Carbonyl Imines from oxaziridines: Generation and cycloaddition of N-O=C dipoles

Article abstract of DOI:10.1002/anie.200905801

Dipoles apart: Unusual 1,3-dipolar carbonyl imines are generated in the presence of a bulky scandium(III) catalyst by undergoing a Lewis acid catalyzed rearrangement of N-sulfonyl oxaziridines. The 1,3-dipolar carbonyl imines then undergo subsequent cyclo

Full text of DOI:10.1002/anie.200905801

Communications
DOI: 10.1002/anie.200905801
Heterocycles
Carbonyl Imines from Oxaziridines: Generation and Cycloaddition of
À =
N O C Dipoles**
Katherine M. Partridge, Ilia A. Guzei, and Tehshik P. Yoon*
Cycloaddition reactions of nitrones,[1] azides,[2] and related
1,3-dipolar species[3] are recognized as powerful tools for the
construction of compounds with great utility in organic
synthesis, chemical biology, and materials science. The con-
ceptual model that underpins our understanding of this broad
class of reactions was first formalized by Huisgen in 1963.[4]
This seminal contribution to organic chemistry provided
chemists with a unified framework for understanding the
structure and reactivity of 1,3-dipolar compounds, and also
predicted the structures of several elementary 1,3-dipoles that
were unknown at that time. Subsequent research has identi-
fied cycloaddition reactions of many of these predicted 1,3-
dipolar species;[5] however, carbonyl imines (e.g., 3;
Scheme 1) have remained elusive since Huisgenꢀs initial
We recently reported the TiCl4-catalyzed rearrangement
of N-sulfonyl oxaziridines[7] (“Davis oxaziridines”, e.g. 1) to
produces a new class of electron-deficient N-sulfonyl nitrones
2 (Scheme 1).[8] These highly unstable nitrones can efficiently
undergo in situ cycloaddition with a variety of styrenic olefins
to afford 1,2-isoxazolidines with very high cis selectivity (5;
Table 1, entry 1). During the course of investigating this novel
Table 1: Lewis acid optimization for carbonyl imine cycloaddition.[a]
Entry
Catalyst
T [8C]
Yield [%][b]
4/5[b]
1[c]
2
3
4
5
6
7
8
TiCl4
BF3·OEt2
Sc(OTf)3
Yb(OTf)3
La(OTf)3
Sc(OTf)3·6
Sc(OTf)3·7
(ScCl2)SbF6·7
(ScCl2)SbF6·7
23
23
23
23
23
23
23
23
40
69
32
65
40
26
30
38
41
80
1:>10
1:5
1:2
8:1
9:1
>10:1
>10:1
>10:1
>10:1
Scheme 1. Lewis acid dependent formation of nitrones or carbonyl
imines from N-sulfonyl oxaziridines. Ns=4-nitrobenzenesulfonyl.
prediction. Although these unusual dipoles have been postu-
lated to be intermediates in the photochemical decomposition
of nitroarenes,[6] the synthesis of dipolar cycloaddition
products of carbonyl imines has not yet been reported.
Herein, we demonstrate that carbonyl imines can be effi-
ciently generated by the Lewis acid catalyzed rearrangement
of N-sulfonyl oxaziridines, and trapped by cycloaddition with
a variety of dipolarophiles.
9
[a] 10 mol% catalyst and 4 equiv oxaziridine unless otherwise noted.
[b] Yields and product ratios were determined by 1H NMR analysis using
a calibrated internal standard. [c] 2 equiv oxaziridine.
reactivity, we made the surprising observation that a variety of
other Lewis acid catalysts (Table 1, entries 2–5) produce a
side product that we identified as the regio- and diastereo-
meric 1,2-isoxazolidine 4.[9] The production of 4 suggested
that the Lewis acid catalyzed rearrangement of 1 could be
used to generate either N-sulfonyl nitrones or N-sulfonyl
carbonyl imines, and that the partitioning between these two
1,3-dipoles could be controlled by the identity of the catalyst.
Intrigued by this observation, we sought to develop con-
ditions that would result in exclusive formation of carbonyl
imine cycloadduct 4, which would enable us to explore this
reactivity in greater detail.
