Aqueous-phase hydroformylation of 1-octene using hydrophilic sulfonate salicylaldimine dendrimers

Article abstract of DOI:10.1039/c2dt31471a

Water-soluble dendritic ligands based on tris-2-(5-sulfonato salicylaldimine ethyl)amine (5) and DAB-(5-sulfonato salicylaldimine) (6) (DAB = diaminobutane) were synthesized by means of Schiff base condensation and sulfonation reactions. These dendritic l

Full text of DOI:10.1039/c2dt31471a

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PAPER
Aqueous-phase hydroformylation of 1-octene using hydrophilic sulfonate
salicylaldimine dendrimers†‡
Emma B. Hager, Banothile C. E. Makhubela and Gregory S. Smith*
Received 5th July 2012, Accepted 5th September 2012
DOI: 10.1039/c2dt31471a
Water-soluble dendritic ligands based on tris-2-(5-sulfonato salicylaldimine ethyl)amine (5) and DAB-
(5-sulfonato salicylaldimine) (6) (DAB = diaminobutane) were synthesized by means of Schiff base
condensation and sulfonation reactions. These dendritic ligands were fully characterized by 1H NMR,
13C NMR and FT-IR spectroscopy, elemental analysis and mass spectrometry. Dendritic ligands (5 and 6)
in combination with [RhCl(COD)]2 (COD = 1,5-cyclooctadiene) were evaluated in aqueous biphasic
hydroformylation of 1-octene. New water-soluble mononuclear 5-sulfonato propylsalicylaldimine Rh(I)
complexes (7 and 8) were synthesized and characterized using 1H NMR, 13C NMR and FT-IR
spectroscopy, elemental analysis as well as mass spectrometry. These complexes were applied as catalyst
precursors in aqueous biphasic hydroformylation reactions. All the catalyst precursors were active in the
hydroformylation of 1-octene under the investigated conditions. Optimal conditions were realized at
75 °C (40 bars), where the best selectivity for aldehydes was noticed. Catalyst recycling was achieved up
to 5 times with minimal loss in conversion and consistent chemoselectivities and regioselectivities. Less
Rh leaching was observed in the dendritic systems (5 and 6)/[RhCl(COD)]2 as compared to mononuclear
catalyst precursors (7 and 8) as determined by inductively coupled plasma-mass spectrometry (ICP-MS).
Biphasic catalysis has found success industrially: the
Rûhrchemie/Rhône Poulenc process utilizes a water-soluble
Introduction
Aqueous biphasic catalysis (Fig. 1) combines the advantages of
homogeneous and heterogeneous catalysis. This type of system
involves the presence of two phases in one reactor: the aqueous
phase in which the catalyst is dissolved, and the organic phase
containing the substrate. During the catalytic process, heat,
pressure and vigorous stirring allow the two phases to come into
contact and the catalytic reaction to occur. At the end of the reac-
tion the organic phase, now containing the products, can be de-
canted from the immobilized aqueous catalyst, which can then
be recycled into further catalytic reactions.1–3 The process, then,
allows the facile separation of a homogeneous catalyst from the
reaction products.1–3
This type of catalytic process is attractive because it reduces
the need for hazardous organic solvents and makes use of water
which is non-toxic and environmentally friendly. Recently there
has been great interest in the development of water-soluble metal
complexes as catalyst precursors for a wide range of reactions,
including hydrogenation, hydroformylation, carbonylation and
alkene metathesis.4–16
rhodium catalyst containing sulfonated phosphine ligands
(Fig. 2) in the hydroformylation of propene to butyralde-
hyde.1a,2a This process has been found to be extremely efficient,
with the catalyst showing high activity and selectivity for the
desired product, the linear aldehyde. It is economically advan-
tageous, there is low catalyst leaching and high catalyst recy-
cling. Furthermore, the use of water as solvent and the avoidance
of separation processes such as distillations address some safety,
environmental stewardship and economical considerations.1,2
Unfortunately, there is one major drawback to the process
described above. The substrate must be sufficiently soluble in the
aqueous phase in order for chemical transformation to occur.
The Rûhrchemie/Rhône Poulenc reaction is limited to the shorter
chain olefins propene and 1-butene due to the poor solubility of
higher olefins, such as 1-octene, in water.1,2,17 An interesting
strategy for overcoming this particular challenge lies in the use
of so-called thermoregulated biphasic systems, whereby the cata-
lyst switches from one phase to the other depending on the temp-
erature.18,19 Alternatively, the use of amphiphilic ligands could
increase the affinity of the catalyst for the organic phase during
the catalytic reaction, while allowing the catalyst to retain its
water-soluble nature. The use of sulfonated phosphines incorpor-
ating long aliphatic pendant arms, for example, has yielded good
results in the hydroformylation of 1-tetradecene.20
Department of Chemistry, University of Cape Town, Rondebosch 7701,
Cape Town, South Africa. E-mail: Gregory.Smith@uct.ac.za
†Dedicated to Professor David Cole-Hamilton on the occasion of his
retirement and for his outstanding contribution to transition metal
catalysis.
In this paper, in situ generation of water soluble Rh(I) metallo-
dendrimers based on tris-2-(iminoethyl)amine and poly(propyl-
eneimine) dendritic scaffolds (bearing sulfonate groups) for the
‡Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt31471a
This journal is © The Royal Society of Chemistry 2012
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Fig. 1 Diagram showing principle of aqueous biphasic catalysis.
aqueous biphasic hydroformylation of 1-octene is described.
