Novel rhenium(i) catalysts for the isomerization of propargylic alcohols into α,β-unsaturated carbonyl compounds: An unprecedented recyclable catalytic system in ionic liquids

Article abstract of DOI:10.1039/c1cc10768b

Carbonyl rhenium(i) complexes are efficient catalysts for the regioselective isomerization of terminal propargylic alcohols into α,β-unsaturated aldehydes or ketones which can be used as an unprecedented recyclable catalytic system (up to 10 consecutive runs) in the ionic liquid [BMIM][PF₆].

Full text of DOI:10.1039/c1cc10768b

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COMMUNICATION
Novel rhenium(I) catalysts for the isomerization of propargylic alcohols
into a,b-unsaturated carbonyl compounds: an unprecedented recyclable
catalytic system in ionic liquidsw
´
Joaquın Garcıa-Alvarez,* Josefina Dıez, Jose Gimeno and Christine M. Seifried
´ ´ ´
´
Received 9th February 2011, Accepted 14th April 2011
DOI: 10.1039/c1cc10768b
Carbonyl rhenium(I) complexes are efficient catalysts for the
regioselective isomerization of terminal propargylic alcohols into
a,b-unsaturated aldehydes or ketones which can be used as an
unprecedented recyclable catalytic system (up to 10 consecutive
runs) in the ionic liquid [BMIM][PF6].
Rhenium(I) catalyzed reactions have focused a growing
attention during the last few years,1 providing unique modes
of reactivity1–3 and disclosing performances which can be
competitive with efficient procedures attained by other transi-
tion metal catalysts. Despite that rhenium(I) and ruthenium(II)
are isoelectronic d6 species4 and feature parallel coordination
chemistry, they display a different organometallic chemistry.
Although these features may open up new efficient synthetic
methodologies, the number of rhenium(I) catalyzed reactions
still remains much more limited as compared with conven-
tional ruthenium(II) based catalysts.
Scheme 1 The Meyer–Schuster and Rupe rearrangements of terminal
propargylic alcohols.
catalysts is achieved and no addition of co-catalysts (TFA) is
needed,5a and (iii) the catalytic isomerization also proceeds
efficiently in ionic liquids as reaction media allowing for the
first time the catalyst recycling (Scheme 2).
The rhenium(I) complexes 3a–b9 have been readily prepared
by the treatment of [ReBr(CO)5] (2) with an equimolecular
amount of 1a–b, in refluxing THF, followed by bromine
extraction with AgSbF6.
Following our interest in ruthenium(II) catalyzed reactions
with atom-economy,5 we were interested to find out
whether rhenium(I) derivatives can improve the catalytic
performances. Recently, we have described the synthesis of
(Z6-arene)-ruthenium(II) complexes containing the hemi-
labile heterotridentate iminophosphorane-phosphane ligands
Ph2PCH2P{QNP(QS)(OR)2}Ph2.6 Encouraged by their catalytic
activity6 we set up the synthesis of analogous rhenium(I)
complexes. Herein, we report a new type of cationic rhenium(I)
complexes [Re(k3-P,N,S-Ph2PCH2P{QNP(QS)(OR)2}Ph2)-
(CO)3][SbF6] (3a,b) which are efficient catalysts for the regio
and stereoselective isomerisation of propargylic alcohols into
a,b-unsaturated aldehydes (Meyer–Schuster (M–S) rearrange-
ment) or ketones (Rupe rearrangement)7 (see Scheme 1). It is
important to note that: (i) these are the first rhenium catalysts
which are active in both M–S and Rupe rearrangements,8 (ii) a
higher catalytic efficiency than that observed for ruthenium(II)
Scheme 2 Coordination of the iminophosphorane-phosphane ligands
Ph2PCH2P{QNP(QS)(OR)2}Ph2 1a,b to the Re(I) precursor [ReBr(CO)5].
Conductance measurements for 3a,b in acetone show that
these complexes are 1 : 1 electrolytes (110–120 OÀ1 cm2 molÀ1).
