N -Sulfonyl α-imino ester-derived chiral oxaziridines: Catalytic asymmetric synthesis and application as a modular chiral organic oxidant

Article abstract of DOI:10.1039/c7cc02502e

A novel class of chiral N-sulfonyl oxaziridines is introduced for use as structurally modifiable chiral oxidants. These oxaziridines are readily prepared from N-sulfonyl α-imino esters in a highly enantioenriched form by oxidation with hydrogen peroxide using l-isoleucine-derived triaminoiminophosphorane as a catalyst. The distinct advantage of their structural modularity is demonstrated through the identification of an optimal oxaziridine that exhibits high reactivity and enantiospecificity in the asymmetric oxidation of a silyl enol ether and N-sulfonyl allylic and homoallylic amines.

Full text of DOI:10.1039/c7cc02502e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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N‐Sulfonyl α‐Imino Ester‐Derived Chiral Oxaziridines: Catalytic
Asymmetric Synthesis and Application as a Modular Chiral
Organic Oxidant†
Received 00th January 20xx,
Accepted 00th January 20xx
Naoya Tanaka,§a Ryosuke Tsutsumi,§a Daisuke Uraguchi,a and Takashi Ooi*ab
DOI: 10.1039/c6cc00000x
www.rsc.org/chemcomm
A novel class of chiral
N
-sulfonyl oxaziridines is introduced
for use as structurally modifiable chiral oxidants. These
oxaziridines are readily prepared from -sulfonyl α-imino
esters in a highly enantioenriched form by oxidation with
hydrogen peroxide using -isoleucine-derived
N
Figure 1. (a) General structure of oxaziridines (b) Structure of oxazridines
designed in this study
L
triaminoiminophosphorane as a catalyst. The distinct
advantage of their structural modularity is demonstrated
through the identification of an optimal oxaziridine that
exhibits high reactivity and enantiospecificity in the
olefins, α-hydroxylation of carbonyl compounds via metal
enolates, and oxidation of heteroatoms.4
Although
enantioselective oxidations have been achieved using chiral,
non-racemic, -sulfonyl oxaziridines, their structures are
N
asymmetric oxidations of a silyl enol ether and
allylic and homoallylic amines.
N-sulfonyl
practically confined to those derived from natural products,5
primarily due to the lack of reliable methods for the asymmetric
construction of the oxaziridine ring core. This limitation not only
renders the design and preparation of an oxaziridine capable of
Oxaziridines are three-membered heterocycles consisting of
oxygen, nitrogen and carbon atoms (Fig. 1a). The ring strain and
weakness of the N–O bond allow oxaziridines to transfer their
oxygen or nitrogen atoms to other nucleophilic functionalities.1
The position of the nucleophilic attack on an oxaziridine depends
on the electronic properties of the substituent on the 2-nitrogen
atom. The presence of an electron-withdrawing group increases
the leaving ability of the nitrogen atom and directs a nucleophile
to the oxygen atom, endowing the oxaziridine with the character
of a neutral organic oxidant. For example, perfluorinated
oxaziridines, bearing perfluoroalkyl groups on their 2-nitrogen
atoms, show greater oxidizing ability than the corresponding
nonfluorinated congener, while preparation and handling of
oxidizing
a
given substrate with
a
high degree of
enantioselectivity difficult, but also hampers the fine tuning of
the inherent reactivity of the oxaziridine.6 Hence, the potential
utility of
N-sulfonyl oxaziridines as structurally modifiable
chiral organic oxidants remains unexplored.
