Platinum complexes of malonate-derived monodentate phosphines and their application in the highly chemo- and regioselective hydroformylation of styrene

Article abstract of DOI:10.1016/j.jorganchem.2008.10.025

Neutral complexes of the formula PtCl₂L₂ (where L = diethyl 2-diphenylphosphino-malonate (1), diethyl 2-methyl-2-diphenylphosphinomalonate (2), dibenzyl 2-diphenylphosphinomalonate (3), 1,3-dihydroxy-2-methyl-2-diphenylphosphinopropane (4)) were prepared. These proved to be precursors to active catalysts for the hydroformylation of styrene. The platinum-containing catalytic systems prepared from ligand 4 provided the highest activity, while the platinum compounds prepared from other ligands all showed similar levels of reactivity to each other. The matching of high chemo- and regioselectivities were observed in most cases. Surprisingly, the complexes were practically inactive in imidazolium-type ionic liquids. ³¹P NMR studies on the PtCl₂L₂ complexes revealed that the stereoselectivity of the cis/trans geometrical isomers is strongly dependent on the structure of the ligand.

Full text of DOI:10.1016/j.jorganchem.2008.10.025

Journal of Organometallic Chemistry 694 (2009) 219–222
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Platinum complexes of malonate-derived monodentate phosphines and their
application in the highly chemo- and regioselective hydroformylation of styrene
a
a
b
b
b,
*
}
György Petocz , Gábor Rangits , Megan Shaw , Henriëtte de Bod , D. Bradley G. Williams
,
László Kollár a,
*
a Department of Inorganic Chemistry, University of Pécs, Ifjúság u. 6., H-7624 Pécs, Hungary
b Department of Chemistry, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 August 2008
Received in revised form 6 October 2008
Accepted 14 October 2008
Available online 22 October 2008
Neutral complexes of the formula PtCl2L2 (where L = diethyl 2-diphenylphosphino-malonate (1), diethyl
2-methyl-2-diphenylphosphinomalonate (2), dibenzyl 2-diphenylphosphinomalonate (3), 1,3-dihy-
droxy-2-methyl-2-diphenylphosphinopropane (4)) were prepared. These proved to be precursors to
active catalysts for the hydroformylation of styrene. The platinum-containing catalytic systems prepared
from ligand 4 provided the highest activity, while the platinum compounds prepared from other ligands
all showed similar levels of reactivity to each other. The matching of high chemo- and regioselectivities
were observed in most cases. Surprisingly, the complexes were practically inactive in imidazolium-type
ionic liquids. 31P NMR studies on the PtCl2L2 complexes revealed that the stereoselectivity of the cis/trans
geometrical isomers is strongly dependent on the structure of the ligand.
Keywords:
Hydroformylation
Platinum
Monodentate phosphine
NMR
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
halide-free hydroformylation catalysts based on platinum-alkyl/
aryl complexes and boron additives [20].
Carbonylation reactions encompass several types of processes.
Among these, hydroformylation is considered to be that process
with the highest industrial importance [1]. Cobalt- and rhodium-
containing catalysts have successful industrial scale application
[2] and have seen detailed mechanistic investigations [2]. Hydro-
formylation activity has also been noted for platinum-phosphine-
tin(II) halide systems [3]. When based on the application of
PtCl2(chiral diphosphine) precursors, these complexes have proved
efficient enantioselective hydroformylation catalysts [4–11]. The
high regioselectivities obtained with 1,1-disubstituted olefins
[12] and the facile route developed towards optically active 2-
aryl-propanal derivatives, the direct precursors of 2-phenyl-propi-
onic acid derivatives (such as the non-steroidal anti-imflammatory
drug ibuprofen [13–16]) using hydroformylation transformations,
render such systems useful as catalysts of synthetically important
reactions.
This paper describes the synthesis of platinum complexes with
malonate-derived monodentate phosphines and their application
in platinum-catalysed hydroformylation of styrene.
2. Experimental
2.1. General
The PtCl2(PhCN)2 precursor was synthesised from PtCl2 (Al-
drich) according to a standard procedure [21]. Toluene was dis-
tilled and purified by standard methods and stored under argon.
Styrene was freshly distilled immediately before use. All reactions
were carried out under argon using standard Schlenk techniques.
The 1H and 31P NMR spectra were recorded on a Varian Inova
400 spectrometer or a Varian Gemini 300 MHz instrument. Chem-
ical shifts are reported in ppm relative to TMS (downfield) or 85%
H3PO4 (0.00 ppm) for 1H and 31P NMR spectroscopy, respectively.
