Catalytic carbonylation of 4-penten-1-ol to ε-caprolactone and oligocaprolactone

Article abstract of DOI:10.1016/j.molcata.2005.06.013

A catalyst consisting of Pd₂(dba)₃ + 2 XANTPhos (dba = dibenzylideneacetone, XANTPhos = 9,9-dimethyl-4,5-bis(diphenylphosphino) xanthene) in toluene catalyzes the alkoxycarbonylation of 4-penten-1-ol to form the cyclic products ε-caprolactone and 2-methylvalerolactone, as well as oligocaprolactone. The conversion and selectivity observed using this catalyst is higher than that observed with other reported systems with the regioselectivity for terminal carbonylation exceeding 70%.

Full text of DOI:10.1016/j.molcata.2005.06.013

Journal of Molecular Catalysis A: Chemical 239 (2005) 185–192
Catalytic carbonylation of 4-penten-1-ol to ␧-caprolactone
and oligocaprolactone
∗
Jeffrey K. Funk, David K. Newsham, David N. Goldstein, Shengsheng Liu, Ayusman Sen
Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
Received 23 February 2005; received in revised form 23 May 2005; accepted 14 June 2005
Available online 20 July 2005
Abstract
A catalyst consisting of Pd
2
3
(dba) + 2 XANTPhos (dba = dibenzylideneacetone, XANTPhos = 9,9-dimethyl-4,5-bis(diphenylphosphino)
xanthene) in toluene catalyzes the alkoxycarbonylation of 4-penten-1-ol to form the cyclic products ␧-caprolactone and 2-methylvalerolactone,
as well as oligocaprolactone. The conversion and selectivity observed using this catalyst is higher than that observed with other reported systems
with the regioselectivity for terminal carbonylation exceeding 70%.
©
2005 Elsevier B.V. All rights reserved.
Keywords: ␧-Caprolactone; Carbonylation; Palladium catalyst; Carbon monoxide; Lactones
1
. Introduction
or hydrogen peroxide [26–29]. From an industrial standpoint,
thisisanundesirableprocessbecauseitisnotcatalyticandthe
oxidation reaction results in the formation of a stoichiomet-
ric amount of acid, as well as some overoxidized byproducts
[30–33].
Transition metal-catalyzed carbonylation reactions have
been extensively studied for the formation of both small
molecules and polymeric materials [1–7]. Of particular
interest has been the use of palladium-based catalysts for
the formation of esters and lactones. Recently, research
has been done on a methoxycarbonylation system utiliz-
ing (DtBPX)Pd(dba) + MSA as a promoter (DtBPX = bis(di-
tert-butylphosphino)-ortho-xylene, MSA = methanesulfonic
acid) for the synthesis of methyl propionate from ethene,
CO and methanol [8–12]. In contrast to this intermolecular
esterification of alkenes, most of the research on lactone for-
mationtodatehasfocusedontheformationoffive-membered
rings from the intramolecular carbonylation of allylic alco-
hols [13–22].
One particularly important lactone that is not a five-
membered ring is ␧-caprolactone. This lactone is used
as a monomer for the production of numerous polyester-
based products including sutures and plasticizers [23–25].
Currently, ␧-caprolactone is synthesized mainly by the
Baeyer–Villiger oxidation of cyclohexanone via a peroxyacid
Interestingly, there are some examples of the synthesis of
seven-membered lactones by intramolecular cyclocarbonyla-
tionreactions. Inoneexample, Alperandco-workersreported
that 2-allylphenols could be selectively cyclocarbonylated to
five- or seven-membered lactones using a catalyst consist-
ing of either [(Cy3P)2Pd(H)(H2O)][BF4] or Pd(OAc)2 + 4 eq.
dppb (dppb = 1,4-bis(diphenylphosphino)butane) in toluene,
under an atmosphere of CO/H2 [34,35]. More importantly,
it was found in the patent literature that a catalyst sys-
tem consisting of (Ph3P)2PdCl2 + 4 eq. SnCl2 in MIBK
(MIBK = methyl isobutyl ketone) under an atmosphere of
pureCOcouldcatalyzethecyclocarbonylationof4-penten-1-
ol for the synthesis of ␧-caprolactone with up to 77% selectiv-
ity [36]. However, few details of the reaction, its products, or
how it compares to other alkoxycarbonylation systems were
given.