In addition, new water-soluble mononuclear complexes prepared
by the reaction of [RhCl(COD)]2 (COD = 1,5-cyclooctadiene)
with sulfonated propylsalicylaldimine ligands are reported and
their catalytic behaviour in the hydroformylation reaction is also
investigated and discussed.
Fig. 2 Water-soluble rhodium catalyst used in the industrial hydro-
formylation of lower olefins.
Results and discussion
Synthesis and characterization of 5-sulfonatosalicylaldehydes
(1 and 2)
Both monomeric and dendritic ligands based on propylamine,
tris-2-(aminoethyl)amine and diaminobutane (DAB) have been
investigated. The dendrimer ligands were of interest as metallo-
dendrimers often display an enhanced catalytic activity and
stability (known as positive dendritic effect) relative to their
mononuclear counterparts.21 In addition, there have been few
reports on the use of water-soluble metallodendrimers as catalyst
precursors in aqueous biphasic catalytic reactions.22,23
The anionic sulfonated salicylaldehyde starting materials
(1 and 2) were prepared according to modified literature pro-
cedures as shown in Scheme 1.24,25 Both sulfonated salicylalde-
hydes (1 and 2) were afforded as crystalline solids in good yields
(63 and 87% respectively) and have been characterized using
1H NMR, 13C NMR, FT-IR spectroscopy, elemental analysis
(C, H, N and S) and mass spectrometry.
Scheme 1 Outline of the preparation of monosodium 5-sulphonato-
salicylaldehydes (1 and 2).
1H NMR spectra for compounds (1 and 2) displays signals for
the hydroxyl hydrogen at ca. δ 9.96 ppm as well as aromatic
peaks in the region of δ 8.11–7.70 ppm. In the 13C NMR
spectra, the carbonyl carbons exhibit characteristic signals at
ca. δ 192 ppm, while the aromatic carbons are seen between
ca. δ 165–123 ppm. FT-IR spectroscopic results show absorption
bands at 1665 cm−1 for (2) and 1667 cm−1 for (1) due to the car-
bonyl functionality as well as strong ν(O–H) bands.
formation of imines by the strong ν(CvN) bands at 1636 cm−1
1
(4) and 1638 cm−1 (3). The H NMR spectra of ligands (3 and
4) exhibit imine signals at ca. δ = 8.60 ppm for both complexes.
13C NMR revealed the expected number of peaks in the aromatic
and aliphatic regions, as well as imine peaks in the expected
region, above δ = 170 ppm. C, H, N and S analysis is in agree-
ment with the proposed structures and the respective mass
spectra (ESI-MS) showing the [M − Na + 2H]+ (m/z = 244.10,
3) and [M − Na]+ (m/z = 298.10, 4) ions, are also in support of
the formation of these ligands.
Synthesis and characterization of 5-sulfonato propylsalicyl-
aldimine ligands (3 and 4)
Subsequently, N^O bidentate monomeric 5-sulfonato salicylaldi-
mine ligands (3 and 4) were prepared by the Schiff base con-
densation reactions of n-propylamine with 5-sulfonato
salicylaldehydes (1 or 2) and were isolated as yellow, air-stable
solids (eqn (1)). Compound (3) is soluble in water and dimethyl-
sulfoxide (DMSO) while (4), containing a hydrophobic substitu-
ent, is soluble in alcohols such as methanol and ethanol as well.
Infrared spectroscopy confirmed the disappearance of the starting
aldehyde (ν(CvO) = 1665 cm−1 and 1667 cm−1) and the
ð1Þ
13928 | Dalton Trans., 2012, 41, 13927–13935
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1H NMR, 13C NMR, FT-IR spectroscopy, elemental analysis
(C, H, N and S) and mass spectrometry.
Synthesis and characterization of tris-2-(5-sulfinatosalicylal-
dimine ethyl)amine ligand (5)
The infrared spectrum displays the characteristic CvN imine
The tris-2-(aminoethyl)amine analogue of (3) was prepared in a
similar manner; however a longer reflux time and a slight excess
of the starting aldehyde ensured complete functionalization of
the pendant amino groups (eqn (2)). This low generation dendri-
tic ligand (5) was isolated as a powdery yellow hygroscopic
solid in good yield (96%).
1
stretching frequency at 1635 cm−1. The H NMR spectrum dis-
plays peaks following a similar trend to the corresponding mono-
meric analogue (3), except for peak broadening in the aliphatic
region, due to the macromolecular nature of the dendritic ligand
(6). Therefore, there is a signal at δ = 8.55 ppm assigned to the
imine protons, broad aliphatic signals in the region of δ =
1.30–3.60 ppm and aromatic signals ranging from δ =
6.78–7.69 ppm in the 1H NMR spectrum of ligand (6). Similarly
in the 13C NMR spectrum, a distinct imine carbon signal is
evident at ca δ = 166 ppm along with aromatic and aliphatic
carbon signals between ca. 24–162 ppm. ESI-MS results show
the presence of the [M − 4Na − 4H]+ (m/z = 1060.58) ion,
while fitting elemental analysis results could not be obtained
here too due to the hygroscopic nature of the dendritic ligand
and possible solvent inclusion within the dendritic arms.28a–d
ð2Þ
The compound is soluble in water and DMSO and has been
characterized using elemental analysis (C, H, N and S), FT-IR,
1H NMR and 13C NMR spectroscopy as well as mass
spectrometry.
Synthesis and characterization of substituted and unsubstituted
5-sulfonato propylsalicylaldimine Rh(I) complexes (7 and 8)
The monomeric ligands (3 and 4) were reacted with the Rh(I)
precursor [Rh(COD)Cl]2 according to eqn (4).
