Hydroformylation of styrene in the presence of platinum(II) complex catalysts incorporating cyclic phosphines and phosphonous diesters as P-ligands

Article abstract of DOI:10.1016/j.jorganchem.2010.11.045

The hydroformylation activity of various 5- and 6-membered P-heterocycles was investigated in platinum-catalysed hydroformylation of styrene. All of the tested ligands, such as the phosphole-, oxaphosphorine- and phoshinine-based ligands proved to be catalytically active. Especially good aldehyde selectivities were obtained with the chelating diphenylphosphino- tetrahydrophosphinine and diphenylphosphino-hexahydrophosphinine ligands. While the two aldehyde regioisomers were formed close to equimolar amounts with the monodentate ligands, high branched selectivities were observed with the chelating ones. The effect of the 4-substitution of the parent styrene on chemo- and regioselectivity was investigated.

Full text of DOI:10.1016/j.jorganchem.2010.11.045

Journal of Organometallic Chemistry 696 (2011) 2234e2237
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Hydroformylation of styrene in the presence of platinum(II) complex catalysts
incorporating cyclic phosphines and phosphonous diesters as P-ligands
,
b
b
b
b
Péter Pongrácz a, László Kollár a , Andrea Kerényi , Viktória Kovács , Viktória Ujj , György Keglevich
*
a Department of Inorganic Chemistry, University of Pécs, Ifjúság u. 6 (PO Box 266), H-7624 Pécs, Hungary
b Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, H-1521 Budapest, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
The hydroformylation activity of various 5- and 6-membered P-heterocycles was investigated in platinum-
catalysed hydroformylation of styrene. All of the tested ligands, such as the phosphole-, oxaphosphorine-
and phoshinine-based ligands proved to be catalytically active. Especially good aldehyde selectivities were
obtained with the chelating diphenylphosphino-tetrahydrophosphinine and diphenylphosphino-hex-
ahydrophosphinine ligands. While the two aldehyde regioisomers were formed close to equimolar
amounts with the monodentate ligands, high branched selectivities were observed with the chelating
ones. The effect of the 4-substitution of the parent styrene on chemo- and regioselectivity was
investigated.
Received 15 May 2010
Received in revised form
3 November 2010
Accepted 28 November 2010
Keywords:
Platinum
Phospholene
Phosphinine
Oxaphosphorine
Hydroformylation
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
halide catalysts, much better results were obtained with these
catalytic systems in the enantioselective hydroformylation of
Many mono- and bidentate ligands, most of them with phos-
phorus donor(s), have already been tested in transition metal-cat-
alysed hydroformylation reactions [1,2]. The ‘in situ’ and ‘preformed’
complexes of cobalt, rhodium and platinum containing tertiary
phosphines have been used most successfully.
Due to their high practical importance in the selective synthe-
sis of aldehydes such as n-butanal and 2-arylpropanal derivatives
using propene and styrene as substrates, respectively, the im-
provement of hydroformylation catalysts received particular
attention. As well known, n-butanal is a key intermediate in the
production of plasticizers and 2-aryl propanals can be considered
as potential intermediates of 2-arylpropionic acids used as
nonsteroidal anti-inflammatory agents, NSAIs exemplified by
ibuprofen.
vinylaromatics (mainly styrene) and, especially, in that of activated
1,1-disubstituted alkenes such as unsaturated carboxylic acid
derivatives (acrylates and itaconates) [3,4].
As for the hydroformylation of styrene, rhodium-containing
catalysts usually provide higher aldehyde selectivity and, consid-
ering the two aldehyde regioisomers, higher branched selectivity
than platinum catalysts of related structure. However, several
platinumephosphineetin(II)halide systems provided regiose-
lectivity of practical importance towards branched aldehydes
accompanied by excellent enantioselectivity regarding the 2-phe-
nylpropanal (branched) regioisomer.
To find an appropriate balance of chemo-, regio and enantio-
selectivities (in asymmetric hydroformylation), as well as a satis-
factory catalytic activity, several unusual type of ligands were also
tested. Although phosphines as ligands bound to various transition
metals play a dominant role in the reaction concerned, achiral
[5e11] and chiral [12e21] phospholes have also been thoroughly
investigated.
Although notable ee’s for the branched 2-methylbutanal isomer
were obtained in the asymmetric hydroformylation of butene
isomers as test substrates using Ptechiral diphosphineetin(II)
During our recent investigations, platinum(II) complexes of
aryl-1H-phospholes were prepared and tested as catalysts in the
hydroformylation of styrene [22]. Phosphorylated phospholes were
also synthesized to fine tune the steric and electronic properties of
the ligands [23]. These latter species were used in in situ rhodium
* Corresponding author. Department of Inorganic Chemistry, University of Pécs,
Ifjúság u. 6 (PO Box 266), H-7624 Pécs, Hungary. Tel.: þ36 72 503600; fax: þ36 72
501527.
E-mail address: kollar@ttk.pte.hu (L. Kollár).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.11.045

