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Recent Advances in Catalytic Enantioselective Aminations and Oxygenations of Carbonyl Compounds.

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Minireviews
J. M. Janey
Synthetic Methods
Recent Advances in Catalytic, Enantioselective
a Aminations and a Oxygenations of Carbonyl
Compounds**
Jacob M. Janey*
Keywords:
amination · asymmetric catalysis · Lewis acids ·
organocatalysis · synthetic methods
The direct introduction of either a nitrogen or oxygen atom adjacent
to a carbonyl group in a catalytic, enantioselective manner using both
chiral Lewis acid and Lewis base catalysis has been described
recently. The enantiomerically enriched products of these reactions,
such as a-amino acids, represent fundamental building blocks for the
construction of complex natural products and other important bioactive molecules. This Minireview provides a synopsis of this evergrowing field and highlights some of the challenges that still remain.
1. Introduction
The literature concerning a amination[1] and a oxygenation[2] of ketones, aldehydes, and esters contains many
examples of nonstereoselective and auxiliary-controlled
methodologies. The more prominent, well-established methods of enantioselective a oxygenation include the use of
Davis oxaziridine,[3] Sharpless dihydroxylation of enol
ethers,[4] manganese–salen epoxidation of enol ethers,[5] and
Shi epoxidation of enol ethers.[6] It is only rather recently that
direct catalytic, asymmetric variants have been reported.[7]
This review will focus only upon these contemporary aminations and oxygenations, although it should be noted that the
catalytic, asymmetric a halogenation of carbonyls may also
serve as an alternative entry to enantioenriched a-amino and
a-hydroxy carbonyls.[8]
The basic reaction manifold consists of an enolate (or
enamine) reacting with an electrophilic nitrogen or oxygen
atom (Scheme 1). The enolate may either be preformed (such
as a silyl enol ether) or generated in situ through “soft”
[*] Dr. J. M. Janey
Department of Process Research
Merck Research Laboratories
Merck & Co., Inc.
P.O. Box 2000
Rahway, NJ 07065 (USA)
Fax: (+ 1) 732-594-5170
E-mail: jacob_janey@merck.com
[**] The author would like to thank Prof. Hisashi Yamamoto (University
of Chicago) for helpful discussions.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
enolization involving a Lewis acidic
metal and a base. Enamines are also
competent nucleophiles and form from
the reversible condensation between
an aldehyde or ketone and a secondary amine. The heteroatom electrophile is either an azodicarboxylate for aminations, or nitrosobenzene for both oxygenation and amination
reactions, depending upon the reaction conditions.
Scheme 1. General reactivity pattern.
The reactions can be catalyzed by either chiral Lewis acids
or Lewis bases. For Lewis acid catalysis, there are two modes
for activation and stereochemical induction (Figure 1). The
chiral Lewis acid metal complex may generate a chiral metal
enolate (A) by either “soft” enolization of the carbonyl or by
transmetalation of a preformed enolate. The chiral Lewis acid
Figure 1. Modes for catalyst activation.
DOI: 10.1002/anie.200462314
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Angewandte
Chemie
Asymmetric Catalysis
may also serve to activate the electrophile by coordination to
either an oxygen or nitrogen lone pair (B). It is also possible
that the Lewis acid complex plays both roles at once by
forming a metal enolate, which may then bind to and activate
the electrophile. Chiral Lewis bases catalyze these reactions
by the reversible formation of a chiral enamine (C). The
asymmetry is then transferred upon reaction between this
enamine and the electrophile, followed by imine hydrolysis of
the product, which allows for catalyst turnover.
