Base-Free Conditions for Rhodium-Catalyzed Asymmetric Arylation to Produce Stereochemically Labile α-Aryl Ketones

Article abstract of DOI:10.1002/anie.201601709

The asymmetric arylation of 2,2-dialkyl cyclopent-4-ene-1,3-diones with aryl boronic acids was found to be efficiently catalyzed by a chiral diene-rhodium μ-chloro dimer, [{RhCl((R)-diene?)}₂], in the absence of bases in toluene/H₂O to give 2,2-dialkyl 4-aryl cyclopentane-1,3-diones in high yields with high enantioselectivity. Such compounds can not be obtained with high enantiomeric purity under the standard basic conditions used for rhodium-catalyzed asymmetric arylation because the α-aryl ketone products undergo racemization under the basic conditions. Forget about bases, score a home run: The asymmetric arylation of 2,2-dialkyl cyclopent-4-ene-1,3-diones with aryl boronic acids was efficiently catalyzed by a chiral diene-rhodium μ-chloro dimer without a base in toluene/H₂O (see scheme). The resulting α-aryl ketones can not be obtained with high ee values under the standard basic conditions for rhodium-catalyzed asymmetric arylation owing to their racemization in the presence of a base.

Full text of DOI:10.1002/anie.201601709

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International Edition: DOI: 10.1002/anie.201601709
German Edition: DOI: 10.1002/ange.201601709
Asymmetric Catalysis
Base-Free Conditions for Rhodium-Catalyzed Asymmetric Arylation
To Produce Stereochemically Labile a-Aryl Ketones
Xiaowei Dou, Yixin Lu,* and Tamio Hayashi*
Abstract: The asymmetric arylation of 2,2-dialkyl cyclopent-4-
ene-1,3-diones with aryl boronic acids was found to be
efficiently catalyzed by a chiral diene–rhodium m-chloro
dimer, [{RhCl((R)-diene*)}2], in the absence of bases in
toluene/H2O to give 2,2-dialkyl 4-aryl cyclopentane-1,3-
diones in high yields with high enantioselectivity. Such com-
pounds can not be obtained with high enantiomeric purity
under the standard basic conditions used for rhodium-cata-
lyzed asymmetric arylation because the a-aryl ketone products
undergo racemization under the basic conditions.
A
symmetric conjugate arylation of electron-deficient olefins
is one of the most efficient methods of constructing stereo-
genic carbon centers at the benzylic position,[1] and the
rhodium-catalyzed asymmetric arylation is currently attract-
ing considerable attention owing to its high enantioselectivity
and high reliability as well as the advantages of using
organoboron reagents as nucleophiles.[2] The olefinic sub-
strates used for the rhodium-catalyzed asymmetric arylation
=
have the general structure RCH CH(EWG), in which EWG
stands for an electron-withdrawing group (Scheme 1a). The
substituent R, which is connected to the stereogenic carbon
center of the products, is an alkyl or aryl group in most
cases;[2] heteroatom substitution, whereby R is a silyl,[3]
amino,[4] or alkoxy[5] group, has also been reported. Rho-
dium-catalyzed conjugate arylation has generally been per-
formed under basic conditions, typically with KOH (0.1–
1.0 equiv) as a base, which has been proposed to generate
a hydroxo–rhodium species and/or a reactive borate species.[2]
Although cyclopent-4-ene-1,3-diones have recently been
studied as substrates for asymmetric desymmetrization,[6] to
the best of our knowledge, their rhodium-catalyzed asym-
metric arylation has not been reported, probably owing to the
stereochemical instability of the arylation products containing
stereogenic carbon centers substituted with aryl and carbonyl
groups. Because of the acidic hydrogen atom, racemization of
the 4-arylated products readily takes place under the basic
conditions used for rhodium-catalyzed asymmetric arylation
Scheme 1. Rhodium-catalyzed asymmetric arylation of electron-defi-
cient olefins. dan=1,8-naphthalenediaminato.
(see below). Herein we report that the asymmetric arylation
of cyclopent-4-ene-1,3-diones with high enantioselectivity
(mostly ꢀ 98% ee) is possible with a chiral diene–rhodium
m-chloro dimer catalyst, [{RhCl((R)-diene*)}2], under non-
basic conditions (Scheme 1b). As related substrates giving
products with a stereogenic center at the a-position to
a carbonyl group, fumarates[7] and maleimides[8] have been
reported to be applicable to the rhodium-catalyzed arylation
under standard basic conditions (Scheme 1c). Products with
high ee values are obtained without difficulty because the
ester and imide products are more stable against racemization
under basic conditions.
