Remarkable enhancement of catalyst activity of trialkylsilyl sulfonates on the Mukaiyama aldol reaction: A new approach using bulky organoaluminum cocatalysts [14]

Full text of DOI:10.1021/ja981464o

J. Am. Chem. Soc. 1998, 120, 8271-8272
8271
Remarkable Enhancement of Catalyst Activity of
Table 1. a Aldol Reaction of 2-Phenylpropanal and Silyl Enol
Ether 3A
Trialkylsilyl Sulfonates on the Mukaiyama Aldol
Reaction: A New Approach Using Bulky
Organoaluminum Cocatalysts
Masataka Oishi, Seiji Aratake, and Hisashi Yamamoto*
entry
catalystb
SiOTf
% yieldc
syn/antid
Graduate School of Engineering, Nagoya UniVersity, CREST
Japan Science and Technology Corporation (JST)
Chikusa, Nagoya, 464-8603, Japan
1
2
3
4
5
6
7
8
9
Me
Me
Me
3
3
3
15
0
75:25
SiI
e
SiOTf/B(OTf)
3
43
41
32
47
62
76
7
0
4
0
78
0
90:10
89:11
65:35
61:39
91:9
89:11
42:58
ReceiVed April 29, 1998
B(OTf)
3
i
Silylium ion is currently one of the most challenging topics in
the chemistry of carbenium ion analogues and has attracted
Me
Me
3
SiOTf/ Bu
3
Al
3
SiOTf/Et AlCl
2
1,2
3
Me SiOTf/MAD
widespread interest in related areas. During the continual efforts
to isolate the absolutely naked silylium cation from basic species
Me
3
SiOTf/MABR
3
MABR
containing a counteranion, solvent and π-electron donor, the
representative ionic silicon-based catalysts, Me SiOTf (1) and
3
10
11
12
13
Me
Me
Me
Me
3
3
3
3
SiCl
SiCl/MABR
SiOFs
SiOFs/MABR
39:61
91:9
4
3
Me SiClO
4
(2) have been well documented in organic synthesis.
f
The activity of these catalysts, however, depends on the reaction
f
employed.5 Recently, Davis and co-workers reported that the
g
14
15
Me SiOMs
3
SiOMs/MABRg
64
88:12
combination of trialkylsilyl triflates or chlorides with B(OTf)
3
Me
3
displays high catalyst activity on the Sakurai allylation and the
Mukaiyama aldol reaction.6 Unfortunately, modification of triflate
to a more electron-deficient group often causes the unavoidable
formation of byproducts.7 Recent mechanistic studies imply that
a
The reaction was carried out at -78 °C for 1 h in the following
scale: aldehyde (10 mmol), silyl enol ether (11 mmol), and dichlo-
b
romethane (20 mL) were used. Of the catalyst or catalyst pair (1:1),
mol % was employed. c Isolated yield. d Determined by 300 MHz
5
1
e
some metal triflates, Zn(OTf)
2
, Sn(OTf)
2
, Yb(OTf)
3
, Cp
2
Ti(OTf)
2
,
H NMR analysis. See ref 6b; literature result: 44% yield, syn/anti
f
g
a
and TrOTf behave as initiators or promotors but that in situ-
) 8:1 FsOH:fluorosulfonic acid. MsOH:methanesulfonic acid. The
generated 1 is likely to be the true catalyst in the process.8 In
reaction was carried out at -78 °C for 1 h in the following scale:
aldehyde (10 mmol), silyl enol ether (11 mmol), and dichloromethane
addition, the aldol methodology remains a major limitation in
the catalytic cross-aldol reaction of ketones with currently
available Lewis acids.9 Hence, we focused on developing a new
and clean system with high activity toward ketones as well as
b
(
20 mL) were used. Of the catalyst or catalyst pair (1:1), 5 mol %
was employed. Isolated yield. d Determined by 300 MHz H NMR
c
1
e
analysis. See ref 6b; literature result: 44% yield, syn/anti ) 8:1
f
FsOH:fluorosulfonic acid. g MsOH:methanesulfonic acid.
(
1) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1393.
(
2) (a) Maerker, C.; Kapp, J.; Schleyer, P. v. R. In Organosilicon
toward less reactive aldehydes, while suppressing side reactions.