Under these circumstances, catalytic systems for the
asymmetric synthesis of
N-sulfonyl oxaziridines by the oxidation
of parent -sulfonyl imines were independently reported by
N
Jørgensen and Yamomoto’s groups.7–9 We also developed a
broadly applicable, preparative method for the imine oxidation
based on the use of chiral P-spiro triaminoiminophosphoranes of
type 110-15 as catalysts (Fig. 2).16 The emergence of these
those fluorinated oxaziridines are somewhat difficult.2
N-
methodologies unlocks the door to access a diverse range of
Sulfonyl oxaziridines, wherein strongly electron-withdrawing
sulfonyl groups are attached on the 2-nitrogen atom, also have
moderate oxygen-transfer ability. These are regarded as the most
commonly utilized class of oxaziridines owing to their stability
optically active
N-sulfonyl oxaziridines. This would ensure an
otherwise unattainable possibility of their structural modification
for precisely tuning the reactivity and selectivity in the
application to asymmetric oxidations. In exploring the synthetic
potential of such tailor-made chiral organic oxidants, we became
and ease of preparation.1,3
N-Sulfonyl oxaziridines have been
applied to various electrophilic oxidations such as epoxidation of
a. Institute of Transformative Bio-Molecules (WPI-ITbM) and Department of
Molecular and Macromolecular Chemistry, Graduate School of Engineering,
Nagoya University, Furo-cho D2-1, Chikusa, Nagoya 464-8601, Japan. E-mail:
tooi@chembio.nagoya-u.ac.jp.
b. CREST, Japan Science and Technology Agency (JST), Nagoya University, Nagoya
464-8601, Japan.
† Electronic Supplementary Information (ESI) available: Experimental procedures
and characterizations of compounds. See DOI: 10.1039/c6cc00000x
Figure 2. P‐Spiro chiral triaminoiminophosphoranes
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interested in the structural features of oxaziridines derived from
COMMUNICATION
(
1e), enabled the isolation of 3a in 82% yield with 97% ee (entry
View Article Online
DOI: 10.1039/C7CC02502E
N
-sulfonyl α-arylimino esters for the following reasons: 1) the 6). We also confirmed that the incorporation of a strongly
presence of the electron-withdrawing alkoxycarbonyl group electron-withdrawing trifluoromethyl group, instead of a fluoro
would be beneficial in facilitating the oxygen atom-transfer functionality, was ineffective in terms of both catalytic activity
event; 2) the steric and electronic attributes of the aromatic and stereoselectivity (entry 7).
substituent (Ar) could be manipulated for rigorous control of the
With the simple asymmetric oxidation protocol and optically
stereochemical outcome of the oxidation (Fig. 1b).17 Herein, we active 3a (97% ee) in hand, we sought to evaluate the
report a catalytic enantioselective preparation of this novel class performance of 3a as a chiral oxidant. For this purpose, we
of
N
-sulfonyl oxaziridines and demonstrate their utility as selected the Rubottom oxidation of ketone-derived silyl enol
as a model system (Chart 1).20 Upon treatment of
with
modular chiral oxidants in the asymmetric oxidations of a silyl ether
4
4
enol ether (Rubottom oxidation) and
homoallylic amines.
N
-sulfonyl allylic and
At the outset of our investigation, we chose methyl (
Z
)-2-
phenyl-2-(tosylimino)acetate (2a) as a precursor and exposed it
to each two equivalents of 35% aqueous hydrogen peroxide
(H2O2) and trichloroacetonitrile (Cl3CCN) in toluene at 0 °C in
the presence of a catalytic amount of
triaminoiminophosphorane 1a (5 mol%), which are the optimal
conditions for the asymmetric oxidation of -sulfonyl
aldimines.16a The reaction proceeded smoothly to afford the
L-isoleucine-derived
N
corresponding
N-sulfonyl oxaziridine 3a in good yield with
moderate enantioselectivity (Table 1, entry 1).18 To our surprise,
Chart 1. A Comparison of Reactivity and Selectivity of Representative
Chiral Oxaziridines in the Rubottom Oxidation of Silyl Enol Ether
= 1‐naphthyl)
Table 1. Optimization of Catalysta
4 (1‐Np
3a (1.2 equiv) in dichloromethane at 0 °C, oxygen atom transfer
occurred smoothly to furnish α-hydroxy ketone in 59% yield
after desilylation with tetrabutylammonium fluoride (nBu4NF).
