Efficient platinum(II) catalyzed hydroformylation reaction in water: Unusual product distribution in micellar media

Article abstract of DOI:10.1002/adsc.201000341

The hydroformylation of a variety of terminal and internal alkenes is efficiently performed by cationic platinum triflate complexes of the type [P₂Pt(H₂O)₂](OTf)₂ under mild conditions in an aqueous micellar medium. The use of surfactants is essential to ensure dissolution of the catalyst and substrate in water with catalysts being positioned on the anionic surface of the micelles. Aldehydes are obtained with linear to branched ratios up to >99:1. With styrene derivatives also the corresponding benzaldehydes are formed. The catalyst can be separated by extraction of the organic products with hexane and recycled for at least four times with only a modest loss of activity and no effect on selectivity.

Full text of DOI:10.1002/adsc.201000341

FULL PAPERS
DOI: 10.1002/adsc.201000341
Efficient Platinum(II) Catalyzed Hydroformylation Reaction in
Water: Unusual Product Distribution in Micellar Media
a
a,
a
a,
Marina Gottardo, Alessandro Scarso, * Stefano Paganelli, and Giorgio Strukul *
a
Dipartimento di Chimica, Universitꢀ Caꢁ Foscari Venezia, Dorsoduro 2137, 30123 Venezia, Italy
Fax : (+39)-041-234-8517; phone: (+39)-041-234-8569 (AS), (+39)-041-234-8931 GS); e-mail: alesca@unive.it or
strukul@unive.it
Received: May 3, 2010; Revised: July 16, 2010; Published online: September 8, 2010
Abstract: The hydroformylation of a variety of ter- With styrene derivatives also the corresponding ben-
minal and internal alkenes is efficiently performed zaldehydes are formed. The catalyst can be separated
by cationic platinum triflate complexes of the type by extraction of the organic products with hexane
[
P Pt ACHTUNGTRENNUNG( H O) ] ACHTUNGTRENNUNG( OTf) under mild conditions in an and recycled for at least four times with only a
2 2 2 2
aqueous micellar medium. The use of surfactants is modest loss of activity and no effect on selectivity.
essential to ensure dissolution of the catalyst and
substrate in water with catalysts being positioned on
the anionic surface of the micelles. Aldehydes are Keywords: diphosphines; hydroformylation; mi-
obtained with linear to branched ratios up to >99:1. celles; platinum; selectivity
Introduction
medium. To circumvent such problems, possible solu-
tions are i) the use of intrinsically water-soluble cata-
The industrial synthesis of fine chemicals and pharma- lysts, ii) the use of water-soluble ligands bearing polar
[
10]
ceutical drugs is witnessing a new era characterized and/or charged moieties
by implementation of sustainability criteria and the water and iii) the employment of supramolecular sys-
to impart solubility in
[
1]
twelve principles of green chemistry. In a chemical tems, like the use of surfactants, whose role is to cope
production, about 80% of the total amount of waste with reagents and catalysts.
The hydroformylation reaction is one of the oldest
covery in only about 70% of cases. Replacement of homogeneous catalytic processes and still a multi-mil-
organic and especially chlorinated solvents with lion tonnes, highly valuable chemical transformation
[2]
is solvent that can be incinerated allowing heat re-
[
3]
[
11]
greener ones is an important quest of green chemistry. characterized by complete atom efficiency to pro-
Among neoteric solvents such as scCO , ionic liquids duce linear and branched aldehydes as intermediates
2
and perfluorinated solvents, water is the only one on the way to plasticizers and detergent alcohols. This
that, together with low cost, negligible impact for the reaction is classically mediated by cobalt and rhodi-
[
2,4]
[12]
environment, zero E factor value
and high safety, um catalysts and to date high catalytic activity, high
often shows extra features like enhanced selectivity regioselectivity for linear/branched products as well
[
13]
(
chemo-, regio-, stereo- and enantio-) if compared to as enantioselectivity have been obtained. With the
[
5]
the use of organic media for the same process.
