Enantioselective relay catalytic cascade intramolecular hydrosiloxylation and mukaiyama aldol reaction

Article abstract of DOI:10.1002/chem.201300702

Cascading chemistry! The first practical relay catalytic cascade intramolecular hydrosiloxylation of arylacetylene and asymmetric Mukaiyama aldol reaction has been established to give synthetically useful products in high yields and with excellent ee (see scheme). Copyright

Full text of DOI:10.1002/chem.201300702

DOI: 10.1002/chem.201300702
Enantioselective Relay Catalytic Cascade Intramolecular Hydrosiloxylation
and Mukaiyama Aldol Reaction
Pu-Sheng Wang, Kang-Nan Li, Xiao-Le Zhou, Xiang Wu, Zhi-Yong Han, Rui Guo, and
Liu-Zhu Gong*[a]
Lignans are an extremely large class of natural products
found in a variety of plants. Although many lignin-contain-
ing plants have been used for centuries, particularly in
Asian communities, as cures and remedies for various ail-
ments, there is still a growing interest in lignans and their
synthetic derivatives due to applications in cancer chemo-
therapy and various other pharmacological effects.[1] From a
chemical point of view, lignans are a class of secondary
plant metabolites produced by oxidative dimerization of two
phenylpropanoid units, and significant quantities of biologi-
cally active lignans contain the units of arylopropane-1,2 or
1,3-diols, which can basically be accessed from an aldol reac-
tion (Figure 1).[2]
ma aldol reaction, but chiral Brønsted acids have also been
developed recently for this type of reaction.[6] Although
highly elegant, these enantioselective transformations basi-
cally required the preformed enolate derivatives. Over the
last decade, the metal/organo combined catalysis,[7–10] which
principally enables unprecedented transformations and es-
sentially avoids additional laborious workup in purification
procedures of intermediates involved, has been an important
research orientation in synthetic organic chemistry. Recent-
ly, Dixon, Che, and our group found that the combination of
gold complexes and chiral Brønsted acids enabled a variety
of asymmetric cascade reactions.[10] Most recently, we estab-
lished the first catalytic formation of siloxyl dienes from
enynes under mild conditions, which avoid the use of a stoi-
chiometric or more amount of strong bases and allowed for
the realization of an otherwise unreachable cascade intra-
molecular hydrosiloxylation/Diels–Alder reaction under the
relay catalysis of gold complex and chiral Brønsted acid.[10f]
In principal, the intramolecular hydrosiloxylation of alkynes
of type 1 would occur to catalytically generate silyl enol
ethers catalyzed by a gold complex, which would subse-
quently participate in the asymmetric Mukaiyama aldol re-
action in the presence of an appropriate chiral Brønsted
acid. Herein, we will report the first asymmetric relay cata-
lytic cascade intramolecular hydrosiloxylation of arylacety-
lene and Mukaiyama aldol reaction by using a gold complex
and chiral Brønsted acid binary system (Scheme 1).
Figure 1. Lignans containing arylopropane-1,2- or -1,3-diols.
The Mukaiyama aldol reaction, a robust tool to form
carbon–carbon bonds and to access chiral b-hydroxyl car-
bonyls, undoubtedly ranks among the most privileged trans-
formations with a wide array of applications in organic syn-
thesis and has thereby been the subject of intensive investi-
gations for the past three decades.[3] Lewis acid[4] or base[5]
catalysts have been successfully developed for the Mukaiya-
[a] P.-S. Wang, K.-N. Li, X.-L. Zhou, Dr. X. Wu, Dr. Z.-Y. Han, R. Guo,
Prof. Dr. L.-Z. Gong
Hefei National Laboratory for Physical Sciences
at the Microscale and Department of Chemistry
University of Science and Technology of China
Hefei, 230026 (P. R. China)
Fax: (+86)551-360-6266
E-mail: gonglz@ustc.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300702.
Scheme 1. Asymmetric relay catalysis of hydrosiloxylation and the Mu-
kaiyama aldol reaction.
