Rhodium(III)-catalysed decarbonylative coupling of maleic anhydrides with alkynes

Article abstract of DOI:10.1039/c4ra06452f

A formal [5 - 1 + 2] annulation for the preparation of substituted α-pyrones is reported. The reaction involves the decarbonylative coupling of substituted maleic anhydrides with internal alkynes in the presence of a rhodium(III) catalyst and a copper(II)

Full text of DOI:10.1039/c4ra06452f

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Rhodium(III)-catalysed decarbonylative coupling of
maleic anhydrides with alkynes†
Cite this: RSC Adv., 2014, 4, 37138
*
Takanori Matsuda and Kentaro Suzuki
Received 30th June 2014
Accepted 5th August 2014
DOI: 10.1039/c4ra06452f
www.rsc.org/advances
A formal [5 ꢀ 1 + 2] annulation for the preparation of substituted rhodium(III) catalyst is the most effective for this reaction
a-pyrones is reported. The reaction involves the decarbonylative (entry 4). A silver salt, Ag2CO3, also worked as the oxidant, albeit
coupling of substituted maleic anhydrides with internal alkynes in the with a lower yield (entry 5). t-Amyl alcohol was found to be the
presence of a rhodium(III) catalyst and a copper(II) salt, affording tri- best solvent. When the reaction was performed in DMF, 3a was
and tetrasubstituted a-pyrones.
obtained in 69% yield (entry 6). The reaction was unsuccessful in
tolueneꢁ(entry 7). The optimal reaction temperature was found to
be 120 C (entry 8). The reaction was considerably less efficient
when it was performed with a substoichiometric amount of
Cu(OAc)2 under an oxygen atmosphere (entry 9).
Then, the scope of alkynes 1 in this reaction with 2a under
the optimal reaction conditions was investigated (Table 2). The
reactions of 2a with diaryl alkynes 1b–e reached completion
within 5 h, affording the corresponding triaryl pyrones 3b–e in
good yields, whereas the reaction with di(2-thienyl)acetylene
a-Pyrones (pyran-2-ones) are important substructures in many
natural products and biologically active compounds1 and also
serve as dienes in Diels–Alder cycloadditions.2 Transition-
metal-catalysed annulation, cycloaddition, and ring expansion
reactions have emerged as efficient strategies for the prepara-
tion of substituted pyrone derivatives.3,4 The nickel-catalysed
decarbonylative addition of cyclic anhydrides to alkynes,
affording a-pyrones and isocoumarins, has been reported in
2008.3m During our investigations on the rhodium(III)-catalysed
oxidative transformations of C–H bonds,5 we discovered that
maleic anhydrides and alkynes undergo a similar coupling
reaction, producing a-pyrones under the rhodium(III)-catalysed
conditions, which is reported herein.
Table 1 Optimisation of the rhodium(III)-catalysed pyrone synthesisa
When diphenylacetylene (1a) and phenylmaleic anhydride
(2a, 1.5 equiv. to 1a) were heated in t-amyl alcohol at 120 ꢁC
under a nitrogen atmosphere in the presence of 5 mol%
[Cp*Rh(MeCN)3](SbF6)2 (Cp* ¼ pentamethylcyclopentadienyl)
and 1.5 equiv. Cu(OAc)2 for 2 h, the formal [5 ꢀ 1 + 2] annulation
of 2a with 1a occurred to give 3,5,6-triphenyl-a-pyrone (3a) in
85% yield (Table 1, entry 1).6 The reaction was regioselective, and
the possible 4,5,6-triphenyl isomer was not observed in the crude
reaction mixture. The reaction failed when the rhodium(III)
catalyst was not used (entry 2); however, the reaction in the
absence of the copper(II) salt afforded 3a in 24% yield even when
using 30 mol% Rh(III) catalyst (entry 3).7 When [Cp*RhCl2]2 was
used instead of [Cp*Rh(MeCN)3](SbF6)2, the product was
obtained in 42% yield, showing that the preformed cationic
Entry Rh(III) catalyst
Oxidant
Solvent
Yieldb (%)
1
[Cp*Rh(MeCN)3](SbF6)2 Cu(OAc)2