aim of developing an easy catalyst recycling, water
Water as an alternative reaction medium is highly de- has been introduced as solvent for this reaction
sirable and is witnessing a sort of renaissance be- almost 25 years ago leading to a commercial process
cause of its many advantages compared to other developed by Ruhrchemie-Rhꢂne Poulenc in 1984 for
media. The hydrophobic effect, that is peculiar for propene hydroformylation using tris-sulfonated tri-
aqueous solutions, is responsible for most of the in- phenylphosphine (TPPTS) as intrinsically water-solu-
creased selectivities observed in chemical transforma- ble ligand. The system can only be applied to short-
tions performed by reactants both dissolved in water chain olefins because of the very poor solubility of
and under “on water” conditions. However, it also longer ones in water. To circumvent the problem pos-
[14,15]
[6]
[7]
[8]
[
16]
[
9]
implies some challenging drawbacks, in primis the sible approaches have been based on the use of host-
[
17]
[18]
generally low solubility of organic substrates and of ing scaffolds like cyclodextrins or calix[4]arenes
the majority of metal complex catalysts, most of able to dissolve substrates in water and capable of
which are also inactivated/decomposed by this steering substrate selectivity, but the size of the host
Adv. Synth. Catal. 2010, 352, 2251 – 2262
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2251

Marina Gottardo et al.
FULL PAPERS
cavity is a strong limitation for large substrates. Micel- Results and Discussion
lar media are even simpler solutions to this problem
improving substrate solubility in water via sequestra- The Catalysts
tion in the apolar core of the self-assembled micellar
aggregate, with concomitant increase of catalytic ac- As mentioned above, Pt(II) hydroformylation reac-
tivity and some particular effects also on selectivity, tions have so far been performed starting from chloro
as observed for rhodium catalysts bearing water-solu- complexes with the addition of SnCl under anhy-
2
[19,20]
ble phosphines.
drous conditions as a labilizing ligand to activate the
Indeed micellar media can do much more. Our catalyst (Scheme 1). Few exceptions are known with
[
30d]
group has recently demonstrated that surfactants can tin dodecarborate
instead of SnCl or based on
2
[
21]
[22]
be used to dissolve in water Pt(II),
Co(II)
and dimethyl Pt(II) complexes [(PP)Pt ACHTUNTREGNNUN(G CH ) ] and
3 2
[33]
[23]
Ru
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(III) organometallic complexes bearing ordinary
B
A
H
U
G
R
N
N
(C F ) .
Our idea was to use bis-triflate Pt(II)
6
5 3
phosphine or salen ligands developed to work in complexes which can be easily prepared by halide ab-
common organic solvents. This prevents the need of straction from the corresponding dichloro complex
modifying the ligands with water-soluble tags like sul- with AgOTf (Scheme 1). Triflate is a weakly coordi-
fonated moieties that often can negatively affect the nating anion and such Pt(II) species can be better de-
[
10]
performance of the catalyst due to modification of scribed as bis-aquo complexes with external triflate
electronic and steric effects. Supramolecular interac- counteranion. They can be used directly in water in
tions between catalyst and micelles via ion pairing as the presence of suitable surfactants in which they
well as hydrophobic effect are responsible for the dis- have been already tested in the Baeyer–Villiger oxi-
[
21c]
solution of the catalyst in the micelle taking advant- dation of ketones
where they behaved as soft
capable of easy interaction with soft
More- Lewis bases like alkenes or CO and H2.
over, enhancement of both catalytic activity and enan- As shown in Scheme 1 the classical and the alterna-
tioselectivity was observed in several oxidation reac- tive (here reported) routes to hydroformylation start
[
21b]
age of a higher local reactants concentration and ena- Lewis acids
[
21b]
bling in some cases also catalyst recycling.