Several attempts to obtain good elemental analysis of ligand
(5) were unsuccessful due its highly hygroscopic nature. The
infrared spectrum of ligand (5) shows a strong characteristic
imine band at 1636 cm−1 as well as a ν(O–H) absorption band.
1
Both the H and 13C NMR spectra indicate the formation of the
ð4Þ
imine moiety through signals at δ = 8.56 ppm (1H) and ca. δ =
167 ppm (13C) and all other expected signals are observed. Elec-
tron impact mass spectrometry (EI-MS) agrees with the for-
mation of this ligand by displaying a peak at m/z = 704.72 for
the [M − 3Na − 2H]+ ion.
The metal complexes were isolated as yellow solids and are
somewhat less soluble in water than the free ligands, probably
due to the presence of the hydrophobic rhodium COD moiety.
However, they do retain some aqueous solubility, and are also
quite soluble in DMSO and methanol. These new complexes
have been fully characterised using elemental analysis (C, H, N
Synthesis and characterization of DAB-(5-sulfinatosalicylal-
dimine) ligand (6)
1
and S), FT-IR and H NMR and 13C NMR spectroscopy as well
as mass spectrometry.
A first-generation poly(propyleneimine) ligand was prepared by
the reaction of sodium 5-sulfonato salicylaldehyde with the first-
generation DAB dendrimer in ethanol (DAB = diaminobutane)
(eqn (3)). Although there are several examples of water-soluble
Schiff base ligands prepared by the condensation of sodium
5-sulfonato salicylaldehyde with various amines,24–27 to our
knowledge there are no reports of the preparation of water-
soluble DAB dendrimers by this method.
Elemental analysis results gave C, H, N and S percentage
values corresponding to the calculated percentages. Infrared
spectroscopy of complexes (7 and 8) indicates coordination of
the metal centre to the imine nitrogen, where υ(CvN) has
shifted to 1606 cm−1 and 1607 cm−1, from 1638 cm−1 in ligand
(3) and 1636 cm−1 in ligand (4), respectively. The 1H NMR
spectra of the monomeric complexes exhibit some interesting
features. Firstly, there is an upfield shift of the imine proton to
approximately δ = 8.10 ppm in the proton spectrum (Fig. 3).
Secondly, there is splitting of the vinylic COD protons, due
probably to the asymmetric environment induced by the chelat-
ing N–O ligand. The 13C spectrum supports this, where two
separate signals are observed for the vinylic carbons. In some
instances these signals are observed as doublets, due to coupling
to the NMR-active 103Rh (1JRh–C = 13 Hz), but in this particular
spectrum the vinylic carbon peaks are rather broad, and the split-
ting is not always clear. Carbon NMR also shows an upfield
ð3Þ
Dendrimer (6) was isolated as a yellow solid (yield: 66%)
which like compound (5) is water soluble and highly hygro-
scopic. This dendritic ligand has been characterized fully using
shift of the CvN peak. All remaining expected peaks were
̲
1
observed in the correct regions for both the H and 13C NMR
spectra.
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Fig. 3 1H NMR spectrum of rhodium complex (7) conducted at 25 °C in D2O.
Table 1 Aqueous biphasic hydroformylation of 1-octene at different temperatures over 8 hoursa
Entry
Cat.
Temp. (°C)
Conversion (%)
Aldehydes (%)
Iso-octenes (%)
n : isob
TOFc (h−1
)
1
2
3
4
5/[RhCl(COD)]2
6//[RhCl(COD)]2
7
8
75
75
75
75
79
99
72
75
52
37
85
56
48
63
15
44
72 : 28
57 : 48
56 : 44
74 : 26
189
134
309
203
5
6
7
8
5/[RhCl(COD)]2
6//[RhCl(COD)]2
7
8
95
95
95
95
89
92
77
87
64
71
88
63
36
29
12
37
46 : 54
39 : 61
46 : 54
31 : 69
233
258
320
229
a Reactions carried out with (CO : H2) (1 : 1) at 30 bars in distilled H2O (5 ml) and toluene (5 ml) with 6.37 mmol of 1-octene and 2.87 × 10−3 mmol
Rh catalyst precursor (error estimate: (5/RhCl(COD)]2) = 0.16; (6/RhCl(COD)]2) = 0.14; (7) = 0.16 and (8) = 0.12). GC conversions obtained
using n-decane as an internal standard in relation to authentic standard iso-octenes and aldehydes. b Regioselectivity calculated at 2 hours. c TOF =
(mol product/mol cat.) × h−1 and is based on total aldehydes produced.
Effect of temperature. Previous studies using iminophosphine
and iminopyridyl based Rh(I) catalyst precursors in the hydro-
formylation of 1-octene revealed 75 °C and 95 °C (30 bars) as
suitable temperatures and pressure to obtain high yields of alde-
hydes.28d,29 Therefore, initial reactions in this study were carried
out under these conditions and the results are summarized in
Table 1.
All the catalyst precursors are active at 75 °C (30 bars) with
conversion in the ranges of 72–99%. The dendritic systems
(5 and 6)/[RhCl(COD)]2 display better conversion compared to the
mononuclear systems (7 and 8) (Table 1, entries 1–4). Aldehyde
formation is most favoured when using catalyst precursor (7)
while, (8) and (5)/[RhCl(COD)]2 yield near equal amounts of
aldehydes and iso-octenes. The linear aldehyde (nonanal) is
favoured in all the reactions. Turnover frequency (TOF) calcu-
lations based on total aldehydes formed indicate that the mono-
nuclear systems perform better than their dendritic counterparts,
possibly due to lesser steric bulk in (7 and 8) as compared to the
macromolecular dendritic systems (5 and 6)/[RhCl(COD)]2.
