Online ATR-IR investigations and mechanistic understanding of the carbonylation of epoxides - The selective synthesis of lactones or polyesters from epoxides and CO

Article abstract of DOI:10.1016/j.jorganchem.2003.12.030

In situ ATR-IR spectroscopy is applied as a powerful tool to study the factors that control the reaction of epoxides with carbon monoxide in the presence of [Lewis acid]⁺ [Co(CO)₄]^- salts. Based on these investigations, a consistent mechanistic scheme is presented, comprising the main lactone and polyester products and minor components, e.g., acetone and crotonic acid derivatives. β-Alkoxy-acyl-cobalttetracarbonyl species are shown to be key intermediates from which two reaction routes start in dependence of the applied Lewis acid (LA). Labile LA-alkoxy combinations favor a backbiting process of the oxygen function on the Co-acyl bond, primarily producing lactone products. More stable LA-alkoxy entities are unreactive toward PO conversion and afford a polymerization reaction after the addition of a nucleophile. In that case, the Lewis acid remains bonded to the chain end.

Full text of DOI:10.1016/j.jorganchem.2003.12.030

Journal of Organometallic Chemistry 689 (2004) 971–979
www.elsevier.com/locate/jorganchem
Online ATR-IR investigations and mechanistic understanding
of the carbonylation of epoxides – the selective synthesis
of lactones or polyesters from epoxides and CO
a
Markus Allmendinger , Manuela Zintl , Robert Eberhardt , Gerrit A. Luinstra
a
a
b,
*
,
b,
a, ,1
*
Ferenc Molnar *, Bernhard Rieger
a
Department of Materials and Catalysis, University of Ulm, Albert-Einstein-Allee 11, Ulm 89069, Germany
b
Polymer Research, BASF AG, Ludwigshafen 67056, Germany
Received 2 June 2003; accepted 1 December 2003
Abstract
In situ ATR-IR spectroscopy is applied as a powerful tool to study the factors that control the reaction of epoxides with carbon
monoxide in the presence of [Lewis acid]þ[Co(CO)4]ꢀ salts. Based on these investigations, a consistent mechanistic scheme is
presented, comprising the main lactone and polyester products and minor components, e.g., acetone and crotonic acid derivatives.
b-Alkoxy-acyl-cobalttetracarbonyl species are shown to be key intermediates from which two reaction routes start in dependence of
the applied Lewis acid (LA). Labile LA–alkoxy combinations favor a backbiting process of the oxygen function on the Co–acyl
bond, primarily producing lactone products. More stable LA–alkoxy entities are unreactive toward PO conversion and afford a
polymerization reaction after the addition of a nucleophile. In that case, the Lewis acid remains bonded to the chain end.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Online ATR-IR spectroscopy; Catalytic carbonylation; Epoxides; Lactones; Aliphatic polyesters
1. Introduction
thenium-based (phosphane) catalysts [8]. Carbonylation
reactions in the absence of hydrogen also found ways
into technical application and are nowadays connected
to important industrial synthesis like the ‘‘Monsanto
process’’ for the production of acetic acid from metha-
nol [9] and the ‘‘Reppe carbonylation’’ to yield car-
boxylic acids from olefins and alkynes [10]. Other
substrate molecules like epoxides or aziridines were also
subjected to carbonylation and hydroformylation
conditions during the time of pioneering studies on car-
bonylative transformations in the 1950s and 1960s [11–
13]. Little attention was given to these starting materials
thereinafter. Nevertheless, putative derived product
structures such as lactones, polyesters or polyamides are
of substantial technical importance. Aliphatic polyesters
for example result from epoxides and CO either directly
or via a lactone intermediate. The latter in turn is pro-
duced by a ring expanding carbonylation of epoxides
[14]. In the last two years, important progress was
achieved in the carbonylation of epoxides to lactones
[15] and directly to polyesters [16]. We present here a
The insertion of CO into organic substrates through
transition metal catalyzed reactions is of importance for
numerous transformations in heterogeneous and ho-
mogeneous catalysis [1–5]. In addition, hydroformyla-
tion – the addition of carbon monoxide and hydrogen to
olefins – is a widely industrially applied homogeneous
catalytic carbonylation process [3,6]. The catalytic re-
action itself was already discovered in 1938 by Roelen,
using the system Co2(CO)8/CO/H2 [7]. Since then, hyd-
roformylation research has led to a profound under-
standing of the elementary steps of the catalytic cycle
and from this perception, significant improvements were
achieved by the application of cobalt, rhodium or ru-
* Corresponding authors. Tel.: +49-731-502-3038; fax: +49-731-502-
3039.
E-mail address: bernhard.rieger@chemie.uni-ulm.de (B. Rieger).
1
Present address: Anorganische Chemie II, Universitaet Ulm, Ulm
D-89069, Germany.
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.12.030

