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Diarylprolinol Ethers Expanding the Potential of EnamineIminium-Ion Catalysis.

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Highlights
DOI: 10.1002/anie.200602943
Organocatalysis
Diarylprolinol Ethers: Expanding the Potential of
Enamine/Iminium-Ion Catalysis**
Claudio Palomo* and Antonia Mielgo
Keywords:
domino reactions · enamines · iminium ions ·
organocatalysis
Catalytic asymmetric synthesis is providing chemists with new and powerful
tools for the efficient synthesis of complex molecules.[1] Although for many
years asymmetric catalysis has been
linked to the use of chiral transitionmetal complexes,[2] recently, processes
catalyzed by metal-free species—organocatalytic methods—have emerged as
a complementary tool.[3] While chiral
Brønsted acids[4] and bifunctional chiral
Brønsted acid/Brønsted base catalysts[5]
have been employed, the asymmetric
catalysis of carbonyl transformations via
iminium ion[6] and enamine intermediates[7] using chiral amines has grown
most remarkably. One of the main goals
for chemists is to devise a general
organocatalyst suitable for a broad
range of transformations; in many cases
this requires tedious screening of a large
number of catalysts because only a few
stand out for their broad utility.[8] Relevant examples are MacMillan1s imidazolidinones[6] and the amino acid proline (1), which have often been described as fairly general and efficient
amine-based catalysts (see Figure 2).
Indeed, enamine catalysis using 1
has been applied to both intermolecular
and intramolecular nucleophilic addition reactions with a variety of electrophiles.[7] In addition to carbonyl com[*] Prof. Dr. C. Palomo, Dr. A. Mielgo
Departamento de Qu/mica Org0nica I
Facultad de Qu/mica
Universidad del Pa/s Vasco
Apdo. 1072, 20080 San Sebasti0n (Spain)
Fax: (+ 34) 943-015-270
E-mail: claudio.palomo@ehu.es
[**] This work was supported financially by The
University of the Basque Country (UPV/
EHU) and Ministerio de EducaciBn y
Ciencia (MEC, Spain).
7876
pounds (C=O),[9] these include imines
(C=N) in Mannich reactions,[10] azodicarboxylates (N=N),[11] nitrosobenzene
(O=N),[12] and Michael acceptors.[13] In
these processes the configuration of the
final adducts is generally controlled by a
hydrogen-bond interaction between the
acidic proton of proline and the incoming electrophile (A in Figure 1). Thus,
Although (S)-2,2-diphenylprolinol
(2 a; Figure 2) may promote reactions
with a good level of stereocontrol, the
Figure 2. Developed organocatalysts.
Figure 1. Pictogram showing how the complementary pyrrolidine catalysts function: model
A: l-proline, through a hydrogen bond; model
B: diarylprolinol ethers, through steric control.
this interaction, whilst activating the
electrophile, guides its approach from
the upper face of the enamine. A similar
pattern is also followed by protonated
amine-based catalysts[14] and, in general,
by catalysts bearing a hydrogen-bond
donor at the a position of the pyrrolidine nitrogen.[7d,e, 15]
Recently, other pyrrolidine derivatives, diarylprolinol ethers, have
emerged as potentially general enamine
organocatalysts. Most significantly, with
these catalysts the configuration of the
final adduct is controlled by the steric
hindrance of the substituent a to the
pyrrolidine nitrogen (B in Figure 1).
This forces the approach of the electrophile to the lower face of the enamine,
thus affording products of opposite
configuration compared to those obtained with l-proline as catalyst.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
processes are characterized by low catalyst turnover. This fact has been mainly
ascribed to the formation of the relatively stable and unreactive hemiaminal
species, which removes a significant
amount of the catalyst from the catalytic
cycle. To sidestep hemiaminal formation, trimethylsilyl (TMS) ethers 3 and
methyl ether 4 have been developed.
Diarylprolinol silyl ethers 3 have
been applied successfully to the a-functionalization of aldehydes, and a large
variety of transformations such as C X
(X = F, Br, S) bond formation, a-aminations, and Mannich reactions have been
reported (Scheme 1).[7d,e, 16] The reactions all proceed in good to high yields
and with excellent enantioselectivities.
