Ring-opening of epoxides promoted by organomolybdenum complexes of the type [(η5-C5H4R)Mo(CO)2(η3-C3H5)] and [(η5-C5H5)Mo(CO)3(CH2R)]

Article abstract of DOI:10.1016/j.jorganchem.2015.09.022

The cyclopentadienyl molybdenum carbonyl complexes [(η⁵-C₅H₄R)Mo(CO)₂(η³-C₃H₅)] and [(η⁵-C₅H₅)Mo(CO)₃(CH₂R)] (R = H, COOH) have been shown to promote acid-catalysed reactions in liquid phase, under moderate conditions. The catalytic alcoholysis of styrene oxide with ethanol at 35 °C gave 2-ethoxy-2-phenylethanol in 100% yield within 30 min for the dicarbonyl complexes and 3-6 h for the tricarbonyl complexes. Steady catalytic performances were observed in consecutive runs with the same catalytic solution, suggesting fairly good catalytic stability. In the second acid-catalysed reaction studied, the isomerization of α-pinene oxide at 55 °C gave campholenic aldehyde and trans-carveol in a total yield of up to 86% at 100% conversion. Chemoselectivity is shown to be solvent dependent.

Full text of DOI:10.1016/j.jorganchem.2015.09.022

Journal of Organometallic Chemistry 799-800 (2015) 179e183
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Ring-opening of epoxides promoted by organomolybdenum
5
3
5
complexes of the type [(
Mo(CO) (CH R)]
h
-C H R)Mo(CO) (h -C H )] and [(h -C H )
5 4 2 3 5 5 5
3
2
a
a
b
a, **
Sofia M. Bruno , Ana C. Gomes , Marta Abrantes , Anabela A. Valente
,
a
a, *
Martyn Pillinger , Isabel S. Gonçalves
Department of Chemistry, CICECO e Aveiro Institute of Materials, University of Aveiro, Campus Universit aꢀ rio de Santiago, 3810-193 Aveiro, Portugal
Centro de Química Estrutural, Complexo Interdisciplinar, Instituto Superior T ꢀe cnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal
a
b
a r t i c l e i n f o
a b s t r a c t
The cyclopentadienyl molybdenum carbonyl complexes [( 5
Mo(CO) (CH
h
3
5
Article history:
Received 15 May 2015
Received in revised form
5 4 2 3 5 5 5
-C H R)Mo(CO) (h -C H )] and [(h -C H )
3
2
R)] (R ¼ H, COOH) have been shown to promote acid-catalysed reactions in liquid phase,
ꢀ
under moderate conditions. The catalytic alcoholysis of styrene oxide with ethanol at 35 C gave 2-
ethoxy-2-phenylethanol in 100% yield within 30 min for the dicarbonyl complexes and 3e6 h for the
tricarbonyl complexes. Steady catalytic performances were observed in consecutive runs with the same
8
September 2015
Accepted 19 September 2015
Available online 28 September 2015
catalytic solution, suggesting fairly good catalytic stability. In the second acid-catalysed reaction studied,
-pinene oxide at 55 C gave campholenic aldehyde and trans-carveol in a total yield
of up to 86% at 100% conversion. Chemoselectivity is shown to be solvent dependent.
ꢀ
the isomerization of
a
Keywords:
Molybdenum carbonyl complexes
Acid catalysis
©
2015 Elsevier B.V. All rights reserved.
Epoxides
Ring-opening reactions
Isomerization
Alcoholysis
1
. Introduction
due to the high product selectivities often reachable together with
straightforward catalyst optimization through ligand exchange
and/or functionalization [7].
Epoxides are important organic intermediates, commonly syn-
thesized from olefins, with patented technologies including the use
of homogeneous Mo catalysts, such as those derived from mo-
Several group 6 (Mo, W) metal carbonyl complexes are OLAs [8]
and have been used in a series of acid-catalysed reactions [9e15].
