A new type of imido group donor: Synthesis and characterization of sulfonylimino-Δ3-bromane that acts as a nitrenoid in the aziridination of olefins at room temperature under metal-free conditions

Article abstract of DOI:10.1021/ja075811i

A stable sulfonylimino-λ³-bromane, CF₃SO₂N^--Br⁺C₆H₄-p-CF₃, has been synthesized and structurally characterized for the first time. X-ray crystallographic analyses indi

Full text of DOI:10.1021/ja075811i

Published on Web 10/04/2007
A New Type of Imido Group Donor: Synthesis and Characterization of
Sulfonylimino-λ3-bromane that Acts as a Nitrenoid in the Aziridination of
Olefins at Room Temperature under Metal-Free Conditions
Masahito Ochiai,*,† Takao Kaneaki,† Norihiro Tada,† Kazunori Miyamoto,† Hiroshi Chuman,†
Motoo Shiro,‡ Satoko Hayashi,§ and Waro Nakanishi*,§
Graduate School of Pharmaceutical Sciences, UniVersity of Tokushima, 1-78 Shomachi,
Tokushima 770-8505, Japan, Rigaku Corporation, 3-9-12 Matsubara, Akishima, Tokyo 196-8666, Japan, and
Department of Materials Science and Chemistry, Faculty of Systems Engineering, Wakayama UniVersity,
930 Sakaedani, Wakayama 640-8510, Japan
Received August 2, 2007; E-mail: mochiai@ph.tokushima-u.ac.jp
Phenyl(sulfonylimino)-λ3-iodanes PhIdNSO2Ar are readily ac-
Scheme 1
cessible from sulfonamides by the reaction with phenyl-λ3-iodanes1
and have found many applications in modern organic synthesis.2
Because of the hyper-leaving-group ability of aryl-λ3-iodanyl
groups,3 they serve as excellent nitrene (nitrenoid) progenitors either
in the aziridination of alkenes or in the amidation of alkanes via
C-H insertion using copper or rhodium catalysts.4 In the past
decade, the research activity in this area has surged.5
with σp of 0.96,9 which stabilizes the negative charge at the nitrogen
On the other hand, little is known about the chemistry of the
related group 17 sulfonylimino-λ3-bromanes because a method for
atom. Less acidic p-toluenesulfonamide, however, showed no
evidence for formation of the corresponding iminobromane under
our conditions.
their syntheses is not available. We report herein, for the first time,
the synthesis, isolation, and characterization of a stable sulfo-
nylimino-λ3-bromane 1 whose structure was firmly established by
X-ray crystal analysis. The imino-λ3-bromane 1 serves as an
efficient imido group donor and directly undergoes aziridination
of olefins stereospecifically with retention of original stereochem-
istry. It should be noted that the aziridination proceeds at room
temperature using limiting amounts of alkenes under transition-
metal-free conditions.
The solid-state structure of imino-λ3-bromane 1 (Figure 1),
obtained by recrystallization from dichloromethane/hexane at 5 °C,
illustrates a centrosymmetric dimer with a Br2O2 rhomboid structure
(Br1‚‚‚O1 3.0361(18) and Br1‚‚‚O1* 3.0603(19) Å),10 which stands
in marked contrast to the polymeric zigzag chain structure of
PhINTs with an I-N‚‚‚I-N‚‚‚ backbone.7 The Br1-N1 distance
of 1.846 Å is in a good agreement with the sum of the covalent
radii for Br (1.14 Å) and N (0.70 Å)11 and comparable to the
reported bond lengths for N-bromobenzamide (1.843 Å), N-
bromosuccinimide (1.817 Å), and bromamine-T (TsNBrNa, 1.870
Å).12 The result suggests little double-bond character for the “ylidic”
Br-N bond of 1 as well as a negligibly small electrostatic attraction
between the oppositely charged Br1 and N1.13 The C1-Br1-N1
bond angle of 100.94° is considerably greater than that (95.8°)
reported for PhINTs,7a probably because of the increased nonbonded
repulsions between the two substituents on Br(III) with a decreased
atomic size.
