Iron catalyzed asymmetric oxyamination of olefins

Article abstract of DOI:10.1021/ja3046684

The regioselective and enantioselective oxyamination of alkenes with N-sulfonyl oxaziridines is catalyzed by a novel iron(II) bis(oxazoline) complex. This process affords oxazolidine products that can be easily manipulated to yield highly enantioenriched free amino alcohols. The regioselectivity of this process is complementary to that obtained from the analogous copper(II)-catalyzed reaction. Thus, both regioisomers of enantioenriched 1,2-aminoalcohols can be obtained using oxaziridine-mediated oxyamination reactions, and the overall sense of regiochemistry can be controlled using the appropriate choice of inexpensive first-row transition metal catalyst.

Full text of DOI:10.1021/ja3046684

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Communication
Iron Catalyzed Asymmetric Oxyamination of Olefins
Kevin Scott Williamson, and Tehshik Peter Yoon
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja3046684 • Publication Date (Web): 13 Jul 2012
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Iron Catalyzed Asymmetric Oxyamination of Olefins
Kevin S. Williamson, Tehshik P. Yoon*
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Department of Chemistry, University of Wisconsin–Madison, 1101 University Avenue, Madison, Wisconsin,
53706.
Supporting Information Placeholder
and good enantioselectivity (eq 1).12 In a separate study,
ABSTRACT: The regioselective and enantioselective
oxyamination of alkenes with N-sulfonyl oxaziridines is
catalyzed by a novel iron(II) bis(oxazoline) complex. This
we also discovered that the opposite regioisomer is acces-
sible using iron salts (eq 2).15 Recognizing that the ability
to control both the stereochemistry and the regiochemis-
process affords oxazolidine products that can be easily
try of intermolecular oxyamination processes with chiral
manipulated to yield highly enantioenriched free amino
first-row transition metal catalysts would provide an at-
alcohols. The regioselectivity of this process is comple-
tractive alternative to state-of-the-art osmium-catalyzed
mentary to that obtained from the analogous copper(II)-
aminohydroxylation methods, we initiated a study fo-
catalyzed reaction. Thus, both regioisomers of enanti-
cused on the development of an enantioselective iron-
oenriched 1,2-aminoalcohols can be obtained using oxa-
ziridine-mediated oxyamination reactions, and the over-
catalyzed oxyamination.
all sense of regiochemistry can be controlled using the
appropriate choice of inexpensive first-row transition
metal catalyst.
Table 1. Optimization studies for olefin oxyamination.
Ar
Ns
O
N
O
N
Ns
Ar
H
5% Fe(X)2
10% ligand
4-MePh
Ar= 2,4-Cl2Ph
3
+
O
2
benzene
14 h
The Sharpless asymmetric aminohydroxylation reaction is
a powerful method for the stereocontrolled construction
+
Ns
1 2
,
of 1,2-aminoalcohols. Nevertheless, the poor regioselec-
tivity of this reaction3 and the requirement for a toxic and
precious osmium catalyst has inspired significant efforts
over the last decade to design alternative oxyamination
processes based on palladium,4 platinum,5 rhodium,6
gold,7 copper,8 hypervalent iodine,9 and radical reactions.10
Several enantioselective osmium-free oxyaminations have
2,4-Cl2Ph
N
4-MePh
H
4
Me
O
Me
N
Me
N
Me
O
Me
N
Me
N
O
O
O
O
N
R
R
N
R
R
R
R
8
11
b,
been reported in an intramolecular context.
However,
5a (R=i-Pr)
5b (R=t-Bu)
5c (R=Ph)
5e (R = Ph)
5f (R = 1-Naphth)
5d
with the exception of a recent report from our laborato-
ry,12 no enantioselective intermolecular oxyaminations
have been reported to date. In this communication, we
describe the oxaziridine-mediated enantioselective oxy-
amination of alkenes catalyzed by a novel iron(II)
bis(oxazoline) complex. This reaction is highly regioselec-
tive and enantioselective for a range of alkenes, and it is a
rare example of a highly enantioselective oxidative trans-
formation catalyzed by iron.