Steric hindrances can impede activity and the rate of hydroformy-
lation. Increasing the temperature to 95 °C (30 bars) results in
increased conversion of 1-octene and more aldehydes forming
(Table 1, entries 5–8). Branched aldehydes are dominant at this
The mass spectral data for complexes (7 and 8) agrees
with the proposed structures, where peaks corresponding to
[M +/− Na]+ are observed.
Aqueous biphasic hydroformylation of 1-octene
The catalytic behaviour of Rh(I) metallodendrimers generated
in situ from [RhCl(COD)]2 and the water-soluble dendritic ligands
(5 and 6) were evaluated in the aqueous biphasic hydroformyla-
tion of 1-octene. In each experiment, the substrate (1-octene)
was in toluene while the catalyst precursor was immobilized in a
water layer (eqn (5)).
ð5Þ
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Table 2 Aqueous biphasic hydroformylation of 1-octene at different pressures over 8 hoursa
Entry
Cat.
Pressure (bars)
Conversion (%)
Aldehydes (%)
Iso-octenes (%)
n : isob
TOFc (h−1
)
1
2
3
4
5
5/[RhCl(COD)]2
6/[RhCl(COD)]2
7
8
40
40
40
40
40
94
90
98
96
99
81
86
96
92
99
19
14
4
8
1
60 : 40
61 : 39
56 : 44
62 : 38
49 : 51
294
312
349
334
359
[RhCl(COD)]2
6
7
8
9
5/[RhCl(COD)]2
6//[RhCl(COD)]2
7
8
50
50
50
50
82
74
88
90
46
46
62
51
54
54
38
49
66 : 34
59 : 41
54 : 46
61 : 39
167
167
225
185
a Reactions carried out with (CO : H2) (1 : 1) at 75 °C in distilled H2O (5 ml) and toluene (5 ml) with 6.37 mmol of 1-octene and 2.87 × 10−3 mmol
Rh catalyst precursor (error estimate: (5/RhCl(COD)]2) = 0.13; (6/RhCl(COD)]2) = 0.11; (7) = 0.10 and (8) = 0.09). GC conversions obtained
using n-decane as an internal standard in relation to authentic standard iso-octenes, aldehydes, alcohols and n-octane. b Regioselectivity calculated at
2 hours. c TOF = (mol product/mol cat.) × h−1 and is based on total aldehydes produced. Trace amounts of alcohols and n-octane were detected by GC
for reactions at 50 bars.
temperature and these are afforded via initial isomerization reac-
tions as observed by monitoring the reactions using gas chro-
matography (GC) every 2 hours.29 However, the catalyst
precursors all display decomposition to a black species in solu-
tion at 95 °C (30 bars).
Effect of pressure. Further experiments were carried out to
explore how increasing the syngas pressure affects hydroformy-
lation activity and selectivity (Table 2).
Increasing the pressure from 30 bars (Table 1, entries 1–4) to
40 bars (75 °C) results in a marked improvement in conversion
of 1-octene and TOFs thus leading to high yields in aldehydes
Fig. 4 Recycle runs of catalyst precursors (5 and 6)/[RhCl(COD)]2 and
(Table 2, entries 1–4). The linear aldehyde (nonanal) is favoured
(7 and 8). Reactions carried out with (CO : H2) (1 : 1) at 75 °C (40 bars)
while there is a slight decrease in the average n : iso ratio in com-
in distilled H2O (5 ml) and toluene (5 ml) with 6.37 mmol of 1-octene
parison to reactions carried out at 30 bars (75 °C). The mono-
and 2.87 × 10−3 mmol Rh catalyst precursor (error estimate: (5/RhCl-
(COD)]2) = 0.14; (6/RhCl(COD)]2) = 0.12; (7) = 0.10 and (8) =
nuclear systems (7 and 8) tend to perform better than the
dendritic systems (5 and 6)/[RhCl(COD)]2 in producing the most
0.11).
aldehydes and therefore higher TOFs here. This is indicative of
better accessibility of the substrate to active Rh sites in the
mononuclear systems and therefore increased hydroformylation
observation that has been reported.29,31,32 The N^O based chelat-
ing systems in the current study show inferior regioselectivity as
compared to N^N and N^P based chelating systems that we have
recently reported for hydroformylation of 1-octene.29 There are
no significant differences between reactions carried out using
dendritic catalyst precursors (5 and 6)/[RhCl(COD)]2 and mono-
nuclear analogues (7 and 8) with regards to chemo- and regio-
selectivities, however the dendritic scaffolds appear to stabilize
and restrict Rh leaching to a greater extent than their mono-
nuclear counterparts (vide infra). This Rh retention may be pro-
moted by a combination of the N^O chelate moiety and the N
atoms within the dendritic scaffolds.
rates. A reaction carried out using the dimeric precursor [RhCl-
(COD)]2 (entry 5) also displays excellent conversion of 1-octene
to aldehydes. However there is no outright regioselectivity in
this case. Moreover, this catalyst precursor is soluble in the
organic layer and therefore cannot be recycled and reused by
means of aqueous/organic phase separations, as is the intention
of this study. Further increasing the pressure to 50 bars (75 °C)
produces a slight drop in 1-octene conversion and a more pro-
nounced decrease in aldehyde formation (Table 2 entries 6–9).
