Hybrid metal/organo relay catalysis enables enynes to be latent dienes for asymmetric diels-alder reaction

Article abstract of DOI:10.1021/ja3007148

The hybrid Au(I)/Bronsted acid binary catalyst system enables enynes to serve as latent 1,3-silyloxydienes capable of participating in the first cascade hydrosiloxylation of an enynyl silanol/asymmetric Diels-Alder reaction. A variety of polycyclic compounds bearing multistereogenic centers were obtained in high yields and excellent enantioselectivities from the relay catalytic cascade reaction between (2-(but-3-en-1-ynyl)phenyl) silanols and quinones catalyzed by the combined achiral gold complex and chiral N-triflyl phosphoramide.

Full text of DOI:10.1021/ja3007148

Communication
pubs.acs.org/JACS
Hybrid Metal/Organo Relay Catalysis Enables Enynes To Be Latent
Dienes for Asymmetric Diels−Alder Reaction
Zhi-Yong Han, Dian-Feng Chen, Ya-Yi Wang, Rui Guo, Pu-Sheng Wang, Chao Wang,
and Liu-Zhu Gong*
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science and
Technology of China, Hefei, 230026, China
S
* Supporting Information
development of asymmetric relay catalytic transformations by
using metal/organo binary catalyst systems.4−6 Indeed, such a
ABSTRACT: The hybrid Au(I)/Brønsted acid binary
catalyst system enables enynes to serve as latent 1,3-
silyloxydienes capable of participating in the first cascade
hydrosiloxylation of an enynyl silanol/asymmetric Diels−
Alder reaction. A variety of polycyclic compounds bearing
multistereogenic centers were obtained in high yields and
excellent enantioselectivities from the relay catalytic
cascade reaction between (2-(but-3-en-1-ynyl)phenyl)
silanols and quinones catalyzed by the combined achiral
gold complex and chiral N-triflyl phosphoramide.
strategy has rendered the birth of a large number of
unprecedented enantioselective processes and thereby has
become an important research orientation in synthetic organic
chemistry. We have found that gold(I) complexes and
phosphoric acids are highly compatible and, more importantly,
can synergistically afford some asymmetric cascade reac-
tions.5d,h,k Moreover, the gold-catalyzed hydroxylation of a
carbon−carbon triple bond has been well established.7 Thus,
we envisioned that the gold complex might be able to catalyze
an intramolecular hydrosiloxylation of enynyl silanol of type A,
giving a reactive silyloxydiene C, which could subsequently
participate in an asymmetric Diels−Alder reaction with an
electron deficient olefin B catalyzed by a chiral Brønsted acid
(Scheme 2). As such, an asymmetric Diels−Alder reaction
using enynes as latent 1,3-silyloxydienes turns out to be feasible.
he Diels−Alder reaction is undoubtedly one of the most
T
important and fascinating transformations in organic
chemistry.1 1,3-Silyloxydienes have proven to be extraordinarily
useful building blocks due to their high reactivity in a Diels−
Alder reaction, providing an efficient entry to cyclohexanone
derivatives.2 So far, the most used method to prepare dienes of
this type is the silylation of unsaturated carbonyls (Scheme 1).3
Scheme 2. Combination of Catalytic Diene Generation and
Asymmetric Diels−Alder Reaction in One Operation
through Metal/Organo Relay Catalysis
Scheme 1. Classical Synthesis of 1,3-Silyloxydienes for
Diels−Alder Reaction
Although it is reliable and practical, the method basically
requires a stoichiometric or more amounts of strong bases. A
Diels−Alder reaction of 2-siloxy-1,3-butadiene catalytically
generated from readily available precursors would be a highly
appealing alternative and is arguably valuable in organic
synthesis. However, an approach to realize such a Diels−
Alder reaction has not been available, yet. As a proof of
concept, we will herein describe a hybrid metal/organo relay
catalytic reaction that comprises the first catalytic formation of
1,3-silyloxydiene from enynes and a Brønsted acid catalyzed
asymmetric Diels−Alder reaction.