Molybdenum-based catalysts are especially attractive due to the
ready availability and low cost of this element. We recently started
to explore organomolybdenum complexes for the catalytic isom-
VI
lybdenum carbonyl catalyst precursors [1e3]. Epoxides can un-
dergo ring-opening reactions in the presence of acid catalysts to
give a plethora of useful intermediates and products for the fine
chemicals industry [4e6]. Commonly used Lewis acid catalysts
such as metal halide salts present numerous drawbacks such as
corrosion problems, fast deactivation, the production of large vol-
umes of acidic waste, and the need for high catalyst/substrate ra-
tios. Hence, there is much interest in the development of novel
reaction processes catalysed by highly active and highly selective
Lewis acids that significantly reduce acidic wastes. The use of
organometallic Lewis acids (OLAs) is promising since they are
playing an increasingly important role in modern organic synthesis
erization of
This is an important reaction from two perspectives. Firstly, PinOx
is a renewable chemical obtained from -pinene, which is produced
a-pinene oxide (PinOx) to campholenic aldehyde (CPA).
a
on an industrial scale either by tapping trees (gum turpentine) or as
a byproduct of paper pulping. Secondly, CPA is useful as an aroma
chemical and as a synthetic intermediate for other aroma chemicals
and also for some pharmaceuticals. In particular, it is used for the
manufacture of sandalwood-like fragrances [16]. The isomerization
of PinOx to CPA tends to be promoted by Lewis acid catalysts, while
Brønsted acids give a mixture of compounds in low yields, such as
trans-carveol (TCV), trans-sobrerol, and p-cymene. Apart from be-
ing expensive ingredients in the flavour industry (e.g., TCV is a
constituent of the Valencia orange essence oil), these compounds
*
Corresponding author.
** Corresponding author
E-mail addresses: atav@ua.pt (A.A. Valente), igoncalves@ua.pt (I.S. Gonçalves).
http://dx.doi.org/10.1016/j.jorganchem.2015.09.022
022-328X/© 2015 Elsevier B.V. All rights reserved.
0

180
S.M. Bruno et al. / Journal of Organometallic Chemistry 799-800 (2015) 179e183
have attracted interest due to their biological activities [17e19]. We
obtained encouraging results for the isomerization of PinOx to CPA
previously by Ascenso et al. [29], involving the reaction of [(
h
3-
COOH)
)] (2) was obtained from the reaction of 1 with n-
BuLi and solid CO , followed by aqueous work-up [30,31]. The tri-
carbonyl complex [CpMo(CO) CH ] (3) was synthesized by the re-
action of [CpMo(CO) ]Na with CH I [32]. The reaction of NaCp with
Mo(CO) , followed by addition of 2-chloroacetamide, gave the
complex [CpMo(CO) CH COONH ], which was subsequently
hydrolysed to give [CpMo(CO) CH COOH] (4) [33].
5
3 5 2 3 2 5 4
C H )Mo(CO) (NCCH ) Cl] with LiCp. The acid [(h -C H
5
3
in the presence of the indenyl allyl dicarbonyl derivative [(
h
-Ind)
2 3 5
Mo(CO) (h -C H
3
Mo(CO)
2
(h
-C
3
H
5
)] [20]: 56% CPA yield at 97% PinOx conversion, 3 h
2
ꢀ
reaction at 35 C. In a second investigation [21], even better per-
3
3
5
formance was obtained using the dimeric complex [{(
Mo(CO) -Cl)} ]: 67% CPA yield at 100% PinOx conversion, 30 min
reaction at 35 C. The latter result is roughly comparable to those
reported for the salts ZnCl and ZnBr [22], used as catalysts in
h
-Ind)
3
3
2
(m
2
ꢀ
6
3
2
2
2
2
3
2
patented technology for the production of CPA [23,24].
Another chemical route for the valorization of epoxides involves
ring-opening via alcoholysis to give b-alkoxy alcohol products [25],
2.3. Catalytic tests
which are important precursors for a broad range of pharmaceu-
ticals [26]. These reactions can be promoted by Lewis acid catalysts
The catalytic reactions were carried out under air in a magnet-
ically stirred, closed borosilicate reaction vessel (10 mL capacity)
[
27,28], for which molybdenum carbonyl complexes are interesting
candidates.
In this work, we set out to explore the catalytic potential of
cyclopentadienyl molybdenum carbonyl complexes of the type
ꢀ
which was immersed in an oil bath thermostatted at 35 or 55 C.
