Room temperature activation of aryloxysulfonyl azides by [Co(II)(TPP)] for selective radical aziridination of alkenes via metalloradical catalysis

Article abstract of DOI:10.1016/j.tetlet.2015.01.186

Aryloxysulfonyl azides can be effectively activated by commercially available cobalt(II) complex of meso-tetraphenylporphyrin ([Co(TPP)]) at room temperature under neutral and nonoxidative conditions for selective radical aziridination of alkenes via meta

Full text of DOI:10.1016/j.tetlet.2015.01.186

Tetrahedron Letters xxx (2015) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Room temperature activation of aryloxysulfonyl azides by
[Co(II)(TPP)] for selective radical aziridination of alkenes via
metalloradical catalysis
⇑
Velusamy Subbarayan, Li-Mei Jin, Xin Cui, X. Peter Zhang
Department of Chemistry, University of South Florida, Tampa, FL 33620-5250, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Aryloxysulfonyl azides can be effectively activated by commercially available cobalt(II) complex of meso-
tetraphenylporphyrin ([Co(TPP)]) at room temperature under neutral and nonoxidative conditions for
selective radical aziridination of alkenes via metalloradical catalysis. The [Co(TPP)]-catalyzed radical
aziridination system is suitable for different combinations of olefin substrates and aryloxysulfonyl azides,
producing various N-aryloxysulfonyl aziridine derivatives in good to excellent yields. In addition to
generating the environmentally benign N2 as the only byproduct, this Co(II)-based metalloradical
aziridination process features mild reaction conditions and operational simplicity.
Received 4 December 2014
Revised 27 January 2015
Accepted 29 January 2015
Available online xxxx
Keywords:
Metalloradical
Radical
Ó 2015 Elsevier Ltd. All rights reserved.
Cobalt
Aziridination
Azide
Porphyrin
Introduction
ly as metalloradical catalyst for radical olefin aziridination with
azides when the concern of enantioselectivity is unnecessary. This
As stable metalloradicals with well-defined open-shell doublet
d7 electronic structure, cobalt(II) complexes of porphyrins,
[Co(Por)], have shown to be effective catalysts for aziridination of
a broad spectrum of alkenes with different classes of nitrogen
sources via a unique stepwise radical process (Scheme 1).1,2 Of
various Co(II)-based metalloradical catalysts, Co(II) complexes sup-
ported by porphyrin ligands bearing amide functionalities
[Co(AmidoPor)] have shown to be particularly effective in activat-
ing various organic azides, including phosphoryl azides,3 sulfonyl
azides,4 and aryl azides,5 for radical olefin aziridination, producing
the corresponding aziridines in high yields under mild conditions.
The high catalytic efficiency of [Co(AmidoPor)] metalloradical cat-
practical desire is translated to the fundamental question of
whether [Co((TPP)], which lacks of the hydrogen-bonding capa-
bility, can effectively activate azides to generate the corresponding
a-Co(III)-aminyl radical (Scheme 1: A2) to serve as active interme-
diates for the radical aziridination process.
Several different types of organic azides have been previously
investigated as nitrogen sources for catalytic olefin aziridination
by metalloradical [Co(TPP)], including diphenylphosphoryl azide3a
and 4-nitrophenyl azide.6,7 In addition to limited substrate scope
and low product yields, these existing [Co(TPP)]-based systems
typically required the use of high catalyst loading (8-10 mol%) and
elevated reaction temperature (75–100 °C).3a,6 Driven by the desire
to utilize the commercially available [Co(TPP)] as a practical catalyst
for olefin aziridination, we embarked on a project to search for more
reactive organic azides that can be activated by [Co(TPP)] to gener-
alysts is attributed to the stabilization of the key a-Co(III)-aminyl
radical (also known as Co(III)-nitrene radical) intermediate
through hydrogen-bonding interaction with the amide group of
the porphyrin ligand (Scheme 1: A1).1–5 While the effectiveness
of [Co(AmidoPor)] has been clearly demonstrated (especially in
the case of enantioselective aziridination), it would be practically
more desirable if the commercially available Co(II) complex of
meso-tetraphenylporphyrin ([Co((TPP)]) could be applied effective-
ate the corresponding key a-Co(III)-aminyl radicals under mild con-
ditions. As the outcome of this effort, we herein report our findings
that aryloxysulfonyl azides (ArOSO2N3) are a new type of organic
azides that can be effectively activated by [Co(TPP)] even at room
temperature for selective radical aziridination (Eq. (1)). The newly
developed Co(II)-catalyzed aziridination can utilize ArOSO2N3 with
different aryl substituents and is suitable for a wide range of aro-
matic olefins with diverse electronic and steric properties, affording
a series of N-aryloxysulfonyl aziridines in good to excellent yields. In
⇑
Corresponding author. Tel.: +1 813 974 7249; fax: +1 813 974 3203.
E-mail address: xpzhang@usf.edu (X. P. Zhang).
URL: http://chemistry.usf.edu/faculty/zhang/ (X. P. Zhang).
http://dx.doi.org/10.1016/j.tetlet.2015.01.186
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Subbarayan, V.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.01.186