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M. Allmendinger et al. / Journal of Organometallic Chemistry 689 (2004) 971–979
comprehensive mechanistic outline of the carbonylation
reaction of epoxides yielding lactones, polyesters, ace-
tone and crotonic acid derivatives in various amounts
in dependence of catalysts, cocatalysts and reaction
conditions.
activates epoxides through protonation and subsequent
ring opening by the Co(CO)ꢀ4 nucleophile (Scheme 1).
Mixtures of HCo(CO)4 and epoxides were studied
with the objective of gaining insight into individual
mechanistic steps in the absence of hydrogen. It is
known, that HCo(CO)4 decomposes easily to H2,
Co2(CO)8, higher carbonyl clusters and other products.
For this reason, it was decided to prepare HCo(CO)4 in
situ from Et4NCo(CO)4 and HBF4. Quantitative for-
mation of HCo(CO)4 [17] was detected after the addi-
tion of HBFꢁ4OEt2 to a solution of Et4NCo(CO)4 in
diglyme under 20 bar carbon monoxide pressure
(Fig. 2). Significant changes were instantaneously ob-
served in the IR spectrum after addition of propylene
oxide to this reaction mixture (Fig. 3). New absorptions
appeared at 2107, 2041, 2022, 2003 and 1710 cmꢀ1; si-
multaneously, the bands associated with HCo(CO)4
disappeared, indicating the quantitative formation of an
acyl-cobaltcarbonyl species (Fig. 4).
2. Results and discussion
An ATR-IR spectrometer attached to a high-pressure
autoclave unit was used for in situ online investigation
in order to gain insight into rates and product distri-
butions of epoxide carbonylation reactions. This ana-
lytical tool gives a variety of information under reaction
conditions (high pressure and temperature) relevant to
catalysis: identity and quantity of key structures, kinet-
ics with no sample handling, and online recording of
possibly short-lived (organometallic) intermediates (see
Fig. 1).
Fig. 5 is a time-dependent ATR-IR stack-plot of the
above reaction sequence. The cobalttetracarbonylate
anion of Et4NCo(CO)4 was observed (1889 cmꢀ1) in a
first stage. The addition of one equivalent HBF4 at 0 °C
initiated the immediate formation of a mixture of free
Co(CO)ꢀ4 -anions and HCo(CO)4 (1889 cmꢀ1, resp. 2117,
3. ATR-IR investigation of the carbonylation process
Basic information of epoxide carbonylation reactions
was taken from well-studied hydroformylations.
HCo(CO)4 is presumed to be a potential catalyst that
Fig. 1. ATR-IR/autoclave setup for online measurements.
Scheme 1. Formation of acyl-cobalttetracarbonyl and further reaction with H2.

