Highly enantioselective one-pot, three-component imino-reformatsky reaction

Article abstract of DOI:10.1002/anie.200462757

(Chemical Equation Presented) A key to molecular diversity is the preparation of new frameworks in multi-component condensations of three or more reactants. A new highly enantioselective one-pot, three-component, nickel-catalyzed, imino-Reformatsky reacti

Full text of DOI:10.1002/anie.200462757

Communications
Scheme 1. Rhodium-catalyzed Reformatsky reaction developed by
Honda and co-workers.
generally obtained. Recently, this problem was solved by
[
8]
using imines derived from 2-methoxyaniline. In view of the
increasing importance of b-amino acids as valuable synthetic
[
9]
tools we were attracted by the interesting work published by
[
8b]
Adrian and co-workers, who presented an efficient, nickel-
catalyzed Reformatsky-type three-component condensation
that combines aldehyde, 2-methoxyaniline, and a bromocar-
bonyl reagent (esters, ketones, and amides) in a one-pot
reaction. The method was successfully applied on the micro-
and macroscale and to combinatorial synthesis. Herein, we
present the first practical one-pot three-component enantio-
selective imino-Reformatsky reaction, which is based on the
use of N-methylephedrine as a cheap and recoverable chiral
[
10]
ligand.
The efficient three-component Reformatsky reaction
[
8b]
Multicomponent Reactions
developed by Adrian and co-workers uses Me Zn in the
2
presence of a catalytic amount of inexpensive [NiCl -
2
[
11]
Highly Enantioselective One-Pot, Three-
Component Imino-Reformatsky Reaction**
(PPh ) ]. On the basis of detailed electrochemical study
3 2
by Pꢀrichon, Sibille, and co-workers, a reasonable mecha-
nistic picture was presented by Adrian and co-workers. The
mechanism involved the reduction of the Ni complex to a Ni
complex, oxidative addition of the bromoester to the Ni
complex, and a Ni /Zn exchange, which leads to an organo-
zinc Reformatsky reagent. The proposed catalytic cycle
(Scheme 2) was our starting point in developing an enantio-
selective variant. We reasoned that, contrary to other nickel-
[
12]
II
0
Pier Giorgio Cozzi* and Eleonora Rivalta
0
II
II
[
1]
The Reformatsky reaction, discovered more than 115 years
[
2]
ago, is still widely used in synthesis. High functional-group
tolerance and the in situ preparation of the reagent have
contributed to its success. The reagent is prepared by the
activation of zinc metal through a variety of different
[
13]
catalyzed
processes, a chiral ligand able to coordinate
nickel was not crucial to transmit chiral information. Instead,
we decided to surround the zinc enolate formed by trans-
metalation and the chelating imine with a chiral ligand able to
coordinate zinc. Carreira and co-workers have developed a
[
3]
methods. Recently, renewed interest in the catalytic redox
[
4]
reaction has positively influenced further studies in the
[
5]
field. Honda and co-workers have developed a new,
interesting variant of the Reformatsky reaction, which uses
[
6]
Et Zn and [RhCl(PPh ) ] in catalytic amounts (Scheme 1).
2
3 3
Imines are suitable substrates for the Reformatsky
reaction, and the so-called imino-Reformatsky reaction was
[
7]
first described by Gilman 60 years ago. Although the use of
this reaction in synthesis has great potential, it can be
problematic, as a mixture of b-aminoesters and b-lactams is
[
*] Prof. Dr. P. G. Cozzi, Dr. E. Rivalta
Dipartimento di Chimica “G. Ciamician”
Universitꢀ di Bologna
Via Selmi, 2, 40126 Bologna (Italy)
Fax: (+39)051-2099-456
E-mail: piergiorgio.cozzi@unibo.it
[
**] The European Commission through the project FP6-505267-1-
LIGBANK), the MIUR (Progetto Nazionale Stereoselezioni in
Chimica Organica: Metodologie ed Applicazioni), and the FIRB
Progettazione, preparazione e valutazione biologica e farmaco-
(
(
logica di nuove molecole organiche quali potenziali farmaci
innovativi) are acknowledged for financial support for this research.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 2. Catalytic cycle for the imino-Reformatsky reaction mediated
by nickel.
3
600
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200462757
Angew. Chem. Int. Ed. 2005, 44, 3600 –3603

