Supramolecular encapsulated rhodium catalysts for branched selective hydroformylation of alkenes at high temperature

Article abstract of DOI:10.1002/adsc.201200790

This study reveals that, in contrast to expectations, supramolecular encapsulated rhodium catalysts can be used at industrially relevant temperatures (75-80 °C), providing highly active catalysts. The unprecedented regioselectivity for the branched aldehydes can be retained if high carbon monoxide (CO) concentrations are used.

Full text of DOI:10.1002/adsc.201200790

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DOI: 10.1002/adsc.201200790
Supramolecular Encapsulated Rhodium Catalysts for Branched
Selective Hydroformylation of Alkenes at High Temperature
Tatiana Besset,a David W. Norman,b and Joost N. H. Reeka,*
a
Supramolecular and Homogeneous Catalysis, van’t Hoff Institute for Molecular Sciences, University of Amsterdam,
Science Park 904, 1098 XH, Amsterdam, The Netherlands
Fax : (+31)-20-525-5604; phone: (+31)-20-525-6437; e-mail: j.n.h.Reek@uva.nl
Eastman Chemical Company, 200 South Wilcox Drive, Kingsport, TN, USA
b
Received: September 3, 2012; Revised: October 14, 2012; Published online: January 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200780.
Abstract: This study reveals that, in contrast to ex-
pectations, supramolecular encapsulated rhodium
catalysts can be used at industrially relevant tem-
peratures (75–808C), providing highly active cata-
lysts. The unprecedented regioselectivity for the
branched aldehydes can be retained if high carbon
monoxide (CO) concentrations are used.
One of the examples that we reported recently is
based on a template-ligand approach to encapsulate
the catalytically active metal centers. In the self-as-
sembled 1·(2)3 adduct (Figure 1, a) the phosphorus
Keywords: high temperature processes; high-pres-
sure infrared spectroscopy (HP-IR); hydroformyla-
tion; regioselectivity; supramolecular assembly
The development of efficient catalytic systems dis-
playing atypical reactivity and selectivity is one of the
key challenges of modern catalysis. While traditional-
ly, this has been approached by ligand variation,[1]
more recently supramolecular strategies have been
added to the available palette of tools to control the
selectivity of transition metal catalysts.[2] Supramolec-
ular bidentate ligands formed by self-assembly of
building blocks were shown to be particularly suitable
for combinatorial approaches.[3] Catalyst encapsula-
tion in this case led to conversion of substrates in re-
stricted environments, which evidently gave unusual
selectivities and often enhanced activities. Non-cova-
lent interactions such as hydrogen-bonds, metal-
ligand interactions, ionic and pi-pi interactions are
prevalent in these supramolecular assemblies and
play crucial roles in the formation of the catalyst
system.[4,5] As these weak interactions depend on the
conditions that are applied, especially solvent and
temperature, most experiments with these systems are
carried out at relatively low temperature and general-
ly in apolar solvents. In order to extend the potential
application and general utility of these classes of cata-
lysts it is important to broaden their window of opera-
tion.
Figure 1. a) Self-assembly of Zn(II)TPP units and m-PPy3.
b) Synthesis of encapsulated monophosphine rhodium spe-
cies 3. c) Different coordination modes of active phosphine-
based rhodium catalysts. ee=equatorial-equatorial, ea=
equatorial-apical, acac=acetylacetonate.
348
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Adv. Synth. Catal. 2013, 355, 348 – 352

Supramolecular Encapsulated Rhodium Catalysts for Branched Selective Hydroformylation
this elevated temperature we observed much higher
conversions but at the expense of branched aldehyde
selectivity (l/b=1.5 at 808C).[7b] Although the linear
product was dominant at this temperature, more
branched aldehyde was still formed than in the ab-
sence of porphyrin. We anticipated that the drop in
selectivity could be a result of the formation of bis-
phosphine ligated complexes. Whereas mono-phos-
phine ligation is enforced at room temperature by the
encapsulation of the trispyridyl template ligand,
Scheme 1. Rhodium-catalyzed hydroformylation of 1-octene.