Table 1 summarizes the results obtained for the rhodium-
catalyzed asymmetric addition of PhB(OH)2 (2a) to 2,2-
diethylcyclopent-4-ene-1,3-dione (1a). In the first set of
experiments, a rhodium complex coordinated with (R)-
diene*,[9,10] which is readily accessible from (À)-a-phellan-
drene and is one of the most enantioselective chiral diene
ligands for the asymmetric conjugate arylation, was used as
a catalyst. The reaction in the presence of KOH (20 mol%) in
dioxane/H2O (10:1) at 508C, which are commonly used
reaction conditions for the addition to maleimides and other
electron-deficient olefins,[2,8] proceeded well to give the
[*] Dr. X. Dou, Prof. Dr. Y. Lu, Prof. Dr. T. Hayashi
Department of Chemistry, National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
E-mail: chmlyx@nus.edu.sg
chmtamh@nus.edu.sg
Prof. Dr. T. Hayashi
Institute of Materials Research and Engineering, A*STAR
2 Fusionopolis Way, Singapore 138634 (Singapore)
E-mail: tamioh@imre.a-star.edu.sg
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201601709.
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Table 1: Rhodium-catalyzed asymmetric addition of PhB(OH)2 (2a) to 2,2-diethylcyclopent-4-ene-1,3-
dione (1a).[a]
1 mol % of the Rh catalyst was
a mixture of toluene and H2O in
a ratio of 10:5; under these condi-
tions, 3aa was obtained in 91%
yield with > 99% ee (Table 1,
entry 7). Under the same condi-
tions, the rhodium complexes with
(R,R)-Ph-bod[14] and (S,S)-Fc-tfb[15]
also catalyzed the phenylation reac-
tion, but the yields were lower
Entry Rh catalyst (mol% Rh)
Additive
Solvent [mL]
Yield ee
[%][b] [%][c]
1
2
3
4
5
6
7
8
9
[{RhCl((R)-diene*)}2] (5)
[{RhCl((R)-diene*)}2] (5)
[{Rh(OH)((R)-diene*)}2] (5)[d]
[{RhCl((R)-diene*)}2] (5)
[{RhCl((R)-diene*)}2] (5)
[{RhCl((R)-diene*)}2] (1)
[{RhCl((R)-diene*)}2] (1)
[{RhCl((R,R)-Ph-bod)}2] (1)
[{RhCl((S,S)-Fc-tfb)}2] (1)
[{RhCl((R)-binap)}2] (1)[e]
[{RhCl((R)-diene*)}2] (1)
[{RhCl((R)-diene*)}2] (1)
KOH (20 mol%)
Cs2CO3 (20 mol%) dioxane/H2O (1.0/0.1) 96
–
–
–
–
–
–
dioxane/H2O (1.0/0.1) 93
<1
<1
dioxane/H2O (1.0/0.1) 93
dioxane/H2O (1.0/0.1) 14
toluene/H2O (1.0/0.1) 85
toluene/H2O (1.0/0.1) 65
toluene/H2O (1.0/0.5) 91
toluene/H2O (1.0/0.5) 70
toluene/H2O (1.0/0.5) 38
35 (entries 8 and 9). With (R)-
91
>99
>99
>99
99
94
96
binap,[16] the reaction was very
slow (entry 10). Even in the tolu-
ene/H2O (10:5) solvent system, the
use of bases caused the racemiza-
tion of 3aa, although the racemiza-
tion was much slower than in diox-
ane/H2O (entries 11 and 12).
–
–
10
11
12
toluene/H2O (1.0/0.5)
9
KOH (4 mol%)
Cs2CO3 (20 mol%) toluene/H2O (1.0/0.5) 93
toluene/H2O (1.0/0.5) 91
54
83
The
product
(R)-3aa
(> 99% ee) was treated with
Cs2CO3 in dioxane/H2O and tolu-
ene/H2O under conditions similar
to those for the rhodium-catalyzed
[a] Reaction conditions: 1a (0.15 mmol), 2a (0.30 mmol), Rh catalyst (1 or 5 mol% Rh), 508C, 14 h.
