Catalytic asymmetric [3+2] annulation of allylsilanes with isatins: Synthesis of spirooxindoles

Article abstract of DOI:10.1002/anie.201105739

Silyl-inspired spirocycle: The title reaction is the first example of a catalytic asymmetric [3+2] annulation reaction with allylsilanes. The annulation reaction utilizes a chiral ScCl₂(SbF₆)/L catalyst and TMSCl as a promoter to aff

Full text of DOI:10.1002/anie.201105739

Angewandte
Chemie
DOI: 10.1002/anie.201105739
Synthetic Methods
Catalytic Asymmetric [3+2] Annulation of Allylsilanes with Isatins:
Synthesis of Spirooxindoles**
Nadine V. Hanhan, Nicolas R. Ball-Jones, Ngon T. Tran, and Annaliese K. Franz*
Allylsilanes are readily available, nontoxic, and versatile
reagents for organic synthesis. Although fairly weak nucleo-
philes, allylsilanes exhibit a dynamic reactivity pattern that is
dependent upon the electronic and steric properties of the
silyl group, the electrophilic partner, and the reaction
conditions.[1] Two primary pathways are known for the
silanes.[5] This enantioselective annulation represents a par-
ticular challenge because Lewis acid catalysts often favor the
competing allylation pathway, and a balance of reactivity and
product selectivity is required. Herein we report the first
example of a catalytic asymmetric [3+2] annulation of
allylsilanes and develop a method to access silyl- and
hydroxy-substituted spirooxindoles with superb enantioselec-
tivity.
We chose to study the catalytic asymmetric annulation of
allylsilanes with isatin electrophiles as a route to access
spirooxindoles because of the important biological activities
of this class of compounds.[6,7] We examined a wide variety of
chiral Lewis acid metal complexes (Pd, Cu, Ti, Sc, In, etc.) to
find a catalyst that would provide sufficient activation of the
electrophile to compensate for the relatively weak nucleo-
philicity of allylsilanes without favoring formation of the
allylation product. On the basis of our previous studies
investigating asymmetric additions to isatins, we envisioned
that a scandium(III)/L complex[8,9] could be optimized for the
annulation reaction with allyltriisopropylsilane (Table 1);
however, several variations of the Sc(OTf)3/L complex did
not provide the necessary reactivity (entries 1, 2).
=
addition of an allylsilane to a C X p electrophile in the
presence of a Lewis acid: 1) an elimination pathway to afford
allylation products (Hosomi–Sakurai reaction), and 2) a
pathway wherein the allylsilane acts as a three-carbon unit
in a [3+2] annulation reaction to afford cyclized products
(Scheme 1).[2] The silyl group enhances the nucleophilicity of
the alkene and stabilizes the b-carbocation, which is formed
We proceeded to investigate additives to enhance the
reactivity of the scandium(III) catalysts, and found that upon
addition of both AgSbF6 and TMSCl the annulation and
allylation products (3 and 4, respectively) were observed in a
ratio of 39:61, each with excellent enantioselectivity (Table 1,
entries 3–5). By using a ScCl3 complex in place of Sc(OTf)3,[10]
the annulation product was obtained as the major product in
an 85:15 ratio using CH2Cl2 solvent (entry 8). The annulation
product is obtained with a consistently high (95:5) diastereo-
selectivty. Although the counterion is essential for the desired
reactivity, additional investigations demonstrated that
increasing the amount of the counterion favors the competing
allylation pathway, even with the bulky allyltriisopropylsilane
(entries 8–10). Using NaSbF6 demonstrated that there is no
specific dependency on silver salts (entry 13).[11] Finally, other
additives were also investigated (entries 7, 11, 14, and 15), but
TMSCl was ideal for both rate enhancement and product
ratio.[12,13] Through this screening process, it was established
that: 1) a silyl chloride is an essential activator for this
reaction, 2) the halide ligands and counterion of the scandium
complex play a significant role in controlling the pathway
selectivity between the competing annulation and allylation
reactions, and 3) the diastereoselectivity and enantioselectiv-
ity for the annulation product are excellent under all reaction
conditions.
Scheme 1. Mechanism of allylation versus annulation pathways.