Typically, the reactor was loaded with molybdenum complex,
substrate (StyOx or PinOx) and a solvent. For the StyOx alcoholysis
reaction, the reactor was loaded with molybdenum complex
5
3
5
[
5 4 2 3 5 5 5 3 2
(h -C H R)Mo(CO) (h -C H )] and [(h -C H )Mo(CO) (CH R)]
(
80
isomerization reaction, the reactor was loaded with molybdenum
complex (17 mol), PinOx (170 mol), and a solvent (0.5 mL; DCE,
mmol), StyOx (0.82 mmol), and ethanol (2.0 mL). For the PinOx
(
R ¼ H or COOH) for acid-catalysed ring-opening reactions of ep-
oxides, namely the isomerization of PinOx, and alcoholysis of sty-
rene oxide (StyOx) with ethanol, under mild reaction conditions
m
m
toluene or ethanol). The reactor containing the complex and the
solvent was pre-heated for 10 min at the reaction temperature, and
subsequently the substrate (also pre-heated) was added to the
mixture (the reaction time was counted from this instant). The
evolutions of the reactions were monitored using a Varian 3800 GC
equipped with a BR-5 (Bruker) capillary column (30 m ꢃ 0.25 mm;
(Fig. 1). To the best of our knowledge, this is the first report on the
catalytic performance of metal carbonyl complexes in the alco-
holysis of StyOx.
2
. Experimental
0
2
.25 mm) and a flame ionization detector, using H as the carrier gas.
2
.1. Materials and methods
Cyclododecane epoxide was used as an internal standard. The
products were identified by GCeMS (Trace GC 2000 Series (Thermo
Quest CE Instruments) e DSQ II (Thermo Scientific)), equipped with
Transmission FT-IR spectra (KBr pellets) were measured on a
ꢁ
1
Mattson 7000 FT-IR spectrometer with 4 cm resolution. Styrene
oxide (StyOx, > 97%, Fluka), -pinene oxide (PinOx, > 95%, TCI), 1,2-
a capillary DB-5 type column (30 m ꢃ 0.25 mm; 0.25
mm), using He
a
as the carrier gas.
dichloroethane (DCE, 99%, Aldrich), toluene (ꢂ99.9%, Aldrich) and
absolute ethanol (EtOH, 99.8%, Scharlau) were purchased from
commercial sources and used as received.
3
. Results and discussion
.1. Styrene oxide ring-opening reaction via alcoholysis
Complexes 1e4 were explored for the acid-catalysed ring-
3
2.2. Synthesis of molybdenum carbonyl complexes 1e4
The
p
-allyl complex [CpMo(CO)
2
(
h
3-C
3
H
5
)] (1) (Cp ¼
h
5-C
5
H
5
)
opening reaction of StyOx with ethanol (also used as solvent) in the
temperature range of 35e55 C. All four complexes are completely
ꢀ
was synthesized according to the general procedure described
Fig. 1. Acid-catalysed epoxide ring-opening reactions studied in this work.

S.M. Bruno et al. / Journal of Organometallic Chemistry 799-800 (2015) 179e183
181
soluble under the conditions used. In separate tests carried out in
species involved, which was possibly not considerably affected by
the presence of the carboxylic acid group on one of the ligands.
the absence of substrate, solutions of complexes 1e4 in ethanol
ꢀ
were left at 35 or 55 C for 3 h, and then the solutions were
evaporated to dryness under reduced pressure. The FT-IR spectra
3.2. a-Pinene oxide ring-opening reaction via isomerization
(
not shown) of the resultant solids were unchanged from those of
ꢀ
the as-synthesized complexes, suggesting that the complexes are
stable under the catalytic reaction conditions used. The only reac-
tion product obtained in the alcoholysis reaction was 2-ethoxy-2-
phenylethanol, i.e. selectivity was always 100% (Fig. 1, Table 1).