Our original strategy involved shifting the equilibrium in the
following complexation by preferentially trapping triflate anion
Chemistry: from Molecules to Materials; Auner, N., Weis, J., Eds.; VCH:
Weinheim, 1996; Vol. II. (b) Schleyer, P. v. R. Science 1997, 275, 39. (c)
Olah, G. A.; Heiliger, L.; Li, X.-Y.; Prakash, G. K. S. J. Am. Chem. Soc.
1
990, 112, 5991. (d) Lickiss, P. D. J. Chem. Soc., Dalton Trans. 1992, 1333.
e) Pauling, L. Science 1994, 263, 983. (f) Olah, G. A.; Rasul, G.; Li, X.-Y.;
Buchholz, H. A.; Sandford, G.; Prakash, G. K. S. Science 1994, 263, 983.
(
(3) (a) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science
1
2
993, 260, 1917. (b) Reed, C. A.; Xie, Z.; Bau, R.; Benesi, A. Science 1993,
62, 402. (c) Lambert, J. B.; Zhang, S. Science 1994, 263, 984. (d) Reed, C.
A.; Xie, Z. Science 1994, 263, 985. (e) Lambert. J. B.; Zhang, S. J. Chem.
Soc., Chem. Commun. 1993, 383. (f) Xie, Z.; Manning, J.; Reed, R. W.;
Mathur, R.; Boyd. P. D. W.; Reed, C. A. J. Am. Chem. Soc. 1996, 118, 2922.
(
g) Lambert, J. B.; Zhao, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 400.
(
4) For reviews of 1 and 2 in organic synthesis see: (a) Noyori, R.; Murata,
S.; Suzuki, M. Tetrahedron 1981, 37, 3899. (b) Emde, H.; Domsch, D.; Feger,
H.; Frick, U.; G o¨ tz, A.; Hergott, H. H.; Hofmann, K.; Kober, W.; Kr a¨ geloh,
K.; Oesterle, T.; Steppan, W.; West, W.; Simchen, G. Synthesis 1982, 1. (c)
Murata, S.; Suzuki, M.; Noyori, R. Tetrahedron 1988, 44, 4259. (d) Simchen,
G. In AdVances in Silicon Chemistry; Larson, G. L., Ed.; JAI: London, 1991;
Vol. 1, pp 189-301.
rather than Lewis basic substrates and products with Lewis acid-
type receptors possessing high molecular recognition ability.
Indeed, according to our preliminary 13C NMR results, the
equilibrium of a benzaldehyde/1 complex in CDCl
seems to be located mostly on the left side since the shifts of the
carbonyl carbon and the Me Si group of the complex are both
3
at -50 °C
(5) For the scope and limitation of 1 as a Lewis acid catalyst, see: (a)
Mukaiyama aldol reaction of silyl enol ethers, Mukai, C.; Hashizume, S.;
Nagami, K.; Hanaoka, M. Chem. Pharm. Bull. 1990, 38, 1509. (b) Sakurai
reaction, see ref 4a. (c) Cyanosilylation of aldehydes and ketones, Kaur, H.;
Kaur, G.; Trehan, S. Synth. Commun. 1996, 26, 1925.
3
negligible. We report here the remarkable rate enhancement on
the trialkylsilyl triflate-catalyzed Mukaiyama aldol reaction of silyl
enol ethers by using a bulky organoaluminum reagent, i.e.,
(
6) (a) Davis, A. P.; Jaspars, M. Angew. Chem., Int. Ed. Engl. 1992, 31,
4
2
9
70. (b) Davis, A. P.; Plunkett, S. J. J. Chem. Soc., Chem. Commun. 1995,
173. (c) Davis, A. P.; Muir, J. E.; Plunkett, S. J. Tetrahedron Lett. 1996, 37,
401.
(11) Maruoka, K.; Sato, J.; Yamamoto, H. J. Am. Chem. Soc. 1992, 114,
4422.
(
7) Significant amounts of nonpolar byproducts are produced, particularly
(12) Trimethylsilyl sulfonates can be prepared from allyltrimethylsilane and
the corresponding sulfonic acid in dichloromethane without triflic acid. (a)
Morita, T.; Okamoto, Y.; Sakurai, H. Synthesis 1981, 745. (b) Lipshutz, B.
