Unifying metal- and organocatalysis for asymmetric oxidative iminium activation: A relay catalytic system enabling the combined allylic oxidation of alcohols and prolinol ether catalyzed iminium reactions

Article abstract of DOI:10.1002/chem.201102551

A multicatalytic system consisting of tetrapropylammonium perruthenate/N-methylmorpholine N-oxide (TPAP/NMO) as oxidant, and diarylprolinol TMS-ether as chiral amine catalyst, has been developed and applied in the efficient construction of valuable chiral

Full text of DOI:10.1002/chem.201102551

FULL PAPER
DOI: 10.1002/chem.201102551
Unifying Metal- and Organocatalysis for Asymmetric Oxidative Iminium
Activation: A Relay Catalytic System Enabling the Combined Allylic
Oxidation of Alcohols and Prolinol Ether Catalyzed Iminium Reactions
Magnus Rueping,* Henrik Sundꢀn, and Erli Sugiono[a]
Abstract: A multicatalytic system con-
sisting of tetrapropylammonium per-
generation of a,b-unsaturated alde-
hydes from allylic alcohols and their
subsequent use in various asymmetric
transformations (e.g., cyclopropana-
tion, Michael addition, Michael addi-
tion/acetalization). TPAP as a sub-
strate-selective redox catalyst is well
tolerated by the amine catalyst and the
domino reactions proceed in good
yields and high enantioselectivities.
The compatibility of metal and organo-
catalysis presented herein widens the
scope of asymmetric iminium catalysis.
ruthenate/N-methylmorpholine
N-
oxide (TPAP/NMO) as oxidant, and di-
arylprolinol TMS-ether as chiral amine
catalyst, has been developed and ap-
plied in the efficient construction of
valuable chiral molecules. The one-pot
domino reactions elaborated in the
present study are based on the in situ
Keywords: asymmetric catalysis
·
cascade reactions · iminium activa-
tion · oxidation
Introduction
[Eq. (1)].[6] The efficient one-pot, oxidative and enantiose-
lective cross-coupling reaction of aldehydes and nitrome-
thane using 2,3-dichloro-5,6-dicyanoquinone (DDQ) as oxi-
dant and diphenyl-prolinol silyl ether as catalyst has been
reported by Hayashi et al. [Eq. (2)].[7] Catalytic enantiose-
lective one-pot domino oxidation procedures where the oxi-
dized intermediate is further elaborated are highly desirable
because they offer a potential one-flask-access to demanding
and highly functionalized molecules in higher oxidation
states.
In organic synthesis costly protecting-group strategies and
lengthy purification procedures after each synthetic step
remain an obstacle. The desire to circumvent these problems
has led to the development of several multi-component
domino reactions.[1–2] Asymmetric one-flask combinations of
multiple catalytic cycles that allow the formation of complex
molecules in a small number of operational steps have at-
tracted considerable interest within the field of organic syn-
thetic chemistry and industrial chemistry. In addition, by de-
signing reaction protocols where several bond forming reac-
tions take place in one flask highly desirable demands such
as, time, cost and environmental savings can be met. In this
context many highly enantioselective transformations in-
volving the combination of enamine and iminium catalysis
have been reported.[1–2] However, the asymmetric one-flask
combinations of multiple catalytic cycles based on the com-
bination of a chiral organocatalyst and a achiral metal cata-
lyst still remain a challenge.[3–5] Recently, Alexakis and
Mazet reported an elegant catalytic reaction sequence for
the highly asymmetric a-functionalization of aldehydes. The
strategy involves the achiral iridium catalyzed isomerization
of primary alcohols in the first step and the organocatalyzed
a-functionalization of the aldehydes in the second step
[a] Prof. Dr. M. Rueping, Dr. H. Sundꢀn,+ Dr. E. Sugiono+
Institute of Organic Chemistry, RWTH-Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Fax : (+49)241-80-92665
E-mail: magnus.rueping@rwth-aachen.de
[+] Contributed equally to this work.