Complexes 3a,b (93–99% yields) were fully characterized by
IR and 1H, 13C and 31P{1H} NMR spectroscopy (see ESIw). In
particular, the k3-P,N,S-coordination of ligands 1a,b to
rhenium is clearly reflected in the 31P{1H} NMR spectrum of
3a,b by: (i) an appreciable downfield shift of both the PPh2 and
Ph2PQN signals, and (ii) a slight high-field shifting of the
(RO)2PQS resonances (see ESIw for further data).10
Moreover, the formation of the cationic complexes 3a,b was
unambiguously confirmed by a single-crystal X-ray diffraction
study of complex 3a (see ESIw).
´
´
´
Departamento de Quımica Organica e Inorganica, Instituto
´
´
Universitario de Quımica Organometalica ‘‘Enrique Moles’’
(Unidad Asociada al CSIC), Facultad de Quı´mica, Universidad de
Oviedo, E-33071 Oviedo, Spain. E-mail: garciajoaquin@uniovi.es;
Fax: +34 985103446; Tel: +34 985102985
w Electronic supplementary information (ESI) available: For the
synthesis of compounds 3a–b, catalytic procedure and crystallographic
data for 3a. CCDC 806226. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c1cc10768b
The catalytic activity of complexes 3a,b was firstly evaluated
by using the isomerization of the commercially available
c
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Table 1 Isomerization of propargylic alcohols 4a–i into a,b-unsaturated
aldehydes 5a–i catalyzed by complex 3ba
Table 2 Isomerization of propargylic alcohols 6a–e into a,b-unsaturated
ketones 7a–e catalyzed by complex 3ba
Time/ GC yield [%],
isolated (%)
Entry R1
R2
Product min
Time/ GC yield [%],
isolated (%)
Entry Substrate
Product min
1
2
3
4
5
6
7
8
9
Ph
p-F(C6H4)
p-Cl(C6H4)
p-MeO(C6H4) p-MeO(C6H4) 5d
p-Me(C6H4) p-Me(C6H4) 5e
Ph
Naphthyl
p-Cl(C6H4)
Ph
p-F(C6H4)
p-Cl(C6H4)
5a
5b
5c
5
15
60
10
5
99(91)
97(94)
98(93)
99(94)
97(94)
96(92)
97(91)
99(85)
97(93)
1
2
7a
7b
75
97(93)
H
H
H
5f
5g
5h
5i
5
120
98(91)
90
30
15
p-MeO(C6H4) H
3
4
7c
7d
165
75
99(94)
94(91)
a
Reactions were performed under a N2 atmosphere in a CEM
Discovers S-Class microwave synthesizer at 80 1C through modera-
tion of the initial power (300 MW). 1 mmol of the corresponding
alkynol was used.
1,1-diphenyl-2-propyn-1-ol (4a) into 3,3-diphenylpropenal (5a)
as a model reaction (see Table 1). Thus, in a typical experiment,
the Re(I) precursor (5 mol% of Re) was added to a 1 M solution
of the propargylic alcohol 4a in THF, using as heating source
microwave irradiation at 80 1C, the course of the reaction being
monitored by gas chromatography.11 Both cationic complexes
were found to be active and selective catalysts in the isomeriza-
tion process providing the enal 5a as the unique reaction
product. The best result was obtained for complex 3b, which
led to the enal 5a in quantitative yield (99%) after only
5 minutes of reaction vs. 10 min of reaction for complex 3a.12
As observed for 1,1-diphenyl-2-propyn-1-ol (entry 1,
Table 1), complex 3b was also found to be an efficient catalyst
for the selective isomerisation of other tertiary (entries 2–5)
and secondary (entries 6–9) propargylic alcohols to the corres-
ponding enals (stereoselective E enals were obtained for the
latter).13 Influence of the electronic properties of the aryl
rings on the reaction rates was observed. Thus, alkynols
with electron-withdrawing groups showed less reactivity
(entries 2 and 3) as compared to the substrates with electron-
donating groups (entries 4 and 5).14 No transformation
was observed when internal propargylic alcohols such as
PhCRCC(OH)Ph2 or MeCRCCH2(OH) were used as
substrates.15 The catalytic activity of complex 3b was then
tested in the isomerisation of propargylic alcohols which
contain a C–H bond in the b-position with respect to the
alcohol group (6a–e). Gratifyingly, the process proceeds in a
different way, giving rise to the selective formation of
a,b-unsaturated methyl ketones (7a–e) in high yield via a formal
Rupe-type rearrangement of the alkynol (see Table 2). This
catalytic transformation can be also applied successfully to more
elaborated substrates such as the hormonal steroid mestranol
(6d, entry 4) and ethistherone (6e, entry 5). The corresponding
enones 7d,e which are important building blocks in the chemistry
of steroids have been obtained selectively in a pure form with
excellent yields (94–99%). As far as we are aware, complex 3b
represents the first example of a rhenium catalyst active in the
Rupe-type rearrangement of propargylic alcohols.
