Strained silacycle-catalyzed asymmetric Diels-Alder cycloadditions: the first highly enantioselective silicon Lewis acid catalyst

Article abstract of DOI:10.1016/j.tet.2006.06.076

The first highly enantioselective silicon Lewis acid catalyst for an asymmetric organic transformation has been developed. The catalyst derives its activity from the strain induced in the silicon center by virtue of being constrained in a five-membered ri

Full text of DOI:10.1016/j.tet.2006.06.076

Tetrahedron 62 (2006) 11397–11401
Strained silacycle-catalyzed asymmetric Diels–Alder
cycloadditions: the first highly enantioselective silicon
Lewis acid catalyst
*
Katsumi Kubota, Christopher L. Hamblett, Xiaolun Wang and James L. Leighton
Department of Chemistry, Columbia University, New York, NY 10027, USA
Received 24 April 2006; revised 20 June 2006; accepted 21 June 2006
Available online 12 July 2006
Abstract—The first highly enantioselective silicon Lewis acid catalyst for an asymmetric organic transformation has been developed. The
catalyst derives its activity from the strain induced in the silicon center by virtue of being constrained in a five-membered ring. A simple
tridentate ligand has been developed and the derived chlorosilane complex catalyzes the Diels–Alder cycloaddition of methacrolein and cyclo-
pentadiene with 94% ee.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Diels–Alder reaction of enals with cyclopentadiene was
chosen as the proving ground for this idea.
The possibility that silicon—with all of its inherent practical
advantages—might serve as a useful Lewis acid for asym-
metric synthesis, has long captured the imagination of
organic chemists. Three strategies have emerged for render-
ing otherwise unreactive silanes Lewis acidic for the promo-
tion/catalysis of organic reactions: (1) attachment of
strongly electron-withdrawing groups to the silicon center
(e.g., TMSOTf),1 (2) Lewis base activation of otherwise
only weakly Lewis acidic silanes such as RSiCl3 and
SiCl4,2 and (3) constraining the silicon in a small ring
(strained silacycles).3 Despite some creative ideas,4 the first
approach has yielded no highly enantioselective chiral cata-
lysts for asymmetric synthesis. In contrast, the second
approach has led to the development of several highly enan-
tioselective reactions.5 It is typically the Lewis base that
serves as the catalyst, however, while the silane component
is employed as a stoichiometric reagent. We have recently
reported the first examples of highly enantioselective chiral
silicon Lewis acid-mediated carbon–carbon bond forming
reactions based on the third approach (Scheme 1),6 and it
was natural to wonder whether the same concept might be
applied for the discovery of the first highly enantioselective
silicon Lewis acid catalyst. Because of its prominent role
in the development of chiral Lewis acid catalysis,7 the
Ph
O
N
Ph
Cl
Si
NHBz
NHBz
Ar
Me
(S,S)-1
N
HN
Me
+
ArH
i-PrO2C
H
i-PrO2C
PhCH3, -30 °C
87-95% ee
Ph
O
N
Ph
Cl
Si
Bz
NHBz
Me
(S,S)-1
N
HN
N
Me
+
Ot-Bu
R
H
R
Ot-Bu
90-98% ee
PhCH3 , 23 °C
Scheme 1. Two reactions mediated by silane 1.
Silane Lewis acid 1 (prepared and employed as a 2:1 mixture
of diastereomers) has proven effective both for Friedel–
Crafts alkylations of acylhydrazones and [3+2] enol ether–
acylhydrazone cycloadditions (Scheme 1).8 Mechanistic
investigations have revealed that these reactions proceed
by way of covalent attachment of the hydrazone to the silane
Lewis acid with chloride displacement and protonation of
the amino group of the pseudoephedrine. Such reactions
are therefore mechanistically distinct from the proposed
simple Lewis acid activation of methacrolein for a Diels–
Alder cycloaddition with cyclopentadiene, and there was
no reason a priori to believe that silane 1 would prove effec-
tive for this reaction. Indeed, 1 is wholly ineffective as a cat-
alyst for this process, and replacement of the phenyl group
Keywords: Silicon; Lewis acid; Catalyst; Diels–Alder; Enantioselective.
* Corresponding author. Tel.: +1 212 854 4262; fax: +1 212 932 1289;
e-mail: leighton@chem.columbia.edu
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.06.076

11398
K. Kubota et al. / Tetrahedron 62 (2006) 11397–11401
on the silane with various other alkyl, aryl, and heteroatom
substituents proved fruitless as well.
