Strong counteranion effects on the catalytic activity of cationic silicon lewis acids in Mukaiyama aldol and Diels-Alder reactions

Article abstract of DOI:10.1021/ol052206g

(Graph Presented) A toluene-coordinated silyl borate, [Et ₃Si(toluene)]B(C₆F₅)₄, demonstrated catalytic activities significantly higher than those of Me₃SiOTf and Me₃SiNTf₂ in Mu

Full text of DOI:10.1021/ol052206g

ORGANIC
LETTERS
2005
Vol. 7, No. 25
5621-5623
Strong Counteranion Effects on the
Catalytic Activity of Cationic Silicon
Lewis Acids in Mukaiyama Aldol and
Diels−Alder Reactions
Kenji Hara, Ryuto Akiyama, and Masaya Sawamura*
DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo
060-0810, Japan, and PRESTO, Japan Science and Technology Agency
sawamura@sci.hokudai.ac.jp
Received September 13, 2005
ABSTRACT
A toluene-coordinated silyl borate, [Et3Si(toluene)]B(C6F5)4, demonstrated catalytic activities significantly higher than those of Me3SiOTf and
Me3SiNTf2 in Mukaiyama aldol and Diels Alder reactions.
−
Various Lewis acids have been employed as catalysts for
synthetic organic reactions. Although silicon Lewis acids are
typically silyl chlorides or silyl triflates (R3SiOTf),1 several
other new types of silicon Lewis acids have been utilized.
For example, silyl bis(trifluoromethansulfonyl)imides 1 (R3-
SiNTf2) can act as stronger Lewis acids than Me3SiOTf and,
consequently, have been used as an efficient catalyst for
Friedel-Crafts alkylation,2a allylation of ketal,2a Diels-Alder
reaction,2b-d Mukaiyama aldol reaction,2e and Sakurai-
Hosomi allylation.2e Another new type of the silicon Lewis
acid is the cationic silicon species such as silyl borates [R3-
Si(L)]BAr4 (2), where Ar is a flourine-containing aromatic
substituent and L is a donor group that can coordinate to
the silicon center.3,4
Although several previous studies have focused on the
catalytic activity of silyl borates 2, their cationic characters
were far from those of a “true silyl cation”. One of these
catalysts was used in a strong donor solvent (CH3CN),5 while
others were coordinated with an intramolecular donor group.6
In both cases, the coordination would decrease the inherent
(1) Dilman, A. D.; Loffe, S. L. Chem. ReV. 2003, 103, 733.
(2) (a) Ishii, A.; Kotera, O.; Saeki, T.; Mikami, K. Synlett 1997, 1145.
(b) Mathieu, B.; Ghosez, L. Tetrahedron Lett. 1997, 38, 5497. (c) Mathieu,
B.; de Fays, L.; Ghosez, L. Tetrahedron Lett. 2000, 41, 9561. (d) Mathieu,
B.; Ghosez, L. Tetrahedron 2002, 58, 8219. (e) Ishihara, K.; Hiraiwa, Y.;
Yamamoto, H. Synlett 2001, 1851.
(3) For reviews, see: (a) Lambert, J. B.; Kania, L.; Zhang, S. Chem.
ReV. 1995, 95, 1191. (b) Reed, C. A. Acc. Chem. Res. 1998, 31, 325.
(4) Lambert, J. B.; Zhang, S. J. Chem. Soc., Chem. Commun. 1993, 383.
(5) Johannsen, M.; Jørgensen, K. A.; Helmchen, G. J. Am. Chem. Soc.
1998, 120, 7637.
10.1021/ol052206g CCC: $30.25
© 2005 American Chemical Society
Published on Web 11/12/2005