entry a
Fe(X)2 (mol%)
Ligand (mol%)
Yieldb
eec
1
FeCl2 (5)
5a (10)
5a (10)
5a (10)
5b (10)
5c (10)
5d (10)
5e (10)
5e (20)
5f (20)
5f (20)
5f (20)
46%
62%
61%
31%
80%
38%
65%
85%
53%
76%f
76%f
91%
43%
13%
5%
55%
55%
72%
76%
85%
92%
95%
2
3
4
Fe(OTf)2 (5)
Fe(NTf2)2 (5)
Fe(NTf2)2 (5)
Fe(NTf2)2 (5)
Fe(NTf2)2 (5)
Fe(NTf2)2 (5)
Fe(NTf2)2 (10)
Fe(NTf2)2 (10)
Fe(NTf2)2 (10)
Fe(NTf2)2 (10)
5
6
7
Bs
Ph
5 mol% Cu(F6acac)2
Bs
8
9
O
N
15 mol% (R)-PhBOX
N
(1)
(2)
+
Ar
O
10d
11d,e
Ph
H
acetone, 23 ºC
5% Fe(acac)3
Ar
R
1
a
Ar
Unless otherwise noted, reactions were performed using 0.2 mmol
olefin and 2.5 equiv of oxaziridine in benzene (0.1 M) at 23 °C.
Determined by 1H NMR using TMS2Ph as an internal standard unless
noted. Enantiomeric excess determined by SFC analysis. Per-
formed at 0.05 M with 400 wt% MgO added as drying agent. e Reac-
tion started at 0 ºC and allowed to warm to room temperature. Iso-
Ns
b
O
N
O
+
R
N
MeCN
0 ºC → rt
Ns
Ar
H
c
d
Ar= 2,4-Cl2Ph
2
f
lated yield.
For the past several years, our group has been investigat-
ing reactions between N-sulfonyl oxaziridines13 and al-
kenes in the presence of various transition metal cata-
Despite significant interest in the discovery of new cata-
lytic reactions based on iron catalysts,16 relatively few en-
antioselective iron-catalyzed processes have been report-
ed to date.17 In particular, the development of highly en-
antioselective oxidative transformations catalyzed by iron
lysts.14 Recently, we reported that
a
copper(II)
bis(oxazoline) complex catalyzes the oxaziridine-mediated
oxyamination of alkenes with excellent regioselectivity
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has been challenging due to the limited number of effec- despite the reduced steric demand of an alkene compared
1
2
3
4
5
6
7
8
tive ligand frameworks that possess good oxidative stabil-
ity and also provide superior levels of stereocontrol for
iron-catalyzed reactions.18 Based upon Corey’s report of
asymmetric Diels–Alder reactions catalyzed by an iron
bis(oxazoline) complex,19 and given the success of this
to an arene ring. Aliphatic olefins, however, failed to react
under these conditions (entry 15) and are a current limita-
tion of the method.
Table 2. Scope of iron catalyzed aminohydroxylation.
20
ligand class in a variety of oxidative reactions,8b, we be-
Ar
10% Fe(NTf2)2
Ns
N
O
O
gan our own investigations by screening combinations of
iron salts and known bis(oxazoline) ligands for their abil-
ity to catalyze the reaction between 4-methylstyrene and
oxaziridine 2 (Table 1). We quickly found that oxazolidine
3 could be synthesized with excellent enantioselectivity
using FeCl2 and ligand 5a (entry 1). This system, however,
suffered from limited substrate scope and generally poor
yields. Upon closer analysis of the reaction byproducts, we
found that rapid decomposition of 2 to sulfonylamide 4
occured at a rate competitive with the desired transfor-
mation.21
20% 5f
+
R
N
Ns
H
Ar
R
benzene
400 wt% MgO
0 ºC → rt
9
Ar= 2,4-Cl2Ph
2
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entrya
olefin
product
time yieldb,c eeb,d
Ar
O
R
N
Ns
R
1
2
3
4
5
6
7
8
R = Ph
3 h
3 h
75% 92%
76% 95%
68% 91%
62% 93%
64% 90%
63% 85%
81% 95%
71% 91%
R = 4-MePh
R = 3-MePh
R = 2-MePh
R = 4-ClPh
4 h
The undesired amide 4 presumably arises from the one-
electron reduction and rearrangement of 2; indeed, the
iron(II)-catalyzed radical rearrangement of oxaziridines
to amides by this mechanism was described by Emmons
in 1957.22 Hypothesizing that a less reducing iron catalyst
would retard the formation of this by-product, we elected
to investigate catalysts bearing less-coordinating counter-
ions. We were pleased to find that the triflate and triflim-
ide23 complexes provided higher yields (entries 2 and 3)
and that unreacted oxaziridine was still present after the
completion of the reaction. Reinvestigation of common
bis(oxazoline) ligands showed that the combination of
Fe(NTf2)2 and 4,5-diphenyl-substituted bis(oxazoline) 5e
provided good yields and reasonable asymmetric induc-
tion (entries 3–7). Increasing the loading of the catalyst
(entry 8) and using the bulkier napthalene-substituted
ligand 5f resulted in synthetically useful enantioselec-
tivites (entry 9). Addition of a drying agent and lowering
the concentration increased the selectivity and reproduci-
bility of this process (entry 10). Finally, noticing that the
reaction warms slightly at the beginning of the reaction,
we began the reaction at 0 ºC and allowed the reaction to
warm slowly to ambient temperature over the course of
the reaction (entry 11), which provided optimal levels of
stereoinduction.