No Rh(III) catalyst Cu(OAc)2
[Cp*Rh(MeCN)3](SbF6)2 No oxidant t-AmOH 24
t-AmOH 85 (82)c
t-AmOH NRd
2
3e
4
[Cp*RhCl2]2
Cu(OAc)2
t-AmOH 42
t-AmOH 73
DMF
Toluene Trace
t-AmOH 57
t-AmOH 29
5
6
[Cp*Rh(MeCN)3](SbF6)2 Ag2CO3
[Cp*Rh(MeCN)3](SbF6)2 Cu(OAc)2
[Cp*Rh(MeCN)3](SbF6)2 Cu(OAc)2
[Cp*Rh(MeCN)3](SbF6)2 Cu(OAc)2
[Cp*Rh(MeCN)3](SbF6)2 Cu(OAc)2
69
7
8f
9g
a
Alkyne 1a (0.10 mmol), anhydride 2a (0.15 mmol), Rh(III) catalyst
(5.0 mmol) and oxidant (0.15 mmol) were reacted in solvent (1.0 mL)
Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka,
Shinjuku-ku, Tokyo 162-8601, Japan. E-mail: mtd@rs.tus.ac.jp; Fax: +81 3 5261 4631
† Electronic supplementary information (ESI) available: Experimental procedures
and characterisation data for new compounds. See DOI: 10.1039/c4ra06452f
b
c
at 120 ꢁC under N2 atmosphere. Isolated yield. The reaction was
d
e
performed on a 1.0 mmol scale. No reaction. 30 mol% Rh catalyst
f
g
was used. The reaction was performed at 100 ꢁC. The reaction was
performed with 30 mol% Cu(OAc)2 under O2 atmosphere.
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Table 2 Substrate scope of alkynes 1a
Table 3 Substrate scope of maleic anhydrides 2a
a
Alkyne
1
(0.10
mmol),
anhydride
2a
(0.15
mmol),
[Cp*Rh(MeCN)3](SbF6)2 (5.0 mmol) and Cu(OAc)2 (0.15 mmol) were
a
b
reacted in t-amyl alcohol (1.0 mL) at 120 ꢁC under N2 atmosphere.
See Table 2 for reaction conditions. The reaction was performed
c
b
using Ag2CO3 instead of Cu(OAc)2. 0.20 mmol 2h was used.
The reaction was performed in DMF using Ag2CO3 instead of
d
c
1
Regioisomeric ratio (4-Me : 3-Me).
Cu(OAc)2. Regioisomeric ratio (5-Me : 6-Me) determined by H NMR.
reactions of 1a with the disubstituted maleic anhydrides 2i–k
were sluggish.10 The reaction of unsymmetrically disubstituted
anhydride 2k with 1a gave a mixture of 4-methyl and 3-methyl
derivatives 3u in 24% combined yield with a regioisomeric ratio
of 61 : 39. The similar coupling of 1a with 4,5,6,7-tetra-
uorophthalic anhydride (2l) gave isocoumarin 3v in 51% yield.
The decarbonylative coupling of 1a with the parent maleic
anhydride (2m) or the succinic anhydride derivatives 2n–p
failed (Fig. 1).
Miura and Satoh proposed a mechanism for the formation of
pyrones from maleic acids and alkynes.3r Based on that mech-
anism, we propose a possible mechanism for the formation of
pyrones from maleic anhydrides and alkynes in the presence of
(1f) for 19 h afforded product 3f in only 6% yield. Aliphatic
alkynes were also compatible with this annulation. Although
alkoxy- and hydroxy-substituted alkynes (1h and 1i) did not
react with 2a under the optimal reaction conditions,8 the reac-
tions performed in DMF with Ag2CO3 as the oxidant afforded
the corresponding products (3h and 3i) in moderate yields. In
the case of an unsymmetrical alkyne, 1-phenyl-1-propyne (1j), a
69 : 31 mixture of pyrone regioisomers were obtained, with the
5-methyl-6-phenyl isomer predominating.9
The substrate scope of diverse maleic anhydrides 2b–k in
this reaction with alkynes 1a and 1g was investigated (Table 3).
Triaryl pyrones 3k–m were obtained in excellent yields by the
reaction of 1a with monoaryl maleic anhydrides 2b–d. The
reaction of methylmaleic anhydride (2e) with 1a and 1g afforded
3n and 3o in 84% and 68% yields, respectively. The reaction of
1a with CF3-substituted anhydride 2f required Ag2CO3 instead
of Cu(OAc)2 to achieve an acceptable yield. Methoxy- and
bromo-substituted maleic anhydrides 2g and 2h also partici-
pated in the pyrone annulation, allowing an access to hetero-
Fig. 1 Substrates that failed to participate in the rhodium(III)-catalysed
atom-substituted pyrones 3q and 3r, respectively. However, the coupling reaction with 1a.
