Asymmetrie aldol reaction of ketones with alkenyl trichloroacetates catalyzed by dibutyltin dimethoxide and binap·silver(i) complex: construction of a chiral tertiary carbon center

Article abstract of DOI:10.1002/adsc.200900437

A novel aldol reaction of alkenyl trichloroacetates with a-keto esters was realized by using dibutyltin dimethoxide as a catalyst, which was regenerated by the addition of methanol. The reaction was found to be remarkably accelerated by the addition of a

Full text of DOI:10.1002/adsc.200900437

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DOI: 10.1002/adsc.200900437
Asymmetric Aldol Reaction of Ketones with Alkenyl
Trichloroacetates Catalyzed by Dibutyltin Dimethoxide and
BINAP·Silver(I) Complex: Construction of a Chiral Tertiary
Carbon Center
Akira Yanagisawa,a,* Yuuki Terajima,a Kazuma Sugita,a and Kazuhiro Yoshidaa
a
Department of Chemistry, Graduate School of Science, Chiba University, Inage, Chiba 263-8522, Japan
Fax : (+81)-43-290-2789; e-mail: ayanagi@faculty.chiba-u.jp
Received: June 24, 2009; Published online: July 31, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900437.
Abstract:
A
novel aldol reaction of alkenyl
the asymmetric transformation has recently attracted
the attention of synthetic organic chemists, because
products possessing a chiral tertiary alcohol moiety
are synthetically useful. However, there are still only
few examples of the catalytic asymmetric aldol reac-
tion of ketones due to their low reactivity toward
enolACTHNUTRGENUaGN tes as well as to the facile retro-aldol reaction of
the aldol adducts. Here we describe a novel asymmet-
ric aldol reaction of a-keto esters with alkenyl
trichloroACTHNUTRGNEUGaN cetates catalyzed by dibutyltin dimethoxide
trichloroacetates with a-keto esters was realized by
ACHTUNGTRENNUNG
using dibutyltin dimethoxide as a catalyst, which
was regenerated by the addition of methanol. The
reaction was found to be remarkably accelerated by
the addition of a catalytic amount of a bidentate
phosphine·silver(I) complex. Use of the BINAP·sil-
ver triflate (AgOTf) complex as the chiral co-cata-
lyst resulted in the formation of optically active
aldol products possessing a chiral tertiary carbon
with up to 93% ee. This catalytic method was fur-
ther applied to the asymmetric reaction of diketene
with methyl benzoylformate.
and BINAP·silver triflate in the presence of methanol
(Scheme 1).
We have previously found that dibutyltin dimethox-
ide acts as a catalyst in the aldol reaction of alkenyl
trichloroacetates with aldehydes.[5] The tin compound
is efficiently regenerated from the aldol adduct in the
presence of MeOH and thus the reaction takes place
catalytically. This protocol has been improved in
terms of environmental friendliness because the
amount of toxic organotin(IV) compound[6] is reduced
Keywords: aldol reaction; alkenyl esters; asymmet-
ric catalysis; ketones; silver; tin
The enantioselective aldol reaction provides a benefi- to a catalytic amount. In addition, as dibutyltin di-
cial route to non-racemic b-hydroxy carbonyl com- methoxide is more reactive than tributyltin methox-
pounds that are often utilized as synthetic intermedi- ide, the former tin catalyst has solved the problem
ates of natural products or biologically active com- that aliphatic aldehydes show almost no reactivity in
pounds.[1] In order to obtain aldol products in high the presence of the latter tin catalyst.[5] We expected
yield and satisfactory optical purity, various methods that Bu2SnACTHNUTRGEN(NUG OMe)2 could also catalyze the addition of
using chiral catalysts have been developed.[2–4] In par- alkenyl trichloroacetates to ketones which are recog-
ticular, the application of ketones as electrophiles to nized to be less reactive electrophiles than aldehydes
Scheme 1. The asymmetric aldol reaction of a-keto esters with alkenyl trichloroacetates catalyzed by Bu2SnACTHNUTRGEN(NUG OMe)2 and
BINAP·AgOTf.
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Akira Yanagisawa et al.