2. a Oxygenations Catalyzed by Chiral Lewis Acids
In recent years a growing number of reports have detailed
the use of nitrosobenzene as a versatile electrophile for
catalytic enantioselective nucleophilic additions. Nitrosobenzene can best be thought of as the nitrogen equivalent of an
aldehyde in terms of reactivity and, as such, displays many of
the same reactivity patterns, although with some unique and
obvious deviations. Of particular note is the proclivity for
nitrosobenzene to undergo nucleophilic attack at either
oxygen or nitrogen. As depicted in Scheme 2, depending
Scheme 2. Divergent regioselectivity in reactions with nitrosobenzene.
of a Lewis acid (Scheme 3). Coordination of a metal at one
oxygen then favors nucleophilic attack at the other. Yamamoto et al. also considered the nitrogen coordination in
monomer E, but ancillary evidence argues against this mode
Scheme 3. Possible modes of nucleophilic attack: a complex of the
dimer (D) versus a complex of the monomeric nitrosobenzene (E).
of activation. Upon addition of Lewis acid they detected the
dimer by in situ IR spectroscopy, and also a peak in the ESI
mass spectra corresponded to a Lewis acid coordinated to the
dimer. It should be noted that this cannot rule out monomer E
as the actual kinetically active species in the reaction pathway.
In addition to the divergent reactivity of enol ethers observed
with Lewis acids, enamines may also exhibit such behavior.
For example, morpholine-derived enamines give predominantly 2, whereas pyrrolidine enamines give mainly 3.[11]
Although no explanation for this selectivity has been posited,
it is known that enamines derived from pyrrolidine are orders
of magnitude more nucleophilic than those from morpholine.[12]
The first Lewis acid catalyzed, enantioselective addition
to nitrosobenzene was reported by Yamamoto and co-workers [Eq. (1)].[13] They report that silver(i)–binap complexes
effectively catalyze the enantioselective reaction between
tin(iv) enolates 4 and nitrosobenzene giving the a-oxygenated
carbonyl 3 with excellent regio- and enantioselectivity for a
variety of cyclic ketones (Scheme 4). Although the N O bond
upon the nature of the enolate and presence or absence of a
catalyst, either the a-oxygenated or a-aminated product may
emerge.[9]
Hisashi Yamamoto and co-workers have extensively
explored this “nitroso-aldol” reaction and report that tin(iv),
lithium, and zinc(ii) enolates, in the absence of any Lewis acid
catalyst, give predominantly the aminated product 2.[9b] In
addition, morpholine-derived enamines may also generate
2.[10] By contrast, silyl enol ethers and tin(iv) enolates in the
presence of a Lewis acid give mainly the a-oxygenated
product 3. This divergent reactivity pattern is explained by the
facile formation of a nitrosobenzene dimer D in the presence
Jacob Janey was born in Minnesota in
1976. He received his BS degree in
chemistry from the University of Chicago in
1998, where he worked in the laboratory of
Viresh H. Rawal. In 2003 he earned a PhD
in organic chemistry from Harvard University under the guidance of David A. Evans.
His thesis dealt with the development and
application of chiral Lewis acids for ketene
cycloadditions and Suga–Ibata oxazole aldol
reactions. He is currently a Senior Research
Chemist in the Department of Process Research at Merck & Co., Inc.
Angew. Chem. Int. Ed. 2005, 44, 4292 –4300
Scheme 4. a Oxygenation of tin(iv) enolates catalyzed by a silver(i)–
binap complex.
of 5 is easily cleaved under a variety of hydrogenation
conditions, Yamamoto et al. report that catalytic copper(ii)
sulfate in methanol rapidly gives the free alcohol 6 with no
racemization [Eq. (2)].
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. M. Janey
3. a Aminations Catalyzed by Chiral Lewis Acids
The O selectivity of reactions with nitrosobenzene can be
completely reversed, as reported recently by Yamamoto and
co-workers.[14] They detail conditions under which tin(iv)
enolates 4 react with nitrosobenzene to give almost exclusively the a-amino product 2 in a catalytic, enantioselective
manner [Eq. (3)]. As shown in Scheme 5, a range of cyclic
Figure 2. X-ray structure of [{Ag(OTf)}2{m-(R)-binap}{Ag(H2O)2}].