donor atom is shielded by the three porphyrins, such making formation of bis-ligated species 5 sterically
that monophosphorus complexes form upon coordina- impossible, weakening of the Zn(II)TPP-pyridine in-
tion to rhodium (complex 3, Figure 1, b). This encap- teraction at elevated temperatures could lead to a par-
sulated rhodium complex displayed unusual reactivity tial dissociation of Zn(II)TPP enabling formation of
and selectivity for the hydroformylation[6] of unfunc- species 5. As the formation of complexes 3–6
tionalized terminal and internal alkenes.[7] For the hy- (Figure 1, b and c) can be monitored by IR, we decid-
droformylation of 1-octene (Scheme 1) this system ed to study the catalytic species formed under hydro-
provided an unprecedented selectivity for the formylation conditions at 758C using in situ high-pres-
branched aldehyde (linear/branched aldehyde ratio or sure infrared spectroscopy (HP-IR). For this purpose,
l/b=0.6 at room temperature). While linear aldehydes the encapsulated rhodium complex 3 was examined
are generally preferred for a lot of bulk processes, the under hydroformylation reaction conditions using the
regioselective formation of branched aldehydes from same concentration as in catalysis experiments. At
non-functionalized alkenes could impact the chemical room temperature using 20 bar of syngas (CO/H2 =
industry due to their importance in chemical produc- 1/1), only three bands (2089, 2039 and 2011 cmÀ1) are
tion of fragrances and pharmaceuticals, next to the al- obtained, corresponding to the mono-ligated complex
ready existing hydroformylation route to isobutyl al- [RhH(CO)3P] 4.[7b,10] When 3 was placed under syngas
cohol. The coordination of three zinc-porphyrins was (CO/H2 =2/1) at 20 bar and 758C in dichloromethane,
demonstrated to be crucial, as control experiments carbonyl peaks typical for complex [RhH(CO)3P] 4
performed with bis-3-pyridylphenylphosphine, which (2088, 2037 and 2008 cmÀ1) were observed, but also
also formed monophosphorus rhodium complexes, led those for the bis-ligated complex [RhH(CO)2P2] 5
to a significantly different selectivity (l/b ratio of (2058 and 1990 cmÀ1).[11] After the formation of spe-
1.1).[7b] Besides [Rh]-1·(2)3 catalyst, there has only re- cies 4 and 5, 1-octene was injected in the autoclave
cently been published a second report of a branched and, as expected, the aldehyde product was formed
C-2 selective catalyst for terminal alkenes;[8a] these re- with the linear aldehyde being the major compound
ports contrast with other methods that can provide (l/b ratio of 1.36, see Table S4 in the Supporting Infor-
a low amount of linear aldehydes, as in these exam- mation). Control experiments performed in the ab-
ples several branched aldehyde products (C-2, C-3 sence of Zn(II)TPP at 758C (20 bar, CO/H2 =2/1)
and C-4 ones) are formed as a result of isomerization gave four vibrations (2058, 2006, 1987 and 1953 cmÀ1)
of the substrate.[8b,c]
in accordance with what was observed at room tem-
While the high selectivity obtained by the self-as- perature (20 bar, CO/H2 =1/1).[7b] These peaks are
sembled catalyst towards branched aldehydes is aca- consistent with the more commonly observed bis-
demically interesting,[9] for practical industrial applica- phosphine rhodium complex in which the phosphorus
tions these catalysts need to perform at higher tem- ligands are positioned in the equatorial-equatorial
peratures. This is mainly motivated by the need for (ee) and equatorial-axial (ea) coordination modes,
higher catalyst activities, which is a general economic complexes 5 and 6, respectively.[7b] Upon injection of
driver. The elevated temperature is also useful for pu- the 1-octene substrate in this experiment the linear al-
rification of the aldehyde products by distillation. We dehyde was, as expected, the major compound
therefore explored the experimental window in which (l/b ratio=2.27, see Table S4 in the Supporting Infor-
the supramolecular catalyst could be applied while mation).
maintaining the unusual selectivity observed at room
These IR experiments demonstrate that at least
temperature. Herein we report that indeed, under cer- part of the explanation of the loss in selectivity at ele-
tain conditions, the rhodium-catalyzed hydroformyla- vated temperatures is the formation of bis-ligated
tion of non-functionalized terminal olefins can be car- rhodium complexes. We realized that the equilibrium
ried out at elevated temperature while retaining high between mono-ligated complex 4 and bis-phosphine
selectivity for the branched aldehyde.
rhodium species 5 (Scheme 2) also depends on the
The starting point of the research was the previous- CO concentration, and it was anticipated that at ele-
ly obtained results at room temperature and 808C. At vated CO concentration (pressure) the equilibrium
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349

Tatiana Besset et al.
