Tandem hydroformylation/acyloin reaction - The synergy of metal catalysis and organocatalysis yielding acyloins directly from olefins

Article abstract of DOI:10.1002/adsc.201401031

A novel, atom efficient, orthogonal tandem catalysis was developed yielding acyloin products (α-hydroxy ketones) directly from olefins under hydroformylation conditions. The combination of a metal-catalysed hydroformylation and an organocatalysed acyloin reaction provides three atom efficient C-C bond formations to linear, multifunctional molecules via linkage of the intermediate n-aldehydes. Additionally, the rhodium catalyst system gives a high n/bra ratio with an exclusive conversion of the terminal double bond in the hydroformylation and the n-aldehydes are converted selectively to their acyloins.

Full text of DOI:10.1002/adsc.201401031

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DOI: 10.1002/adsc.201401031
Tandem Hydroformylation/Acyloin Reaction – The Synergy of
Metal Catalysis and Organocatalysis Yielding Acyloins Directly
from Olefins
Karoline A. Ostrowski,a Thiemo A. Faßbach,a and Andreas J. Vorholta,*
a
Department of Bio- and Chemical Engineering, TU Dortmund, 44227 Dortmund, Germany
Fax: (+49)-231-755-2311; phone: (+49)-231-755-2313; e-mail: andreas.vorholt@bci.tu-dortmund.de
Received: October 29, 2014; Revised: January 21, 2015; Published online: April 23, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201401031.
Abstract:
A
novel, atom efficient, orthogonal
in few examples[4] with a main focus on tandem hy-
droformylation/aldol reaction[4a–d]
Hydroformylation reactions are well-known for
being one of the most important homogeneous transi-
tion metal-catalysed processes in the chemical indus-
try, which produce aldehydes from olefins and syngas
on a 107 ton scale each year.[5]
tandem catalysis was developed yielding acyloin
products (a-hydroxy ketones) directly from olefins
under hydroformylation conditions. The combina-
tion of a metal-catalysed hydroformylation and an
organocatalysed acyloin reaction provides three
.
À
atom efficient C C bond formations to linear, mul-
tifunctional molecules via linkage of the intermedi-
ate n-aldehydes. Additionally, the rhodium catalyst
system gives a high n/bra ratio with an exclusive
conversion of the terminal double bond in the hy-
droformylation and the n-aldehydes are converted
selectively to their acyloins.
Nucleophilic acyloin reactions with an N-heterocy-
clic carbene (NHC) as organocatalyst are less popu-
lar,[6] although the enzyme-catalysed reaction is of
great importance in our human metabolism.[7] This re-
action delivers linear multifunctional molecules via
À
C C bond linkage of aldehydes yielding a-hydroxy
ketones catalysed by NHC.[8] These a-hydroxy ke-
tones can be further functionalised to amines, diols,
epoxides or to deoxygenated compounds.[9,10]
Keywords: carbenes; high-pressure chemistry; ho-
mogeneous catalysis; hydroformylation; organoca-
talysis
A successful combination of both catalyses, hydro-
formylation and acyloin reaction, into a single orthog-
onal tandem catalysis would allow an atom efficient
access to linearly linked molecules directly from ter-
À
minal olefins within three C C bond forming reac-
The application of two or more different catalysed re- tions. Applying functionalised terminal olefins (e.g.,
actions in one reaction vessel is called tandem cataly- unsaturated esters), should give a,w-functionalised
sis, which is still a quite new and little explored field linked molecules which could be used as monomers.
in chemistry.[1] Tandem catalyses save effort, time,
Herein, we present the first tandem hydroformyla-
energy and resources by not isolating or purifying in- tion/acyloin reaction, which forms acyloins directly
termediates. Due to these advantages, tandem cataly- from terminal alkenes and syngas within one prepara-
sis has become a flagship for sustainability, which is tive step (Figure 1). To the best of our knowledge, no
outlined in the “12 principles of green chemistry”.[2] If acyloin products have ever been reported in the
different catalysts operate simultaneously in one reac-
tor, the tandem reaction is called orthogonal tandem
catalysis according to the definition of Dos Santos
and Fogg.[1c] The development of such sequences is
rather challenging as interferences between different
catalysts are very likely. Therefore, the combination
of transition metal-catalysed hydroformylation with
further reactions is still a field of high interest in
chemistry as seen in reports that have been recently
published.[3] The specific combination of hydroformy-
Figure 1. Acyloin reaction under hydroformylation condi-
lation with organocatalytic reactions is displayed only tions in tandem catalysis.