5
7e
275
99(91)
a
Reactions were performed under a N2 atmosphere in a CEM
Discovers S-Class microwave synthesizer at 80 1C through modera-
tion of the initial power (300 MW). 1 mmol of the corresponding
alkynol was used.
Given that, as far as we know, no catalyst recovery has been
reported to date for MS and Rupe rearrangements we decided
to explore the recycling of catalyst 3b by using ionic liquids
(IL) as solvent.16 The reaction rate was found to be dependent
on the IL nature, the highest rate being attained by using
[BMIM][PF6] (BMIM = 1-butyl-3-methylimidazolium) as
solvent.17 It is important to note that the reaction is selective
since no side products could be detected by means of GC or
NMR spectra.18 Under the optimized reaction conditions (see
Table 3),19 we have found that the catalytic system remains
active (97–99% yield) after recycling up to 10 times. As
expected, a gradual decreasing of the activity is found after
each recycling, 10–40 min for the first four cycles (entries 1–4)
while two hours of heating is required after the fifth cycle.
Table 3 Re(I)-catalyzed (3b) Meyer–Schuster isomerization of 4a into
5a using [BMIM][PF6] as solventa
Cycle
t/min
Yieldb
TON
Cycle
t/h
Yieldb
TONc
1
2
3
4
5
10
20
30
40
99
99
99
99
99
20
40
60
80
99
6
7
8
9
10
2
3.3
6.5
8
97
97
99
99
99
118
138
158
178
200
120
10
a
Reactions were performed under a N2 atmosphere at 80 1C using
1 mmol of the alkynol 4a in 1 g of [BMIM][PF6] with 5 mol% of
b
catalyst 3b. Determined by GC. Cumulative TON values (turnover
c
number = (mol product/mol Re)).
c
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´
ez, J. Garcıa-Alvarez and J. Gimeno, Organometallics, 2004,
´ ´
The catalytic activity of 3b using [BMIM][PF6] as solvent was
then tested in the Rupe-type rearrangement of 6b (1-ethynyl-
cyclohexanol) into the a,b-unsaturated methyl ketone 7b. A
higher temperature (130 1C) is required with respect to the
Meyer–Schuster rearrangement (80 1C) to attain quantitative
transformations. Moreover, the catalyst 3b could only be
recycled up to 4 times [1st cycle: 1.5 h (99%); 2nd cycle
3.5 h (99%); 3rd cycle 5 h (98%); 4th cycle 20 h (97%)], with
reaction times between 90 minutes to 20 hours.
J. Dı
23, 3425.
7 (a) K. H. Meyer and K. Schuster, Ber. Dtsch. Chem. Ges., 1922, 55,
819; (b) H. Rupe and E. Kambli, Helv. Chim. Acta, 1926, 9, 672.