Lewis acidities of the silicon center. In contrast, a silyl borate
that possesses a strong cationic character is [Et3Si(toluene)]B-
(C6F5)4 (2a), a silyl cation coordinated by toluene7 that was
first reported by Lambert et al. (eq 1).4
Table 1. Mukaiyama Aldol Reaction with Acetophenone
Catalyzed by Silicon Lewis Acidsa
amount of
catalyst (mol %)
entry
catalyst
solvent
yield (%)b
Despite their use as catalysts for the hydrosilylation of
1,1-diphenylethene8 and for the hydrodefluorination of
benzylic C-F bonds,9 the application of such silyl borates
for the formation of C-C bonds remains uncommon.
Mukaiyama et al. have used silyl borate Me3SiBAr4 (Ar )
C6F5), generated in situ from Me3SiCl and AgBAr4, as a
catalyst for the aldol-type reaction between an enol ester and
an aldehyde; unfortunately, the improved catalytic activity,
in comparison to that of Me3SiOTf, was not clearly dem-
onstrated.10 Herein, we report on the improved catalytic
activity of [Et3Si(toluene)]B(C6F5)4 (2a), in comparison to
Me3SiOTf and Me3SiNTf2, as a precatalyst in Mukaiyama
aldol and Diels-Alder reactions.
The catalytic performance of toluene-coordinated silyl
borate 2a was first examined in the Mukaiyama aldol reaction
of ketone, for which only a limited number of effective
catalysts has been reported to date.2b,6,11 In the presence of
2a (1.0 mol %), the reaction of acetophenone (3a) with enol
silyl ether 4a was carried out at -78 °C and was complete
within 1 h to afford, after hydrolytic workup, aldol product
5aa in a quantitative yield (Table 1, entry 1). In contrast,
conversion was not observed using Me3SiOTf catalyst (1 mol
%) at -78 °C (entry 9). The catalytic activities of Me3SiNTf2
(1a) and Et3SiNTf2 (1b) were higher than that of Me3SiOTf,
in accordance with previously reported results, but afforded
only 12% and 8% yields, respectively, under the same
reaction conditions (entries 10 and 11).
1
2a
2a
2a
2a
2a
2a
2a
2a
1.0
1.0
0.5
0.1
1.0
1.0
1.0
1.0
1.0
1.0
1.0
toluene
toluene
toluene
toluene
CH2Cl2
Et2O
CH3CN
no solvent
toluene
toluene
toluene
96
97
97
34
12
0
6
98
0
2c
3d
4e
5
6
7f
8g
9
Me3SiOTf
Me3SiNTf2 (1a)
Et3SiNTf2 (1b)
10
11
12
8
a Reaction conditions: 3a (0.50 mmol), 4a (1.1 equiv), 2a (1.0 mol %),
toluene (1.5 mL), -78 °C, 1 h. b Determined by NMR analysis of the crude
product using CH2Br2 as an internal standard. c 2,6-Di-tert-butylpyridine
(2.0 mol %) was added. d Reaction at 1.0 mmol scale. e Reaction at 5.0
mmol scale. f Reaction at -40 °C. g Reaction at 25 °C.
In terms of the solvent, switching from toluene to CH2-
Cl2, Et2O, or CH3CN caused a drastic decrease in the yields
(entries 5-7). The negative effect of Et2O or CH3CN is
attributable to the coordination of the solvent molecules. In
the case of CH2Cl2, the low catalytic activity is most likely
due to the abstraction of a chloride by the cationic silicon
center.12 As a note, the solvent-free reaction attained complete
conversion (98% yield) at 25 °C in 1 h (entry 8).
To explore the range of the reaction, the Mukaiyama aldol
reaction in the presence of 2a was carried out using other
substrates (Table 2). Enol silyl ether 4a reacted with both
acyclic (3b) and cyclic (3c) aliphatic ketones at -78 °C to
afford the corresponding products quantitatively (entries 1
and 2). Benzaldehyde (3d) afforded aldol product 5da in
96% yield without any trace of byproducts (entry 3).
R-Monosubstituted enol silyl ether 4b was converted to
addition product 5ab in 94% yield in 8 h with 72% threo-
selectivity (entry 4). The reaction of ketene silyl acetal 4c
afforded hydroxy ester 5ac with consecutive, quarternary
carbons in 95% yield (entry 5).
To determine the possibility of a proton-promoted reaction,
the experiment was repeated, but with a proton scavenger,
2,6-di-tert-butylpyridine. As shown in entry 2, the presence
of such a scavenger did not affect the yield, thus excluding
such proton-promoted reactions. At lower catalyst loading
(0.5 mol %), the catalyst remained effective (97% yield)
(entry 3); however, 0.1 mol % resulted in an incomplete
conversion (34% yield) (entry 4).
Next, the catalytic performance of 2a for Diels-Alder
reactions was evaluated. Results for the reaction of 1,3-
cyclohexadiene and methyl acrylate in the presence of
various silicon-based catalysts (toluene, 0 °C, 1 h) are
summarized in Table 3. As shown in entry 1, Me3SiOTf (10
mol %) did not exhibit any catalytic activity. The presence
of 1a (1 mol %) or 1b (1 mol %) did afford cycloadduct 6a,
but in yields of merely 13% (entry 2) and 6% (entry 3),
respectively. Using 2a (1 mol %) as a precatalyst, however,
almost complete conversion of 97% (entry 4) was observed.
Similarly as above, the presence of a proton scavenger did
not affect the yield (entry 5).
(6) Hatanaka, Y.; Tanaka, M. Abstracts of Papers, 81st Spring Meeting
of the Chemical Society of Japan, Tokyo; Chemical Society of Japan: Tokyo,
2001; 2G608.
(7) Olah and other researchers have pointed out that 2a is not a
coordinated “silyl cation” but silylated toluenium. See: Olah, G. A.; Rasul,
G.; Purkash, G. K. S. J. Am. Chem. Soc. 1999, 121, 9615 and references
therein.
(8) Lambert, J. B.; Zhao, Y.; Wu, H. J. Org. Chem. 1999, 64, 2729.
(9) Scott, V. J.; C¸ elenligil-C¸ etin, R.; Ozerov, O. V. J. Am. Chem. Soc.
2005, 127, 2852.
(10) Yanagisawa, M.; Shimamura, T.; Iida, D.; Matsuo, J.; Mukaiyama,
T. Chem. Pharm. Bull. 2000, 48, 1838.
(11) For selected papers on catalytic Mukaiyama aldol reactions with
ketones, see: (a) Hollis, T. K.; Robinson, N. P.; Bosnich, B. Tetrahedron
Lett. 1992, 33, 6423. (b) Ohki, H.; Wada, M.; Akiba, K. Tetrahedron Lett.
1988, 29, 4719. (c) Marx, A.; Yamamoto, H. Angew. Chem., Int. Ed. 2000,
39, 178. (d) Oishi, M.; Aratake, S.; Yamamoto, H. J. Am. Chem. Soc. 1998,
120, 8271.
(12) Kira, M.; Hino, T.; Sakurai, H. J. Am. Chem. Soc. 1992, 114, 6697.
Org. Lett., Vol. 7, No. 25, 2005
5622