H.; Burgess-Henry, J.; Roth, G. P. Tetrahedron Lett. 1993, 34, 995.
(13) Stability Constants of Metal-ion Complexes; Special Publication No.
7; The Chemical Society: London, 1964; Section 1.
in Sakurai allylation. (a) See ref 6a. (b) Ishii, A.; Kotera, O.; Saeki, T.; Mikami,
K. Synlett 1997, 1145.
(
8) (a) Carreira, E. M.; Singer, R. A. Tetrahedron Lett. 1994, 35, 4323.
(
b) Hollis, T. K.; Bosnich, B. J. Am. Chem. Soc. 1995, 117, 4570.
(
9) For cross-aldol reactions of ketones via Reformatsky-type reactions,
see: (a) Maruoka, M.; Hashimoto, S.; Kitagawa, Y.; Yamamoto, H.; Nozaki,
(14) (a) Stoichiometric reaction: Mukaiyama, T.; Banno, K.; Narasaka,
K. J. Am. Chem. Soc. 1974, 96, 7503. (b) Zr(IV)- and Ti(IV)-catalyzed
reactions at room temperature: Hollis, T. K.; Robinson, N. P.; Bosnich, B.
H. J. Am. Chem. Soc. 1977, 99, 7705. (b) Wessjohann, L.; Wild, H. Synthesis
1
997, 512.
(
10) (a) Maruoka, K.; Araki, Y.; Yamamoto, H. J. Am. Chem. Soc. 1988,
3
Tetrahedron Lett. 1992, 33, 6423. (c) BiCl -catalyzed or -mediated reaction
1
1
10, 2650. (b) Maruoka, K.; Nagahara, S.; Yamamoto, H. J. Am. Chem. Soc.
990, 112, 6115. (c) Maruoka, K.; Saito, S.; Yamamoto, H. J. Am. Chem.
at room temperature: Wada, M.; Takeichi, E.; Matsumoto, T. Bull. Chem.
Soc. Jpn. 1991, 64, 990.
Soc. 1992, 114, 1089.
(15) See Supporting Information.
S0002-7863(98)01464-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/30/1998

8272 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Communications to the Editor
Table 2. Catalytic Aldol Reaction of Silyl Enol Ether 3A or 3b
a
with Aldehyde or Ketone
time yieldb
entry
substrates
Ph(CH CHO/3a
catalyst (mol %)
(h)
(%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
)
2
Me
Me
3
3
3
SiOTf(1)
SiOTf-MAD(1)
SiOTf-MABR(1)
0.5
20
61
83
14
72
2
Me
t
BuMe
BuMe
2
SiOTf(1)
t
2
SiOTf -MABR(1)
(CH
3
)
)
3
3
CCHO/3a
CCHO/3b
Me
Me
Me
Me
Me
Me
Me
Me
Me
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
SiOTf(5)
SiOTf-MABR(5)
SiOTf(5)
SiOTf-MABR(5)
SiOTf(5)
SiOTf-MAD(5)
SiOTf-MABR(5)
SiOTf(5)
SiOTf-MAD(5)
SiOTf-MABR(5)
SiOTf(5)
SiOTf-MAD(5)
SiOTf-MABR(5)
SiOTf(5)
1
74
0
87
(CH
3
1
c
Figure 2. 13C NMR spectra at -50 °C in CDCl
of (a) free PhCHO, (b)
3
MAD/PhCHO (1:1) complex, (c) MAD/PhCHO/1 (1:1:1) complex, and
(d) MAD/PhCHO/1 (1:1:2) complex.