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Synergy in Tandem Hydroformylation/Acyloin Reaction
Figure 2. The coenzyme thiamine alias vitamin B1 as hydro-
chloride salt (C1) and its corresponding free carbene (C1-
NHC).
course of hydroformylation reactions, either as de-
sired products or as (known) side-products.
Key to success in this new orthogonal tandem catal-
ysis is the combination of a rhodium-catalysed hydro-
formylation yielding aldehydes with high n/bra ratio
and the selective carbine-catalysed transformation of
these intermediates to acyloin products. Rh(CO)2acac
is used as precursor together with a phosphine ligand
providing a catalyst system for hydroformylation with
high activities.[11] The applied organocatalyst is de-
rived readily from the coenzyme thiamine/vitamin B1
(C1, Figure 2) by in situ deprotonation with base, for
example, triethylamine. C1 has been used before as
catalyst precursor in acyloin reactions and was chosen
due to having no GHS hazard statements and its wide
availability, for example, in yeast.[12] The olefin 1-
octene (1) is used as model substrate, often applied in
hydroformylation studies.
Figure 4. Different ligands applied.
At first, the rhodium precursor and the in situ gen-
Figure 3 displays the intended tandem reaction erated C1-NHC from C1 with base (Figure 2) were
pathway starting from 1. A selective hydroformylation applied together with different ligands (Figure 4) de-
of 1 yields nonyl aldehyde (2) as intermediate. A sub- veloping a possible tandem hydroformylation/acyloin
sequent carbene-catalysed acyloin reaction forms the reaction (Table 1). Gratifyingly, 1 could be converted
a-hydroxy ketone 3, the acyloin, respectively. The iso- to the desired acyloin product 3 in almost every ex-
aldehyde (2-methyloctanal) 4 is formed as a side perimental approach. Notably, no branched aldehydes
product due to an unselective hydroformylation of 1. 6 were observed (entries 1.1–1.10), assuming an exclu-
The terminal alkene 1 can undergo isomerisation to sive hydroformylation of terminal double bonds, since
other octene isomers 5, which may be hydroformylat- isomers 5 are not converted into branched aldehydes
ed to yield 4 or 6.
6. Therefore, it can be assumed, that iso-aldehyde 4 is
Figure 3. Possible pathways in the tandem hydroformylation/acyloin reaction.
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Karoline A. Ostrowski et al.
Table 1. Development of the tandem hydroformylation/acy-
ence of L8 increases aldehyde conversion to 90%
with good yields of 73% for the desired acyloin 3
(entry 1.10). Surprisingly, more aldehydes were gener-
ated with YS(2+3) =82% and a higher n/bra ratio of
>99 was achieved, as well; the possible reasons for
this will be discussed later.
loin reaction system.[a]
With the optimised reaction conditions in hand
(entry 1.10), the scope of the developed tandem catal-
ysis was explored by applying different 1-alkenes. To
prove synthetic usefulness, besides aliphatic 1-alkenes,
also different functionalised 1-alkenes were converted
(XOlefin) to their corresponding acyloins (YAcyl) and the
observed products were analysed in detail (Table 2).
In general, no aldehydes from internal olefins,
traces of iso-aldehydes (Yiso-Ald) and high conversion
of the intermediate n-aldehydes (Xn-Ald) to 3 are con-
Entry Ligand YS(2+3) Y2
Y3
Y4
Y6 n/bra X2
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
–
<1
72
74
<1
21
43
15
37
68
82
<1 <1 <1
34
30
–
–
–
–
–
–
–
–
–
–
–
3
3
–
2
3
8
58
96
–
L1
L2
L3
L4
L5
L6
L7
L8
38
44
24
25
53
59
–
19
40
33
43
34
<1 <1 <1
17
26
10
21
45
9
4
17
5
16
23
73
9
16
2
1
<1
<1
sistently observed, for every applied olefin (Xn-Ald
=
78–90%). The shorter 1-hexene gives less acyloins
with 41%, however the conversion of the n-aldehyde
is quite similar with 84% (entry 2.1). Neohexene
shows an excellent reactivity in hydroformylation re-
action forming 98% n-aldehydes [YS(n-Ald+Acyl)) with an
expected excellent regioselectivity giving 76% yield
for the acyloin product (entry 2.2). More interesting
acyloin products are furnished by applying methyl 10-
undecenoate (entry 2.3), a renewable feedstock de-
rived from castor oil[14] and 5-hexenenitrile
1.10[b] L8
>99 90
[a]
Reaction conditions: 6 mmol 1, 0.03 mmol Rh(CO)2acac,
0.09 mmol bidentate or 0.18 mmol monodentate ligand,
0.3 mmol C1, 1.2 mmol Et3N, 30 bar CO/H2 (1:1), 5 mL
DMF, 608C, 16 h. Results determined by GC in % with
calibrated substances. <1=traces are detectable.
