department of General and Analytical Chemistry, Technical University of Budapest, H-1521 Budapest, Hungary.
1Corresponding author (telephone: (36) 88 422022 (4355); fax: (36) 88 427492; e-mail: bakos@almos.vein.hu).
Can. J. Chem. 79: 725–730 (2001)
DOI: 10.1139/cjc-79-5/6-725
© 2001 NRC Canada

726
Can. J. Chem. Vol. 79, 2001
the chiral backbone (7). Thus, for example, by using
(2S,4S)-bis[((S)-dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphe-
pin-2-yl)oxy]pentane 4 (matched structure) as the ligand re-
markably high catalytic activity and enantioselectivity
(91% ee in CH2Cl2 at 17°C) were achieved, however, the
hydrogenation activity of this heterobimetallic Pt–Sn cata-
lytic system was still rather high (7a). In contrast to this, the
use of the diastereomeric pair of 4, (2R,4R)-bis(S) (mis-
matched constellation), leads to the opposite prevailing con-
figuration of the product with a considerably lower
asymmetric induction.
Materials and methods
Enantiomerically pure pentane-2,4-diol was synthesized
by asymmetric hydrogenation of 2,4-pentanedione over
Raney nickel catalyst modified with the corresponding enan-
tiomer of tartaric acid and NaBr (TA-NaBr-MRNi) (8). 2,2′-
Dihydroxy-1,1′-binaphthyl (BINOL) was obtained from
Merck–Schuchardt. 2,2′-Dihydroxy-5,5′,6,6′,7,7′,8,8′-octahydro-
1,1′-binaphthyl (H8-BINOL) was prepared according to the
literature procedure (9).
Catalytic experiments
We have established that phosphites with enhanced π-ac-
ceptor ability give higher hydrogenation activity. The best
chemoselectivity for hydroformylation was achieved when
bisphosphites, based on the pentanediol backbone associated
with dioxaphosphorinane terminal units, were used in the
hydroformylation of styrene catalyzed by platinum com-
plexes (7b). Based on these findings, we set up our hypothe-
sis that retaining the appropriate structure of 4 and
decreasing the π-acceptor ability of the terminal groups
could improve chemoselectivity and regioselectivity and si-
multaneously retain high enantioselectivity. With this con-
sideration in mind, we synthesized a new diphosphite 3 and
the previously described diphosphite 4 (7a).
This paper describes the preparation and the catalytic
properties of the chiral bisphosphites 3 and 4 in asymmetric
hydroformylation of styrene. Both diphosphites 3 and 4 bear
two methyl groups on the backbone and two atropisomeric
1,1′-bitetralin or 1,1′-binaphthyl frameworks on the phospho-
rus atoms.
In a typical experiment, Pt(PhCN)2Cl2 (0.0125 mmol),
ligand (0.014 mmol), SnCl2 (0.0125 mmol), CH2Cl2 (2 mL),
and then styrene (25 mmol) (for a substrate:catalyst mol ra-
tio of 5000:1, 62.5 mmol of styrene was used) and decane
(5 mmol) (as an internal standard) were placed into a
Schlenk-tube under argon.
A stainless-steel autoclave
(20 mL) was filled with this solution and then purged with
syn gas (CO:H2 (1:1)) and pressurized to the appropriate ini-
tial pressure with this gas mixture. At the end of the reaction
the autoclave was cooled and depressurized. The reaction
mixture was directly vacuum distilled to remove the catalyst.
The reaction mixture and the distilled products were ana-
lyzed by gas chromatography. The enantiomeric excess of
the aldehyde was determined by GC analysis of the distilled
product (Hewlett–Packard HP 4890 gas chromatograph,
split–splitless injector, β-DEX 225, 30 m, internal diameter
0.25 mm, film thickness 0.25 µm, carrier gas: 100 kPa nitro-
gen, FID detector; the retention times of the enantiomers are
30.5 min (R), 32.1 min (S)). The configuration of the pre-
vailing enantiomer in the product was determined by the
sign of optical rotation of the corresponding aldehyde. Con-
versions and composition of the reaction mixture (5:6:7,
branched:linear:hydrogenated) were determined by GC
(SPB-1) using decane as an internal standard.
