Rhodium-catalyzed intermolecular [2+2+2] cross-trimerization of aryl ethynyl ethers and carbonyl compounds to produce dienyl esters

Article abstract of DOI:10.1002/anie.201105519

Positive thinking: A cationic rhodium(I)/H₈-binap complex catalyzes the chemo-, regio-, and stereoselective completely intermolecular [2+2+2] cross-trimerization of two aryl ethynyl ethers with both electron-deficient and electron-rich carbonyl

Full text of DOI:10.1002/anie.201105519

Communications
DOI: 10.1002/anie.201105519
[2+2+2] Cross-Trimerization
Rhodium-Catalyzed Intermolecular [2+2+2] Cross-Trimerization of
Aryl Ethynyl Ethers and Carbonyl Compounds To Produce Dienyl
Esters**
Yuta Miyauchi, Masayuki Kobayashi, and Ken Tanaka*
Transition-metal-catalyzed [2+2+2] cycloaddition reactions
À
of alkynes and Csp heteroatom multiple bonds, such as
nitriles and heterocumulenes, are valuable methods for the
synthesis of substituted heterocycles in a highly atom
economical manner.[1] However, the analogous transition-
À
metal-catalyzed [2+2+2] cycloaddition of alkynes and Csp2
heteroatom multiple bonds, such as ketones and aldehydes,
have been reported in a limited number of examples.[2–6] The
pioneering example of such a catalysis was the nickel(0)/
monophosphine complex catalyzed cycloaddition of diynes
and aldehydes.[2] After this initial report, the [Cp*Ru(cod)Cl]-
catalyzed cycloaddition of diynes and electron-deficient
Scheme 1. Transition-metal-catalyzed [2+2+2] cycloaddition reactions
ketones,[3] the nickel(0)/imidazolylidene complex catalyzed
of alkynes and carbonyl compounds.
cycloaddition of diynes and electron-rich aldehydes and
ketones,[4] and the neutral rhodium(I) complex catalyzed
cycloaddition of diynals were reported.[5] However, a catalyst
that can be used for both electron-deficient and electron-rich
carbonyl compounds has not been developed. Our research
group discovered that a cationic rhodium(I)/H8-binap com-
plex is able to catalyze the [2+2+2] cycloaddition of diynes
and both electron-deficient and electron-rich carbonyl com-
pounds at room temperature.[6] Unfortunately, these exam-
ples are strictly limited to partially or completely intra-
molecular reactions between tethered diynes and carbonyl
compounds. The development of the transition-metal-cata-
lyzed completely intermolecular [2+2+2] cycloaddition of
two untethered alkynes and a carbonyl compound[7,8] is one of
the most challenging targets (Scheme 1).[9]
in high yields, they reacted with symmetrical electron-
deficient alkynes (diethyl acetylenedicarboxylates) in very
low yields.[10] Aryl ethynyl ethers showed high reactivity
towards polar unsaturated compounds, so we anticipated that
aryl ethynyl ethers would show high reactivity towards the
À
polar C O double bonds of carbonyl compounds. Herein, we
report the rhodium-catalyzed chemo-, regio-, and stereose-
lective completely intermolecular [2+2+2] cross-trimeriza-
tion of two aryl ethynyl ethers and carbonyl compounds at
room temperature.
We first investigated the reaction of ethynyl 2-naphthyl
ether (1a) and a-ketoester 2a in the presence of various
[Rh(cod)2]BF4/bisphosphine complexes (Table 1). After
Our research group recently reported that the cationic
rhodium(I)/H8-binap complex is able to catalyze the com-
pletely intermolecular [2+2+2] cycloaddition of two aryl
ethynyl ethers with nitriles,[10] isocyanates,[10] and alkynyles-
ters.[11] Interestingly, although aryl ethynyl ethers[12] reacted
with unsymmetrical electron-deficient unsaturated com-
pounds (nitriles,[10] isocyanates,[10] and ethyl 2-butynoate)[11]
screening a range of bisphosphine ligands (Scheme 2;
Table 1, entries 1–7;), we found that H8-binap is the best
ligand, as when using this ligand dienyl ester 3aa was obtained
in the highest yield with complete regio- and stereoselectivity
(Table 1, entry 1). The catalyst loading could be reduced to
0.025 equiv with only a slight erosion of the product yield
(Table 1, entry 8).[13]
With the optimized reaction conditions established, we
explored the scope of the ketones in reactions with 1a
(Scheme 3).[13] Not only a-ketoester 2a but also a-ketoester
2b, a-ketoamide 2c, acylphosphonate 2d, and 1,2-diketones
2e and 2 f reacted with 1a to give the corresponding dienyl
esters in moderate to good yields. In the previously reported
[2+2+2] cross-trimerization of tethered diynes with carbonyl
compounds catalyzed by the cationic rhodium(I)/H8-binap
complex, electron-rich ketones showed lower reactivity than
electron-deficient ketones.[6a,c] However, interestingly, 1a
reacted with acetone (2g) to give the product in a yield of
88%, which is significantly higher than the yield of the
reaction of a 1,6-diyne with acetone (51%).[6a,c] Not only
[*] Y. Miyauchi, M. Kobayashi, Prof. Dr. K. Tanaka
Department of Applied Chemistry, Graduate School of Engineering
Tokyo University of Agriculture and Technology
Koganei, Tokyo 184-8588 (Japan)
E-mail: tanaka-k@cc.tuat.ac.jp
Homepage: http://www.tuat.ac.jp/~tanaka-k/
[**] This work was supported partly by a Grant-in-Aid for Scientific
Research (No. 20675002) from MEXT (Japan). We are grateful to
Takasago International Corporation for the gift of H8-binap and
segphos, and Umicore for generous support in supplying a rhodium
complex.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105519.