0.6 mmol C1, 2.4 mmol Et3N.
[b]
generated only by the hydroformylation of 1. Mono- (entry 2.4). The acyloin products contain a diester or
dentate phosphorus ligands lead to low n/bra ratios a dinitrile moiety being interesting for polymer appli-
(entries 1.2, 1.3, 1.5). L3 shows no activity (entry 1.4), cations. Both substrates show increased isomerisation
probably due to its steric demand. The application of activity which results in olefin isomers (Yisom). Unde-
monodentate ligands L1 and L2 leads to higher con- sired hydrogenated olefins and undesired aldol prod-
version of 1 to 2 (X2), therefore higher amounts of 2 ucts, typical side products of hydroformylation reac-
have been produced [YS(2+3)], hence more acyloin tions, are playing a minor role, since they were consis-
product 3 was generated. These high yields for alde- tently observed in traces (<1%) unless otherwise
hydes [YS(2+3) =72–74%] and conversions of 2 (X2 = stated.
53–59%) indicate a higher reaction activity in hydro-
Since we observed a higher hydroformylation activ-
formylation in comparison to the used bidentate li- ity if more C1 and base Et3N were added (entries 1.9
gands (entries 1.6–1.9). Since we observed an exclu- and 1.10), the hydroformylation step was investigated
sive conversion of the n-aldehyde 2 to its acyloin 3 in terms of possible interactions between the metal
without any branched acyloin formed by the partici- complex and reactants C1 and base Et3N forming C1-
pation of iso (4)- or branched (6) aldehydes, a high n/ NHC in situ (Table 3). The investigations show that
bra ratio is desirable. Therefore monodentate phos- the sole use of Rh(CO)2acac in DMF as solvent gives
phorus ligands are unsuitable for the desired tandem poor yields of 22% for the desired aldehyde 2 with
catalysis.
a low n/bra ratio of 2 (entry 3.1). In the presence of
The application of sterically demanding bidentate base (entry 3.2), C1 (entry 3.3), C1-NHC (formed by
ligands DPEphos (L6) (entry 1.7) and DPPB (L5) C1 and base) (entry 3.4) or Et3NHCl (S1, formed by
(entry 1.6) does not lead to higher regioselectivity or C1 and base as well) (entry 3.5) the rhodium catalyst
yields. The bidentate phophorus ligand Xantphos (L7) is inactive for hydroformylation.
leads to a higher n/bra ratio of 58 (entry 1.8). Howev-
According to Dupont, a possible reason for the in-
er, the hydroformylation activity with 37% aldehydes activation is the equilibrium between the carbene salt
and the present n/bra ratio are low if compared to Bi- and the corresponding free carbene (NHC) in solu-
phephos (L8) [entry 1.9, YS(2+3) =68%, n/bra=96], tion, which was determined in labelling experiments
which is a known ligand gaining high regioselectivities of an imidazolium salt in methanol.[15] This NHC can
in hydroformylation.[13] A higher load of base and coordinate to the rhodium complex by deactivating
10 mol% C1 (being a standard catalyst concentration the catalyst for hydroformylation, although the equi-
in current organocatalyses investigations[8a,b]) in pres- librium should be on the side of the imidazolium salt.
