Asymmetric hydroformylation of styrene catalyzed by platinum(II)- Alkyl complexes containing atropisomeric diphosphines

Article abstract of DOI:10.1021/om960429s

The complexes [Pt(CH₃)Cl(P-P)] 1-3 (1, P-P = (S)-6,6′-(dimethoxybiphenyl)-2,2′-diylbis-(diphenylphosphine) ((S)-MOBIPH); 2, P-P = OR)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl ((R)-BINAP); 3, P-P = (2S,3S)-2,3-O-isopropylidene-2,3-dihydroxy-1

Full text of DOI:10.1021/om960429s

Organometallics 1996, 15, 4687-4694
4687
Articles
Asym m etr ic Hyd r ofor m yla tion of Styr en e Ca ta lyzed by
P la tin u m (II)-Alk yl Com p lexes Con ta in in g Atr op isom er ic
Dip h osp h in es
Alberto Scrivanti,* Valentina Beghetto, Alessandra Bastianini, and
Ugo Matteoli*
Dipartimento di Chimica, Universita` di Venezia, Calle Larga S. Marta 2137,
30123 Venezia, Italy
Gloria Menchi
Dipartimento di Chimica Organica, Universita` di Firenze, Via G. Capponi 9,
50121 Firenze, Italy
Received J une 3, 1996X
The complexes [Pt(CH3)Cl(P-P)] 1-3 (1, P-P ) (S)-6,6′-(dimethoxybiphenyl)-2,2′-diylbis-
(diphenylphosphine) ((S)-MOBIPH); 2, P-P ) (R)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl
((R)-BINAP); 3, P-P ) (2S,3S)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-(diphenylphosphino)-
butane ((S,S)-DIOP)) in the presence of SnCl2 catalyze the asymmetric hydroformylation of
styrene. The reaction proceeds under mild conditions (50 °C, P(H2) ) P(CO) ) 50 atm) to
give the desired branched aldehyde with moderate regioselectivity. Good enantioselectivities
(up to 75%) have been obtained using [Pt(CH3)Cl{(S)-MOBIPH}]. The influence of solvent,
temperature, P(H2), and P(CO) has been studied. An impressive influence of the solvent
has been observed: using [Pt(CH3)Cl{(R)-BINAP}], the chirality of 2-phenylpropanal obtained
in toluene or in tetrahydrofuran is opposite to that of 2-phenylpropanal produced in
dichloromethane or acetone. Using [Pt(CH3)Cl{(S)-MOBIPH}] or [Pt(CH3)Cl{(R)-BINAP}],
an unusual increase of the rate and enantioselectivity of the reaction with increasing P(CO)
is observed. In order to get information on the reaction mechanism, the carbonylation of
[Pt(CH3)(SnCl3){(S)-MOBIPH}] (4) has been studied. This reaction carried out at room
temperature and atmospheric pressure affords an equilibrium mixture containing the cationic
alkyl complex [Pt(CH3)(CO){(S)-MOBIPH}]+[SnCl3]- (6) and the neutral acyl species
[Pt(COCH3)(SnCl3){(S)-MOBIPH}] (7). The carbonylation of [Pt(CH3)(SnCl3){(R)-BINAP}]
(5) proceeds in the same fashion to give [Pt(CH3)(CO){(R)-BINAP}]+[SnCl3]- (8) and
[Pt(COCH3)(SnCl3){(R)-BINAP}] (9).
Sch em e 1
In tr od u ction
The asymmetric hydroformylation of prochiral olefins
provides a very convenient entry to chiral aldehydes,
which are useful intermediates employed for the prepa-
ration of important biologically active compounds.1,2 For
example, the asymmetric hydroformylation of vinyl-
arenes has been widely studied to produce optically pure
2-aryl-substituted propanals, which are readily oxidized
to the corresponding 2-arylpropanoic acids (Scheme 1),
constituting an important class of nonsteroidal analge-
sics.3,4
Until the important results recently obtained by
X Abstract published in Advance ACS Abstracts, September 15, 1996.
(1) Botteghi, C.; Paganelli, S.; Schionato, A.; Marchetti, M. Chirality
1991, 3, 355.
(2) Giordano, C.; Villa, M.; Panossian, S. In Chirality in IndustrysThe
Commercial Manufacture and Applications of Optically Active Com-
pounds; Collins, A. N., Sheldrake, G. N., Crosby, J ., Eds.; J . Wiley &
Sons: Chichester, England, 1992; pp 303-323.
(3) Rieu, J .-P.; Boucherle, A.; Cousse, H.; Mouzin, G. Tetrahedron
1986, 42, 4095.
Takaya and co-workers using Rh(I) complexes,5 the most
widely used catalysts in asymmetric hydroformylations
were systems based on platinum(II) complexes of the
type [PtCl2(P-P)] (P-P ) chiral chelating diphosphine
(4) Sonawane, H. R.; Bellur, N. S.; Ahuja, J . R.; Kulkarni, D. G.
Tetrahedron Asymmetry 1992, 3, 163.
(5) Sakai, N.; Mano, S.; Nozaki, K.; Takaya H. J . Am. Chem. Soc.
1993, 115, 7033.
S0276-7333(96)00429-3 CCC: $12.00 © 1996 American Chemical Society

4688 Organometallics, Vol. 15, No. 22, 1996
Scrivanti et al.
Ta ble 1. In flu en ce of Solven t on th e Hyd r ofor m yla tion of Styr en e Usin g th e Ca ta lytic System
a
[P t(CH3)Cl{(R)-BINAP }]/Sn Cl2
product composition (%)
solvent
t (h)
conversn (%)
aldehydes
ethylbenzene
polymer
b/ n
opb (%)
config
toluene
acetone
THF
118
186
498
42
79.8
44.4
17.2
13.2
84.0
72.8
52.3
84.1
3.9
4.5
7.0
7.6
12.1
22.7
40.7
8.3
0.58
1.03
0.56
1.08
41.6
23.2
20.0
15.0
R
S
R
S
CH2Cl2
a
Styrene 50 mmol; [Pt(CH3)Cl{(R)-BINAP}] 0.05 mmol; SnCl2 0.05 mmol; substrate/catalyst 1000/1; solvent 35 mL; P(H2) ) P(CO) 50
b
atm; T 50 °C. op, optical purity.