M. Allmendinger et al. / Journal of Organometallic Chemistry 689 (2004) 971–979
973
Fig. 2. ATR-IR spectroscopic monitoring of the in situ synthesis of HCo(CO)4 (a: 2117, 2053 and 2024 cmꢀ1) from Et4NCo(CO)4 (b: 1887 cmꢀ1) and
HBF4.
Fig. 3. ATR-IR spectroscopic detection of the formation of the acyl complex HO–CH(CH3)CH2CO–Co(CO)4 (a: 2107, 2041, 2022, 2003 and 1710
cmꢀ1) from HCo(CO)4 (b: 2117, 2053 and 2024 cmꢀ1) by PO addition under CO.
2053, 2024 cmꢀ1). This mixture remained unchanged for
20–30 min. HCo(CO)4, apparently stable at 0 °C, was
present in the IR-spectrum after the addition of another
0.5 equivalents of HBF4 as the only species. Finally, b-
alkoxyacyl-cobalttetracarbonyl was formed within 2
min after the subsequent addition of propylene oxide (50
equivalents relative to Et4NCo(CO)4 at 0 °C).
75 °C to induce the formation of carbonylation products
(Table 1, entry 6). However, only decomposition of acyl-
cobalttetracarbonyl and the simultaneous formation
of Co2(CO)8 (2072, 1847 cmꢀ1, Fig. 6) was observed
under these conditions. It is presumed, that the strong
acidic condition were responsible for the decomposition
reactions.
The reaction mixture comprising acyl-cobalttetra-
Therefore, a second set of reaction conditions was
used to study a mixture of acyl-cobalttetracarbonyl,
carbonyl, HBF4 and propylene oxide was heated to

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M. Allmendinger et al. / Journal of Organometallic Chemistry 689 (2004) 971–979
lactone formation was relatively low, but the reaction is
remarkably selective.
We further tested a variety of tetracarbonyl cobaltate
salts for the carbonylation of propylene oxide to learn
more about the dependence of activity on the nature of
the LA-fragment (Table 1) [15].
All experiments that were conducted to induce car-
bonylation reactions of epoxides by using Co(CO)ꢀ4 -
salts with cations of low Lewis acidity (e.g.,
PPNCo(CO)4 or Et4NCo(CO)4, PPN: bis(triphenyl-
phosphoranylidene)ammonium) failed [15b] (Table 1,
entries 1 and 2). The characteristic absorption of the
Co(CO)ꢀ4 -anion (1889 cmꢀ1) was invariable over several
hours; no carbonylation product was detected. 1H
NMR spectroscopy of the reaction mixture confirmed
this observation (PO and solvent signals only). These
results are consistent with theoretical considerations
[15a] and suggest that the Co(CO)ꢀ4 -anion by itself is
not nucleophilic enough to induce ring opening of the
epoxide substrate [19].
Fig. 4. Characteristic IR absorptions of the acyl-cobalttetracarbonyl
complex resulting from treatment of HCo(CO)4 with PO and carbon
monoxide. Synthesis, characterization and reactions of acyl-cobalt-
carbonyl complexes are already reviewed in literature [18].
excess Et4NCo(CO)4 and PO for its carbonylation ac-
tivity. In contrast to the previous experiments, less than
one equivalent HBF4 was added to Et4NCo(CO)4 and
PO in diglyme at 0 °C. After heating to 75 °C, now a
slow formation of b-butyrolactone (1829 cmꢀ1) is ob-
served and no decomposition of the acyl-complex oc-
curred within 1 h (Fig. 7) (Table 1, entry 7). The rate of
Fig. 5. ATR-IR stack plot for the conversion of Et4NCo(CO)4 (1889 cmꢀ1) to HCo(CO)4 (2117, 2053 and 2024 cmꢀ1) and further to the acyl complex
HO–CH(CH3)CH2CO–Co(CO)4 (1407 cmꢀ1: characteristic PO absorption).
Table 1
All reactions were carried out at 75 °C and 60 bar CO pressure
No.
Catalyst
Epoxide
Reaction time [h]
Results
1
2
3
4
5
6
7
8
Et4NCo(CO)4
PPNCo(CO)4
PO
PO
PO
PO
PO
PO
PO
PO
20
20
20
20
20
4
No reaction
No reaction
No reaction
Hg[Co(CO)4]2
Zn[Co(CO)4]2
NaCo(CO)4
5–10% PO consumption
5–10% PO consumption
Acyl-Co(CO)4/no carbonylation
Acyl-Co(CO)4/slow lactone formation
Acyl-Co(CO)4 (stable)
HCo(CO)4/excess HBF4
HCo(CO)4/excess Et4NCo(CO)4
Ph3SiCo(CO)4
2
4