Experimental
General
Preparation of (S)-2-chloro-5,5′,6,6′,7,7′,8,8′-octahydro-
1,1′-dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepine ((S)-2)
All reaction were carried out in oven-dried glasswork us-
ing Schlenk techniques under an atmosphere of argon. Ben-
zene and toluene were distilled from sodium–benzophenone.
Triethylamine and dichloromethane were distilled from
CaH2 and stored under an atmosphere of argon. Infrared (IR)
spectra were recorded on a Specord IR-75A spectrometer.
31P (202.45 MHz), 13C (125.77 MHz), and 1H (500.13 MHz)
NMR spectra were obtained on a Bruker DRX-500 or on
Varian Unity 300 spectrometer (121 MHz for 31P, 75.4 MHz
(S)-H8-BINOL
1 (4.2 g, 14 28 mmol) was dried
azeotropically with toluene (3 × 20 mL), and then the sol-
vent was evaporated. The residue was dissolved in dry THF
(50 mL) and added slowly (30 min) to a solution of PCl3
(1.6 mL, 18.6 mmol) and Et3N (4.4 mL, 31.4 mmol) in THF
(20 mL) at 0°C. The reaction mixture was stirred overnight
at room temperature. The formed amine salts were removed
by filtration. Evaporation to dryness gave the pale yellow
phosphorochloridite, which was washed with ether (2 ×
30 mL) to give the product as a white solid (4.92 g, 95.9%).
This product was used without any further purification. mp
109–111°C, [α]2D0 = +391.52° (c = 1.18, CH2Cl2). EI-MS
for 13C, and 300.15 MHz for H). Optical rotations were
1
measured on Schmidt Haensch 21245. Gas chromatographic
analyses were run on a Hewlett–Packard 5830A gas
chromatograph (SPB-1 30 m column, film thickness 0.1 µm,
carrier gas 2 mL min–1). Mass spectrometric measurements
were performed using a PE SCIEX API 2000 triple
quadrupole instrument. Electrospray ionization (ES) was
used and positive ions were detected. Samples were dis-
solved in acetonitrile. LiI was added to the solution to obtain
abundant adduct ion with the molecule, because protonoted
molecular ions were observed only in very low abundance.
Mass spectrum of (S)-2 was obtained using a VG ZAB2-
SEQ instrument with electron impact ionization (EI). Elec-
tron energy was 70 eV. The sample was introduced via a di-
rect inlet system.
1
m/z: 358 (M+). H NMR (CDCl3, 300.15 MHz) δ: 1.48–1.64
(m, 2H, CH2), 1.70–1.85 (m, 6H, CH2), 2.26 (dt, J =
16.6, 6.4 Hz, 2H, CH2), 2.58–2.70 (m, 2H, CH2), 2.76–2.88
(m, 4H, CH2), 6.91 and 7.01 (d, J = 8.3 Hz, 2H, H3, H3′),
7.09 (d, J = 8.3 Hz, 2H, H4, H4′). 13C NMR (CDCl3,
75.42 MHz) δ: 23.00 (s, CH2), 23.06 (s, CH2), 23.15 (s,
CH2), 23.24 (s, CH2), 28.5 (s, CH2), 29.8 (s, CH2), 29.9 (s,
CH2), 119.42 (d, JP,C = 15.1 Hz), 119.42 (d, JP,C = 18.1 Hz),
128.3 (d, JP,C = 3.0 Hz), 129.8 (d, JP,C = 5.0 Hz), 129.9 (s),
130.5 (s), 135.7 (d, JP,C = 2.0 Hz), 136.7 (d, JP,C = 2.0 Hz)
139.5 (d, JP,C = 2.0 Hz), 145.9 (d, JP,C = 5.0 Hz) 147.4 (d,
© 2001 NRC Canada

Bakos et al.