To get a better understanding of this effect we
synthesized the alkyl platinum complexes of formulation
[Pt(CH3)Cl(P-P)] (1-3) (1, P-P ) (S)-6,6′-dimethoxy-
biphenyl-2,2′-diylbis(diphenylphosphine) ((S)-MOBIPH);
2, P-P ) (R)-2,2′-bis(diphenylphosphino)-1,1′-binaph-
thyl ((R)-BINAP); 3, P-P ) (2S,3S)-2,3-O-isopropyl-
idene-2,3-dihydroxy-1,4-(diphenylphosphino)butane
((S,S)-DIOP)). Their catalytic behavior in the presence
of SnCl2 has been fully investigated by studying the
influence of some variables, such as P(CO), P(H2),
temperature, and nature of the solvent. Finally, the
catalytic activity of complexes 1-3 has been compared
with that of the analogue dichloro-Pt(II) complexes.
F igu r e 1. Structures of the chiral ligands used.
ligand) promoted by SnCl2.6 With this type of catalyst,
very high asymmetric inductions have been achieved
using dibenzophospholyl-substituted diphosphines7,8 or
atropisomeric ligands such as BINAP9,10 (Figure 1).
Unfortunately, the high enantioselectivities obtained
Since it is reasonable to hypothesize that the P(CO)
with these Pt(II) catalysts are frequently accompanied
plays a crucial role in the formation of the acyl species,
by low regioselectivities to the desired branched alde-
we have also studied the insertion of CO into the Pt-
hyde and poor reaction rates,6 so there is a constant
CH3 bond of complexes 1 and 2 in the presence of SnCl2
impulse to test and develop more active and selective
to get information on the mechanism and clarify the
catalytic systems.
In a previous study we observed that when the
influence of P(CO) on the enantioselectivity.
Ca ta lytic Stu d ies. The platinum catalytic precur-
hydroformylation of styrene is carried out in the pres-
sors were synthesized by reacting complex [Pt(CH3)Cl-
ence of the system [PtCl2{(S)-MOBIPH}]/SnCl2 the
(1,5-COD)] (1,5-COD ) 1,5-cyclooctadiene) with an
enantioselectivity increases with increasing the carbon
equimolecular amount of the appropriate diphosphine.14
monoxide partial pressure.10 Since such a trend con-
The characterization of the resulting complexes
trasts with that usually found with similar systems,6
[Pt(CH3)Cl{(S)-MOBIPH}] (1), [Pt(CH3)Cl{(R)-BINAP}]
we were prompted to further investigate the reaction
and its mechanism.
(2), and [Pt(CH3)Cl{(S,S)-DIOP}] (3)15 was done by
elemental analysis and spectroscopic techniques (IR, 1H
Essential steps6c,11,12 of the mechanism of the platinum-
and 31P NMR). The data, reported in the Experimental
catalyzed hydroformylation are (i) insertion of the
Section, are in keeping with the proposed formulation.
alkene into a Pt-H bond to give both linear and
Owing to our interest in the synthesis of 2-arylpro-
branched alkyl-Pt complexes, (ii) insertion of CO into
panoic acids, we have studied the hydroformylation of
the Pt-C bond to give the corresponding acyl species,
styrene.
and (iii) reaction of the acyl intermediates with molec-
The catalytic experiments were carried out as de-
ular hydrogen and/or acid species to afford the alde-
scribed in the Experimental Section by combining in situ
hydes and restore the starting platinum hydride.
equimolecular amounts of the complex 1, 2, or 3 and
Since the asymmetric induction occurs, as suggested
SnCl2. The composition of the raw mixtures at the end
in the literature,8,13 during the formation of the Pt-
of reactions was determined by GC using an internal
alkyl intermediates, no substantial influence on the
standard. This procedure allowed us to quantify both
optical purity should be played by the CO partial
the expected aldehydes and ethylbenzene and the
pressure. The favorable influence of this parameter on
polystyrene not detected by GC. The formation of the
the enantioselectivity reported by us10 is unprecedented
polymer was confirmed by IR spectroscopy. The optical
and needs to be rationalized.
purity of the branched aldehyde was determined by
polarimetry using the specific rotatory power values
(6) For recent reviews, see: (a) Agbossou, F.; Carpentier, J .-F.;
Mortreux, A. Chem. Rev. 1995, 95, 2485. (b) Gladiali, S.; Bayon, J . C.;
Claver, C. Tetrahedron Asymmetry 1995, 6, 1453. (c) Consiglio, G. In
Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH Publishers: New
York, 1993; pp 273-302.
reported by Consiglio.16
Since the use of Pt(II)-alkyl complexes 1-3 as
catalyst precursors was unprecedented, we have first
studied the influence of the solvent. The relevant data
we obtained using complex 2 are reported in Table 1.
They indicate that the solvent plays a peculiar role in
influencing all the aspects of the catalytic activity. The
(7) Stille, J . K.; Su, H.; Brechot, P.; Parrinello, G.; Hegedus, L. S.
Organometallics 1991, 10, 1183.
(8) Consiglio, G.; Nefkens, S. C. A.; Borer, A. Organometallics 1991,
10, 2046.
(9) Kollar, L.; Sandor, P.; Szalontai, G. J . Mol. Catal. 1991, 67, 191.
(10) Scrivanti, A.; Zeggio, S.; Beghetto, V.; Matteoli, U. J . Mol. Catal.
A 1995, 101, 217.
(11) Scrivanti, A.; Botteghi, C.; Toniolo, L.; Berton, A. J . Organomet.
Chem. 1988, 344, 261.
(12) Scrivanti, A.; Paganelli, S.; Matteoli, U.; Botteghi, C. J . Organo-
met. Chem. 1990, 385, 439.
(14) Clark, H. C.; Manzer, L. E. J . Organomet. Chem. 1973, 59, 411.
(15) Payne, N. C.; Stephan, D. W. J . Organomet. Chem. 1982, 228,
203.