The ligands 1–4 and their borane adducts were synthesised as
follows.
The search for higher-activity platinum hydroformylation cata-
lysts has led to the application of novel achiral diphosphines [17].
Xantphos [18] and its analogues [19] have been tested in plati-
num–phosphine–tin(II) chloride systems. Investigations into the
role of the co-catalyst have resulted in the development of tin(II)
2.2. General procedure for the synthesis of the malonate ligands
Diethylmethylmalonate (0.22 mL, 0.43 mmol) was placed into a
Schlenk tube and dissolved in 21 mL of dry THF. The solution was
cooled to 0 °C and 65 mg (0.516 mmol) of NaH were added and the
mixture was stirred for half an hour. The solution was cooled to
* Corresponding authors. Tel.: +36 72 327622 4153; fax: +36 72 327622 4680
(L. Kollár).
E-mail address: kollar@ttk.pte.hu (L. Kollár).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.10.025

}
G. Petocz et al. / Journal of Organometallic Chemistry 694 (2009) 219–222
220
À78 °C and 0.16 mL (0.29 mmol) of chlorodiphenylphosphine were
added and the reaction mixture was left to stir overnight. The solu-
tion was brought to 0 °C using an ice bath and 1.02 mL (0.34 mmol)
of borane–THF complex were added. The mixture was left to stir
for 2 h after which it was warmed to room temperature and stirred
for a further 10 min. TLC was performed to check the progress of
the reaction. Slow addition of water quenched the reaction and ex-
cess THF solvent was removed in vacuo. The reaction mixture was
extracted with EtOAc and washed twice with water. The organic
layer was dried over MgSO4 and the excess solvent removed in
vacuo.
acid and brine. The organic layer was dried over MgSO4 and excess
solvent was removed in vacuo. The desired pure product was ob-
tained via flash chromatography as a white solid, (55%).
TLC: Rf 0.35 (1:1 hexane:EtOAc); m.p: 79–81 °C; IR: mmax
(CHCl3)/cmÀ1 3485, 3008, 2385, 1437, 1067; 1H NMR: (300 MHz,
CDCl3) dH 7.92–7.86 (m, 4H), 7.51–7.42 (m, 6H), 4.03 (dd, 2H,
J = 11.7 and 8.7 Hz), 3.64 (dd, 2H, J = 15.5 and 11.9 Hz), 2.46 (br s,
2H), 1.12 (d, 3H, J = 12.9 Hz), 1.65–0.50 (v br m, 3H); 13C NMR:
(75 MHz, CDCl3) dC 134.0 (d, 4C, J = 8.6 Hz), 131.5 (d, 2C,
J = 2.6 Hz), 128.7 (d, 4C, J = 9.9 Hz), 126.2 (d, 2C, J = 53.6 Hz), 66.5
(d, 2C, J = 5.6 Hz), 41.4 (d, 1C, J = 29.6 Hz), 17.4 (1C); 31P NMR:
(121 MHz, CDCl3) dP 25.2 (br d, 1P, J = 69.5 Hz); CIMS: m/z 287
(M+À1, 50%), 275 (M+ÀBH3+H, 100%), 257 (M+ÀBH3O, 15%); EIMS:
m/z 287 (M+À1, 5%), 274 (MÀBH3, 50%), 183 (MÀC4H11O2B, 60%),
108 (MÀC10H17O2B, 100%).
2.2.1. Diphenyl(diethylmalonyl)phosphine borane (1)
The pure product was isolated in the form of a white solid (54%).
TLC: Rf 0.15 (10:1 hexane:EtOAc); m.p: 74–76 °C; IR: mmax
(CHCl3)/cmÀ1 3009, 2390, 1732, 1256; 1H NMR: (300 MHz, CDCl3)
dH 7.84–7.78 (m, 4H), 7.49–7.39 (m, 6H), 4.58 (d, 1H, J = 11.1 Hz),
3.97 (q, 4H, J = 7.1 Hz), 0.99 (t, 6H, J = 7.2 Hz), 1.62–0.58 (v br m,
3H); 13C NMR: (75 MHz, CDCl3) dC 164.2 (2C), 133.1 (d, 4C,
J = 9.9 Hz), 131.7 (d, 2C, J = 2.3 Hz), 128.5 (d, 4C, J = 10.5 Hz),
126.5 (d, 2C, J = 55.0 Hz), 62.2 (2C), 52.3 (d, 1 C, J = 21.6 Hz), 13.4
(2C); 31P NMR: (121 MHz, CDCl3) dP 24.9 (br d, 1P, J = 41.1 Hz);
CIMS: m/z 357 (M+À1, 15%), 345 (M+ÀBH3+H, 100%); EIMS: m/z
357 (M+À1, 5%), 344 (M+ÀBH3, 35%), 201 (M+ÀC7H9O4, 100%).