O red, S turquoise, F green.
Scheme 5. a Amination of tin(iv) enolates catalyzed by a AgI2·binap
complex.
tin(iv) enolates react with nitrosobenzene giving 2 with
uniformly high levels of both N selectivity and enantioselectivity when conducted in the presence of 10 mol % of a
preformed catalyst composed of a 2:1 mixture of silver(i)
triflate and (R)-binap. This catalyst also showed pronounced
solvent effects with THF giving predominantly the oxygenated product, albeit with little enantioselectivity. For
aminations, the optimal solvent was found to be ethylene
glycol diethyl ether. The reactions in other diether solvents
were equally N selective, although the enantioselectivity was
moderate at best.
Although no model has been offered to explain why the
1:1 complex of silver(i) and binap is O selective while the 2:1
complex is N selective and gives product 2, X-ray structures of
the 1:2, 1:1, and 2:1 silver(i)–binap complexes were reported.[14] In the 2:1 structure, each phosphine of binap is
coordinated to a separate AgI center; one of the metal
centers also bears two triflates and the other bears two water
molecules (Figure 2). In contrast, the 1:1 complex has a
tetragonal AgI center bound in a chelate by both phosphines
of binap in addition to the coordinated counterion (Figure 3).
Based on these structures it is not immediately apparent what
factors govern O versus N selectivity, as solvent appears to
play an important role in this regard.
Azodicarboxylates also serve as ready electrophiles for
the Lewis acid catalyzed amination reaction. In a seminal
paper on this topic, David A. Evans and Scott G. Nelson[15]
reported that a chiral magnesium Lewis acid complex 8 in the
presence of a complimentary base will enolize aryl-substitut-
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. X-ray structure of [Ag(OCOCF3){(R)-binap}]. The CF3 group
was not localized.[14]
ed carboximides 7, which are then aminated by di-tert-butyl
azodicarboxylate [Eq. (4)]. As shown in Table 1, the catalyst
provides the expected a-amino acid derivatives 9 with
excellent levels of enantioselectivity and yield. The enantiomeric purity can be upgraded with a single recrystallization. It
is postulated that the azodicarboxylate acts as the base for the
soft enolization between MgII and 7. A kinetic investigation
indicates that the N-methyl-p-toluenesulfonamide additive
acts as a weak acid to protonate the hydrazide conjugate from
the MgII complex, allowing for liberation of the catalyst. The
sense of asymmetric induction was rationalized based upon a
chelated tetrahedral magnesium enolate where the C2-symmetric chiral backbone of the bis(sulfonamide) ligand gears
the aryl substituents to shield one face of the bound enolate.
Chiral C2-symmetric copper(ii) bis(oxazoline) Lewis acids
also are capable of effecting a “soft” enolization–amination
reaction [Eq. (5)].[16] The reaction proceeds quite well for a
variety of a-keto esters 10 and dibenzyl azodicarboxylates
catalyzed by 11 (Table 2). Since the products 12 were quite
prone to racemization, the crude reaction mixture was
immediately treated with l-selectride. The reaction most
likely proceeds through a complex in which the distorted
square-planar CuII center is chelated to the a-keto ester
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Chemie
Asymmetric Catalysis
Table 1: a Amination of carboximides catalyzed by a magnesium
bis(sulfonamide) complex.[a]
Table 3: a Amination of b-keto esters catalyzed by a copper(ii)–
bis(oxazoline) complex.
Yield [%]
ee [%]
92
97
93
86
90
86
4
85
82
5
84
80
6
87
82
Entry
Ar
1
2
3
Ph
4-F-C6H4
4-MeO-C6H4
Entry
R1
R2
R3
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
10
Me
Et
Ph
iPr
Bn
Me
Me
Me
Me
Me
Me
Me
allyl
Me
Et
Et
Et
tBu
tBu
tBu
tBu
Et
Et
Et
91
98
81
89
79
80
86
96
96
70
96
98
87
98
98
98
98
99
99
99
(CH2)3
(CH2)4
(CH2)5
[a] Boc = tert-butyloxycarbonyl, Ts = toluenesulfonyl.