Complexes 1 and 2 led to a very fast alcoholysis reaction, with
The reaction of PinOx in the presence of complexes 1e4, at 55 C
with DCE as solvent, gave mainly CPA, together with TCV, trans-
pinocarveol (PCV), and iso-pinocamphone (IPC) (Fig. 1, Table 2). As
found for ethanol, separate solubility tests performed for DCE in the
absence of substrate confirmed that the complexes are stable in this
ꢀ
ꢀ
ꢀ
1
00% conversion reached at 10 min/55 C or 30 min/35 C. Com-
solvent at 55 C (exemplified in Fig. 2 for complexes 3 and 4). The
plexes 3 and 4 led to a slower alcoholysis reaction, with 100%
conversion being reached at 2 h/55 C or 6 h/35 C for 3, and
best catalytic results in terms of CPA and TCV total yield at 24 h
were obtained for complex 4 (51% CPA yield and 21% TCV yield at
100% conversion). These results are superior to the total yields of
CPA and TCV reported previously by some of us for indenyl com-
ꢀ
ꢀ
ꢀ
ꢀ
9
0 min/55 C or 3 h/35 C for 4. Without catalyst, the reaction was
ꢀ
sluggish (3% StyOx conversion at 4 h/55 C), which emphasizes the
catalytic role of the molybdenum complexes. To the best of our
knowledge, there are no reports on the catalytic performance of
metal carbonyl complexes in the alcoholysis of StyOx. According to
the literature, the Lewis acid catalysed alcoholysis of StyOx may
involve the coordination of the oxygen atom of the epoxide to the
transition metal centre by an acidebase interaction, leading to an
increase in the electrophilicity of the carbon atom attached to the
phenyl group (polarisation of the CeO bond of the oxirane ring),
making it susceptible to nucleophilic attack by the alcohol (ROH) to
give the product [34,35].
5
3
plexes of the type [(
Si(CH ) [20], and somewhat inferior to those obtained with [{(
Ind)Mo(CO) -Cl)} ] [21] (Table 2). The observed differences in
h
-IndR)Mo(CO)
2
(
h
-C
3
H
5
)] (R ¼ H, CH
3
,
h -
5
3
)
3
2
(
m
2
catalytic results may be partly due to differences in catalyst solu-
bility, stability and acidity of active species.
As mentioned in the introduction, the isomerization of PinOx to
CPA tends to be promoted by Lewis acid catalysts, while Brønsted
acidity can favour TCV [36,37]. Hence, the catalytic results for 1e4
suggest that the active species are of Lewis acid type. Complex 2 led
to slower reaction than 1 and, on the other hand, complex 4 led to
faster reaction than 3. Thus, for each pair of complexes no direct
relationship between the Brønsted acidity associated with the
carboxylic acid groups and catalytic activity can be established.
The reaction of PinOx in the presence of complex 2 was further
examined using toluene or ethanol as solvent instead of DCE
(Table 2). The reaction rate decreased in the order
ethanol > toluene > DCE. On the other hand, the total yield of CPA
and TCV at 100% conversion was higher for toluene than for
ethanol. Unidentified reaction products were formed in ethanol,
which were not detected for the other solvents. Possibly, ethanol
acts as a non-innocent solvent (as is the case for the alcoholysis of
StyOx), broadening the product spectrum. A comparison of the
catalytic results for DCE and toluene as solvents shows that the
molar ratio TCV/CPA at 55e62% conversion is higher for the latter
(ca. 2.8 compared with 0.3 for DCE). The DCE solvent molecules
The catalytic stability was investigated by re-loading the sub-
strate to the same reactor where the catalyst was maintained. These
ꢀ
experiments were carried out at 35 C for complexes 1 and 2, and at
ꢀ
5
5 C for (less active) 3 and 4. For all complexes, at least 95% StyOx
conversion was reached, and 2-ethoxy-2-phenylethanol selectivity
was always 100% for four runs (Table 1). In general, an increase in
initial reaction rate was observed from run 1 to run 2, and for each
complex in runs 2e4 the activity was somewhat steady. Hence, the
increasing concentration of 2-ethoxy-2-phenylethanol in the re-
action medium did not have a negative effect on the kinetics of
consecutive runs. The catalytic results were roughly comparable for
each pair of complexes 1 and 2, or 3 and 4, and thus the carboxylic
acid groups of complexes 2 and 4 do not seem to exert a significant
influence on the alcoholysis reaction. The catalytic activity may be
associated with the Lewis acidity of the metal centre of the active
(
dielectric constant ¼ 10.36; dipole moment ¼ 1.75 D [38]) may
unfavourably interact with the carboxylic acid function of the
catalyst (e.g. via H-bonding), affecting TCV formation. These effects
may be less important for the much less polar solvent toluene
Table 1
a
Alcoholysis of styrene oxide with ethanol in the presence of complexes 1e4.