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Chem. Eur. J. 2013, 19, 6234 – 6238

COMMUNICATION
Table 2. Evaluation of Brønsted acid catalysts and optimization of reaction condition-
s.[a]
The dimethyl silyl compounds are convenient to
be converted into other molecules by using classical
procedures, such as the Hiyama coupling reaction,[11]
Tamao–Fleming oxidation,[12] and so on. Therefore,
our initial investigation began with an intramolecu-
lar hydrosiloxylation of 1a catalyzed by a variety of
gold complexes to find the most efficient catalyst
(Table 1). The gold catalysts along with either silver
salts or Brønsted acids were explored. In the pres-
ence of [AuClACHTUNGTRENNUNG(PPh3)]/AgNTf2 and [AuClCAHUTNGTRENN(UGN PPh3)]/
AgOTf, the starting material 1a was completely
consumed within 30 min, but the chemoselectivity
favoring enol silyl ether M1 was moderate as report-
ed previously (Table 1, entries 1 and 2).[10f,13] Al-
though a gold phosphate, generated in situ from
[Au(Me)ACHTUNGTRENNUNG
(PPh3)] and phosphoric acid A1,[14] was also
able to accelerate the intramolecular hydrosiloxyla-
tion reaction, the chemoselectivity was still moder-
ate (Table 1, entry 3). Interestingly, a gold phosphor-
À
Entry
R
L
B* H
Solvent
Yield [%][b]
d.r.[c]
ee [%][d]
0 (17)[e]
0 (15)
14 (43)
29 (13)
30 (20)
53 (32)
84 (43)
84 (44)
84 (43)
86 (38)[g]
87 (40)[g,h]
1
2
3
4
5
6
7
8
9
Et
Et
Et
Et
L1
L1
L1
L1
L1
L1
L1
L2
L3
L2
L2
A2
A2
A3
A4
A5
A3
A3
A3
A3
A3
A3
DCM
DCM
DCM
DCM
DCM
DCM
toluene
toluene
toluene
toluene
toluene
19
91
93
71
12
52
51
97
82
90
94
1.3:1
1.3:1
1.5:1
1.3:1
1.3:1
3:1
5:1
5:1
5:1
5:1
ACHTUNGTRENNUNGamide, in situ generated from [Au(Me)HCATUNGTREN(NGUN PPh3)] and
N-triflyl phosphoramide (A2),[15] could dominantly
provide the desired enol silyl ether M1, but the re-
action proceeded incompletely (Table 1, entry 4). To
our delight, either the gold complex with a bulky
ligand or N-triflyl phosphoramide with bulky sub-
stituents was able to provide a cleaner reaction, effi-
ciently inhibiting the homocoupling process
(Table 1, entries 5 and 6). Notably, the use of N-het-
erocyclic carbene (NHC) L2 as a the ligand[16] for
the gold complex rendered a cleaner reaction in
nearly perfect yield and chemoselectivity (Table 1,
entry 7).
Et
Flu[f]
Flu
Flu
Flu
Flu
Flu
10
11
5:1
[a] The reaction was carried out in 0.025m of 1a. [b] Isolated yield. [c] Determined by
1H NMR spectroscopy. [d] Determined by HPLC analysis and the ee values in paren-
thesis were observed for the minor diastereomer. [e] 5% A2 was used. [f] Flu=fluo-
renyl. [g] The reaction was carried out at 258C. [h] 15% A3 was used.
After establishing the optimized conditions for ef-
ficient and chemoselective generation of the enol
silyl ether, we turned our attention to the asymmetric relay
catalytic cascade reaction between the silanol 1a and glyox-
ylate 2 (Table 2). Unfortunately, the gold phosphorACTHNUGRTNEUNGamide, in
situ generated from 5 mol% of [Au(L1)(Me)] and 5 mol%
of N-triflyl phosphoramide A2, turned out to be ineffective
for the reaction, giving a poor yield and low stereoselecti-
vites (Table 2, entry 1). To our delight, a drastic increase in
the yield was observed when using 5 mol% of
[Au(L1)(Me)] and 10 mol% of N-triflyl phosphoramide A2
(Table 2, entry 2), which implied that the Brønsted acid can
dramatically accelerate the aldol reaction. Encouraged by
these preliminary results, various structurally diverse chiral
N-triflyl phosphoramides (A2–A5) derived from 3,3’-disub-
stituted 1,1’-bi-2-naphthols (BINOLs) were evaluated to
identify the best organocatalyst. However, only moderate
entioselectivity and very low diastereoselectivity was ob-
tained (Table 2, entries 2–5). Fluorenyl glyoxylate was found
to provide a higher stereoslectivity probably due to the
bulky substitution effect, but with a significant erosion of
the yield (Table 2, entry 6). Fortunately, the variation of the
solvent from dichloromethane to toluene led to an implausi-
ble improvement in the reaction performance (Table 2,
entry 7). Interestingly, the achiral ligands of gold complexes
exerted a considerable effect on the yield, but exhibit very
slight influence on the stereoslectivity (Table 2, entries 7–9).