(16) Consiglio, G.; Pino, P.; Flowers, L. I.; Pittman, C. U., J r. J .
Chem. Soc., Chem. Commun. 1983, 612.
(13) Consiglio, G.; Pino, P. Top. Curr. Chem. 1982, 105, 77.

Asymmetric Hydroformylation of Styrene
Organometallics, Vol. 15, No. 22, 1996 4689
Ta ble 2. In flu en ce of Tem p er a tu r e on th e Hyd r ofor m yla tion of Styr en e Usin g th e Ca ta lytic System
a
[P t(CH3)Cl{(S)-MOBIP H}]/Sn Cl2
product composition (%)
T (°C)
t (h)
conversn (%)
aldehydes
ethylbenzene
polymer
b/ n
opb (%)
config
30
50
80
240
164
17
52.7
86.7
97.8
84.6
74.7
76.9
2.5
3.7
6.6
12.9
21.6
16.5
1.09
0.73
0.77
75.8
56.3
0.1
S
S
S
a
Styrene 50 mmol; [Pt(CH3)Cl{(S)-MOBIPH}] 0.05 mmol; SnCl2 0.05 mmol; substrate/catalyst 1000/1; toluene 35 mL; P(H2) ) P(CO)
b
50 atm. op ) optical purity.
Ta ble 3. In flu en ce of Tem p er a tu r e on th e Hyd r ofor m yla tion of Styr en e Usin g th e Ca ta lytic System
a
[P t(CH3)Cl{(R)-BINAP }]/Sn Cl2
product composition (%)
T (°C)
t (h)
conversn (%)
aldehydes
ethylbenzene
polymer
b/ n
opb (%)
config
30
50
80
688
118
40
33.5
79.8
93.4
62.7
84.0
66.5
3.3
3.9
4.9
34.0
12.1
28.6
0.50
0.58
0.64
58.4
41.6
3.9
R
R
R
a
Styrene 50 mmol; [Pt(CH3)Cl{(R)-BINAP}] 0.05 mmol; SnCl2 0.05 mmol; substrate/catalyst 1000/1; toluene 35 mL; P(H2) ) P(CO) 50
b
atm. op ) optical purity.
Ta ble 4. In flu en ce of Tem p er a tu r e on th e Hyd r ofor m yla tion of Styr en e Usin g th e Ca ta lytic System
a
[P t(CH3)Cl{(S,S)-DIOP }]/Sn Cl2
product composition (%)
T (°C)
t (h)
conversn (%)
aldehydes
ethylbenzene
polymer
b/ n
opb (%)
config
30
50
80
66
22
2
46.8
52.6
56.6
67.1
70.2
67.3
4.1
5.1
8.8
28.8
24.7
23.9
0.29
0.41
0.59
29.2
27.2
20.7
R
R
R
a
Styrene 50 mmol; [Pt(CH3)Cl{(S,S)-DIOP}] 0.05 mmol; SnCl2 0.05 mmol; substrate/catalyst 1000/1; toluene 35 mL; P(H2) ) P(CO) 50
b
atm. op ) optical purity.
reaction rate, the regioselectivity, the optical purity, and
even the absolute configuration of the branched alde-
hyde are greatly affected by the solvent. It is interesting
to note that in toluene and tetrahydrofuran the catalyst
gives 2-phenylpropanal with prevailing (R) configura-
tion and a branched/normal ratio (b/n) of ∼0.6, while
in acetone and dichloromethane the (S) branched alde-
hyde prevails and the isomeric ratio is close to 1.
To the best of our knowledge, a similar behavior has
not been reported in literature.6a It is likely to be
connected to the particular nature of the atropisomeric
BINAP ligand rather than to the nature of the platinum
complex itself. In fact, experiments not reported in the
tables and carried out using the DIOP containing
complex 3 demonstrate that the branched aldehyde
produced in dichloromethane has the same (R) config-
uration of that formed in toluene.
Even if several studies have shown that the regio- and
the enantioselectivity of the platinum-catalyzed asym-
metric hydroformylation can be very sensitive to the
nature of the solvent,6a it is difficult to account for the
behavior of complex 2.
Since toluene gives the highest reaction rate and the
best enantioselectivity, it was used as routine solvent
for all subsequent investigations.
The data relevant to the effect of the temperature on
the catalytic activity of the systems 1/SnCl2, 2/SnCl2,
and 3/SnCl2 are reported in Tables 2-4, respectively.
The chemoselectivity and the regioselectivity of the
reaction show only little variations over the temperature
range used. As usual, on lowering the temperature the
enantioselectivity improves at the expense of the reac-
tion rate. Accordingly, when working with complex 2
at 30 °C a good optical purity is obtained, but exceed-
ingly long reaction times are necessary to get reasonable
yields (Table 3). At variance, at 30 °C complex 1 still
displays a fairly good catalytic activity accompanied by
an excellent optical purity (Table 2). When complexes
1 and 2 are used at 80 °C, the enantioselectivity
decreases to values very close to zero, as previously
found with the dichloro analogues [PtCl2{(S)-MOBI-
PH}]10 and [PtCl2{(S)-BINAP}]9 and some other Pt(II)
catalytic precursors.17,18
Complex 3 displays the highest activity regardless of
the reaction temperature, however the optical purity of
the branched aldehyde reaches at most 30% (Table 4).
Owing to the above reported results, we deemed that
only complexes 1 and 2 were worth further investiga-
tion.
The effect of the hydrogen partial pressure on the
catalytic activity of complexes 1 and 2 is reported in
Tables 5 and 6, respectively. With both catalysts, an
increase of P(H2) produces moderate effects concerning
all the main features of the reaction: (i) the hydro-
formylation rate is enhanced; (ii) the hydrogenation of
the substrate to ethylbenzene is also enhanced though
the chemoselectivity of the reaction is not greatly
affected; (iii) the branched/normal isomeric ratio and the
enantioselectivity are scarcerly affected.