Table 2: a Amination of a-keto esters catalyzed by a copper(ii)–
bis(oxazoline) complex.[a]
Entry
R
1
2
3
4
5
6
7
8
Bn
Me
pentyl
allyl
but-3-enyl
iBu
iPr
(c-hexyl)methyl
Yield [%]
ee [%]
55
45
63
62
52
53
78
54
77
90
93
93
92
96
95
96
synthesized the azodicarboxylate derivative 17, which is
capable of forming a chelate with the CuII complex 18
[Eq. (7)]. This reacts cleanly with a variety of silyl enol ethers
16 giving aminated products 19 with uniform high selectivities
and yields (Table 4). Cyclic silyl enol ethers 20 are also good
nucleophiles for this transformation (Scheme 6). The key to
efficient catalyst turnover in this reaction was found to be the
addition of CF3CH2OH. This alcohol additive serves to break
down a hetero-Diels–Alder intermediate that binds strongly
to the CuII catalyst. The rationale for the observed stereoselectivity is consistent with that seen for other reactions
catalyzed by copper(ii)–bis(oxazoline) complexes.[19]
Table 4: a Amination of enolsilanes catalyzed by a copper(ii)–
bis(oxazoline) complex.[a]
[a] Bn = benzyl, Cbz = benzyloxycarbonyl, Tf = trifluoromethanesulfonyl.
enolate. The sense of asymmetric induction can be rationalized by electrophilic attack of the azodicarboxylate onto the
less hindered face of the copper(ii) enolate.
This type of amination reaction was extended to b-keto
ester substrates (13) as well [Eq. (6)].[17] Once again, a C2symmetric copper(ii)–bis(oxazoline) complex, 14, was found
to be the optimal catalyst with very low loadings (0.5 mol %).
The catalyst shows exceptional levels of selectivity for a
variety of cyclic and acyclic keto esters (Table 3). The
proposed mechanism and model for stereoinduction is similar
to that previously described: the chelating ability of the
substrate explains the high levels of enantioselectivity.
Lewis acid activation of the azodicarboxylate electrophile
has also been reported by Evans and co-workers.[18] They
Angew. Chem. Int. Ed. 2005, 44, 4292 –4300
Entry
R1
R2
1
2
3
4
5
6
7
8
9
10
11
12
13
Ph
4-MeO-C6H4
6-MeO-Nap
Ph
Ph
Ph
Ph
4-MeO-C6H4
4-MeO-C6H4
4-MeO-C6H4
pyrrolyl
pyrrolyl
pyrrolyl
Me
Me
Me
Et
allyl
iBu
iPr
tBu
Bn
Ph
Me
allyl
iPr
Yield [%]
ee [%]
95
96
96
93
92
92
86
84
88
95
93
73
65
99
99
99
98
97
98
99
98
91
91
98
98
99
[a] Troc = 2,2,2-trichloroethoxycarbonyl.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
J. M. Janey
Scheme 6. a Aminations of cyclic silyl enol ethers. Ox = 2-oxazolidinone, Troc = 2,2,2-trichloroethoxycarbonyl.