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Table 1. Additive effects on aldol reaction of methyl benzoylformate with an alkenyl trichloroacetate of cyclohexanone cata-
lyzed by Bu2Sn
(OMe)2.[a]
ACHTUNGTRENNUNG
Entry
Additive
t [h]
Yield[b] [%]
dr[c]
1
2
3
none
2
3
2
27
88
60
71:29
57:43
22:78
dppp (10 mol%), AgOTf (20 mol%), MS 3 ꢁ
dppf (10 mol%), AgOTf (20 mol%), MS 3 ꢁ
[a]
Unless otherwise noted, the reaction was performed using Bu2SnACHTNUTRGNE(UNG OMe)2 (10 mol%), 1-trichloroacetoxycyclohexene
(2 equiv.), and methyl benzoylformate (1 equiv.) in dry THF containing MeOH (5 equiv.) at room temperature (22–248C)
for 2–3 h.
Isolated yield.
Determined by H NMR analysis.
[b]
[c]
1
in the conventional Lewis acid-promoted Mukai- ence of 10 mol% of dppp and 20 mol% of AgOTf
yama-type aldol reaction of silyl enolates or ketene with MS 3 ꢁ (entry 2). Employment of dppf in place
silyl acetals.[7,8] First, we tested the catalytic activity of of dppp also resulted in a significant acceleration of
Bu2Sn(OMe)2 in the reaction of 1-trichloroacetoxycy- the aldol reaction, but the opposite diastereoselectiv-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
clohexene (2 equiv.) with methyl benzoylformate ity was obtained probably due to the ligand bite angle
(1 equiv.), and found that the targeted b-hydroxy effects,[9] which might affect the transition state struc-
ketone was obtained in 27% yield with a diastereo- ture (entry 3).
meric ratio of 71:29 in the presence of 10 mol% of
the catalyst and 5 equiv. of MeOH in THF at room trichloro
temperature (22–248C) for 2 h (entry 1, Table 1). We esters using Bu2SnAHCTUNTGRENNUNG
We next performed the aldol reaction of 1-
acetoxycyclohexene with various a-keto
(OMe)2 and dppp·AgOTf as cata-
AHCTUNGTRENNUNG
then examined the utility of a bidentate phosphine·sil- lysts (Table 2). From ethyl benzoylformate, the de-
ver(I) complex as an additive in an effort to increase sired aldol product was furnished in 43% yield; how-
the yield of the product. As a result, we found that ever, the corresponding methyl ester was also formed
the yield of the product improved to 88% in the pres- in 14% yield probably due to transesterification pro-
Table 2. Aldol reaction of a-keto esters with an alkenyl trichloroacetate of cyclohexanone catalyzed by Bu2SnACTHNUTRGEN(NUG OMe)2 and
dppp·AgOTf.[a]
Entry
R1
R2
t [h]
Yield [%][b]
A
B
1
2
Ph
Ph
CH3
CH3
Et
4
4
18
13
43
40
80
36
14
23
6
Bn
Bn
Bn
3
4[c]
2
[a]
Unless otherwise noted, the reaction was performed using dppp (10 mol%), AgOTf (20 mol%), Bu2SnACTHUNTRGENNG(U OMe)2 (8 mol%),
1-trichloroacetoxycyclohexene (2 equiv.), and a-keto ester (1 equiv.) in dry THF containing MeOH (20 equiv.) at room
temperature (22–248C) for 4–18 h.
Determined by H NMR analysis.
[b]
[c]
1
Without MS 3 ꢁ.
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Asymmetric Aldol Reaction of Ketones with Alkenyl Trichloroacetates
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Table 3. The asymmetric aldol reaction of a-keto esters with alkenyl trichloroacetates catalyzed by Bu2Sn
(R)-BINAP·AgOTf.[a]
ACHTUNGRTEN(NUNG OMe)2 and
Entry
Alkenyl trichloroacetate
R3
t [h]
Yield [%][b]
dr[c]
ee [%][d]
1
2
3
Ph
4
4
5
99
99
86
66:34
53:47
83:17
74[e]
61[f]
90[g]
4-BrC6H4
4-MeOC6H4
4
Ph
24
58
97:3
89
5
6
Ph
CH3
24
24
38
40
>99:1
52:48
93
60[h]
[a]
Unless otherwise noted, the reaction was performed using (R)-BINAP (10 mol%), AgOTf (20 mol%), Bu2SnACHTUNGTRENNUNG(OMe)2
(8 mol%), alkenyl trichloroacetate (2 equiv.), and a-keto ester (1 equiv.) in dry THF containing MeOH (5 equiv.) at
À208C for 4–24 h.