Recently, there have been numerous reports in the patent
literature of attempts to find a new route towards the synthesis
of ␧-caprolactone starting with1,3-butadiene according to the
proposed reaction shown in Scheme 1 [37–51].
∗
Corresponding author. Tel.: +1 8148632460; fax: +1 8148651543.
E-mail address: asen@chem.psu.edu (A. Sen).
1
381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.06.013

1
86
J.K. Funk et al. / Journal of Molecular Catalysis A: Chemical 239 (2005) 185–192
Scheme 1. Proposed synthesis of ␧-caprolactone via a two-step sequence from 1,3-butadiene, carbon monoxide, and dihydrogen.
In this reaction sequence, the synthesis of ␧-caprolactone
split capillary column (Restek) connected to an FID detector.
Gel permeation chromatography was obtained on a Shimadzu
size exclusion chromatograph using a flow rate of 1 mL/min
and a three-column bed (Styragel HR 7.8 mm × 300 mm
columns with a 5 ␮m bead size: 100–10,000, 500–30,000
and 5000–6,000,000 Da), a Shimadzu RID 10A differen-
tial refractometer and SPD-10A UV–vis detector. Samples
were run in CHCl3 at ambient temperature and calibrated
to polystyrene standards obtained from Aldrich. All work
involving air- and/or water-sensitive compounds was carried
out using standard Schlenk and/or dry box techniques under
a dinitrogen atmosphere.
is potentially catalytic and utilizes the relatively cheap
starting materials of 1,3-butadiene, dihydrogen and carbon
monoxide. With the exception of the (Ph3P)2PdCl2 + 4 SnCl2
system, there are no reports to date on a possible catalyst
for the second step of the above synthetic scheme. Herein
we report progress towards a new synthetic method for the
preparation of ␧-caprolactone and polycaprolactone via the
carbonylation of 4-penten-1-ol.
2
. Experimental
2
.1. Materials
2.3. Synthesis of transesterification product
Toluene (ACS Grade) was dried over a Grubbs-type
purification system and stored over molecular sieve packets
Aldrich) [52]. 4-Penten-1-ol (99%, Acros), cis-3-penten-1-
In a 100 mL round bottom flask, 100 mmol ␧-
caprolactone, 100 mmol 4-penten-1-ol and 200 mmol triethy-
lamine were refluxed overnight. The products were then
dissolved in 100 mL 5% HCl (aq) and purified by liq/liq
extraction (3× 100 mL hexane). The slightly reddish solu-
tion was reduced under vacuum and purified by column
(
ol (97+%, GFS), ␧-caprolactone (99%, Acros) and 1,1,2,2-
tetrachloroethane (TCE, 99%, Aldrich) were dried over
CaH2, distilled and degassed (freeze, pump, thaw) three
times prior to use. Polycaprolactone (average 10k Mw,
Aldrich), hexanes (ACS grade, EMS), isopropanol (ACS
grade, EMS), toluene-d8 (“100.0%”, Aldrich) and triethy-
lamine (99%, EM Science) were used as received. Tris(diben-
zylideneacetone)dipalladium (0) (Pd2(dba)3), palladium
1
chromatography (15% isopropanol: 85% hexanes). H NMR
(
CDCl3) (ppm): 5.72 (m, 1H), 4.96 (td, 2H), 4.01 (t, 2H),
3
1
1
2
.53 (t, 2H), 2.25 (t, 2H), 2.01 (q, 3H), 1.56 (m, 6H),
.32 (m, 2H); 13C NMR (CDCl3) (ppm) 173.77, 138.17,
15.57, 64.25, 62.14, 34.56, 32.88, 30.64, 28.47, 26.01,
5.37.
