Rhodium-catalyzed double 1,4-addition of arylboronic acids to β-aryloxyacrylates involving β-oxygen elimination

Article abstract of DOI:10.1002/adsc.201100253

The rhodium-catalyzed addition reaction of arylboronic acids to β-aryloxyacrylates gives 3,3-diarylpropanoates in good yields. The double arylation reaction proceeds via the intermediate cinnamates generated by β-oxygen elimination. Copyright

Full text of DOI:10.1002/adsc.201100253

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DOI: 10.1002/adsc.201100253
Rhodium-Catalyzed Double 1,4-Addition of Arylboronic Acids to
b-Aryloxyacrylates Involving b-Oxygen Elimination
Takanori Matsuda,a,* Shigeru Shiose,a and Yuya Sudaa
a
Department of Applied Chemistry, Tokyo University of Science, 1–3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
Fax : (+81)-3-5261-4631; e-mail: matta@rs.kagu.tus.ac.jp
Received: April 8, 2011; Revised: May 12, 2011; Published online: August 4, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100253.
Abstract: The rhodium-catalyzed addition reaction
of arylboronic acids to b-aryloxyacrylates gives 3,3-
diarylpropanoates in good yields. The double aryla-
tion reaction proceeds via the intermediate cinna-
mates generated by b-oxygen elimination.
arylation occurred exclusively. An analogous double
arylation was previously observed as a side reaction
in the rhodium-catalyzed arylation of some acryl-
ates.[8] When the reaction was performed with one
equivalent of 2a, a 56:44 mixture of 3a and methyl
(E)-3-(4-methylphenyl)acrylate (4a) was obtained in a
combined yield of 19% along with 74% of recovered
1a (entry 2). Acrylate 1b bearing an electron-donating
aryloxy group provided 3a in 83% yield (entry 3),
Keywords: 1,4-addition; boron; esters; b-oxygen
elimination; rhodium
Table 1. Rhodium-catalyzed reaction of b-aryloxyacrylates
1a–c with 4-methylphenylboronic acid (2a).[a]
À
The rhodium-catalyzed carbon carbon bond-forming
reactions of organoboron compounds have become an
indispensable tool in organic syntheses.[1] In particu-
lar, the rhodium-catalyzed 1,4-addition reactions to
electron-deficient alkenes have been extensively stud-
ied.[2] However, the 1,4-additions to a,b-unsaturated
carbonyl compounds with oxygen functionality at the
b position have been limited to cyclic substrates.[3]
Herein, we report a rhodium-catalyzed reaction of b-
aryloxyacrylates and arylboronic acids that leads to
the formation of 3,3-diarylpropanoates. The double
arylation proceeds via intermediate formation of cin-
namates by b-oxygen elimination.
Entry
1
Equiv. of 2a
mol% Rh
Yield[b]
Methyl (E)-b-aryloxyacrylates were readily pre-
pared from the reaction between methyl propiolate
and the corresponding phenols (1.1 equiv.) in THF in
the presence of a catalytic amount of 1,4-diazabicyclo-
1
2
3
4
1a
1a
1b
1c
1c
1c
5
1
5
5
5
5
6
6
6
6
3
6
88%
11%[c]
83%
ACHTUNGTRENNUNG
[2.2.2]octane.[4] Initially, the reaction of b-aryloxyacry-
99%
5
32%[d]
95%
lates 1a–c and 4-methylphenylboronic acid (2a) was
performed in the presence of catalytic amounts of
[Rh(OH)ACHTUNGTRENNUNG(cod)]2 (cod=cycloocta-1,5-diene; Table 1).