Encouragingly, the linear aldehyde is favoured in these reactions
too. In addition, trace amounts of alcohols and n-octane were
observed by GC analysis when operating at 50 bars (75 °C) and
40 bars (95 °C).30
Recyclability and Rh leaching studies. The catalyst precursors
exhibit good recyclability over at least five cycles (Fig. 4) and
chemoselectivities as well as regioselectivities remained consist-
ent throughout the cycles. After simple decantation of the
organic layer, the aqueous layer containing the active catalyst
was reused in subsequent reactions. There is an average loss in
conversion of ca. 6% for (5 and 6)/[RhCl(COD)]2 and ca. 14%
for (7 and 8) after the first run and this may be attributed to Rh
leaching from the aqueous layer and catalyst decomposition.
Chemoselectivity and regioselectivity. These studies have
revealed that the current catalyst precursors produce the best
chemoselectivity for aldehydes at 40 bars (75 °C), where the
aldehydes yielded range from 81–96% (Table 2). On the other
hand, slightly more iso-octenes are formed at 50 bars (75 °C) as
well as trace amounts of hydrogenation products. Overall, nonanal
is preferred at 75 °C (30 and 40 bars) (Tables 1 and 2), an
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ICP-MS analysis of the aqueous layer shows that 11% and 9%
Rh has leached from the aqueous layer during reactions invol-
ving (5)/[RhCl(COD)]2 and (6)/[RhCl(COD)]2 respectively.
Even greater quantities of Rh, 19% and 22% for (7) and (8)
respectively, are leached from the reactions involving the mono-
nuclear catalyst precursors due to reasons proposed earlier in the
text. Additionally, the use of hard and strong donors such as the
imine and phenoxide may destabilize the catalyst precursor thus
resulting in more leaching in comparison to pi-acidic ligands
(e.g. traditionally used phosphine-based ligands).
coupled plasma-mass spectrometry was obtained using a Perkin-
Elmer Elan600 quadrupole ICP-MS with a Cetax LSX-200 UV
laser module. Catalysis products were analysed using a Perkin
Elmer Clarus 580 GC.
Synthesis of monosodium 5-sulfonatosalicylaldehydes (1 and 2)
The sulfonated salicylaldehydes (1 and 2) were prepared accord-
ing to modified literature procedures in a stepwise manner as out-
lined below.
Step 1: synthesis of N-phenyl-salicylaldimine. Salicylaldehyde
(5.02 g, 41.10 mmol) and aniline (5.84 g, 41.20 mmol) were
stirred in 50 cm3 methanol for 90 minutes. Distilled water was
then added dropwise, with stirring, to the yellow solution until
the solution remained cloudy for a few seconds before turning
translucent again. The mixture was cooled in an ice-bath and the
resultant yellow crystalline product was filtered and washed with
small portions of cold water and methanol. Yield 7.51 g (93%).
Conclusions
Water-soluble dendritic ligands derived from tris-2-(aminoethyl)-
amine and diaminobutane (DAB) cores were synthesized and
these were characterized by a combination of analytical and
spectroscopic techniques. These ligands were successfully
employed as precursors to water-soluble Rh(I) metallodendrimers
for aqueous biphasic hydroformylation of 1-octene. Additionally,
new model water-soluble mononuclear complexes were obtained
and fully characterized for application in the hydroformylation
reaction. All the catalyst precursors proved active and selective
in the hydroformylation of 1-octene to aldehydes (major pro-
ducts). These systems possess the ability to interact with a long
chain α-olefin during catalysis under aqueous biphasic con-
ditions and we believe that the aliphatic groups in compounds
(5–8) play a role in ensuring this interaction. Optimal conditions
were established at 40 bars (75 °C) where the most aldehydes
were afforded. Branched aldehydes were predominantly obtained
at 30 and 40 bars (95 °C) and nonanal at 30 and 40 bars (75 °C).
This phenomenon introduces the idea of thermal and pressure
regulated regioselectivity. Incorporation of sulfonate groups pres-
ents promising catalyst precursors which can be immobilized in
water, a green solvent, and subsequently recycled up to five
times with consistent activities and selectivities.
Step 2: synthesis of N-phenyl-5-sulfonato-salicylaldimine.
Concentrated sulfuric acid (10 cm3 of 98% H2SO4) was placed
in a round-bottomed flask fitted with reflux condenser and
3.50 g (17.70 mmol) N-phenyl-salicylaldimine was added
slowly with stirring. The mixture was heated at 100–105 °C for
2.5 hours. The hot solution was poured carefully into a beaker
containing ca. 100 cm3 ice water. A yellow product precipitated
immediately. The suspension was then reheated until all the
product had dissolved to form a bright orange solution. Undis-
solved particles were filtered by gravity from the hot solution
and the filtrate left to stand at room temperature. The yellow-
brown microcrystalline product was filtered and washed with
small portions of cold water. Yield 2.79 g (57%).
Step 3: synthesis of monosodium 5-sulfonatosalicylaldehyde
(1). N-Phenyl-5-sulfonato-salicylaldimine (1.49 g, 5.39 mmol)
and Na2CO3 (0.60 g, 5.70 mmol) were boiled vigorously in an
open flask containing 10 cm3 distilled water for two hours. The
water was replenished as necessary. Glacial acetic acid (6 cm3)
was then added to the cooled solution. The same amount of
ethanol was added and the solution cooled in an ice-bath for
several hours. The fine beige crystals of (1) were filtered and
washed with cold ethanol, and dried in vacuo. Yield: (0.76 g,
63%). mp.: 331–334 °C. FT-IR (νmax/cm−1, KBr): 3460s (O–H),
1667s (CvO). δH (400 MHz, D2O, 25 °C) (ppm) = 9.95 (s, 1H,
Experimental
General details
All reactions were carried out in air unless otherwise stated. Sol-
vents used were reagent grade and were not distilled prior to use
unless otherwise stated. RhCl3·3H2O was obtained from Anglo
Platinum Corporation. All other chemicals were purchased from
Aldrich and used as received. [Rh(COD)Cl]2 was prepared
according to a literature procedure.13
4
3
C(O)H), 8.11 (d, J = 2.39 Hz, 1H, Ar), 8.07 (d, J = 8.78 Hz,
1H, Ar), 7.92 (dd, 3J = 8.78 Hz, 4J = 2.39 Hz, 1H, Ar).