727
JP,C = 3.0 Hz). 31P{1H} NMR (CDCl3, 121.42 MHz) δ:
171.5 (s).
Results and discussion
H8-BINOL 1 can be readily derived from BINOL using
the protocol of Cram et al. (9). The reaction of racemic H8-
BINOL with PCl3 in the presence of triethylamine in THF
gave phosphorochloridite, (S,R)-2-chloro-5,5′,6,6′,7,7′,8,8′-
octahydro-dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepine
((S,R)-2). The 31P{1H} NMR spectrum of the compound
showed one absorption at 171.5 ppm, which is in the region
expected for a chlorodioxaphosphepine. It is noteworthy that
there is a substantial upfield 31P chemical shift relative to the
BINOL-based chlorophosphepine (179.0 vs. 171.5 ppm,
∆δ = 7.5 ppm) (10). A similar upfield shift was observed
when the chemical shift of phosphorochloridite based on
2,2′-bisphenol was compared to the value of that based on
2,2′-bisphenol bearing bulky tert-butyl substituents at the
ortho and para positions (180.2 vs. 172.1 ppm) (5b).
Two equivalents of the racemic chloro-compound (S,R)-2
were reacted with optically pure (2S,4S)-pentane-2,4-diol in
the presence of triethylamine to give a diastereomeric mix-
ture of 3 (Scheme 1). In the 31P{1H} NMR spectrum of the
product at ambient temperature, six lines are observed with
relative intensities and splittings consistent with the expected
three diastereomers. An obvious explanation is that a pair of
doublets arises from the diastereomer, (2S,4S)-(S,R) =
(2S,4S)-(R,S). The absolute configuration of the
diastereomers is given in a shorthand notation as (2S,4S)-
(R,S), (2S,4S)-bis(R), and (2S,4S)-bis(S). Where (2S,4S) re-
fer to the absolute configurations of carbon atoms C(2) and
C(4) in the pentane-2,4-diol backbone, and (R) or (S) refer
to the configuration of the axial chirality. The 2D
homonuclear 31P COSY NMR spectrum of the product con-
firms the existence of coupling (6JP,P = 8.8 Hz) between the
P atoms (δ 140.9 and 145.7 ppm) six bonds apart (11). Two
singlets (δ 146.2 for (2S,4S)-bis(S)-3 and 141.1 for (2S,4S)-
bis(R)-3) are attributed to the presence of the other two
diastereomers. This was supported by the NMR spectrum of
the optically pure diastereomer, (2S,4S)-bis(S)-3 obtained
from (S)-2 and (2S,4S)-pentane-2,4-diol. In this case the
other diastereomers were not detected by 31P{1H} NMR, im-
plying that diastereomer 3 was obtained in optically pure
form.
The “like” (S,S,S) diastereomers of 3 and 4 give 31P reso-
nances at 146.2 and 153.6 ppm, respectively. The “unlike”
(S,S,R) diastereomers of 3 and 4 give signals at 141.1 and
147.1 ppm, respectively. In both cases there are substantial
upfield shifts relative to the BINOL based bisphosphite
(146.2 to 153.6 ppm, ∆δ = 7.4 ppm and 141.6 to 147.1 ppm,
∆δ = 6.0 ppm), which might be primarily due to the change
of the electronic character of the phosphorus, rather than to
an increase of strain of the partially hydrogenated
dioxaphosphepine ring.
Phosphites 3 and 4 form phosphite–platinum complexes.
For example, treatment of 4 with PtCl2(PhCN)2 in CD2Cl2
resulted in displacement of PhCN by the ligand. The
31P{1H} NMR spectrum of PtCl2[(2S,4S)-bis(S)-4] consists
of a singlet supplemented with J 195Pt–31P satellites (δ =
87.5, JPt,P = 5672 Hz). The phosphorus resonance is
66.1 ppm upfield from that of the free ligand (153.6 ppm).