When the reaction of phenoxyacrylate 1a with 2a
(5 equiv. to 1a) was conducted with 3 mol%
[Rh(OH)ACHTUNGTRENNUNG(cod)]2 (6 mol% Rh) at room temperature
in MeOH for 2 h, methyl 3,3-bis(4-methylphenyl)pro-
panoate (3a) was isolated in 88% yield (entry 1).[5–7]
The expected product [i.e., methyl 3-(4-methylphen-
yl)-3-phenoxypropanoate] was not observed; double
6[e]
[a]
Reaction conditions: 1, 2 (1 or 5 equiv.), [Rh(OH)ACHTUNGTRENNUNG(cod)]2
(1.5 or 3 mol%), MeOH (0.1M), room temperature, 2 h.
Isolated yield by preparative TLC.
[b]
[c]
[d]
[e]
Isolated along with 4a (8%) and 1a (74%).
Isolated along with 4a (18%).
4-Methylphenylboronic acid pinacol ester was used in-
stead of 2a, and the reaction was performed in refluxing
MeOH.
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Takanori Matsuda et al.
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whereas when 4-acetylphenoxy derivative 1c was em-
Then, b-(4-acetylphenoxy)acrylate 1c was subjected
ployed, 3a was obtained in 99% yield (entry 4). A cat- to the reaction with various arylboronic acids, and the
alyst loading of 3 mol% Rh resulted in incomplete results are summarized in Table 2. Phenyl-, 3-methyl-
conversion after 2 h, and the product 3a was obtained phenyl-, and 4-tert-butylphenylboronic acids (2b–d)
in 32% yield along with 4a (18%; entry 5). When 4- provided the corresponding 3,3-diarylpropanoates
methylphenylboronic acid pinacol ester was used as (3b–d) in high yields (entries 1–3). The reaction of the
the arylating agent, a higher temperature was re- sterically demanding 2-methylphenylboronic acid (2e)
quired to facilitate the reaction (entry 6).
was inefficient at room temperature, and 3,3-diaryl-
ACHTUNGTRENpNUNG ropanoate 3e and cinnamate 4e were formed in 19%
Based on the finding that cinnamate 4 was formed
as a by-product under certain conditions, the mecha- and 38% yields, respectively; selective formation of
nism for the formation of double arylation product 3 3e was achieved when the reaction was performed in
appears to follow a tandem sequence involving 1,4-ad- refluxing MeOH (entry 4). In the case of mesitylbor-
dition/b-oxygen elimination[9–11] forming 4 followed onic acid (2f), the second arylation was hampered,
by 1,4-addition/protonation (Scheme 1). The absence even with reflux, by the severe steric hindrance of the
of the usual 1,4-addition product in the reaction mix- mesityl group, furnishing cinnamate 4f as the sole
ture indicates that b-oxygen elimination occurred so product in 100% yield (entry 5).[13] In contrast to bor-
smoothly that the possible protonation of intermedi- onic acids 2h and 2i having an electron-donating me-
ate A was completely suppressed. The present reac- thoxy group that gave 3,3-diarylpropanoates in good
tion stands in stark contrast to the rhodium(I)-phos- yields at room temperature (entries 7 and 8), the reac-
phane-catalyzed reaction of a b-acetoxyacrylate and tions with 2j–m having electron-withdrawing substitu-
an arylboronic acid that formed a cinnamate as the ents were sluggish at room temperature; low-to-
cross-coupling product.[12]
The reaction of methyl b-benzyloxyacrylate (1d) af-
forded a mixture of double arylation product 3a
Table 2. Double arylation of 1c.[a]
(11%) and methyl 3-benzyloxy-3-(4-methylphenyl)-
propanoate (5, 16%) (Scheme 2). The reactivity and
eliminating ability of the benzyloxy group of 1d are
comparatively poorer than those of the aryloxy coun-
terparts.
A control experiment revealed that the reaction of
methyl propiolate with 2a failed to form the double
arylation product 3a under the present reaction condi-
tions, demonstrating the potential utility of b-aryloxy-
Entry
2
Ar
Temp.
3
Yield[b]
1
2
3
4
5
6
7
8
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
Ph
r.t.
r.t.
r.t.
reflux
reflux
r.t.
r.t.
r.t.
reflux
reflux
reflux
reflux
r.t.