Summing up, an increase of P(H2) favorably affects
the reaction as usually observed with platinum-based
catalysts.6a,b
The data showing the influence of the CO partial
pressure on the catalytic activity of complexes 1 and 2
are reported in Tables 7 and 8, respectively. With both
complexes an increase of P(CO) produces several useful
effects: (i) the hydroformylation rate is enhanced. It
is worth noting that if the yield in aldehydes per hour
(17) Kollar, L.; Bakos, J .; Toth, I.; Heil, B. J . Organomet. Chem.
1988, 350, 277.
(18) Toth, I.; Guo, I.; Hanson, B. E. Organometallics 1993, 12, 848.

4690 Organometallics, Vol. 15, No. 22, 1996
Scrivanti et al.
Ta ble 5. In flu en ce of P (H2) on th e Hyd r ofor m yla tion of Styr en e Usin g th e Ca ta lytic System
a
[P t(CH3)Cl{(S)-MOBIP H}]/Sn Cl2
product composition (%)
P(H2) (bar)
t (h)
conversn (%)
aldehydes
ethylbenzene
polymer
b/ n
opb (%)
config
20
50
80
164
164
82
56.3
86.7
66.5
100
58.8
74.7
73.4
73.2
2.0
3.7
5.3
7.1
39.2
21.6
21.3
19.7
0.79
0.73
0.75
0.92
54.0
56.3
60.2
58.5
S
S
S
S
110
87
a
Styrene 50 mmol; [Pt(CH3)Cl{(S)-MOBIPH}] 0.05 mmol; SnCl2 0.05 mmol; substrate/catalyst 1000/1; toluene 35 mL; P(CO) 50 atm;
b
T 50 °C. op ) optical purity.
Ta ble 6. In flu en ce of P (H2) on th e Hyd r ofor m yla tion of Styr en e Usin g th e Ca ta lytic System
a
[P t(CH3)Cl{(R)-BINAP }]/Sn Cl2
product composition (%)
P(H2) (bar)
t (h)
conversn (%)
aldehydes
ethylbenzene
polymer
b/ n
opb (%)
config
50
110
118
88
79.8
82.7
84.0
75.2
3.9
11.9
12.1
12.9
0.58
0.75
41.6
48.4
R
R
a
Styrene 50 mmol; [Pt(CH3)Cl{(R)-BINAP}] 0.05 mmol; SnCl2 0.05 mmol; substrate/catalyst 1000/1; toluene 35 mL; P(CO) 50 atm; T
b
50 °C. op ) optical purity.
Ta ble 7. In flu en ce of P (CO) on th e Hyd r ofor m yla tion of Styr en e Usin g th e Ca ta lytic System
a
[P t(CH3)Cl{(S)-MOBIP H}]/Sn Cl2
product composition (%)
P(CO) (bar)
t (h)
conversn (%)
aldehydes
ethylbenzene
polymer
b/ n
opb (%)
config
20
50
80
148
139
65
70.0
86.7
60.7
74.5
68.4
74.7
69.7
70.7
8.6
3.7
2.0
1.6
23.0
21.6
28.3
27.7
0.79
0.73
0.86
0.96
28.8
56.3
67.1
72.1
S
S
S
S
110
74
a
Styrene 50 mmol; [Pt(CH3)Cl{(S)-MOBIPH}] 0.05 mmol; SnCl2 0.05 mmol; substrate/catalyst 1000/1; toluene 35 mL; P(H2) 50 atm;
b
T 50 °C. op ) optical purity.
Ta ble 8. In flu en ce of P (CO) on th e Hyd r ofor m yla tion of Styr en e Usin g th e Ca ta lytic System
a
[P t(CH3)Cl{(R)-BINAP }]/Sn Cl2
product composition (%)
P(CO) (bar)
t (h)
conversn (%)
aldehydes
ethylbenzene
polymer
b/ n
opb (%)
config
20
50
80
132
118
104
88
75.5
79.8
81.9
77.7
80.0
84.0
87.1
84.8
8.2
3.9
2.3
2.2
11.8
12.1
10.6
13.0
0.50
0.58
0.63
0.72
23.1
41.6
55.2
66.8
R
R
R
R
110
a
Styrene 50 mmol; [Pt(CH3)Cl{(R)-BINAP}] 0.05 mmol; SnCl2 0.05 mmol; substrate/catalyst 1000/1; toluene 35 mL; P(H2) 50 atm; T
b
50 °C. op ) optical purity.
F igu r e 3. Influence of P(CO) on the hydroformylation rate
in the presence of complex 2.
F igu r e 2. Influence of P(CO) on the hydroformylation rate
in the presence of complex 1.
is plotted vs P(CO), a linear trend is recognized (Figures
2 and 3); (ii) the chemoselectivity is affected to a very
little extent; (iii) the regioselectivity improves; (iv) the
optical purity of the branched aldehyde increases.
Indeed, the influence of P(CO) on the enantioselec-
tivity of the reaction is impressive: the optical purity
almost triples over the P(CO) range we used. A plot of
the optical purity of the branched aldehyde vs P(CO) is
reported in Figure 4.
The influence of P(CO) on the catalytic activity of 1
and 2 strongly resembles that observed in the case of
the dichloro complex [PtCl2{(S)-MOBIPH}].6 As we

Asymmetric Hydroformylation of Styrene
Organometallics, Vol. 15, No. 22, 1996 4691
Pt-H bond of Pt(II)-trichlorostannate complexes,11,21
the possibility of interconversion of the diastereomers
increases and the enantioselectivity of the reaction is
likely to be determined by the difference in the ther-
modynamic stability of the two diastereomeric Pt-alkyl
intermediates.22 If this latter is smaller than the
difference in the activation energies that lead to the
formation of the two diastereomeric Pt-alkyl complexes,
the enantioselectivity at low P(CO) will be lower than
that at high P(CO).
Stu d y of Mod el Rea ction s. To substantiate our
mechanistic proposal we studied the carbonylation of
complexes 1 and 2 in the presence of SnCl2. The study
was carried out using dichloromethane as the solvent
owing to the low solubility of complexes 1 and 2 in
toluene.