From the magnitude of the coordination shift and the cou-
pling constant (JPt,P) a cis P-Pt-P disposition in a square pla-
Preparation of (2S,4S)-2,4-bis[(S)-5,5′,6,6′,7,7′,8,8′-
octahydro-dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphos-
phepin-2-yloxy]-pentane ((2S,4S)-bis(S)-3)
To a solution of 2 (2.00 g, 5.57 mmol) and Et3N
(2.25 mL, 16.0 mmol) in toluene (20 mL) was added slowly
a
solution of optically pure (S,S)-pentane-2,4-diol
(281.0 mg, 2.7 mmol, azeotropically dried with toluene, 3 ×
3 mL) in toluene (25 mL) at 0°C. The reaction mixture was
stirred for 30 min at 0°C and then at room temperature over-
night. The formed hydrochloride salt was removed by filtra-
tion. Evaporation to dryness gave a pale yellow foam, which
was washed with ether (2 × 30 mL) to give the product as a
white solid (1.92 g, 95.0%). mp 158–160°C, [α]D20
=
+269.23° (c = 1.04, CH2Cl2). EI-MS m/z: 755 ([M + Li]+).
1H NMR (CDCl3, 500.13 MHz) δ: 1.38 (d, J = 6.3 Hz, 6H,
CH3), 1.56–1.65 (m, 4H, CH2), 1.76 (m, 2H, CH2), 1.79–
1.86 (m, 12H, CH2), 2.29–2.36 (m, 4H, CH2), 2.66–2.74 (m,
4H, CH2), 2.77–2.89 (m, 8H, CH2), 4.65 (m, 2H, CH) 6.88
(d, J = 8.2 Hz, 2H, CH), 7.05 (d, J = 8.2 Hz, 2H, CH) 7.06
(d, J = 8.2 Hz, 2H, CH), 7.09 (d, J = 8.2 Hz, 2H, CH). 13C
NMR (CDCl3, 125.75 MHz) δ: 22.91 (s), 22.95 (s), 23.10
(s), 23.16 (s), 23.76 (s), 28.21 (s), 29.61 (s), 29.65 (s), 46.60
(m, CH2), 69.00 (t, JP,C = 12.1 Hz, CH), 119.20 (s), 119.60
(s), 125.70 (s), 128.50 (s), 128.60 (s), 129.3 (s), 129.40 (s),
129.70 (s), 129.90 (s), 131.20 (s), 134.10 (s), 134.90 (s),
137.50 (s), 137.90 (s), 138.60 (s), 146.40 (s) 146.90 (s).
31P{1H} NMR (CDCl3, 202.45 MHz) δ: 146.2 (s).
(S)-2-chloro-dinaphhtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepine
This compound was prepared according to the literature
procedure (10). mp 113–115°C, [α]2D0 = +709.25° (c = 1.135,
CH2Cl2).