The greatest challenge that needs to circumvent for the
validation of the concept is to find mild conditions that tolerate
both the catalytic generation of dienes and the subsequent
asymmetric Diels−Alder reaction. However, the classical
synthesis of 1,3-silyloxydienes including the method shown in
Scheme 1 is apparently far from requisites to realize the one-pot
sequential diene formation and asymmetric Diels−Alder
reaction. Recently, many endeavors have been devoted to the
Although the addition of either water or alcohols across a
carbon−carbon triple bond has long been established by using
transition metal catalysts,7,8 surprisingly, similar addition
reactions using silanols as nucleophiles that produce syntheti-
cally useful silyl enol ethers or 1,3-silyloxydienes (with enynes)
have been rarely reported.9 To validate the concept, an
intramolecular hydrosiloxylation of (2-(but-3-en-1-ynyl)-
phenyl)dimethylsilanol (1a) was first explored using gold
Received: January 21, 2012
Published: April 4, 2012
© 2012 American Chemical Society
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Communication
complexes as catalysts.10 However, the initial trials had met
with problems. Under the catalysis of either Ph3PAuOTf or
Ph3PAuNTf2, the enynyl silanol 1a was completely converted
into silicon ether 3a at room temperature (Table 1, entries 1
Table 2. Evaluation of Brønsted Acid Catalysts and
Optimization of Reaction Conditions
a
Table 1. Gold Catalyzed Intramolecular Hydrosiloxylation of
a
(2-(But-3-en-1-ynyl)phenyl) Dimethylsilanol (1a)
b
entry
Au(I)
time (h)
solvent
conv. (%)
2a/3a
1
2
3
4
5
6
Ph3PAuOTf
5
5
5
5
8
8
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
PhCH3
PhF
>99
>99
33
<1:99
<1:99
Ph3PAuNTf2
c
Ph3PAuMe/6a
50:50
85:15
>95:5
>95:5
b
c
entry
1
solvent
B*−H
time (h)
yield (%)
ee (%)
8
8
8
>99
>99
>99
1
1a
1a
1a
1a
1a
1a
1a
1b
1c
1c
1c
1c
1c
1c
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
toluene
PhF
6a
6b
6c
7a
7b
7c
7d
7c
7c
7c
7d
7d
7d
−
72
72
72
72
72
72
72
96
72
48
9
54
3
2
trace
30
−
a
3
27
13
56
71
65
67
79
83
Unless indicated otherwise, the reaction of 1a (0.05 mmol) was
carried out in presence of a gold catalyst (6 mol %) at room
temperature. The conversion of the reaction and the ratio of 2a/3a
were determined by H NMR. 6 mol % of 6a was used.
4
52
b
5
22
c
1
6
82
7
76
8
57
and 2). This homocoupling of silanols probably impeded the
exploration of the hydrosiloxylation reaction between silanol
and alkynes. Encouragingly, a gold phosphate, which was
generated in situ from Ph3PAuMe and phosphoric acid 6a,11
was able to yield a mixture of the desired 1,1-dimethyl-3-vinyl-
1H-benzo[c][1,2]oxasiline 2a and the silicon ether 3a although
the reaction proceed sluggishly (entry 3). To our delight, the
gold complex 8 prepared from a bulky phosphine ligand12 was
able to efficiently inhibit the homocoupling process and
provided the silyloxydiene 2a in majority (entry 4). The
screening of solvents found that either toluene or fluoroben-
zene was suitable media to give a high chemoselectivity in favor
of the desired diene 2a (>95:5, entries 5−6).
9
68
10
11
12
13
14
65
d
81
88
de
,
PhF
12
11
240
65
89
89
−
d f
−
PhF
85
ef
,
PhF
trace
a
Unless indicated otherwise, the reaction of 1 (0.05 mmol) and 4 (0.1
mmol) was carried out at 35 °C in the presence of gold catalyst (6 mol
%) and Brønsted acid (10 mol %). Isolated yield. The ee was
determined by HPLC. 15 mol % of Brønsted acid was used. The
reaction was carried out at room temperature. 0.2 mmol of 4 was
b
c
d
e
f
used.
After establishing the optimized conditions for the efficient
and chemoselective generation of the 1,3-silyloxydiene, we
turned our attention to the asymmetric relay catalytic cascade
reaction between the enynyl silanol 1a and naphthoquinone 4
in the presence of the combined catalyst of the gold complex 8
and a chiral Brønsted acid (Table 2). Delightedly, the chiral
phosphoric acid 6a indeed rendered the cascade reaction to
proceed in dichloroethane. The desired polycyclic compound
5a was isolated as a single diastereomer in a moderate yield, but
with poor enantioselectivity (entry 1). More interestingly, in
comparison with the usual Diels−Alder reaction, an olefin
migration occurred in this case presumably due to the
conjugated stabilization energy between the aryl group and
the carbon−carbon double bond. However, an effort to identify
the optimal catalyst by evaluating 3,3′-disubstituted BINOL-
based phosphoric acid failed to attain satisfactory results
(entries 2 and 3). Thus, we turned our attention to examining
chiral N-triflyl phosphoramides,13 which have proven to be
efficient catalysts for the asymmetric Diels−Alder reaction
between linear 1,3-silyloxydienes and ethyl vinyl ketone as
demonstrated by Yamamoto and co-workers.14 To our delight,
a drastic increase in both yield and enantioselectivity was
observed using these stronger Brønsted acids compared to the
corresponding phosphoric acids (entries 4−7). Notably, upon
exploitation of phosphoramide 7c as a catalyst, the product 5a
can be obtained in 82% yield and with 71% ee (entry 6). We
then investigated the reaction involving (2-(but-3-en-1-ynyl)-
phenyl)diethylsilanol (1b) and (2-(but-3-en-1-ynyl)phenyl)-
diphenylsilanol (1c) under the catalysis of 7c (entries 8 and 9).