M. Allmendinger et al. / Journal of Organometallic Chemistry 689 (2004) 971–979
975
Fig. 6. Reactivity of HO–CH(CH3)CH2CO–Co(CO)4 in the presence of excess HBF4 and PO. Co2(CO)8 (2072 and 1847 cmꢀ1) is detected instead of
PO carbonylation products. (The absorptions of Co2(CO)8 (2042 and 2001 cmꢀ1) overlap with the bands of the acyl-cobaltcarbonyl species (2041,
2022 and 2003 cmꢀ1). For this reason the free absorptions (2072 and 1847 cmꢀ1) are used for characterization.)
Fig. 7. Reactivity of HO–CH(CH3)CH2CO–Co(CO)4 in the presence of excess Et4NCo(CO)4 and PO. Selective b-butyrolactone formation (1829
cmꢀ1) is observed (PO carbonylation product).
We further considered the reactivity of Hg[Co(CO)4]2
and Zn[Co(CO)4]2. The two components differ in Lewis
acidity. Hg[Co(CO)4]2 is a covalent complex of low
polarity, while Zn[Co(CO)4]2 has considerable ionic
character [20]. Hg[Co(CO)4]2 proved to be inactive for
epoxide ring opening and consecutive carbonylation
reactions (Table 1, entry 3). In contrast, a PO car-
bonylation experiment with Zn[Co(CO)4]2 in diglyme
resulted in CO and epoxide consumption (1H NMR:
10% PO consumed after 20 h, 75 °C). The product is a
mixture of b-butyrolactone, ester oligomers and acetone
(Table 1, entry 4).
Recently, we reported that experiments indicating the
possibility of a direct alternating copolymerization reac-
tion of epoxides and CO to generate polyesters (Scheme
2, B) [16a]. In order to suppress ‘‘backbiting’’ sequences
of the acyl-cobalttetracarbonyl species 3 that would af-
ford lactone, the LA–alkoxy oxygen moiety has to be
sufficiently unreactive to form the end-group of a growing
polyester chain. The compound Ph3SiCo(CO)4 (Fig. 8)
was found to be an excellent catalysts-precursor in this
regard [21]. The corresponding acyl-cobalttetracarbonyl
complex (Ph3Si–OCH(CH3)–CH2CO–Co(CO)4; Fig. 8,
2107, 2043, 2024, 2005 and 1710 cmꢀ1) is quantitatively

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M. Allmendinger et al. / Journal of Organometallic Chemistry 689 (2004) 971–979
Scheme 2. Catalytic cycle for the organometallic carbonylation of epoxides.
obtained by adding PO to the solution of Ph3SiCo(CO)4
ing the polyester product (5) [24]. Hydrolysis by resid-
ual or intermediate water was demonstrated to be a
chain-transfer mechanism (3b ! 5), and thermal
degradation of the formed PHB to crotonic acid deriva-
tives (6) influence the molecular weight of the polymer
product.
2
3
in toluene (Fig. 7, A) [22]. This acyl-species is stable
for several hours at 50 °C or at 75 °C in CO atmosphere
[23]. Addition of pyridine to this reaction mixture results
in the formation of polyhydroxybutyrates. All investi-
gations show that pyridine plays a prominent, not yet
fully understood role in the further insertion of epoxide
monomers into cobalt–acyl bonds (3 ! 3a ! 3b) yield-
4. Mechanistic considerations
2
Toluene instead of diglyme was used because Ph3SiCo(CO)4 is
known to react with cyclic and linear ethers.
The results of this study are cumulated in Scheme 2.
Polarization of the epoxide (1) is achieved by interaction
of a Lewis acidic species with the epoxide oxygen atom.
A strong Lewis acid results in an immediate ring open-
ing of the cyclic ether (Scheme 2, C), affording the
3
Alternative methods for the synthesis of acyl- or alkyl-cobalttet-
racarbonyl complexes were performed by adding NaCo(CO)4 to acyl-
or alkyl-chlorides (e.g., ethyl-bromoacetate). Partially, these methods
suffer from significant decomposition.