Although not directly related to the aforementioned
topics, Yamamoto et al. have also explored the use of
arylnitroso compounds as heterodienophiles in the Diels–
Alder reaction of various cyclic dienes [Eq. (8)].[20] They
catalysis of the aldol reaction by chiral Lewis bases by means
of reversible formation of enamines, a series of reports have
appeared recently which detail the use of proline to catalyze
the addition of simple aldehydes and ketones to nitrosobenzene. The primary advantage of this approach is that it avoids
the added step of first forming a metal enolate; in addition,
proline catalysis tolerates benign reaction conditions (ambient temperature and atmosphere, wet solvents). The first
reports of this methodology all appeared within three months
of each other from the laboratories of Guofu Zhong,
David W. C. MacMillan, and Yujiro Hayashi, and differ only
in choice of solvent and the substrates examined.[22] As shown
in Equation (9), enolizable aldehydes 23 with varying degrees
of branching and functionality react quite well with nitrosobenzene in the presence of catalytic amounts of l-proline.
Since the initial a-alkoxy aldehydes produced are oligomeric,
subsequent reduction with NaBH4 is necessary to give diol 24.
Table 5 shows that a variety of aldehydes, from bulky ones
Table 5: a Oxygenation of enolizable aldehydes catalyzed by l-proline.
report quantitative yields and excellent enantioselectivity for
forming cycloadduct 22 when they use a chiral Lewis acid
catalyst composed of a copper(i) hexafluorophosphate salt
and the axially chiral bisphosphine (S)-segphos. The key to
this reaction is the use of 2-nitrosopyridines 21, as these are
capable of forming a five-membered chelate with the chiral
CuI catalyst. This provides a high level of organization in the
transition state, thus leading to exceptional levels of stereoselectivity. The reported model to explain the stereochemical
outcome of this Diels–Alder reaction with (S)-binap as a
ligand assumes a tetrahedral CuI center with a “filled” first
and third quadrant according to the mnemonic developed by
Noyori (Figure 4).[21] Cycloadduct 22 can be transformed over
a five-step sequence in 48 % overall yield to a fully protected
1,4-amino alcohol.
Figure 4. Transition-state model for the hetero-Diels–Alder reaction
catalyzed by copper(i)–(S)-binap.
4. a Oxygenations Catalyzed by Chiral Lewis Bases
Many of the aforementioned amination and oxygenation
reactions relied upon pregenerated, stable metal enolates as
reaction partners, but as stated in the introduction, enamines
are also known to react cleanly with nitrosobenzene. Given
this precedent and the burgeoning literature concerning
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Entry
R
Solv.
l-Proline
[mol %]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
Me
nBu
iPr
CH2CH=CH2
Bn
Ph
(CH2)2OTIPS[a]
(N-methylindol3-yl)methyl
nPr
CH2=CHCH2CH2
BnOCH2
BocNH(CH2)4
Et
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
5
5
5
5
5
5
5
5
88
79
85
80
95
60
76
83
97
98
99
99
97
99
98
98
DMSO
DMSO
DMSO
DMSO
CH3CN
20
20
20
20
30
71
73
54
61
87
99
99
99
94
99
9
10
11
12
13
TIPS = triisopropylsilyl.
(Table 5, entry 3) to aldehydes containing other functional
groups (entries 7, 8, 11, and 12), are well tolerated and
produce the expected diol with excellent enantioselectivity
and moderate-to-high yields. MacMillan and co-workers
report that chloroform is the best solvent in terms of catalyst
efficiency. They found that l-proline loadings could be
dropped to 1 mol % with little loss in reaction efficiency. In
other recent reports photochemically generated singlet oxygen was used as an electrophile for this transformation, albeit
with moderate enantioselectivity.[23]
Ketones have proven to be more challenging substrates
for this reaction.[24] Besides their diminished ability to
reversibly form enamines with proline, they have two
enolizable carbon atoms, which may compromise chemoselectivity in the oxygenation reaction. To avoid this complication, a large excess (2–10 equiv) of ketone 25 is often used
and nitrosobenzene is added slowly with a syringe pump
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Chemie
Asymmetric Catalysis
[Eq. (10)]. This protocol prevents the formation of any C2symmetric diaddition products and cleanly gives 26, although
in the case of acyclic ketones (Table 6, entries 1–4) minor
amounts of the regioisomer 27 arising from N addition are
The proline-catalyzed reaction has also been applied to
the desymmetrization of meso ketones 29 a and 29 b
[Eq. (12)].[24b] The degree of catalyst control is very high as
both the cis (30 a,b) and trans ketones (31 a,b) are produced
with high ee.
