Rhodium-catalyzed asymmetric arylative cyclization of meso-1,6-dienynes leading to enantioenriched cis-hydrobenzofurans

Article abstract of DOI:10.1002/anie.201300137

In tandem: The title reaction of cyclohexadienone-containing meso-1,6-dienynes with arylboronic acids through a tandem arylrhodation/ conjugate addition sequence has been realized, and provides a novel approach to the enantioenriched cis-hydrobenzofurans with both excellent yields and enantioselectivities. binap=2,2′-bis(diphenylphosphanyl)-1,1′- binaphthyl, Boc=tert-butoxycarbonyl. Copyright

Full text of DOI:10.1002/anie.201300137

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DOI: 10.1002/anie.201300137
Asymmetric Catalysis
Rhodium-Catalyzed Asymmetric Arylative Cyclization of meso-1,6-
Dienynes Leading to Enantioenriched cis-Hydrobenzofurans**
Zhi-Tao He, Bing Tian, Yuki Fukui, Xiaofeng Tong, Ping Tian,* and Guo-Qiang Lin*
In memory of Wei-Shan Zhou
Chiral cis-hydrobenzofurans represent a unique motif existing
in numerous natural products, for instance, isoambrox,[1]
haterumaimide I,[2] rosenonolactone,[3] incarviditone,[4] and
millingtonine A[5] (Scheme 1a). Owing to their diverse bio-
logical activities,[1–5] a great deal of attention has been paid to
the development of efficient methods toward their enantio-
selective syntheses. One of the most straightforward and
powerful ways to construct such a framework is the catalytic
asymmetric desymmetrization of cyclohexadienones (Sche-
me 1b). In recent elegant reports, Rovis and co-workers
demonstrated the feasibility of using chiral-NHC-catalyzed
intramolecular Stetter reactions for the asymmetric desym-
metrization of cyclohexadienones to prepare chiral cis-hydro-
benzofuranones.[6] Very recently, Sasai and co-workers de-
scribed the use of bifunctional chiral phosphinothiourea
catalysts in a intramolecular Rauhut–Currier reaction for
the enantioselective discrimination of cyclohexadienones.[7]
Although these protocols[6–8] proved to be highly effective,
their efforts were mainly focused on the application of chiral
organocatalysts in intramolecular reactions with a limited
substrate scope. Moreover, transition metal catalyzed asym-
metric desymmetrization of cyclohexadienones is quite
scarce.[9]
During our continuous efforts in exploring rhodium/chiral
diene-catalyzed asymmetric arylation,[10] we envisioned that
a rhodium-catalyzed tandem arylrhodation/conjugate addi-
tion reaction of cyclohexadienone-containing[13,14] 1,6-dien-
ynes,[11,12] which are accessible from dearomatization of
corresponding phenols,[15] would provide a novel approach
to these enantioenriched cis-hydrobenzofurans (Scheme 1c).
However, two major concerns need to be addressed in this
rhodium-catalyzed asymmetric desymmetrization process.
Scheme 1. Catalytic asymmetric desymmetrization of cyclohexa-
dienones for preparing chiral cis-hydrobenzofurans. a) cis-Hydrobenzo-
furans as structural motifs in natural products. b) Organocatalytic
asymmetric desymmetrization of cyclohexadienones. c) Our approach
to enantioenriched cis-hydrobenzofurans.
[*] Z.-T. He, B. Tian, Y. Fukui, Prof. Dr. P. Tian, Prof. G.-Q. Lin
Key Laboratory of Synthetic Chemistry of Natural Substances,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Road, Shanghai 200032 (China)
E-mail: tianping@sioc.ac.cn
lingq@sioc.ac.cn
B. Tian, Prof. Dr. X. Tong
One is the competitive reaction between the arylrhodation
of the carbon–carbon triple bond and two conjugate addition
reactions of the cyclohexadienone with the arylboronic acid.
The other is whether the chiral ligand coordinating to
rhodium could efficiently discriminate between the meso-
cyclohexadienones in the cyclization step.