Remarkably significant is the Mannich
reaction with glyoxylate imines, which
turned out to be highly anti diastereoselective.[16] It is worth noting that only
two other papers by Barbas et al. and
Maruoka et al. have presented (3R,5R)5-methylpyrrolidine-3-carboxylic acid[17]
and an axially chiral aminosulfonaAngew. Chem. Int. Ed. 2006, 45, 7876 – 7880
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promoting this reaction in the presence
of hydrogen peroxide as oxidant
(Scheme 3).[20] In this case the reaction
occurs through the iminium ion intermediate C; the steric bulk of the a
substituent also forces the approach of
Scheme 4. Conjugate addition of malonates to
a,b-unsaturated aldehydes catalyzed by 3 b.
Scheme 1. a-Functionalization of aldehydes
catalyzed by 3 b. PMP = p-methoxyphenyl.
mide,[18] respectively, as catalysts for
promoting the anti Mannich reaction
with high diastereo- and enantioselectivities. These results complement those
reported with l-proline where high syn
diastereoselectivities are observed.[10] In
the same context, Gellman and Chi have
recently described the a-aminomethylation of aldehydes through a catalytic
asymmetric Mannich reaction involving
a formaldehyde-derived iminium electrophile and by using 3 a as organocatalyst together with AcOH and
LiCl[19] (Scheme 2). The expected adducts are reduced in situ to afford the gaminoalcohols in good yields and excellent enantioselectivities.
Diarylprolinol silyl ethers have also
been investigated as organocatalysts for
the epoxidation of a,b-unsaturated aldehydes. Jørgensen et al. have demonstrated that catalyst 3 b is efficient in
Scheme 2. Enantioselective aminomethylation
of aldehydes catalyzed by 3 a. Bn = benzyl.
Angew. Chem. Int. Ed. 2006, 45, 7876 – 7880
Scheme 3. Epoxidation of a,b-unsaturated aldehydes catalyzed by 3 b.
the hydrogen peroxide to the opposite
face. Further nucleophilic attack of the
resulting enamine carbon atom at the
electrophilic peroxygen atom, followed
by hydrolysis of the iminium intermediate results in epoxide formation. Other
oxidants can also be employed in these
reactions. For example, Lattanzi has
reported the epoxidation of a,b-enones
with tert-butyl hydroperoxide and catalyst 2 a to yield the corresponding epoxides although with moderate enantioselectivities.[21]
Asymmetric conjugate additions for
C C bond formation are another important challenge in organic synthesis.
The enantioselective version of this
process[22] has also been explored with
diarylprolinol ether derivatives by both
enamine and iminium-ion strategies. A
recent example is the conjugate addition
of malonates to a,b-unsaturated aldehydes[23] catalyzed by 3 b (Scheme 4)
providing the addition products by iminium-ion activation in good yields and
excellent enantioselectivities. Likewise,
Hayashi et al.[24] have documented the
Michael addition of aldehydes to nitroalkenes catalyzed by 3 a (Scheme 5). In
this case an enamine pathway is followed, which is more effective with
nitrostyrenes, while with b-alkyl-substituted nitroalkenes moderate yields are
obtained. In a related context, Gellman
Scheme 5. Conjugate addition of aldehydes to
nitroalkenes catalyzed by 3 a.
and co-workers[25] showed the utility of
diphenylprolinol methyl ether 4 in the
conjugate addition of aldehydes to vinyl
ketones (Scheme 6). Optimum results
were achieved when the reactions were
carried out in the presence of a catechol
derivative as a cocatalyst, which apparently electrophilically activates the
Scheme 6. Conjugate addition of aldehydes to
vinyl ketones catalyzed by 4.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7877
Highlights
enone by hydrogen-bond donation to
the carbonyl oxygen.