Angewandte
Chemie
very efficient and simple method based on the use of N-
the aqueous alkaline layer. The two solvents tested in the
model reaction, CH Cl and toluene, gave comparable
methylephedrine for the addition of acetylides to alde-
2
2
[
14]
[18]
hydes, and Tan et al. have used N-alkylephedrine deriva-
results. The crucial difference that determines the choice
of solvent concerns the use of the nickel catalyst. [NiCl2-
(PPh ) ] is soluble in CH Cl but is poorly soluble in toluene.
tives to add zinc acetylides to a ketone precursor of the anti-
[
15]
HIV drug efavirenz. N-Methylephedrine can be considered
3
2
2
2
[
16]
as a privileged ligand for zinc derivatives. For this reason we
We have also used [Ni(acac) ] hydrate, which is poorly soluble
2
explored the Me Zn-mediated one-pot three-component
in both solvents. To compare results and guarantee reprodu-
cibility, we decided to add Ni salts as a solid to the reaction
2
[
17]
II
imino-Reformatsky reaction with N-methylephedrine
as
the chiral ligand. Me Zn plays several important roles in the
mixture containing Me Zn, imine, bromoester, and N-meth-
2
2
reaction: a) Me Zn is the dehydrating agent responsible for
ylephedrine (see Supporting Information for details), at the
indicated reaction temperature. The Ni salts are quickly
2
II
the formation of the imine in situ; b) Me Zn can reduce the
2
II
0
Ni salt to Ni ; c) Me Zn reacts with the incipient nickel
dissolved in the reaction mixture and initiate the catalytic
cycle. The [RhCl(PPh ) ] complex also promotes the reaction,
2
enolate to form the reactive zinc enolate; and, finally,
3
3
d) Me Zn can coordinate N-methylephedrine, the amino
although the product was isolated with lower ee values than
those obtained in the presence of nickel complexes (Table 1,
entry 6). We also examined the enantiomeric excess attained
in the model reaction with different bromoesters. Apart from
the hindered tert-butyl bromoacetate (Table 1, entry 14),
which gave a lower ee value than the others, similar
enantiomeric excesses were attained with all type of esters
examined. Toluene was chosen as the solvent owing to the
slightly better results obtained, and the scope of the reaction
was investigated.
2
alcohol used as a chiral ligand. We have chosen the imine
obtained in situ from 2-methoxyaniline and p-chlorobenzal-
dehyde as a model substrate, in conjunction with different
II
bromoacetates, Ni salts in catalytic amounts, and N-methyl-
ephedrine. We adjusted the protocol developed by Adrian
[
8b]
and Snapper to investigate several experimental conditions
of our process in detail (Table 1). In light of its multiple roles
[
19]
in this reaction, 4 equivalents of Me Zn was used. To ensure
2
good enantiomeric excess, 1.5–1.6 equivalents of N-methyl-
ephedrine were employed in our process (Table 1, entries 2–
In Table 2 we report the results collected with different
aldehydes. The reaction shows broad scope, since aromatic,
aliphatic, unsaturated, and heterocyclic aldehydes are reac-
tive and result in good to excellent
5
). N-methylephedrine can easily be separated from the
adducts by acidic workup and recovered after extraction of
enantioselectivites (up to 92% ee).
Table 1: Enantioselective one-pot imino-Reformatsky reaction of 4-chlorobenzaldehyde with 2-methoxy-
aniline and bromoacetates.
Yields were generally only moder-
ate, but no by-products were iso-
lated. Better yield was obtained by
increasing the reaction tempera-
ture, but at the expense of enan-
tiomeric excess. We optimized the
reaction temperature with all the
different aldehydes so as to reach a
compromise between yield and
enantiomeric excess. Notably, a,b-
unsaturated aldehydes are good
reaction partners in this reaction
and also lead to good enantiomeric
[
a]
II
[b]
[c]
Entry
Ni
R
Solvent
CH Cl
Yield [%]
ee [%]
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
d,e]
e]
f]
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
[NiCl (PPh ) ]
[NiCl (PPh ) ]
[NiCl (PPh ) ]
[NiCl (PPh ) ]
[NiCl (PPh ) ]
[RhCl(PPh ) ]
[NiCl (PPh ) ]
[Ni(acac)2]
Et
Et
Et
Et
Et
Et
Et
Et
Me
Me
iPr
Bn
Bn
tBu
Et
82
92
85
75
88
70
75
57
57
60
66
70
60
50
73
68
75
81
86
86
72
86
84
84
86
83
82
87
76
78
2
3
2
2
2
2
2
2
2
2
CH Cl
2
3
2
2
CH Cl
2
3
2
2
f,g]
f,h]
i]
CH Cl
2
3
2
2
CH Cl
2
3
2
2
[20]
CH Cl
excess. The absolute configura-
tion of the products obtained with
3
3
2
f]
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
2
3 2
f]
(1S,2R)-N-methylephedrine as
a
f]
[Ni(acac)2]
[NiCl (PPh ) ]
2 3 2
[NiCl (PPh ) ]
[Ni(acac)2]
[NiCl (PPh ) ]
[NiCl (PPh ) ]
[NiCl (PPh ) ]
chiral ligand was established in
the case of aliphatic and aromatic
aldehydes (isopropyl and phenyl),
as shown in Scheme 3, following
the general procedure of deprotec-
tion of 2-methoxyamines, devel-
oped by Josephsohn, Snapper, and
Hoveyda (see Supporting Informa-
f]
1
1
1
1
1
1
f]
2
3 2
f]
f]
2
3 2
f]
2
3 2
j]
2
3 2
[
a] All the reactions were carried out at 08C for 3 h in the presence of N-methylephedrine (1.6 equiv) and
II
the Ni salt (7.5–8 mol%). [b] Yield of isolated product after chromatographic purification. [c] Deter-
[
21]
mined by HPLC analysis (see Supporting Information for details). [d] N-methylephedrine: 1 equiv.
tion for details).
[
8b]
II
[
e] [NiCl (PPh ) ] was added as a solution in CH Cl as reported by Adrian and co-workers. [f] The Ni
2
3
2
2
2
In conclusion, we have devel-
salt was added as solid to the reaction mixture. [g] N-methylephedrine: 1.8 equiv. [h] N-methylephe-
oped the first practical and highly
drine: 2 equiv. [i] Wilkinson catalyst, [RhCl(PPh ) ], (5 mol%) was used and added as a solution in
3
3
[22]
[
8b]
efficient, enantioselective
one-
CH Cl . The reaction was stirred for 13 h at 08C. [j] Recycling experiment: N-methylephedrine,
2
2
pot three-component imino-Refor-
matsky reaction, which gives b-
aminoesters in moderate to good
recovered from the reaction, was dried under high vacuum for 3 h. The crude N-methylephedrine
containing 2-methoxyaniline (10%) (checked by NMR and ES-MS) was used in another model reaction
without any purification. The reaction was performed at À108C for 14 h with [NiCl (PPh ) ] (8 mol%).
2
3 2
Angew. Chem. Int. Ed. 2005, 44, 3600 –3603
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3601