The reaction of 1 or 2 with SnCl2 in dichloromethane
(or dichloromethane-d2) affords the corresponding trichlo-
rostannate complexes [Pt(CH3)(SnCl3){(S)-MOBIPH}]
(4) or [Pt(CH3)(SnCl3){(R)-BINAP}] (5). These species
have been characterized in solution by low-temperature
1H and 31P NMR spectroscopies since at room temper-
ature their spectra are dynamic owing to the equilibria
involving the trichlorostannate moiety.23 The relevant
data (see Experimental Section) are in keeping with the
proposed structures and agree with those of similar
complexes.11,24
Upon treatment with 1 atm of carbon monoxide,
complex 4 reacts to form two new Pt(II) species as
indicated by IR and NMR spectroscopies (Scheme 4).
The most abundant species is the cationic alkyl
complex [Pt(CH3)(CO){(S)-MOBIPH}]+[SnCl3]- (6), defi-
nitely identified since its spectroscopic data closely
match those of an authentic sample of [Pt(CH3)(CO){(S)-
MOBIPH}]+[BF4]- (6a ) synthesized by reacting 1 with
AgBF4 under CO (Scheme 5), as described in the
Experimental Section.
F igu r e 4. Influence of P(CO) on the optical purity of
2-phenylpropanal in the presence of complexes 1 and 2.
Sch em e 2
have outlined in the Introduction, this behavior is
remarkable and deserves some comment since usually
with platinum catalysts both the hydroformylation rate
and the enantioselectivity are unaffected or decrease
with increasing P(CO).6a Such negative influence has
been attributed to the existence of different catalytic
species, each one having its own activity and enantio-
selectivity.19 In our opinion, to account for the data of
Tables 5-8, it is not necessary to invoke the presence
of different catalysts. As a matter of fact, the positive
influence of P(H2) on the reaction rate has been already
reported16,20 and indicates that the rate-determining
step of the reaction is the hydrogenolysis of an acyl
complex as depicted in Scheme 2.
The formation of a second Pt(II) species is confirmed
1
by the H NMR spectrum of the solution obtained from
the carbonylation reaction which shows, along with the
signals stemming from complex 6, a singlet at 1.93 ppm
which can be attributed to a COCH3 moiety.25 In
keeping, the IR spectrum displays a broad absorption
of medium intensity at 1625 cm-1 attributable to the
C-O stretching of an acetyl group of the type Pt-
In keeping with Scheme 2, a positive effect of P(CO)
on the reaction rate suggests that over the pressure
range we have investigated [Pt-COR] is dependent on
P(CO). Such dependence can account for the enhance-
ment of the enantioselectivity of the reaction observed
with increasing P(CO). The competitive pathways that
produce the two enantiomeric branched aldehydes are
reported in Scheme 3.
COCH3.11,26,27 Finally, the low-temperature (198 K) 31
P
{1H} NMR data allowed us to formulate the second
species formed in the carbonylation of 4 as the acyl
complex [Pt(COCH3)(SnCl3){(S)-MOBIPH}] (7) (see Ex-
perimental Section for the relevant data).
It is important to point out that the carbonylation
reaction occurs in two stages since the formation of the
cationic species 6 takes place in a few minutes, while
Using very high P(CO), the Pt-alkyl complexes are
rapidly converted into the corresponding Pt-acyl com-
plexes. These latter are then hydrogenated to give the
final aldehydes. Accordingly, under high P(CO), the
diastereomeric Pt-alkyls have little probability to inter-
convert and the enantioselectivity of the reaction is
likely to be determined by the difference in the activa-
tion energy of the processes that lead to the formation
of the two diastereomeric Pt-alkyl complexes; that is,
the enantioselectivity of the reaction is kinetically
controlled. To the contrary, at low P(CO) values, the
carbonylation is slower and the concentration of the
diastereomeric Pt-alkyl species likely increases. Owing
to the reversibility of the insertion of an olefin into a
(21) Gomez, M.; Muller, G.; Sainz, D.; Sales, J . Organometallics
1991, 10, 4036.
(22) Consiglio, G.; Arber, W.; Pino, P. Chim. Ind. (Milan) 1978, 60,
396.
(23) Pregosin, P. S.; Sze, S. N. Helv. Chim. Acta 1978, 61, 1848.
(24) Toth, I.; Kegl, T.; Elsevier, C. J .; Kollar, L. Inorg. Chem. 1994,
33, 5708.
(25) Dekker, G. P. C. M.; Buijs, A.; Elsevier, C. J .; Vrieze, K.; van
Leeuwen, P. W. N. M.; Smeets, W. J . J .; Spek, A. L.; Wang, Y. F.; Stam,
C. H. Organometallics 1992, 11,1937.
(26) Dent, S. P.; Eaborn, C.; Pidcock, A. J . Organomet. Chem. 1975,
97, 307.
(19) Haelg, P.; Consiglio, G.; Pino, P. J . Organomet. Chem. 1985,
296, 281.
(20) Hayashi, T.; Kawabata, Y.; Isoyama, T.; Ogata I. Bull. Chem.
Soc. J pn. 1981, 54, 3438.
(27) In ref 24 the ν(CO) for the acetyl moiety of [Pt(COCH3)(CO){(S,S)-
BDPP}]+[SnCl3]- is reported to be ∼1710 cm-1; the conflict with our
data is to be attributed to the use of 13C-enriched CO by Toth and
co-workers.

4692 Organometallics, Vol. 15, No. 22, 1996
Scrivanti et al.
Sch em e 3
Sch em e 4
(diphenylphosphino)pentane), recently reported by Toth,24
proceeds in a similar way: first a cationic carbonyl
species of formulation [Pt(CH3)(CO){(S,S)-BDPP}]+-
[SnCl3]- is rapidly and quantitatively formed and then
slow CO insertion takes place to give a mixture of
[Pt(CH3)(CO){(S,S)-BDPP}]+[SnCl3]- and of the cationic
acyl species [Pt(COCH3)(CO){(S,S)-BDPP}]+[SnCl3]-.
The main difference is that in our case the acyl species
has to be formulated as [Pt(COCH3)(SnCl3)(diphosphine)]
rather than [Pt(COCH3)(CO)(diphosphine)]+[SnCl3]-.
This difference is likely due to the fact that our model
reactions were carried out under 1 atm of CO while Toth
performed his investigations using P(CO) ) 25 atm.
