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УDesigner AcidsФ Combined Acid Catalysis for Asymmetric Synthesis.

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Reviews
H. Yamamoto and K. Futatsugi
> Designed Catalysis
“Designer Acids”: Combined Acid Catalysis for
Asymmetric Synthesis
Hisashi Yamamoto* and Kentaro Futatsugi
Keywords:
asymmetric catalysis · Brønsted acids ·
catalysis design · cooperative
effects · Lewis acids
Angewandte
Chemie
1924
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200460394
Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
Angewandte
Combined Acid Catalysis
Chemie
Lewis and Brønsted acids can be utilized as more-effective tools for
chemical reactions by sophisticated engineering (“designer acids”).
The ultimate goal of such “designer acids” is to form a combination of
acids with higher reactivity, selectivity, and versatility than the individual acid catalysts. One possible way to take advantage of such
abilities may be to apply a “combined acids system” to the catalyst
design. The concept of combined acids, which can be classified into
Brønsted acid assisted Lewis acid (BLA), Lewis acid assisted Lewis
acid (LLA), Lewis acid assisted Brønsted acid (LBA), and Brønsted
acid assisted Brønsted acid (BBA), can be a particularly useful tool for
the design of asymmetric catalysis, because combining such acids will
bring out their inherent reactivity by associative interaction, and also
provide more-organized structures that allow an effective asymmetric
environment.
1. Introduction
In 1923 Lewis laid the foundation for our modern
understanding of acid catalysis. His definition of the
valence-bond principle encompasses all chemical phenomena. The designation of electron-pair donors and acceptors as
Lewis bases and Lewis acids, respectively, is firmly ingrained
in the chemical language. However, the true power and
elegant applications of this reagent to asymmetric synthesis[1]
has only begun to be realized. The developments continue
unabated, and nowadays many different Lewis acid catalysts
have been elaborated and cover almost all the metals in the
periodic table.
This Review summarizes a program of research that has
been carried out in our laboratories at Nagoya and Chicago
over the past two decades with the objective of extending the
research of Lewis acid catalysis to new domains, specifically
targeting combined acid catalysis for asymmetric synthesis
and a much deeper understanding of the fine mechanistic
details and pathways of these reagents. Many other research
groups have also been active in this field of asymmetric
synthesis, and the science of organic synthesis has benefited
greatly from the development of new reagents and catalysts
for other types of enantioselective reactions.
2. Combined Acid Catalysis: Introduction and
Background
Coordination of a ketone or aldehyde to a Lewis acid can
promote enolization by virtue of the enhanced acidity of the
a-hydrogen atoms in the complex. The practical utility of
these types of Lewis acid activation is well developed. For
example, changes in acidities of up to 24 pKa units are
observed by combining the measured gas-phase acidities with
the calculated solvation energies for these species
(Figure 1).[2] This example can be described as a typical case
for a Lewis acid activation of a weak Brønsted acid.
Similar activations can be readily found in various reports
as combined acid complexes. Clark showed that the catalytic
Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
From the Contents
1. Introduction
1925
2. Combined Acid Catalysis:
Introduction and Background
1925
3. Brønsted Acid Assisted Lewis
Acid Catalysts (BLA)
1926
4. Lewis Acid Assisted Lewis Acid
Catalysts (LLA)
1931
5. Lewis Acid Assisted Brønsted
Acid Catalysts (LBA)
1937
6. Brønsted Acid Assisted
Brønsted Acid Catalysts (BBA)
1940
7. Conclusions and Outlook
1941
Figure 1. Lewis acid activation of a weak Brønsted acid.
activity of silica-supported AlCl3 arose from Lewis acid
activation of the Brønsted acidic site on the silica surface
(Figure 2).[3]
Figure 2. Brønsted acidity arising from Lewis acid complexation on a
silica surface.
Negishi reported the principle of activation of Lewis acids
or electrophiles through association (Figure 3).[4] He described that coordinatively unsaturated monomers are far
more Lewis acidic than doubly bridged coordinatively
saturated dimers. However, the same compounds can readily
generate a species that is even more Lewis acidic than the
monomer through the formation of a singly bridged dimer.
This species is also an example of a combined acid catalyst.
[*] Prof. H. Yamamoto, K. Futatsugi
Department of Chemistry
University of Chicago
5735 South Ellis Avenue, Chicago IL 60637 (USA)
Fax: (+ 1) 773-702-0805
E-mail: yamamoto@uchicago.edu
DOI: 10.1002/anie.200460394
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1925
Reviews
H. Yamamoto and K. Futatsugi
Figure 4. Plausible involvement of dimeric acids in acid-catalyzed
addition of HCl to alkenes.
more or less intramolecular assembly of such combined
systems rather than intermolecular arrangements. Thus, a
correct design of the catalyst structure is essential for success.
Figure 3. State of association of Lewis acid M-X.
Classic acid-catalyzed reactions, such as the addition of
HCl to alkenes, often show a second-order dependence on the
acids. In that sense, the involvement of dimeric acids such as
HCl···HCl might be speculated (Figure 4).[5] The same principle of activation of Brønsted acids plays an important role in
many cases, a number of which are regarded as Brønsted acid
assisted Brønsted acid systems.
These combined acid catalysts can be classified as shown
in Table 1. It should be emphasized that we anticipated a
3. Brønsted Acid Assisted Lewis Acid Catalysts (BLA)
When Lewis acids and Brønsted acids are combined,
conjugate Friedel–Crafts acids are produced (e.g. anhydrous
HF·BF3 and HCl·AlCl3). They effectively catalyze hydrocarbon transformations. In the early 1960s, much stronger
acids were prepared. Magic acid (HSO3F·SbF5) and fluoroantimonic acid (HF·SbF5) are two well-known examples.[6]
Table 1: General classifications of combined acid catalysis.