δC (75 MHz, DMSO-d6, 25 °C) (ppm) = 191.9, 165.3, 162.0,
136.8, 131.8, 125.5, 123.3. Elemental Analysis (calculated for
C7H5NaO5S): C, 37.51; H, 2.28; S, 14.30. Found: C, 37.28;
H, 2.16; S, 14.04%. EI-MS (m/z) = 200.07 ([M − Na − H]+, 59%).
1H and 13C NMR spectra were recorded on a Varian Mercury-
300 MHz (1H: 300 MHz; 13C: 75.5 MHz; 31P: 121 MHz) or
Varian Unity-400 MHz (1H: 400 MHz; 13C: 100.6 MHz)
spectrometer and values are reported relative to the internal stan-
dard tetramethylsilane (δ 0.00). FT-IR spectra were recorded as
KBr pellets using a Perkin Elmer 100 Spectrum One spectro-
meter. Elemental analyses were conducted with a Fision EA 110
CHNS Analyzer. Melting points were determined using a Kofler
hot stage microscope (Riechter Thermo). Electrospray Ionization
(ESI) mass spectrometry was carried out on a Waters API
Quattro Micro triple quadrupole mass spectrometer in the posi-
tive or negative-ion mode. Electron Impact mass spectrometry
was conducted on an Agilent 6890 N spectrometer. Inductively
Step 1: synthesis of N-phenyl-3-tbutyl-salicylaldimine. 3-tert-
Butyl-2-hydroxybenzaldehyde (0.94 g, 5.27 mmol) and aniline
(0.49 g, 5.30 mmol) were stirred in 20 cm3 of ethanol. Anhy-
drous magnesium sulfate was added and the reaction mixture
stirred overnight. The solvent was removed to yield a yellow oil;
this was used directly as obtained in the following reaction.
Step 2: synthesis of N-phenyl-3-tbutyl-5-sulfonatosalicylaldi-
mine. The round-bottomed flask containing N-phenyl-3-tbutyl-
13932 | Dalton Trans., 2012, 41, 13927–13935
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3
4
salicylaldimine was submerged in an ice-bath; 6 cm3 concen-
trated (98%) sulfuric acid was added slowly with stirring. The
orange solution was then heated at 105–110 °C for 2.5 hours.
The solution was allowed to cool and ice pieces were added to
induce crystallization. A peach coloured precipitate formed; this
was filtered and washed with ice-cold water and dried in vacuo.
Yield 0.77 g (44% based on initial aldehyde used).
4J = 2.17 Hz, 1H, Ar), 3.56 (td, J = 6.81, J = 1.08 Hz, 2H,
3
–NCH2), 1.67 (sext., J = 7.20 Hz, 2H, CH2propyl), 1.38 (s, 9H,
t
3
CH3 butyl), 0.94 (t, J = 7.37 Hz, 3H, CH3propyl). δC (75 MHz,
DMSO-d6, 25 °C) (ppm)= 174.1, 166.8, 162.3, 137.5, 136.4,
127.7, 127.1, 116.9, 59.1, 34.8, 29.4, 23.9, 21.8, 11.8. Elemental
Analysis (calculated for C14H20NNaO4S): C, 52.32; H, 6.27; N,
4.36; S, 9.98. Found: C, 52.86; H, 6.79; N, 4.11; S, 10.68%.
ESI-MS (m/z) = 298.10 ([M − Na]+), 66%).
Step 3: synthesis of monosodium 3-tbutyl-5-sulfonatosalicyl-
aldehyde (2). The N-phenyl-3-tbutyl-5-sulfonatosalicylaldimine
was dissolved in hot distilled water and an equimolar amount of
Na2CO3 was added slowly over several minutes. The solution
was left to boil in an open flask for three hours, with periodic
replenishment of water, as needed. Then 3 cm3 each of acetic
acid and ethanol were added. The product did not precipitate
after extended cooling, therefore the solution was heated again
until all solvent had evaporated. Ethanol was added and the light
brown crystalline product (2) was filtered and washed well with
ethanol to remove all traces of acetic acid. Yield: (0.71 g, 87%).
mp 158–161 °C. FT-IR (νmax/cm−1, KBr): 3441s (O–H), 1665s
(CvO). δH (400 MHz, D2O, 25 °C) (ppm) = 9.96 (s, 1H, C(O)
H), 7.79 (d, J = 2.60 Hz, 1H, Ar), 7.70 (d, J = 2.62 Hz, 1H,
Ar), 1.31 (s, 9H, CH3). δC (75 MHz, DMSO-d6, 25 °C) (ppm) =
192.1, 164.8, 160.9, 135.7, 132.4, 126.7, 126.4, 34.3, 30.1,
28.5, 25.6. Elemental Analysis (calculated for C11H13NaO5S):
C, 47.14; H, 4.68; S, 11.44. Found: C, 46.92; H, 4.12; S,
10.96%. EI-MS (m/z) = 256.13 ([M − Na − H]+, 63%).