The decomplexation of the borane adducts using DABCO (1,4-
diaazabicyclocyclo [2,2,2]octane) was carried out on the basis of
previous studies [22] and, owing to several observed differences
in reactivity, is detailed below.
2.3. General method for the synthesis of PtCl2(monophosphine)2
complexes (monophosphine = 1–4)
To a three-necked flask equipped with a gas-inlet and a reflux
condenser with a balloon at the top was added a degassed solution
of PtCl2(PhCN)2 (236 mg, 0.5 mmol) and the borane adduct of 1
(1.05 mmol; or of 2, 3 or 4) in benzene (20 mL). This solution
was heated to reflux under argon. A bright yellow homogeneous
mixture resulted. Upon addition of DABCO (1.05 mmol) a white
precipitate formed. The mixture was heated for 24 h. The small
amount of precipitate was filtered off, the benzene was evaporated
from the filtrate and the residue dried under vacuum for 2 h. The
pale yellow highly viscous material was crystallised from a hex-
ane–chloroform mixture. The target complexes were obtained as
white powder-like solid materials.
cis-PtCl2(1)2 (1b). Yield: 67%. Anal. Calc. for C38H42O8P2Cl2Pt
(954.68): C, 47.81; H, 4.43. Found: C, 47.96; H, 4.59%. For NMR data
see Table 1.
trans-PtCl2(2)2 (2a). Yield: 70%. Anal. Calc. for C40H46O8P2Cl2Pt
(982.73): C, 48.89; H, 4.72. Found: C, 48.97; H, 4.89%. For NMR data
see Table 1.
2.2.2. Diphenyl(methyldiethylmalonyl)phosphine borane (2)
The pure product was obtained via flash chromatography as a
turbid oil (64%).
TLC: Rf 0.18 (10:1 hexane:EtOAc); IR: mmax (CHCl3)/cmÀ1 3011,
2395, 1730, 1256, 1105; 1H NMR: (300 MHz, CDCl3) dH 7.91–7.85
(m, 4H), 7.48–7.37 (m, 6H), 4.05 (q, 4H, J = 7.1 Hz), 1.71 (d, 3H,
J = 14.7 Hz), 1.06 (t, 6H, J = 7.1 Hz), 1.58–0.61 (v br m, 3H); 13C
NMR: (75 MHz, CDCl3) dC 168.3 (2C), 134.1 (d, 4C, J = 9.4 Hz),
131.4 (d, 2C, J = 2.6 Hz), 128.2 (d, 4C, J = 10.5 Hz), 127.1 (d, 2C,
J = 54.3 Hz), 62.3 (2C), 55.3 (d, 1C, J = 20.2 Hz), 20.3 (d, 1C,
J = 2.6 Hz), 13.5 (2C); 31P NMR: (121 MHz, CDCl3) dP 34.7 (broad
d, 1P, J = 39.7 Hz); CIMS: m/z 371 (M+À1, 25%), 359 (MÀBH3+H,
100%); EIMS: m/z 371 (M+À1, 10%), 358 (MÀBH3, 20%), 329
(MÀC2H7B, 100%), 188 (MÀC8H17O4B, 50%).
2.2.3. Diphenyl(dibenzylmalonyl)phosphine borane (3)
The pure product was isolated in the form of a clear sticky oil
(67%).
trans-PtCl2(3)2 (3a) and cis-PtCl2(3)2 (3b) (isolated as a mixture).
Yield: 79%. Anal. Calc. for C58H50O8P2Cl2Pt (1202.96): C, 57.91; H,
4.19. Found: C, 58.06; H, 4.30%. For NMR data see Table 1.
trans-PtCl2(4)2 (4a) and cis-PtCl2(4)2 (4b) (isolated as a mixture).