P. Pongrácz et al. / Journal of Organometallic Chemistry 696 (2011) 2234e2237
2235
Cl
P
Cl
P
Cl
P
Cl
P
Cl
P
Cl
P
Ph
Ph
Ph
Ph
Ph
Ph
Pt
Pt
Pt
3
1
2
Cl
Cl
Cl
Cl
Cl
Cl
O
P
O
P
O
P
O
P
O
P
O
P
Pt
Pt
Pt
i
i
O Pr
PrO
OMe
OEt
MeO
EtO
4
5
6
Fig. 1. The platinum complexes (1e6) containing phosphole- and oxaphosphorine-derived monodentate ligands used as catalytic precursors in this work.
catalytic systems [24]. The rhodium(III) complex of 1-(2,4,6-triiso-
propylphenyl-)3-methyl-1H-phosphole was also prepared and
used as a catalyst in the hydroformylation of styrene [25]. The
regioselectivities obtained on the systematic investigation of the
complexes of phosphole derivatives [22,24,25] encouraged us to
turn to novel phosphorus containing heterocycles.
3. Results and discussion
3.1. Hydroformylation of styrene in the presence of
platinumephospholene/phospholaneetin(II)chloride and
platinumedibenzooxaphosphorineetin(II)chloride ‘in situ’ catalysts
In the present paper, in addition to the application of mono-
dentate P-ligands such as phospholene and phospholane deriva-
tives in platinum-catalysed hydroformylation, that of the recently
developed phosphinine-based ligands with diphenylphosphino
substituents will be described.
Styrene as model substrate was reacted in the presence of in situ
platinum catalysts with CO/H2 (1/1) at 50 or at 100 ꢀC at a pressure
of 80 bar.
It has to be noted, that no acceptable conversions could be
obtained below 50 ꢀC and some reduction of Pt(II) to Pt(0), i.e., the
formation of some platinum black can be observed above 125 ꢀC
using the catalytic precursors below. On the basis of previous
investigations with similar catalysts [34], the pressure of 80 bar
(CO/H2 1/1) was chosen because negligible conversion was
obtained under 40 bar in comparable reaction times.
2. Materials and methods
The catalytic system was formed directly in the reaction mixture
from the appropriate PtCl2(ligand)2 type complexes containing the
corresponding monodentate 5- or 6-membered P-heterocycles. In
this way, the catalytic precursors 1e3, containing 1-phenyl-3,4-
dimethyl-3-phospholene, 1-phenyl-3,4-dimethylphospholane and
1-phenyl-3-methyl-3-phospholene [23], respectively, and catalytic
precursors 4e6, containing 1-methoxy-dibenzo-2,1-oxaphosp-
horine,1-ethoxy-dibenzo-2,1-oxaphosphorine and 1-isopropyloxy-
dibenzo-2,1-oxaphosphorine [24], respectively, were used (Fig. 1).
In addition to the formation of the two formyl regioisomers,
2-phenylpropanal (A) and 3-phenylpropanal (B), ethylbenzene (C)
as a hydrogenation product was also expected (Eq. (1)).
2.1. General
The 31P, 13C and 1H NMR spectra were taken on a Bruker DRX-500
spectrometer operating at 202.4, 125.7 and 500 MHz, respectively.
Chemical shifts are downfield relative to 85% H3PO4 or TMS. GLC
analyses were carried out with an HP-5890/II gas chromatograph
using a 15 m HP-5 column (temperature program: initial tempera-
ture: 200 ꢀC (2 min), rate: 10 ꢀC/min, final temperature: 300 ꢀC).
The catalytic precursors 1e3 [26], 4e6 [27], 7 [28e30] and
8 [31e33] were prepared according to procedures published previ-
ously. Toluene was distilled under argon from sodium in the presence
of benzophenone. Styrene was freshly distilled before use.
CO / H2
PhCH CH2
PhCH(CHO)CH3 + PhCH2CH2CHO + PhCH2CH3
(Eq. 1)
B
C
A
2.2. Hydroformylation experiments
As for the chemoselectivity, the formation of the aldehydes is
preferred by using all precursors, however, the use of the phosp-
hole-derived ligands resulted in slightly higher aldehyde yields,
that is, lower extent of hydrogenation. Chemoselectivities higher
than 85% and 77e84% were obtained with the 5-membered P-
precursors (entry 1e4) and 6-membered P-precursors (entry 5e7),
respectively (Table 1). The similar selectivity towards aldehydes
was also observed with phospholes of various structures [20].
The branched selectivity is favoured with all of the phosphole-
based ligands 1e3 (entries 1e3). However, the chemo- and
regioselectivity can be finely tuned by the P-substituents of the
oxaphosphorine ring. That is, the preferred formation of the linear
In a typical experiment a solution of 0.02 mmol of 7 and
0.04 mmol of tin(II)chloride in 10 mL toluene containing 1 mmol of
styrene was transferred under argon into a 100 mL stainless steel
autoclave. The reaction vessel was pressurized to 80 bar total
pressure (CO/H2 ¼1/1) and placed in an oil bath of appropriate
temperature and the mixture was stirred with a magnetic stirrer for
the appropriate reaction time. The pressure was monitored
throughout the reaction. After cooling and venting of the autoclave,
the pale yellow solution was removed and immediately analysed
by GC.