(
II) acetate (Pd(OAc)2), tetrakis(acetonitrile)palladium(II)
tetrafluoroborate ((MeCN)4Pd(BF4)2), 9,9-dimethyl-4,5-bis
diphenylphosphino)xanthene (XANTPhos), alpha, alpha’-
(
bis(di-tert-butylphosphino)-ortho-xylene (97+%) (DtBPX),
tricyclohexylphosphine (97+%) (PCy3), and 1,4-bis(diphen-
ylphosphino)butane (dppb) were purchased from Strem and
used as received. 2-Methylvalerolactone [53–55] and 2-
ethylbutyrolactone [56] were synthesized according to lit-
erature procedures.
2.4. General procedure for the carbonylation of
penten-1-ol
A mixture of 0.050 mmol Pd (dba) , 0.10 mmol XANT-
2
3
Phos, 10 mmol penten-1-ol, 2.0 mmol TCE (GC standard),
and 15 mL toluene was added to a thin-necked glass liner. A
small sample was removed for initial GC analysis. The glass
liner was then placed in a 125 mL Parr high-pressure reactor,
sealed and charged with 300 psi CO. The reactor was then
2
.2. General experimental methods
1
13
◦
◦
All H NMR (at ambient temperature), C NMR (at
placed on a 140 C oil bath (100 C internal temperature) for
3 h. The high pressure reactor was then removed from the
oil bath, cooled in an ice bath, depressurized and analyzed
by GC. To determine the polymer content of the reaction,
the sample was subsequently placed in a scintillation vial,
reduced under vacuum, dissolved in CDCl3 and analyzed by
ambient temperature), and 31P NMR (at −80 C) spectra
were obtained on a Bruker DPX-300 spectrometer using
an external capillary standard of 1,4-dioxane referenced to
6.9 ppm ( C NMR) or standard of Xantphos referenced to
17.5 ppm ( P NMR). Gas chromatography analysis was
◦
13
6
−
31
13
obtained on an Agilent 5890 Series II GC using a RTX-5
C NMR spectroscopy.

J.K. Funk et al. / Journal of Molecular Catalysis A: Chemical 239 (2005) 185–192
187
Table 1
Carbonylation of 4-penten-1-ol: comparison of catalyst systemsa
Run Catalysta
Solvent
Conversion (%)
b
(%)
(%)
(%) Ester (%) Oligomer (%)
c
1
2
3
4
5
Pd2(dba)3 + XANTPhos
Pd(OAc)2 + XANTPhos
(MeCN)4Pd(BF4)2 + XANTPhos
Pd(OAc)2 + 4 dppb + 300 psi H2
(MeCN)4Pd(BF4)2 + 2
Toluene 62
Toluene 41
Toluene 78
Toluene <5
Toluene <5
0
0
13
–
17
11
23
–
16
10
6
–
–
13
18
15
–
16
2
21
–
–
–
–
–
PCy3 + 300 psi H2
6
7
8
9
(COD)PdCl2 + 2 PPh3 + 4 SnCl2
(COD)PdCl2 + 2 PPh3 + 4 SnCl2
Pd2(dba)3 + DtBPX + 2 MSA
Pd2(dba)3 + DtBPX + 2 MSA
Toluene 27
5
2
–
0
3
3
–
3
8
–
16
1
–
0
22
–
MIBK
36
Toluene <5
Anisole 99.5d
0.5
4.5
0.1
0
a
b
c
◦
Reaction conditions: 1% catalyst loading, 10 mmol 4-penten-1-ol, 0.2 mmol TCE (GC Std), 15 mL solvent, 300 psi CO, 100 C, 3 h.
Conversion determined by gas chromatography of the remaining penten-1-ol isomers. Lactone and ester products determined by gas chromatography.
Oligomeric products determined by C NMR analysis. Gel permeation chromatography analysis of the oligomeric species indicated that they all had a
13
Mw < 600 as calibrated to polystyrene standards.
d
91.5% of the starting 4-penten-1-ol converted to valeraldehyde.