Yield: 57%. Anal. Calc. for C32H38O4P2Cl2Pt (814.58): C, 47.18; H,
4.70. Found: C, 47.32; H, 4.84%. For NMR data see Table 1.
TLC: Rf 0.19 (5:1 hexane:EtOAc); IR: mmax (CHCl3)/cmÀ1 3009,
2390, 1734, 1216; 1H NMR: (300 MHz, CDCl3) dH 7.74–7.67 (m,
4H, aromatic H), 7.49–7.43 (m, 2H), 7.37–7.22 (m, 12H), 7.11–
7.07 (m, 4H), 4.98 (d, 2H, J = 17.1 Hz), 4.94 (d, 2H, J = 17.4 Hz),
4.68 (d, 1H, J = 10.8 Hz), 1.80–0.56 (v br m, 3H); 13C NMR:
(75 MHz, CDCl3) dC 164.0 (d, 2C, J = 2.5 Hz), 134.3 (2C), 133.1 (d,
4C, J = 10.0 Hz), 131.7 (d, 2C, J = 2.5 Hz), 128.6 (d, 4C, J = 10.6 Hz),
128.4 (4C), 128.4 (4C), 128.3 (2C), 126.1 (d, 2C, J = 55.2 Hz), 68.1
(2C), 52.3 (d, 1C, J = 20.5 Hz); 31P NMR: (121 MHz, CDCl3) dP 25.7
(br d, 1P, J = 20.1 Hz); CIMS: m/z 481 (M+À1, 3%), 469 (M+ÀBH3+H,
20%), (M+ÀC22H20O4PB, 100%).
3. Hydroformylation experiments
In a typical experiment, a solution of 0.01 mmol of PtCl2(mono-
phosphine) (monophosphine = ligands 1–4) and 0.02 mmol of
tin(II) chloride in 10 mL toluene containing 1 mmol of styrene (7)
Table 1
2.2.4. (2-Methyl-1,3-propanediol-2-yl)diphenylphosphine borane (4)
A suspension of 244 mg (6.44 mmol, 4 equiv.) of lithium alu-
minium hydride and 22 mL of ether was stirred at room tempera-
ture for 45 min. The suspension was cooled to 0 °C and a solution
of 2 (600 mg, 1.61 mmol, 1 equiv.) in 14 mL of ether was added
drop wise to the LiAlH4/ether mixture using Schlenk syringe meth-
ods. The mixture was stirred at room temperature for 30 min, after
which it was heated under reflux for a further 8 h. TLC was per-
formed to assess the reaction progress. Slow addition of small
pieces of ice destroyed the residual lithium aluminium hydride
to quench the reaction. The product was extracted with DCM, citric
NMR data of the platinum complexes containing monodentate phosphines 1–4.a
1J(Pt,P) (Hz)
Yield of isomerb (%)
b
Complex
dP (ppm)
trans-PtCl2(1)2 (1a)
cis-PtCl2(1)2 (1b)
trans-PtCl2(2)2 (2a)
trans-PtCl2(3)2 (3a)
cis-PtCl2(3)2 (3b)
trans-PtCl2(4)2 (4a)
cis-PtCl2(4)2 (4b)
27.0
3.1
29.7
27.5
3.3
2845
3551
2872
2530
3578
2980
4079
5
95
100
45
55
47
53
39.0
25.4
a
Measured in CDCl3 (room temperature).
Obtained by reacting 2 equiv. of phosphine with PtCl2(PhCN)2.
b

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G. Petocz et al. / Journal of Organometallic Chemistry 694 (2009) 219–222
221
was transferred under argon into a 100 mL stainless steel auto-
clave. The reaction vessel was pressurised to 80 bar total pressure
(CO/H2 = 1:1) and placed in an oil bath and the mixture was stirred
with a magnetic stirrer for the given reaction time. The pressure
was monitored throughout the reaction. After cooling and venting
of the autoclave, the pale yellow solution was removed and imme-
diately analysed by GC–MS.
It is worth noting that the 1J(195Pt,31P) coupling constants for
the ligands possessing ester functionalities (1–3) fall in the typical
ranges cited above. In contrast, both the cis and trans complexes of
the dihydroxy derivative 4 possess larger couplings by about 400–
500 Hz. Accordingly, direct interaction of the hydroxy functional-
ities with the central metal cannot be excluded.
4.2. Hydroformylation reactions
4. Results and discussion
Pre-formed complexes PtCl2(L)2 (where L = 1–4, Fig. 1) were
used as catalysts for the hydroformylation of styrene (Scheme 1)
in the presence of 2 equiv. of tin(II) chloride per Pt under ‘oxo-con-
ditions’ (p(CO) = p(H2), 100 °C, as described in Table 2).
In addition to the aldehyde product regioisomers 6 and 7, some
hydrogenation product (ethylbenzene, 8) may also be expected.
