Organocatalytic enantioselective formal conjugate addition of a hydroxymoyl anion to α,β-unsaturated aldehydes

Article abstract of DOI:10.1002/chem.201100605

The conjugate addition reaction is a fundamental transformation in organic chemistry. The wide variety of reagents amenable to participate as Michael donors or acceptors makes this reaction extremely versatile, allowing the preparation of many different classes of highly functionalized compounds in a very easy and reliable way. Moreover, the conjugate addition reaction also occurs very often with the concomitant generation of one or more stereocenters and, in this context, intense research has focused on the development of catalytic enantioselective approaches for carrying out this important reaction in a stereocontrolled way. Although most of the methodologies reported to date involve the use of transition-metal catalysts, several organocatalytic enantioselective variants of the conjugate addition reaction have been developed in the last few years; many of which have performed excellently with regard to chemical efficiency and stereocontrol. On the other hand, many research groups have also directed their attention to finding novel reactivity patterns applied to the conjugate addition reaction by using umpoled reagents, which have expanded the array of compounds which can be used as Michael donors or acceptors, to further increase the scope of this methodology. In particular, the enantioselective conjugate addition of acyl or formyl anions to a,b-unsaturated carbonyl compounds or related derivatives entails a particular interest because this transformation allows the preparation of 1,4-dicarbonyl compounds, which are classically much more difficult to access by standard C-C bond forming reactions than the corresponding 1,3- or 1,5-dicarbonyl compounds.[4] However, despite the enormous potential of the oxime moiety and the key role played by this functional group in plenty of organic reactions, the development of umpoled reagents that behave as hydroxymoyl anion equivalents remains unexploited

Full text of DOI:10.1002/chem.201100605

DOI: 10.1002/chem.201100605
Organocatalytic Enantioselective Formal Conjugate Addition of a
Hydroxymoyl Anion to a,b-Unsaturated Aldehydes
Beatriz Alonso, Efraꢀm Reyes, Luisa Carrillo, Jose L. Vicario,* and Dolores Badꢀa*[a]
In memory of our colleague and friend Rafael Suau
The conjugate addition reaction is a fundamental transfor-
mation in organic chemistry. The wide variety of reagents
amenable to participate as Michael donors or acceptors
makes this reaction extremely versatile, allowing the prepa-
ration of many different classes of highly functionalized
compounds in a very easy and reliable way. Moreover, the
conjugate addition reaction also occurs very often with the
concomitant generation of one or more stereocenters and,
in this context, intense research has focused on the develop-
ment of catalytic enantioselective approaches for carrying
out this important reaction in a stereocontrolled way.[1] Al-
though most of the methodologies reported to date involve
the use of transition-metal catalysts, several organocatalytic
enantioselective variants of the conjugate addition reaction
have been developed in the last few years;[2] many of which
have performed excellently with regard to chemical efficien-
cy and stereocontrol. On the other hand, many research
groups have also directed their attention to finding novel re-
activity patterns applied to the conjugate addition reaction
by using umpoled reagents,[3] which have expanded the
array of compounds which can be used as Michael donors or
acceptors, to further increase the scope of this methodology.
In particular, the enantioselective conjugate addition of acyl
or formyl anions to a,b-unsaturated carbonyl compounds or
related derivatives entails a particular interest because this
transformation allows the preparation of 1,4-dicarbonyl
compounds, which are classically much more difficult to
In fact, even for the related case of the conjugate addition
of acyl/formyl anion equivalents, enantioselective versions
for this particular transformation under metal-free condi-
tions are very scarce (see Scheme 1). For example, the Stet-
ter reaction using N-heterocyclic carbenes (NHC) as cata-
lysts can be an extremely effective approach for the formal
enantioselective conjugate addition of acyl anions,[5] but it is
ineffective when trying to use formaldehyde as the Michael
donor.
À
access by standard C C bond forming reactions than the
corresponding 1,3- or 1,5-dicarbonyl compounds.[4] However,
despite the enormous potential of the oxime moiety and the
key role played by this functional group in plenty of organic
reactions, the development of umpoled reagents that behave
as hydroxymoyl anion equivalents remains unexploited.
Scheme 1. Examples of organocatalytic enantioselective conjugate addi-
tion of acyl/formyl anion equivalents used previously and the conjugate
hydroxymoylation described in this work.