[b]
[c]
[d]
Isolated yield.
1
Determined by H NMR analysis.
The value corresponds to the major diastereomer. Determined by HPLC analysis (Daicel Chiralcel OD-H, Chiralpak
AD-H, or AS-H).
[e]
[f]
The minor isomer: 67% ee.
The minor isomer: 57% ee.
The minor isomer: 79% ee.
The minor isomer: 60% ee.
[g]
[h]
moted by the tin methoxide (entry 1).[10] With benzyl major diastereomer had 74% ee. The substituent at
benzoylformate, the transesterification could not be the para-position of methyl benzoylformate influ-
suppressed either (entry 2). In contrast, use of benzyl enced to some extent the diastereomeric ratio and the
pyruvate led to a satisfactory result in terms of selec- enantiomeric excess of the product (entries 2 and 3).
tivity although a longer reaction time was required to For the p-methoxy derivative, in particular, the enan-
gain a higher yield (entry 3). It is noteworthy that MS tioselectivity of the reaction reached 90% ee
3 ꢁ plays an important role in the present aldol reac- (entry 3). The electron-donating group might enhance
tion; in fact, the chemical yield of the targeted aldol the Lewis basicity of the carbonyl oxygen, which is
adduct was decreased to 36% in the absence of MS anticipated to more strongly coordinate with the
3 ꢁ (entry 4).
The above-mentioned results further prompted us tion state structure for asymmetric induction. The
to use the BINAP·silver(I) complex as a chiral cata- utility of the present Bu2Sn(OMe)2- and
silver Lewis acid leading to a more favorable transi-
AHCTUNGTRENNUNG
lyst for the dibutyltin dimethoxide-catalyzed aldol re- (R)-BINAP·AgOTf-catalyzed asymmetric aldol reac-
action. We have shown before that the tion was then demonstrated by using alkenyl trichlor-
BINAP·AgOTf complex is a good chiral catalyst for oacetates prepared from diverse ketones (entries 4–
the asymmetric aldol reaction of aldehydes with al- 6). In addition to cyclic substrates, an acyclic substrate
kenyl trichloroacetates in the presence of a catalytic was also allowed to react with a-keto esters enantio-
amount of Bu2SnACHTUNGTRENNUNG
(OMe)2.[11] Treatment of methyl ben- selectively, although long reaction times were necessa-
zoylformate with 1-trichloroacetoxycyclohexene in ry to produce satisfactory yields (entries 5 and 6).
the presence of (R)-BINAP (10 mol%), AgOTf Worthy of note is that the reaction with methyl ben-
(20 mol%), dibutyltin dimethoxide (8 mol%), and zoylformate showed nearly exclusive diastereoselec-
MeOH (5 equiv.) in dry THF at À208C for 4 h gave a tivity and the highest enantioselectivity (93% ee,
66:34 diastereomeric mixture of optically active aldol entry 5). In contrast, the reaction with methyl pyru-
adduct in 99% combined yield (Table 3, entry 1). The vate proceeded with almost no diastereoselectivity
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Scheme 2. The asymmetric aldol reaction of diketene with methyl benzoylformate catalyzed by Bu2Sn
(R)-BINAP·AgOTf.
ACHTUNGRTEN(NUNG OMe)2 and
(entry 6). Furthermore, we investigated the reactivity À208C for 3 h it furnished a 62:38 diastereomeric
of classical ketones such as acetophenone and cyclo- mixture of the target product in 97% combined yield
hexanone under the standard reaction conditions (Scheme 3). The absolute configuration of the major
(À208C, 4–20 h); however, these simple ketones did diastereomer was assigned to be 2R,2’S by compari-
1
not afford the desired product in the reaction with son of its H NMR, 13C NMR, chiral HPLC retention
the alkenyl trichloro
tetralone, respectively.