Having recorded a proof of concept with silane 8a, we turned
to a survey of the sulfonamide substituent (R1) as well as the
substituents on the phenol ring (R2) in an effort to optimize
both for efficiency and enantioselectivity (Table 1). Al-
though in some cases the silanes could be isolated in reason-
able purity, it was more convenient to employ them in situ.
Thus, the precursor sulfonamidoaminophenols 7a–f were
treated with SiCl4 and DBU and the resulting solutions of
silanes 8a–f were simply treated with cyclopentadiene and
2. Results and discussion
A breakthrough was achieved when we examined chiral
ligands that carry three functional groups for attachment to
the silane. Ephedrine-derived aminophenol 2 was treated
with SiCl4 and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
to give silane 3 as a single (unassigned) diastereomer
(Scheme 2). We were then delighted to discover that 3 is
indeed a competent (albeit non-selective) catalyst for the
Diels–Alder reaction between cyclopentadiene and meth-
acrolein, producing aldehyde 4(R) in 58% yield and 6% ee.
In this and in every case, the exo diastereomer was produced
highly selectively (>95:5). The diphenylethylenediamine-
derived aminophenol 5 was next examined, and the derived
silane 6 was also found to be a competent catalyst providing
aldehdye 4(S) as the major product in 46% yield and 37% ee.
The commonly employed cyclohexanediamine ligand motif
was examined in the form of 7a, which was employed to gen-
erate silane 8a (the illustrated and observed NOE interaction
allowed the assignment of relative configuration at the silicon
center in 8a). Gratifyingly, 8a proved to be a significantly im-
proved catalyst, giving 4(S) in 75% yield and 56% ee, and
providing our first true lead catalyst for optimization. Finally,
we note that silane 9 is completely inactive under otherwise
identical conditions, clearly supporting the hypothesis that
the strain induced by the five-membered rings in 3, 6, and
8a is an essential component of their Lewis acidity and
catalytic activity.
Table 1. Optimization of catalyst structure
SO2R1
NH
SO2R1
N
Cl
Si
SiCl4
DBU
NH OH
N
O
R2
R2
7a-f
8a-f
R2
CHO
R2
Me
CHO
20 mol % 8a-f
+
CH2Cl2, -78 °C
Me
4(S)
Entry (cat.)
R1
R2
Yield (%)
ee (%)
1 (8a)
2 (8b)
3 (8c)
4 (8d)
5 (8e)
6 (8f)
p-Tolyl
Me
Mesityl
p-Tolyl
p-F–C6H4
p-Tolyl
t-Bu
t-Bu
t-Bu
Br
H
H
72
75
58
69
56
78
ꢀ56a
ꢀ75a
ꢀ33a
57
88
94
a
Major enantiomer was 4(R).
Ph
OH
Ph
O
N
Cl
O
SiCl4
DBU
Me
CHO
20 mol % 3
Si
58%;
+
+
CHO
6% ee
Me
NH OH
Me
CH2Cl2
-78 °C, 8 h
4(R)
Me
t-Bu
t-Bu
3
2
t-Bu
t-Bu
Ts
Ph
Ph
NHTs
Ph
Ph
CHO
46%;
N
Me
CHO
Cl
O
20 mol % 6
SiCl4
DBU
Si
CH2Cl2
-78 °C, 8 h
Me
37% ee
N
NH OH
4(S)
t-Bu
t-Bu
6
5
t-Bu
t-Bu
Ts
CHO
Me
CHO
20 mol % 8a
NHTs
+
N
Cl
75%;
SiCl4
DBU
t-Bu
CH2Cl2
-78 °C, 8 h
Me
Si
56% ee
4(S)
NH OH
N
O
t-Bu
Ts
7a
8a
N
Cl
O
nOe
t-Bu
H
Si
O
H
N
t-Bu
t-Bu
N
Si
Ts
t-Bu
t-Bu
N
H
9
8a
Cl
Inactive -
No catalysis
t-Bu
Scheme 2. Initial discovery of competent silane Lewis acid catalysts.