Exposure of trifluoromethanesulfonamide (TfNH2) to p-trifluo-
romethylphenyl(difluoro)-λ3-bromane6 (1 equiv) in acetonitrile at
0 °C for 10 min in a Teflon PFA vessel under argon afforded
trifluoromethylsulfonylimino-λ3-bromane 1 in 98% yield as a white
solid, after repeated decantation with hexane (Scheme 1). This is
the first example for the synthesis of imino-λ3-bromanes. Because
of a highly aggregated polymeric nature,7 the iminoiodane PhINTs
is insoluble in organic media such as dichloromethane and
acetonitrile, being employed for transition-metal-catalyzed aziri-
dination and C-H insertion. The insoluble nature makes it difficult
to improve catalytic efficiencies and complicates mechanistic
studies.8 In marked contrast, the imino-λ3-bromane 1 is readily
soluble in these solvents. The imino-λ3-bromane 1 is fairly stable
in the solid state and can be stored for more than 2 months without
any decomposition in a refrigerator (at 4 °C) under argon. In
solution (0.02 M in CDCl3), however, it decomposes at 23 °C with
a half-life time (t1/2) of 1 h and produces a mixture of triflylamide
and p-(trifluoromethyl)bromobenzene quantitatively. It is more
stable in a coordinating solvent such as acetonitrile (t1/2 ) 42 h at
23 °C). Brief heating to melting (at 85 °C) resulted in the complete
disappearance of 1 with formation of the amide (80%) and the
bromoarene (82%). The 1H NMR spectrum in CDCl3 showed a set
of signals assigned to the disubstituted phenyl group [δ 8.12 (o)
and 7.92 (m) ppm]. The 13C resonance of the ipso carbon atom
appeared at δ 138.9 ppm.
A fully optimized structure of phenyl(triflylimino)-λ3-bromane
PhBrdNSO2CF3 at the MP2/6-311+G(d) level in the Gaussian 03
program14 was found to be essentially superimposable on the X-ray
dimeric structure of 1 shown in Figure 1, except for the CF3 group
(Figure S1). The calculated dimeric structure is by 18.6 kcal mol-1
more stable than that of the monomer. Natural charges via the
natural population analysis (Br, +0.825 and N, -1.004) indicate a
highly polarized Br-N single bond (Table S1).15 NBO calculations
predict that negative hyperconjugation of the nitrogen lone pair
orbital with the σ* Br1-C1 and S1-C8 bond orbitals probably
plays an important role in stabilizing the imino-λ3-bromane in the
gas phase:16 donor-acceptor interaction energies (E(2)) 7.9 and 20.6
kcal mol-1, respectively (Table S2). In fact, the observed dihedral
angles C1-Br1-N1-S1 (99.84°) and Br1-N1-S1-C8 (90.11°)
in the solid-state structure of 1 probably reflect these negative
hyperconjugation.
Key to the success of the synthesis of imino-λ3-bromane 1 seems
to be the presence of a highly electron-withdrawing triflyl group
Aziridines possess a strained ring and, thus, are susceptible to
ring-opening reactions, which makes aziridines useful intermediates
in organic synthesis.17 The imino-λ3-bromane 1 readily undergoes
transfer of the sulfonylimido group to olefins even at room
† University of Tokushima.
‡ Rigaku Corporation.
§ Wakayama University.
9
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J. AM. CHEM. SOC. 2007, 129, 12938-12939
10.1021/ja075811i CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
importantly, the olefin aziridination of both stereoisomers was found
to be highly stereospecific, which is a relatively rare example of
stereochemical retention in the aziridinations19 and strongly suggests
a concerted reaction mechanism.
Rates for aziridination of cis-cyclooctene with imino-λ3-bromane
1 were measured spectrophotometrically at 50 °C in acetonitrile
solution. The observed rate constants kobsd are proportional to
concentration of the olefin and afforded k2 value of 1.14 × 10-1
M-1 s-1 (Table S3). These results most likely suggest the involve-
ment of a bimolecular transition state such as 3 (Scheme 2), in
which an olefin attacks the σ* N-Br orbital (LUMO) of 1 in the
nitrenoid transfer process.