Table 6: a Oxygenation of ketones catalyzed by l-proline.
Entry
Product 26
26/27
Yield [%]
ee [%]
1
81:19
93
> 99
2
98:2
66
99
3
88:22
87
> 99
4
90:10
64
> 99
5
> 99:1
91
> 99
6
> 99:1
96
> 99
7
> 99:1
84
> 99
8
> 99:1
53
96
9
> 99:1
44
99
formed. It is also interesting to note that in the case of
nonsymmetric ketones, such as entries 1, 3, and 4, it is the
more substituted carbon that undergoes reaction with nitrosobenzene. Cyclic ketones (entries 5–9) are excellent substrates, giving 26 with complete regio- and stereoselectivity
regardless of any substitution at the 4-position. In addition,
Yamamoto et al. have described a more efficient proline–
tetrazole catalyst 28, which mediates the oxygenation of both
aldehydes and ketones in high yields (65–97 %) and enantioselectivities [Eq. (11)].[11] Shorter reaction times, lower catalyst loadings, and faster addition times of nitrosobenzene
were reported with catalyst 28.
An interesting extension to this chemistry is the heteroDiels–Alder reaction between cyclic enones and nitrosobenzene catalyzed by chiral Lewis bases.[25] A variety of 2cyclohexene-1-ones 32 react with nitrosobenzenes 33 catalyzed by proline tetrazole 28 giving cycloadducts 34 with
moderate yields and very high enantioselectivity [Eq. (13)].
This reaction appears to be insensitive to substituents at the 4position of enones 32 and nitrosobenzene 33. Although
formally a hetero-Diels–Alder reaction, this appears to be a
more stepwise process. After a typical nitroso-aldol reaction,
the nitrogen undergoes a conjugate addition to the enone
double bond to give the cycloadduct 34. Attempts at reversing
the regioselectivity of this reaction with silver(i) Lewis acids
were unsuccessful as the oxygen is reluctant to add in a
conjugate manner to the enone.
The purported mechanism of this proline-catalyzed nitroso-aldol reaction is similar to that of the proline-catalyzed
aldol reaction (Figure 5).[26] The proline enamine is believed
to form an organized six-membered chairlike transition state
with nitrosobenzene. A hydrogen bond between the nitrogen
of nitrosobenzene and the acid functionality of proline
accounts for the observed facial bias. According to calculations performed by Houk and co-workers, the O-syn transition state, which leads to the observed major enantiomer, is
favored by 3.3 kcal mol 1 over the O-anti transition state. A
Figure 5. Possible transition state in the reaction of a proline enamine
with nitrosobenzene.
Angew. Chem. Int. Ed. 2005, 44, 4292 –4300
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angewandte
Chemie
J. M. Janey
competing pathway that is only 2.5 kcal mol 1 higher in
energy proceeds via the N-anti transition state, which gives
rise to the undesired a-aminated product. Although the
observed selectivities lend support to this model to explain
the stereoinduction, kinetic data suggests a more complex
catalytic cycle.[27] A calorimetric analysis of the reaction
kinetics clearly indicates a selectivity-enhancing autoinductive process in which the rate and ee of the reaction increase
with each cycle of the catalytic species. This implies that a
product–proline adduct of some sort (either an enamine or
hydrogen-bonded species) gives rise to an improved catalyst
over the original, sparingly soluble proline.