The enantiomeric excess of was determined to be 70% ee by
chiral stationary phase HPLC, and the enantiospecificity of the
oxidation process was calculated to be 72% es (es = [% ee of
5
entry
1
additive
Cl3CCN
none
none
none
none
none
none
yield (%)b
ee (%)c
69
96
94
95
94
97
95
5
1d
2
3
4
5
6
7
1a
1a
1b
1c
1d
1e
1f
73
78
59
78
80
82
69
5
]/[% ee of
intended product was detected in the reaction with oxaziridines
derived from -(+)-10-camphorsulfonic acid, and was
formed, but with much less enantiospecificity, when
benzaldehyde-derived
was employed as an oxidant.1,3,16a
3]*100). In sharp contrast, no formation of the
6
D
5
a The reaction was performed with 0.2 mmol of 2a, 2 equiv of 35% aqueous H2O2,
and 5 mol% of 1 in toluene (4.0 mL) at 0 °C. b Isolated yield. c Enantiomeric
excesses were analyzed by chiral stationary phase HPLC. d 2 equiv of Cl3CCN was
added as an additive.
7
These results reveal the superior reactivity and enantio-
differentiating ability of 3a compared to the previously
accessible, representative chiral oxaziridines.
however, the oxygen atom transfer from H2O2 occurred with
comparable efficiency even in the absence of Cl3CCN, yielding
3a with a substantially higher enantiomeric excess (entry 2).
This observation suggests that the catalytically generated
An additional salient feature of
3 as a chiral oxidant is its
structural modularity, allowing us to pursue modification of the
aromatic and ester substituents for improving the
enantiospecificity of the present oxidation. Since the basis for
taking full advantage of this possibility is the sufficient
generality of the catalytic method for the asymmetric preparation
of 3, various α-arylimino esters 2 with different aromatic and
ester substituents were subjected to oxidation with a chiral
triaminoiminophosphorane 1e-H2O2 system (Table 2, eq 1). As
listed in Table 2, the corresponding oxaziridines 3 were obtained
aminophosphonium hydroperoxide provides
a
chiral
environment superior to that created by the peroxyimidate
counterpart for discriminating prochiral faces of 2a in the
nucleophilic attack by the hydroperoxide ion. In addition, the
subsequent intramolecular N–O bond formation is facilitated
probably because of the geminal substitution on the 3-carbon
atom of 2a (Thorpe-Ingold effect), despite the poor leaving
ability of the hydroxide ion.19 This initial yet intriguing finding
generally in good chemical yields with a high level of
enantiocontrol, irrespective of the mode of substitution on the
led us to modify the structure of iminophosphorane catalyst 1,
aromatic moiety of
include slightly diminished enantioselectivity observed with
2 (entries 1–11). The trends to be noted
specifically with respect to the aromatic substituents (Ar), for
further selectivity enhancement (entries 3–7). While a slight
2
,
having highly electron-deficient aromatic substituents (entries 9
and 10), and insensitivity of the stereochemical outcome to the
steric bulk of the ester moiety (entry 3 vs. entry 12–14). The
decrease in enantioselectivity was observed with 1b and 1d
,
bearing para- and meta-tolyl groups, respectively, as catalysts
(entries 3 and 5), the introduction of an electronegative fluoro
appendage to the aromatic nucleus, particularly at the 3-position
library of enantioenriched oxaziridines
3 thus obtained was
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Table 2. Substrate Scope of 1e‐Catalyzed Asymmetric Oxaziridine Synthesis and Enantiospecificity of the Resulting Oxaziridine in the Rubottom Oxidationa
View Article Online
DOI: 10.1039/C7CC02502E
eq 1
eq 2
entry
1
2
3
4
5
6
7
8
Ar'
4-MeC6H4
4-FC6H4
R
2
yield (%)b
68
ee (%)c of 3
3
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
time (h)
50
50
12
12
72
72
50
12
15
12
72
12
12
12
yield (%)b
37
ee (%)c of 5
es (%)d
79
87
91
88
77
76
83
86
84
87
–
Me 2b
Me 2c
Me 2d
Me 2e
Me 2f
Me 2g
Me 2h
Me 2i
Me 2j
Me 2k
Me 2l
Et 2m
iPr 2n
tBu 2o
98
95
96
97
96
97
95
95
85
82
99
95