Catalyst system
General structure
Examples
Brønsted acid assisted Lewis acid catalyst (BLA)
Enhancement of Lewis acidity by the combination with Brønsted acid
Lewis acid assisted Lewis acid catalyst (LLA)
Enhancement of Lewis acidity by the combination with Lewis acid
Lewis acid assisted Brønsted acid catalyst (LBA)
Enhancement of Brønsted acidity by the combination with Lewis acid
Brønsted acid assisted Brønsted acid catalyst (BBA)
Enhancement of Brønsted acidity by the combination with Brønsted acid
Hisashi Yamamoto received his BSc from
Kyoto University and his PhD from Harvard
University (Prof. E. J. Corey). He held academic positions at Kyoto University and at
the University of Hawaii, before he moved
to Nagoya in 1980, where he became Professor (1983). In 2002, he was appointed
Professor at the University of Chicago. He
has been honored with the Prelog Medal
(1993), the Chemical Society of Japan
Award (1995), the Max-Tishler Prize
(1998), Le Grand Prix de la Fondation
Maison de la Chimie (2002), the National
Prize of the Purple Medal (Japan) (2002),
and the Yamada Prize (2004).
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Kentaro Futatsugi received his BSc from
Nagoya University in 2001 under the supervision of Professor Hisashi Yamamoto. He is
currently carrying out research towards his
PhD in organic chemistry at the University
of Chicago under the supervision of Professor
Hisashi Yamamoto. His research is directed
toward the development of stereoselective
reactions with a Lewis acid assisted chiral
Lewis acid (LLA) system and their application to the synthesis of biologically important
compounds.
Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
Angewandte
Combined Acid Catalysis
Chemie
Our results suggested that several characteristics of the
chiral Lewis acid catalyst are enhanced significantly by
attachment of a Brønsted acid. In 1986 we reported the
asymmetric Diels–Alder reaction of naphthoquinone derivatives and various dienes in the presence of a chiral boron
reagent derived from B(OMe)3 and (R,R)-(+)-tartaric acid
diamide (Scheme 1).[7] The reagent has been described as a
Scheme 2. Asymmetric reactions promoted by CAB catalysts.
Scheme 1. Asymmetric Diels-Alder reaction promoted by a tartaric acid
diamide derived chiral boron reagent.
model for the enantioselective preparation of important
tetracycline natural products. Both the rate enhancement and
the high enantioselectivity observed for this type of auxiliary
can be ascribed to the novel intramolecular hydrogen bonding
between the hydrogen atom of the amide and the oxygen
atom attached to the boron center.
In 1988 we first reported a chiral boron catalyst based on a
tartaric acid ligand (Figure 5).[8] The high reactivity of the
chiral acyloxyborane (CAB) catalysts 1 a–c may originate
Brønsted acid assisted chiral Lewis acid
(BLA) 2 was proposed to promote high
selectivity through the dual effects of intramolecular hydrogen-bonding interactions and
attractive p–p donor–acceptor interactions in
the transition state (Scheme 3).[9] Extremely
high enantioselectivity (up to > 99 % ee) and
exo selectivity (up to > 99 % exo) are obtained
for cycloadditions of a-substituted a,b-enals
with dienes in the presence of BLA 2. The
absolute stereochemical preference in the
Figure 6. CAB–
methacrolein
complex.
Figure 5. Representative CAB (chiral acyloxyborane) catalysts.
from intramolecular hydrogen bonding of the terminal
carboxylic acid to the alkoxy oxygen atom. The CAB catalysts
are uniformly effective for aldol, Diels–Alder, and allylation
reactions (Scheme 2). Based on a series of NOE experiments,
it was established that the effective shielding of the CABcoordinated aldehydes arises from p stacking of 2,6-diisopropoxybenzaldehyde ring and the coordinated aldehyde
(Figure 6).
Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
Scheme 3. Diels–Alder reaction catalyzed by BLA 2.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1927
Reviews
H. Yamamoto and K. Futatsugi
reaction can be easily understood in terms of the most
favorable transition-state assembly shown in Scheme 3. The
coordination of a proton of the 2-hydroxyphenyl group with
an oxygen atom of the adjacent B O group in the complex
plays an important role in asymmetric induction; this hydrogen-bonding interaction through a Brønsted increases the
Lewis acidity of boron and the p basicity of the phenoxy
moiety.
Diels–Alder reactions of a-unsubstituted a,b-enals with
BLA 2 as well as most other chiral Lewis acids exhibit low
enantioselectivity and/or reactivity. We developed BLA 3,
which was prepared from a chiral triol and 3,5-bis(trifluoromethyl)benzeneboronic acid to overcome this deficiency.[10]
This catalyst was extremely effective in the enantioselective
cycloaddition of both a-substituted and -unsubstituted a,benals with various dienes (Figure 7). The Brønsted acid in the
new BLA 3 catalyst clearly improves the scope of this
cycloaddition.
Scheme 4. Asymmetric aza-Diels–Alder reaction promoted by BLA 4.
Scheme 5. Mannich-type reaction promoted by BLA 4.
Scheme 6. Asymmetric Michael addition reaction catalyzed by (R,R)La–binol. DME = 1,2-dimethoxyethane, Bn = benzyl.
Figure 7. Asymmetric Diels–Alder reactions catalyzed by BLA 3.
BLA 4, which is prepared from a trialkyl borate and
optically pure binol (1,1’-binaphthol) in a 1:2 molar ratio, is
also an excellent chiral promoter for the aza-Diels–Alder
reaction of chiral imines with the Danishefsky diene
(Scheme 4).[11] The structure of this BLA 4 was identified by
X-ray crystallographic analysis.