[b] Yield of 3aa isolated by GPC. [c] The ee value was determined by HPLC analysis on a chiral-stationary-
phase column. The absolute configuration (R) was determined by X-ray crystal-structure analysis of the
related compound 3cb (see Table 2). [d] The catalyst was generated in situ from [{Rh(OH)(coe)2}2] and
(R)-diene*. [e] The catalyst was generated in situ from [{RhCl(coe)2}2] and (R)-binap. coe=cyclooctene.
asymmetric
phenylation
(Scheme 2). As expected, complete
racemization was observed within
1 h in dioxane/H2O. In toluene/
H2O, the racemization was much
slower. The slow racemization in
this biphasic system may be as-
phenylation product, 4-phenylcyclopentane-1,3-dione 3aa, in
93% yield (Table 1, entry 1). However, unfortunately, 3aa
isolated by gel permeation chromatography (GPC) was
racemic. Product 3aa and some other 4-aryl cyclopentane-
1,3-diones described herein should not be subjected to silica-
gel chromatography because a considerable extent of race-
mization occurs on exposure to silica gel. Racemic 3aa was
also produced with Cs2CO3 in place of KOH (Table 1,
entry 2). Less basic conditions with [{Rh(OH)((R)-
diene*)}2] as the catalyst without an additional base gave
enantiomerically enriched 3aa, although with only 35% ee
(entry 3). We found that 3aa was obtained with a high
ee value of 91%, albeit in low (14%) yield, with [{RhCl((R)-
diene*)}2] as the catalyst in the absence of a base[11] (entry 4).
It may be concluded that the low ee value of the product
obtained under basic conditions is due to the racemization of
3aa, originally formed with high enantioselectivity in the
catalytic asymmetric hydrophenylation. We studied solvent
effects on the catalytic activity and racemization, and found
that a biphasic solvent system consisting of toluene and
H2O[12,13] greatly improved the catalytic activity and main-
tained the high original ee value of product. Thus, the reaction
with [{RhCl((R)-diene*)}2] (5 mol% Rh) in toluene/H2O
(10:1) gave the product 3aa in 85% yield with > 99% ee
(Table 1, entry 5). The amount of catalyst was successfully
reduced to 1 mol% by adding more water to the reaction
system (entries 6 and 7). The best solvent system with
Scheme 2. Racemization of 3aa under basic conditions.
cribed to the low concentration of the inorganic base in the
toluene phase, which contains most of the organic product
3aa.
Under the optimal conditions found for the asymmetric
phenylation of 1a (Table 1, entry 7), reactions of two other
cyclopent-4-ene-1,3-dione substrates and several aryl boronic
acids were performed (Table 2). In reactions of 1a, the
enantiomeric purity of the products was very high (> 99% ee)
for all aryl boronic acids substituted with electron-withdraw-
ing groups, which would promote the racemization, as well as
those with electron-donating groups (Table 2, entries 2–6).
The enantioselectivity in the reactions of 2,2-diphenyl sub-
strate 1b and spiro compound 1c was lower than that for 1a,
but the products were still formed with over 90% ee (entries 7
and 8).