(2S,4S)-2,4-bis[(S)-dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxa-
phosphepin-2-yloxy]-pentane ((2S,4S)-bis(S)-4)
To
a solution of (S)-2-chloro-dinaphhtho[2,1-d:1′,2′-
f][1,3,2]-dioxaphosphepine (5.3 g, 15.11 mmol) and Et3N
(6.3 mL, 16.0 mmol) in toluene (10 mL) was added slowly a
solution of optically pure (S,S)-pentane-2,4-diol (0.755 g,
7.25 mmol, azeotropically dried with toluene, 3 × 8 mL) in
toluene (30 mL) at 0°C. The reaction mixture was stirred for
30 min at 0°C and then at room temperature overnight. The
formed hydrochloride salt was removed by filtration. Evapo-
ration to dryness gave a pale yellow residue, which was
washed with ether (2 × 20 mL) to give the product as a
white solid (5.2 g, 97.9%). mp 100–102°C, [α]2D0 = +509.43°
1
(c = 1.06, CH2Cl2). EI-MS m/z: 739 ([M + Li]+). H NMR
(CDCl3, 500.13 MHz) δ: 1.43 (d, J = 6.3 Hz, 6H, CH3), 1.78
(m, 2H, CH2), 4.80 (m, 2H, CH), 7.19–7.34 (m, 4H, CH),
7.41–7.50 (m, 10H, CH), 7.60 (d, J = 8.8 Hz, 2H, CH),
7.94–8.01 (m, 8H, CH). 13C NMR (CDCl3, 125.75 MHz) δ:
23.85 (s, CH3), 46.50 (m, CH2), 69.71 (t, JP,C = 10.9 Hz,
CH), 122.30 (s), 122.40 (s), 123.40 (s), 124.80 (s), 125.20
(s), 125.3 (s), 125.70 (s), 126.50 (s), 126.60 (s), 127.50 (d,
JP,C = 3.6 Hz), 128.70 (s), 128.80 (s), 129.40 (s), 130.10 (s),
130.70 (s), 131.60 (s), 132.00 (s), 133.10 (s), 133.30 (s),
148.00 (s) 148.30 ppm (s). 31P{1H} NMR (CDCl3,
202.45 MHz) δ: 153.5 (s).
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Can. J. Chem. Vol. 79, 2001
Scheme 1.
O
O
O
O
P
P
O
O
OH
OH
PCl3
THF
HO OH
Et3N, toluene
O
O
P
Cl
3
2
1
Table 1. Hydroformylation of styrene with ligands 3 and 4.
Run
Ligand
Time (h)
T (°C)
Conv. (%)a
RCHO (%)b
5:6a
ee of 5 (%)c
1
2
3
3
3
3
3
3
3
4
4
4
4
4
4
1
1.5
2
20
4
20
2
4
80
60
40
23
23
23
60
60
17
100
60
17
93
95
77
62
27
65
68
69
71
63
62
53
53
46
41
45
45
57:43
70:30
80:20
85:15
84:16
84:16
59:41
58:42
60:40
59:41
60:40
58:42
54 (R)
67 (R)
79 (R)
86 (R)
94 (R)
88 (R)
71 (R)
75 (R)
91 (R)
62 (R)
73 (R)
88 (R)
4
5d
6d
7
83
100
100
90
100
100
76
8e
9e
10d
11d
12d
70
1
2
67
Note: Reaction conditions: solvent CH2Cl2; substrate:Pt ratio = 2000.
aConversions and composition of the reaction mixture of branched:linear:hydrogenated (5:6:7) were determined by GC (SPB-1) using decane as an
internal standard.
bAldehydes/(aldehydes + ethylbenzene).
cDetermined by GC analysis of the distilled product (β-DEX 225, 30 m, id 0.25 mm). Stereochemisty determined by the sign of optical rotation of the
corresponding aldehyde.
dSolvent, toluene.
eSubstrate:Pt ratio = 5000.
nar arrangement can be confidently assigned (7). Both
diphosphites form eight-membered chelate rings upon coor-
dination of the Pt metal center, and afford only the cis com-
plexes without any oligomeric species. Oligomeric platinum
complexes of diphospholes, dibenzophospholes, and
bisnaphthophospholes have been reported with trans P-Pt-P
arrangement (4d and e).
nated ethylbenzene product 7. Some representative results
are given in Table 1.
CHO
CO/H2
*
CHO
+
+
PtLCl2/SnCl2
[1]
7
5
6
Several studies (2) have shown that regio- and enantio-
selectivity of platinum-catalyzed asymmetric hydroformylation
can be very sensitive to the nature of the solvent. Dichloro-
methane and toluene, having medium polarity, give the high-
est reaction rate and the best enantioselectivity and therefore
were chosen as the routine solvents. It is interesting to note
that the chemoselectivity and the activity of the catalysts are
significantly lower in toluene than in dichloromethane. For
example, although the highest ee of 94% was obtained at a
conversion of 27% in toluene (Table 1, run 5) the enantio-
selectivity is not constant in time, i.e., after 20 h (run 6) at a
conversion of 83% the ee is 88%.