BLA 4 is also very useful in the double stereochemical
differentiation of Mannich-type reactions of chiral imines.
The Mannich reaction with the trimethylsilyl ketene acetal
derived from tert-butyl acetate proceeds with high diastereoselectivity (Scheme 5).
In 2000 Shibasaki and co-workers successfully developed
a stable, storable, and reusable La–binol complex for the
asymmetric Michael reaction (up to > 99 % ee) with broad
generality (Scheme 6).[12] After 4 weeks of storage, this
powdered complex was still highly effective in the catalytic
asymmetric Michael reaction. Furthermore, the complex is
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
recoverable and recyclable still gives the desired product with
very high ee values even after the fourth cycle. These catalysts
are typically considered to be activated by Brønsted acids
located in favored positions. On the other hand, the intermolecular activation of the Lewis acid catalysts by external
Brønsted acids could be achieved by the appropriate choice of
1) a chiral Lewis acid that has Brønsted basic sites in the
optimum position to activate the Lewis acidic center, and
2) an external Brønsted acids of sufficient Brønsted acidity
for the desired reactivity.
Mukaiyama and co-workers reported that proline derivatives combined with BBr3 produce a promising catalyst for
Diels–Alder reactions (Scheme 7).[13] The chiral catalyst is
believed to be the HBr adduct salt of the amino boron
derivative.
In 2002 Corey and co-workers reported that chiral
proline-derived oxazaborolidines combined with trifluoromethanesulfonic acid (TfOH) produce very reactive catalysts
5 a and 5 b (Figure 8) for Diels–Alder reactions (Scheme 8).[14]
The high activity of the catalysts is derived from the cationic
nature of the nitrogen ligand and the fact that a very strong
acid is required for their efficient generation. The absolute
stereochemical outcome of the Diels–Alder reactions medi-
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Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
Angewandte
Combined Acid Catalysis
Chemie
ated with these catalysts was successfully predicted on the
basis of the pre-transition-state assemblies shown in Figure 9.
Figure 9. Proposed pre-transition-state assemblies for 5 b.
Scheme 7. Chiral proline-based catalyst reported by Mukaiyama and
co-workers.
Figure 8. Cationic oxazaborolidines generated by protonation of chiral
proline-derived oxazaborolidines with strong Brønsted acids (TfOH or
Tf2NH).
Scheme 8. Diels–Alder reactions catalyzed by cationic oxazaborolidine
5 b (see Figure 8).
Later, Corey reported that the strong Brønsted acid
triflimide ((CF3SO2)2NH) protonates chiral oxazaborolidines
to form superactive, stable, chiral Lewis acids 6 a and 6 b
(Figure 8), which are highly effective catalysts for a wide
variety of enantioselective Diels–Alder reactions that were
previously beyond the reach of synthetic chemists
(Scheme 9).[15]
The hydrogen bonding within BLA catalysts participates
not only in the activation of the Lewis acidic center but also in
the effective transmission of the chiral scaffold to other
molecules. Kobayashi and co-workers developed a series of
chiral Lewis acid catalysts in which Brønsted acids are utilized
in an intramolecular fashion to transmit chiral information to
the other organic molecules (Figure 10). Kobayashi and coworkers also reported the chiral Lewis acid catalyst 7 a
(prepared from Sc(OTf)3, (R)-binol, and a tertiary amine),
Figure 10. Chiral Diels–Alder catalysts (Kobayashi and co-workers).
Scheme 9. Representative examples for the Diels–Alder reaction catalyzed by cationic oxazaborolidine 6 a. TBS = tert-butyldimethylsilyl,
TIPS = triisopropylsilyl.
Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1929
Reviews
H. Yamamoto and K. Futatsugi
which was quite effective in the enantioselective Diels–Alder
reactions of acyl-1,3-oxazolidin-2-ones with dienes in
dichloromethane (Scheme 10).[16] A unique coordination
mode, which has never been observed in other Lewis acid
droquinoline derivatives in high yields with high diastereoand enantioselectivities (Scheme 11).[18] This is the first
example of aza-Diels–Alder reactions in the presence of a
catalytic amount of a chiral source.
Scheme 11. Aza-Diels–Alder reaction catalyzed by chiral Yb catalyst.
DTBP = 2,6-di-tert-butylpyridine.
Later, catalytic enantioselective 1,3-dipolar cycloadditions of nitrones with alkenes in the presence of a novel
heterochiral YbIII catalyst were developed by Kobayashi and
Kawamura (Scheme 12).[19] The combination of (S)-binol and
the newly prepared chiral amine (R)-MNEA, N-methylbis[(R)-1-(1-naphthyl)ethyl]amine, led to the endo adduct in
excellent yield (92 %), enantioselectivity (96 % ee), and
diastereoselectivity (endo/exo = 99:1) by. The chiral YbIII
catalyst thus prepared has two independent chiral units
(heterochiral YbIII catalyst, vide below), and it was found that
the sense of the chiral induction in these reactions was mainly
Scheme 10. Diels–Alder reaction catalyzed by chiral Diels–Alder
catalysts 7 a and 7 b (see Figure 10).
catalysts, was indicated from several experiments. In the
chiral catalyst, the axial chirality of (R)-binol is postulated to
be transferred to the amine part, which would work as a
“wall” in the transition state to shield one side of the
dienophile.
Using a similar design, Kobayashi and co-workers found
that both enantiomers of the Diels–Alder reaction between 3acyl-1, 3-oxazolidin-2-ones and cyclopentadiene could be
prepared in the presence of chiral lanthanide (iii) 7 b from the
same chiral source, (R)-binol (Scheme 10).[17] It should be
noted that the chiral catalysts with reverse enantiofacial
selectivities could be prepared by using the same chiral source
with an appropriate choice of achiral ligands.