3
. Results and discussion
bulky bidentate phosphine-coordinated palladium catalyst
that is activated by the addition of a strong acid such as a
sulfonic acid to produce a palladium-hydride species, usu-
ally in an ether solvent [1–3,8–12,39–44,50–51]. The role
of the ether solvents seems to be very important in stabiliz-
ing the acidic species as the same reaction run in toluene
versus anisole resulted in very different results (entry 8 ver-
sus entry 9). However, the addition of acid to this catalyst
species resulted in a very active alkene isomerization cata-
lyst, isomerizing over 90% of the initial 4-penten-1-ol species
to valeraldehyde.
3
.1. Carbonylations of 4-penten-1-ol: comparison of
catalytic systems
A catalyst system consisting of Pd2(dba)3 + 2 XANTPhos
was found to produce ␧-caprolactone and oligocaprolactone
from 4-penten-1-ol in moderate yield and moderate selec-
tivity in comparison to the other cyclic lactone products
as shown in Eq. (1) (products of the Pd(II)-catalyzed car-
bonylation of 4-penten-1-ol). A comparison to other known
carbonylation systems is shown in Table 1.
(1)
Since the ␧-caprolactone, the ester, and oligocaprolactone
are all formed through alkoxycarbonylation with the terminal
regioselectivity, the desired regioselectivity for carbonylation
exceeds 70% for entries 1 and 2, Table 1. The Alper system,
which is useful for the cyclocarbonylation of 2-allylphenols,
gave very low yields with the present substrate (entries 4 and
3.2. The carbonylation of 4-penten-1-ol: variations of
the catalyst system andmechanistic considerations
Inordertofurtherprobethereactivityofthiscarbonylation
system, a series of control reactions were performed and are
summarized in Table 2.
5
). More importantly, the use of (Ph3P)2PdCl2 (produced in
situ from addition of (COD)PdCl2 + 2 PPh3) + 4 SnCl2 under
similar reaction also resulted in low yield and selectivity
In the Alper systems for the cyclocarbonylation of
allylphenol, the use of H2 was necessary, presumably to form
the starting palladium-hydride. In contrast, the Pd2(dba)3 + 2
XANTPhos system does not require the use of H2, although
its addition did significantly increase the substrate conversion
without affecting the regioselectivity (Table 2, entry 1 versus
entry 2).
(entry 6). Moreover, using the more coordinating solvent of
MIBK (methyl isobutyl ketone) reported in the patent did not
result in a significant improvement (entry 7).
The current catalysts for the methoxycarbonylation of
ethene to produce methyl propionate all utilize a sterically

1
88
J.K. Funk et al. / Journal of Molecular Catalysis A: Chemical 239 (2005) 185–192
Table 2
Variations in the catalyst systema
Entry
1
Catalyst system variation
Conversion (%)
62
(%)
(%)
(%)
Ester (%)
13
Oligomer (%)
16
None (4-penten-1-ol as substrate,
0
17
16
300 psi CO, no acid)
2
3
4
300 psi CO and 300 psi H2
cis-3-Penten-1-ol as substrate
cis-3-Penten-1-ol as substrate,
98
16
87
0
2
25
31
6
28
27
1
0
9
6
8
35
1
26
0.2 mmols MSA added
5
6
7
2 eq. MSA added to Pd catalyst
␧-Caprolactone as substrate
5 mmol 4-penten-1-ol and 5 mmol
99
<5
56
4
–
0
9
–
4
3
–
43
6
–
5
77
–
4
␧
-caprolactone as substrates
b
8
9
Ester as substrate
22
96
0
0
0
0
0
4
17
0
5
92
␧-Caprolactone as substrate, no Pd
catalyst, 0.2 mmol MSA added
␧-Caprolactone as substrate, no Pd
catalyst, 0.4 mmols SnCl2 added
1
0
98
0
0
10
0
90
a
Where not specified, reaction conditions same as in Table 1.
b
2.60 mmol starting material.
In comparison to the reactivity of the terminal 4-penten-
-ol, use of the internal isomer cis-3-penten-1-ol produced
five-memberedtwo-ethylbutyrolactoneisomercanbeformed
analogously.