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Table 1: Optimization of reaction conditions for the rhodium-catalyzed
[2+2+2] cross-trimerization reaction of 1a and 2a.[a]
Entry
Ligand
Catalyst [equiv]
Yield [%][b]
E/Z
1
2
3
4
H8-binap
binap
segphos
biphep
dppf
dppb
dppe
H8-binap
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.025
44
30
19
20
24
21
17
41
1:>99
7:93
1:>99
7:93
1:>99
1:>99
10:90
1:>99
5[c]
6
7[d]
8[e]
[a] [Rh(cod)2]BF4 (0.0075 mmol), ligand (0.0075 mmol), 1a (0.15 mmol),
2a (0.15 mmol), and CH2Cl2 (2.0 mL) were used. [b] Yield of the isolated
products. [c] For 3 h. [d] [Rh(nbd)2]BF4 was used. [e] 1a (0.30 mmol) and
2a (0.30 mmol) were used.
Scheme 2. Structures of bisphosphine ligands.
acetone but also 3-pentanone (2h) and cyclohexanone (2i)
reacted with 1a in good yields. Next, the use of aldehydes
instead of ketones was examined. A variety of aromatic and
aliphatic aldehydes 2j–p were able to react with 1a to give the
corresponding dienyl esters in good to high yields. It is
noteworthy that all reactions in Scheme 3 proceeded to
completion at room temperature in only 1 hour, and complete
regioselectivity and excellent stereoselectivity were observed.
This exceptionally high reactivity of the aryl ethynyl ether
Scheme 3. Rhodium-catalyzed [2+2+2] cross-trimerization reactions of
ethynyl 2-naphthyl ether (1a) and carbonyl compounds 2a–p.
[Rh(cod)2]BF4 (0.0075 mmol), H8-binap (0.0075 mmol), 1a
(0.30 mmol), 2a–p (0.30–3.0 mmol), and CH2Cl2 (2.0 mL) were used.
Cited yields are of the isolated products. [a] 2g–i were used as a
solvent instead of CH2Cl2. [b] [Rh(cod)2]BF4 (0.015 mmol) and H8-
binap (0.015 mmol) were used. [c] (CH2Cl)2 was used as the solvent.
À
1a towards the C O double bonds prompted our investiga-
tion into the chemoselectivity in the reactions of aryl ethynyl
ethers and carbonyl compounds, bearing an additional
reaction site.[14] Cationic rhodium(I)/biaryl bisphosphine
complexes are highly effective catalysts for the [2+2+2]
cycloaddition of 1,6-diynes and electron-deficient alkynes,[15]
and thus the reaction of 1,6-diyne and alkynyl ketone 2q using
the cationic rhodium(I)/H8-binap complex as a catalyst
exclusively furnished the corresponding substituted benze-
We have also reported that the reaction of 1,6-diynes and
aryl ketones in the presence of a cationic rhodium(I)/biaryl
bisphosphine complex, as the catalyst, exclusively furnished
the corresponding dienylation products through carbonyl-
ne.[6a,c] In contrast, 1a exclusively reacted with the C O
À
double bond of 2q using the same rhodium catalyst to give not
benzene 4, but the corresponding dienyne ester 3aq in high
yield (Scheme 4).[13]
[16]
À
directed aromatic C H bond activation.