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Synergy in Tandem Hydroformylation/Acyloin Reaction
Table 2. Investigation of the tandem hydroformylation/acyloin reaction with other substrates.[a]
Entry
1.10
Olefin
Product
XOlefin
YS(n-Ald+Acyl)
Yn-Ald
YAcyl
Yiso-Ald
YIsom
n/bra
Xn-Ald
100
82
9
73
<1
18
>99
90
2.1
74
98
49
98
8
41
76
1
25
—
49
84
78
2.2
22
—
>99
2.3[b]
100
81
45
36
6
5
34
31
1
1
53
45
40
36
85
86
2.4
[a]
Reaction conditions: 6 mmol olefin, 0.03 mmol Rh(CO)2acac, 0.09 mmolL8, 0.6 mmol C1, 2.4 mmol Et3N, 30 bar CO/H2
(1:1), 5 mL DMF, 608C, 16 h. Results determined by GC in % with calibrated substances. <1=traces are detectable.
5% aldol condensation product, 3% hydrogenated olefin.
[b]
Table 3. Hydroformylation in the presence of C1 and base.[a] (entry 3.2) and the salt Et3N·HCl (entry 3.5) are in-
hibiting the hydroformylation, probably due to a coor-
dination of the nitrogen to the metal complex.
For the application of the orthogonal tandem catal-
ysis it is crucial to overcome the inhibiting effect of
the base and C1-NHC on the hydroformylation activi-
Entry Add. Et3N YS(2+3) Y2
Y3
Y4
Y6
n/bra
ty of rhodium. The hydroformylation activity can be
regained if the coordinating C1-NHC is replaced by
a phosphine ligand as already seen in Table 1.
3.1
3.2
3.3
3.4
3.5
À
À
+
À
+
À
22
22
<1
<1
–
–
–
11
<1 <1
<1
–
2
–
–
–
–
À
<1
<1
<1
<1
Table 4 gives a more detailed investigation by ap-
plying base or C1 in the presence of Biphephos (L8).
The sole use of Rh(CO)2acac and L8 in DMF give
very good yields of 81% for 2 with a low n/bra ratio
of 5 (entry 4.1), due to the observed branched alde-
hydes 6, indicating low selectivity at the given condi-
tions. The presence of base increases the n/bra ratio
to 85 (entry 4.2) indicating a coordination of Et3N to
the complex. In contrast to the observations of
Dupont[15] (no changes in hydroformylation selectivity
in the presence or absence of carbene salt) we see
C1
C1
S1
–
–
–
<1 <1 <1
<1
–
–
[a]
Reaction conditions: 6 mmol 1, 0.03 mmol Rh(CO)2acac,
0.3 mmol additive, 1.2 mmol Et3N, 30 bar CO/H2 (1:1),
5 mL DMF, 608C, 16 h. Results determined by GC in %
with calibrated substances. The presence/addition of
a compound is marked as “++” and the absence is marked
as “À”. <1=traces are detectable. Add.=additive.
Herein, we see the same effect with the thiazolium a significant improvement of the n/bra ratio of >99, if
salt C1 and DMF as solvent indicating C1-NHC is carbene salt C1 is applied (entry 4.3). Isomers 5 are
formed in sufficient amount and coordinating to the not converted in the hydroformylation step, therefore
rhodium complex by deactivating the active catalyst no branched aldehydes 6 were observed, showing an
species for the hydroformylation reaction (entry 3.4), excellent selectivity. Due to different studies referring
since a ten-fold excess of C1 is added compared to to Rh/NHC complexes it can be assumed that the
the amount of used rhodium complex. Also the base formed C1-NHC from C1 remains in the coordination
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Karoline A. Ostrowski et al.
Table 4. Detailed investigation of the tandem hydroformyla-
Table 5. Tandem hydroformylation/acyloin reaction with thi-
amine derivative C2.[a]
tion/acyloin reaction.[a]
Entry C1 Et3N YS(2+3) Y2 Y3 Y4
Y6 n/bra
Entry L8 C2
mmol
Et3N
mmol
YS(2+3) Y2 Y3 Y4 n/
bra
4.1
4.2
4.3
4.4
À
À
+
+
À
+
À
+
81
85
65
68
81
85
65
–
–
–
8
1
<1
<1
8
1
–
–
5
85
>99
96
5.1
5.2
5.3
5.4
À
+
+
+
0.3
À
<1
63
<1 – <1 –
0.3
0.3
0.6
À
63
–
1
2
1
96
32
58
45 23
0.6
1.2
80
66
65 15
11 45
[a]
Reaction conditions: 6 mmol 1, 0.03 mmol Rh(CO)2acac,
0.09 mmolL8, 0.3 mmol C1, 1.2 mmol Et3N, 30 bar CO/
H2 (1:1), 5 mL DMF, 608C, 16 h. Results determined by
GC in % with calibrated substances. The presence/addi-
tion of a compound is marked as “+” and the absence is
marked as “À”. <1=traces are detectable.