Communications
Table 2: Enantioselective imino-Reformatsky reaction with different aldehydes.
other inexpensive homochiral aminoalco-
[
24]
hols are available. Further studies of this
reaction are in progress in our laboratory
and will be reported in due course.
Received: November 30, 2004
Revised: February 25, 2005
Published online: May 6, 2005
[
a]
1
[b]
[c]
Entry
R
R
T [8C]
t [h]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
4-ClPh
4-ClPh
4-ClPh
Et
Me
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
À25
À25
À20
À28
À28
À20
À30
À30
À20
À30
À30
À28
À5
3
3
3
16
6
48
48
3
62
60
90
30
41
60
63
50
85
40
76
49
40
72
60
57
52
67
90
90
83
90
90
85
83
81
57
82
74
92
84
74
66
64
74
80
Keywords: aminoesters · asymmetric
synthesis · imines · multicomponent
reactions · zinc
[
d,e]
.
4-CF Ph
3
C6F5
PhCH=CH
Ph
Ph
Ph
[
[
d]
[1] S. Reformatsky, Ber. Dtsch. Chem. Ges.
d,f]
4
1887, 20, 1210; for a review, see: R.
Ocampo, W. R. Dolbier, Jr., Tetrahedron
2004, 60, 9325 – 9374.
10
11
12
13
14
15
16
17
18
tBuPh
tBuPh
2-thiophene
2-naphtyl
2-naphtyl
4-MeOPh
iPr
iPr
cyclohexyl
24
48
16
16
1
48
48
48
3
[2] a) A. Fꢁrstner, Synthesis 1989, 571 – 590;
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York, 2002, pp. 579 – 664.
25
[
d]
À15
À10
À20
À20
[
d]
[
a] All the reactions were performed at the indicated temperature with N-methylephedrine (1.6 equiv) as
a chiral ligand (see Supporting Information for experimental details).[b] Yield of isolated product after
flash chromatography. [c] Determined by chiral HPLC analysis (Chiralcel OD column). [d] The reaction
was slowly allowed to reach 08C in 3–4 h before quenching. [e] A solution of Et Zn in toluene (1m;
2
[
3] a) Rieke Zn: R. D. Rieke, S. J. Uhm, Syn-
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Santaniello, A. Manzocchi, Synthesis 1977,
4
equiv) was used. GC–MS analysis performed on the crude reaction mixture after quenching revealed
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reaction was performed with the preformed purified imine in the presence of Me Zn (3 equiv) and N-
methylephedrine (1 equiv) as chiral ligand.
2
698 – 699; Zn/Ag graphite: R. Csuk, A.
Fꢁrstner, H. J. Weidmann, Jr., J. Chem.
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[
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[
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[
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[
23]
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entry 15).
The imines are prepared in situ, in the same
reaction flask, and Me Zn is used as a dehydrating agent.
2
More importantly, the reaction shows broad scope, since
aromatic, heterocyclic, aliphatic and unsaturated aldehydes
are suitable substrates for the reaction. We believe that the
enantioselectivity (64–92% ee) can be improved as many
3
602
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 3600 –3603