With this in mind, several varieties of chiral ligands
(Scheme 2) were evaluated for this rhodium-catalyzed asym-
metric tandem arylrhodation/conjugate addition of (4-
Shanghai Key Laboratory of Functional Materials Chemistry
East China University of Science and Technology
130 Meilong Road, Shanghai 200237 (China)
[**] Financial support for this work was generously provided by the 973
Program (2010CB833302), NSFC (21232009, 21102161), and the
Chinese Academy of Sciences (CAS). We thank Dr. Hanqing Dong
for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300137.
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Table 1: Evaluation of chiral ligands for rhodium-catalyzed asymmetric
tandem arylrhodation/conjugate addition of 2b to 1a.[a]
Entry
L
x (mol%)
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L6
L7
L8
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
6.0
7.5
10.0
12.5
17.0
17.0
17.0
17.0
17.5
17.5
17.0
17.0
16.0
22.0
16.0
22.0
16.0
16.0
22.0
17.0
7.0
14
29
38
27
29
16
65
71
24
<5
56
54
36
24
38
71
95
99
88
10
À24
59
À57
57
20
À75
96
9
L9
83
–
93
10
11
12
13
14
15
16
17
18
19
L10
L11
L12
L13
L14
L15
L8
L8
L8
L8
À90
93
83
À60
96
97
99
99
6.0
12.0
[a] The reaction was carried out with 1a (0.1 mmol), 2b (0.3 mmol),
[{RhCl(C2H4)2}2] (2.5 mol%), and chiral ligand (L*, x mol%) in toluene/
H2O (10:1, 4 mL) at 408C. [b] Yield of the isolated product. [c] Deter-
mined by HPLC analysis using a chiral stationary phase.
Scheme 2. Structures of chiral ligands.
observed when L10 was used, thus indicating that the p-acidic
nature of the bisphosphine ligands play a decisive role in this
tandem reaction. This observation is quite similar to the
principle in the Hayashi–Miyaura reaction wherein a strong
p-accepting ability can significantly accelerate the rate-
determining transmetalation step and conjugate addition
reaction.[19] From the above preliminary screening results,
the chiral ligand (R)-binap (L8) was the best choice in this
tandem reaction. Finally, the catalyst loading was optimized
(entries 16–19, Table 1). By employing 10 mol% of (R)-
binap, both nearly perfect yield (99%) and enantioselectivity
(99% ee) were achieved.
With the optimal reaction conditions identified, the scope
of various arylboronic acids 2 was investigated in this Rh/(R)-
binap-catalyzed asymmetric tandem arylrhodation/conjugate
addition to 1a. All 4-substituted and 3-substituted arylboronic
acids, regardless the electron-donating or electron-withdraw-
ing ability of the substitutent at the phenyl ring, gave both
excellent yields and enantioselectivities (entries 2–9, Table 2).
As for the 3,5-disubstituted arylboronic acid 2j and 2-
naphthylboronic acid (2k), the reaction proceeded smoothly,
thus providing the corresponding products with excellent
yields and enantioselectivities (entries 10 and 11, Table 2).
Given the highly enantioselective nature of this method,
we investigated the addition of different 4-substituted aryl-
boronic acids 2 to various meso-1,6-dienynes 1. As for
substrate 1b (R1 = Me, R2 = Et), the tandem reactions with
methoxyphenyl)boronic acid (2b) to the meso-1,6-dienyne
substrate 1a. The reaction was conducted in the presence of
2.5 mol% of [{RhCl(C2H4)2}2] and 5 mol% of a chiral ligand,
and the screening results are summarized in Table 1. We
began with a representative set of chiral diene ligands (L1–
L5;[16] entries 1–5, Table 1). The desired product 3ab was
obtained despite low yield and low to moderate ee values,
which might result from the emulative coordination of 1a and
the chiral diene ligand to rhodium. The sulfoxide olefin hybrid
ligand L6[17] and phosphine olefin hybrid ligand L7[18] were
subsequently examined in this reaction (entries 6 and 7,
Table 1). L7 furnished the cyclization product with better
enantioselectivity, thus indicating that the strong coordination
of the phosphorus atom to rhodium played a role in achieving
good enantioselectivity. Therefore the bisphosphine ligand
(R)-binap (L8) was used in this catalytic tandem reaction. To
our delight, both the reaction yield and the enantioselectivity
were dramatically improved to 71% and 96% ee, respectively
(entry 8, Table 1).