The carbonylation of Pt(II) complexes containing
monodentate phosphine ligands has been extensively
investigated.28-30 In particular, the carbonylation of
complexes of the type [Pt(Ph)(SnCl3)P2] (P ) PPh3 or
PMePh2) was studied by Anderson.30 These authors
demonstrated that the presence of the trichlorostannate
ligand favors a nondissociative route which allows a
rapid and quantitative formation of the corresponding
Pt-benzoyl complexes. Moreover, they showed that the
reaction can be easily reversed in absence of CO.
The carbonylation of Pt(II)-alkyl species containing
chelating diphosphine ligands has received less atten-
tion. Anderson, who studied the carbonylation of a
series of complexes of the type [Pt(Ph)Cl(P-Y)]31 (P-Y
) chelating ligand), observed that the carbonylation
does not take place at all when P-Y is 1,2-bis(diphen-
ylphosphino)ethane while it proceeds smoothly to afford
quantitatively the acyl complex [Pt(COPh)Cl(P-Y)]
when P-Y is 1,3-bis(diphenylphosphino)propane.
Our previous studies on the carbonylation of cis
Pt(II)-alkyl species containing diphosphine ligands
were stimulated by the interest in the role played by
Sch em e 5
the formation of the acyl species 7 requires some hours.
At room temperature, the relative concentration of 6 and
7 is 1.4/1, respectively, as it can be readily established
by integration of the resonances relevant to the methyl
groups in the 1H NMR spectrum. The formation of both
complexes 6 and 7 is reversible, and complex 4 can be
restored by passing an argon or nitrogen stream through-
out the solution.
The carbonylation of complex 5 proceeds in a strictly
analogous way forming the cationic alkyl complex
[Pt(CH3)(CO){(R)-BINAP}]+[SnCl3]- (8) and the neutral
acyl complex [Pt(COCH3)(SnCl3){(R)-BINAP}] (9) in a
1/1 relative ratio. As in the case of 6, the formulation
of 8 is supported by the comparison of its spectral
features with those of [Pt(CH3)(CO){(R)-BINAP}]+-
[BF4]- (8a ) synthesized as described in the Experimen-
tal Section.
These findings indicate that the carbonylation of the
alkyl species 4 and 5 is a slow process which does not
quantitatively afford the corresponding Pt-acyl com-
plexes. Probably, high CO pressures are necessary to
speed up and drive the reaction to completion. IR-based
studies at variable P(CO) and temperature are currently
in progress to gain a deeper insight into carbonylation
equilibria. The reluctance of complexes 4 and 5 to
undergo CO insertion gives some support to our mecha-
nistic rationalization on the influence of P(CO). How-
ever, taking into account that the model reactions have
been studied in dichloromethane (due to the low solubil-
ity of 4 and 5 in toluene), no absolute conclusions can
be drawn.
(28) Anderson, G. K.; Cross, R. J . Acc. Chem. Res. 1984, 17, 67.
(29) Anderson, G. K.; Cross, R. J .; J . Chem. Soc., Dalton Trans. 1980,
1434.
(30) Anderson, G. K.; Clark, H. C.; Davies, J . A. Organometallics
1982, 1, 64.
It is worth noting that the carbonylation of [Pt(CH3)-
(SnCl3){(S,S)-BDPP}] ((S,S)-BDPP ) (2S,4S)-2,4-bis-
(31) Anderson, G. K.; Lumetta, G. J . Organometallics 1985, 4, 1542.

Asymmetric Hydroformylation of Styrene
Organometallics, Vol. 15, No. 22, 1996 4693
SnCl2 in the Pt-catalyzed hydroformylation of olefins.11
We observed that at atmospheric pressure the carbon-
ylation of complexes of the type [Pt(C2H5)(SnCl3)(P-P)]
(P-P ) 1,3-bis(diphenylphosphino)propane or 1,4-bis-
(diphenylphosphino)butane) affords quantitatively and
very rapidly the corresponding cis Pt-acyl complexes
[Pt(COC2H5)(SnCl3)(P-P)].11 Therefore, it appears that
the nature of the ancillary ligands strongly influences
the course of the carbonylation reaction. It should be
deduced that the behavior of 4 and 5 cannot be at-
tributed to the peculiar nature of the atropisomeric
ligands we employed. Since the same behavior is
observed with ligands having different basicity and
which give rise to chelate rings of different size,24 the
factors that set the course of the insertion of CO are
not clear.
styrene (5.8 mL, 50 mmol), toluene (35 mL), Pt complex (0.05
mmol), and SnCl2 (0.05 mmol). The reactor was flushed with
nitrogen and then pressurized with H2 and CO. The reactor
was maintained at the desired temperature ((1 °C) by
circulating a thermostatic fluid. At the end of the reaction,
the residual gases were vented off and the composition of the
raw reaction mixture was determined by GC. The branched
aldehyde was isolated from the reaction mixture by distillation
under reduced pressure, and its optical purity was determined
by polarimetry using the specific rotatory power values
reported by Consiglio.16
[P t(CH3)Cl{(S)-MOBIP H}] (1). A dichloromethane solu-
tion of (S)-MOBIPH (584 mg, 1 mmol in 10 mL) was added
dropwise to a dichloromethane solution of [Pt(CH3)Cl(1,5-
COD)] (354 mg, 1 mmol in 20 mL). The resulting solution was
stirred for 3 h under argon. Addition of n-hexane afforded the
product as a white precipitate that was collected, washed with
n-hexane, and dried in vacuo (yield: 650 mg, 78 %). Anal.
Calcd for C39H35ClO2P2Pt: C, 56.56; H, 4.26. Found: C, 56.73;
H, 4.34. IR spectrum (Nujol mull, CsI windows): 298 cm-1
(w, Pt-Cl). 31P NMR (CDCl3, 298 K): 17.32 (d, J (P-P) ) 17
Hz, J (Pt-P) ) 4437 Hz), 22.71 ppm (d, J (Pt-P) ) 1758 Hz).