5 h
48 h
48 h
3 h
R = 4-BrPh
R = 4-AcOPh
R= 2-Naphth
5 h
Ar
Me
Ph
O
Ph
9
6 h
52% 88%
N
Ns
Me
Ar
O
Me
10
48 h
--
--
N
Ph
Ns
Ph
Me
Ar
O
N
Ns
X
X
11
X = OTIPS
X = NHTs
8 h
78% 92%
62% 91%
12
10 h
Ar
O
13
14
15
3 h
4 h
82% 91%
51% 94%
Ph
N
Ns
Ph
Ar
O
n-Hex
N
Ns
n-Hex
Table 2 summarizes experiments probing the scope of this
oxyamination process. As in our copper chemistry and
racemic iron system, styrenes proved to be excellent sub-
strates. Substitution at all positions of the aryl ring is well
tolerated (entries 2–4). Styrenes bearing substituents at
the α-position of the olefin also provided high enantiose-
lectivities (entry 9), although β-substituted styrenes gave
no oxyamination product (entry 10). The reaction is tol-
erant of electronic perturbation; styrenes bearing elec-
tron-withdrawing (entries 5 and 6) and electron-donating
substituents (entry 7) react with good yield and high en-
antioselectivity. Fused polycyclic aromatics (entry 8) are
also good substrates for this reaction. The functional
group compatibility of the process is good; halides, esters,
and protected oxygen and nitrogen moieties were easily
tolerated without modification of the reaction conditions
(entries 5, 6, 11, and 12). 1,3-Dienes also undergo efficient
oxyamination, and we observed exclusive chemoselectivi-
ty for the terminal olefin (entries 13 and 14). Notably, the
enantioselectivities remained high for these substrates
Ar
O
48 h
--
--
n-Hex
N
Ns
n-Hex
a
Reactions were performed using 0.3 mmol olefin and 2.5 equiv of
oxaziridine. b Data represent the averaged results of two reproducible
c
d
experiments. Isolated yield. Enantiomeric excess determined by
chiral SFC analysis.
We were intrigued to observe that the oxyamination
products in all cases were formed with excellent cis dia-
stereoselectivity. In our previously reported copper-
catalyzed method for preparation of the regioisomeric
aminals, we had found that the oxyamination products
were isolated as mixtures of aminal diastereomers, and
that the two diastereomers were generally formed with
different ee's. To test whether the high diastereoselectivi-
ty of this iron-catalyzed process arises from Lewis acid-
catalyzed stereomutation of the aminal group, we pre-
pared the trans isomer of 324 and subjected it to the iron
triflimide catalyst (eq 3). After 3 h, no formation of the cis
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Supporting Information. Experimental procedures and
diastereomer was observed. We speculate, therefore, that
1
2
3
4
5
6
7
8
characterization data for all new compounds. This material
is available free of charge via the Internet at
http://pubs.acs.org.
the high diastereoselectivity observed in this reaction is a
consequence of selective reaction of one of the enantio-
mers of the racemic oxaziridine 2 in a kinetic resolution
process. Consistent with this hypothesis, after completion
of the oxyamination reaction, we find that the remaining
unreacted oxaziridine can be re-isolated with significant
optical enrichment (80% ee, eq. 4).25
AUTHOR INFORMATION
Corresponding Author
* tyoon@chem.wisc.edu
10% Fe(NTf2)2
20% ligand 5f
Ar
Ar
O
O
9
ACKNOWLEDGMENT
(3)
N
N
benzene
10
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12
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Ns
Ns
4-MePh
4-MePh
Financial support for this research was provided by the NIH
(GM084022). The NMR spectroscopy facility at UW–
Madison is funded by the NIH (S10 RR04981-01) and NSF
(CHE-9629688).
