Asymmetric hydroformylation catalysed by platinum complexes of new chiral bisphosphites

Article abstract of DOI:10.1039/a608436b

Asymmetric hydroformylation of styrene using bis(phosphite)PtCl₂-SnCl₂ systems gives branched aldehydes with high enantioselectivity (up to 91%).

Full text of DOI:10.1039/a608436b

View Article Online / Journal Homepage / Table of Contents for this issue
Asymmetric hydroformylation catalysed by platinum complexes of new chiral
bisphosphites
Stefa´nia Csere´pi-Szu¨cs and Jo´zsef Bakos*
Department of Organic Chemistry, University of Veszpre´m, H-8201 Veszpre´m, PO Box 158, Hungary
Asymmetric hydroformylation of styrene using bis(phos-
phite)PtCl2–SnCl2 systems gives branched aldehydes with
high enantioselectivity (up to 91%).
succinamide as a host compound8 or (8S,9R)-(2)-N-benzylcin-
chonidium chloride as a resolving agent.9
Phosphites 1 and 2 in CD2Cl2 form phosphite–platinum
complexes. The 31P NMR{1H} spectrum of PtCl2[(2S,4S)-
bis(S)-1] yields a characteristic satellite ‘triplet’ pattern (d 87.5,
JPt–P = 5672 Hz) and of PtCl(SnCl3)[2S,4S)-bis(S)-1] exhibits
a P–P coupling pattern (JP–P = 20 Hz) with well separated
chemical shifts (d 98.0 and 89.1) and large inequivalent P–Pt
coupling constants (5580 and 4420 Hz) as expected for
inequivalent P atoms in positions trans to the chloro and SnCl3
ligands, respectively. Reaction of PtCl2(PhCN)2 with both 1 and
2 afforded only the expected cis mononuclear eight-membered
complexes without any oligomeric species. Oligomeric plati-
num complexes of diphospholes, dibenzophospholes and bis-
binaphthophospholes have been reported with trans P–Pt–P
disposition.10
In a typical experiment the autoclave was filled with solvent,
alkene and the catalyst precursor. Catalysts were prepared in
situ by simple mixing PtCl2(PhCN)2 in toluene or in dichloro-
methane with the ligands, while anhydrous SnCl2 was used as
co-catalyst. It was then purged with syn gas [CO–H2 (1:1)] and
pressurised. Catalytic reactions were carried out under classical
conditions to give a mixture of the branched (b, 2-phenyl-
propanal) and the normal (n, 3-phenylpropanal) regioisomers
and hydrogenated (ethylbenzene) products. At the end of the
reaction the autoclave was cooled and depressurised. The
reaction mixture was directly vacuum distilled to remove the
catalyst. The reaction mixture and the distilled products were
analysed by GC. The enantiomeric excess was determined by
GC using a chiral capillary column.
Asymmetric hydroformylation is a convenient synthetic method
for obtaining optically active aldehydes from olefinic sub-
strates.1 These aldehydes are attractive building blocks in
organic synthesis.2 Most compounds which have been deve-
loped for chiral modification of catalysts are chiral derivatives
of aryl- or alkyl-phosphines; less work has been done with less
electron-rich ligands such as phosphites. For asymmetric
hydroformylation of olefins, chiral diphosphine–RhI,3 di-
phosphine–PtII,4 bisphosphite–RhI5 or phosphinephosphite–
RhI6 have mostly been used. Surprisingly, however, there have
been no reports on the application of bisphosphite–PtII–SnCl2
complexes for asymmetric hydroformylation. This system may
reasonably be expected to represent an alternative to the
diphosphine system because the less electron-rich phosphites
may cause dramatic changes in the reactivity and selectivity.
Here we report the synthesis and spectroscopic characteri-
zation of new bisphoshites, which afforded highly active
platinum catalysts for asymmetric hydroformylation. The
highest enantioselectivity (91%) was obtained with the plati-
num(ii)–SnCl2 catalytic system associated with (2S,4S)-bis(S)-
1 and its enantiomer (2R,4R)-bis(R). These ligands are of
particular interest, because they contain a chiral centre in the
backbone and a chiral axis in the terminal groups. In this regard,
these model ligands show the exploration of chiral cooper-
ativity.
The reaction of racemic 1,1A-bi-2-naphthol with excess PCl3
in toluene catalysed by 1-methylpyrrolidin-2-one gave phos-
phorochloridite (S,R)-6-chlorodinaphtho[d,f][1,3,2]dioxaphos-
phepine. Two equivalents of the racemic dioxaphosphepine
were reacted with optically pure (2R,4R)-pentane-2,4-diol in the
presence of triethylamine to give a diastereoisomeric mixture
of 2,4-bis[dinaphtho[d,f][1,3,2]dioxaphosphepin-6-yloxy]-
pentane. Diastereoisomers were obtained in a 1:2:1 ratio
showing that the chiral pentanediol does not induce diaster-
eoselectivity in the bisphosphite formation. Separation of the
diastereoisomers was not successful.
Some representative results are given in Table 1. When 1 or
2 was used as the ligand in platinum catalysed asymmetric
hydroformylation of styrene, the absolute configuration of
central chirality in the bridge is of primary importance in
asymmetric induction (Table 1). In the case of (2S,4S)-bis(S)-1,
there is a cooperative effect of the axial chirality of the terminal
1,3,2-dioxaphosphepine groups and the central chirality of the
In the 31P NMR{1H} spectrum of the product at ambient
temperature, six lines are observed with relative intensities and
splitting consistent with the expected three diastereoisomers.
An explanation is that four resonances arise from the meso form,
(2R,4R)-(S,R) = (2R,4R)-(R,S). The 2D homonuclear 31P
COSY NMR spectrum of the product confirms the existence of
coupling (6JP–P 9.5 Hz) between the P atoms (d 149.0 and
153.6) six bonds apart.7 Two singlets [d 153.6 for (2R,4R)-
bis(R)-1 and 147.1 for (2R,4R)-bis(S)-2]) are attributed to the
presence of the other two diastereoisomers. This was supported
by the NMR spectra of the optically pure diastereoisomers
O
toluene
P
Cl
PCl3
O
OH
OH
Me
Me
Et3N, toluene
OH OH
Me
Me
O
obtained
from
(S)-6-chlorodinaphtho[d,f]-
O
O
O
O
[1,3,2]dioxaphosphepine and (2S,4S)- (Scheme 1) or (2R,4R)-
pentane-2,4-diol, respectively. In these cases the other two
diastereoisomers were not detected by 31P NMR. This implies
that the diastereoisomers (1 and 2) were obtained in optically
pure form. Optical resolution of 1,1A-bi-2-naphthol was carried
out by using (R,R)-(+)-2,3-dimethoxy-N,N,NA,NA-tetramethyl-
P
P
O
(2S, 4S)-bis(S)-1
Scheme 1
Chem. Commun., 1997
635