1H NMR (CDCl3, 298 K): 0.54 (dd, 3H, J (H-Pcis) ) 4.0 Hz;
J (H-Ptrans) ) 7.5 Hz, J (H-Pt) ) 56 Hz), 3.42 (s, 3H), 3.49 (s,
3H), 6.2-8.0 ppm (m, 26 H).
Con clu sion s
The present work demonstrates that complexes of the
type [Pt(CH3)Cl(chelating diphosphine)] in the presence
of SnCl2 are effective catalysts for the hydroformylation
of olefins. Their catalytic activity and their chemo-,
regio-, and enantioselectivity do not differ substantially
from those of the corresponding dichloro species. As far
as the enantioselectivity of the reaction is concerned,
both the atropisomeric ligands we tested allow one to
obtain good to excellent results. Unfortunately, these
catalysts do not provide any improvement on the regio-
selectivity of the reaction which, as in the case of most
of the Pt(II)-based catalysts, is disappointing since the
main product is the undesired linear aldehyde.
The results of our investigations emphasize the role
played by the diphosphine ligands: their nature influ-
ences not only the enantioselectivity but also the overall
course of the reaction. In particular it becomes clear
that when atropisomeric ligands such as BINAP are
used, the process becomes very sensitive to the nature
of the solvent.
[P t(CH3)Cl{(R)-BINAP }] (2). Complex 2 was prepared
according to the method used for the (S)-MOBIPH analogue
(yield: 720 mg, 83%). Anal. Calcd for C45H35ClP2Pt: C, 62.25;
H, 4.06. Found: C, 62.52; H, 3.98. IR spectrum (Nujol mull,
CsI windows): 300 cm-1 (w, Pt-Cl). 31P NMR spectrum
(CDCl3, 298 K): 23.69 (d, J (P-P) ) 17 Hz, J (Pt-P) ) 1760
Hz), 18.06 ppm (d, J (Pt-P) ) 4380 Hz). 1H NMR: 0.33 (dd,
3 H, J (H-Pcis) ) 4.1 Hz, J (H-Ptrans) ) 7.5 Hz, J (H-Pt) ) 57
Hz), 6.5-8.0 ppm (m, 32 H).
[P t(CH3)Cl{(S,S)-DIOP }] (3). Complex 3 was synthesized
similarly to the (S)-MOBIPH analogue (yield: 560 mg, 75%).
Anal. Calcd for C32H35ClO2P2Pt: C, 51.65; H, 4.74. Found:
C, 51.77; H, 4.64. IR spectrum (Nujol mull, CsI windows): 298
cm-1 (w, Pt-Cl). 31P NMR spectrum (CDCl3, 298 K): 9.75 (d,
J (P-P) ) 14 Hz, J (Pt-P) ) 1683 Hz), 6.80 ppm (d, J (Pt-P)
) 4340 Hz). 1H NMR: 0.54 (dd, 3 H, J (H-Pcis) ) 4.9 Hz, J (H-
P
trans) ) 7.3 Hz, J (H-Pt) ) 55 Hz), 1.16 (s, 6 H), 2.50 (m, 2
H), 3.15 (m, 2 H), 3.88 (m, 2 H), 6.40-7.40 ppm (m, 20 H).
[P t(CH3)(Sn Cl3){(S)-MOBIP H}] (4). A 33 mg sample of
1 (0.04 mmol) was stirred in dichloromethane (or dichloro-
methane-d2) with an equimolecular amount of anhydrous
SnCl2 (8 mg) until a clear yellow solution formed (∼1-2 h).
31P NMR (CD2Cl2, 203 K): 24.63 (P trans to SnCl3, d, J (P-P)
) 22 Hz, J (P-Pt) ) 3707 Hz, J (P-117,119Sn) ) 4030, 3850 Hz),
16.23 ppm (P atom trans to CH3, d, J (P-P) ) 22 Hz, J (P-Pt)
) 1963 Hz, J (P-117,119Sn) ) 217 Hz (average value of J (P-
117Sn) and J (P-119Sn)). 1H NMR: 0.71 (t, 3 H, J (H-Pcis) ≈
J (H-Ptrans) ≈ 6.2 Hz, J (H-Pt) ) 55 Hz), 3.44 (s, 3 H), 3.49 (s,
3 H), 6.2-8.0 ppm (m, 26 H).
[P t(CH3)(Sn Cl3){(S)-BINAP }] (5). A 34 mg sample of 2
(0.04 mmol) was stirred in dichloromethane (or dichloro-
methane-d2) with an equimolecular amount of anhydrous
SnCl2 (8 mg) until a clear yellow solution formed (∼1-2 h).
31P NMR (CD2Cl2, 203 K): 25.58 (P trans to SnCl3, d, J (P-P)
) 21 Hz, J (P-Pt) ) 3702 Hz, J (P-117,119Sn) ) 4047, 3869 Hz),
17.13 ppm (P trans to CH3, d, J (P-Pt) ) 1986 Hz, J (P-
117,119Sn) ) 215 Hz (average value of J (Px-117Sn) and J (Px-
119Sn)). 1H NMR: 0.50 (t, J (H-Pcis) ≈ J (H-Ptrans) ≈ 6.0 Hz,
J (H-Pt) ) 57 Hz), 6.40-7.40 ppm (m, 20 H).
Exp er im en ta l Section
All the operations were carried out under argon in Schlenk-
type glassware. Commercial solvents (Carlo Erba) were
purified following methods described in the literature.32
AgBF4 and anhydrous SnCl2 were commercial products
(Fluka). CDCl3 and CD2Cl2 were purchased from Aldrich. The
chiral ligands (S)-MOBIPH,33 (R)-BINAP,34 (S,S)-DIOP,35 and
complex [Pt(CH3)Cl(1,5-COD)]14 were synthesized by literature
methods. 1H and 31P {1H} NMR were obtained on a Bruker
AC 200 spectrometer operating at 200.13 and 81.01 MHz,
respectively. 31P NMR chemical shifts are reported with
positive values downfield from 85% H3PO4. IR spectra were
registered on a Nicolet 750 FT-IR interferometer.
Elemental analyses were carried out by the Department of
Organic Chemistry of the University of Florence. GC analyses
were carried out on a Hewlett-Packard 5830 II series gascro-
matograph. Optical rotations were measured with a Perkin-
Elmer 241 polarimeter.