400 wt% MgO
0 ºC → rt, 3 h
(Ar= 2,4-Cl2Ph)
no isomerization
observed
Ar
O
REFERENCES
N
4-MePh
Ns
4-MePh
10% Fe(NTf2)2
3
20% ligand 5f
+
+
(4)
benzene
Ns
N
Ns
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O
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2
2
racemic
80% ee
Finally, we examined the stereochemical integrity of the
oxyamination product upon deprotection to reveal the
corresponding free amino alcohol (Scheme 1). Subjecting
2-vinylnaphthalene-derived aminal 6 to standard acid-
catalyzed hydrolysis conditions cleanly removed the ami-
nal to give the N-nosyl protected amino alcohol 7 in 89%
yield. Thiol-mediated removal of the nosyl group yielded
the free amino alcohol 8 in high yield. Importantly, no
erosion of ee was observed in either step. In addition, we
were able to confirm the absolute stereochemistry of 8 by
comparison of its optical rotation to known values,26 veri-
fying the selectivity of the oxyamination process.
3
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7
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89% yield, 92% ee
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OH
NH2
PhSH, K2CO3
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92% yield, 92% ee
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8
9
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catalyzed by an iron(II) bis(oxazoline) complex. When
used in conjunction with our previously reported copper
chemistry, this method allows regio- and stereochemical
control in the generation of a variety of 1,2-amino alco-
hols. Current efforts in our lab are underway to better
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16
For reviews of iron catalysis in organic synthesis, see: (a)
Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004,
104, 6217–6254. (b) Enthaler, S.; Junge, K.; Beller, M. Angew.
Chem., Int. Ed. 2008, 47, 3317–3321. (c) Correa, A.; Garcia
Mancheño, O.; Bolm, C. Chem. Soc. Rev. 2008, 37, 1108–1117.
(d) Sherry, B. D.; Fürstner, A. Acc. Chem. Res. 2008, 41, 1500–
1511. (e) Fürstner, A. Angew. Chem., Int. Ed. 2009, 48, 1364–
1367. (f) Czaplik, W. M.; Mayer, M.; Cvengroš, J.; Jacobi von
Wangelin, A. ChemSusChem 2009, 2, 396–417.
22 Emmons, W. D. J. Am. Chem. Soc. 1957, 79, 5739–5754.
23
Sibi, M. P.; Petrovic, G. Tetrahedron: Asymm. 2003, 14,
2879–2882.
24
The racemic trans oxazolidine was isolated from an unse-
lective oxyamination reaction catalyzed by Fe(acac)3. See ref
15a.
17
25
For recent reviews on asymmetric iron catalysis see: (a)
Darwish, M.; Willis, M. Catal. Sci. Technol. 2012, 2, 243–255.
(b) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282–2291.
At the suggestion of a reviewer, we also examined the ste-
reospecificity of the oxyamination with respect to the olefin by
subjecting (Z)-β-deuterostyrene to the optimized reaction con-
ditions. The oxazolidine product was obtained as a 1:1.5 ratio of
deuterated diastereomers, consistent with a step-wise oxyami-
nation process.
18
For examples of enantioselective iron-catalyzed oxidative
transformations, see: (a) Collman, J. P.; Wang, Z.; Straumanis,
A.; Quelquejeu, M.; Rose, E. J. Am. Chem. Soc. 1999, 121, 460–
461. (b) Cheng, Q. F.; Xu, X. Y.; Ma, W. X.; Yang, S. J.; You, T.
P. Chin. Chem. Lett. 2005, 16, 1467–1470. (c) Gelalcha, F. G.;
Bitterlich, B.; Anilkumar, G.; Tse, M. K.; Beller, M. Angew.
Chem., Int. Ed. 2007, 46, 7293–7296. (d) Suzuki, K.; Olden-
26
Cho, B. T.; Kang, S. K.; Shin, S. H. Tetrahedron: Asymm.
2002, 13, 1209–1217.
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10 mol%
1-Naph
Me
Me
Ns
O
O
O
N
1-Naph
Ar
N
N
Ar
H
O
Fe
1-Naph
1-Naph
Ar= 2,4-Cl2Ph
Tf2N
NTf2
N
Ns
8
9
R
+
MgO, benzene, 0 °C
rt
88–95% ee
one regioisomer
13 examples
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