M. Allmendinger et al. / Journal of Organometallic Chemistry 689 (2004) 971–979
977
Fig. 8. Stack plot for the reaction of Ph3SiCo(CO)4 (2094, 2032, 1999 cmꢀ1) in toluene with PO (1407 cmꢀ1; addition after 0.6 h) under CO-
atmosphere. Product is the acyl complex Ph3Si–OCH(CH3)–CH2CO–Co(CO)4 (2107, 2043, 2024, 2005, 1710 cmꢀ1).
propylene oxide isomer acetone or inducing a ring
opening polymerization (ROP) reaction of the epoxide
to polyethers. Less acidic fragments activate the epoxide
substrate and an alkyl-cobalttetracarbonyl species
(Scheme 2, 2) is formed after backside attack of the
Co(CO)ꢀ4 -nucleophile. Carbon monoxide is subse-
quently inserted to yield the corresponding acyl-cobalt-
tetracarbonyl species (3), which is stable under CO
atmosphere. The intermediate 3 can now react according
to two different routes, depending on the Lewis acid
cocatalyst to produce lactone (4) or polyester (5) prod-
ucts after the addition of pyridine. A backbiting reaction
of the alkoxy oxygen on the acyl moiety of 3 results in a
lactone formation process provided that the LA–oxygen
entity is labile enough. In addition, the catalyst ([Lewis
acid]þ[Co(CO)4]ꢀ) is re-generated (A) if the lactone ef-
fectively de-coordinates from the Lewis acid. ROP of the
lactone intermediate gives secondary polyester products
or rearrangement to crotonic acid derivatives (6), feasi-
ble reactions in the presence of a Lewis acid [25].
importance. Several reaction channels are accessible for
the reactive epoxides, ranging from isomerization or
polyether formation with potent Lewis acids, fast ring
opening and formation of stable cobalt acylium b-alk-
oxy complexes with Lewis acids of intermediate
strength, or catalytic lactone formation with even
weaker acidic species to complete interactivity. Polyester
is formed under the action of pyridines on unreactive
cobalt acylium species. It may be anticipated, that pyr-
idine induces the formation of Lewis acid after reaction
with the cobalt acyl compound. The exact reaction
pathway is still under investigation. A comprehensive
reaction scheme (Scheme 2) shows the whole of the
currently established reaction modes.
6. Experimental
Racemic propylene oxide (rac-PO) was supplied by
BASF AG (water <15 ppm). Diglyme was purchased
water free from Fluka, degassed before use. NaCo(CO)4
[26], Et4NCo(CO)4 and PPNCo(CO)4 [15a], Hg-
[Co(CO)4]2 [27], Zn[Co(CO)4]2 [28] and Ph3SiCo(CO)4
[29] were synthesized according to literature procedures.
The applied ReactIR1000e from Mettler Toledo
is based on mid-infrared Fourier transform (FTIR)
spectrometer. The sampling system includes a dipper,
connected to the autoclave, equipped with a choice
of DiCompe (diamond sensor for highest chemical
5. Conclusions
In situ ATR-IR spectroscopy has proven to be a
powerful tool to gain insight in cobalt carbonylate cat-
alyzed carbonylation reactions of propylene oxide at
elevated pressure and temperature. It became clear, that
the Lewis acidity of the cocatalyst is of eminent

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M. Allmendinger et al. / Journal of Organometallic Chemistry 689 (2004) 971–979
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General carbonylation procedure. Reactions were
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conducted in a 250-ml Buchi reactor equipped with the
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the autoclave. The system was cooled to 0 °C and the
catalyst-precursors (3.47 mmol of Co(CO)ꢀ4 -anion salt)
were added. HBF4 and propylene oxide (30 ml) were
transferred to the autoclave through a pressure burette
with a carbon monoxide pressure pulse. The carbony-
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2–20 h while pressurizing the reactor with 60 bar CO.
The carbonylation reaction was quenched by cooling the
ATR-IR-reactor setup to ambient temperature and
ventilation of the gaseous components. Yields were de-
termined by online-IR and by NMR analysis of the
resulting solutions.
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