The data show that the in situ platinum catalyst is most ef-
fective for hydroformylation of styrene when the reaction is
performed at room temperature. As usual, on lowering the
temperature enantioselectivity improves at the expense of re-
action rate (Table 1, runs 1–4 compared to runs 10–12). At
higher temperature the activity of the catalyst increases from
83 to 1200 mol mol–1 h–1 (runs 4–1, TOF in mol aldehydes
O
O
O
O
P
P
O
O
4
Catalysts were prepared in situ from PtCl2(PhCN)2 and
1.12 equiv. of ligand in dichloromethane using a 6.25 mM
solution of platinum–diphosphite catalyst. It is noteworthy
that, in contrast to rhodium systems modified with
phosphines or phosphinephosphites, there is no need for an
excess of phosphite ligand in the platinum system. Anhy-
drous SnCl2 was used as a co-catalyst. Addition of SnCl2 is
essential for catalytic activity (12). Catalytic reactions were
carried out under classical conditions to give a mixture of
the branched 2-phenylpropanal 5, the normal 3-
phenylpropanal aldehyde regioisomers 6, and the hydroge-
© 2001 NRC Canada

Bakos et al.
729
(mol Pt)–1 h–1 determined at the reaction time indicated in
Table 1). In dichloromethane the highest enantioselectivity
is obtained (91%, R) when ligand 4 is used, but in case of
ligand 3 the enantioselectivity is only slightly lower (86%).
At the same time, chemoselectivity to hydroformylation
(71%) and regioselectivity to branched aldehyde (85%) of
the catalyst improved substantially (run 4). Thus, an impor-
tant feature of this diphosphite system in combination with
platinum is the relatively high regioselectivity of the reac-
tion. The branched isomer forms with these ligands in a sig-
nificantly higher amount than with the usual mono- and di-
tertiary phosphines (4e).
Regioselectivity of the reaction depends largely on the re-
action temperature in case of 3; with decreasing temperature
regioselectivity to branched aldehyde increases markedly.
This can be explained by the enhanced rate of isomerization
of the initially formed Pt-i-alkyl complex to the thermody-
namically more stable n-alkyl complex, which leads to a
higher percentage of n-aldehyde. It is interesting to note that
the regioselectivity is less dependent on temperature in the
case of ligand 4.
We have shown recently that with the platinum phosphite
systems the relative configuration of the stereogenic ele-
ments in the backbone and in the terminal units play crucial
roles in the degree of enantioselectivities and the sense of
enantioface discrimination. In the case of styrene, the latter
is mainly determined by the configuration of the backbone
of the ligand. Accordingly, both ligands used in this study
gave (R)-2-phenylpropanal as the prevailing enantiomer. It is
not surprising because 3 and 4 can be considered “like”
sytems (2S,4S)-bis(S) and the relative spatial arrangement
around the stereogenic elements of the backbone is the
same.
the electronic effects on rhodium diphosphine catalyzed
hydroformylation were investigated by Casey et al. (16) and
van Leeuwen et al. (17).