[21,22]
tions
in the micellar medium compared to from the common [P PtCl ] nominal species. Diphos-
2 2
common organic solvents.
phines used include two alkyl diphosphines with dif-
Pt(II) complexes have emerged in recent years as ferent steric hindrance (complexes 1a and b) and tet-
valuable hydroformylation catalysts with interesting raaryldiphosphines with increasing bite angle (com-
[24,25,26]
results
in terms of enantioselectivity even if plexes 1c–f and ligands 1L-h and 1L-j). A bridging hy-
with a lower catalytic activity compared to rhodium droxo Pt(II) dimer (1g) was also tested as catalyst.
[27]
species.
All Pt(II) catalysts reported were mono-
meric chloro derivatives bearing a diphosphine or two
[
28]
monophosphines or other ligands
activated with Surfactant Screening and Choice of the Optimum
SnCl always tested in organic media or in ionic liq- Conditions
2
[
29]
uids.
The role of tin chloride is still incompletely
[30]
understood, although it is assumed that the ÀSnCl
Preliminary catalytic tests to check the effect of dif-
3
moiety formed by interaction with PtÀCl is more ferent surfactants on the activity and selectivity were
labile compared to ÀCl itself and can be easily dis- carried out with styrene as substrate and 1e as catalyst
placed giving access to substrate interaction with Pt.
as indicated in Scheme 2. Reactions were performed
We have recently developed some efficient Pt(II) under moderate experimental conditions (708C,
catalysts for the isomerization of terminal olefins into 80 bar) and the results are summarized in Table 1.
[
31]
internal ones under mild experimental conditions.
Catalytic activity resulted to be highly dependent
Insight into the mechanism of the reaction showed on the medium used, with reaction in organic media
that the resting state of the catalyst was an agostic characterized by low yields in hydroformylation prod-
Pt(II)-alkyl species that can be easily formed from ucts as observed in Table 1 (entries 1 and 2) while in
[
32]
[
(PP)Pt(H)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(H O) ]
A
H
U
G
R
N
N
and the alkene. Such spe- water with SDSU, SHSU, SDBS and SDS as surfac-
2
2
cies are common intermediates in several catalytic re- tants the yield markedly increased (entries 7–11) with
actions involving alkenes, including hydroformylation. a good linear/branched (l/b) ratio. It is worth noting,
This observation prompted us to develop a new but will be extensively discussed later, that benzalde-
Pt(II)-based hydroformylation system in water using hyde 2a was produced as by-product in all tests per-
surfactants to promote both catalyst and substrate sol- formed with styrene. To the best of our knowledge
ubilization.
the presence of benzaldehyde in the hydroformylation
of styrene has never been observed before. Different
categories of surfactants were tested, with neutral
ones like TritonX100 and SPAN60 leading only to low
conversions (entries 4 and 5) similarly to cationic
2252
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Adv. Synth. Catal. 2010, 352, 2251 – 2262

Efficient Platinum(II) Catalyzed Hydroformylation Reaction in Water
Scheme 1. Different strategies for the preparation of Pt(II) hydroformylation catalysts and structure of the Pt(II) complexes
1a–g used in the hydroformylation reactions in water in the presence of different surfactants.
CTA-NO (entry 6) and zwitterionic species (entry 3) crease in selectivity for the linear aldehyde is ob-
3
providing hydroformylation products up to 15%. served (Figure 1). Best results were obtained with
Anionic surfactants turned out to be the best ones in SDS as surfactant with quantitative yield and an 82:9
promoting catalyst solubilization and this is not sur- l/b selectivity (Table 1, entry 11).
prising in view of the cationic nature of the catalyst.
Sulfonate tensides (SDSU, SHSU, SDBS) showed the
formation of significant amounts of benzaldehyde Effect of the Diphosphine Ligand
that can be reduced with sulfate SDS tenside. An in-
crease in the reaction temperature to 1008C did not Once established the best experimental conditions, we
improve product formation or selectivity (Table 1, screened
a
series of different Pt(II) catalysts
entry 10), while a screening of the effect of syn gas (Scheme 1, Table 2). In agreement with the general
pressure showed that better yields were possible by behaviour of the hydroformylation reactions alkyl di-
working above 60 bar, where a concomitant slight de- phosphines a and b (Table 2, entries 1 and 2) showed
Adv. Synth. Catal. 2010, 352, 2251 – 2262
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2253

Marina Gottardo et al.