K. Kubota et al. / Tetrahedron 62 (2006) 11397–11401
11399
methacrolein. As shown, simple changes to the sulfonamide
group and the substituents on the phenol ring produced dra-
matic changes in the enantioselectivity. Indeed, it was possi-
ble to obtain either moderately good enantioselectivity for
the R enantiomer (entry 2) or excellent enantioselectivity
for the S enantiomer (entry 6) using the same enantiomer
of the cyclohexanediamine core. As shown, the optimal
catalyst was the simple tosylamide-unsubstituted phenol
8f. Using 20 mol % of this catalyst and the in situ procedure,
4(S) was produced in 78% yield and 94% ee. Silane 8f thus
represents the first highly enantioselective silicon Lewis acid
catalyst developed for asymmetric synthesis.
dramatic drop-off in enantioselectivity observed with etha-
crolein (Table 2, entry 4). The added methyl group (relative
to methacrolein) in this dienophile must swing away from
the approaching cyclopentadiene in the transition state, but
cannot do so without being forced into a costly steric inter-
action with the phenyl ring. The likely result is a shift to
a greater population of the s-cis form of the enal, and in
turn eroded levels of enantioselectivity.
H
Ts
H
N
Ts
N
H
H
8f
N
N
Si
CHO
O
Si
O
O
A brief survey of the scope with respect to the enal structure
in Diels–Alder reactions with cyclopentadiene catalyzed by
8f was carried out (Table 2). Whereas b-substitution necessi-
tated extended reaction times, such substrates were neverthe-
less well tolerated from the standpoint of enantioselectivity
(entries 2 and 3). In contrast, and surprisingly, a-substitution
beyond a simple methyl group caused a dramatic drop in
enantioselectivity (entry 4).
Cl
H
Cl
Me
4(S)
O
H
Me
Me
Scheme 3. A model for asymmetric induction.
3. Conclusion
In an attempt to construct a mechanistic model for stereo-
chemical induction, we began with two assumptions: (1) in
order for the five-membered diazasilacyclopentane ring to
be accommodated in the presumed trigonal bipyramidal
(tbp) silane–aldehyde complex, one of the two nitrogens
must occupy an apical position, and this will likely be the
more electron-poor tosylamide nitrogen, and (2) the alde-
hyde oxygen will bind to the silane opposite this tosylamide
nitrogen so as to occupy the other apical position in the com-
plex (Scheme 3). If a hydrogen bond between the amine and
the formyl hydrogen—a type of interaction proposed by
Corey to be an essential element in the success of numerous
enantioselective chiral Lewis acid catalyzed reactions9—is
further invoked as depicted, an intriguingly simple model
for asymmetric induction emerges as the phenyl ring clearly
provides highly effective shielding of the back face of the di-
enophile. This model can also explain the surprisingly
We have developed the first highly enantioselective silicon
Lewis acid catalyst for asymmetric synthesis. The catalyst
is easily and inexpensively prepared and may be employed
in situ. Lewis acid catalysts for the Diels–Alder reaction
are legion,7 however, and a powerful new strategy for enal
and enone activation in Diels–Alder reactions has recently
emerged.10 The significance of this work is therefore not
specifically tied to the Diels–Alder reaction, but rather lies
in the potential for the development of improved silicon
Lewis acid catalyst designs for a broader range of transfor-
mations. The present work establishes the validity of this
notion and delineates a strategy for achieving such catalysts.
4. Experimental
4.1. General
Table 2. Survey of enal scope
R1
R2
CHO
CHO
20 mol % 8f
+
All reactions were carried out under an atmosphere of nitro-
gen in flame- or oven-dried glassware with magnetic stirring
unless otherwise indicated. Flash chromatography was
performed on EM silica gel 60 (230–240 mesh). Degassed
solvents were obtained from the solvent purification system
by passage through a column of activated alumina. 1H
NMR spectra were recorded on a Bruker DRX-300
(300 MHz), Bruker DRX-400 (400 MHz), or DMX-500
(500 MHz) spectrometer, and are reported in parts per mil-
lion from CDCl3 (7.26 ppm), C6D6 (7.15 ppm), or DMSO-
d6 (2.50 ppm) as internal standard. Data are reported as
follows: s ¼ singlet, d ¼ doublet, t ¼ triplet, q ¼ quartet,
quin ¼ quintet, m ¼ multiplet, dd ¼ doublet of doublets,
dt ¼ doublet of triplets, br ¼ broad; integration; coupling
constant(s) in Hertz; assignment. 29Si NMR spectra were
recorded on a Bruker DRX-300 (60 MHz) spectrometer and
are reported in parts per million from tetramethylsilane as in-
ternal standard. 19F NMR spectra were recorded on a Bruker
DRX-300 (338.6 MHz) spectrometer and are reported in
parts per million from CFCl3 as internal standard. Proton
decoupled 13C NMR spectra were recorded on a Bruker
R1
R2
CH2Cl2, -78 °C
Entry
1
Enal
Product
CHO
t (h) Yield (%) ee (%)
Me
CHO
8
78
94
4(S)
Me
Me
Me
CHO
CHO
2
48
87
90
9
Me
Me
CHO
CHO
CHO
3
4
24
16
79
88
86
54
10
Et
CHO
11
Et

11400
K. Kubota et al. / Tetrahedron 62 (2006) 11397–11401
DRX-300orBrukerDRX-400(100 MHz)spectrometerusing
CDCl3 (77.0 ppm), C6D6 (128.0 ppm), or DMSO-d6
(40.5 ppm) as internal standard. Infrared spectra were re-
corded on a Perkin Elmer Paragon 1000 FTIR spectrometer.