[*] K. M. Partridge, I. A. Guzei, Prof. T. P. Yoon
Department of Chemistry, University of Wisconsin-Madison
1101 University Avenue, Madison, WI 53706 (USA)
Fax: (+1)608-265-4534
E-mail: tyoon@chem.wisc.edu
Homepage: http://yoon.chem.wisc.edu
[**] We thank Kevin Williamson for the preparation of [D]-8. Financial
support for this research has been provided by Abbott Laboratories
(fellowship to K.M.P.), the NIH (R01-GM084022), and the NSF
(CHE-0645447). The NMR spectroscopy facility at UW-Madison is
funded by the NIH (S10 RR04981-01) and NSF (CHE-9629688).
Noting that the sterically demanding lanthanide triflate
Lewis acids favored formation of 4, albeit in poor yields, we
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905801.
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speculated that the coordination of a bulky ligand to a more
reactive, but poorly selective, transition metal salt (e.g.,
Sc(OTf)3; Table 1, entry 3) might reverse the selectivity.
Indeed, coordination of either bipyridyl ligand 6 or bis(oxazo-
line) ligand tmbox (7)[10] resulted in the exclusive formation of
carbonyl imine cycloadduct 4, although the reactivity of these
complexes was attenuated (Table 1, entries 6 and 7). The
scandium complex, [Sc(tmbox)Cl2]SbF6, proved to be a more
reactive catalyst (Table 1, entry 8),[11] and afforded good
yields of the desired cycloadduct at elevated temperatures
without loss of selectivity (Table 1, entry 9). These conditions
for the formation of 4 were then used to further investigate
this rearrangement–cycloaddition sequence.
Both theoretical and experimental data support the
postulated formation of carbonyl imine intermediate 3 in
these reactions. The rearrangement of oxaziridines into
carbonyl imines was first proposed in a series of computa-
tional studies reported by Rzepa et al.[12] Those studies
characterized the oxaziridine rearrangement to carbonyl
imines as a thermally allowed, conrotatory electrocyclic ring
opening, and predicted that the presence of electron-with-
drawing N substituents would significantly stabilize and
increase the lifetime of the carbonyl imines. Given this
theoretical support, we felt that the formation of N-nosyl
carbonyl imine intermediate 3 was a reasonable explanation
to account for the formation of 4.
The intermediacy of a carbonyl imine is also supported by
the stereospecificity of the cycloaddition (Scheme 2). The
reaction of 1 with cis-b-deuterostyrene [D]-8 (96 atom%
Scheme 3. Stereochemical probe in support of carbonyl imine inter-
mediate 3 (Mechanism A) over Mechanism B.
intermediate such as 3, and is not consistent with the
stereospecific nucleophilic attack suggested by Mechanism
B. We can also exclude the alternate possibility that this result
arises from rapid Lewis acid catalyzed racemization of
oxaziridine 1*; when the reaction is halted before completion,
the remaining oxaziridine can be re-isolated in enantioen-
riched form, albeit with reduced ee, along with the racemic
cycloadduct [Eq. (2)]. This result indicates that the rate of
formation of racemic cycloadduct 9 is faster than the rate of
racemization of 1*, which is not consistent with Mechanism B
but is fully consistent with the slow, reversible Lewis acid
catalyzed rearrangement of 1 to 3 followed by rapid cyclo-
addition with styrene, as proposed in Mechanism A.
Scheme 2. Cycloaddition of deuterium-labeled styrene.
[D])[13] afforded isoxazolidine [D]-9 without any detectable
loss of stereochemical fidelity of the deuterium label. This
result is what would be expected from a stereospecific syn
addition of a carbonyl imine with an olefin in a concerted
cycloaddition process.
Finally, to distinguish between the proposed carbonyl
imine mechanism (Scheme 3, Mechanism A) and an alternate
mechanism involving the direct nucleophilic attack of the
dipolarophile on the Lewis acid activated oxaziridine
(Scheme 3, Mechanism B), we investigated the cycloaddition
of enantioenriched oxaziridine 1*.[14] The cycloaddition of 1*
(55% ee) with styrene afforded only the racemic cycloadduct
9 [Eq. (1)], which is consistent with reaction via an achiral
Table 2 summarizes the range of dipolarophiles that were
used to investigate the scope of the carbonyl imine cyclo-
addition.[15] Styrenes are very good substrates for this process,
and substitution at the para (Table 2, entries 2–5), and meta
(Table 2, entries 6–8) positions have little impact on the
efficiency of the reaction. Conversely, large ortho substituents
diminish the efficiency and diastereoselectivity of the reaction
(Table 2, entries 9–11), and styrenes that have substituents on
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Table 2: Dipolarophile scope in carbonyl imine cycloadditions.[a]
the olefin do not undergo cycloaddition in syntheti-
cally useful yields (Table 2, entry 12).