FULL PAPERS
Scheme 2. Hydroformylation reaction of alkenes in water in the presence of surfactant mediated by monomeric Pt(II) com-
plex 1e.
Table 1. Screening of the reaction medium in the hydrofor- vours the reaction in agreement with that observed in
[a]
mylation of styrene catalyzed by 1e.
the literature for diphosphine ligands with large bite
angle such as Xantphos. Unfortunately, attempts to
prepare bis-aquo triflate complexes from Xantphos or
[
24]
[b]
[c]
[c]
#
Reaction Medium
Temp.
8C]
Yield
[%]
2a:2b:2c
[
1,6-bis-diphenylposphinohexane failed, and even at-
1
2
3
4
DCE
toluene
70
70
70
70
12
8
2
8:69:23
27:48:25
–
–
tempts to form the catalyst in situ by reaction of the
corresponding dichloro complexes with two equiva-
lents of AgOTf did not seem to work (entry 10). The
use of the dimeric complex 1g resulted in poor activi-
ty and selectivity. Interestingly the use of SDS as sur-
factant instead of SDBS in two cases (entries 6 and 8)
led to a significant increase in yield and a drastic re-
duction of the amount of benzaldehyde produced.
H O/zwitterionic
2
H O/TRITON
2
2
X100
5
6
7
8
9
1
1
H O/SPAN 60
70
70
70
70
70
100
70
15
11
30
93
88
78
100
25:54:21
100:0:0
17:80:3
10:83:7
18:72:10
33:60:7
9:82:9
2
H O/CTA-NO3
2
H O/SDSU
2
H O/SHSU
2
H O/SDBS
2
0 H O/SDBS
1 H O/SDS
2
Catalyst Positioning
2
[
a]
À2
Experimental conditions: 1e 1% mol, [sub]=8.6ꢄ10 M,
As recently disclosed in oxidation reactions per-
formed with Pt(II) catalysts in micellar media,
NOESY and DOSY experiments allow us to demon-
strate the presence of reciprocal interactions between
À2
[
surfactant]=7.5ꢄ10 M, P 80 bar, H /CO 1:1, H O
2
2
1
0 mL time 22 h.
[
b]
DCE: 1,2-dichloroethane; zwitterionic: N-dodecyl-N,N-
dimethyl-3-ammonio-1-propanesulfonate; TritonX100:
[
21b]
polyoxyethylene(10)isooctyl phenyl ether; SPAN60: sor- the micelles and catalyst.
These experiments were
bitan monostearate; CTANO : hexadecyltrimethylammo- carried out for 1e in D O in the presence of SDS, the
3
2
nium nitrate; SDS: sodium dodecylsulfonate; SHSU: catalyst being completely insoluble in water. The
3
1
sodium hexadecylsulfonate; SDBS: sodium dodecylben-
zensulfonate.
Determined by H NMR integration.
P NMR spectrum of 1e (Figure 2, A) shows a singlet
at 3.22 ppm with a slight shift to higher field with re-
spect to the same complex in CDCl (2.84 ppm). The
[
c]
1
3
lack of evidence for P-Pt coupling could be easily due
moderate yields (43 and 16%, respectively) while to a rapid H O exchange between the complex and
2
1
with bis-diphenylphosphines characterized by differ- the reaction medium. The H NMR spectrum is
ent bite angles an increasing trend between catalytic shown in Figure 2, B, while DOSY experiment
activity and diphosphine bite angle was observed (Figure 2, C) clearly shows that the catalyst has a sim-
(
Table 2, entries 3–8) with best results obtained with ilar diffusion coefficient compared to the micelles. No
catalysts 1e and 1f. The latter, bearing a ferrocenyldi- cross-peaks were present in the NOESY spectrum be-
phoshine and characterized by higher rigidity com- tween 1e and SDS resonances. The use of a more con-
pared to dppb (1,4-bis-diphenyphosphinobutane), fa- centrated solution (100 mM in SDS and saturated
2254
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Adv. Synth. Catal. 2010, 352, 2251 – 2262

Efficient Platinum(II) Catalyzed Hydroformylation Reaction in Water
Figure 1. Effect of the total pressure on the reaction yield and product distribution in the hydroformylation of styrene medi-
ated by 1e in water/SDBS.