95
96
77
83
87
85
74
74
79
82
71
71
–
71
79
77
73
67
63
58
62
52
85
70
75
88
95
88
75
37
85
82
82
99
<5
95
90
4-ClC6H4
4-BrC6H4
3-MeC6H4
3-MeOC6H4
3-FC6H4
3-ClC6H4
3-CF3C6H4
3,5-F2C6H3
2-MeC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
9
10
11
12
13
14
88
90
92
93
95
96
77
90
a The reaction 1 was performed with 0.2 mmol of 2, 2 equiv of 35% aqueous H2O2, and 5 mol% of 1e in toluene (4.0 mL) at 0 °C. The reaction 2 was conducted with 3
which was isolated from the reaction 1, 1 equiv of 4 in CH2Cl2 (0.1 M) at 0 °C, and then, whole reaction mixture was treated with a solution of nBu4NF in THF (1.2 equiv,
1.0 M) at 0 °C for 0.5 h. b Isolated yield. c Enantiomeric excesses were analyzed by chiral stationary phase HPLC. d es = 100*[% ee of 5]/[% ee of 3]
utilized
reactivity/selectivity relationship in the Rubottom oxidation of
(Table 2, eq 2). As expected, the electronic nature of was found
to be responsible for the reaction profile, and , possessing an
for
facile
assessment
of
the
structure-
4
3
3
electron-deficient aromatic group, exerted higher reactivity and
stereoselectivity (entries 1–10). The position of the substituents
Scheme 1. Dual Catalytic System for Asymmetric Oxidation of Silyl Enol
Ether
by in situ‐Generated Chiral Oxaziridine 3o22
on the aromatic ring also appeared critical, as oxaziridines
prepared from 4-substituted α-arylimino esters gave
3
5
4
The potential utility of oxaziridine 3o (96% ee) was further
demonstrated in the application to the oxidation of more
challenging substrates such as non-activated olefins (Chart 2).
consistently with better enantioselectivity and the steric
hindrance caused by the ortho-tolyl group completely
suppressed the oxidation (entry 11). Importantly, however, an
increase in the steric demand of the ester substituent led to a
significant improvement in enantioselectivity without any
detrimental impact on the reactivity, and 3o with a tert-butyl
ester substituent was identified as the optimal oxidant capable of
transferring an oxygen atom to
enantiospecificity (entries 12–14).
4
with excellent
The stereoselective Rubottom oxidation relies on the
electrophilic nature of chiral oxaziridine 3o that is converted into
parent α-imino ester 2o after transferring an oxygen atom to the
electron-rich substrate
4
.
Considering this aspect and the
nucleophilic character of aminophosphonium hydroperoxide, the
key reactive species in the asymmetric oxidation of 2o with the
chiral iminophosphorane 1e-H2O2 system, we reasoned that
these two oxidation processes could be merged through the
establishment of a dual catalysis using H2O2 as a stoichiometric
Chart 2. Asymmetric Oxidation of Simple Olefins
8 possessing N‐
Sulfonylated Amines by Chiral Oxaziridine 3o (Ar1 = 3,5‐(CF3)2C6H3)22
oxygen source.21 Indeed, treatment of silyl enol ether
H2O2 in the presence of each catalytic quantity of 1e and 2o in
toluene at 0 °C resulted in the production of α-hydroxy ketone
4 with
For instance, 3o acted as a prominent chiral oxidant for the
epoxidation of prenyl amine derivatives 8a and 8b, giving rise to
the desired epoxides 9a and 9b with excellent enantiospecificity.
5
Not only
N-sulfonyl tiglic amine 8c but also N-sulfonyl
in high yield with rigorous enantiocontrol (Scheme 1). This
catalytic Rubottom oxidation involves the conversion of 2o into
3o by the action of the 1e-H2O2 system (nucleophilic asymmetric
oxidation) and subsequent stereoselective oxygen atom transfer
homoprenyl amine 8d were also epoxidized by 3o with
satisfactory levels of efficiency and stereochemical control.
Although three alkyl substituents on a double bond are essential
for ensuring adequate conversion, this represents a rare example
of the highly enantioselective epoxidation of simple olefins by
chiral oxaziridines.8,23
from the in situ generated 3o to
4 (electrophilic asymmetric
oxidation), with the concomitant regeneration of 2o, thus
highlighting the distinctive feature of our approach.