Low-resolutionmassspectrawereobtainedonaJEOLHX110
mass spectrometer in the Columbia University Mass Spec-
trometry Laboratory. Optical rotations were obtained on a
JASCO DIP-1000 polarimeter using a 10 cm path length cell.
chromatography (100% pentane/20% Et2O/pentane) to af-
ford the pure bicyclic aldehyde products in the yields and
enantioselectivities indicated in Table 2.
4.2.1. Diels–Alder reaction products—characterization
and determination of enantioselectivity.
4.2.1.1. Table 2, entry 1. The physical and spectral data
for this compound matched previously reported data.12 The
diastereoselectivity (exo:endo ratio ¼ 96:4) was determined
1
4.1.1. Synthesis of ligand 7f. The Schiff base formed by
condensation of cyclohexanediamine monotosylamide and
salicylaldehyde has been prepared and characterized previ-
ously.11 We prepared the (R,R)-cyclohexanediamine-derived
Schiff base according to this procedure. To a cooled (0 ꢁC)
solution of this Schiff base (3.0 mmol) in CH2Cl2 (4.0 mL)
and MeOH (12.0 mL) was added NaBH4 (337 mg,
9.0 mmol). The reaction mixture was warmed to room tem-
perature after 15 min and stirred for an additional 3 h. The re-
action was then quenched by the addition of 1 M NaOH
(15 mL). The aqueous layer was extracted with CH2Cl2
(3ꢂ10 mL) and the combined organic layers were washed
with brine (1ꢂ5 mL), dried over MgSO4, filtered, and
concentrated. The solid residue was then purified by recrys-
tallization (Et2O/CH2Cl2) to afford 1.01 g (90%) of ligand
by H NMR (400 MHz, CDCl3) integration: d 9.69 (s, 1H,
CHO, exo), 9.39 (s, 1H, CHO, endo). The enantioselectivity
(ee ¼ 94%) was determined by reduction with NaBH4 to the
corresponding alcohol, conversion to the (R)-MTPA ester
1
1
derivative, and H NMR integration: H NMR (500 MHz,
CDCl3) d 4.34 (d, 1H, one of CH2O-(R)-MTPA, major),
4.31 (d, 1H, one of CH2O-(R)-MTPA, minor), 4.25 (d, 1H,
one of CH2O-(R)-MTPA, minor), 4.22 (d, 1H, one of
CH2O-(R)-MTPA, major).
4.2.1.2. Table 2, entry 2. The physical and spectral data
for this compound matched previously reported data.13 The
diastereoselectivity (exo:endo ratio>98:2) was determined
1
by H NMR (400 MHz, CDCl3) integration: d 9.62 (s, 1H,
CHO, exo), 9.37 (s, 1H, CHO, endo). The enantioselectivity
(ee ¼ 90%) was determined by reduction with NaBH4 to the
corresponding alcohol, conversion to the (R)-MTPA ester
1
7f as a white solid. [a]2D1 +9.77 (c 1.00, CH2Cl2); H NMR
(400 MHz, CDCl3) d 7.74 (dd, 2H, J¼5.0 Hz, 1.6 Hz,
SO2C6(o-H2)(m-H2)CH3), 7.24 (d, 2H, J¼7.9 Hz,
SO2C6(o-H2)(m-H2)CH3), 7.15 (dt, 1H, J¼1.5 Hz, 7.7 Hz,
one of NCH2C6H4), 6.91 (dd, 1H, J¼1.3 Hz, 6.1 Hz, one of
NCH2C6H4), 6.81–6.73 (m, 2H, two of NCH2C6H4), 4.67
(br s, 1H, NHSO2Ar), 3.93 (d, 1H, J¼13.9 Hz, one of
NCH2Ar), 3.84 (d, 1H, J¼13.9 Hz, one of NCH2Ar), 3.02–
2.95 (br m, 1H, CHNHSO2Ar), 2.37 (s, 3H, SO2C6H4CH3),
2.31–2.26 (m, 1H, CHNHCH2Ar), 2.14–2.08 (br m, 1H,
Cy–H), 1.70–1.58 (br m, 3H, Cy–H3), 1.23–1.12 (m, 4H,
Cy–H4); 13C NMR (75 MHz, CDCl3) d 158.0, 143.7,
137.6, 129.8, 129.8, 128.5, 128.0, 126.9, 126.9, 123.1,
119.0, 116.4, 60.9, 57.1, 50.0, 33.4, 31.1, 24.8, 24.0, 21.5;
IR (KBr) 3319, 3237, 3043, 2932, 2857, 1613, 1590, 1478,
1449, 1411, 1322, 1262, 1150, 1091, 934, 912, 748, 666,
547 cmꢀ1; HRMS (FAB+) calculated for C20H27N2O3S
[M+H]+ 375.1742, found 375.1733.