Origin of Benzaldehyde and Reaction Mechanism
Table 2. Pt(II) 1a–g catalysts screening in the hydroformyla-
tion of styrene in water/SDBS.
[a]
Benzaldehydes formation in the hydroformylation re-
action occurs only with styrene and substituted styr-
enes (see Table 3). However benzaldehyde is not a
side product coming from decomposition of the pri-
mary linear and branched products of the reaction as
was demonstrated in blank experiments using 2-phe-
nylpropanal and 3-phenylpropanal as substrates under
hydroformylation conditions and showing no reaction.
In a second experiment indene was used as sub-
strate. In this case (Scheme 3) a total 7% aldehyde
products was observed. These were identified by
[
b]
[b]
#
Catalyst P-P Bite Angle Yield [%] 2a:2b:2c
1
2
3
4
5
6
7
8
9
1
1a
1b
1c
–
–
43
16
0
59
88
99
75
99
6
33:54:13
54:41:5
–
4:73:23
18:72:10
9:82:9
100:0:0
7:83:10
59:26:15
–
85
91
98
98
96
96
98
111.4
1d
1e[
1e
1f[
c]
c]
1f
1g
0
1L-h
0
1
H NMR and GC-MS analyses. The major species
shows a doublet at 9.72 ppm typical of hydroformyla-
[
a]
Experimental conditions: catalyst 1% mol, [sub]=8.6ꢄ
À2
À2
1
0
M, [SDBS]=7.5ꢄ10 M, P 80 bar, H /CO 1:1, tem- tion products A1 and A2, while the minor one shows
2
perature 708C, H O 10 mL, time 22 h.
Determined by H NMR integration.
SDS was employed instead of SDBS.
a singlet at 9.84 ppm that can be only attributed to a
benzaldehyde as in B1 and B2. GC-MS analyses of
the two products were in agreement with the struc-
ture proposed confirming that benzaldehyde comes
from the splitting of the single bond between the aryl
2
[
[
b]
1
c]
with 1e corresponding to approximately 4 times the group and vinyl residues on the adjacent C atom in a
concentration reported in Figure 2, C) and different Pt-alkyl intermediate that is involved in the formation
mixing times (900, 600 and 300 ms, respectively) led of the linear aldehyde.
in the latter case to the observation of extremely
A plausible mechanism for the hydroformylation of
weak cross-peaks between the aromatic residues of 1e styrene is shown in Scheme 4 (cycle A) and is based
[34]
and the first methylene of SDS. This means that the on the known chemistry of Pt in this reaction.
It
catalyst, probably because of charge pairing, interacts also accounts for the formation of benzaldehyde
with the surface of the anionic micelles without being (cycle B) starting from the Pt alkyl intermediate. In
buried in the hydrophobic core, acting as counter fact such species can either insert CO or, alternative-
cation for the sulfate anions of the surfactant. Minor ly, driven by the large bite angle of the diphosphine
secondary species are also present but not assigned to favour the b-aryl migration of the phenyl group to the
specific complexes. Similar features were observed metal center leading to a Pt-aryl species that further
also for complex 1f.
insert CO eventually providing benzaldehyde
Adv. Synth. Catal. 2010, 352, 2251 – 2262
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Marina Gottardo et al.
FULL PAPERS
31
1
Figure 2. A) P, B) H and C) DOSY spectra of catalyst 1e (0.9 mM) in D O with SDS (75 mM).