In a previous investigation on iminium catalysis we and
others discovered that trace amounts of acid, abundant in
the a,b-unsaturated aldehydes or ketones, have a detrimen-
tal effect on both, the reactivity and enantioselectivity.[8]
Therefore, the aldehydes have to be distilled or recrystal-
lized prior to use. Hence, we became attracted to the idea of
forming the aldehydes in situ. We envisioned that allylic al-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102551.
Chem. Eur. J. 2012, 18, 3649 – 3653
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3649

cohols could be oxidized to their aldehyde counterparts with
a substrate-selective redox catalyst as a first step in an asym-
metric oxidative iminium activation cascade (AOIC)
[Eq. (3)].[9]
idant had an accelerating effect on the catalytic system.
After an optimization of solvent, temperature and catalysts,
efficient protocols for the enantioselective formation of for-
mylcyclopropanes, addition cyclization reaction of 1,3-dike-
tones and the Michael addition of malonates were devel-
oped.
Results and Discussion
The double catalytic asymmetric oxidative formation of
formylcyclopropanes[12] from allylic alcohols and diethyl bro-
momalonate proceeds in high yields with high enantioselec-
tivities in the presence of 1–7 mol% of TPAP, 1.5 equiv of
NMO and 10 mol% of amine catalyst 4a (Table 1, entries 1–
8). Cinnamyl alcohol (2a) is converted to 3a in 78% yield
with 94% ee (Table 1, entry 1). Compounds 3b and 3c bear-
ing aryl groups with nitro electron withdrawing groups in
the 2- and 4-position can be isolated in 98% yield with 95%
ee and 72% yield with 92% ee, respectively (Table 1, en-
tries 4 and 5).
p-Tolylpropenol is readily converted into the correspond-
ing cyclopropane 3e in 77% yield with 95% ee (Table 1,
entry 7). The reactions are finished within 7 h and the prod-
ucts can be obtained after silica gel purification. Moreover
the TPAP loading can be decreased to 1 mol% without af-
fecting the stereoselectivity but to the expense of a longer
reaction time (Table 1, entry 3).
Central to the implementation of an asymmetric oxidative
iminium cascade is that the catalytic oxidation is adaptable
to the catalytic cycle of the chiral amine catalyst, the chiral
amine catalyst is not oxidized, the reactions can be per-
formed at room temperature and the terminal oxidant is
mild, cheap and easy to handle. The robust tetrapropylam-
monium perruthenate TPAP and N-methylmorpholine N-
oxide (NMO) system[10] meets these requirements and there-
fore became the substrate-selective redox catalyst of choice.
Initial oxidation of the alcohol by the TPAP/NMO system
generates the aldehyde and one equiv of N-methylmorpho-
line NMM (Scheme 1). The aldehyde then forms the imini-
um ion with the amine catalyst and simultaneously NMM
acts as a base and activates the nucleophile. The activated
nucleophile then adds to the iminium ion upon formation of
the chiral enamine. Depending on the nucleophile, several
pathways are possible. To survey the scope of the asymmet-
ric oxidative iminium cascade strategy we decided to investi-
gate three reaction types. The domino iminium enamine
mechanism was probed using bromo malonate as nucleo-
phile, a Michael addition cyclization reaction with 1,3-dike-
tones[8a–d,h] and a Michael addition using malonates.
Table 1. Oxidative enantioselective amine catalyzed formation of formyl-
cyclopropanes.
In the initial studies, the TPAP/NMO system proved to
have good compatibility with the chiral amine catalyst di-
phenylprolinol TMS-ether (TMS-DPP)[11] without any nota-
ble degradation of either TPAP or the TMS-DPP. Further-
more, the NMM formed as a by-product of the sacrificial ox-
Entry[a]
3
R1
Yield [%][b]
ee [%][c]
1
3a
3a
3a
3b
3c
3d
3e
3 f
Ph
Ph
Ph
2-NO2-Ph
4-NO2-Ph
4-Br-Ph
4-Me-Ph
4-CF3-Ph
78
64
78
89
75
66
77
70
94
94
94
95
92
95
95
94
2[d]
3[e]
4
5
6
7
8
[a] Reactions were performed with diethyl bromomalonate 1 (1.3 equiv),
alcohol 2 (1.0 equiv), 10 mol% 4a, TPAP (0.07 equiv), NMO (1.5 equiv)
in CH2Cl2 at RT for 6–24 h. [b] Yield of isolated product after column
chromatography. [c] ee determined by HPLC. [d] Performed with
6 mol% of 4a (24 h). [e] Performed with 1 mol% of TPAP (36 h).