In 1996 Kobayashi reported that in the presence of a
catalytic amount of chiral ytterbium Lewis acid, which was
prepared from Yb(OTf)3, (R)-binol, DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), and an additive, achiral imines reacted
with achiral dienophiles to afford the corresponding tetrahy-
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 12. 1,3-Dipolar cycloaddition in the presence of heterochiral
YbIII catalyst.
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Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
Angewandte
Combined Acid Catalysis
Chemie
determined by binol and that the chiral amine increased or
decreased the induction relatively. Actually, the existence of
hydrogen bonds between the phenolic hydrogen atoms of (S)binol and the nitrogen atoms of (R)-MNEA was confirmed by
the IR spectrum of the catalyst.
organoaluminum
reagents
and
binol
derivatives
(Scheme 14).[21] The catalytic activity of this catalyst is
significantly greater than that of monometallic reagents. The
catalyst achieves high reactivity and selectivity by an intramolecular interaction of two aluminum Lewis acids.
4. Lewis Acid Assisted Lewis Acid Catalysts (LLA)
Reactive Lewis acid assisted Lewis acid catalysts are
relatively well known. Electron-deficient metal compounds
can be further activated as electrophiles through hetero- and
homodimeric associative interaction. In several instances,
naked metal cations can be generated. In fact, this has been
widely encountered in many acid-catalyzed reactions including the Friedel–Crafts reaction and the Ziegler–Natta polymerization. However, its full recognition as a synthetically
powerful tool does not appear to be widespread as of yet. It
may be further extended to include asymmetric catalysis
design.
These associative interactions are crucial not only for
higher reactivity but also for the construction of the wellorganized chiral environment. Representative examples of
Lewis acid assisted Lewis acid catalysts that have higher
reactivities and/or highly organized transition states by the aid
of associative interactions will be discussed in this section.
Our results suggested that several characteristics of Lewis
acid catalysts are enhanced significantly in the presence of
another Lewis acid. In 1998 we reported the remarkable
enhancement of the catalyst activity of trialkylsilyl triflate in
the Mukaiyama aldol synthesis by the addition of the bulky
aluminum reagent MABR (Scheme 13).[20] Thus, a strongly
Lewis acidic species forms from two different Lewis acids,
namely the bulky organoaluminum reagent and Me3SiOTf.
One plausible active species in which MABR might act as an
activator for Me3SiOTf by coordinating to the triflate counter
anion is shown in Scheme 13. Thus, MABR might act as MAO
does in Ziegler–Natta catalysts.
A chiral dialuminum Lewis acid, which is effective as an
asymmetric Diels–Alder catalyst, has been prepared from
Scheme 14. Chiral dialuminum Lewis acid as an asymmetric Diels–
Alder catalyst. R = iBu.
Similarly, a chiral trialuminum compound was quantitatively formed from optically pure 3-(2,4,6-triisopropylphenyl)binaphthol (2 equiv) and Me3Al (3 equiv) in CH2Cl2 at
room temperature (Scheme 15).[21] The novel structure of this
Scheme 15. Asymmetric Diels–Alder reaction catalyzed by a chiral
trialuminum catalyst.
Scheme 13. Mukaiyama aldol reaction catalyzed by MABR–TMSOTf.
Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
complex was ascertained by 1H NMR spectroscopic analysis
and measurement of the methane gas evolved. This trinuclear
chiral aluminum complex is effective for the Diels–Alder
reaction of methacrolein with cyclopentadiene. Diels–Alder
adducts were obtained in 99 % yield with 92 % exo selectivity.
Under optimum reaction conditions, the exo adduct was
obtained with 75 % ee. Intramolecular association of aluminum Lewis acids as shown in Scheme 15 might lead to
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1931
Reviews
enhanced Lewis acidity of the aluminum atom located in the center of the
chiral scaffold (shown in the gray
circle).
Catalytic enantioselective Streckertype reactions of aldimines with tributyltin cyanide (Bu3SnCN) proceeded
smoothly in the presence of a novel
chiral zirconium catalyst to afford aamino nitrile derivatives with high
enantioselectivities
(Scheme 16).[22]
Later, hydrogen cyanide (HCN) was
successfully used instead of Bu3SnCN
as the cyanide source.[23] The catalytic
asymmetric Strecker amino acid synthesis starting from achiral aldehydes,
amines, and HCN in the presence of
chiral zirconium catalyst has also been
H. Yamamoto and K. Futatsugi
Scheme 17. Chiral bis-TiIV oxide catalyzed asymmetric allylation reaction.
asymmetric approach provides a very useful way for obtaining
high reactivity and selectivity by the simple introduction of
the M-O-M unit in the design of chiral Lewis acid catalysts.
The authors proposed that the high reactivity of this chiral bisTiIV oxide might be ascribed to the intramolecular coordination of one isopropoxy oxygen atom to the other titanium
center, thereby enhancing the otherwise weak Lewis acidity
of the original TiIV center for carbonyl activation. This mode
of activation is a typical example of the Lewis acid assisted
Lewis acid mechanism. Alternatively, a dual activation of the
carbonyl group by the simultaneous coordination of two Ti
centers was also proposed as the origin of the high reactivity.
Marhwald et al. recently reported that ligand exchange of
titanium(iv) alkoxides with optically active a-hydroxy acids
presents an unexpected and novel approach to enantioselective direct aldol reactions of aldehydes and ketones
(Scheme 18).[25] The aldol products were isolated with a high
degree of syn diastereoselectivity. High enantioselectivities
were observed when using simple optically pure a-hydroxy
acids. Based on the X-ray crystallographic analysis of this
Ti(OiPr)4/(R)-mandelic acid complex and on the absolute
configuration found in the aldol products, they proposed the
LLA-like species shown in Scheme 18.