2
(
Scheme 4, cycle B). Although b-aryl migration is un- not possible; in this case no benzene was formed sug-
+
common the inverse reaction that is, alkene insertion gesting that PtÀC cleavage by H O is faster in Pt-
3
into a Pt-aryl species was recently observed by us in alkyls than in Pt-aryls. Under hydroformylation condi-
the isomerization of olefins with [(P-P)Pt CAHTUNGTRNEU(GN C F ) tions neither ethylbenzene nor propanal (from the hy-
6 5
[
31]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(H O)]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OTf) complexes.
Further support to droformylation of ethene) were observed.
2
Scheme 4 was found in the following experimental ob-
servations: (i) the isomerization of allylbenzene to cis-
and trans internal alkenes at 508C catalyzed by 1e in Substrate Scope
the presence of 1 bar of hydrogen suggesting the for-
mation of a Pt hydride intermediate; (ii) the existence This catalytic system was applied to a wide range of
of a Pt-alkyl intermediate with agostic interaction substrates with different kinds of C=C bond (Table 3).
3
1
1
1
(
P NMR 18.3 ppm, JP,Pt 5266 Hz and 14.9 ppm, J
As already mentioned above, a series of styrenes sub-
156 Hz) as resting state in the same reaction, similar- stituted in the 4 position like p-Cl, p-Me and p-OMe
(C F ) (entries 2–4) led to the formation of the expected hy-
P,Pt
3
ly to what was already observed with [(dppb)PtCAHTUNGTRENNNUG
ACHTUNGTRENNUNG( H O)] ACHTUNGTRENNUNG( OTf); (iii) the hydrogenation of styrene to droformylation products in good yields along with the
6
5
[31]
2
ethylbenzene in the absence of CO suggesting a short- corresponding benzadehyde. It is worth noting that
cut in cycle A when CO coordination and insertion is there are no examples in the literature of such a kind
2256
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ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 2251 – 2262

Efficient Platinum(II) Catalyzed Hydroformylation Reaction in Water
[a]
Table 3. Substrate screening in the hydroformylation of alkenes in H O/SDS catalyzed by 1f.
2
[b]
[b]
#
Substrate
Products
Yield [%]
l/b
[
c]
1
99 (7)
99 (3)
89:11
87:13
91:9
[c]
2
3
[
c]
99 (11)
83 (17)
[
c]
4
5
91:9
[c]
57 (0)
100:0
6
7
8
99
79
99
96:4
95:5
99:1
9
1
1
58
69
21
100:0
100:0
92:8
0
1
[d]
12
8
13
14
15
0
4
0
–
–
–
1
1
6
7
0
–
85
95:5
Adv. Synth. Catal. 2010, 352, 2251 – 2262
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Marina Gottardo et al.
FULL PAPERS
Table 3. (Continued)
[b]
[b]
#
Substrate
Products
Yield [%]
l/b
1
8
9
19
82:18
77:23
1
99
[
a]
À2
À2
Experimental conditions: 1f 1% mol, [sub]=8.6·10 M, [SDS]=7.5·10 M, P 80 bar, H /CO 1:1, H O 10 mL, 708C, time
2
2
2
2 h.
[
[
b]
c]
1
Determined by H NMR.
Number in parentheses refer to the yield in benzaldehyde. Several unassigned aldehydes.
[d]
of reactivity in hydroformylation reactions, even with Catalyst Recycling
other metal catalysts and conversions of styrene de-
rivatives directly into the corresponding benzalde- In principle, the use of micellar media to dissolve the
hydes have been reported only in two contributions catalyst implies the possibility to separate and recycle
both under oxidative conditions characterized both by the catalyst from the products by extraction of the
[35]
low selectivity. Even the highly congested 2,4,6-tri- latter with a water-immiscible solvent followed by
methylstyrene (entry 5) reacted but, because of the phase separation. Vinylcyclohexane was chosen as
high steric hindrance imparted by the substituents on model substrate and a few tests were performed to
the aromatic ring, only the linear aldehyde was choose the most appropriate catalyst concentration,
formed.