For the particular case of the conjugate addition of a
formyl anion equivalent, there is only one single example re-
lated to an enantioselective organocatalytic version; this in-
volves the use of N,N-dialkylhydrazones as Michael donors,
which undergo conjugate addition to b,g-unsaturated a-ke-
toesters under chiral Brønsted-acid catalysis to give moder-
ate levels of enantioselection.[6] Another relevant example
[a] B. Alonso, Dr. E. Reyes, Prof. L. Carrillo, Prof. J. L. Vicario,
Prof. Dr. D. Badꢀa
Departamento de Quꢀmica Orgꢁnica II
Facultad de Ciencia y Tecnologꢀa
Universidad del Paꢀs Vasco/
Euskal Herriko Unibertsitatea (UPV/EHU)
P.O. Box 644, 48080 Bilbao (Spain)
Fax : (+34)94-601-5454
involves oxazol-5ACTHNUTRGNEUNG(2H)-ones as aryloyl anion equivalents in
the conjugate addition to enoylphosphonates,[7] which also
relies on Brønsted-acid catalysis. In addition, one report by
Enders uses a-aminonitriles as Michael donors in the enan-
E-mail: joseluis.vicario@ehu.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100605.
6048
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Chem. Eur. J. 2011, 17, 6048 – 6051

COMMUNICATION
Table 1. Conjugate addition of 1 to a,b-unsaturated aldehydes 2.
tioselective b-glyoxylation of a,b-unsaturated aldehydes
under iminium catalysis, which proceeded with enantioselec-
tivities of about 85% ee, to give the final 1,4-dicarbonyl
compounds in moderate overall yield.[8] Other related um-
polung reagents that are the source of hydroxycarbonyl or
alkoxycarbonyl anions, such as cyanides or azlactones, have
also been reported using hydrogen-bonding catalysis[9] and
phase-transfer catalysis[10] as the vehicle for enantiocontrol.
However, as mentioned before, despite the efforts in the
field, as far as we know there is no example in the literature
dealing with the development of a hydroxymoyl anion
equivalent for use in catalytic enantioselective conjugate ad-
dition reactions neither under organocatalytic conditions
nor by transition-metal catalysis.
In this context, we have found that N-nitromethylphthali-
mide, which can be easily prepared from commercially avail-
able starting materials in a very simple way,[11] constitutes an
excellent hydroxymetanimidoyl anion equivalent to be used
as a Michael donor in enantioselective conjugate addition
reactions with a,b-unsaturated aldehydes under iminium ac-
tivation (see Scheme 1).[12] On one hand, N-nitromethylph-
thalimide is an acidic compound, which can participate as
pronucleophile under the neutral or slightly basic reaction
conditions associated to iminium catalysis,[13] therefore al-
lowing the use of a chiral secondary amine catalyst to ach-
ieve enantiocontrol. On the other hand, we have also found
suitable conditions that allow the transformation of the ni-
tro(phthalimido)methyl moiety into the hydroxymetanimi-
doyl group by a simple and efficient procedure.
Entry
Compound
R
Yield
[%][c]
d.r.[d]
ee
[%][e]
1
2
3
4
5
6
7
8
4a[a]
4b[a]
4c[a]
4d[a]
4e[a]
4 f[a]
4g[a]
4h[b]
4i[b]
Ph
2-MeO
4-MeOACHTNUGTRENNUNG
3-AcO-4-MeO
2-furyl
3,4-OCH2O
Me
Et
nPr
nBu
80
84
80
78
80
65
60
88
80
88
82
73
78
70
70
10:3
5:1
5:1
5:1
5:4
3:1
1:1
1:1
5:4
1:1
1:1
5:4
1:1
1:1
5:4
90
92
96
96
88
90
90
90
93
93
92
95
94
92
93
ACHTUNGTRENNUNG
ACHTUGNTERN(NUGN C6H3)
ACHTUGNTERN(NUGN C6H3)
9
10
11
12
13
14
15
4j[b]
4k[b]
4l[b]
nC5H11
nC6H13
(Z)-hex-3-enyl
nC7H15
4m[b]
4n[b]
4o[b]
nC8H17
[a] Reactions carried in EtOH at 48C. [b] Reactions carried out in the
presence of DABCO (10 mol%) in CHCl3 at 48C. [c] Combined yield of
both diastereoisomers after column chromatography purification.
[d] d.r.=diastereoisomeric ratio; determined by NMR spectroscopic
analysis of the crude reaction mixture. [e] Determined by HPLC on
chiral stationary phase (see the Supporting Information for details). The
ee values for both diastereoisomers were found to be identical in all cases
tested (see ref. [15]). DABCO=1,4-diazabicycloACHTUNTRGNEU[GN 2.2.2]octane.