COMMUNICATIONS
tries 4–7). Indeed, we were able to achieve a higher
selectivity (l/b=0.7) when the hydroformylation reac-
tion was performed at 758C and at 80 bar with CO/
H2 =2/1 (entry 6). However, no improvement was ob-
served when the CO concentration was further raised
(CO/H2 =3/1, entry 7), which suggested that the maxi-
mum equilibrium shift was already obtained with
a syngas ratio of 2/1.
Scheme 2. Equilibrium between 4 and 5.
should shift back towards 4. An HP-IR experiment
Further experiments revealed that the system is
performed at an increased syngas pressure of 80 bar rather sensitive to small temperature changes. An in-
(CO/H2 =2/1) and at 758C, showed that only the crease in reactor temperature of 58C (from 75 to
mono-ligated species 4 is formed. Apparently, the 808C) has a strong effect on the selectivity displayed
equilibrium between 4 and 5 involves a subtle balance by the catalyst as the l/b ratio changed from 0.7 to
and mono-coordination can be enforced at high CO 0.91 (entries 6 and 8). Interestingly, we observed an
pressure only. These results prompted us to further effect of raising the pressure while at elevated tem-
evaluate the performance of the catalyst system at perature. For example, at 160 bar (CO/H2 =2/1) and
elevated temperature and various pressures of CO.
808C the selectivity for the branched aldehyde was
We pursued our investigations by studying in detail significantly higher (l/b=0.76, entry 9). Even at 858C
the effect of syngas pressure and, more particularly, a high branched selectivity was obtained (l/b=0.79,
the CO partial pressure on the hydroformylation of 1- entry 10). When an excess of the porphyrin (Zn/P=6
octene in the presence of [Rh
ACHTUNGTRENNUNG
and the supramolecular capsule 1·(2)3 (Table 1).[12,13]
When the hydroformylation reaction was per- b of 0.66 (entry 11), which is comparable to that ob-
formed under 20 bar of syngas (CO/H2 =1/1) and tained at room temperature (l/b=0.6).
758C in toluene, the catalyst system favored the
To study the effect of these parameters on the ac-
linear aldehyde (l/b=1.54, entry 1). Increasing the tivity of the catalytic system at 758C and 808C, the re-
syngas pressure to 60 and 80 bar resulted in an in- actions were monitored over time. These experiments
creased preference for the branched aldehyde (en- show that under high temperature conditions the cata-
tries 2 and 3, l/b=0.9 and 0.83). The system could be lyst activity is remarkably high, giving turnover fre-
further tuned by modulation of the syngas ratio (en- quencies (TOF) as high as 4000 hÀ1 and full conver-
sion (5000 equiv. of substrate) after only 2 h.[11] As
a control experiment, kinetic studies were performed
Table 1. Initial optimization study for the hydroformylation
of 1-octene.[a]
at a lower temperature (558C), conditions known to
provide high conversion (97% after 16 h). In this ex-
periment a TOF of 1700 hÀ1 was obtained which is, as
expected, much lower than observed at 758C and
808C. These experiments show that at 758C the cata-
lytic system is highly active while keeping a good se-
lectivity. Also in the reaction of related 1-alkenes an
increase of the temperature is beneficial for the con-
version that was obtained after 16 h (see Table S4 in
the supporting information).
Since the selectivity strongly depends on tempera-
ture and CO concentration, we anticipated that stir-
ring rate may also be of importance, as CO depletion
may occur if gasses are mixed inefficiently with the
solvent. A series of experiments in which the stirring
rate was varied were conducted at 558C and 80 bar
syngas (CO/H2 =2/1). Results showed that the
l/b ratio indeed is affected by the stirring speed. At
a stirring rate of 1100 rpm the l/b was 0.7, which im-
proved to 0.59 at higher rates of 1800 and 2600 rpm.
