Rhodium-catalyzed hydroaroylation of α,β-unsaturated esters using aroyl chlorides and Et2MeSiH

Article abstract of DOI:10.1016/j.tetlet.2013.06.009

Hydroaroylation of methyl acrylate 2a to give the α-aroyl esters 4 took place in the three-component reaction of 2a, aroyl chlorides 1, and Et ₂MeSiH in the presence of 1 mol % of [Rh(cod)(PR₃) ₂]OTf (cod = 1,5-cyclooctadi

Full text of DOI:10.1016/j.tetlet.2013.06.009

Tetrahedron Letters 54 (2013) 4309–4312
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Rhodium-catalyzed hydroaroylation of
aroyl chlorides and Et2MeSiH
a,b-unsaturated esters using
⇑
Takako Muraoka, Eiji Hiraiwa, Minami Abe, Keiji Ueno
Division of Molecular Science, Faculty of Science and Technology, Gunma University, Kiryu 376-8515, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydroaroylation of methyl acrylate 2a to give the
a-aroyl esters 4 took place in the three-component
Received 3 May 2013
Revised 30 May 2013
Accepted 4 June 2013
Available online 11 June 2013
reaction of 2a, aroyl chlorides 1, and Et2MeSiH in the presence of 1 mol % of [Rh(cod)(PR3)2]OTf
(cod = 1,5-cyclooctadiene, OTf = OSO2CF3, R = Ph (3a), OPh (3b)) in CH2Cl2. GC and 1H NMR investigation
revealed that the rhodium-catalyzed hydroaroylation proceeds via two successive transformations, that
is, hydrosilylation of 2a to afford silyl enol ether 5a followed by C–C bond formation between 5a and 1a.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Rh-catalyzed
Three-component coupling
Aroyl chloride
Hydrosilane
Introduction
Transition-metal catalyzed multicomponent reactions (MCRs)
involving C–C bond formation have been recognized as versatile
procedures for constructing the polyfunctional organic molecules.1
Neutral and cationic rhodium(I) complexes with monodentate or
bidentate phosphines as spectator ligands are widely investigated
as the catalyst for MCRs.2,3 In particular, RhX(PR3)n and [Rh(die-
ne)(PR3)2]+ types of complexes have been used for catalytic,
three-component reductive aldol condensations with aldehydes
Scheme 1. Catalytic reductive aldol condensations.
or ketones, a,b-unsaturated carbonyl compounds, and reducing re-
agents such as hydrosilanes or hydrogen gas (Scheme 1).4,5 The rho-
dium-catalyzed reactions with hydrosilanes proceed via the in situ
generation of enolate species which attacks aldehydes or ketones to
give the C–C bond formation products. Thus, the three-component
systems have significant advantages over the conventional two-
component aldol reactions since, in the latter case, the enolate spe-
cies should be prepared independently from carbonyl compounds
using a stoichiometric amount of strong bases.
Development of the Rh-catalyzed systems applicable to a vari-
ety of electrophiles instead of aldehydes and ketones would pro-
vide efficient synthetic procedures for carbonyl compounds,
however, such examples are limited to only an aldimine and a ket-
imine as the electrophiles.6 We and co-workers have investigated
on rhodium-catalyzed three-component couplings of electrophiles
Scheme 2. Rh-catalyzed hydroaroylation of
chlorides 1 and Et2MeSiH.
a,b-unsaturated esters 2 with aroyl
a-amido-, and b-amino carbonyls, are selectively produced owing
to preferential enolate generation followed by selective C–C bond
formation with electrophiles. During the course of the study, we
found that aroyl chloride 1 is applicable as an electrophile to the
rhodium-catalyzed three-component reaction system. Aroyl chlo-
rides have been known as efficient substrates for introducing a
keto group to aromatic substrates electrophilically, that is, Lewis
acid induced/catalyzed Friedel–Crafts acylation.8 Aroyl chlorides
have also been used for palladium-catalyzed cross-couplings with
organometallic reagents, including organoboron, organotin, and
organozinc reagents, to afford aromatic ketones.9 However, the
organometallic reagents required for the catalytic cross-coupling
(allyl carbonates, aryl isocyanates, and aldimines),
a,b-unsaturated
carbonyl compounds, and hydrosilanes.6a,7 In these reactions,
functionalized carbonyl compounds, such as
c,d-unsaturated-,
⇑
Corresponding author. Tel./fax: +81 277 30 1260.
E-mail address: ueno@gunma-u.ac.jp (K. Ueno).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.009