Table 2. Mukaiyama Aldol Reaction Catalyzed by
[Et3Si(toluene)]B(C6F5)4 (2a)a
Table 4. Diels-Alder Reaction Catalyzed by Silyl Borate 2aa
a Reaction conditions: dienophile (0.50 mmol), diene (2.0 equiv), 2a
(1.0 mol %), toluene (1.5 mL), 0 °C, 1 h. b Isolated yield. c endo:exo )
98:2. d endo:exo ) 99:1. e The diene containing 100 ppm (w/w) of 2,6-di-
tert-butyl-4-methylphenol was used to inhibit the polymerization of the
dienes.
a Reaction conditions: 3 (0.50 mmol), 4 (1.1 equiv), 2a (1.0 mol %),
toluene (1.5 mL), -78 °C, 1 h. b Isolated yield. c Reaction for 8 h.
d Diastereoselectivity of the product 5ab was 72:28 (threo:erythro).
°C to afford adduct 6b in 93% yield in 13 h with 98% endo-
selectivity (entry 1). It should be noted that the methyl
crotonate is a dienophile of poor reactivity and that the
catalytic reaction of this particular substrate with simple diene
is rare.13 Both methyl vinyl ketone and crotonaldehyde
reacted with 1,3-cyclohexadiene to afford cycloadducts 6c
and 6d almost quantitatively without any trace of byproducts
(entries 2 and 3, respectively). The reaction of 2,3-dimethyl-
1,3-butadiene with methyl acrylate gave addition product 6e
in a high yield (entry 4).
Results of Diels-Alder reaction catalyzed by 2a using
combinations of other substrates are summarized in Table
4. Methyl crotonate reacted with 1,3-cyclohexadiene at 25
Table 3. Diels-Alder Reaction Catalyzed by Silicon Lewis
Acidsa
In summary, the high catalytic activity of [Et3Si(toluene)]B-
(C6F5)4 (2a), in comparison to those of Me3SiOTf and Et3-
SiNTf2 (1b), was clearly demonstrated in Mukaiyama aldol
and Diels-Alder reactions. Studies to explore the scope of
the strongly cationic silyl borate catalyst are currently
underway in our laboratory.
entry
catalyst
mol %
yield (%)b
1
2
3
4
5c
Me3SiOTf
Me3SiNTf2 (1a)
Et3SiNTf2 (1b)
[Et3Si(toluene)]B(C6F5)4(2a)
[Et3Si(toluene)]B(C6F5)4(2a)
10
0
6
13
97
97
Supporting Information Available: Experimaental pro-
cedures and characterization data for the products. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1.0
1.0
1.0
1.0
a Reaction conditions: methyl acrylate (0.50 mmol), 1,3-cyclohexadiene
(2.0 equiv), silicon catalyst (10 or 1.0 mol %), toluene (1.5 mL), 0 °C, 1 h.
b Yields were determined by NMR analysis of the crude product using
CH2Br2 as an internal standard. c 2,6-Di-tert-butylpyridine (2.0 mol %) was
added.
OL052206G
(13) Reported catalytic Diels-Alder reactions of methyl crotonate with
simple diene were conducted with higher catalyst loading (10 mol %). For
details, see ref 2c,d and Ishihara, K.; Kobayashi, J.; Inagawa, K.; Yamamoto,
H. Synlett 2001, 394.
Org. Lett., Vol. 7, No. 25, 2005
5623