Scheme 16. Enantioselective Strecker reactions in the presence of
chiral dinuclear zirconium catalysts.
achieved. Although mechanistic details of this catalytic
reaction are not yet clear, the two binuclear zirconium
complexes shown in Scheme 16 are postulated to be active
chiral catalysts in the reactions of aldimines with Bu3SnCN
and in the three-component reactions.
A new chiral bis-TiIV oxide was successfully designed by
Maruoka and co-workers and can be utilized for strong
activation of aldehydes, thereby allowing a new catalytic
enantioselective allylation of aldehydes with allyltributyltin
(Scheme 17).[24] The chiral bis-TiIV catalyst can be readily
prepared either by treatment of bis(triisopropoxy)titanium
oxide [(iPrO)3Ti–O–Ti(iOPr)3] with (S)-binol or by reaction
of ((S)-binaphthoxy)isopropoxytitanium chloride with silver(i) oxide. The reaction of 3-phenylpropanal with allyltributyltin (1.1 equiv) under the influence of chiral bis-TiIV oxide
(10 mol %) generated in situ in CH2Cl2 at 0 8C for 4 h afforded
1-phenyl-5-hexen-3-ol in 84 % yield with 99 % ee. This
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Scheme 18. Asymmetric direct aldol reaction promoted by a chiral TiIV
catalyst.
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Combined Acid Catalysis
Chemie
Oh and Reilly reported that chiral Lewis acids derived
from 1,8-naphthalenediylbis(dichloroborane), a novel bidentate Lewis acid, have been found to be active catalysts for the
asymmetric Diels–Alder reaction (Scheme 19).[26] Chiral
The development of chiral heterobimetallic catalysts,
which could be considered as examples of LLAs, has been
studied extensively.[27a] In this section, representative examples will be disclosed.
The chiral heterobimetallic complex,[27a, b] LaLi3-tris(binaphthoxide) (LLB), is an efficient asymmetric catalyst for
direct aldol reactions of aldehydes and unmodified ketones
(Scheme 20).[28] The LLB catalyst functions not only as a base
to remove an a proton from the ketone, but also as a Lewis
acid, giving the aldol product in high yield with high
enantioselectivity. The LLB catalyst also works well for the
direct aldol reaction of aldehydes with a-hydroxy ketones to
provide 1,2-dihydroxyketones with high enantiomeric
excess.[29]
A practical and efficient Michael addition for the largescale synthesis of enantiomerically pure (R)-3-[bis(methoxycarbonyl)methyl]cyclohexanones in the presence of an (R)Al-Li-bis(binaphthoxide) complex ((R)-ALB)[30] was developed (Scheme 21).[31] The Michael reaction of 2-cyclohexe-
Scheme 19. Asymmetric Diels–Alder reaction catalyzed by chiral
dinuclear boron catalyst.
ligands derived from amino acids resulted in a range of
enantioselectivities in the reaction of cyclopentadiene with
various a,b-unsaturated aldehydes. The authors proposed a
transition-state assembly that includes chloride bridging
between two boron atoms. Such an interaction should trigger
the enhancement of the Lewis acidity of the ligand-free
borane center.
Scheme 21. Asymmetric Michael addition catalyzed by (R)-ALB.
Scheme 20. Asymmetric direct aldol reaction in the presence of (R)-LLB. KHMDS = potassium hexamethyldisilazide.
Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
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H. Yamamoto and K. Futatsugi
none (6.0 mol, 581 mL) with dimethyl malonate (6.0 mol,
686 mL) was completed in 24 h at ambient temperature when
using 0.1 mol % of the catalyst with 0.09 mol % of KOtBu and
4- molecular sieves to afford more than 1 kg of the
enantiomerically pure product in 91 % yield after three
successive crystallizations.
The direct catalytic asymmetric Mannich reaction of
unmodified ketones was achieved by the cooperative catalysis
of a heterobimetallic asymmetric complex of ALB and
La(OTf)3·n H2O in the presence of 3- molecular sieves
(Scheme 22).[32] The reactivity and selectivity were greatly
improved by addition of La(OTf)3·n H2O to ALB. The
association of ALB and La(OTf)3·n H2O was indicated by
means of LDI-TOF mass spectrometry.
A variety of epoxide-opening reactions were found to
proceed smoothly in the presence of a bridged (R,R)-Ga-Li–
binol complex (3–10 mol %), affording products in 67–94 %
yield with 66–96 % ee (Scheme 23).[33] The structure of the
LiCl-free Ga- and Li-linked binol complex was elucidated by
X-ray crystallographic analysis and was shown to have a
monomeric tetracoordinated structure similar to that of ALB.
A mixture of Et2Zn and bridged (S,S)-binol (4:1) with 3-
molecular sieves was developed as an effective catalyst system
for the direct aldol reaction of hydroxyketones
(Scheme 24).[34] As little as 0.1 mol % of the bridged (S,S)binol species and 0.4 mol % of Et2Zn promoted the direct
aldol reaction smoothly, using only 1.1 equiv of hydroxyketones as a donor (substrate/ligand = 1000:1). In terms of
catalyst loading, this is the most efficient asymmetric catalyst
for the direct catalytic asymmetric aldol reaction.