Similarly, Ir(I)- and Ru(II)-complexes of H8-BINAP
served as more effective catalyst precursors for asymmetric
hydrogenation of some substrates, than did the analogous
complexes of BINAP (18). Furthermore, the Pd(II) complex
of (R,S)-H8-BINAPHOS is a more active catalyst compared
to that of (R,S)-BINAPHOS for alternating copolymerization
of propene with carbon monoxide. A completely isotactic
copolymer was obtained in high optical purity with (R,S)-
H8-BINAPHOS (19). It is noteworthy that in [Rh{(S)-H8-
BINAP}(COD)]ClO4 the dihedral angle between the two
phenyl rings of the bitetralin moiety (18a) is markedly larger
(80.3°) than that between the two naphthalene rings in
[Rh{(R)-BINAP}(NBD)]ClO4 (74.4°) (20). Recently, we
have reported the X-ray structure of the complex [PtCl2(2,2′-
bis[(4R,6R)-4,6-dimethyl-1,3,2-dioxaphosphorinan-2-yloxy]-
1,1′-binaphtyl)] and that of the free ligand (7c). It is interest-
ing to note that the dihedral angle defined by the planes of
the naphthyl rings is 77.6° in the complex and 103.0° in the
free ligand, while in the free ligand of the octahydro ana-
logue it is 98.2° (7d). The dihedral angles in these structures
suggest that ligands having H8-BINOL based backbone pos-
sess a quite different axial flexibility, however, in our case
the seven-membered dioxaphosphepine ring restricts this
flexibility. Unfortunately, suitable crystals of the free ligand
3 or its platinum complex for X-ray analysis could not be
obtained.
Conclusion
When 3, a partially hydrogenated analogue of 4, was used
as the bisphosphite ligand for the catalyst precursor, the
branched:linear ratio of hydroformylation products was
found to increase from ~1.5 to 5.7 (i.e., 85:15).
As an the extension of our study on asymmetric
hydroformylation, we reported herein a comparative study of
the catalytic performance of bisphoshites 3 and 4. Chiral
bisphosphites with C2 symmetry afforded highly active plati-
num catalysts for asymmetric hydroformylation of styrene.
High enantioselectivities (86–91%) have been obtained with
the platinum(II)–SnCl2 catalytic system containing ligands 3
and 4. Retaining the favorable structure of ligand 4 but in-
creasing the electron density in the terminal group signifi-
cantly improved the catalytic performance, i.e., higher
regioselectivity to branched aldehyde (85%) and lower hy-
drogenation activity were achieved with almost unchanged
enantioselectivity. Although the chemoselectivity of the pro-
cess could be improved, the level is still modest. The ob-
served changes of activity, chemoselectivity, and
regioselectivity may have their origin in electronic effects,
but steric factors like the bite angle of the ligands cannot be
excluded.
Enhancing the electron-donating ability of the ligand re-
sulted in an increase of chemoselectivity to
hydroformylation from 53% with 4 (run 7) to 68% with 3
(run 2) at 60°C. The regioselectivity to branched aldehyde
were 59 and 70%, respectively. The increase of the
branched:linear ratio with increasing phosphite basicity
(ligand 3 vs. ligand 4) probably results from the different be-
haviour of branched and linear platinum alkyls toward β-hy-
drogen elimination. The linear alkyls, the anti-Markovnikov
products of the insertion of styrene into the Pt–H bond, are
mainly converted to linear aldehyde, while the branched
alkyls partially regenerate styrene through enhanced β-hy-
drogen elimination. Selective β-hydrogen elimination from
the branched alkyl metal intermediate was already observed
by Lazzaroni et al. (13) for rhodium systems. In our case in-
creasing electrophilicity of the platinum center may result in
higher reactivity of the branched alkyl platinum species to-
ward β-hydrogen elimination (14).
Recently, RajanBabu and Ayers (15) studied the electronic
effect of the ligands on the enantioselectivity of
Acknowledgments
hydroformylation
of
vinylarenes
using
rhodium
Financial support from Hungarian National Science Foun-
dation (OTKA T 032 004) is gratefully acknowledged. We
are grateful to Mr. Béla Édes for skillful assistance in the
synthetic and catalytic experiments.
diphosphinites and found that the introduction of electron-
withdrawing aryl groups on the phosphorus increased the
enantioselectivity of the catalyst. The mechanistic aspects of
© 2001 NRC Canada

730
Can. J. Chem. Vol. 79, 2001
Inorg. Chim. Acta, 296, 222 (1999); (e) R. Kadyrov, D. Heller,
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