Hydroformylation experiments were carried out in a mag-
netically stirred stainless steel autoclave (total volume ∼150
mL). In a typical experiment, the reactor was charged with
[P t(CH3)CO{(S)-MOBIP H}]+[Sn Cl3]- (6). IR spectrum
(CH2Cl2, NaCl windows): 2114 cm-1 (s, C-O). 31P NMR
(CD2Cl2, 198 K): 18.30 (P trans to CO, d, J (P-P) ) 27 Hz,
J (P-Pt) ) 3316 Hz), 14.35 ppm (P trans to CH3, d, J (P-Pt) )
1721 Hz). 1H NMR (298 K): 0.75 (t, Pt-CH3, J (H-Pcis) ≈
J (H-Ptrans) ≈ 6.0 Hz, J (Pt-H) ) 55 Hz), 3.34 (s, OCH3), 3.37
(s, OCH3), 6.2-8.0 ppm (aromatics).
(32) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, England, 1988.
(33) Schmid, R.; Foricher, J .; Cereghetti, M.; Schonholzer, P. Helv.
Chim. Acta 1991, 74, 370.
(34) Takaya, H.; Akutagawa, S.; Noyori, R. Org. Synth. 1988, 67,
20.
(35) Kagan, H. B.; Dang, T.-P. J . Am. Chem. Soc. 1972, 94, 6429.
[P t (COCH 3)(Sn Cl3){(S)-MOBIP H }] (7). IR spectrum

4694 Organometallics, Vol. 15, No. 22, 1996
Scrivanti et al.
(CH2Cl2, NaCl windows): 1625 cm-1 (m, C-O). 31P NMR
(CD2Cl2, 198 K): 8.56 (P trans to SnCl3, d, J (P-P) ) 28 Hz,
J (P-Pt) ) 4400 Hz, J (P-117,119Sn) ) 3217, 3078 Hz), 7.44 ppm
(P trans to COCH3, d, J (P-Pt) ) 1637 Hz, J (P-117,119Sn) )
285 Hz (average value of J (P-117Sn) and J (P-119Sn)). 1H NMR
(298 K): 1.93 (s, COCH3), 3.39 (s, OCH3), 3.42 (s, OCH3), 6.2-
8.0 ppm (m, aromatics).
stirred for 1 h. Treatment of the suspension with activated
charcoal and filtration gave a clear solution which was
concentrated under reduced pressure. Addition of diethyl
ether afforded 6a as an off-white powder (yield: 130 mg, 80%).
Anal. Calcd for C40H35O3P2BF4Pt: C, 52.94; H, 3.89. Found:
C, 53.02; H, 3.91. IR spectrum (CH2Cl2, NaCl windows): 2114
cm-1 (s, C-O). 31P NMR (CDCl3, 298 K): 18.34 (d, J (P-P) )
27 Hz, J (P-Pt) ) 3360 Hz), 14.56 ppm (d, J (P-Pt) ) 1713
Hz). 1H NMR: 0.83 (t, J (H-Pcis) ≈ J (H-Ptrans) ) 6.0 Hz, J (H-
Pt) ) 55 Hz), 3.34 (s, 3 H), 3.38 (s, 3 H), 6.3-8.0 ppm (m, 26
H).
[P t(CH3)CO{(S)-BINAP }]+[BF 4]- (8a ). Complex 8a was
synthesized in a 80% yield similarly to complex 6a . Anal.
Calcd for C46H35OP2BF4Pt: C, 58.30; H, 3.72. Found: C, 58.14;
H, 3.66. IR spectrum (CH2Cl2, NaCl windows): 2113 cm-1 (s,
C-O). 31P NMR (CDCl3, 298 K): 18.82 (d, J (P-P) ) 27 Hz,
J (P-Pt) ) 3371 Hz), 16.80 ppm (d, J (P-Pt) ) 1720 Hz). 1H
NMR: 0.73 (t, J (H-Pcis) ≈ J (H-Ptrans) ) 6.0 Hz, J (Pt-H) )
57 Hz), 6.2-8.0 ppm (m, 32 H).
[P t (CH 3)CO{(S)-BINAP }]+[Sn Cl3]- (8). IR spectrum
(CH2Cl2, NaCl windows): 2113 cm-1 (s, C-O). 31P NMR
(CD2Cl2, 198 K): 18.68 (P trans to CO, d, J (P-P) ) 27 Hz,
J (P-Pt) ) 3329 Hz), 15.81 ppm (P trans to CH3, d, J (P-Pt) )
1735 Hz). 1H NMR (298 K): 0.71 (t, Pt-CH3, J (H-Pcis) ≈
J (H-Ptrans) ≈ 6.0 Hz, J (H-195Pt) ) 57 Hz), 6.3-8.0 ppm (m,
aromatics).
[P t (COCH 3)(Sn Cl3){(R)-BINAP }] (9). IR spectrum
(CH2Cl2, NaCl windows):1634 cm-1 (m, C-O). 31P NMR
(CD2Cl2, 198 K): 12.50 (P trans to SnCl3, d, J (P-P) ) 26 Hz,
J (P-Pt) ) 3941 Hz, J (P-117,119Sn) ) 4100, 3920 Hz), 8.03 ppm
(P trans to COCH3, d, J (P-Pt) ) 1659 Hz, J (P-117,119Sn) )
289 Hz (average value of J (P-117Sn) and J (P-119Sn)). 1H NMR
(298 K): 2.02 ppm (s, COCH3).
[P t(CH3)CO{(S)-MOBIP H}]+[BF 4]- (6a ). Under a CO
atmosphere, a methanol solution of AgBF4 (35 mg, 0.18 mmol
in 5 mL) was added to a dichloromethane solution of [Pt(CH3)-
Cl{(S)-MOBIPH}] (1) (150 mg, 0.18 mmol in 20 mL). A white
precipitate was immediately formed. The suspension was then
Ack n ow led gm en t. This work was supported by
Italian CNR (Progetto Strategico Tecnologie Chimiche
Innovative).
OM960429S