1
a significantly lower amount of carbonylation products and
with a higher selectivity toward the five- and six-membered
lactone rings (entry 3). This suggests that the rate of isomer-
ization of the C C bond is slow compared to the carbonyla-
tion reaction. In order to promote the isomerization, an acid
was added to increase the concentration of palladium(II)-
hydride species. Acids are also frequently added to palladium
catalystsystemsaspromoterssuchasinthemethoxycarbony-
lationsystems[8–12]. Additionof2 eq. MSAtocis-3-penten-
The carbonylation of 4-penten-1-ol also results in the for-
mation of open-chain oligomeric products. Gel permeation
chromatography analysis of these species indicate that they
are low molecular weight oligomers with a Mw < 600 versus
polystyrene standards. These oligoesters are formed via com-
petitive intermolecular versus intramolecular nucleophilic
attack on the Pd-acyl species according to Scheme 4 [57]. In
the case of intermolecular nucleophilic attack, the resulting
ester species also contains an alkene functionality, this can
potentially coordinate to the Pd catalyst to be carbonylated,
producing higher molecular weight oligomeric species.
It is also conceivable that the oligomeric species could be
formed by ring opening polymerization of the initially syn-
thesized ␧-caprolactone (Scheme 5). Because palladium(II)
is an electrophilic species, the metal center may coordinate to
␧-caprolactone, initiatingcationicpolymerization(Scheme5,
path a) [58–65]. Alternatively, the electrophilic palladium(II)
species could interact with the penten-1-ol species to form
a palladium(II)-alkoxy species which are known to initiate
ring-opening polymerization (Scheme 5, path b) [66–67]. In
order to determine if palladium species present in the cat-
alytic system are able to initiate ring opening polymerization,
a series of control reactions starting with ␧-caprolactone were
run. In one reaction, ␧-caprolactone was heated in the pres-
ence of the catalyst system (Table 2, entry 6). This resulted
in no decrease in the concentration of ␧-caprolactone or the
formation of oligomeric species, suggesting that the catalyst
systemitselfwasnotabletoinitiatering-openingpolymeriza-
tion of the ␧-caprolactone (Scheme 5, path a). However, it is
also possible that the palladium catalyst served to activate ␧-
caprolactone toward nucleophilic attack by the penten-1-ol in
solution, resulting in ring opening polymerization (Scheme 5,
path b). To test this possibility, an equimolar amount of ␧-
1
-ol as a substrate did significantly increase its reactivity, but
still failed to produce the desired regioselectivity (entry 4).
In comparison, the addition of 2 eq. MSA to 4-penten-1-ol
resulted in increased conversion, predominantly to the oligo-
caprolactone (entry 5).
A reasonable mechanism for the Pd2(dba)3 + 2 XANT-
Phos carbonylation system is similar to that proposed by
Alper and co-workers for the cyclocarbonylation of 2-
allylphenols [34]. Shown in Scheme 2, the reaction starts
with the formation of a palladium(II)-hydride species via
oxidative addition of adventitious water or the ROH group.
This palladium-hydride species then coordinates and inserts
the vinyl functionality to form a palladium-alkyl species,
which is subsequently carbonylated to form a palladium(II)-
acyl species. This species then undergoes intramolecular
nucleophilic attack by the alcohol functionality, producing
␧
-caprolactone and regenerating the palladium(II)-hydride
species.
The six- and five-membered lactone ring products are
also formed by a similar mechanism as shown in Scheme 3.
Formation of the six-membered two-methylvalerolactone
isomer could result from either initial palladium-hydride
mediated isomerization of the terminal alkene or by initial
2,1-insertion of the alkene into the Pd-hydride bond. The

J.K. Funk et al. / Journal of Molecular Catalysis A: Chemical 239 (2005) 185–192
189
Scheme 2. Proposed mechanism for the cyclocarbonylation of 4-penten-1-ol to form ␧-caprolactone.
caprolactone and 4-penten-1-ol were heated in the presence
of the palladium catalyst under catalytic conditions (Table 2,
entry 7). If the formation of oligomeric species resulted from
ring opening polymerization of the ␧-caprolactone in solu-
tion, then this reaction should have resulted in a formation of
a greater amount of oligomeric species in comparison to the
standard reaction because of the greater initial concentration
of ␧-caprolactone. If, on the other hand, the ␧-caprolactone
is not susceptible to ring opening polymerization under these
conditions, then there should be no extra oligomers formed.