observing that in order to ensure high productivity
Terminal alkenes are highly reactive as confirmed 1% mol of 1f was required (with 0.5% mol: yield
observing entries 6–8, with good results even with vi- 53% l/b 99:1; 0.1% mol: yield 10%, l/b 99:1). In
nylcyclohexane as a sterically demanding substrate. Table 4 are reported the results observed in four cata-
Dienes as in entry 9 are hydroformylated only in the lytic cycles for the hydroformylation of vinylcyclohex-
terminal position, while hydroformylation of allylben- ane as substrate with 1f in water with SDS as surfac-
zene (entry 10) occurs readily without formation of tant for 22 h at 708C. At the end of each catalytic
benzaldehyde or similar by-products in agreement cycle the system was extracted with hexane to remove
with Scheme 4. Catalytic activity decreases depending the reaction products and the pale yellow aqueous
on the presence of polar functional groups on the sub- phase containing the catalyst was reused with the ad-
strate (entries 11–16) that will hamper dissolution of dition of substrate, CO and H for the next catalytic
2
the substrate in the apolar core of the micelle, thus cycle. After four cycles only a small decrease in pro-
limiting the contact between substrate and catalyst. ductivity and the same high level of selectivity was
Internal alkenes are suitable substrates too, as shown observed. The total TON was 379. No activity was
in entries 17 and 18 leading to high yields in linear hy- shown by the organic layer.
droformylation products as the result of a prior iso-
merization process that equilibrates all the possible
[31]
alkene isomers. Under these circumstances the hy-
droformylation process occurs only on the more reac- Table 4. Recycling of the aqueous phase in the hydroformy-
[a]
tive terminal alkenes with the overall effect to pro- lation of vinylcyclohexane catalyzed by 1f in H
O/SDS.
2
vide mostly linear aldehydes. Norbornene (entry 19)
[
b]
[b]
#
Recycle
Yield [%]
Linear/branched
containing a double bond that cannot be isomerized
yields aldehydes in high conversion and with a 77/23
endo/exo ratio. Finally, substituted alkenes like a-
methylstyrene, b-pinene , a-isophorone and camphene
were not suitable substrates probably because of their
difficult coordination to the metal center, in agree-
ment with the trend known for the association con-
1
2
3
4
fresh
99
98
90
92
99:1
99:1
99:1
99:1
st
1
2
3
nd
rd
[a]
À2
Experimental conditions: 1f 2% mol, [sub]=8.6ꢄ10 M,
SDS]=7.5ꢄ10 M, P 80 bar, H /CO 1:1, temperature
À2
[
2
[
36]
stants to such Pt complexes.
708C, H O 40 mL, time 22 h.
2
[b]
1
Determined by H NMR.
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Adv. Synth. Catal. 2010, 352, 2251 – 2262

Efficient Platinum(II) Catalyzed Hydroformylation Reaction in Water
1
Scheme 3. Low-field portion of the H NMR spectrum and mass spectra of the products of the hydroformylation of indene
catalyzed by complex 1f in H O/SDS.
2
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2259

Marina Gottardo et al.
FULL PAPERS
Scheme 4. Proposed hydroformylation mechanism accounting for the formation of benzaldehyde. Triflate omitted for clarity.
Conclusions
Experimental Section
In the present contribution we demonstrated that General Remarks
simple bis-cationic Pt(II) complexes bearing diphos-
phines with large natural bite angles can be success-
fully employed as catalysts for the hydroformylation
of alkenes in water and in the presence of anionic sur-
1
31
1
H NMR, and P{ H} NMR spectra were recorded at 298 K,
unless otherwise stated, on a Bruker AVANCE 300 spec-
trometer operating at 300.15, 121.50 MHz, respectively. d
values in ppm are relative to SiMe and 85% H PO . All re-
4
3
4
1
factants, in particular SDS. The system shows catalytic actions were monitored by H NMR.