To further investigate the scope of the asymmetric oxida-
tive iminium cascade, allylic alcohols and 1,3-diketones were
subjected to TPAP/NMO and diphenylprolinol ether 4. The
1,4-addition/cyclization reaction proceeds with high selectiv-
ity for both aliphatic and aromatic allylic alcohols (Table 2).
Hexenol (2, R1 =C3H7) affords at room temperature the cor-
responding 2-hydroxychromenone 6a in 59% yield with
94% ee (Table 2, entry 1). Long alkyl chains are also well
tolerated and decenol (2, R1 =C7H15) could be converted to
Scheme 1. General overview of the oxidative iminium activation cascade.
3650
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Chem. Eur. J. 2012, 18, 3649 – 3653

Metal- and Organocatalysis
FULL PAPER
Table 2. Formation of chromenones through an asymmetric oxidative
iminium activation cascade.
Table 4. Oxidative enantioselective amine-catalyzed addition of methyl
malonate to allylic alcohols.
Entry[a]
10
R1
Yield [%][b]
ee [%][c]
1
2
3
4
5
10a
10b
10c
10d
10e
Ph
61
73
71
63
72
93
93
86
92
77
2-NO2-Ph
4-NO2-Ph
4-Br-Ph
4-Me-Ph
Entry[a]
4
6
R1
R2
Yield [%][b]
ee [%][c]
1
4b
4b
4b
4a
4a
6a
6b
6c
6d
6e
C3H7
C3H7
C7H15
2-Cl-Ph
2-Br-Ph
H
Me
H
H
H
59
50
51
66
53
94
90
96
92
80
2[d,e]
3
[a] Reactions were performed with methyl malonate 9 (3.0 equiv), alco-
hol 2 (1.0 equiv), 20 mol% 4a, TPAP (0.07 equiv), NMO (1.5 equiv) in
CH2Cl2 at RT for 24–48 h. [b] Yield of isolated product after column
chromatography. [c] ee determined by HPLC.
4[d]
5[d,e]
[a] Reactions were performed with diketone 5 (1.0 equiv), alcohol 2
(5.0 equiv), 20 mol% 4, TPAP (0.05 equiv), NMO (1.0 equiv) in CH2Cl2
at RT for 48 h. [b] Yield of isolated product after column chromatogra-
phy. [c] ee determined by HPLC. [d] Performed at 108C. [e] 96 h.
groups in the p- or o-position of the aromatic ring. Apart
from halogen and alkyl substituents, nitro-aromatics are tol-
erated well in this reaction giving the addition products in
high yields up to 73% and enantioselectivities up to 93% ee
(Table 4, entries 2 and 3).
The ultimate goal for catalytic oxidations is to use oxygen
as the terminal oxidant due to the high efficiency of weight
per oxidant and the environmental benefits.[14] Both Ley[15]
and Brown[16] have reported that molecular oxygen in addi-
tion to NMO can be used in TPAP oxidations, thus, the oxi-
dative cyclopropanation of allylic alcohols was conducted
under an oxygen atmosphere to yield cyclopropane 3a in
55% yield with 94% ee (Scheme 2).
6c in 51% yield with 96% ee (Table 2, entry 3). Halo-substi-
tuted cinnamyl alcohols were smoothly converted to the het-
erocyclic enones 6d and 6e in good yields and very high se-
lectivities from the double catalytic process (Table 2, en-
tries 4 and 5).