Monosubstituted aliphatic alkenes are also
excellent substrates for this cycloaddition reaction.
Olefins bearing both linear and branched alkyl
substituents react smoothly (Table 2, entries 13–15),
although tertiary alkyl substituents diminish the
yield of the cycloadduct (Table 2, entry 16). A
variety of functional groups are successfully toler-
ated, including ethers, halides, esters, silanes, and
protected amines and alcohols (Table 2, entries 17–
22). Notably, despite the Lewis acidity of the
cationic scandium(III) complex, the reaction con-
ditions tolerate the presence of sensitive acetal
functional groups (Table 2, entry 23). We found that
internal aliphatic alkenes are unreactive using this
methodology; therefore, the highly chemoselective
cycloaddition of terminal olefins is observed in
substrates bearing multiple olefinic bonds (Table 2,
entry 24).
Entry Dipolarophile
Product
Yield
trans/cis[c,d]
[%][b,d]
1
2
Ar=Ph
65
66
74
69
70
69
52
55
19[e]
56
62
>10:1
>10:1
>10:1
>10:1
>10:1
>10:1
>10:1
>10:1
7:1
Ar=4-AcOC6H4
Ar=4-ClC6H4
Ar=4-BrC6H4
Ar=4-MeO2CC6H4
Ar=3-MeOC6H4
Ar=3-FC6H4
Ar=3-ClC6H4
Ar=2-ClC6H4
Ar=2-MeC6H4
Ar=2-FC6H4
3
4
5
6
7
8
9
10
11
9:1
>10:1
The highest occupied molecular orbital
(HOMO) and lowest energy molecular orbital
(LUMO) energies of these highly electron-deficient
carbonyl imines are presumably to be quite low, and
are likely poorly matched to electron-deficient
dipolarophiles. Indeed, fumarates, succinimides,
and acrylates fail to produce the desired cyclo-
adducts. Conversely, heteroatom-containing dipo-
larophiles are good substrates for cycloaddition; the
reaction of aldehydes, ketones, or imines (Table 2,
entries 25–27) all proceed in good yields, and with
good diastereoselectivity.[16]
Although the scope of the dipolarophile is quite
broad, the scope of the oxaziridine is somewhat
more limited (Table 3). Oxaziridines that have
aliphatic substituents at carbon do not successfully
undergo this reaction (Table 3, entries 1 and 2), and
the use of N-sulfonyl groups that have electron-
withdrawing substituents is critical to the success of
the rearrangement–cycloaddition process (Table 3,
entries 3–5). This observation is fully consistent
with the computation studies reported by Rzepa
and co-workers,[12] which suggested that the intro-
duction of electron-withdrawing N-substituents
would significantly increase the propensity of the
oxaziridine to rearrange to the isomeric carbonyl
imine. A variety of C-aromatic N-nosyl oxaziridines
are excellent carbonyl imine precursors; experi-
ments using para or meta substituents proceeded
smoothly (Table 3, entries 6–8), although dimin-
ished yields were observed using ortho-substituted
oxaziridines (Table 3, entry 9). Electron-donating
substituents increased the propensity of the oxazir-
idine to react; however, N-nosyl oxaziridines bear-
ing even weakly electron-donating groups at the
para position (such as methyl groups) are very
unstable and can decompose violently.[17]
12
26
>10:1
13
14
15
16
R=nHex
R=Bn
R=cyclohexyl
R=1-adamantyl
78
68
>10:1
>10:1
>10:1
>10:1
60
24[e]
17
18
19
20
21
22
74
79
78
65
73
72
>10:1
>10:1
>10:1
>10:1
>10:1
>10:1
23
24
79
73
>10:1
>10:1
The superior reactivity of oxaziridines that have
electron-deficient N-sulfonyl groups confers an
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Table 2: (Continued)
ring-opened upon reduction of 9 with sodium
Entry Dipolarophile
Product
Yield
trans/cis[c,d]
naphthalenide to afford N-nosyl-protected anti-
1,3-aminoalcohol 11 in high diastereomeric purity
[Eq. (4)]. Importantly, the isomeric cis-1,2-isoxazo-
lidines that are formed from our previously
reported TiCl4-catalyzed nitrone cycloadditions,
are also amenable to these synthetic manipula-
tions.[8] Therefore, using two complementary sets of
conditions, the reaction of N-sulfonyl oxaziridines
with olefins can provide access to deprotected cis-
and trans-N-isoxazolidines and syn and anti 1,3-
aminoalcohols with high levels of control over the
chemoselectivity and the stereoselectivity of the
two-step processes.