0
1
2
3
4
5
6
7
8
9
0
cyclohexanone/3a
0.5
30
86
90
3
PhCOCH
3
/3a
2
60
72
1
77
91
seen in Table 2, aliphatic and sterically hindered aldehydes
undergo smooth aldol addition with silyl enol ether 3a or 3b into
the corresponding aldol adducts (entries 1-9). Surprisingly, the
present system supersedes any previously reported Lewis acid
catalysis, and makes the catalytic cross-aldol reaction between
simple ketones feasible under mild reaction conditions (entries
Me
3
(CH )
2
CHCOCH
3
/3a Me
Me
1.5
2
Me
Me
Me
d
PhCOCH
3
/3b
14
d
SiOTf-MABR(5)
90
1
0-18).14 For example, exposure of isopropyl methyl ketone
a
The reaction was performed at -78 °C in the following scale:
and 3a to 5 mol % of 1 and MABR (1:1) in dichloromethane at
-78 °C for 1.5 h followed by acidic workup gave the corre-
sponding â-hydroxy ketone in 91% yield without the formation
of a dehydrated byproduct, while a trace amount of the adduct
was produced in the absence of MABR. Furthermore, catalytic
aldehyde or ketone (10 mmol), 3a or 3b (11 mmol), dichloromethane
20 mL) were used. Isolated yield. The syn/anti ratio was determined
by 300 MHz H NMR to be 15:85. The syn/anti ratios were
determined by 300 MHz H NMR to be 35:65 (entry 19) and 28:72
entry 20), respectively. The stereochemical assignment was made by
comparison with the H NMR spectrum of the authentic sample (Ghera,
b
c
(
1
d
1
(
1
E.; Shoua, S. J. Org. Chem. 1972, 37, 1292).
cross-aldol coupling between acetophenone and cyclohexanone
leads to a desired aldol product in high yield, as illustrated below.
The reaction profile of the aldol reaction of pivalaldehyde with
15
3b clearly shows a multiplier effect of 1 and MABR (Figure 1).
This new finding prompted us to explore the active species in
the catalyst system. Accordingly, a 13C NMR spectral study of
benzaldehyde/1/MAD complexes was carried out at -50 °C in
CDCl , where the downfield shift of carbonyl carbon in the
3
complex from the aldehyde coordinated with MAD was observed
(204.4 ppm) as revealed in Figure 2.15 The peak (201.7 ppm) of
the carbonyl carbon based on the benzaldehyde-MAD complex
disappeared completely when 2 equiv of 1 were added (Figure
2d). Similar results were obtained for the addition of benzalde-
hyde and 1 in the reverse order. These observations indicate that
a more strongly Lewis acidic species forms from two different
Lewis acids of the bulky organoaluminum reagent and 1 in the
presence of an aldehyde.
Figure 1. Reaction profile of the catalytic aldol addition of 3b (20 mmol)
to pivalaldehyde (20 mmol) in dichloromethane (40 mL) at -78 °C.
methylaluminum bis(4-substituted-2,6-di-tert-butylphenoxide), MAD
or MABR, as a cocatalyst.10,11
Initially, we examined the relative activities of various com-
bined catalysts on the aldol reaction of less reactive 2-phenyl-
propanal and 3a under the reaction conditions shown in Table 1.
3
The catalyst pair of 1 and B(OTf) resulted in a moderate yield
In conclusion, we have demonstrated that the new combined
system of trialkylsilyl sulfonate-MAD or -MABR exhibits
exceedingly high activity on the catalytic Mukaiyama aldol
reaction of silyl enol ethers with aldehydes and ketones, where
MAD and MABR are currently thought to be cocatalysts
stabilizing a silyl counteranion. Studies on the development of
chiral super Lewis acid catalysts are now underway.
similar to that of B(OTf) (entries 3 and 4). In marked contrast
3
to the disappointing results with both 1 and MABR (entries 1
and 9), the combination of 1 with MAD or MABR appeared to
be an optimal choice to attain higher chemical yield (entries 7
and 8). Other organoaluminum reagents gave less satisfactory
results with a lower syn/anti ratio (entries 5 and 6). It should be
3 3
noted that Me SiOFs and Me SiOMs are also employable as active
catalysts in the presence of an equimolar amount of MABR
Supporting Information Available: Experimental procedures and
C NMR data (3 pages, print/PDF). See any current masthead page for
ordering information and Web access instructions.
reagent (entries 13 and 15), indicating that interaction between
13
aluminum and fluorine is not necessarily important.12,13
This remarkably active catalyst system was then applied to a
range of less reactive carbonyl substrates (Table 2). As can be
JA981464O