8 Although [BunN][ReO4] as well as oxo-rhenium and other oxo-
complexes have also been used as catalysts in M–S rearrangements,
no applications in Rupe isomerizations have been described to
date. (a) K. Narasaka, H. Kusama and Y. Hayashi, Chem. Lett.,
1991, 1413; (b) C. Y. Lorber and J. A. Osborn, Tetrahedron Lett.,
1996, 37, 853; (c) B. D. Sherry, A. T. Radosevich and F. D. Toste,
J. Am. Chem. Soc., 2003, 125, 6076; (d) M. R. Luzun and
F. D. Toste, J. Am. Chem. Soc., 2003, 125, 15760;
(e) B. M. Trost and C. K. Chung, J. Am. Chem. Soc., 2006, 128,
10358; (f) M. Stefanoni, M. Luparia, A. Porta, G. Zanoni and
G. Vidari, Chem.–Eur. J., 2009, 15, 3940; (g) K. A. Nolin,
R. W. Ahn, Y. Kobayashi, J. Kennedy-Smith and F. D. Toste,
Chem.–Eur. J., 2010, 16, 9555; (h) K. Saito, Y. Onizawa and
N. Iwasawa, Chem.–Eur. J., 2010, 16, 4716.
9 (a) A. Hagenbach, S. Athenstaedt, H. E. Daroczi, U. Abram and
R. Alberto, Z. Anorg. Allg. Chem., 2004, 630, 2709; (b) M. Hecht,
S. Saucedo Anaya, A. Hagenbach and U. Abram, Inorg. Chem.,
2005, 44, 3172.
10 These data are in accordance with those reported previously in the
k3-P,N,S-coordination of 1a,b to Ru(II) fragments see ref. 6.
11 A parameter study of the catalytic activity of complex 3b with
substrate 4a using different solvents, heating sources and acid
co-catalyst is shown in Table S1 in ESIw.
In summary, in this work, we have shown that the cationic
Re(I) complex [Re(k3-P,N,S-Ph2PCH2P{QNP(QS)(OPh)2}Ph2)-
(CO)3][SbF6] (3b) is a highly efficient catalyst for the isomer-
ization of terminal propargylic alcohols into a,b-unsaturated
carbonyl compounds, which can be obtained in excellent yield
and in a time-scale reaction of minutes. This catalyst has also
proven to promote chemoselective transformations producing
either enals (Meyer–Schuster rearrangement) or enones (Rupe
rearrangement) depending on the nature of the propargylic
alcohol. As far as we are aware, complex 3b is the first example
of a rhenium catalyst active in the Rupe-type rearrangement of
propargylic alcohols. Moreover, 3b is also an efficient catalyst
in the ionic liquid [BMIM][PF6]. The use of this environ-
mentally friendly solvent allows the catalyst recovery disclosing
an unprecedented appealing synthetic approach for the chemo-
selective isomerisation of propargylic alcohols with practical
utility.20 Mechanistic studies and further synthetic applications of
this catalytic system as an appealing alternative to the widely used
ruthenium(II) catalysts are presently underway.
12 Significantly, isomerisation of the alkynol 4a to the corresponding
enal 5a with the rhenium(V)-catalyst [ReOCl3(OPPh3)(SMe2)] in a
5 mol% loading at 80 1C requires (20 h) (see ref. 8f). This
isomerisation with our previously reported catalytic systems
Ru(II)/CF3CO2H requires 1.5 h (see ref. 5a).
13 This total E-stereoselectivity was also previously observed for the
rhenium(V)-catalyst [ReOCl3(OPPh3)(SMe2)] (ref. 8f).
14 The limitation of this methodology concerns to the use of primarily
propargylic alcohols as the propargylic alcohol (HCRCCH2OH)
give rise to a polymer material.
We are indebted to the MICIIN of Spain (Projects CTQ2008-
00506), Consolider Ingenio 2010 (CSD2007-00006) for financial
support. J.G.-A. thanks the MEC and the European Social
Fund for the award of a ‘‘Juan de la Cierva’’ contract.
15 This fact appears to be in accord with the required formation of
intermediate hydroxyvinylidene species ([Re]+QCQC(H)C(OH)R2),
only accessible via tautomerisation of terminal alkynols:
(a) D. A. Engel and G. B. Dudley, Org. Biomol. Chem., 2009, 7,
4149; (b) V. Cadierno, P. Crochet, S. E. Garcıa-Garrido and
´
J. Gimeno, Dalton Trans., 2010, 39, 4015; (c) Metal Vinylidenes and
Allenylidenes in Catalysis: From Reactivity to Applications in Synthesis,
ed. C. Bruneau and P. H. Dixneuf, Wiley-VCH, Weinheim, 2008.