2236
P. Pongrácz et al. / Journal of Organometallic Chemistry 696 (2011) 2234e2237
Table 1
Table 2
Hydroformylation of styrene in the presence of in situ catalysts formed from Pt
Hydroformylation of styrene and its 4-substituted derivatives in the presence of in
situ catalysts formed from Pt(PeP heterobidentate ligand) complexes (7 or 8) and tin
(II)chloride.a
(Pemonodentate ligand) complexes (1e6) and tin(II)chloride.a
c
Entry
Complex
Conv. (%)
Rcb (%)
Rbr (%)
Entry Complex Substrate (Ar) Temp. [ꢀC] R. time [h] Rcb [%] Rbrc [%]
1
2
3
4
5
6
7
1
2
83
87
62
62
89
70
52
87
90
85
84
77
79
84
61
60
53
64e
44
50
60
1
2
3
4
5
6
7
8
7
7
7d
7
7
7
7
7
7
8
8
C6H5
C6H5
C6H5
4-CH3eC6H4
4-CH3eC6sH4
4-OAceC6H4
4-OAceC6H4
4-CleC6H4
4-CleC6H4
C6H5
50
100
100
50
100
50
100
50
100
50
96
24
24
96
24
96
24
96
24
96
24
96
95
88
90
89
80
78
53
84
90
95
94
85
83e
65
51
59
46
78
42
82
70
3
3d
4
5
6
a
Reaction conditions: Pt/SnCl2/styrene ¼ 1/2/50; p(CO) ¼ p(H2) ¼ 40 bar; 100 ꢀC;
solvent: toluene; reaction time ¼ 24 h.
9
10
11
b
Chemoselectivity towards aldehydes (A, B). [(moles of A þ moles of B)/(moles of
A þ moles of B þ moles of C) ꢁ 100].
C6H5
100
c
Regioselectivity towards branched aldehyde (A). [moles of A/(moles of
a
A þ moles of B) ꢁ 100].
Reaction conditions: Pt/SnCl2/styrene ¼ 1/2/50; p(CO) ¼ p(H2) ¼ 40 bar; solvent:
toluene; conversion >98% in all cases.
d
Complex 3 contains an optically active ligand.
ee < 3% (S) was obtained for 2-phenylpropanal (determined by chiral GC on
e
b
Chemoselectivity towards aldehydes (A, B). [(moles of A þ moles of B)/(moles of
a Cyclodex column).
A þ moles of B þ moles of C) ꢁ 100].
c
Regioselectivity towards branched aldehyde (A). [moles of A/(moles of
A þ moles of B) ꢁ 100].
aldehyde was observed with the P-methoxy-oxaphosphorine
derivative 4 (entry 5) which is shifted towards the branched alde-
hyde in the presence of the corresponding ethoxy (5) and
isopropyloxy (6) derivatives (entries 6 and 7). Similarly, the alde-
hyde selectivity was slightly increased by moving from 4 to 6, while
the conversion changed in the opposite way, that is, the larger
P-substituents resulted in catalysts of lower activity.
The catalytic precursor 3 was also isolated containing the
optically active 1-phenyl-3-methyl-3-phospholene. Its application
in hydroformylation resulted in low optical induction. ee lower
than 3% was obtained (entry 4).
d
e
25
Complex 7 contains an optically active ligand ([
a
]
¼ þ47.7 (c 1, CHCl3)).
D
ee ¼ 5.5% (S) was obtained for 2-phenylpropanal (determined by chiral GC on
a Cyclodex column).
The two phosphinine-based ligands applied differ thoroughly
both in electronic and in steric properties [25,26,28,29]. Obviously, it
is due to the presence and absence of 4-chloro-3-ene moiety in tet-
rahydro- and hexahydrophosphinine, respectively. Both in situ cata-
lysts formed from the platinum precursors (7 or 8) and tin(II)chloride
were active under the given conditions (see Materials and Methods).
Practically complete conversions have been obtained in all cases both
at 100 ꢀC and at 50 ꢀC in 24 h and 96 h, respectively (Table 2).
As for the chemoselectivity towards the formation of aldehydes,
it should be noted that unexpectedly high preference for aldehydes
was observed in this platinum-catalysed hydroformylation reaction
resulting in chemoselectivities higher than 90%. Regarding the
aldehyde regioisomers, the branched aldehyde (A) predominates
over the linear one under all conditions.
3.2. Hydroformylation of styrene (and its 4-substituted derivatives)
in the presence of platinumetetra- and hexahydrophosphinineetin
(II)chloride ‘in situ’ catalysts
As above, the hydroformylation of styrene as model substrate
was carried out in the presence of in situ platinum catalysts
prepared from the starting complexes 7 [28e30] or 8 [31e33] and
tin(II)chloride. In these cases, the hydroformylation of styrene
derivatives was also studied (Eq. (2)).
The structural differences of the two ligands have a remarkable
effect on the regioselectivity. Although the formation of the
CO / H2
ArCH(CHO)CH3 + ArCH2CH2CHO + ArCH2CH3
(Eq. 2)
ArCH CH2
B
C
A
Ar = C6H5, 4-CH3-C6H4, 4-AcO-C6H4, 4-Cl-C6H4
The catalytic precursors 7 and 8 contain diphenylphosphino-tet-
rahydrophosphinine and diphenylphosphino-hexahydrophosphi-
nine derivatives as heterobidentate ligands, respectively (Fig. 2).
The same reaction conditions (pressure, temperature) were used as
above in order to obtain comparable results. Therefore, reaction
temperatures 50 ꢀC and 100 ꢀC and CO/H2 1/1 mixture (p(CO) ¼ p
(H2)¼40 bar) were chosen.
branched aldehyde (A) is highly preferred in both cases, its pref-
erence is undoubtedly higher when the structurally more rigid
ligand in precursor 7 was used (entries 1e3). It is worth noting
that the regioselectivity towards branched aldehyde is much
higher than the ones obtained with the most widely used Ptedi-
phosphine precursors [3,4]. In accordance with the previous results
obtained in platinum-catalysed hydroformylation of styrene, the
reactions carried out at lower temperature resulted in the preferred
formation of the branched regioisomer, 2-phenylpropanal (A)
(compare entry 1 with 2 and entry 10 with 11).
Cl
H
H
H
Me
Similar effect of the temperature on chemo- and regioselectiv-
ities has been observed using 4-substituted styrenes such as
4-methylstyrene and 4-acetoxystyrene (Table 2, entries 4 and 5, as
well as entries 6 and 7). Slightly higher chemoselectivity has
been obtained at 50 ꢀC than at 100 ꢀC with 4-acetoxystyrene, while
in case of 4-methylstyrene an opposite trend has been observed.
With both substrates a higher regioselectivity towards branched
aldehydes was obtained at 50 ꢀC. The effect of the temperature on
Ph2P
Ph2P
Me
P
P
Pt
Pt
Ph
Ph
Cl
Cl
Cl
Cl
7
8
Fig. 2. The platinum complexes (7 and 8) containing phosphinine-derived hetero-
bidentate ligands used as catalytic precursors in this work.