Based on the transition-state model initially proposed by
Noyori and co-workers, Shibasaki developed a new enantioselective catalyst for the addition of Me2Zn to a-ketoesters
(Scheme 25).[35] The introduction of the cis hydroxy group
into the prolinol ligand might induce more organized
aggregation of Zn species, resulting in high catalyst activity
and enantioselectivity. (The original concept stems from the
increased nucleophilicity of the alkyl zinc species by the
Scheme 23. Catalytic enantioselective epoxide-opening reaction with
(R,R)-Ga,Li-linked binolate.
proximal zinc alkoxide, which might act as an additional
Lewis base).
Trost and co-workers reported a series of novel dinuclear
zinc complexes (Figure 11), which are effective catalysts for
asymmetric direct aldol reactions,[36a,b, c] Mannich-type reactions,[36d] and desymmetrizations of 1,3- and 1,4-diols, as
shown in Scheme 26.[36e] These catalysts might be regarded as
a type of LLA, because the synergistic effect discussed above
could be assumed for the addition stage. In these systems, the
intramolecular interaction of the Lewis acids through the
attached heteroatom should enhance the Lewis acidity
efficiently in the chiral environment. In contrast, the intermolecular activation of the Lewis acid assisted by either
catalytic or stoichiometric amounts of the other Lewis acid is
also possible.
The discovery that Group IV metallocenes can be activated by methylaluminoxane (MAO),[37] an extremely strong
Lewis acid and cocatalyst in olefin polymerization, has
Scheme 22. Asymmetric direct Mannich reaction catalyzed by (R)-ALB and La(OTf)3·n H2O.
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Scheme 24. Asymmetric direct aldol reaction catalyzed by an (S,S)-Zn–binol complex.
Scheme 25. Asymmetric addition of ZnMe2 with a chiral prolinolderived ligand.
stimulated a renaissance in Ziegler–Natta catalysis. These
combined Lewis acid catalysts could also be regarded as a
typical type of LLAs (Figure 12). The (R)-zirconocene
catalyst/MAO reported by Pino et al. for the asymmetric
oligomerization of propylene, 1-pentene, and 4-methyl-1pentene using hydrogen as a chain-transfer reagent is a
representative example (Scheme 27).[38]
Highly enantioselective carboalumination of alkenes was
achieved with the Erker non-bridged chiral zirconocene
catalyst by Negishi and co-workers (Scheme 28).[39] An
impressive range of alkyl aluminum complexes can be
Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
Figure 11. Structure of dinuclear zinc catalysts and their plausible
synergistic effect.
added with high efficiency and excellent enantioselectivity.
A different reaction medium changes the mechanism so that
the catalytic alkylations proceed through carbometallation of
the alkene (direct addition of cationic alkylzirconium to the
olefin, followed by Zr–Al ligand exchange), rather than
involving the formation of a metallacyclopentane.
Ishiyama and Miyaura reported that the addition of
pinacol allylboronic esters to aromatic and aliphatic aldehydes occurred smoothly at 78 8C in toluene in the presence
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Scheme 26. Some applications of dinuclear zinc catalysts to asymmetric synthesis.
of a catalytic amount of AlCl3 or Sc(OTf)3
(10 mol %) to give the corresponding homoallyl
alcohols in high yields (Scheme 29).[40] The reactions proceeded regio- and diastereospecifically,
yielding the isomerically pure syn and anti homoallyl alcohols from (Z)- and (E)-allylboronic esters,
respectively. The protocol was also applied to
enantioselective reactions in the presence of a
chiral Lewis acid catalyst. Although the origin of
activation remains unclear, the chiral Lewis acid
might coordinate to the oxygen atom of the allyl
boron reagent, resulting in dramatic increase in the
Lewis acidity of the boron center.
Hall and co-workers reported a remarkably
general and practical aldehyde allylation method
based on the Sc(OTf)3-catalyzed reaction of stable
chiral allylboronates (Scheme 30).[41a,b] This
approach is unrivaled in many ways, particularly
in its efficient control of both diastereo- and
Figure 12. Mode of activation for Ziegler–Natta catalysts.
Scheme 29. Asymmetric allylation with achiral allyl boron reagent
catalyzed by a chiral Lewis acid. L* L* = (S)-binol.
Scheme 27. Asymmetric oligomerization catalyzed by (R)-zirconocene/
MAO.
Scheme 30. Asymmetric allylation with a chiral allyl boron reagent
catalyzed by Sc(OTf)3.
Scheme 28. Enantioselective carboalumination of alkenes in the presence of a chiral zirconocene catalyst.
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enantioselectivity. The precise mechanistic nature of this
Lewis acid catalyzed process and the mode of stereoinduction
are unknown. On the basis of preliminary arguments presented earlier and the fact that the diastereospecificity of the
uncatalyzed reaction is preserved, the allylboration is thought
to proceed via the usual cyclic transition state, with electrophilic boron activation by metal coordination to the boronate
oxygen atoms.[41b]
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Promising results obtained by Itsuno and co-workers in
the 1980s with mixtures of chiral 1,2-amino alcohols and
borane paved the way for the discovery of oxazaborolidines as
chiral catalysts for the borane-mediated enantioselective
reduction of a wide variety of achiral ketones (CBS reduction) (Scheme 31).[42] In the general mechanistic model, the
coordination of the electrophilic BH3 to the nitrogen atom of
the oxazaborolidine serves to activate BH3 as a hydride donor
and also to increase strongly the Lewis acidity of the
endocyclic boron atom. The strongly Lewis acidic complex
then readily binds to the ketone at the more sterically
accessible electron lone pair and cis to the vicinal BH3 group.