Bn=benyzl, Ts=4-toluenesulfonyl.
upon initial attack, through s–p hyperconjugation (referred to
as the b-silyl effect).[3] The [3+2] allylsilane annulation
reaction represents a powerful method for efficient stereose-
lective synthesis of complex heterocycles and carbocycles;
however, catalytic asymmetric variants have remained elu-
sive.[4] In contrast, numerous methods have been reported
that describe enantioselective allylation reactions with allyl-
[*] N. V. Hanhan, N. R. Ball-Jones, N. T. Tran, Prof. A. K. Franz
Department of Chemistry, University of California
One Shields Avenue, Davis, CA 95616 (USA)
E-mail: akfranz@ucdavis.edu
Homepage: http://chemgroups.ucdavis.edu/~franz/
[**] This research was supported by the University of California, Davis
and the Donors of the American Chemical Society Petroleum
Research Fund (49I8I-DNI1). A.K.F. acknowledges the 3M Corpo-
ration for a Nontenured Faculty Award. N.V.H. is a recipient of the
Eugene Cota-Robles, the Bradford Borge, and the Bryan Miller
Graduate Fellowships, and N.T.T. is a recipient of the Bradford
Borge Chemistry Fellowship and a Department of Education
GAANN Fellowship. We would also like to acknowledge Dr. James C.
Fettinger for consultations regarding X-ray analysis.
With optimized reaction conditions for the selective
formation of the spirocyclic annulation product 3 in hand,
we explored the scope of isatins for this reaction (Table 2). In
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105739.
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Table 1: Optimization of enantioselective allylsilane annulation.
Table 2: Scope of isatins for the allylsilane annulation.[a]
Entry
1
R1
R2
t [h]
3
Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
9
1a CH3
1b CH3
1c CH3
1d CH3
1e CH3
5-Br
5-Cl
4-Cl
5-F
5-OCH3 72
5-OCF3
5-F
24
24
24
48
3a 71
3b 74
3c 62[d]
3d 67
3e 47[e]
99
99
97
99
99
99
99
99
97
97
Entry
X
Counterion
source (mol%)
Additive
Solvent 3/4[a]
ee [%][b] (3)
1
2
3
4
5
6
7
8
OTf
–
–
–
CH2Cl2 n.r.
CH2Cl2 n.r.
CH2Cl2 n.r.
–
–
–
OTf AgSbF6 (10)
OTf
1 f
Bn
48
48
72
24
48
3 f
82
–
TMSCl
TMSCl
TMSCl
TMSCl
1g PMB
1h CH3
3g 55
3h 47
OTf AgSbF6 (10)
OTf AgSbF6 (10)
Cl AgSbF6 (10)
Cl AgSbF6 (10)
Cl AgSbF6 (10)
Cl AgSbF6 (17)
Cl AgSbF6 (25)
Cl AgSbF6 (10)
Cl[d] AgSbF6 (10)
Cl[d] NaSbF6 (10)
Cl[d] NaSbF6 (10)
Cl[d] NaSbF6 (10)
CH2Cl2 31:69 96[c]
CH3CN 39:61 99
CH3CN 58:42 99
H
H
1i
1j
Ph
3i
3j
81
72
10
CH2CCH 5-F
PhMe2SiCl CH3CN 47:53 99
[a] All reactions performed under argon with ScCl3(THF)3, NaSbF6, and
3 equiv of the allylsilane 2a with ligand L. Thermal ellipsoids of the X-ray
structure of 3h are shown at 50% probability. [b] Yield of isolated
annulation product containing trace amounts of the minor diastereomer
as determined by 1H NMR spectroscopy. [c] The ee value was determined
by HPLC analysis using an AD-H column. [d] A 71:29 mixture of
diastereomers was isolated as determined by HPLC analysis; ee value is
reported for the major diastereomer. [e] Reaction performed using
AgSbF6. PMB=para-methoxybenzyl.
TMSCl
TMSCl
TMSCl
TMSOTf
TMSCl
TMSCl
CH2Cl2 85:15 99
CH2Cl2 68:32 99
CH2Cl2 48:52 99
9
10
11
12
13
14
15
CH3CN 0:100
–
CH2Cl2 78:22 99
CH2Cl2 82:18[e] 99
PhMe2SiCl CH2Cl2 78:22 99
PhCOCl
CH2Cl2 90:10[f] 99
[a] Product ratios were determined using 1H NMR spectroscopy or HPLC
analysis. [b] The ee value was determined by HPLC analysis using a chiral
AD-H column. [c] Allylation proceeded with 90% ee. [d] Entries 6–11
were performed using ScCl3, and entries 12–15 were performed using a
ScCl3(THF)3 complex. [e] The product ratio is an average of five
experiments. [f] PhCOCl provides rate enhancement, compared to
conditions in entry 2, but only a 50% yield of 3 and with an 88:12
diastereomeric ratio (average of two experiments). L=2,6-bis[(3aS,8aR)-
3a,8a-dihydro-8H-indeno[1,2-d]oxazolin-2-yl]pyridine, n.r.=no reaction,
Tf =trifluoromethanesulfonyl, TMS=trimethylsilyl.