16 A large number of organometallic catalysts have been successfully
applied to a variety of organic transformations using ionic liquids
(IL) as environmentally friendly solvents. (a) H. Olivier-Bourbigou
and L. Magna, J. Mol. Catal. A: Chem., 2002, 182–183, 419;
(b) P. Wasserscheid, in Transition Metal Catalysis in Ionic Liquids,
ed. P. Wasserscheid and W. Keim, Wiley-VCH, Weinheim,
Germany, 2003, p. 213; (c) N. Jain, A. Kumar, S. Chauhan and
S. M. S. Chauhan, Tetrahedron, 2005, 61, 1015; (d) S. Liu and
J. Xiao, J. Mol. Catal. A: Chem., 2007, 207, 1; (e) W. L. Wong,
K.-C. Cheung, P.-H. Chan, Z.-Y. Zhou, K.-H. Lee and
K.-Y. Wong, Chem. Commun., 2007, 2175; (f) R. Sarma and
D. Prajapati, Synlett, 2008, 3001; (g) Y. Liu, S.-S. Wang,
W. Liu, Q.-X. Wan and G.-H. Gao, Curr. Org. Chem., 2009, 13,
1322; (h) J. W. Lee, J. Y. Shin, Y. S. Chun, H. B. Jang, C. E. Song
and S.-G. Lee, Acc. Chem. Res., 2010, 43, 985.
Notes and references
1 For a review on C–H and C–C activation see: Y. Horino, Angew.
Chem., Int. Ed., 2007, 46, 2144.
2 For leading references on catalytic activity of carbonyl rhenium(I)
compounds see: (a) R. Umeda, K. Kaiba, T. Tanaka,
Y. Takahashi, T. Nishimura and Y. Nishiyama, Synlett, 2010,
3089 and references therein; (b) N. Chatani, K. Kataoka and
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H. Yamabe, Y. Onizawa, T. Hoshino and N. Iwasawa, Angew.
Chem., Int. Ed., 2005, 44, 468; (d) H. Chen and J. F. Hartwig,
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Y. Tokunaga, A. Kawata and K. Takai, J. Am. Chem. Soc.,
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K. Takai, J. Am. Chem. Soc., 2008, 130, 14062; (g) Y. Kuninobu,
T. Matsuki and K. Takai, Org. Lett., 2010, 12, 2948 and references
therein.
3 A different chemoselectivity has been observed when using
rhenium vs. ruthenium catalyzed reactions; see for instance
ref. 2c,e,f.
4 Advanced Inorganic Chemistry, ed. F. A. Cotton, G. Wilkinson,
C. A. Murillo and M. Bochmann, Wiley Interscience, 6th edn, 1999.
´
5 For recent reports see: (a) V. Cadierno, S. E. Garcıa-Garrido and
17 We have investigated the M–S rearrangement of the alkynol 4a
using two different ionic liquids, such as [BMIM][PF6] (BMIM =
1-butyl-3-methylimidazolium) and [BMIM][BF4] as solvents (see
Table S2 in ESIw for details).
18 The presence of catalytic amounts (5 mol%) of complex 3b was
found to be essential for the reaction outcome. Note that in the
absence of the catalyst the reaction did not generate any product
(see Table S2 in ESIw).
19 In our case, the use of MW as heating source slowed down the
reaction; as an example, using 5 mol% of complex 3b under a
N2 atmosphere at 80 1C with 300 MW, only 35% conversion
of 1,1-diphenyl-2-propyn-1-ol into 3,3-diphenylpropenal in
[BMIM][PF6] was achieved after 1 hour.
J. Gimeno, Adv. Synth. Catal., 2006, 348, 101; (b) V. Cadierno,
J. Gimeno and N. Nebra, ChemCatChem, 2010, 2, 519;
(c) V. Cadierno, S. E. Garcı
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Inorg. Chim. Acta, 2010, 363, 1912; (d) M. A. Ferna
B. Lastra-Barreira, M. Scheele, J. Dı
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6 (a) V. Cadierno, P. Crochet, J. Dı
S. E. Garcı
M. A. Rodrıguez, Dalton Trans., 2003, 3240; (b) V. Cadierno,
ez, J. Garcıa-Alvarez,
´ ´
´
a-Garrido, J. Gimeno, S. Garcı
´
a-Granda and
20 The oxo-rhenium(V) catalyst [ReOCl3(OPPh3)(SMe2)] remains
active only for two cycles requiring 20 h, see ref. 8f.
´
c
6472 Chem. Commun., 2011, 47, 6470–6472
This journal is The Royal Society of Chemistry 2011