The reaction has been applied in numerous syntheses.[43]
Noyori and co-workers reported that in the presence of
( )-3-exo-(dimethylamino)isoborneol (DAIB), the reaction
of dialkyl zinc complexes and aldehydes is markedly accelerated to give, after hydrolysis, the corresponding S alcohols
with high enantiomeric purity (Scheme 32).[44] The alkylation
proceeds via a dinuclear zinc species containing the DAIB
auxiliary, an aldehyde ligand, and three alkyl groups; it is the
bridging alkyl group, rather than the terminal alkyls, that
migrates from zinc to the aldehyde carbon atom. The
operation of a synergistic effect in the bimetallic complex
serves to activate the central Zn as a Lewis acid while
concomitantly increasing the nucleophilicity of the diorganozinc reagent in the transition state.
A possible transition state for titanium-catalyzed enantioselective additions of alkyl groups to aldehydes was
proposed by Walsh and co-workers (Scheme 33).[45] Because
of the tendency of the binolate–titanium complexes to
associate, the formation of a binuclear intermediate was
proposed.
Carreira and co-workers demonstrated the unique reactivity of a fluorotitanium complex as a catalyst for the
asymmetric addition of Me3Al to aldehydes (Scheme 34).[46]
Importantly, in contrast to known alkyl metal additions to
aldehydes involving TiIV catalysts, the fluorotitanium complex
obviates the need for stoichiometric quantities of Ti(OR)4 as
additive. It is conceivable that fluoride on a Lewis acid center
could also serve to participate as a bridging component and
Scheme 32. Enantioselective addition of diethylzinc to aldehydes with
DAIB.
Scheme 33. Asymmetric alkylation with dialkylzinc reagent catalyzed by
Ti–binol.
provide bimetallic complexes that function in a manner
similar to the dialkyl zinc complexes described above.
5. Lewis Acid Assisted Brønsted Acid Catalysts (LBA)
The combination of Lewis acids and Brønsted acids gives
Lewis acid assisted Brønsted catalysts and provide an
opportunity to design a “unique proton”,[47] that is, the
Scheme 31. Representative examples of chiral oxazaborolidine-mediated asymmetric reactions.
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Scheme 34. Enantioselective alkylation with Me3Al catalyzed by chiral
titanium complex.
coordination of a Lewis acid to the heteroatom of the
Brønsted acid could increase the acidity of the latter.
In 1994, we reported that such LBAs can be generated
in situ from optically pure binol and SnCl4 in toluene and is
stable in solution, even at room temperature.[48a,b] In the
presence of a stoichiometric amount of (R)-LBA, the protonation of the silyl enol ether derived from 2-phenylcyclohexanone proceeded at 78 8C to give the S isomer with 97 % ee
(Scheme 35). This reagent is also applicable to various ketene
Scheme 36. Catalytic enantioselective protonation with chiral LBA
generated from monoprotected binol derivative and SnCl4.
achiral proton source and a catalytic amount of the chiral
LBA was possible (Scheme 36).[48a,c]
The synthetic utility of LBAs is not only limited to
asymmetric synthesis. We also developed the regio- and
stereoselective isomerization of a “kinetic” silyl enol ether to
a “thermodynamic” one catalyzed by an achiral LBA
(Scheme 37).[48d] “Kinetic” TBDMS enol ethers were isomer-
Scheme 37. Isomerization of silyl enol ethers catalyzed by achiral LBA.
TBDMS = tert-butyldimethylsilyl.
Scheme 35. Enantioselective protonation with chiral LBA generated
from (R)-binol and SnCl4.
bis(trialkylsilyl) acetals derived from a-aryl carboxylic acids.
The sense of stereoinduction can be understood in terms of
the proposed transition-state assembly shown in Scheme 35.
The trialkylsiloxy group is directed opposite to the binaphthyl
moiety to avoid any steric interaction, and the aryl group is
stacked on this naphthyl group. In further studies, enantioselective protonation with a stoichiometric amount of an
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ized to “thermodynamic” species in the presence of catalytic
amounts of the coordinate complexes of SnCl4 and the
monoalkyl ethers of binol or biphenol. For the various
structurally diverse substrates, the isomerization proceeded
cleanly in the presence of 5 mol % of the achiral LBA.
In 2003, optically active 1,2-diarylethane-1,2-diol·SnCl4
derivatives were designed as a new type of LBA for
enantioselective protonation (Scheme 38).[47e] A variety of
optically active 1,2-diarylethane-1,2-diols could be readily
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Figure 13. X-ray structure of LBA generated from monomethylated
chiral hydrobenzoin and SnCl4.
Scheme 38. Enantioselective protonation with chiral LBA generated
from chiral hydrobenzoin derivative and SnCl4.
prepared by asymmetric syn dihydroxylation, which is advantageous to the use of binol for the flexible design of a new
LBA. The most significant finding is that we were able to
specify the conformational direction of the H O bond of
LBA by X-ray diffraction analysis (Figure 13). The stereo-
chemical outcome of the enantioselective protonation of silyl
enol ethers with this LBA could be controlled by a linear OH–
p interaction with in the initial step (Scheme 38).
Despite extensive studies on acid-catalyzed diastereoselective polyene-cyclizations, the corresponding enantioselective processes have not yet been reported. Very recently, we
succeeded in the first enantioselective biomimetic cyclization
Scheme 39. Biomimetic cyclizations catalyzed by chiral LBAs.
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of polyprenoids catalyzed by chiral LBA (Scheme 39). This is
the first example of a proton-induced enantioselective ene
cyclization in synthetic chemistry.[49a, b] Geranyl phenyl ethers,
o-geranylphenols, and geranylacetone derivatives were
directly cyclized at 78 8C in the presence of (R)-binol
derivatives and SnCl4. During the cyclization, unusual [1,3]
Claisen rearrangements often took place, with up to 90 % ee.
These chiral LBAs recognize a trisubstituted terminal olefin
enantiotopically and generate site-selective carbocations on
the substrates.
