Iron(IV) corroles are potent catalysts for aziridination of olefins by Chloramine-T

Article abstract of DOI:10.1016/S0040-4039(01)01717-8

Iron(III) corroles were found to be more selective catalysts than analogous porphyrins for the aziridination of olefins by PhI=NTs, and, most important, the iron(IV) corrole 1 displays the unique ability of utilizing Chloramine-T as nitrogen atom source.

Full text of DOI:10.1016/S0040-4039(01)01717-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8089–8092
Iron(IV) corroles are potent catalysts for aziridination of olefins
by Chloramine-T
Liliya Simkhovich and Zeev Gross*
Department of Chemistry and Institute of Catalysis Science and Technology, Technion-Israel Institute of Technology,
Haifa 32000, Israel
Received 15 July 2001; revised 5 September 2001; accepted 12 September 2001
Abstract—Iron(III) corroles were found to be more selective catalysts than analogous porphyrins for the aziridination of olefins
by PhIꢀNTs, and, most important, the iron(IV) corrole 1 displays the unique ability of utilizing Chloramine-T as nitrogen atom
source. © 2001 Elsevier Science Ltd. All rights reserved.
3
d
In 1998, Sharpless and co-workers discovered a very
interesting aziridination system, based on the reaction
of Chloramine-T (TsN-ClNa) with olefins in the pres-
not well established. The more classical aziridination
reagent is PhIꢀNTs (Ts=tosylate), which was intro-
duced in the early 1980s by the groups of Breslow and
Mansuy. They showed that heme enzymes and syn-
thetic metalloporphyrins are efficient catalysts for
transferring the NTs moiety of PhIꢀNTs into CꢁH and
−
1
ence of 10 mol% of Br . The same authors also
3
reported in that publication that they had searched
sporadically (and unsuccessfully) over two decades for
a transition-metal-catalyzed aziridination process using
Chloramine-T. This challenge had been met earlier that
year by Komatsu and co-workers, who demonstrated
that CuCl was able to serve as catalyst for that reac-
tion, providing N-tosyl aziridines in yields of 18–75%
4
CꢀC bonds. This methodology was subsequently
extended to many other metal complexes, which has
also included the asymmetric versions thereof. The
reactive intermediate in this reaction is considered to be
a high valent metalꢀNTs complex, a conclusion that
also holds for the related manganese porphyrin system.
5
2
6
with 5–10 mol% of catalyst. To our knowledge, this
and related work (all with copper) remain the only
examples up to date about the utilization of Chlo-
ramine-T for aziridination of olefins. Some mechanistic
However, there are drawbacks to the use of PhIꢀNTs: it
is an expensive and inconvenient nitrogen source,
iodobenzene is formed in equimolar amounts, and oxy-
3
7
aspects of the copper-catalyzed aziridinations are still
genated hydrocarbons are dominant by-products. This
Ar
Ar
Ar
N
L
N
L
Fen
Fen
N
N
N
N
L'
L'
N
N
Ar
Ar
Ar
Ar
1
4
, 2, 3, 5: Ar = C6F5
, 6: Ar = o-Cl2C6H3 5: n = III, L = Cl (no L')
: n = III, L = Cl (no L')
1
2
3
4
: n = IV, L = Cl (no L')
: n = III, L = L' = OEt2
: n = III, L = NO (no L')
: n = III, L = L' = OEt2
6
Figure 1. The catalyst utilized in this study.
*
0
Corresponding author. Tel.: +972-4-8293954; fax: +972-4-8233735; e-mail: chr10zg@tx.technion.ac.il
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01717-8