NHCs turned out to be better ligands than phosphines,
Table 1. Gold-catalyzed intramolecular hydrosiloxylation.[a]
Entry
AuI
[AuCl
[AuCl(PPh3)]/AgOTf
t [h]
Conv. [%][b]
M1/M2[b]
1
2
3
4
5
6
7
G
(PPh3)]/AgNTf2
0.5
0.5
24
16
16
8
>99
>99
30
40:60
83:17
75:25
E
[Au(Me)
[Au(Me)
U
64
93:7
[Au(L1)(Me)]/A2
[Au(L1)(Me)]/A3
[Au(L2)(Me)]/A3
>99
>99
>99
>99:1
>99:1
>99:1
4
[a] Unless otherwise indicated, the reaction of 1a (0.05m) was carried out
in the presence of a gold catalyst (5 mol%) at 358C (DCM=dichloro-
ACHTUNGTRENNUNGmethACHTUNGTRENNUNGane). [b] The conversion of the reaction and the ratio of M1/M2
1
were measured by H NMR spectroscopy of the crude after removing the
solvent.
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L.-Z. Gong et al.
giving high yields without sacrificing stereoselection
(Table 2, entries 8 and 9). The additional optimization of the
reaction conditions revealed that lowering the temperature
resulted in a slightly enhanced enantioselectivity of 86% ee
(ee=enantiomeric excess), but the yield dropped a small
amount (Table 2, entry 10). Finally, the results (94% yield,
87% ee) could be further improved by using 15 mol% of N-
triflyl phosphoramide A3 (Table 2, entry 11).
synthesis. For example, the Baeyer–Villiger reaction of 3b
with meta-chloroperoxybenzoic acid (mCPBA) and
Na2HPO4·12H2O in dichloroethane (DCE) afforded 4 in a
75% yield with retained stereochemistry. The enantiopure 4
was obtained by a single recrystallization. The conversion of
4 into the Weinreb amide 5, which was suitable to grow a
single crystal for the X-ray crystallographic analysis, allowed
the assignment of the absolute configurations of 3b to be
3S,4S (Scheme 2).[18]
Under the optimized reaction conditions, we explored the
generality for the substrates (Table 3). Encouragingly, 4,5-di-
methoxyphenyl silanol 1b underwent a smooth sequential
Moreover, the removal of the flurenyl group in 4 by hy-
Table 3. Scope of the substrates.[a]
Entry 1 (R1, R2)
3
Yield [%][b] d.r.[c] ee [%][d]
1
2
3
4
5
6
7
8
9
1b (4,5-MeO, Ph)
1c (4,5-MeO, 2’-MeOC6H4) 3c
1d (4,5-MeO, 4’-MeC6H4)
1e (4,5-MeO, 4’-FC6H4)
1 f (4,5-MeO, 3’-MeOC6H4) 3 f
1g (4,5-MeO, 2’-naphthyl)
1h (4,5-MeO, 2’-furanyl)
1i (4,5-Me, Ph)
1j (5-Me, Ph)
1k (5-iPr, Ph)
3b
87
70
78
95
73
92
77
80
97
95
83
90
10:1 93
7:1 94
10:1 87
10:1 88
5:1 85[e]
3:1 76
9:1 77
9:1 94
6:1 90
7:1 87
5:1 88
11:1 94
3d
3e
3g
3h
3i
3j
3k
3l
Scheme 2. Reaction conditions: a) mCPBA, Na2HPO4·12H2O, DCE,
508C, 3 h; b) H2, Pd/C, EtOAc, RT, 5 h; c) MeONHMe·HCl, N-methyl-
morpholine, N’-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydro
AHCTUNGTERGrNNUN ide (EDC·HCl), DCM, 08C, 2 h.