View Article Online
Table 1 Enantioselective hydroformylation of styrene catalysed by bis(phosphite)PtCl2-SnCl2 complex [100 atm. of H2–CO (1:1), P/Pt: 2.05, SnCl2/Pt:
1.0]a
Me
CHO
Et
CO–H2
CHO
*
+
+
PtLCl2–SnCl2
3
4
5
Ligand
t/h
T/°C
Solvent
Substr/Pt
Conv. (%)b
5 (%)b
(b/n)b
Ee of 3 (%)c
Config.d
2
1
1
1
1
1
1
10
67
2
2
70
4
60
17
60
100
17
60
PhMe
PhMe
PhMe
PhMe
CH2Cl2
CH2Cl2
—
2000
2000
2000
2000
5000
5000
5000
65
76
100
100
90
59
55
55
59
54
47
48
62/38
58/42
60/40
59/41
60/40
58/42
58/42
14
88
73
62
91
75
89
S
R
R
R
R
R
R
96
50
22
17
a
Reactions were carried out in the given solvent [ca. 2.5 mol dm23, except in CH2Cl2 (8.35 mol dm23)] in a 20 ml stainless-steel autoclave under an
b
atmosphere of H2 and CO (1:1) at 100 atm. initial total pressure. Conversions and composition of the reaction mixture (b:n:h branched:normal:hy-
c
drogenated) were determined by GC (SPB-1) using decane as an internal standard. Determined by GC analysis (b-DEX, 30 m, id. 0.25 mm) of the
corresponding acid. d Determined by the sign of optical rotation of the corresponding aldehyde.
backbone. The highest enantioselectivity (91% R) was obtained
with the platinum(ii)–SnCl2 catalytic system associated with
(2S,4S)-bis(S)-1 and its enantiomer (2R,4R)-bis(R)-1 (matched
constellation).11 Reactions with (2R,4R)-bis(S)-2 (mismatched
constellation) resulted in much lower enantioselectivities. Up
until now the highest enantioface discrimination (86.3% ee) has
been reported4b for the hydroformylation of styrene to hydra-
tropaldehyde with the platinum–Sn system [(R,R)-BCO-
DBP]PtCl2 [BCO-DBP = 5,5A-(bicyclo[2.2.2]octane-2,3-diyl-
dimethyl)bis(5H-benzo[b]phosphindole)].
References
1 Recent reviews, see S. Gladiali, J. C. Bayo´n and C. Claver,
Tetrahedron: Asymmetry, 1995, 7, 1453; M. Beller, B. Cornils,
C. D. Frohning and V. W. Kohlpainter, J. Mol Catal., 1995, 104, 17;
F. Agbossou, J.-F. Carpentier and A. Mortreux, Chem. Rev., 1995, 95,
2485; G. Consiglio, in Catalytic Asymmetric Syntheses, ed. I. Ojima,
Weinheim, 1993, p. 273.
2 C. Botteghi, S. Paganelli, A. Schionato and M. Marchetti, Chirality,
1991, 3, 355.
3 A. M. Masdeu-Bulto´, A. Orejon, A. Castellanos, S. Castillo´n and
C. Claver, Tetrahedron: Asymmetry, 1996, 7, 1834.
Addition of SnCl2 is essential for catalytic activity. The role
of the SnCl2 co-catalyst in promoting the key steps of Pt–Sn-
catalysed hydroformylation, such as the alkene insertion, the
CO insertion and the hydrogenolysis of acyl complexes, has
been thoroughly investigated.12 A dramatic increase in reaction
rate and a slight decrease in enantioselectivity were found when
the reaction temperature was increased. Regioselectivity was