containing a “removable” aromatic moiety so that the silyl
group can be easily converted into a hydroxy group under
mild oxidation conditions.[16] We screened several allylsilanes
having different steric and electronic properties with the aim
of identifying an allylsilane that favors the annulation path-
way and also contains an oxidizable silane group (Table 3).[17]
All annulation reactions maintained excellent enantioselec-
tivity (99% ee) with these reaction conditions. Although we
expected that various allylsilanes would favor the annulation
product, the allylation pathway is remarkably persistent even
for bulky silyl groups.[18] Even replacing one isopropyl group
on the silicon with a para-anisole group led to a drop in the
selectivity for the annulation product (entry 2). Overall, the
benzhydryl allylsilane provided a balance for the desired
reactivity and product ratio (entry 3), and was optimal given
the ease of preparation.
all cases, the annulation product is obtained as the major
product with 97–99% ee, and the reported structures and
absolute stereochemistry were confirmed by X-ray analysis of
3h.[14] Asymmetric catalysis is often performed at low
temperatures, so the consistently high enantioselectivity of
this reaction at room temperature is noteworthy. The
diastereoselectivity remained high for all isatins except for
the case of 4-chloro-substituted isatin (1c), which afforded a
71:29 mixture of diastereomers while still maintaining a high
97% ee (entry 3). A decrease in reaction rate and yield was
observed with more electron-donating substituents on isatin,
such as a methoxy group (entry 5). Investigations of other
pybox ligands confirmed that the indapybox ligand (L)
provides superior enantioselectivity (99% ee); the isopropyl
and norephedrine variants of the ligand still provide greater
than 90% ee.
Proceeding with the easily oxidizable benzhydryl silyl
À
group, we demonstrated the Si C oxidation and scope with
respect to isatin for obtaining hydroxy-spirooxindoles 5
(Table 4). This reaction utilizes oxidation conditions with
TBAF and hydrogen peroxide to achieve high yields.[17a] The
conversion of the silyl group proceeds with retention of
configuration as confirmed by X-ray structure analysis for the
product 5d,[14] and the high enantiomeric excess is preserved.
The competing protodesilylation reaction with TBAF is
minimized by maintaining the reaction at 08C and controlling
the reaction time.[19]
Silyl spirooxindoles and other organosilicon compounds
are of interest for their potential biological activity,[15] as well
À
as the added synthetic utility derived from oxidation of the C
Si bond. For greatest synthetic utility, the silyl group must be
bulky enough to suppress the elimination pathway, while also
In summary, we have demonstrated the first example of an
asymmetric catalytic [3+2] annulation reaction of allylsilanes
and an efficient enantioselective synthesis for 3’-silyl- and 3’-
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Table 3: Scope and annulation ratios for allylsilanes.[a]
Keywords: allylic compounds · annulation ·
asymmetric catalysis · enantioselectivity · scandium
.
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Entry SiR3
t [h] 3/4
Yield of
ee of
3 [%][b]
3 [%][c]
1
2
3
4
5
6
7
Si(iPr)3
Si(iPr)2(4-MeOC6H4)
SiMe2(CHPh2)
SiMe2(2-MeOC6H4)
SiMe2(2,6-(MeO)2C6H3)
SiMe2(1-Np)
24 82:18 71
72 52:48 39
72 66:34 51[d]
48 38:62 35
99
99
99
99
99
99
–
2
30:70 20[e]
48 17:83 10[f]
192
SiPh3
–
–
[a] All reactions performed under argon, using ScCl3(THF)3, NaSbF6, with
3 equiv of allylsilane. [b] Yield of isolated annulation product. For
entries 1–5, the annulation product is obtained with >95:5 diastereo-
[3] For a review on b-silyl carbocations, see Refs. [1a,b] and also:
J. B. Lambert, Y. Zhao, R. W. Emblidge, L. A. Salvador, X. Y.
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T. Rovis, J. S. Tedrow, J. Am. Chem. Soc. 2001, 123, 12095 –
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[5] For selected examples of catalytic enantioselective allylation
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[8] We have previously reported the enantioselective addition of
heteroaromatic nucleophiles and silyl enol ethers to isatins using
Sc(OTf)3/pybox catalysts. See: N. V. Hanhan, A. H. Sahin, T. W.