[%][b,d]
25
75
1:>10
26
27
85
89
4:1
1:>10
In conclusion, we have found that oxaziridines
undergo highly stereoselective cycloaddition with a
variety of dipolarophiles in the presence of a bulky
scandium(III) catalyst. This reactivity suggests the
formation of a carbonyl imine intermediate, and
this study is the first report of cycloaddition
products arising from this long-elusive class of 1,3-
[a] TIPS=triisopropylsilyl; Phth=phthalimido; TBS=tert-butyldimethylsilyl; See
the Supporting Information for experimental details. [b] Yield of isolated product
unless otherwise noted. [c] Diastereomeric ratios were determined by 1H NMR
analysis. [d] These yields and diastereomeric ratios represent the averaged results of
two experiments. [e] Yield determined by 1H NMR analysis using a calibrated
internal standard.
Table 3: Variation of oxaziridine structure.[a]
Entry Oxaziridine
Product
Yield [%][b,d] trans/cis[c,d]
1
2
R=tBu
R=cyclopropyl
0
0
–
–
Scheme 4. Conversion of N-isoxazolidines into either deprotected N-
isoxazolidines [Eq. (3)] or 1,3-aminoalcohols [Eq. (4)] under comple-
mentary conditions.
3
4
5
Ar1 =Ph
0
18
45
–
>10:1
>10:1
Ar1 =4-ClC6H4
Ar1 =4-CF3C6H4
dipoles. As a procedure for the synthesis of structurally
complex heterocycles, this transformation is a valuable
complement to the TiCl-catalyzed nitrone cycloadditions
previously reported by our laboratory. Cis- or trans-N-
sulfonyl-1,2-isoxazolidines can be selectively produced from
the reaction of N-sulfonyl oxaziridines and olefins, and the
sense of diastereoselectivity can be controlled by the choice of
Lewis acid catalyst utilized. An enantioselective variant of
these reactions would be a powerful tool for the synthesis of
complex molecules. Whilst we have not yet observed good
levels of enantioinduction using a variety of chiral bis(oxazo-
line) ligands that are commonly used in asymmetric cataly-
sis[19] in place of 7, studies towards this goal are currently
ongoing in our laboratory.
6
7
8
9
Ar2 =4-FC6H4
83
81
90
50
>10:1
>10:1
>10:1
10:1
Ar2 =3-MeOC6H4
Ar2 =3,5-Me2C6H3
Ar2 =2-ClC6H4
[a] See the Supporting Information for experimental details. [b] Yield of
isolated product. [c] Diastereomeric ratios were determined by 1H NMR
analysis. [d] Yields and diastereomeric ratios represent the averaged
results of two experiments.
additional practical benefit to this methodology (Scheme 4).
The N-nosyl moiety can be removed from trans-1,2-isoxazo-
lidine cycloadduct 9 using mild deprotection conditions
reported by Fukuyama et al. (PhSH, K2CO3);[18] no unwanted
Received: October 15, 2009
Revised: November 14, 2009
Published online: December 22, 2009
À
cleavage of the sensitive N O bond is observed in the
reaction, and the N-unsubstituted isoxazolidine 10 is isolated
Keywords: carbonyl imines · cycloaddition · heterocycles ·
oxaziridines · scandium
.
in good yields [Eq. (3)]. Alternatively, the heterocycle can be
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Communications
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