Notwithstanding this possibility, the chemoselectivity towards
aldehydes is very high and the formation of 8 is negligible (less
than 0.5% in all cases and typically below than the GC–MS detec-
tion limit).
4.1. Synthesis of the complexes and their NMR investigation
The monodentate ligands (1–4) used in this study are depicted
in Fig. 1. These ligands were prepared by treatment of Ph2PCl with
the relevant malonate anion obtained by reaction of the malonate
ester with sodium hydride. Smooth conversion to the diphenyl-
malonylphosphine ensued and the borane adducts could be readily
prepared by adding borane–THF to the reaction mixture, allowing
isolation of the desired compounds. Reduction to the diol 4 was ef-
fected by treatment of the malonate derivative with LiAlH4 in
diethylether, without loss of the borane protecting group.
Neutral complexes of general formula PtCl2(phosphine)2 were
synthesised by the reaction of PtCl2(PhCN)2 with two equivalents
of the respective phosphine. Since the ligands are air sensitive they
were stored as their borane adducts and deprotected immediately
prior to (during) the preparation of the corresponding platinum
precursors in a one-pot reaction, to avoid formation of the corre-
sponding phosphine oxides. Both diethylamine and DABCO were
used for the deborylation procedure of the R3P Á BH3 adducts; the
DABCO system proved superior, providing clean reactions and
the Pt complexes in acceptable yields.
Rather surprisingly, the diaryl-monoalkyl type phosphines of
similar structure show different selectivities regarding complex
formation. The two diethylmalonate derivatives (1 and 2) reacted
with the Pt(PhCN)2Cl2 precursor selectively leading to the corre-
sponding bis(monophosphino) complexes PtCl2(P)2 with cis-(1b,
95%) and trans-(2a, 100%) geometries, respectively. The use of the
dibenzylmalonate 3 and the dihydroxy phosphine 4 resulted in the
formation of the cis/trans mixture of 55/45 and 53/47, respectively.
The PtCl2(P)2-type complexes 1a–4a and 1b, 3b and 4b each
showed a single central signal (flanked by platinum satellites, in
a ratio of 1:4:1) in the 31P NMR spectra thereof (Table 1). In these
spectra, the coupling constants are diagnostic, showing values of
2800–3000 Hz or 3500–4100 Hz for the P-trans and P-cis com-
plexes, respectively. The 31P NMR spectra of platinum complexes
provide a sensitive probe for the structures of complexes, even in
complicated mixtures. The magnitude of the one-bond interac-
tions, i.e. 1J(195Pt,31P) coupling constants, has been shown to de-
pend strongly on whether the P atom has another P atom or a
chlorine atom as ligand in the position trans to it on the Pt. The P
atom trans to chloro ligand possesses 1J(195Pt,31P) coupling con-
stants larger than 3500 Hz, while P nuclei trans to another P atom
display typical platinum-phosphorus coupling constants of 2500–
3000 Hz [23].
Although the activity of the platinum catalysts described here is
lower than most of the platinum–diphosphine–tin(II)chloride sys-
tems tested to date [20], the novel ligand-containing catalysts
are of interest from several theoretical points of view. The hydro-
formylation activity of the in situ-generated platinum–tin(II)chlo-
ride catalysts, obtained from PtCl2(1), PtCl2(2) and PtCl2(3)
precursors, is moderate and the conversions obtained in 24 h are
nearly the same (Table 1, entries 1, 3 and 5). The regioselectivity
towards branched aldehyde is high in all cases, especially with 1
and 2, which give the highly sought after yet rarely observed exclu-
sive formation of a single isomer. Higher conversions, accompanied
by only a slight decrease in the regioselectivity, were observed in
elevated reaction times as long as 120 h (entries 2, 4 and 6). It is
worth mentioning that the application of the obvious analogues
catalytic precursor, cis-PtCl2(PPh3)2 resulted in much lower regi-
oselectivity (entry 14). Similar results have been obtained by using
the corresponding Pt–PPh3–SnCl2 ‘in situ’ system for the hydrofor-
mylation of styrene [24].
Most interestingly, the analytical data for the Pt complexes of
ligands 1 and 2 possess opposite stereochemistry: while that of 1
is cis, that of 2 is trans. The fact that the product formation is insen-
sitive to this parameter implies one of several points. It is possible
that the complex dissociates one of its P ligands to form structur-
ally similar active catalysts for both instances, or the binding mode
of the alkene to the Pt and the first step in the catalytic cycle is
unaffected by the geometry of the Pt complex or that the geometry
of one of the complexes changes to match the other in situ. It may
be possible to elucidate which of these factors (or some other char-
acteristic) determines this particular feature of the reaction out-
come by attempting to isolate the Pt complex from the solution
and investigating its structure. This aspect forms part of ongoing
work in our laboratories.