Catalytic and stereoselective domino or cascade reactions are tools of
particular interest as they can be used
for the one-pot synthesis of complex
organic molecules with multiple stereogenic centers.[26] In particular, secondary
amines, capable of both enamine and
iminium-ion catalysis, are good candidates for the design of this type of
process.[7c, 27] Four important contributions on this subject which use diarylprolinol silyl ethers as catalysts have
recently appeared. The first, by Jørgensen et al.,[28] presents the pioneering
multicomponent domino conjugate thiol
addition/electrophilic amination reaction that gives access, after in situ
reduction and cyclization of the resulting adducts, to highly functionalized
oxazolidinones in good yields and with
over 99 % ee (Scheme 7). This example
combines first iminium-ion activation to
promote the conjugate addition of thiols
to a,b-unsaturated aldehydes, and then
enamine activation to effect the amination reaction. The second report, from
the same research group, describes the
one-pot Michael addition/aldol/SN2 reaction of a,b-unsaturated aldehydes and
g-chloro-b-ketoesters (Scheme 8) in the
presence of NaOAc followed by further
treatment with K2CO3. Iminium-ion and
enamine activations are successively
combined to afford epoxycyclohexanone derivatives with up to four stereocenters in very good diastereo- and
enantiomeric ratios.[29] The third contribution in this area, by Wang et al.,[30]
describes the organocatalytic asymmetric Michael addition/aldol dehydration
reaction of a,b-unsaturated aldehydes
and 2-mercaptobenzaldehydes in the
presence of benzoic acid through iminium-ion/enamine activation; the corresponding adducts are obtained in excellent yields and enantioselectivities
(Scheme 9). The fourth report, by Enders and co-workers, is an elegant example of the control of four stereocenters in a triple cascade organocatalytic
reaction of a linear aldehyde, a nitroalkene, and an a,b-unsaturated aldehyde with catalyst 3 a to afford tetrasubstituted cyclohexene carbaldehydes
with high diastereoselectivity and essentially
complete
enantiocontrol[31]
(Scheme 10). In contrast to Jørgensen1s
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Scheme 7. Domino addition/amination reaction of a,b-unsaturated aldehydes catalyzed by 3 b.
Scheme 8. One-pot Michael addition/aldol/SN2 reaction of a,b-unsaturated aldehydes and gchloro-b-ketoesters catalyzed by 3 b.
Scheme 9. Cascade Michael addition/aldol dehydratation reactions of a,b-unsaturated aldehydes and 2-mercaptobenzaldehydes catalyzed by 3 b.
and Wang1s work, the catalytic cycle
here starts with enamine activation of
the linear aldehyde D (Figure 3), which
then selectively adds to the nitroalkene
E in a Michael reaction, in analogy to
the mechanism proposed by Hayashi
et al. (see Scheme 5). The subsequent
hydrolysis liberates the catalyst, which is
now able to form the iminium ion of the
a,b-unsaturated aldehyde F to accomplish the conjugate addition with nitroalkane 5.[32] In the subsequent third step,
an enamine activation of the resulting
enamine intermediate 6 leads to an
intramolecular aldol condensation via
7. Hydrolysis completes the catalytic
cycle and releases the tetrasubstituted
cyclohexene carbaldehyde 8. It is note-
worthy that in all four cascade processes
described by Jørgensen, Wang, and
Enders, one of the steps is a conjugate
addition reaction.[33]
In conclusion, diarylprolinol ethers
have greatly contributed to increase the
power of aminocatalysis in asymmetric
C C and C X bond-forming reactions.
The efficiency of these catalysts is based
on the principle of “steric control approach”, and they have assisted in the
understanding of the mechanisms of
reactant activation and reaction stereoselectivity. Nevertheless, aminocatalysis
is still in the first stages of development,
and new powerful enantioselective
transformations, including multicomponent/domino reactions, are waiting to be
Scheme 10. Three-component multistep reaction cascade catalyzed by 3 a.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 7876 – 7880
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[11]
[12]
[13]
[14]
Figure 3. Proposed catalytic cycle of a triple cascade reaction affording substituted cyclohexenes.
discovered.[34] Yet, catalyst recovery has
to be solved and catalyst loading has to
be reduced.
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2006, 45, 7876 – 7880
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