An improved enantioselectivity was observed in the case using
1c as the substrate (entry 9). Variation of the solvent from
dichloromethane to toluene could further improve the
enantioselectivity from 79% to 83% ee (entry 9 vs 10). In
particular, the product 5c could be obtained in 81% yield and
88% ee using 3,3′-di(1-pyrenyl) BINOL-based phosphoramide
7d as the catalyst in fluorobenzene (entry 11). Finally, a further
enhanced result (85% yield, 89% ee) could be achieved by
tuning the equivalent of naphthoquinone and using 15 mol %
of 7d (entries 12−13). Interestingly, the Diels−Alder reaction
between the 1,3-silyloxydiene 2c in situ generated from 1c and
4 did not occur in the presence of the gold complex alone
(entry 14), implying that the Diels−Alder reaction was solely
accelerated by the Brønsted acid.
Under the optimized reaction conditions, we explored the
generality of this asymmetric relay catalytic cascade reaction. As
shown in Table 3, a (2-(but-3-en-1-ynyl)phenyl)diphenylsilanol
derivative 1d bearing a phenyl group at C4 underwent a
smooth sequential intramolecular hydrosiloxylation/asymmet-
ric Diels−Alder reaction to provide product 5d in an excellent
yield and enantioselectivity (95% yield, 94% ee). Significantly,
the asymmetric relay catalytic reaction tolerated a wide
spectrum of enynyl diphenylsilanols bearing either an
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a
In addition to naphthoquinone 4, benzoquinone derivatives
10a−10c also showed good reactivity toward the 1,3-
silyloxydiene catalytically generated from 1q and could
participate in a smooth relay catalytic cascade reaction, after a
one-pot diastereoselective reduction with diisobutylaluminium
hydride to give 11a to 11c in 69−82% yields and with 93−96%
ee (eq 1). The relative configuration of 11 was tentatively
assigned by NOESY analysis (see SI).
Table 3. Substrate Scope
In conclusion, we have demonstrated that the hybrid Au(I)/
Brønsted acid binary catalyst system enables both (2-(but-3-en-
1-ynyl)phenyl) dialkylsilanol and (2-(but-3-en-1-ynyl)phenyl)
diarylsilanol derivatives to be latent 2-siloxy1,3-butadienes and
allows them to participate in the first asymmetric relay catalytic
cascade intramolecular hydrosiloxylation/asymmetric Diels−
Alder reaction. This relay catalytic process provides an
unprecedented alternative to traditional Diels−Alder reactions
for the efficient preparation of highly enantioenriched
polycyclic compounds. Moreover, we developed a new 3,3′-
di-(1-pyrenyl) BINOL-based phosphoramide, which may
potentially be a useful catalyst for asymmetric Diels−Alder
reactions. Further investigations focused on tandem reactions
involving intermolecular alkyne hydrosiloxylation are underway.
ASSOCIATED CONTENT
* Supporting Information
■
S
Complete experimental procedures and characterization data
for the prepared compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
gonglz@ustc.edu.cn
■
a
Unless indicated otherwise, the reaction of 1 (0.05 mmol) and 4 (0.2
Notes
mmol) was carried out at room temperature in the presence of 8 (6
mol %) and 7d (15 mol %) for 12 h.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from NSFC (21172207),
MOST (973 Project 2010CB833300), and the Ministry of
Education.
electron-donating or -withdrawing aryl substituent at C4, giving
rise to the desired polycyclic compound 5e−5k in high yields of
up to 98% and excellent enantioselectivities of up to 96% ee.
The variation of the substituent on the phenylacetylene moiety
was also tolerable (5l and 5m). The enynyl diphenylsilanols
with an alkyl substituent at C3 also underwent the cascade
reaction cleanly, furnishing the desired products in high yields
(67−94%) and excellent enantioselectivities (91% ee). Interest-
ingly, enynyl dimethylsilanols 1q and 1r with an aryl substituent
at C4 also provided the products 5q and 5r with satisfactory
results (90−92% yields and 95% ee's). Notably, scale-up
reactions of 1d (up to 804 mg) with naphthoquinone 4 also
proceeded smoothly to give 5d in high yield without erosion of
the enantioselectivity (see Supporting Information (SI)). The
configurations of 5d were determined by X-ray crystallography
analysis of its monoketoxime derivative 9, and the relative
configuration of 5n was tentatively assigned to be syn by
NOESY analysis (see SI).
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