The enantioselective cyclization of homo(polyprenyl)arenes was also induced by (R)-binol–o-FBn·SnCl4 (Scheme 39).[49c, d] Several optically active podocarpa-8,11,13triene diterpenoids and ( )-tetracyclic polyprenoids of sedimentary origin were synthesized by the enantioselective
cyclization of homo(polyprenyl)benzene derivatives induced
by this catalyst and subsequent diastereoselective cyclization
induced by BF3·Et2O/EtNO2 or CF3CO2H·SnCl4 (75–
80 % ee).
6. Brønsted Acid Assisted Brønsted Acid Catalysts
(BBA)
Hydrogen bonding plays a crucial role in the organization
of the three-dimensional structure of enzymes and is often
involved in the reaction at the active site. An ideal example is
a concerted proton transfer in the rate-limiting step of hemeoxygenase catalysis, which was recently elucidated
(Figure 14).[50]
Such an elegant principle could be applicable to asymmetric catalysis. Especially for Brønsted acid catalysis, the
Figure 14. Concerted proton transfer in the rate-limiting step of heme
oxygenase.
design of these catalysts would result not only in the
formation of a highly organized chiral cavity but also in an
increase in the Brønsted acidity of the terminal proton in a
much milder way than that of the LBA system.
An example of intramolecular hydrogen bonding is also
observed in the small organic molecule taddol[51] (Figure 15),
which is one of the most efficient chiral
backbones for asymmetric synthesis.
Within this molecule, one of the hydrogen atom of the alcohol group participates in an intramolecular hydrogen
bond and the other one is free for
intermolecular interactions. These Figure 15. Intramolecunique features have been confirmed ular hydrogen bonding within taddol.
by X-ray crystallographic analysis.
This excellent candidate was utilized as a chiral BBA catalyst by Rawal and co-workers.[52a]
They showed that hetero-Diels–Alder reactions in the
presence of taddol are highly enantioselective and generate
only one of the enantiomers of the dihydropyran product (e.g.
Scheme 40, top). This type of catalysis mimics the action of
enzymes and antibodies, in contrast to traditional, metalbased catalysts used in organic chemistry. An interesting
feature of this catalyst is that the monomethyl ether
derivatives of taddol showed poor enantioselectivity, indicating that the intramolecular hydrogen bonding network within
the catalyst must be crucial for higher asymmetric induction.
This BBA catalyst is also highly effective for the
enantioselective Diels–Alder reactions of a, b-unsaturated
aldehydes (Scheme 40, bottom).[52b] After the two-step transformations from the initial cycloadducts, a variety of chiral
cyclohexenones were obtained with high enantioselectivities
(up to 92 % ee).
A proposed working model for the taddol-catalyzed
Diels–Alder reactions is shown in Figure 16. Well-defined
internal hydrogen bonding may organize the asymmetric
environment of the taddol derivative, coupled with an
increase in Brønsted acidity of the other hydroxy proton,
which coordinates to the carbonyl oxygen atom.[53] The
resulting electron-deficient dienophile is stabilized through
Scheme 40. Asymmetric Diels–Alder reactions catalyzed by a taddol derivative. LAH = lithium aluminum hydride.
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Figure 16. Proposed working model for the taddol-catalyzed Diels–
Alder reactions.
p–p donor–acceptor interactions with the proximal 1-napthyl
ring, which shields one enantiofacial side of the dienophile.
Chiral binol-derived Brønsted acids catalyze the enantioselective asymmetric Morita–Baylis–Hillman (MBH) reaction of cyclohexenone with aldehydes (Scheme 41).[54] The
Scheme 41. Asymmetric Baylis–Hillman reaction catalyzed by a chiral
binol derivative.
asymmetric MBH reaction requires 2–20 mol % of the chiral
Brønsted acid and triethylphosphane as the nucleophilic
promoter. The reaction products are obtained in good yields
(39–88 %) and high enantioselectivities (67–96 % ee). The
Brønsted acid catalyzed reaction is the first example of a
highly enantioselective asymmetric MBH reaction of cyclohexenone with aldehydes. Interestingly, the removal of one
Brønsted acid equivalent from the binol-derived catalyst to
give the corresponding monomethyl ether resulted in a lower
catalytic activity and no enantioselectivity. This result also
indicates the importance of the intramolecular hydrogenbonding network within the BBA catalyst.[53]
7. Conclusions and Outlook
In recent years, the power of designer acid catalysis has
greatly increased as a result of the development of catalytic
enantioselective versions described herein. Combined acid
catalysis is still very much in a state of infancy, and there is still
much more to learn with regard to new reactivity. The
ultimate goal is a more reactive, more selective, and more
versatile catalyst. We believe the realization of such an
objective would be a tremendous benefit for the development
of organic synthesis including green chemistry.
Angew. Chem. Int. Ed. 2005, 44, 1924 – 1942
It is a great pleasure to thank the many wonderful co-workers
who have accompanied me on the scientific journey that is
retraced herein. Their names appear in the references cited.
However, special mention needs be made to the following
talented members of our group: S. Aratake, J. Fujiwara, K.
Furuta, Q. Gao, Y. Hiraiwa, K. Inanaga, H. Ishibashi, K.
Ishihara, K. Iwanaga, A. Kanematsu, M. Kaneeda, J. Kobayashi, H. Kurihara, K. Maruoka, T. Maruyama, M. Matsumoto,
Y. Miwa, M. Miyata, M. Mouri, H. Nakamura, S. Nakamura,
D. Nakashima, M. Oishi, and M. Sakurai. I am also grateful to
the Japan Science and Technology Corporation for generous
financial support over the years.
Received: April 21, 2004
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