Indeed, no decrease in the quantity of ␧-caprolactone was
observed, suggesting that the oligomeric species are a result
of direct competitive intermolecular versus intramolecular
nucleophilic attack on the Pd-acyl species (Scheme 4).
Although the above results suggest that palladium species
present in the reaction mixture are not electrophilic enough
to activate the ␧-caprolactone towards ring opening poly-
merization, it is well known that Brønsted and strong
Lewis acids can initiate ring opening polymerization of
␧-caprolactone (Scheme 5, reaction c) [68–71]. To test
the possibility that ␧-caprolactone formed in situ might
be polymerized by the MSA or SnCl2 cocatalyst, ␧-
Scheme 3. Proposed mechanism of formation of isomeric lactones.

1
90
J.K. Funk et al. / Journal of Molecular Catalysis A: Chemical 239 (2005) 185–192
Scheme 4. Intermolecular vs. intramolecular nucleophilic attack for the formation of oligomeric species vs. ring-closed products.
mixtures were examined by 31P NMR spectroscopy. The 31
P
caprolactone was heated with either MSA or SnCl2 with-
out the palladium catalyst under reaction-like conditions
NMR spectrum of the product mixture from Run 2, Table 1
consisted of a dominant singlet at 8.5 ppm, and a doublet
of doublet (4.9 and 1 ppm, JP–P = 59 Hz) The former corre-
sponds to the trans coordination of the two phosphorus atoms
of XANTPhos. Furthermore, the chemical shift is consistent
(Table 2, entries 9 and 10). As expected, the presence of
these acids result in the ring-opening polymerization of ␧-
caprolactone.
Inlightoftheaboveresults, oneimportantadvantageofthe
Pd2(dba)3 + 2 XANTPhos system over several other reported
alkoxycarbonylation systems is that it does not require the use
of an acid cocatalyst, resulting in lower yields of oligomeric
products. Nevertheless, some oligomeric species are formed
via competitive intermolecular nucleophilic attack on the Pd-
acyl intermediate.
+
with that of a (Xantphos)Pd(acyl) species [57]—a likely
resting stage for the catalyst system (see Scheme 2). The
minor doublet of doublet corresponds to a palladium species
with cis coordination of the Xantphos ligand. When DtBPX
was used as the ligand, the species formed quantitatively
was (DtBPX)Pd(CO) [72]. The persistence and stability of
this species is consistent with the very poor catalytic activity
shown by this system.
Finally, in order to probe the nature of the palladium
species present in some of the systems described, the reaction
Scheme 5. Possible mechanisms of oligocaprolactone formation from ␧-caprolactone formed in situ.

J.K. Funk et al. / Journal of Molecular Catalysis A: Chemical 239 (2005) 185–192
191
4
. Conclusion
[27] A. Corma, L.T. Nemeth, M. Renz, S. Valencia, Nature 412 (2001)
23.
4
[
[
[
28] K. Neimann, R. Neumann, Org. Lett. 2 (2000) 2861.
29] M. Renz, B. Meunier, Eur. J. Org. Chem. (1999) 737.
30] E. Ohara, K.-I. Kawazumi, US Patent Application 2004/0087804
As a proposed new method for the synthesis of ␧-
caprolactone, we have demonstrated the viability of a cat-
alytic system consisting of Pd2(dba)3 + 2 XANTPhos for the
carbonylation of 4-penten-1-ol in toluene. The conversion
and selectivity observed using this catalyst is higher than that
observed with other reported systems with the regioselectiv-
ity for terminal carbonylation exceeding 70%. Since in the
present system C C bond isomerization is slower than car-
bonylation, the use of bulkier ligands that favor 1,2-alkene
insertion may further increase the product regioselectivity.
(2004).
[31] K. Tanaka, H. Ogawa, US Patent 6,472,540 (2002).
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