Ligand 4, which is more electron rich than its ester analogues,
enables higher hydroformylation activities to be observed than
its ester counterparts. It is worth noting that the reaction is sensi-
tive to pressure. While the conversion is negligible under 80 bar
pressure (CO/H2 = 1:1), complete conversion may be obtained un-
der 110 bar pressure (compare entries 7, 8 and 9).
O
CH
3
OH
OH
R'
Ph P
OR
OR
Ph P
2
2
CHO
CHO
Pt cat./SnCl
O
2
+
CO/H 1:1
1: R = Et, R' = H
2
4
2: R = Et, R' = Me
3: R = benzyl, R' = H
6
7
5
Fig. 1. The monodentate ligands (1–4) used in this study.
Scheme 1.

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G. Petocz et al. / Journal of Organometallic Chemistry 694 (2009) 219–222
222
Table 2
Hydroformylation of styrene with platinum complexes containing malonate-derived monodentate phosphines (1–4).a
Products formedb
Olig.c (%)
Rbr (%)
d
Run
Catalyst
Solvent
Pressure (bar) p(CO)/p(H2)
Conv. (%)
6 (%)
7 (%)
1
2
3
4
5
6
7
8
PtCl2(1)2 + 2SnCl2
PtCl2(1)2 + 2SnCl2
PtCl2(2)2 + 2SnCl2
PtCl2(2)2 + 2SnCl2
PtCl2(3)2 + 2SnCl2
PtCl2(3)2 + 2SnCl2
PtCl2(4)2 + 2SnCl2
PtCl2(4)2 + 2SnCl2
PtCl2(4)2 + 2SnCl2
PtCl2(4)2 + 2SnCl2
PtCl2(4)2 + 2SnCl2
PtCl2(4)2 + 5SnCl2
PtCl2(4)2 + 5SnCl2
PtCl2(PPh3)2 + 2SnCl2
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
60/60
60/60e
60/60
60/60e
60/60
60/60e
40/40
55/55
55/55e
40/40
40/40
40/40
60/60
60/60
23
67
53
90
23
85
4
76
100
100
39
100
58
66
0
0
0
0
0
0
0
0
23
65
53
88
21
77
2
65
85
0
0
2
0
2
2
8
2
11
15
0
0
0
100
97
100
98
91
90
50
86
85
-
9
Toluene
0
10
11
12
13
14
[BMIM][PF6]
[BMIM][BF4]
[BMIM][BF4]
[BMIM][BF4]
Toluene
100
39
100
55
0
0
0
2
30
-
-
67
45
1
36
a
b
c
Reaction conditions: Pt/styrene = 1:100; T = 100 °C; t = 24 h.
The rest is unconverted substrate.
Dimers, oligomers, polymers [determined by GC (dodecane internal standard)]; the amount of the recovered substrate 1 is not indicated.
Rbr = regioselectivity towards branched aldehyde regioisomer [moles of 6/(moles of 6 + moles of 7) Â 100 ].
Reaction time: 120 h.
d
e
[3] I. Schwager, J.F. Knifton, J. Catal. 45 (1976) 256.
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tems for several reactions including metathesis [25] and hydrofor-
mylation [26] reactions. Ligand 4 was expected to be a suitable
ligand in ionic liquid catalysis due to the presence of the two polar
hydroxyl groups enabling good solubility. Nevertheless, and disap-
pointingly, attempted hydroformylation under standard conditions
set out herein led exclusively to styrene oligomerisation and no
formyl products were detected at all (entries 10–12). Even the in-
crease of the total pressure to 120 bar (p(CO) = p(H2) = 60 bar) re-
sulted in the formation of aldehydes in traces only (entry 13).
In summary, the chemoselectivity of hydroformylation was
excellent throughout, notwithstanding relatively slow rates of
reaction. Importantly, ethylbenzene could be detected by GC–MS
analysis in traces only, implying high degrees of chemoselectivity
and that the catalysts are poor hydrogenation promoters. The reg-
ioselectivity is significantly influenced by the phosphorus ligands,
and the branched aldehyde (2-phenyl-propionaldehyde) prevails
under all conditions investigated, with single isomers (100%
branched product) being observed in some cases.
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