almost independent of reaction temperature.
In spite of the high hydrogenation activity of the catalyst, we
were able to find a catalytic system with remarkably high
hydroformylation activity. For example, in dichloromethane at
60 °C, hydroformylation rates of ca. 1100 and 530 turnovers
h21 at 54 and 88% of conversion were observed in combination
with high enantioselectivity (75% ee), respectively. Surpris-
ingly when Rh(CO)2(acac) was used as a catalytic precursor
(100 atm. and 60 °C) both diastereoisomers afforded the same
preferred R enantiomer in the product with the same re-
gioselectivity (81:19 b/n), enantioselectivity (16% ee) and with
complete chemoselectivity.
4 (a) J. K. Stille, H. Su, P. Brechot, G. Parenillo and L. S. Hegedus,
Organometallics, 1991, 10, 1183; (b) G. Consiglio, S. C. A. Nefkens
and A. Borer, Organometallics, 1991, 10, 2046; (c) A. Scrivanti,
S. Zeggio, V. Beghetto and U. Matteoli, J. Mol. Catal., 1995, 101,
217.
5 J. E. Babin and G. T. W. O. Whiteker, USP 911 518, 1992;
G. J. H. Buisman, P. C. J. Kamer and P. W. N. M. van Leeuwen,
Tetrahedron: Asymmetry, 1993, 4, 1625; G. J. H. Buisman, M. E.
Martin, E. J. Vos, A. Klootwijk, P. C. J. Kamer and P. W. N. M. van
Leeuwen, Tetrahedron: Asymmetry, 1995, 7, 719.
6 N. Sakai, K. Nozaki, K. Mashima and H. Takaya, Tetrahedron:
Asymmetry, 1992, 3, 583; N. Sakai, S. Mano, K. Nozaki and H. Takaya,
J. Am. Chem. Soc., 1993, 115, 7033; T. Horiuchi, T. Ohta, K. Nozaki
and H. Takaya, Chem. Commun., 1996, 155.
7 G. Szalontai, J. Bakos, I. To´th and B. Heil, Magn. Reson. Chem., 1987,
25, 761.
8 F. Toda and K. Tanaka, J. Org. Chem., 1988, 53, 3607.
9 Q.-S. Hu, D. Vitharana and L. Pu, Tetrahedron: Asymmetry, 1995, 6,
2123.
10 J. J. Brunet, M. Go`mez, H. Hajouji and D. Neibecker, J. Organomet.
Chem., 1993, 463, 205; G. Consiglio, S. C. A. Nefkens, Tetrahedron:
Asymmetry, 1990, 1, 417; S. Gladiali, D. Fabri and L. Kolla´r,
J. Organomet. Chem., 1995, 491, 91.
Financial support from the Hungarian National Science
Foundation (OTKA-T016269) and the Ministry of Culture and
Education (MDM-698/95) is gratefully acknowledged. We are
11 S. Masamune, W. Choy, J. S. Petersen and L. R. Sita, Angew. Chem.,
Int. Ed. Engl., 1985, 24, 1.
´
grateful to Mr Be´la Edes for skillful assistance in the catalytic
12 G. K. Anderson, H. C. Clark and J. A. Davies, Organometallics, 1982,
1, 64; P. J. Stang, Z. Zhong and A. M. Arif, Organometallics, 1992, 11,
1017; I. To´th, T. Ke´gl, C. J. Elseiver and L. Kolla´r, Inorg. Chem., 1994,
33, 5708 and references cited therein.
experiments.
Footnote
* E-mail: bakos@almos.vein.hu
Received, 17th December 1996; Com. 6/08436B
636
Chem. Commun., 1997