acetates of cyclohexanone and a- time, and optical rotation with reported data.[8h] Al-
ACHTUNGTRENNUNG
though its optical purity was not sufficiently high
We further applied the present asymmetric catalytic (32% ee), a similar stereochemistry seems to be appli-
process to the reaction of diketene with an a-keto cable to the products shown in Table 3.
ester in which
Bu2Sn(OMe)2 and diketene might be recycled in the reaction of alkenyl trichloroacetates with a-keto
presence of MeOH.[12] Under conventional reaction esters is illustrated in Figure 1. First of all,
conditions for alkenyl trichloroacetates, good enantio- Bu2Sn(OMe)2 reacts with alkenyl trichloroacetate 1 to
a
tin enolate generated from
A plausible catalytic cycle for the asymmetric aldol
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
selectivity (70% ee) was obtained along with good yield tin enolate 2 accompanied by methyl trichloroa-
chemical yield when a mixture of diketene (2 equiv.) cetate. Enolate 2 is then allowed to add to an a-keto
and methyl benzoylformate (1 equiv.) was treated ester enantioselectively in the presence of the
with 10 mol% of (R)-BINAP, 20 mol% of AgOTf, (R)-BINAP·AgOTf complex, affording the tin alkox-
and 8 mol% of Bu2SnACHTUNGTRENNUNG
MeOH (5 equiv.) at À208C for 50 h (Scheme 2).
Finally, we attempted to determine the absolute ste- cally active aldol product 4 and the regeneration of
reochemistry of the asymmetric aldol reaction of a- dibutyltin dimethoxide. The rapid methanolysis of
keto esters with alkenyl trichloroacetates catalyzed by alkoxide 3 promotes the catalytic cycle.
Bu2Sn
we chose (E)-methyl 4-(4-bromophenyl)-2-hydroxy-2- aldol reaction of alkenyl trichloroacetates with a-keto
(2-oxocyclohexyl)but-3-enoate as a suitable product, esters catalyzed by Bu2Sn(OMe)2 and a bidentate
A
In summary, we have presented an example of an
AHCTUNGTRENNUNG
because the absolute configuration of one stereoiso- phosphine·AgOTf complex in the presence of MeOH,
mer has been unambiguously established by Zhao and its asymmetric version using the BINAP·AgOTf
and co-workers.[8h] When (E)-methyl 4-(4-bromophen- complex as a chiral co-catalyst. The main features of
yl)-2-oxobut-3-enoate was reacted with 1-trichloroace- the present method are as follows: (1) the procedure
toxycyclohexene under the influence of (R)-BINAP is operationally simple, employs readily available
(10 mol%), AgOTf (20 mol%), dibutyltin dimethox- chemicals and can provide optically active b-hydroxy
ide (8 mol%), and MeOH (5 equiv.) in dry THF at ketones possessing a chiral tertiary carbon center with
Scheme 3. Determination of absolute stereochemistry of the asymmetric aldol reaction of an a-keto ester with an alkenyl tri-
chloroacetate catalyzed by Bu2SnACHTUNRTGNEUNG(OMe)2 and (R)-BINAP·AgOTf.
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Asymmetric Aldol Reaction of Ketones with Alkenyl Trichloroacetates
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Typical Experimental Procedure for Asymmetric
Aldol Reaction of a-Keto Esters: Synthesis of Methyl
2-Hydroxy-2-(2’-oxocyclohexyl)-2-phenylacetate
(entry 1 in Table 3)[8a]
A mixture of AgOTf (25.7 mg, 0.10 mmol), (R)-BINAP
(31.1 mg, 0.050 mmol), and MS 3 ꢁ (0.3 g) was dissolved in
dry THF (2.5 mL) under an argon atmosphere and with
direct light excluded, and stirred at 208C for 10 min. To the
resulting solution were added dropwise methyl benzoylfor-
mate (70.8 mL, 0.50 mmol), dibutyltin dimethoxide (9.2 mL,
0.040 mmol), MeOH (100 mL, 2.48 mmol), and 1-trichloro-
AHCTUNGTREGaNNNU cetoxycyclohexene (243.5 mg, 1.00 mmol) successively at
À208C. After stirring for 4 h at this temperature, the mix-
ture was treated with MeOH (2 mL). The mixture was then
treated with solid KF (ca. 1 g) and brine (2 mL) at À208C
for 30 min. The resulting precipitate was filtered off with a
glass filter funnel filled with Celiteꢃ and silica gel. The fil-
trate was dried over Na2SO4 and concentrated under
vacuum after filtration. The residual crude product was puri-
fied by column chromatography on silica gel to afford a mix-
ture of the aldol adducts; yield: 130.1 mg (99%). The diaste-
reomeric ratio was determined to be 66:34 by 1H NMR anal-
ysis. The enantioselectivity of the major isomer was deter-
mined to be 74% ee by HPLC analysis using a chiral
column (Daicel Chiralcel OD-H, hexane/i-PrOH=99/1,
flow rate=1.0 mLminÀ1): tmajor =33.1 min, tminor =48.7 min.