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Table 2: Asymmetric arylation of 2,2-disubstituted cyclopent-4-ene-1,3-
diones 1 with ArB(OH)2 (2) catalyzed by [{RhCl((R)-diene*)}2].[a]
Table 3: Asymmetric arylation of 2-benzyl-2-methylcyclopent-4-ene-1,3-
dione (4a) with ArB(OH)2 (2) catalyzed by [{RhCl((R)-diene*)}2].[a]
Entry R, R in 1
Ar in 2
3
Yield [%][b] ee [%][c]
Entry
Ar in 2
5a/6a
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
Et, Et (1a)
Et, Et (1a)
Et, Et (1a)
Et, Et (1a)
Et, Et (1a)
Et, Et (1a)
Ph (2a)
4-MeC6H4 (2b)
4-MeOC6H4 (2c) 3ac 94
4-BrC6H4 (2d)
4-CF3C6H4 (2e)
1-naphthyl (2 f)
3aa 91
3ab 98
>99
>99
>99
>99
>99
>99
96
1
2[e]
3[g]
4
Ph (2a)
Ph (2a)
Ph (2a)
94:6
78:22
54:46
94:6
95:5
95:5
93:7
95:5
>95:5
95:5
94:6
93:7
93:7
94:6
94:6
90:10
95:5
87 (5aa)
92[f] (5aa)
88[f] (5aa)
89 (5ab)
92 (5ag)
94 (5ah)
89 (5ac)
89 (5ai)
89 (5aj)
92 (5ad)
91 (5ae)
79 (5ak)
90 (5al)
86 (5am)
92 (5af)
84 (5an)
87 (5ao)
>99[d]
97[d]
94[d]
3ad 92
3ae 92
3af
3bb 88
3cb 96
4-MeC6H4 (2b)
3-MeC6H4 (2g)
2-MeC6H4 (2h)
4-MeOC6H4 (2c)
3-MeOC6H4 (2i)
4-PhOC6H4 (2j)
4-BrC6H4 (2d)
4-CF3C6H4 (2e)
3-ClC6H4 (2k)
3,5-Me2C6H3 (2l)
3,4-(OCH2O)C6H3 (2m)
1-naphthyl (2 f)
2-naphthyl (2n)
3-thienyl (2o)
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
93
5
6[h]
7
Ph, Ph (1b) 4-MeC6H4 (2b)
(CH2)4 (1c) 4-MeC6H4 (2b)
93
8
9
[a] Reaction conditions: 1 (0.15 mmol), 2 (0.30 mmol), [{RhCl((R)-
diene*)}2] (1 mol% Rh), toluene (1.0 mL), H2O (0.5 mL). [b] Yield of 3
isolated by GPC. [c] The ee value was determined by HPLC analysis on
chiral-stationary-phase columns. The absolute configuration (R) was
determined by X-ray crystal-structure analysis of 3cb.
10
11
12
13
14
15
16
17[h]
The present catalytic system was applied to the asym-
metric arylation of cyclopent-4-ene-1,3-diones with two
different substituents at the 2-position to generate two
stereogenic centers at the 2- and 4-positions (Table 3). The
reaction of 4a, in which the two substituents are methyl and
benzyl, with PhB(OH)2 (2a) in the presence of [{RhCl((R)-
diene*)}2] in a toluene/H2O biphasic solvent system gave the
diastereomeric phenylation products (2S,4R)-5aa and
(2R,4R)-6aa in a ratio of 94:6 with ꢀ 99 and 74% ee,
respectively, in high yield (Table 3, entry 1).[17] The absolute
configuration (2S) of 5aa was determined by oxidation to the
olefinic compound (S)-7[18] whose configuration was deter-
mined by X-ray crystal-structure analysis[19] (Scheme 3). The
minor product 6aa had the same configuration at the 4-
position and the opposite configuration at the 2-position. The
epimerization of (2S,4R)-5aa with Et3N (Scheme 3), for
which the equilibrated ratio of the diastereoisomers was
around 1:1, gave (2S,4S)-6aa, which is the enantiomer of
(2R,4R)-6aa formed as a minor diastereoisomer in the
asymmetric reaction under the conditions in entry 1 of
Table 3. The observed decrease in the 5aa/6aa ratio and the
concurrent decrease in their ee value under basic conditions
(Table 3, entries 2 and 3) are well accounted for by epime-
[a] Reaction conditions: 4a (0.15 mmol), 2 (0.30 mmol), and [RhCl((R)-
diene*)]2 (1 mol% Rh) in toluene (1.0 mL) and H2O (0.5 mL). [b] Yield of
isolated 5a. [c] The ee values of 5a were determined by HPLC analysis on
chiral-stationary-phase columns. The absolute configuration of 5aa was
determined to be 2S,4R by X-ray crystal-structure analysis of (S)-7 (see
Scheme 3). [d] The ee values of 6aa are 74% ee (2R,4R), 62% ee (2S,4S),
and 90% ee (2S,4S) in entries 1, 2, and 3, respectively. [e] The catalyst
was [{Rh(OH)((R)-diene*)}2] (5 mol% Rh) generated in situ from
[{Rh(OH)(coe)2}2] and (R)-diene*. [f] Yield of a mixture of 5aa and 6aa.
[g] The reaction was carried out with [{RhCl((R)-diene*)}2] (5 mol% Rh)
and KOH (20 mol%) in dioxane (1.0 mL) and H2O (0.1 mL). [h] The
reactions was carried out with ArB(OH)2 (0.45 mmol) and [{RhCl((R)-
diene*)}2] (2 mol% Rh).
rization at the 4-position of the diastereoisomers with
opposite absolute configurations at the 2-position.