Moreover, the oxidative addition–acetalization process
could be further elaborated to yield lactones via a stepwise
double oxidative procedure (Table 3). For example, when
meso-compound 7 and allylic alcohol were treated with
TPAP and amine catalyst 4b, lactones 8a and 8b were ob-
tained with 97% ee for both diastereomers (Table 3).
Table 3. Catalytic double oxidation/iminium activation to chiral bicyclic
lactones.
Scheme 2. A direct aerobic oxidative iminium cascade.
Entry[a]
8
R1
Yield [%][b]
ee [%][c]
1
2
8a
8b
C3H7
C7H15
51
50
97/97
97/97
Conclusion
[a] 1stt step: Reactions were performed with diketone 7 (1.0 equiv), alco-
hol 2 (5.0 equiv), 20 mol% 4b, TPAP (0.05 equiv), NMO (1.0 equiv) in
CH2Cl2 at RT for 72 h; 2nd step: Reactions were performed with isolated
lactol (1.0 equiv), TPAP (0.05 equiv), NMO (1.5 equiv) in CH2Cl2 at RT
for 20 min. [b] Yield of isolated product over 2 steps after column chro-
matography. [c] ee determined by HPLC.
In summary, we have developed the first catalytic asymmet-
ric oxidative iminium cascade. TPAP as a substrate-selective
redox catalyst is well tolerated by the amine catalyst and
provides an opportunity to form the a,b-unsaturated alde-
hydes in situ for further transformations. The generality of
this methodology is demonstrated in Tables 1–4. The three
fundamentally different reactions applied, proceed in good
yields and high enantioselectivities. The reactions are con-
ducted with 6–20% of TMS-prolinol ether and the TPAP
loadings can be reduced to 1 mol%. Furthermore, the in
situ oxidation can be conducted with environmentally
benign terminal oxidants such as O2. The compatibility of
one metal catalyst and one organic catalyst presented here
clearly widens the scope of asymmetric iminium catalysis
and combined catalysis in general.
The addition of malonates to allylic alcohols was also suc-
cessful by applying the asymmetric oxidative iminium cas-
cade, and the products 10[13] could be isolated in high yields
with up to 93% ee (Table 4). Cinnamyl alcohol was convert-
ed at room temperature into malonate aldehyde 10a in
61% yield with 93% ee (Table 4, entry 1).
Furthermore, the addition of dimethyl malonate to the ar-
omatic allylic alcohols tolerates a variety of functional
Chem. Eur. J. 2012, 18, 3649 – 3653
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3651

M. Rueping et al.
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Experimental Section
General procedure for the oxidative amine-catalyzed formation of for-
mylcyclopropanes: To a vial containing cinnamyl alcohol (2, R1 =Ph,
0.2 mmol, 1.0 equiv) in dichloromethane (1.0 mL) was subsequently
added, NMO (0.3 mmol, 1.5 equiv), TPAP (0.014 mmol, 0,07 equiv), di-
ethyl bromomalonate (1, 0.26 mmol, 1.3 equiv) and 2-(diphenyl(trime-
thylsilyloxy)methyl)pyrrolidine (4a, 0.1 mmol, 0.1 equiv). The reaction
was stirred at room temperature for 6 h and then put on a silica gel
column and purified using hexane/ethyl acetate as eluent to give cyclo-
propane 3a as a colorless oil. [a]D =ꢀ49.6 (c=1.0 in CHCl3, 94% ee);
1H NMR (300 MHz, CDCl3): d=9.45 (d, 1H, J=4.8 Hz), 7.33–7.22 (m,
5H), 4.38–4.20 (m, 2H), 3.93 (dq, 2H, J=7.2, 1.2 Hz), 3.83 (d, 1H, J=
7.4 Hz), 3.37 (dd, 1H, J=7.5, 4.8 Hz), 1.30 (t, 3H, J=7.1 Hz), 0.93 ppm
(t, 3H, J=7.1 Hz); 13C NMR (75 MHz, CDCl3): d=196.0, 166.0, 164.6,
132.2, 128.5, 128.4, 128.0, 62.5, 62.0, 44.7, 38.2, 35.3, 14.0, 13.7 ppm; IR
(neat): n˜ =3425, 2982, 2935, 2872, 1715, 1639, 1448, 1369, 1287, 1216,
1183, 1143, 1020, 744, 698 cmꢀ1; MS-EI: m/z (%): 261.1 (26) [MꢀEt]+,
216.1 (22), 187.1 (26), 171.1 (17), 170.0 (33), 159.0 (11), 144.1 (16), 130.9
(13), 116.1 (33), 115.1 (100), 105.1 (22), 91.1 (14); HPLC conditions: AS-
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column, n-hexane/2-propanol 85:15, flow rate=0.6 mLminꢀ1
, l=
210 nm; major enantiomer: tR =11.61 min and minor enantiomer: tR =
13.81 min.