Next, various bisphosphine ligands (L9–L14) and the
monophosphine ligand L15 were investigated to further
improve the enantioselectivity. Unfortunately, they lead to
different levels of erosion in yields and ee values (entries 9–
15, Table 1). Interestingly, almost no desired product was
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Table 2: Rh/(R)-binap-catalyzed asymmetric tandem arylrhodation/con-
jugate addition of various arylboronic acids 2 to 1a.[a]
Table 3: Rh/(R)-binap-catalyzed asymmetric tandem arylrhodation/con-
jugate addition of arylboronic Acids 2 to various meso-1,6-dienynes 1.[a]
Entry
2 (Ar)
t [h]
3
Yield [%][b]
ee [%][c]
1[b]
2[b]
3[b]
4[b]
5
6
7
8
2a (C6H5)
6.0
6.0
18.0
11.0
6.0
8.0
8.0
4.0
18.0
3.0
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
99
99
99
99
92
93
92
99
85
91
94
97
99
98
99
97
95
97
97
96
99
98
2b (4-MeOC6H4)
2c (4-MeC6H4)
2d (4-tBuC6H4)
2e (4-FC6H4)
2 f (4-ClC6H4)
2g (4-BrC6H4)
2h (3-MeC6H4)
2i (3-MeOC6H4)
2j (3,5-Me2C6H3)
2k (2-naphthyl)
Entry
1
2
t [h]
3
Yield [%][e]
ee [%][f]
1[b]
2
1b
1b
1b
1b
1c
1c
1c
1d
1d
1d
1e
1e
1e
1 f
2b
2d
2e
2g
2b
2d
2g
2b
2d
2g
2b
2d
2g
2b
2b
13.0
2.0
17.0
17.0
5.0
3bb
3bd
3be
3bg
3cb
3cd
3cg
3db
3dd
3dg
3eb
3ed
3eg
3 fb
3gb
91
90
95
92
97
85
90
99
84
88
81
85
84
81
80
97
98
97
95
97
99
97
99
98
97
98
98
95
98
98
3
4[c]
5[b]
6
9[b]
10
11[b]
1.0
7[c]
8[b]
9
20.0
25.0
3.0
20.0
2.0
1.0
1.0
17.0
17.0
3aj
3ak
15.0
[a] The reaction was carried out with 1a (0.1 mmol), arylboronic acid 2
(0.3 mmol), [{RhCl(C2H4)2}2] (2.5 mol%), and (R)-binap (10.0 mol%) in
toluene/H2O (10:1, 4 mL) at 608C, unless otherwise noted. [b] At 408C.
[c] Yield of the isolated product. [d] Determined by HPLC analysis using
a chiral stationary phase.
10[c]
11
12
13[c]
14[d]
15[c]
1g
[a] The reaction was carried out with 1 (0.1 mmol), arylboronic acid 2
(0.3 mmol), [{RhCl(C2H4)2}2] (2.5 mol%), and (R)-binap (10.0 mol%) in
toluene/H2O (10:1, 4 mL) at 608C, unless otherwise noted. [b] At 408C.
[c] At 808C. [d] [{RhCl(C2H4)2}2] (3.5 mol%) and (R)-binap (14.0 mol%)
were used. [e] Yield of the isolated product. [f] Determined by HPLC
analysis using a chiral stationary phase. Boc=tert-butoxycarbonyl.
several electronically different 4-substituted arylboronic acids
(2b, 2d, 2e and 2g) provided excellent yields and enantio-
selectivities (entries 1–4, Table 3). Despite bulky R2 or R1
substituents in 1c, 1d, and 1e, the reaction still performed
uniformly with outstanding yields and enantioselectivities
(entries 5–13, Table 3). With a heteroatom (O and N) as part
of R2 in substrate 1, the reaction yields and ee values
remained high (entries 14 and 15, Table 3).