Chang, J. C. Fettinger, A. K. Franz, Angew. Chem. 2010, 122,
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[9] The (S,R)-indapybox ligand was synthesized using a modified
procedure. See the Supporting Information and the following
reference: G. Desimoni, G. Faita, M. Guala, C. Pratelli,
Tetrahedron: Asymmetry 2002, 13, 1651 – 1654.
1
selectivity as determined by H NMR spectroscopy. [c] Determined by
HPLC analysis using a chiral AD-H column. [d] Represents an average of
four reactions. [e] The amount of annulation can vary because of the
photosensitivity of the starting material under the reaction conditions;
see Ref. [17b]. [f] An 85:15 mixture of diastereomers was isolated.
Np=naphthyl.
[a]
À
Table 4: Enantioselective allylsilane annulation and C Si oxidation.
Entry R1
R2
Yield of 3 [%][b]
5
Yield of 5 [%] ee of 5 [%][c]
1
2
3
4
CH3 5-Br 51[d]
CH3 5-Cl 53
CH3 5-F 53
5a 92
5b 93
5c 85
5d 85
98
99
99
93
Bn
5-Br 55
[a] All annulation reactions performed under argon, using ScCl3(THF)3,
NaSbF6, with 3 equiv of allylsilane 2c for 72 h. Thermal ellipsoids of the
X-ray structure of 3h are shown at 50% probability. [b] Yield of isolated
annulation product. [c] Determined by HPLC analysis using a chiral
stationary phase. [d] Yield represents an average of four reactions.
TBAF=tetra-n-butylammonium fluoride.
hydroxy-spirooxindoles. This reaction utilizes a chiral cationic
ScCl2(SbF6)/L complex with TMSCl as an essential promoter,
thus allowing the reaction to proceed with high enantiose-
lectivity at room temperature. The role of TMSCl (and
promoters such as ArCOCl) to enhance the reaction rate is
currently under investigation. The optimization and scope of
the enantioselective allylation of isatins with allylic silanes
will be reported in a separate publication. Additional inves-
tigations into the scope of carbonyl electrophiles and
substituted allylic silanes are underway.
[10] a) D. A. Evans, C. E. Masse, J. Wu, Org. Lett. 2002, 4, 3375 –
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[11] AgSbF6 alone does not catalyze the reaction, thus indicating that
a scandium complex is the active catalyst. Alternate counterions
(AgOTf, NaBArf, AgBF4) showed a minimal effect to improve
the ratio in favor of the annulation product, and using AgBF4
resulted in a significant reduction in the reaction rate.
Received: August 14, 2011
Published online: December 12, 2011
Angew. Chem. Int. Ed. 2012, 51, 989 –992
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
[12] Although we cannot comment on the exact role of TMSCl,
several control experiments and NMR studies provide important
insights. When addition of TMSCl to the ScCl2(SbF6)/L complex
Lett. 2009, 11, 357 – 360. For an example where TMSCl is
hypothesized to pre-activate an electrophile or nucleophile, see:
g) P. H. Lee, D. Seomoon, S. Kim, K. Nagaiah, S. V. Damle, K.
Lee, Synthesis 2003, 2189 – 2193. For an example of TMSCl as a
drying agent, see: h) D. M. Volochnyuk, S. V. Ryabukhin, A. S.
Plaskon, O. O. Grygorenko, Synthesis 2009, 3719 – 3743.
1
is monitored by H NMR spectroscopy (in CD2Cl2), an overall
broadening of the indapybox signals is observed. No interaction
or complexation is observed upon mixing TMSCl with isatin.
While the addition of TMSCl has a dramatic effect on the
reaction rate, varying amounts of TMSCl does not affect the
enantioselectivity. TMSCl also does not appear to be acting as a
drying agent based on a series of control experiments regarding
the effects of molecular sieves and water on the reaction rate and
annulation ratio. Other additives (such as silyl-transfer agents
HFIP and diisopropylphenol) were also investigated and do not
enhance the rate of the reaction. Overall, this evidence supports
a hypothesis that TMSCl interacts directly with the scandium
complex by facilitating formation of the cationic catalyst or
additionally enhancing the Lewis acidity of the scandium. See
the Supporting Information for more details.
[14] The structure and absolute stereochemistry was assigned and
confirmed based on X-ray crystallography (anomalous disper-
sion method) of 3h and 5d. CCDC 832206 (3h) and
CCDC 831210 (5d) contain the supplementary crystallographic
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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[18] For relevant discussions of the size and electronic effects of silyl
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992
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