General procedure for the oxidative addition of malonates to allylic alco-
hols: To
a
vial containing cinnamyl alcohol (2, R1 =Ph, 0.2 mmol,
ˇ
6566; g) P. Kukula, V. Matousek, T. Mallat, A. Baiker, Chem. Eur. J.
1.0 equiv) in dichloromethane (1.0 mL) was subsequently added, NMO
(0.30 mmol, 1.5 equiv), TPAP (0.014 mmol, 0.07 equiv), dimethyl malo-
nate (9, 0.26 mmol, 1.3 equiv) and 2-(diphenyl(trimethylsilyloxy)methyl)-
pyrrolidine (4a, 0.2 mmol, 0.2 equiv). The reaction was stirred at room
temperature for 24 h and then put on a silica gel column and purified
using hexane/ethyl acetate as eluent to afford 10a as colorless oil. [a]D =
ꢀ19.9 (c=1.0 in CHCl3; 93% ee); 1H NMR (300 MHz, CDCl3,): d=9.59
(t, 1H, J=1.8 Hz), 7.32–7.22 (m, 5H), 4.06–3.98 (m, 1H), 3.74 (d, 1H,
J=9.8 Hz) 3.74 (s, 3H), 3.50 (s, 3H), 2.94–2.90 ppm (m, 2H); 13C NMR
(75 MHz, CDCl3): d=199.9, 168.3, 167.8, 139.7, 128.8, 128.0, 127.6, 57.3,
52.7, 52.4, 47.2, 39.5 ppm; IR (neat): n˜ = 3427, 3031, 2954, 2846, 1995,
496, 1454, 1434, 1319, 1253, 1197, 1155, 1062, 1022, 767, 701 cmꢀ1; MS-EI:
m/z(%): 264.9 (0.1) [M+H]+, 133.1 (12), 132.0 (38), 131.1 (22), 117.1
(12), 115.1 (28), 105.1 (32), 104.1 (48), 103.1 (28), 100.0 (18), 91.1 (15),
78.1 (16), 77.0 (23), 69.0 (21), 59.0 (100); enantioselectivity measured
after conversion to the corresponding ester.[13c] HPLC conditions: AD-H
column, n-hexane/2-propanol 90:10, flow rate=1.0 mLminꢀ1, l=210 nm.
major enantiomer: tR =12.22 min and minor enantiomer: tR =14.73 min.
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General procedure for the oxidative addition-acetalization: A solution of
allylic alcohol (2, R1 =C3H7, 5.00 equiv) in CH2Cl2 (0.2m) was placed in a
screw capped test tube equipped with stirring bar and NMO (1.5 equiv)
was added to the mixture and stirred for 5 min. TPAP (0.05 equiv) was
added and the resulting mixture stirred at the ambient temperature for
15 min. Diketone (5, R2 =H, 1.0 equiv) and catalyst 4b (0.02 equiv) were
added and the mixture was stirred for 48 h. The crude reaction mixture
was directly charged on silica gel and purified by column chromatogra-
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Acknowledgements
The authors acknowledge European Research Council (BIMOC
GA209437) and the Foundation in Memory of Bengt Lundqvist (stipend
given to H.S.) for financial support.
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Received: August 17, 2011
Published online: February 28, 2012
Chem. Eur. J. 2012, 18, 3649 – 3653
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