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In summary, we have introduced a novel class of chiral
sulfonyl oxaziridines as uniquely reactive and modular chiral
organic oxidants that can be readily prepared by the asymmetric
COMMUNICATION
8
9
J. L. Olivares-Romero, Z. Li and H. Yamamoto,VJie.wAAmrti.clCe Ohnelmine.
Soc., 2012, 134, 5440.
Since the leading works by Jørgensen and Yamamoto, several
enantioselective syntheses of -sulfonyl oxarziridines have
N
-
DOI: 10.1039/C7CC02502E
N
oxidation of α-imino esters with H2O2 using
P-spiro chiral
been reported: (a) S. Dong, X. Liu, Y. Zhu, P. He, L. Lin and
X. Feng, J. Am. Chem. Soc., 2013, 135, 10026; (b) T. Zhang,
W. He, X. Zhao and Y. Jin, Tetrahedron, 2013, 69, 7416; (c)
triaminoiminophosphoranes as catalysts. The ample generality
of this practical method for the synthesis of the enantioenriched
oxaziridines offers an unprecedented opportunity of the fine
tuning of the oxaziridine structure, thereby enabling facile
identification of an optimal chiral oxidant for the
enantioselective oxidation of a silyl enol ether (Rubottom
oxidation). Notably, a catalytic Rubottom oxidation has also
been developed based on the in situ generation of the requisite
chiral oxaziridine by the oxidation of a catalytic quantity of the
parent α-imino ester under the catalysis of the iminophosphorane
with H2O2 as a stoichiometric terminal oxidant. The synthetic
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3602; (h) J. Novacek, J. A. Izzo, M. J. Vetticatt and M. Waser,
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utility of this class of chiral
demonstrated in achieving the highly enantioselective
epoxidation of -sulfonyl allylic and homoallylic amines. We
N-sulfonyl oxaziridines is further
N
believe that this study opens a door to a new avenue for the use
of optically active oxaziridines as tailor-made chiral oxidants in
implementing selective chemical synthesis.
13 For a review on chiral iminophosphorane catalysis, see: H.
Krawczyk, M. Dzięgielewski, D. Deredas, A. Albrecht and Ł.
Albrecht, Chem.–Eur. J., 2015, 21, 10268.
Acknowledgements
14 (a) D. Uraguchi and T. Ooi, J. Synth. Org. Chem. Jpn., 2010,
68, 1185; (b) D. Uraguchi, S. Nakamura, H. Sasaki, Y.
Konakade and T. Ooi, Chem. Commun., 2014, 50, 3491; (c) D.
Financial support was provided by CREST-JST (JPMJCR13L2:
13418441), Program for Leading Graduate Schools "Integrative
Graduate Education and Research Program in Green Natural
Sciences" in Nagoya University, and Grants of JSPS for
Scientific Research. NT and RT thank JSPS for financial support.
Uraguchi, K. Yamada and T. Ooi, Angew. Chem., Int. Ed.
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2015, 54, 9954; (d) M. A. Horwitz, N. Tanaka, T. Yokosaka,
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Notes and references
§ These authors contributed equally.
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2
3
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5
While we focus on non-ionic, isolatable chiral oxaziridines,
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21 (a) B. H. Brodsky and J. Du Bois, J. Am. Chem. Soc., 2005,
127, 15391. (b) N. D. Litvinas, B. H. Brodsky and J. Du Bois,
Angew. Chem., Int. Ed., 2009, 48, 4513.
6
7
For example, oxidation of simple olefins with known N-sulfonyl
oxaziridines generally requires harsher condition (60 °C, 3–12
h) than that adopted for more reactive substrates such as metal
enolates: (a) F. A. Davis, N. F. Abdul-Malik, S. B. Awad and M.
E. Harakal, Tetrahedron Lett., 1981, 22, 917; (b) F. A. Davis and
A. C. Sheppard, J. Org. Chem., 1987, 52, 954.
22 See ESI for detailed reaction conditions.
23 N. Ji, J. Yuan, M. Liu, T. Lan and W. He, Chem. Commun.
,
L. Lykke, C. Rodríguez-Escrich and K. A. Jørgensen, J. Am.
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