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
94%
95%
88%
91%
0%[c]
63%[d]
91%
71%
64%
34%
22%
14%
70%
28%[f]
ACHTUNGTRENNUNGacrylates for the synthesis of 3,3-diarylpropanoates.
3-MeC6H4
4-t-BuC6H4
2-MeC6H4
2,4,6-Me3C6H2
2-naphthyl
4-MeOC6H4
3-MeOC6H4
4-F3CC6H4
3-AcC6H4
3-O2NC6H4
4-NCC6H4
4-FC6H4
9
10
11
12[e]
13
14
3-BrC6H4
r.t.
[a]
Reaction conditions: 1c and arylboronic acid 2 (5 equiv.)
were treated in the presence of [Rh(OH)(cod)]2
ACHTUNGTRENNUNG
Scheme 1. Possible mechanism.
(3 mol%) in MeOH at the indicated temperatures for
2 h.
[b]
[c]
Isolated yield by preparative TLC.
Methyl (E)-3-mesitylacrylate (4f) was obtained in 100%
yield.
[d]
Isolated along with methyl 3-(2-naphthyl)propanoate
(12%).
1a was used instead of 1c.
Isolated along with methyl (E)-3-(3-bromophenyl)acry-
[e]
[f]
Scheme 2. Reaction of 1d with 2a.
late (4o, 9%).
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Rhodium-Catalyzed Double 1,4-Addition of Arylboronic Acids to b-Aryloxyacrylates
solvent was removed under reduced pressure. The crude res-
idue was purified by preparative thin-layer chromatography
on silica gel (hexane:AcOEt=5:1) to afforded methyl 3,3-
di-p-tolylpropanoate (3a) as a pale yellow semisolid; yield:
52.8 mg (99%).
Scheme 3. Double arylation of 6.
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the reaction was performed in refluxing MeOH (en-
tries 9–12). The reaction of 4-fluorophenyl- and 3-bro-
mophenylboronic acids (2n and 2o) afforded the cor-
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phenone (6) can also be used as a substrate for the
double arylation reaction. The reaction of 6 occurred
under the same reaction conditions to produce 3,3-di-
arylpropiophenones 7 in good yields (Scheme 3).
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dergo the second arylation prompted us to examine
the introduction of two different aryl groups in 1 by
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2b and 2h at reflux provided a one-pot access to
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good yields (Scheme 4).
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In conclusion, we have developed a rhodium-cata-
lyzed double 1,4-addition reaction of arylboronic
acids to b-aryloxyacrylates and b-phenoxyacrylophe-
none that leads to the formation of 3,3-diarylpropan-
ACHTUNGTRENNUNGoates and 3,3-diarylpropiophenones, respectively. The
double arylation was achieved by the regeneration of
a,b-unsaturated carbonyl compounds by b-oxygen
elimination.
Experimental Section
General Procedure for the Rhodium(I)-Catalyzed
Double Arylation of b-Aryloxyacrylates 1
To a Schlenk tube under nitrogen were added 1c (43.9 mg,
0.20 mmol), 2a (136.2 mg, 1.00 mmol), and [Rh(OH)ACHTNUTRGNEUNG(cod)]2
(2.8 mg, 6.1 mmol) followed by methanol (2.0 mL). The mix-
ture was stirred at room temperature for 2 h, and then the
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Takanori Matsuda et al.
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[10] For rhodium-catalyzed arylation reactions involving b-
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action of 1a with one equivalent of 2a was performed
in 1,4-dioxane, 3a, (Z)-3-(4-methylphenyl)acrylate
(4’a), and 1a were obtained in 26%, 14%, and 55%
yields, respectively (cf. Table 1, entry 2).
[12] J.-Y. Yu, R. Kuwano, Angew. Chem. 2009, 121, 7353–
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[13] A similar result was also observed at room temperature
(96% yield).
[11] The stereochemistry of the b-oxygen elimination seems
to depend strongly on the solvents used. When the re-
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