The corresponding arylation products 5ab–ao were
obtained from 4a with high enantioselectivity (> 99% ee)
and diastereoselectivity (ꢀ 90:10) with various types of aryl
boronic acids, including those with electron-withdrawing and
electron-donating groups at ortho, meta, and para positions
(Table 3, entries 4–17). Other diketones 4b–f substi-
tuted with methyl and benzylic groups, 4g–j with other
alkyl groups, and the spiro compound 4k were all
found to be appropriate substrates for the asymmetric
phenylation reaction. The corresponding products
were obtained with high enantioselectivity (ꢀ
98% ee; Scheme 4).
Another substrate whose asymmetric arylation
was made possible by the present conditions is 1,2-
dibenzoylethene (10; Scheme 5). Under basic condi-
tions with KOH (20 mol%) in dioxane/H2O, the
phenylation product 11 was not obtained because
the decomposition of 10[20] is faster than the asym-
metric arylation. The reaction with [{RhCl((R)-
diene*)}2] (1 mol% Rh) as a catalyst in toluene/H2O
Scheme 3. Epimerization and transformation of (2S,4R)-5aa.
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The present reaction conditions are also applicable to
other substrates often employed in rhodium-catalyzed asym-
metric arylation reactions.[2] Maleimide, cyclic and acyclic
enones, and a cyclic sulfonylimine were all converted into the
corresponding phenylation products with high enantioselec-
tivity under the conditions used for cyclopent-4-ene-1,3-
diones (Scheme 5).
In summary, the asymmetric hydroarylation was found to
be efficiently catalyzed by a chiral diene-rhodium m-chloro
dimer without a base in toluene/H2O. This reaction system is
particularly useful for the asymmetric synthesis of stereo-
chemically labile a-aryl ketones.
Acknowledgements
We thank the National University of Singapore and the
Ministry of Education (MOE) of Singapore (R-143-000-539-
133) for generous financial support.
Keywords: a-aryl ketones · asymmetric arylation ·
chiral diene ligands · racemization · rhodium
Scheme 4. Asymmetric arylation of 2,2-disubstituted cyclopent-4-ene-
1,3-diones 4 with PhB(OH)2 (2a) catalyzed by [{RhCl((R)-diene*)}2].
Reaction conditions: 4 (0.15 mmol), 2a (0.30 mmol), [{RhCl((R)-
diene*)}2] (1 mol% Rh), toluene (1.0 mL), H2O (0.5 mL). Yields are for
the main diastereoisomer 5 (isolated product). The ee values were
determined by HPLC analysis on chiral-stationary-phase columns. The
absolute configuration was estimated by stereochemical similarity to
the reaction giving 5aa. [a] Yield of a mixture of diastereoisomers 5
and 6. [b] The ee value of the minor diastereoisomer 6ha was 99%.
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6739–6743
Angew. Chem. 2016, 128, 6851–6855
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Scheme 5. Asymmetric phenylation of other substrates.
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this particular substrate, the diene ligand (R)-12[9c] improved
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ditions in toluene/H2O solvent, see: a) Z. Cui, Y.-J. Chen, W.-Y.
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[16] H. Takaya, K. Mashima, K. Koyano, M. Yagi, H. Kumobayashi,
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[17] With 0.15 mmol (1.0 equiv with respect to 4a) of PhB(OH)2, the
yield of 5aa was 66%. When the catalyst loading was reduced to
0.3 mol%, the same yield and selectivity were observed. With
0.1 mol% of the catalyst, the yield was around 10%.
[18] The olefinic compound (S)-7 was further converted into (S)-8
and (1S,2S)-9 with high selectivity to demonstrate its synthetic
utility.
[19] The X-ray crystal structures of (R)-3cb (CCDC 1453040) and
(S)-7 (CCDC 1453039) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[20] Under basic conditions with KOH in dioxane/H2O, 9 undergoes
oligomerization/polymerization, which is probably initiated by
the conjugate addition of hydroxide.
[14] a) Y. Otomaru, K. Okamoto, R. Shintani, T. Hayashi, J. Org.
Chem. 2005, 70, 2503; b) S. Abele, R. Inauen, D. Spielvogel, C.
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Received: February 18, 2016
Revised: March 21, 2016
Published online: April 21, 2016
Angew. Chem. Int. Ed. 2016, 55, 6739 –6743
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6743