8
090
L. Simkho6ich, Z. Gross / Tetrahedron Letters 42 (2001) 8089–8092
is in sharp contrast to Chloramine-T, which is commer-
cially available, cheap and produces only NaCl as
byproduct.
molecular sieves or using CH Cl as solvent caused a
2 2
significant increase in the yield of the undesired oxy-
genated products. Comparison between the various
complexes (Table 1) shows that the corrole-based cata-
lysts (entries 1–3) were more selective than the
analogous iron porphyrin (entry 5). Interestingly, while
the change of meso-aryl substitution from penta-
fluorophenyl in 1 and 5 to 2,6-dichlorophenyl in 4
and 6 was slightly advantageous for the porphyrin
(entries 5 and 6), it resulted in a very large decrease in
catalytic activity for the corrole (entries 1 and 4). In any
case, the best catalyst is the iron(III) corrole 2: it
provides 84% of aziridine together with the largest
selectivity (entry 2). However, the known limitation in
metalloporphyrin catalyzed aziridination of olefins is
still present: lowering the olefin/PhIꢀNTs ratio from
We have recently introduced easily accessible corroles
8,9
and their metal complexes, and shown that they are
much more active catalysts than analogous porphyrin
complexes in the cyclopropanation of olefins by car-
1
0
benoids. Since many complexes that catalyze cyclo-
propanation reactions are also effective aziridination
1
1
catalysts, we have decided to examine several iron
corroles for the latter purpose. The results show that
the iron(III) corroles are more potent catalysts than
analogous porphyrin complexes for aziridination by
PhIꢀNTs and disclose the unique ability of the iron(IV)
corrole to utilize Chloramine-T as the nitrogen source.
1
00:1 to 10:1 and 1:1 results in a very large drop of
First, the reaction of styrene with PhIꢀNTs was exam-
ined with the corrole-based catalysts 1–4 and the iron
porphyrins 5 and 6 (Fig. 1). In all cases, the desired
aziridine was obtained together with significant
amounts of styrene oxide and phenylacetaldehyde. The
optimal results in terms of yield and selectivity for
aziridination versus oxygenation were obtained in ben-
zene in the presence of molecular sieves: omitting the
yield, from 84 to 31 and 0% aziridine (entries 2, 7–9),
respectively.
All these results strongly indicate that the reactions
proceed via an iron–nitrene intermediate, that reacts
either with the olefin to produce the aziridine or
hydrolyses to the (oxo)iron complex that subsequently
oxidizes the olefin. This is shown in Scheme 1, which is
a
Table 1. Aziridination of styrene by PhIꢀNTs, under catalysis by 1 mol% iron corroles and porphyrins
Product yield (%)
Entry
Catalyst
Ph-I
Epoxide+aldehyde
Aziridine
Selectivity
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
96
99
98
53
98
99
90
9
8.4+0
71.0
84.0
78.9
13.0
75.2
76.1
30.9
0
8.5
11.4
8.8
0.5
6.1
6.0
2.8
0
4.2+3.2
5.1+3.9
21+5.6
7.7+4.7
6.3+6.3
7.1+4.0
2.5+0.6
4.4+3.3
b
2
c
2
d
2
71
0
0
a
3
0–56 mg (0.08–0.15 mmol) of PhIꢀNTs was added to dry benzene solutions that were 0.28–0.3 mM in catalyst (0.7–1.3 mg, 0.8–1.5 mmol) and
contained 0.9–1.7 mL (8–15 mmol) styrene, 7 mL (68.5 mmol) nitrobenzene and activated molecular sieves. The chemical yields were determined
by GC after filtration of the reaction mixtures, relative to nitrobenzene.
Catalyst:PhIꢀNTs:styrene=1:100:1000
Catalyst:PhIꢀNTs:styrene=1:1000:1000
Catalyst:PhIꢀNTs:styrene=1:100:100.
b
c
d
Ts
N
H2O
Fen+2
Ph
TsNH2
I
I
Ph
Ph
O
Fen+2
Ph
Ph
Ts N
N
O
CHO
Fen
Ts
+
Ph
Ph
Scheme 1. Proposed catalytic cycles for iron complexes catalyzed reaction of styrene with PhIꢀNTs.

L. Simkho6ich, Z. Gross / Tetrahedron Letters 42 (2001) 8089–8092
8091
Ts
Cl
Cl
IV
N
IV
IV
N-
Na
+
+
N
Fe
Fe
Cl
+
Fe
Cl
-
NaCl
Ts
Ts
Ar
Ar
Scheme 2. Proposed activation of Chloramine-T by 1.
the generally accepted mechanism with metallopor-
phyrin catalysts. Both the sensitivity to humidity and
the need for large excess of olefin are considered to be
different manifestations of the same phenomenon—the
low hydrolytic stability of the metal–nitrene intermedi-
ate. The larger selectivity of corrole versus porphyrin
iron complexes fits within this proposal, as corroles
stabilize high valent metals significantly better than
porphyrins.
struction of a Hammett plot via competition reactions
of couples of substrates with Chloramine-T under low-
conversion conditions (z =−0.5).
4
c
+
These observations are very different from those
obtained in the aziridination by PhIꢀNTs with metal
complexes of porphyrins and other ligands. Therein,
competitive oxygenation is a serious problem and the
12
Hammett’s z values are much larger. In addition,
suggesting the mechanism of Scheme 1 for catalysis by
the iron(IV) corrole 1 would require an unreasonable
iron(VI) intermediate.
Having demonstrated the potential of the iron corroles
as aziridination catalysts, we turned our attention to
Chloramine-T. All six complexes were investigated,
under various reaction conditions (CH CN, CH Cl ,
Based on the acquired data, we propose the mechanism
shown in Scheme 2 for the activation of Chloramine-T
by 1. The polarity of the initial ionic and nucleophilic
NꢁNa bond is inverted upon coordination to the
iron(IV) corrole, allowing it to act as an electrophile for
the reaction with olefins. The low z value is in accord
with significant radical contribution, expected for such
3
2
2
benzene, aerobic and under N , with and without
2
molecular sieves, different styrene/TsN-ClNa ratio).
These examinations exposed the iron(IV) corrole 1 as
the only active catalyst. The best results were obtained
in CH Cl , either under Ar or in aerobic solution in the
2
2
presence of molecular sieves. Interestingly, the disad-
vantageous characteristics of the reactions with
PhIꢀNTs are not found in the 1-catalyzed reactions of
the styrene derivatives with Chloramine-T: there are no
oxygenated by-products and the demand for large
excess of olefin is much smaller. Accordingly, the Chlo-
ramine-T based system is much more practical than the
one based on PhIꢀNTs. This is demonstrated in Table
3d
a four-centered mechanism.
The mechanism also
accounts for the absolute requirement for the strongly
polarizing high valent metal. In conclusion, the proper-
ties of the iron(IV) corrole give rise to a novel and
practical catalyst for aziridination of olefins by
Chloramine-T.
2, which lists the results obtained with five styrene
derivatives.
Acknowledgements
Note that in all these cases; the reported yields are of
isolated products, Chloramine-T trihydrate was used
without prior dehydration (by the potentially haz-
This research (No. 368/00) was supported by the Israel
Science Foundation.
1
ardous procedure), and only 1 mol% of catalyst was
used. The same substrates were also used for the con-
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