In contrast, 1a
À
exclusively reacted with the C O double bond of acetophe-
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Communications
Table 2: Rhodium-catalyzed [2+2+2] cross-trimerization reactions of
terminal alkynes 1a–h and acetone (2g).[a]
Yield
[%][b]
Entry
1
3
1
2
1a
1b
3ag
3bg
88
98
Scheme 4. Chemoselectivity between the ketone carbonyl group and
alkyne triple bond.
3
1c
3cg
76
none (2r) to give not diene 5, but the corresponding dienyl
ester 3ar in high yield (Scheme 5).[13]
Next, we explored the scope of aryl ethynyl ethers in the
4
5
1d
3dg
3eg
74
0
reaction with acetone (2g; Table 2).[13] Electronically and
1e (R1 =Ph)
6
7
8
1 f (R1 =Me)
1g (R2 =Ph)
1h (R2 =Bn)
3 fg
3gg
3hg
0
0
0
[a] [Rh(cod)2]BF4 (0.0075 mmol), H8-binap (0.0075 mmol), 1a–h
(0.30 mmol), and 2g (2.0 mL) were used. [b] Yields of the isolated
products.
Scheme 5. Chemoselectivity between the [2+2+2] cross-trimerization
À
and C H bond functionalization.
sterically different aryl ethynyl ethers 1a–d could be
employed in this process (Table 2, entries 1–4). Interestingly,
electron-rich ethynyl 4-methoxyphenyl ether (1b) showed
excellent reactivity (Table 2, entry 2). Importantly, a direct
connection between the aryloxy group and the triple bond is
essential to promote this reaction. Phenyl and methyl
propargyl ether (1e and 1 f, respectively), benzyl acetylene
(1g), and phenyl acetylene (1h) failed to react with 2g, and
the homo-cyclotrimerization products of 1e–h were gener-
ated (Table 2, entries 5–8).
Scheme 6. Rhodium-catalyzed reaction of 1a, 1 f, and 2g.
Finally, the reaction of 1a and 2g was examined in the
presence of an excess of 1 f (Scheme 6). The yield of the cross-
trimerization product 3ag from two molecules of 1a, and one
molecule 2g was decreased to 22%, and the cross-trimeriza-
tion product 3afg from 1a, 1 f, and 2g was generated in only
7% yield, even though excess 1 f was employed.[17]
Scheme 7 depicts a mechanistic proposal for this rhodium-
catalyzed chemo-, regio-, and stereoselective [2+2+2] cross-
trimerization of aryl ethynyl ethers and carbonyl compounds.
Two molecules of aryl ethynyl ether 1 react with rhodium to
generate rhodacyclopentadiene intermediate A.[13] Insertion
of the carbonyl compound 2 into intermediate A generates
intermediate B.[18] Reductive elimination of rhodium to
Scheme 7. A mechanistic proposal.
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Angew. Chem. Int. Ed. 2011, 50, 10922 –10926

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generate pyrane C followed by electrocyclic ring opening
furnishes dienone 3. The olefin geometry of dienone 3 may be
determined by the thermodynamically controlled electro-
cyclic ring-opening reaction[19] and the subsequent E/Z
isomerization catalyzed by a cationic rhodium(I) complex.[20]
À
In intermediate A, the highly polarized Rh C bond (high-
lighted in bold) may facilitate the carbonyl insertion. Indeed,
the electronic nature of the aromatic substituents of 1a–c,
À
which would affect the polarity of the Rh C bond of
intermediate A, had an appreciable impact on the product
yields (electron density of alkyne: 1b > 1a > 1c, product
yield: 3bg > 3ag > 3cg; Table 2, entries 1–3). The cross-tri-
merization reaction of 1a, 1 f, and 2g could proceed through
moderately electron-rich intermediate A’, bearing one ary-
loxy group, to afford 3afg in low yield, while the less electron-
rich intermediate A’’, bearing no aryloxy group, would not
react with 2g.[17]
[2] T. Tsuda, T. Kiyoi, T. Miyane, T. Saegusa, J. Am. Chem. Soc.
1988, 110, 8570.
With regard to the synthetic utility of the present [2+2+2]
cross-trimerization reactions, it is important to note that 3-
alkoxy-6-oxo-2,4-hexadienoates show biological activity for
various pharmacological targets.[21] The present method
enables the preparation of interesting new analogues.
Although alkyl ethynyl ethers cannot be employed in this
reaction owing to their instability towards the Lewis acidic
cationic rhodium(I) catalyst, 2-naphthyl ester 3ag can be
converted almost quantitatively into the corresponding
methyl ester 6 by treatment with NaOMe in MeOH
(Scheme 8).
[3] Y. Yamamoto, H. Takagishi, K. Itoh, J. Am. Chem. Soc. 2002,
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[7] The completely intermolecular [2+2+2] cycloadditions of two
alkynes and one carbonyl compound using a stoichiometric
amount of transition-metals were reported. For Co, see: a) D. F.