1
1
derivative, and H NMR integration: H NMR (500 MHz,
CDCl3) d 4.31 (d, 1H, one of CH2O-(R)-MTPA, major),
4.25 (d, 1H, one of CH2O-(R)-MTPA, minor), 4.20 (d, 1H,
one of CH2O-(R)-MTPA, minor), 4.14 (d, 1H, one of
CH2O-(R)-MTPA, major). The absolute configuration was
assigned by analogy to the methacrolein reaction.
4.2.1.3. Table 2, entry 3. The physical and spectral data
for this compound matched previously reported data.14 The
diastereoselectivity (exo:endo ratio>96:4) was determined
1
by H NMR (400 MHz, CDCl3) integration: d 9.73 (s, 1H,
CHO, exo), 9.45 (s, 1H, CHO, endo). The enantioselectivity
(ee ¼ 86%) was determined by reduction with NaBH4 to the
corresponding alcohol, conversion to the (R)-MTPA ester
1
1
derivative, and H NMR integration: H NMR (500 MHz,
CDCl3) d 2.68 (br s, 1H, ]CH–CH, major), 2.63 (br s,
1H, ]CH–CH, minor).
4.2. General procedure for the Diels–Alder reactions in
Table 2
4.2.1.4. Table 2, entry 4. The physical and spectral data
for this compound matched previously reported data.13 The
diastereoselectivity (exo:endo ratio>95:5) was determined
To a cooled (ꢀ78 ꢁC) solution of SiCl4 (149 mL, 1.30 mmol)
and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (612 mL,
4.09 mmol) in CH2Cl2 (13.0 mL) is added a solution of 7f
(500 mg, 1.33 mmol) in CH2Cl2 (13.0 mL). The reaction
mixture is slowly warmed to room temperature over the
course of 4 h, and after an additional 4 h at room tempera-
ture, the resulting solution of the catalyst 8f in CH2Cl2
(0.0500 M) is used directly for the Diels–Alder reactions.
1
by H NMR (400 MHz, CDCl3) integration: d 9.70 (s, 1H,
CHO, exo), 9.42 (s, 1H, CHO, endo). The enantioselectivity
(ee ¼ 54%) was determined by reduction with NaBH4 to the
corresponding alcohol, conversion to the (R)-MTPA ester
1
1
derivative, and H NMR integration: H NMR (500 MHz,
CDCl3) d 4.41 (d, 1H, one of CH2O-(R)-MTPA, minor),
4.37 (d, 1H, one of CH2O-(R)-MTPA, minor), 4.33 (d, 1H,
one of CH2O-(R)-MTPA, major), 4.29 (d, 1H, one of
CH2O-(R)-MTPA, minor). The absolute configuration was
assigned by analogy to the methacrolein reaction.
To a cooled (ꢀ78 ꢁC) solution of catalyst 8f (0.05 M in
CH2Cl2, 4.0 mL, 0.20 mmol) and cyclopentadiene (250 mL,
3.8 mmol) is added the unsaturated aldehyde (1.0 mmol)
dropwise. After the indicated reaction time (see Table 2),
the mixture is diluted with CH2Cl2 (1.0 mL) and quenched
with a solution of 4:1 MeOH/H2O (250 mL). The organic
layer is washed with brine (1ꢂ1 mL), dried over Na2SO4,
and concentrated. The residue is purified by flash
Acknowledgements
This work was supported by grants from the National Insti-
tutes of Health (NIGMS GM58133) and the National

K. Kubota et al. / Tetrahedron 62 (2006) 11397–11401
11401
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Science Foundation (NSF CHE-04-53853). We thank the
Sumitomo Corp. for postdoctoral support of K.K.
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