Our studies began with the optimization of the reaction
conditions and the identification of the best catalyst for the
initial conjugate addition of N-nitromethylphthalimide 1 to
a,b-unsaturated aldehydes under iminium catalysis. After
several experiments using cinnamaldehyde as model sub-
strate, it was found that O-TMS diphenyl prolinol 3, with
EtOH as a solvent and at a temperature of 48C, was able to
promote the reaction efficiently (Table 1, entry 1). These
conditions were applied to a set of different a,b-unsaturated
aldehydes, to obtain a variety of Michael addition products
4a–4g in good yields and with excellent enantioselectivities
(Table 1, entries 2–7). However, when working with b-alkyl
substituted enals that were different to crotonaldehyde, the
reaction slowed down considerably under these conditions
to give the conjugate addition products with a significant
lower yield for the same reaction time (20% for 2h and
<5% for 2i), which was attributed to the important steric
hindrance associated to the income of the rather bulky nu-
cleophile derived from 1 when increasing the size of this b-
substituent. As a consequence, conditions had to be found
to overcome this limitation; the inclusion of a catalytic
(Table 1, entries 8–15). All these adducts were isolated as an
inconsequential mixture of diastereoisomers because of the
inherent configurational instability of the stereocentre at the
nitro(phthalimido)methyl moiety. Nevertheless, both diaste-
reoisomers of each adduct 4 were obtained with identical ee
values, higher than 90% in all cases.[15]
With Michael adducts 4a–4o in hand, we next focused on
the conversion of the nitro(phthalimido)methyl moiety into
the target hydroxymetanimidoyl group (Table 2). This trans-
formation required the initial protection of the formyl
moiety, which was carried out in a very easy and straightfor-
ward manner by conversion into the corresponding ethylen-
glycol-based acetal derivatives 5a–5o. These were next re-
acted with hydroxylamine hydrochloride in the presence of
Na2CO3 to give directly the target oximes 6a–6o in good
yields and with very high optical purity. Oximes 6a–6o were
obtained as mixtures of E/Z diastereoisomers in different
proportions, which varied depending on the R substituent.
Nevertheless, both diastereoisomers for each oxime of type
6 were isolated with the same ee values, which were found
to be in the range of 88–96% ee in all cases (see Table 2).
At this point, it should be mentioned that the base em-
ployed to neutralize the hydroxylamine hydrochloride
played a crucial role in this reaction with respect to the con-
servation of the stereochemical integrity of the stereocenter
present in the final products. The use of other bases such as
amount of
a basic additive such as 1,4-diazabicyclo-
ACHTUNGTRENNUNG[2.2.2]octane (DABCO), which presumably activates the Mi-
chael donor by assisting its deprotonation, was shown to ac-
celerate the reaction.[14] These modified conditions also re-
quired the use of CHCl3 as solvent. Under these conditions,
different b-alkyl substituted a,b-unsaturated aldehydes re-
acted efficiently providing adducts 4h–4o in excellent yields
Chem. Eur. J. 2011, 17, 6048 – 6051
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6049

J. L. Vicario, D. Badꢀa et al.
Table 2. Conversion of adducts 4 into oximes 6.
noprotected 2-substituted-1,4-dialdehydes could be obtained
as highly enantioenriched compounds, which shows that this
protocol can also be applied to the formal enantioselective
b-formylation of a,b-unsaturated aldehydes. At this point
we could also assign the absolute configuration of the b-ste-
reocenter generated at the initial conjugate addition reac-
tion by chemical correlation. Comparison of the [a]2D0 value
obtained for compound 7a ([a]2D0 =À11.0, c=0.40, CHCl3)
to that reported in the literature ([a]2D0 =À14.7, c=1.33,
CHCl3, for the R isomer)[16] allowed us to assign a R config-
uration for the a stereocenter to the formyl moiety in com-
pound 7a and this configuration was extended to all com-
pounds 4, 5, 6, and 7 prepared previously, in which the ste-
reocenter had been already installed in the initial conjugate
addition reaction between aldehydes 2a–2o and N-nitrome-
thylphthalimide 1 by using catalyst 3. Moreover, this config-
uration is also in agreement with the configuration of conju-
gate addition products obtained in other Michael-type reac-
tions catalyzed by 3.[17]
Entry
R
5
Yield
[%][a]
6
Yield E/Z[c] ee
[%][b]
[%][d]
1
2
3
4
5
6
7
8
Ph
2-MeO
4-MeOACHTUNGTRENNUNG
3-AcO-4-MeO
2-furyl
3,4-OCH2O
Me
Et
5a 85
5b 83
5c 80
6a
6b
6c
82
65
71
5:3 90
>10:1 92
>10:1 96
>10:1 96
3:1 88
3:1 90
3:1 90
8:1 90
ACHTUNGTRENNUNG