Figure 1. A plausible catalytic mechanism for the asymmet-
ric aldol reaction of a-keto esters catalyzed by
Bu2SnACHTUNGTRENNUNG(OMe)2 and (R)-BINAP·AgOTf.
high enantioselectivity of up to 93% ee; (2) diketene
can be converted into a non-racemic dimethyl 2-hy-
droxy-4-oxohexanedioate derivative by applying this
method; (3) the in situ generated tin enolate reacts
much faster with an a-keto ester than with MeOH
and the resulting tin alkoxide of the aldol adduct is
readily protonated by MeOH to regenerate
1
Spectral data of the product: H NMR (400 MHz, CDCl3, a
Bu2SnACHTUNGTRENNUNG(OMe)2; and (4) this method is environmentally
66:34 mixture of the diastereomers): d=1.51–2.12 (m, 6H, 3
CH2), 2.30–2.47 (m, 2H, CH2), 3.21 (dd, 0.66H, J=5.8,
12.8 Hz, CH), 3.43 (dd, 0.34H, J=6.0, 11.8 Hz, CH), 3.73 (s,
1.02H, CH3), 3.79 (s, 1.98H, CH3), 3.90 (s, 0.34H, OH), 4.20
(s, 0.66H, OH), 7.26–7.38 (m, 3H, aromatic), 7.48–7.51 (m,
1.32H, aromatic), 7.56–7.60 (m, 0.68H, aromatic); 13C NMR
(100 MHz, CDCl3, major isomer): d=25.3, 27.8, 30.5, 43.0,
52.8, 59.5, 79.3, 125.4, 127.9, 128.3, 140.2, 173.3, 212.8; [a]2D1:
À19.78 (c 1.0, CHCl3, major isomer). The above-mentioned
spectral data exhibited good agreement with reported
data.[8a]
benign as the amount of toxic organotin(IV) com-
pound is reduced to a catalytic amount. To the best of
our knowledge, this is the first example of an asym-
metric aldol reaction of ketones using alkenyl esters
as masked enolates. Current efforts in our laboratory
are directed towards the application of the present
catalytic system to other asymmetric reactions.
Experimental Section
General Remarks
Acknowledgements
Column chromatography was conducted with 70–230 mesh
silica gel. 1H NMR spectra were recorded on a 400 MHz
spectrometer. Chemical shifts of the H NMR spectra were
reported relative to tetramethylsilane (d 0). Splitting pat-
terns are indicated as s, singlet; d, doublet; or m, multiplet.
13C NMR spectra were recorded on a 100 MHz spectrome-
ter. Chemical shifts of the 13C NMR spectra were reported
relative to CDCl3 (d=77.0). Analytical high-performance
This work was supported by a Grant-in-Aid for Scientific Re-
search on Priority Area “Advanced Molecular Transforma-
tions of Carbon Resources” from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
1
liquid chromatography (HPLC) was done using a chiral References
column (4.6 mmꢂ25 cm, Daicel Chiralcel OD-H). Optical
rotation was measured on a JASCO P-1020 polarimeter. All
experiments were carried out under an atmosphere of stan-
dard grade argon gas (oxygen <10 ppm). Alkenyl trichloro-
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ketone with trichloroacetic anhydride in the presence of a
catalytic amount of p-toluenesulfonic acid and purified by
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cording to reported procedures.[14] Other chemicals were
used as purchased.
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Akira Yanagisawa et al.
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[8] For examples of organocatalytic asymmetric direct
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1762
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Adv. Synth. Catal. 2009, 351, 1757 – 1762