Figure 1. ORTEP drawing of the dimeric unit of 1 with thermal ellipsoids
at 50% probability. Selected bond lengths (Å) and angles (deg): Br1-N1
1.846(2), Br1-C1 1.931(2), C1-Br1-N1 100.94(11).
Table 1. Aziridination of Olefins with Imino-λ3-bromane 1a
entry
olefin
solvent
time (h) yield (%)b
Z:E
Supporting Information Available: Experimental details, Figures
S1 and S2, Tables S1-S3, complete ref 14, and X-ray crystallographic
data in CIF format for 1. This material is available free of charge via
the Internet at http://pubs.acs.org.
1
2
3
4
5
6
7
8
9
cyclohexene
cyclohexene
MeCN
CH2Cl2
MeCN
MeCN
MeCN
MeCN
CH2Cl2
MeCN
MeCN
MeCN
MeCN
CH2Cl2
MeCN
MeCN
CH2Cl2
CH2Cl2
5
5
3
2
5
2a
2a
2b
2c
2d
2e
2f
2g
2g
2h
2h
2h
2i
67 (75)
76 (85)
97
90
72 (78)
80
cycloheptene
cis-cyclooctene
norbornene
Me2CdCMe2
n-C8H17CHdCH2
Z-PhCHdCHPh
E-PhCHdCHPh
9
References
18
12
48
3
24
8
1
36
6
65 (68)
81
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>99:1
0c
10 Z-PhCHdCHMe
11 E-PhCHdCHMe
12 E-PhCHdCHMe
13 Z-PrCHdCHPr
14 E-PrCHdCHPr
15 E-PrCHdCHPr
16 PhCHdCH2
82 (97) >99:1
(15) 7:93
79 (84) <1:99
89 (98) >99:1
(23)
2i
2i
2j
2l
5:95
53 (88) <1:99
12
36
84 (91)
17 p-ClC6H4CHdCH2 CH2Cl2
92 (95)d
a Conditions: 1:1.2 olefin/bromane 1, acetonitrile, room temperature, Ar.
b Isolated yields. Numbers in parentheses are 1H NMR yields. c trans-
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shown in Table 1. Compared to the aziridination reaction with
sulfonylimino-λ3-iodanes, in which a transition-metal catalyst such
as Cu or Rh(II) salt is indispensable to generate reactive metal imido
species,2,5 our imido transfer reaction does not require any metal
additives. Furthermore, the aziridination proceeds using limiting
amounts of starting olefins, in contrast to most of the reported
reactions with preformed imino-λ3-iodanes, which rely on excess
amounts of substrate (3 or more equiv) to effect high product
conversion.18 Thus, exposure of cyclohexene to imino-λ3-bromane
1 (1.2 equiv) in acetonitrile at room temperature for 5 h afforded
N-triflylaziridine 2a selectively in 67% yield, with no evidence for
formation of an allylic amidation side product (entry 1). A wide
array of cyclic and acyclic alkenes gave the corresponding aziridines
in high yields. Among the challenging substrates, a terminal olefin
1-decene was found to afford a good yield of aziridine.
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) 9.8448(2) Å, c ) 8.9487(2) Å, â ) 93.0748(7)°, V ) 1148.83(4) Å3,
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the geometry of olefins: aziridination of cis-1,2-disubstituted olefins
occurs smoothly, and the isolated aziridines 2g-i were found to
be diastereomerically pure, with complete preservation of the
original cis stereochemistry of the olefins. On the other hand, the
aziridinations of trans isomers in acetonitrile were very sluggish.
Finally, we found that use of dichloromethane as a solvent
dramatically increases the rate and efficiency of the uncatalyzed
reaction (compare entries 10-12 and 13-15). Acetonitrile probably
coordinates to the positively charged bromine atom of imino-λ3-
bromane 1 and stabilizes it, which, in turn, will decrease the
reactivity of 1. Such an adduct is predicted to be more stabilized
than the components by 7.8 kcal mol-1 (Figure S2). Most
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