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Notes and references
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Scheme 1 Proposed catalytic cycle for the rhodium(III)-catalysed
pyrone formation.
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Scheme 2 Rh(III)-catalysed reaction of 1a and 2q.
a rhodium(III)/copper(II) catalyst system (Scheme 1). First, a ve-
membered oxarhodacycle A (ref. 3o, r and 4a–c, j, m) is formed
from the Cp*Rh(III) species11 and 2, presumably by the decar-
bonylation of 2a. The C–C bond cleavage occurs selectively at
the less-substituted site of 2. However, the detailed reaction
mechanism for the formation of A remains unclear.12 Next, the
insertion of alkyne 1 to A generates B, and the subsequent
reductive elimination furnishes pyrone 3. Finally, the Cp*Rh(I)
species is oxidised by the copper(II) salt to regenerate the
catalytically active rhodium(III) species, thus completing the
catalytic cycle.
We extended the rhodium(III)-catalysed pyrone-forming
reaction to compounds other than cyclic anhydrides. The
reaction of 1a with benzoic anhydride (2q, 1a : 2q ¼ 1 : 1)
delivered isocoumarin 3w in excellent yield (Scheme 2). Product
3w may have been formed via the formation of rhodium(III)
benzoate, intramolecular C–H bond activation, alkyne inser-
tion, and reductive elimination.13
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Org. Lett., 2007, 9, 1407; (b) K. Ueura, T. Satoh and M. Miura,
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In summary, we developed a rhodium(III)-catalysed coupling
reaction for preparing tri- and tetrasubstituted a-pyrones from
substituted maleic anhydrides and internal alkynes. The formal
[5 ꢀ 1 + 2] annulation reaction afforded a variety of a-pyrones,
with diverse substitution patterns.
Acknowledgements
This work was supported by JSPS, Japan (Grant-in-Aid for
Scientic Research (C) no. 25410054) and the Sumitomo
Foundation.
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5 For reviews, see: (a) T. Satoh and M. Miura, Chem.–Eur. J.,
2010, 16, 11212; (b) G. Song, F. Wang and X. Li, Chem. Soc.
Rev., 2012, 41, 3651; (c) D. A. Colby, A. S. Tsai,
8 3h (7h, 8%); 3i (no reaction).
9 Use of phenylacetylene resulted in homocoupling to give 1,4-
diphenyl-1,3-butadiyne.
R. G. Bergman and J. A. Ellman, Acc. Chem. Res., 2012, 45, 10 The disubstituted anhydride 2k was relatively reluctant to
814; (d) F. W. Patureau, J. Wencel-Delord and F. Glorius,
ring opening using the Cu(II) salt, and 63% of 2k remained
intact aer 8 h. cf. ref. 7.
¨
Aldrichimica Acta, 2012, 45, 31; (e) N. Kuhl, N. Schroder and
F. Glorius, Adv. Synth. Catal., 2014, 356, 1443.
6 The corresponding diacid and monoester reacted with 1a
under the test conditions to give 3a in 80% and 62%
yields, respectively, whereas no reaction was observed
using the diester. A reaction between maleic acids and
alkynes under similar conditions was recently reported. See
ref. 3r.
11 For effect of cyclopentadienyl ligands in the rhodium(III)-
catalysed reactions, see: (a) T. Uto, M. Shimizu, K. Ueura,
H. Tsurugi, T. Satoh and M. Miura, J. Org. Chem., 2008, 73,
298; (b) T. K. Hyster and T. Rovis, Chem. Sci., 2011, 2, 1606;
(c) Y. Shibata and K. Tanaka, Angew. Chem., Int. Ed., 2011,
50, 10917; (d) T. K. Hyster and t. Rovis, Chem. Commun.,
2011, 47, 11846; (e) B. Ye and N. Cramer, J. Am. Chem. Soc.,
2013, 135, 636.
12 The results obtained under copper-free conditions (Table 1,
entry 3) indicate that copper is not involved in the coupling
process. The results obtained in DMF (Table 1, entry 6)
indicate that ring opening of 2 by alcoholysis is not
essential for the reaction.
7 Cyclic anhydride 2a remained largely unchanged (91%
recovered aer 3 h) aer being heated in t-AmOH at
120 ꢁC. In contrast, treatment with 1 equiv. Cu(OAc)2
under the same conditions caused 2a to be consumed,
ring-opened products to be produced and the recovery of
2a aer 8 h to be only 6%.
13 For a precedent work employing benzoic acids, see ref. 4a–c,
j, m.
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