To date, there have been no reported organocatalytic
reactions of nitrosobenzene that give the product resulting
from attack at nitrogen as the major product, other than the
single example of a stoichiometric morpholine enamine
giving the N adduct.[10] Instead, K. A. Jørgensen et al. as well
as Benjamin List have reported that proline catalyzes the
amination of aldehydes and ketone with azodicarboxylates
(Table 7).[28] These reactions proceed with moderate-to-ex-
rate. The authors characterized an
oxazolidinone (37) that leads to this
acceleration. Although the exact role
and mechanism of this “improved”
catalyst is not yet known, it may
serve as a solublized proline species
that can enter the catalytic cycle immediately, as solid proline
is only slightly soluble in organic solvents such as chloroform.
It has also been proposed that the origin of autocatalysis may
be the improved solubility of proline through hydrogenbonding interactions with a matched product molecule.[30b]
A recent contribution to this area was made by K. A.
Jørgensen and co-workers who report that a quinidinederived alkaloid (b-isocupreidine, b-ICD) serves as an
efficient catalyst for the a amination of a-cyanoacetates and
b-dicarbonyl compounds [Eq. (15)].[31] This highly efficient
reaction proceeds with 5 mol % b-ICD to give the expected
product with excellent levels of enantioselectivity for a variety
of aryl-substituted a-cyanoacetates and a few b-dicarbonyl
compounds (Table 8 and Scheme 7). No mechanistic scenarios were offered, although given the high acidity of the
substrates and the basic nature of the catalyst, an enolate with
a chiral ammonium (b-ICD-H+) counterion is a likely
intermediate.
Table 7: a Amination of enolizable aldehydes catalyzed by l-proline.
Table 8: a Amination of a-cyanoacetates catalyzed by b-isocupreidine.
5. a Aminations Catalyzed by Chiral Lewis Bases
Entry
R1
R2
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
10
11
Me
Et
iPr
tBu
allyl
Bn
iPr
nPr
nBu
Me
Bn
Et
Et
Et
Et
Et
Et
Bn
Bn
Bn
Bn
Bn
67
77
83
57
92
68
99
93
94
97
95
93
95
93
91
93
89
96
95
97
95
95
cellent yields and high enantioselectivity, and appear to be
quite general for a variety of aldehydes and azodicarboxylates. There have also been reports that a,a-disubstituted
aldehydes are competent substrates, although selectivities are
moderate at best.[29]
The mechanism for this transformation exhibits very
interesting and dramatic kinetics.[30] An examination of this
reaction using calorimetry reveals both a nonlinear effect and
an accelerating reaction rate, which is a hallmark of an
autocatalytic reaction wherein the ee is amplified after each
catalytic cycle. It was found that adding product to the
reaction mixture accelerated the reaction. In addition,
premixing proline and the product gave a new catalyst (or
precatalyst) species which led to a much improved reaction
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Entry
Ar
1
2
3
4
5
6
7
8
Ph
2-F-C6H4
3-Me-C6H4
4-Cl-C6H4
4-NO2-C6H4
4-MeO-C6H4
2-naphthyl
2-thienyl
Yield [%]
ee [%]
99
99
99
99
99
95
99
99
> 98
98
97
98
91
89
98
97
Scheme 7. a Amination of b-dicarbonyl compounds.
6. Summary and Outlook
The catalytic, asymmetric a amination and a oxygenation
of carbonyl compounds represents a valuable advance in
synthetic methodology. Although much progress has been
made in developing this chemistry, many challenges and
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Chemie
Asymmetric Catalysis
questions still remain. A deeper understanding of the
mechanism and the factors that govern O versus N selectivity
with nitroso compounds will allow for innovation in catalyst
design and make this methodology a robust and efficient way
to access valuable, enantiopure building blocks with broad
applicatione to the synthesis of biologically important molecules. In addition, more convenient electrophilic sources of
nitrogen and oxygen along with broadened and general
substrate scope would be a welcome advance. This area is
developing at a rapid pace and surely further progress in both
catalyst design and mechanistic insight will be forthcoming.
[9]
[10]
Received: October 14, 2004
Published online: June 9, 2005
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