P. Pongrácz et al. / Journal of Organometallic Chemistry 696 (2011) 2234e2237
2237
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regioselectivity was more pronounced using 4-chlorostyrene, i.e.
78% and 42% branched selectivity was obtained at 50 ꢀC and
100 ꢀC, respectively (entries 8 and 9).
Although the present catalytic studies are not directed towards
mechanistic investigations, it could be stated that the electron-
withdrawing chloride substituent in 4-position has a remarkable
influence on the styrene insertion into platinumehydride bond.
Unlike the electron-releasing groups in the 4-position of styrene,
the CO insertion seems to be favoured at higher temperature
resulting in higher aldehyde selectivity (Table 2, entry 9). Further-
more, the formation of the platinume(branched alkyl) over the
platinume(linear alkyl) intermediate is highly preferred at 50 ꢀC.
Consequently, its inserted product, the platinume(branched acyl)
intermediate leads to the prevailing formation of 2-(4-chlor-
ophenyl)propanal (Table 2, entry 8).
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4. Conclusions
As a part of the systematic investigation of the ligand structure
ecatalytic reactivity (selectivity), five- and six-membered P-het-
erocycles were investigated as ligands in platinum-catalysed
hydroformylation of styrene. The preference of hydroformylation
over hydrogenation was observed by using all ligands. While the
application of monodentate ligands resulted in nearly equimolar
formation of the two formyl regioisomers, that of the bidentate
phosphinine-based ligands provided excellent branched regiose-
lectivity. The tetrahydrophosphinine-derived bidentate ligand
possessing the ‘most planar’ 6-membered P-heterocycle provided
the highest branched aldehyde selectivities using styrene.
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