The absolute configurations of two newly formed chiral
centers in 3ag were unambiguously assigned as S and S by X-
ray crystal crystallography (see the Supporting Informa-
tion).[20] The absolute configuration of other products could
be determined as S and S by chemical correlation with (S,S)-
3ag.
On the basis of the above results, a plausible mechanism is
proposed in Scheme 3. Initiation of the reaction through the
transmetalation of an aryl group from boron to the hydroxy-
rhodium A generates the aryl rhodium B, which subsequently
undergoes syn addition to the carbon–carbon triple bond in
1 to afford the vinyl rhodium intermediate C. The favorable
six-membered chair having an axial H atom and equatorial R2
group in C enabled syn-migratory insertion of the vinyl
rhodium across the carbon–carbon double bond in cyclo-
hexadienone, thus forming the oxa-p-allylrhodium intermedi-
ate D, which is readily hydrolyzed under protic conditions to
regenerate A and liberate the product 3. Throughout the
whole catalytic cycle, rhodium maintains an oxidation state of
+ 1.
Scheme 3. Proposed mechanism for this tandem reaction.
As for 3ab, the a,b-unsaturated double bond was selectively
reduced by hydrogenation, and subsequently the tetrasubsti-
tuted double bond was cleaved by ozonolysis to afford the
optically pure tetrahydrobenzofurandione 5ab, which is the
core structure of isoambrox.[1] The oxygen and nitrogen atoms
in 3 fb and 3gb, respectively, provided a handle to construct
tricyclic skeletons through Michael additions. After removing
the acetyl group in 3 fb under basic conditions, an oxa-
Michael addition reaction occurred in situ in a syn fashion to
To check the practical applicability of this tandem
reaction, a half-gram scale reaction of 1a was carried out,
and the excellent yield and ee value were maintained
(Scheme 4). The cyclization products could be further con-
verted into useful core structures related to natural products.
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0.0025 mmol, 2.5 mol%), (R)-binap (L8, 6.2 mg, 0.01 mmol,
10.0 mol%), KHF2 (2.3 mg, 0.03 mmol, 30 mol%), and 2.0 mL of
anhydrous toluene under argon. The resulting mixture was stirred at
room temperature for 30 min. The 1,6-dienyne substrate 1a (17.6 mg,
0.1 mmol) in anhydrous toluene (2.0 mL) was added, and then 0.4 mL
of degassed water was added. After stirring at 408C for 6.0 h, the
reaction mixture was quenched with saturated aqueous NH4Cl
(20 mL), extracted with ethyl acetate (30 mL ꢀ 3), and the combined
organic phases were washed with brine, dried over anhydrous
Na2SO4, filtered, and concentrated in vacuo. The residue was purified
by flash column chromatography on silica gel (EtOAc/hexanes 1:10)
to afford 3ab (28.4 mg, 99% yield, 99% ee) as a colorless oil.
Received: January 8, 2013
Revised: February 21, 2013
Published online: April 16, 2013
Keywords: asymmetric catalysis · cyclization · heterocycles ·
.
rhodium · synthetic methods
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ynes has been developed with high efficiency, thus providing
optically pure cis-hydrobenzofurans with high to excellent
yields (80–99%) and excellent enantioselectivities (95–99%
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applications of the cyclohexadienone-containing meso-1,6-
dienynes are in progress in our laboratories and will be
reported in due course.
Experimental Section
3ab: A dried Schlenk flask was charged with (4-methoxyphenyl)bor-
onic acid 2b (45.6 mg, 0.3 mmol, 3.0 equiv), [{RhCl(C2H4)2}2] (1.0 mg,
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[20] CCDC 916597 (3ag) contains 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.
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