Synthesis of 5-sulfonatosalicylaldimine dendrimers (5 and 6)
Synthesis of tris-2-(5-sulfinatosalicylaldimine ethyl)amine
ligand (5). Tris-2-(aminoethyl)amine (0.038 g, 0.26 mmol) and
sodium 5-sulfonatosalicylaldehyde (1) (0.24 g, 1.07 mmol) were
refluxed in ethanol for 4 hours. The yellow hygrosopic solid
product (5) was filtered under nitrogen and washed with ethanol.
Yield: (0.190 g, 96%). mp.: decomp without melting, onset
occurs at 291 °C. FT-IR (νmax/cm−1, KBr): 3430s (O–H), 1636s
(CvN). δH (400 MHz, DMSO-d6, 25 °C) (ppm) = 8.56 (s, J =
4
6.57 Hz, 3H, Himine), 7.68 (d, J = 1.81 Hz, 3H, Ar), 7.55 (dd,
4
4
3J = 8.57, J = 2.17, Ar), 6.78 (dd, 3J = 8.56, J = 1.86 Hz, 3H,
4
4
3
3
Ar), 3.70 (t, J = 6.39 Hz, 6H, –NCH2), 2.92 (t, J = 6.69 Hz,
6H, CH2). δC (75 MHz, DMSO-d6, 25 °C) (ppm)= 166.9,
162.3, 138.9, 130.4, 129.3, 117.3, 116.5, 56.6, 55.1. Elemental
Analysis (calculated for C28H31N4Na3O12S3): C, 43.08; H, 4.00;
N, 7.18; S, 12.32. Found: C, 40.59; H, 4.34; N, 8.13; S, 11.30%.
EI-MS (m/z) = 704.72 ([M − 3Na − 2H]+), 27%).
Synthesis of DAB-(5-sulfinatosalicylaldimine) ligand 6. DAB
generation 1 dendrimer (0.72 g, 0.23 mmol) and sodium 5-sulfo-
natosalicylaldehyde (1) (0.23 g, 1.03 mmol) were refluxed in
ethanol for four hours then stirred overnight. The yellow hygro-
sopic precipitate of (6) was filtered under nitrogen and washed
with ethanol. Yield: (0.157 g, 66%). mp.: decomp without
melting, onset occurs at 271 °C. FT-IR (νmax/cm−1, KBr): 3436s
(O–H), 1635s (CvN). δH (400 MHz, DMSO-d6, 25 °C) (ppm)
Synthesis of 5-sulfonatosalicylaldimine ligands (3 and 4)
Synthesis of 5-sulfonato propylsalicylaldimine (3). Propyl-
amine (0.028 g, 0.47 mmol) and sodium 5-sulfonatosalicylalde-
hyde (1) (0.106 g, 0.47 mmol) were refluxed in ethanol for
60 minutes. The yellow precipitate of (3) was filtered and
washed with ethanol. Yield: (0.110 g, 88%). mp.: decomp
without melting, onset occurs at 289 °C. FT-IR (νmax/cm−1
,
4
= δ 8.55 (s, 4H, Himine), 7.69 (d, J = 1.97 Hz, 4H, Ar), 7.55
KBr): 3521s (O–H), 1638s (CvN). δH (400 MHz, DMSO-d6,
(dd, 3J = 8.56, 4J = 1.98 Hz, 4H, Ar), 6.78 (d, 3J = 8.58 Hz, 4H,
Ar), 3.6–1.3 (m, 32H, CH2). δC (75 MHz, DMSO-d6, 25 °C)
(ppm) = 166.0, 162.4, 138.7, 130.3, 129.3, 117.3, 116.4, 56.0,
53.7, 51.1, 28.3 (d), 24.7. Elemental Analysis (calculated for
C45H56N6Na4O16S4): C, 46.71; H, 4.88; N, 7.26; S, 11.08.
Found: C, 44.97; H, 5.16; N, 8.64; S, 10.82%. ESI-MS (m/z) =
1060.58 ([M − 4Na − 4H]+), 38%).
4
25 °C) (ppm) = 13.8 (s, OH), 8.57 (s, 1H, Himine), 7.68 (d, J =
3
4
2.19 Hz, 1H, Ar), 7.54 (dd, J = 8.54, J = 2.18 Hz, 1H, Ar),
6.77 (d, 3J = 8.54 Hz, 1H, Ar), 3.56 (td, 3J = 6.76, 4J = 1.08 Hz,
3
3
2H, –NCH2), 1.66 (sext., J = 7.10 Hz, 2H, CH2), 0.93 (t, J =
7.39, 3H, CH3). δC (75 MHz, DMSO-d6, 25 °C) (ppm) = 173.4,
165.9, 161.8, 137.3, 131.6, 126.7, 122.9, 60.2, 22.2, 11.7.
Elemental Analysis (calculated for C10H12NNaO4S): C, 45.28;
H, 4.56; N, 5.28; S, 12.09. Found: C, 45.48; H, 4.19; N, 5.88; S,
11.86%. ESI-MS (m/z) = 244.10 ([M − Na + 2H]+, 56%).
Synthesis of substituted and unsubstituted 5-sulfonato
propylsalicylaldimine Rh(I) complexes (7 and 8)
Synthesis of 3-tbutyl-5-sulfonato propylsalicylaldimine (4).