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T. Muraoka et al. / Tetrahedron Letters 54 (2013) 4309–4312
systems are not readily available and also more than stoichiome-
tric amounts of organometallic reagents should be employed to
complete the reaction. Except for the aforementioned palladium-
catalyzed reactions, only a few examples have been reported on
the transition-metal catalyzed transformation using aroyl chlo-
rides as a source of the keto functional group.10 We report here
the rhodium-catalyzed three-component reaction of aroyl chloride
Results and discussion
Treatment of benzoyl chloride 1a, methyl acrylate 2a, and Et2
MeSiH in CH2Cl2 in the presence of 1 mol % of [Rh(cod)(PPh3)2]OTf
(3a, cod = 1,5-cyclooctadiene, OTf = OSO2CF3) at 25 °C for 10 h
afforded a
-aroyl ester 4a11 in 61% yield with a concomitant forma-
tion of Et2MeSiCl (Eq. 1). The yield of 4a slightly increased to 74%
when the Rh complex was changed from 3a to its P(OPh)3 ana-
logue, [Rh(cod){P(OPh)3}2]OTf (3b) (entry 3 in Table 1), and
reached 84% under reflux for 5 h (entry 4 in Table 1).
1,
b-keto (
can be categorized as a formal hydroaroylation of 2 in which the
-aroyl ester 4 is formed via regioselective introduction of hydride
a
,b-unsaturated carbonyl compound 2 and Et2MeSiH to afford
a-aroyl) ester 4 in high yield (Scheme 2). This reaction
a
and aroyl groups to the b- and a-positions of 2, respectively.
Table 1
Rhodium-catalyzed hydroaroylation of 2a with 1a and Et2MeSiHa
Entry
Precatalyst
Temp.
Time
h
Yield of 4ab
°C
(%)
1
2
3
4
3a
3a
3b
3b
25
reflux
25
10
5
10
5
61
72
74
84
The scope of the hydroaroylation was explored using 3b as
catalyst, aroyl chloride 1,
a,b-unsaturated ester 2, and Et2MeSiH
reflux
(Table 2). Hydroaroylation to methyl crotonate 2b was sluggish
owing to its b-Me substituent and the corresponding product
a
A CH2Cl2 solution of benzoyl chloride 1a, 2a, Et2MeSiH, and 1 mol% of 3a or 3b
was stirred at 25 °C for 10 h or heated at reflux for 5 h.
b
Isolated yield.
Table 2
Scope of the rhodium-catalyzed hydroaroylationa
Entry
ArCOCl (1)
2
Product 4
time
h
yieldb
(%)
1
2
1a
1a
2a
2b
4a11
5
84
35
4a’12
20
3
4
5
6
7
1a
1b
1c
1d
1e
2c
2a
2a
2a
2a
4a’’13
4b11b
4c11b
4d
24
6
3
50
6
16
81
93
3
86
85
4e11b,14
13
8
1f
2a
4f11b
5
86
9
1g
1h
2a
2a
4g15
4h16
6
67
86
10
10
a
A CH2Cl2 solution of aroyl chloride 1, 2a (or 2b), Et2MeSiH, and 1 mol% of 3b was refluxed for appropriate hours described in each entry.
Isolated yield.
b

T. Muraoka et al. / Tetrahedron Letters 54 (2013) 4309–4312
4311
12
4a0 was formed in low yield after prolonged heating (35%, entry
2). Methyl group at the -position in ,b-unsaturated ester ham-
a
a
pered C–C bond formation. Thus the catalytic hydroaroylation with
methyl methacrylate 2c provided almost no coupling product (en-
try 3). Steric hindrance around the C(=O)Cl functionality in aroyl
chloride 1 also influenced the progress of hydroaroylation. Cata-
lytic reaction with o-toluoyl chloride 1b gave hydroaroylation
product 4b in a modest yield after a mixture of 1b, 2a, and Et2Me-
SiH with 1 mol % of 3b was refluxed for 6 h (entry 4). In contrast to
this result, aroyl chlorides with methyl and bromo substituents on
meta- and para-positions 1c–1f were successfully converted into
corresponding
5–8). Hetero aroyl chlorides, 2-furoyl chloride 1g, and 2-thioyl
chloride 1h, were also transformed into the corresponding -aroyl
a-aroyl esters 4c–4f under usual conditions (entries
Scheme 3. A possible reaction mechanism of Rh-catalyzed hydroaroylation.
a
esters 4g and 4h, respectively, in high yields (entries 9 and 10).
Rhodium-catalyzed reductive aldol reactions have been pro-
posed to proceed via nucleophilic attack of the in situ generated
Rh-enolates to aldehydes and ketones.17 To explore the reaction
mechanism, 3b-catalyzed hydroaroylation of 2a with benzoyl chlo-
ride 1a and Et2MeSiH was monitored by gas chromatography (GC)
and 1H NMR spectroscopy. The reaction profiles are summarized in
Figure 1. Formation of the silyl enol ether 5a,7b along with a small
amount of the final product 4a, was observed immediately after
mixing of substrates and the catalyst. After 1 h, almost all of 2a
and Et2MeSiH were converted into 5a. The yield of 4a gradually in-
creased and reached 86% after 12 h along with consumption of 1a
and 5a. Without the Rh complex, neither the reaction of the three
components nor that between isolated 5a7b and 1a took place.
These observations revealed that the Rh-catalyzed hydroaroylation
proceeds via two successive transformations, i.e., Rh-catalyzed
generation of the silyl enol ether 5a7b by 1,4-hydrosilylation of
2a with Et2MeSiH followed by C–C bond formation between result-
ing silyl enol ether 5a and 1 by the catalysis of the rhodium
complex.
ethers and aroyl chlorides could be activated by rhodium com-
plex.18,19 Thus the [Rh] species activates both aroyl chloride 1
and silyl enol ether 5a for the C–C bond forming reaction to take
place, which affords 4 and Et2MeSiCl as the final products.
Acknowledgment
This work was supported by Grants-in-Aid for Scientific Re-
search (Nos. 23550066 and 24750053) and the ‘‘Element Innova-
tion’’ Project from the Ministry of Education, Culture, Sports,
Science and Technology of Japan. T. M. acknowledges The Japan
Prize Foundation for a research grant.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.06.
009.
A possible reaction mechanism for Rh-catalyzed hydroaroyla-
tion of 2a with aroyl chloride 1 and Et2MeSiH is shown in Scheme 3.
The reaction starts from the oxidative addition of Et2MeSiH to [Rh]
complex to give complex A. The insertion of 2a to the Rh-H or/and
Rh-SiEt2Me bond in A results in the formation of B or/and B0. Isom-
erization to C or/and C0 followed by reductive elimination of 5a
regenerates [Rh] species. It is well established that both silyl enol
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