ACHTUNGTRENNUNGchlo-
10
11
12
1l (5-Cl, Ph)
1m (4-MeO, Ph)
3m
drogenolysis on Pd/C and reduction of the corresponding
acid furnished the primary alcohol 6 in an overall 90%
yield. After the classical transformations, including hydroly-
sis of the benzoyl group and removal of the siloxy group
under mild conditions, the veratryl glycerol 7 was conve-
[a] The reaction was carried out in 0.025m. [b] Isolated yield. [c] Deter-
mined by 1H NMR spectroscopy. [d] Determined by HPLC analysis.
[e] The reaction was carried out at 358C.
intramolecular hydrosiloxylation and asymmetric aldol reac-
tion to provide product 3b in a high yield and with excellent
enantioselectivity (87% yield, 93% ee) and a 10:1 diastereo-
meric ratio (d.r.; Table 3, entry 1). Significantly, the asym-
metric relay catalytic reaction tolerated a wide variety of (2-
arylethynyl)4,5-dimethoxyphenyl silanols, giving rise to the
desired products 3c–h in up to 95% yields with up to
94% ee (Table 3, entries 2–7). (2-Phenylethynyl)aryl silanols
containing both electron-withdrawing and -donating groups
on the aromatic ring proceeded smoothly to give the aldol
adducts in good to excellent yields and with excellent enan-
tioselectivities (Table 3, entries 8–12).
ACHTUNGTRENNUNG
the enol silyl ether, but also an active intermediate for a va-
riety of transformations to construct more complex struc-
tures. For instance, the primary alcohol 6 was able to partici-
pate in a Pd-catalyzed intermolecular Hiyama coupling with
4-iodo-1,2-dimethoxybenzene followed by a hydrolysis of
the benzoyl group to form the a optically active triol 8,
which might serve as a potential intermediate towards the
synthesis of metasequirin B (Scheme 4).
In summary, we have developed a novel and practical
relay catalytic cascade intramolecular hydrosiloxylation of
arylacetylenes and asymmetric Mukaiyama aldol reactions
Notably, when the reaction of 1b and 2b was carried out
on a 10 g scale in the presence
of 1 mol% [Au(L2)(Me)] and
3 mol% A3 it still proceeded
smoothly to give the desired
product in 64% yield and with
a 10:1 d.r. and 91% ee.[17] The
feasibility of scale-up actually
enhances the practical applica-
tions of this protocol in organic
Scheme 3. Enantioselective synthesis of veratryl glycerol: a) H2, Pd/C, EtOAc, RT, 5 h; b) THF, BH3·THF, RT,
12 h; c) Na2CO3, MeOH, RT, 5 h, 78%; d) tetrabutylammonium fluoride (TBAF), THF, refluex, 5 h, 62%.
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Intramolecular Hydrosiloxylation and Mukaiyama Aldol Reaction
COMMUNICATION
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focused on the use of this relay catalytic reaction for the
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Experimental Section
General procedure:
A mixture of [AuACHTUNTGRENUNG(IPr)(Me)] (5 mol%), N-trifyl
phosphoamide (15 mol%), silanol 1 (0.05 mmol), and glyoxylate ester 2
(0.1 mmol) were added in toluene (2 mL) in one portion under argon.
The resulting mixture was allowed to stir at 258C for 36 h, and then the
solution was subjected to flash column chromatography on silica gel (pe-
troleum ether/ethyl acetate) directly to afford product 3.
Acknowledgements
We are grateful for financial support from the NSFC (21232007 and
21172207),
MOST
(973
project
2010CB833300),
FRFCU
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Keywords: Brønsted acids
·
gold catalysis
·
Hiyama
coupling · Mukaiyama aldol reaction · relay catalysis
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[18] CCDC-921638 contains the supplementary crystallographic data for
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data_request/cif.
Received: February 22, 2013
Published online: March 22, 2013
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