ꢀ
Conv. (%)b
Complex
Run
Time (min)
Temp. ( C)
(
dielectric constant ¼ 2.38; dipole moment ¼ 0.36 D [38]).
1
1
1
2
3
4
1
1
2
3
4
1
2
3
4
1
1
2
3
4
1
1/10
1/30
1/30
1/30
1/30
1/10
1/30
1/30
1/30
1/30
1/30/60/120
1/30
1/30
55
35
35
35
35
55
35
35
35
35
55
55
55
55
35
55
55
55
55
35
67/100
65/99
77/99
72/98
71/98
71/99
60/98
83/99
76/98
75/95
7/21/58/100
78/99
77/99
83/97
1/15/99
12/46/93/100
74/100
79/99
77/98
3/64/100
Attempts were made to identify metal species formed during
the catalytic reaction. This was accomplished for complex 4 by
adding pentane to the reaction solution after 3 h and 24 h (in
separate experiments), giving the solids denoted 4-DCE(cat-3h)
and 4-DCE(cat-24h), respectively. The FT-IR spectra of 4-DCE(cat-
2
3
4
3
h) and 4-DCE(cat-24h) were similar to that of 4, suggesting that
the predominant metal species were of the type 4 (Fig. 2). Never-
theless, it is worth mentioning that the colour of the catalytic re-
action solution changed slightly from yellow (initially) to greenish-
yellow after 24 h. Thus, one cannot rule out the possibility that
different active species may be formed from 4. It has been previ-
ously reported that molybdenum(II) carbonyl complexes can un-
dergo decarbonylation in the presence of air/water giving different
species [21]. Very small amounts of air and/or water may be dis-
solved in the reagents/solvent, and influence the stability of the
molybdenum carbonyl complexes.
1/30
1/60/360
1/30/60/90
1/30
1/30
1/30
1/60/180
a
Reaction conditions: 80
mmol of Mo, 0.82 mmol StyOx, 2.0 mL ethanol,
4
. Concluding remarks
[
Mo] ¼ 0.038 M.
b
StyOx conversion. The only product was 2-ethoxy-2-phenylethanol (100%
selectivity).
In this work we have shown that complexes of the type [(h
5-

182
S.M. Bruno et al. / Journal of Organometallic Chemistry 799-800 (2015) 179e183
Table 2
Acid-catalysed conversion of
a
-pinene oxide, in the presence of complexes 1e4, and comparison with literature data for molybdenum carbonyl complexes.a
Catalyst
Solvent
Time (h)
Conv.b (%)
Product yield (%)
Ref.
CPA
TCV
PCV
IPC
1
2
DCE
DCE
Ethanol
Toluene
DCE
DCE
DCE
DCE
DCE
24
24
0.017
6/24
24
99
62
37
27
21
9/27
18
51
68
40
38
33
14
8
37
25/59
4
21
9
17
13
12
11
11
4
1/7
5
10
3
5
4
2
3
1/3
6
1
8
7
7
This work
This work
This work
This work
This work
This work
[21]
[20]
[20]
[20]
100
55/100
50
100
100
92
3
4
24
[
[
[
[
{(
h
5-Ind)Mo(CO)
2
(
h
m
-Cl)}
2
]
0.017
2.5
1
5
3
(
(
(
h
-Ind)Mo(CO)
2
(
-C
3
H
-C
5
)]
5
3
h
-IndCH
3
)Mo(CO)
2
(
h
3
H
-C
5
)]
66
61
4
2
5
3
h
-IndSi(CH
3
)
3
)Mo(CO)
2
(
h
3
H
5
)]
DCE
1
4
a
mol PinOx, 55 ꢀC, 0.5 mL solvent, [Mo] ¼ 0.032 M.
Reaction conditions: 17
Conversion of PinOx.
m
mol Mo, 170 m
b
project FCOMP-01-0124-FEDER-029779 (FCT ref. PTDC/QEQ-SUP/
906/2012, including the research grant with ref. BPD/UI89/4864/
013 to A.C.G.). The FCT and the European Union are acknowledged
1
2
for a post-doctoral grant to S.M.B. (ref. SFRH/BPD/46473/2008) co-
funded by MCTES and the European Social Fund through the pro-
gram POPH of QREN.
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[
[
8
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