activity comparable, and in some cases better, than
[
26]
traditional Pt-Sn systems, with good l/b selectivities
that can rival with the best Rh catalysts. When styr-
Synthesis of the Complexes
enes are employed as substrates the unusual forma- All work was carried out with the exclusion of atmospheric
tion of significant amounts of benzaldehydes is ob- oxygen under a dinitrogen atmosphere using standard
Schlenck techniques. Solvents were dried and purified ac-
cording to standard methods. The catalysts 1a–e and
served via a b-aryl elimination step. The catalytic
system can be successfully applied to a large range of
[
(dppf)PtCl2] (dppf=diphenylphosphino-ferrocene) were
substrates characterized by terminal or internal cis-
double bonds and even with the latter high selectivi-
ties to the most desired linear aldehydes is observed
thanks to a prior isomerization process. The catalyst
can be recycled up to four times employing 1% mol
synthesized following procedures already reported in the lit-
[32]
erature. New compounds are described below.
[(dppf)Pt(H O) ](OTf) (1f): [(dppf)PtCl ] (0.14 mmol)
was dissolved in CH Cl /acetone (25+5 mL, respectively)
A
H
U
G
R
N
U
G
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
2
2
2
2
2
2
and to the solution were added 0.56 mL of a 0.5M solution
as catalyst loading with only a modest decrease in ac- of AgOTf (0.28 mmol) in acetone. After stirring for 2 h at
tivity and no effect on selectivity.
room temperature the mixture was filtered off and the fil-
More generally, given the good catalytic activity trate concentrated to a small volume. Addition of n-hexane
led to precipitation of the complex that was filtered, washed
and selectivity displayed towards a variety of sub-
strates, the present catalytic system constitutes a po-
tential alternative to the use of sulfonated phosphines
to practice hydroformylation reaction in water, with
the advantage of exploiting the huge ligand know-
1
with hexane and dried under vacuum; yield: 69%. H NMR
(
CDCl ): d=7.66–7.47 (20H, m), 2.63 (4H, m), 2.05 (4H,
3
31 1 1
m); P{ H} NMR: d=2.84 (s, J =3868 Hz).
P,Pt
how acquired in years of research in this popular pro- Substrates
cess without the need to modify the ligands.
All the terminal and internal alkenes used as substrates are
commercial products (Aldrich) and were used as received
without any further purification. CO and H2 (>99.9%
purity) were purchased by SIAD. Aldehyde products of the
hydroformylation reactions were identified by GC-MS anal-
ysis and comparison with the NMR data of authentic sam-
ples.
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Adv. Synth. Catal. 2010, 352, 2251 – 2262

Efficient Platinum(II) Catalyzed Hydroformylation Reaction in Water
Catalytic Studies
Acknowledgements
General procedure for hydroformylation reaction with com-
plexes 1a–f in water with surfactants: The hydroformylation
experiments were carried out in a glass-lined, 100-mL stain-
less steel autoclave containing a magnetic stirring bar. In a
typical run, the autoclave was flushed three times with N2
and loaded with 10 mL of bi-distilled water, 0.75 mmol sur-
factant, 0.0086 mmol catalyst and 0.86 mmol of substrate
The authors thank Mi.U.R., the Universitꢀ Caꢁ Foscari Vene-
zia and “Consorzio Interuniversitario Nazionale per la Scien-
za e Tecnologia dei Materiali” for financial support. G.S.
thanks Johnson-Matthey for the loan of platinum. The help
of L. Sperni for GC-MS analysis is gratefully acknowledged.
under N flow. Once closed, the autoclave was flushed twice
2
with syn gas (CO/H 1:1 v/v), then pressurised with a 1/1
2
CO/H mixture at the desired pressure and thermostatted at
2
the chosen temperature for 22 h under vigorous stirring. References
Then the autoclave was cooled down to room temperature,
the pressure was released and the clear pale yellow solution
was extracted three times with 10 mL of ethyl acetate. The
organic phases were combined and concentrated under re-
duced pressure. Conversion, product assignment and distri-
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