Harvey, B. M. Johnson, C. S. Ung, K. P. C. Vollhardt, Synlett
1989, 15; for Zr, see: b) T. Takahashi, Y. Li, T. Ito, F. Xu, K.
Nakajima, Y. Liu, J. Am. Chem. Soc. 2002, 124, 1144.
Scheme 8. Transesterification of 2-naphthyl ester 3ag leading to
methyl ester 6.
In conclusion, the chemo-, regio-, and stereoselective
completely intermolecular [2+2+2] cross-trimerization of
two aryl ethynyl ethers with both electron-deficient and
electron-rich carbonyl compounds, leading to aryloxy-substi-
tuted dienyl esters, has been achieved at room temperature by
using a cationic rhodium(I)/H8-binap complex as a catalyst.
Further utilization of aryl ethynyl ethers in rhodium-cata-
lyzed reactions is underway in our laboratory.
[8] The Rh-catalyzed reductive coupling reaction of acetylene gas
and aldehydes was reported, although this reaction does not
À
involve the C O reductive elimination step, see: a) J. R. Kong,
M. J. Krische, J. Am. Chem. Soc. 2006, 128, 16040; for the closely
related reaction between acetylene gas and N-arylsulfonyl
imines, see: b) E. Skucas, J.-R. Kong, M. J. Krische, J. Am.
Chem. Soc. 2007, 129, 7242.
Received: August 4, 2011
Published online: September 21, 2011
[9] For a recent review of the completely intermolecular [2+2+2]
cycloaddition, see Ref. [1g].
[10] Y. Komine, K. Tanaka, Org. Lett. 2010, 12, 1312.
[11] Y. Komine, Y. Miyauchi, M. Kobayashi, K. Tanaka, Synlett 2010,
3092.
Keywords: alkynes · carbonyl compounds · dienyl esters ·
homogeneous catalysis · rhodium
.
[12] For the pioneering work on the transition-metal-mediated
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b) R. L. Funk, K. P. C. Vollhardt, J. Am. Chem. Soc. 1980, 102,
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Communications
Saegusa, Synth. Commun. 1989, 19, 1575; f) F. E. McDonald,
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Tanaka, K. Takeishi, K. Noguchi, J. Am. Chem. Soc. 2006, 128,
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similar reaction, see: b) K. Tsuchikama, Y. Kuwata, Y. Tahara, Y.
Yoshinami, T. Shibata, Org. Lett. 2007, 9, 3097.
[17] The reaction shown in Scheme 6 furnished cross-cyclotrimeriza-
tion products of 1a and 1 f [ca. 50% yield (1a/1 f = 2:1) and ca.
20% yield (1a/1 f = 1:2)] as major products. Possible pathways
for the formation of these products may involve intermediates A
and A’, which would support the formation of 3ag and 3afg via
intermediates A and A’, respectively (see Scheme 7).
H. Y. H. Zhu, C. R. Holmquist, J. Am. Chem. Soc. 1995, 117,
6605; g) T. Takahashi, JP 11263737, 1999 [Chem. Abstr. 1999,
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Tetrahedron 2009, 65, 5056; for the Ni-catalyzed [3+2+2]
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75, 480.
[13] In the reactions of aryl ethynyl ethers 1a–d and carbonyl
compounds 2a–r (Tables 1–2 and Scheme 3–5), homo-cyclo-
trimerization products of 1a–d were generated as by-products.
These by-products are generated via intermediates A, and
therefore their formation supports the formation of 3 via
intermediates A (Scheme 7).
[14] We have reported that the cationic rhodium(I)/H8-binap com-
plex catalyzes the reaction of a 1,6-diyne with the ketone
carbonyl group and not the ester carbonyl group of a-ketoester
2s (see: Ref. [6c]). However, interestingly, 1a reacted with not
only the ketone carbonyl group but also the ester carbonyl group
of 2s, although products 3as and 3as’ were isolated as an
inseparable mixture.
À
[18] For carbonyl insertion into a Rh C bond, see: C. Krug, J. F.
Hartwig, J. Am. Chem. Soc. 2002, 124, 1674.
[19] J. M. Um, H. Xu, K. N. Houk, W. Tang, J. Am. Chem. Soc. 2009,
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[20] K. Tanaka, T. Shoji, M. Hirano, Eur. J. Org. Chem. 2007, 2687.
[21] Y. Tanaka, T. Kawano, S. M. Islam, H. Nishioka, M. Hatanaka, I.
Ueda, Chem. Pharm. Bull. 1996, 44, 885.
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