Angewandte
Chemie
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2
[
[
11] An imino-Reformatsky reaction based on the use of Et Zn and a
2
catalytic amount of [RhCl(PPh ) ] has been developed: K.
3
3
Kanai, H. Wakabayashi, T. Honda, Heterocycles 2002, 58, 47 – 51.
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[
[
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17] Other chiral ligands were examined: binol, sparteine, chiral
Schiff bases, tartramides gave racemic product; N-methylproli-
nol gave no reaction; cinchonine led to 57% ee but low
conversion.
[
[
18] The use of THF as reaction solvent gives an incomplete
formation of imines compared with toluene and CH Cl .
19] Bromoamides are reactive partners in our reaction but we have
isolated racemic products by using several amides in the reaction
with 4-chlorobenzaldehyde: P. G. Cozzi, E. Rivalta, unpublished
results.
2
2
[
20] Crotyl aldehyde afforded the corresponding b-aminoester
adduct with 64% ee and in 60% yield; other unsaturated
aldehydes are currently under investigation.
[
21] For details, see: N. S. Josephsohn, M. L. Snapper, A. H. Ho-
veyda, J. Am. Chem. Soc. 2004, 126, 3734 – 3735.
[
22] For a diastereoselective imino-Reformatsky reaction catalyzed
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Pharm. Bull. 2002, 50, 307 – 308; b) A. Otaka, J. Watanabe, A.
Yukimasa, Y. Sasaki, H. Watanabe, T. Kinoshita, S. Oishi, H.
Tamamura, N. Fujii, J. Org. Chem. 2004, 69, 1634 – 1645.
23] N-methylephedrine was completely recovered from the reaction
[
[
(
100% yield, > 99% ee) contaminated with 10–16% of o-
1
methoxyaniline (checked by integration of crude H NMR
spectra). We have performed a recycling experiment without
purifying the N-methylephedrine recovered (Table 1, entry 15).
24] Different asymmetric imino-Reformatsky reactions have been
reported: a) Y. Ukaji, S. Takenaka, Y. Horita, K. Inomata, Chem.
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www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3603