[a]
Reaction conditions: 6 mmol 1, 0.03 mmol Rh(CO)2acac,
0.09 mmolL8, 30 bar CO/H2 (1:1), 5 mL DMF, 608C,
16 h. Results determined by GC in % with calibrated
substances. The presence/addition of a compound is
marked as “+” and the absence is marked as “À”. <1=
only in traces detectable.
We see the same effect – deactivating the catalyst
for hydroformylation (compare entry 5.1 with 4.3)
and enhancing the n/bra ratio in presence of Biphe-
phos (L8) (compare entry 5.2 with 4.3) – proving that
the thiazole NHC is responsible for the overall en-
hanced regioselectivity and chemoselectivity. This
effect was described before with various imidazole
Figure 5. Thiamine derivative C2.
sphere of rhodium during the hydroformylation[16] NHCs.[17] In this work, the same effect is described for
thus being responsible for the selective hydroformyla- the first time with thiazole NHCs.
tion of terminal double bonds.
A simultaneous application of C2 and base leads to
It is important to mention that Dupont worked 15% acyloin product (entry 5.3). As seen in
with an imidazolium compound while a thiazolium entry 1.10, a higher load of 10 mol% organocatalyst
compound was applied here. A direct comparison of affords higher conversion of 2 with higher yields of 3
these results is not feasible due to different electronic with 45% (entry 5.4).
properties of thiazole- and imidazole-derived car-
benes.
These results demonstrate that a possible coordina-
tion between the aminopyrimidine ring in C1 and Rh
A full conversion of C1 to C1-NHC is achieved by is not responsible for the selectivity in hydroformyla-
adding a base in presence of a phosphorus ligand, tion. This leads to the justified assumption that both
leading to a successful combination of both reaction the free carbene (C1-NHC and C2-NHC) and Biphe-
steps, hydroformylation and acyloin reaction, was re- phos are coordinating to Rh during hydroformylation.
alised. This orthogonal tandem catalysis yields the de-
In summary, we have described the first tandem hy-
sired acyloins (entry 4.4). Since C1-NHC is applied as droformylation/acyloin reaction yielding atom effi-
an organocatalyst in this acyloin reaction and is also ciently acyloins from olefins and syngas. Due to the
capable as a ligand for the hydroformylation, we ob- synergy effects between the applied metal catalyst
served here a rare example of synergy effects between and the organocatalyst, we achieve an excellent n/bra
metal catalysts and organocatalysts.
ratio of up to >99 with yields up to 98% for the inter-
The application of the thiamine derivative C2 mediate n-aldehyde.
(Figure 5), wherein the aminopyrimidine ring is being
Internal double bonds yielded by an isomerization
substituted by a phenyl group, leads to similar results side-reaction are not converted to their branched al-
with the same high selectivity for the n-aldehyde 2 dehydes, indicating an excellent conversion in hydro-
with no observed branched aldehydes 6 (Table 5). We formylation of only terminal double bonds. Further-
tested C2-NHC in the tandem reaction as well ensur- more, the intermediate n-aldehydes are highly selec-
ing that no coordination effects between the amino- tively converted by an in situ formed NHC, derived
pyrimidine ring and the metal complex were responsi- from vitamin B1 used as an organocatalyst, yielding
ble for the exclusive conversion of the terminal up to 76% of the desired acyloin products. Currently,
double bond.
we are investigating the mechanistic coordination as-
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Synergy in Tandem Hydroformylation/Acyloin Reaction
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Typical Procedure for the Tandem Hydroformylation/
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were dissolved in a 40-mL custom-made stainless steel auto-
clave in 5 mL of DMF. After adding triethylamine
(2.4 mmol) the autoclave was pressurised with 30 bar syngas
(CO/H2 =1:1) and the mixture was stirred at 608C for 16 h
magnetically. After cooling to room temperature the auto-
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(see the Supporting Information).
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Acknowledgements
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