G
6d[e] 72
6e
6 f
6g
6h
6i
75
75
65
65
68
68
90
70
(C6H3)
5 f 80
5g 80
5h 80
9
nPr
5i
83
7:1
93
10
11
12
13
14
15
nBu
nC5H11
nC6H13
(Z)-hex-3-enyl
nC7H15
nC8H17
5j 70
5k 81
6j
6k
6l
6m 82
6n
6o
10:1 93
10:1 92
10:1 95
5:1 94
10:1 92
4:1 93
5l
80
The proposed mechanism for the initial Michael reaction
is given in Scheme 3. The reaction would start with the acti-
vation of the enal by condensation with the catalyst leading
5m 70
5n 82
5o 70
76
70
[a]Combined yield of both diastereoisomers isolated after column chro-
matography purification. [b]Combined yield of both E/Z isomers isolated
after column chromatography purification. [c]Determined by NMR spec-
troscopic analysis of crude reaction mixture. [d]Determined by HPLC on
chiral stationary phase (see the Supporting Information). The ee values
for both E and Z diastereoisomers were found to be identical in all cases
tested. [e]The deacetylated product was obtained (see the Supporting In-
formation). PPTS=pyridinium para-toluenesulfonate
Cs2CO3, K2CO3, or KOH led to the formation of the final
oximes 6a–6o with a significant loss of enantiomeric purity.
Careful selection of the base may be required because of
the configurational instability of the final chiral oximes 6, in
which the enolizable proton present at the stereogenic
centre can undergo deprotonation under basic conditions. In
our case, these results may indicate that the use of a weaker
base, such as Na2CO3, deprotonates the stereogenic centre
more slowly than when other stronger bases (Cs2CO3,
K2CO3, or KOH) are present in the reaction medium.
In addition, the hydroxymetanimidoyl group can also be
considered as an excellent precursor for a formyl moiety. In
our case, we succeeded in transforming a family of represen-
tative oximes 6 into chiral aldehydes 7 by carrying out a
simple ozonolysis process, which also proceeded with no rac-
emization (Scheme 2). By this simple operation, several mo-
Scheme 3. Proposed pathway for the 3-catalyzed reaction between a,b-
unsaturated aldehydes 2a–2o and nitromethylphthalimide 2.
to the formation of the corresponding iminium ion, which
next undergoes conjugate addition with the nitronate anion
derived from 1, which is generated in situ spontaneously be-
cause of the high acidity of the methylenic protons, or alter-
natively, its formation can be assisted by the presence of
DABCO (in the cases in which this reagent is included as
co-catalyst). After the conjugate addition process takes
place, an intermediate enamine is formed, which delivers
the final adduct 4 after the release of the catalyst by hydrol-
Scheme 2. Conversion of oximes 6 into monoprotected dialdehydes 7.
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Organocatalytic Enantioselective Formal Conjugate Addition
COMMUNICATION
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ble relative orientations of the nitronate nucleophile with
respect to the Michael acceptor or, otherwise, a fully diaste-
reoselective Michael reaction can also be considered in
which the configurational instability of this stereocentre
makes an epimerization process to occur fast under the re-
action conditions because of the high acidity of the proton
adjacent to the nitro group.
In conclusion, we have developed a highly efficient proto-
col for the formal catalytic enantioselective b-hydroxyimino-
methylation of a,b-unsaturated aldehydes by using N-nitro-
methylphthalimide as the hydroxymetanimidoyl anion
equivalent and a chiral secondary amine as the organocata-
lyst. The reaction is remarkably wide in scope and allows
access to highly enantioenriched chiral functionalized
oximes in a very simple and straightforward way. Moreover,
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preparation of a-substituted 1,4-dialdehydes, in which both
formyl moieties have been chemically differentiated, with
high enantiomeric excesses. This methodology represents
the first case in which a hydroxymoyl anion equivalent has
been used as Michael donor in a catalytic enantioselective
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Acknowledgements
This work was supported by the UPV/EHU (fellowship to E.R.),
MICINN (CTQ2008-00136/BQU), and EJ/GV (Grupos IT-328-10 and a
fellowship to B.A.)
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were prepared had been formed with the same very high enantio-
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Keywords: asymmetric synthesis
organocatalysis · oximes · umpolung
·
Michael reaction
·
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Received: February 24, 2011
Published online: April 21, 2011
Chem. Eur. J. 2011, 17, 6048 – 6051
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6051