Propylamine (0.046 g, 0.78 mmol) was dissolved in 10 cm3
Synthesis of 5-sulfonato propylsalicylaldimine rhodium(I)1,5-
cyclooctadiene complex (7). Propylamine (0.019 g, 0.32 mmol)
and deprotonated sodium 5-sulfonatosalicylaldehyde (3)
(0.085 g, 0.34 mmol) were stirred in ethanol–water mixture
ethanol;
sodium
3-tbutyl-5-sulfonatosalicylaldehyde
(2)
(0.217 g, 0.78 mmol) was added and the mixture refluxed for
30 minutes. The bright yellow solution was cooled to room
temperature and filtered by gravity. The solvent was removed
in vacuo to reveal an oily yellow product (4) which solidified
upon standing for several hours. The product was collected and
washed with diethyl ether. Yield: (0.202 g, 81%). mp.:
213–215 °C. FT-IR (νmax/cm−1, KBr): 3447s (O–H), 1636s
(CvN). δH (400 MHz, D2O, 25 °C) (ppm) = 8.55 (t, 4J =
under argon. After
5 minutes [Rh(COD)Cl]2 (0.072 g,
0.15 mmol) was added and the reaction stirred at room tempera-
ture for 1 hour. The solvent was removed; methanol and diethyl
ether were added to precipitate the product, which was filtered to
give a yellow solid product (7). Yield: (0.091 g, 66%). mp.:
decomp without melting, onset occurs at 301 °C. FT-IR (νmax
/
4
1.10 Hz, 1H, Himine), 7.58 (d, J = 2.18 Hz, 1H, Ar), 7.54 (d,
cm−1, KBr): 1606s (CvN). δH (400 MHz, D2O, 25 °C) (ppm) =
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4
8.12 (s, 1H, Himine), 7.75 (d, J = 2.52 Hz, 1H, Ar), 7.67 (dd,
[RhCl(COD)]2 was then added and the solution was stirred at
room temperature for 24 hours.
3J = 8.92 Hz, J = 2.53 Hz, 1H, Ar), 6.86 (d, J = 8.94 Hz, 1H,
Ar), 4.40 (br s, 2H, CHCOD), 3.88 (br s, 2H, CHCOD), 3.12
(t, 3J = 7.48 Hz, 2H, –NCH2), 2.41 (br, 4H, CH2COD), 1.90
(m, 4H, CH2COD), 1.68 (sext, 3J = 7.42 Hz, 2H, CH2propyl), 0.84
3
3
Acknowledgements
3
(t, J = 7.34 Hz, 3H, CH3propyl). δC (75 MHz, MeOD, 25 °C)
We would like to thank the University of Cape Town (UCT), the
National Research Foundation and Department of Science and
Technology of South Africa (NRF-DST Centre of Excellence in
Catalysis- c*change), UCT Faculty of Science and the Oppen-
heimer Memorial Trust for financial support. A generous
donation of rhodium trichloride from Anglo American Platinum
Limited is gratefully acknowledged.
(ppm)=165.7, 164.6, 132.5, 130.7, 130.4, 119.6, 117.8, 84.2
(br), 71.0 (br d, J = 13 Hz), 60.0, 30.5, 27.8, 26.1, 9.3. Analysis
(calculated for C18H23NNaO4RhS): C, 45.48; H, 4.88; N, 2.95;
S, 6.75. Found: C, 45.57; H, 4.50; N, 3.18; S, 6.82%. ESI-MS
(m/z) = 498.00 ([M + Na]+, 84%).
Synthesis of 3-tbutyl-5-sulfonato propylsalicylaldimine
rhodium(I)1,5-cyclooctadiene complex (8). Ligand (4) (0.044 g,
0.14 mmol), was dissolved in 10 cm3 ethanol–DCM (1 : 1
volume) mixture. KOH (0.25 cm3 of 1 M ethanol solution) was
added and the reaction was stirred at room temperature for
30 minutes. [Rh(COD)Cl]2 (0.035 g, 0.07 mmol) was added and
the mixture stirred for 90 minutes. The reaction mixture was
filtered by gravity and the solvent was removed to yield a yellow
solid stuck to the sides of the round-bottomed flask. Diethyl
ether was added and the product scratched off the walls of the
flask, then filtered under vacuum. Yield of yellow solid (8)
(0.045 g, 61%). mp.: decomp without melting, onset occurs at
277 °C. FT-IR (νmax/cm−1, KBr): 1607s (CvN). δH (400 MHz,
DMSO-d6, 25 °C) (ppm) = 8.20 (d, J = 1.79 Hz, 1H, Himine),
7.53 (d, J = 2.32 Hz, 1H, Ar), 7.49 (d, J = 2.29 Hz, 1H, Ar),
4.62 (br s, 2H, CHCOD), 3.74 (br s, 2H, CHCOD), 3.09 (t, J =
7.59 Hz, 2H, –NCH2), 2.42 (br s, 4H, CH2COD), 1.86 (m, 4H,
CH2COD), 1.63 (sext., J = 7.56 Hz, 2H, CH2propyl), 1.29 (s, 9H,
CH3 butyl), 0.87 (t, J = 7.34 Hz, 3H, CH3propyl). δC (75 MHz,
DMSO-d6, 25 °C) (ppm)= 166.8, 164.0, 137.9, 134.1, 131.5,
128.8, 117.7, 83.0 (br), 71.4 (br), 60.0, 35.0, 29.9, 28.9, 27.2,
26.1, 11.5. Analysis (calculated for C22H31NNaO4RhS):
C, 49.72; H, 5.88; N, 2.64; S, 6.03. Found: C, 49.15; H, 5.53;
N, 2.22; S, 6.48%. ESI-MS (m/z) = 508.1 ([M − Na]+, 71%).
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Dalton Trans., 2012, 41, 13927–13935 | 13935