The activity was somewhat lower, also in line with
a higher CO concentration at high stirring rates,[11] as-
suming a negative order in CO as is commonly ob-
served in hydroformylation.[1a,6a]
Entry
T
PACHTUNGTRENNUNG
[8C]
N
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
75
75
75
75
75
75
75
80
80
85
80
20 (1/1)
60 (1/1)
80 (1/1)
20 (2/1)
80 (1/2)
80 (2/1)
80 (3/1)
80 (2/1)
160 (2/1)
160 (2/1)
160 (2/1)
10
30
40
13
27
53
60
53
99
3
1
1
8
2
3
<1
2
<1
<1
<1
1.54
0.9
0.83
1.42
1.35
0.7
0.77
0.91
0.76
0.79
0.66
>99
>99
99
>99
>99
>99
>99
>99
>99
>99
9
107
107
107
10
11[e]
[a]
[RhACHTUNGTRENNUNG
(acac)(CO)2]=0.5 mmolLÀ1 in toluene, [P]/[Rh]=5,
[Zn]/[P]=3, [1-octene]/[Rh]=5000, [DIPEA]/[Rh]=30,
16 h at temperature and syngas pressure specified in the
table; DIPEA=N,N-diisopropylethylamine.
Percentage conversion calculated using the GC method.
Percentage of internal alkenes from the isomerization of
1-octene.
[b]
[c]
[d]
Nonanal/2-methyloctanal ratio (l/b), only traces (<5%)
of other aldehydes (2-ethylheptanal, 2-propylhexanal)
were detected, estimated error of Æ0.01.
[e]
An excess of Zn(II)TPP is used ([Zn]/[P]=6).
350
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Supramolecular Encapsulated Rhodium Catalysts for Branched Selective Hydroformylation
Table 2. Evaluation of substrate scope for the hydroformyla-
mixture to reach a total volume of 4 mL. The autoclave was
flushed with argon, subsequently with syngas (2ꢁ25 bar,
CO/H2) and the solution was then injected. The system was
flushed once again (25 bar, CO/H2). Then the autoclave was
pressurized to the required pressure, immersed into a pre-
heated oil bath and the mixture stirred. After 16 h, the
system was cooled down in an ice bath, then the autoclave
was opened after the pressure had been released. 20 mL n-
tributylphosphite were added to the reaction mixture to
quench the active rhodium catalyst. The aliquots of the reac-
tion mixture (50 mL) were diluted with dichloromethane and
injected into GC directly without work-up or product isola-
tion.
tion of terminal alkenes.[a]
Entry
Alkenes
Conv.[b]
Ald.[ c]
Iso.[d]
l/b[e]
1
2
3
4
1-octene
>99
>99
>99
>99
97
3
0.7
1-hexene
1-decene
1-dodecene
>99
>99
>99
<1
<1
<1
0.87
0.84
0.81
[a]
[RhACHTUNGTRENNUNG
(acac)(CO)2]=0.5 mmolLÀ1 in toluene, [P]/[Rh]=5,
[Zn]/[P]=3, [substrate]/[Rh]=5000, [DIPEA]/[Rh]=30,
16 h, 758C and 80 bar (CO/H2, 2/1); DIPEA=N,N-diiso-
propylethylamine.
Percentage conversion calculated using the GC method.
Percentage of all aldehydes.
Percentage of internal alkenes from the isomerization of
1-octene.
Nonanal/2-methyloctanal ratio (l/b), only traces (<5%)
of other aldehydes (2-ethylheptanal, 2-propylhexanal)
were detected, estimated error of Æ0.01.
[b]
[c]
[d]
Acknowledgements
[e]
´
We thank Dr. Vladica Bocokic for helpful discussions and
Eastman Chemical Company is kindly acknowledged for fi-
nancial support.
With our optimal conditions in hand, the catalytic
system was successfully applied to other substrates.
Both the smaller 1-hexene as well as the longer chain
1-decene and 1-dodecene were efficiently converted
with high selectivity for the branched aldehyde
(Table 2). Importantly, in none of the systems the for-
mation of isomerized alkenes was observed.
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formylation catalyst displays a high selectivity for the
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Experimental Section
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