2
V. Subbarayan et al. / Tetrahedron Letters xxx (2015) xxx–xxx
R1
5R
R3
R1
R
R
N
N
N
N
N
N
R4
O
O
O
Co
N H
R1 H N
N2
N
R2
[Co(Por)]
Co(II)-metalloradical
N
N
N
c
a
Co
N
H N
N H
R
O
3R R4
R2
R
B
A1
A
R1
1R
N2
N
N
R5
Co(Por)
-Co(III)-alkyl radical
R1
Co(Por)
N
N
N
N
α-Co(III)-aminyl radical
γ
R3
R4
2R
5R
Co
N
A2
b
Scheme 1. (Left) Pathways of proposed mechanism of radical aziridination via Co(II)-based metalloradical catalysis: (a) generation of
metalloradical activation of azide by [Co(Por)]; (b) formation of -Co(III)-alkyl radical through radical addition of -Co(III)-aminyl radical onto olefin; and (c) production of
aziridine via 3-exo-tet radical cyclization of -Co(III)-alkyl radical. (Right) Structures of postulated -Co(III)-aminyl radical intermediates supported by amidoporphyrins (A1)
and TPP (A2). For clarity, one 3,5-DitBu-phenyl group on the meso-position of the amidoporphyrin (A1) and one phenyl group on the meso-position of TPP (A2) were omitted.
a-Co(III)-aminyl radical via
c
a
c
a
addition to generating the environmentally benign N2 as the only
byproduct, the new metalloradical aziridination system enjoys sev-
eral practical attributes associated with neutral and nonoxidative
condition as well as the commercial availability of the catalyst
[Co(TPP)].
one-pot procedure by following the literature methods.4b,10 For
example, azides 2a–e (Table 1), which contain H, Cl, Br, F, and Ph
substituents, respectively, at the para-position of the aryl ring,
were readily prepared from the reactions of the commercially
available phenols with SO2Cl2 in the presence of pyridine, followed
by treatment of the resulting aryloxysulfonyl chlorides with
NaN3.10
With 4-phenylphenoxysulfonyl azide (2e) as a model azide
reagent, we studied the catalytic aziridination reaction of styrene
and found that [Co(TPP)] was a highly effective metalloradical
catalyst to activate the azide even at room temperature for the pro-
duction of the corresponding aziridine 4ea (Table 2). Further
experiments demonstrated that the effectiveness of the
[Co(TPP)]-catalyzed aziridination reaction could be dramatically
impacted by the molar ratio of the azide reagent to styrene sub-
strate. When 10 equivalents of styrene were used, aziridine 4ea
was obtained in 95% yield (Table 2, entry 1). The yield slightly
decreased to 82% in the presence of 5 equivalents of styrene
(Table 2, entry 2). The aziridination by [Co(TPP)] became much less
effective when a stoichiometric amount of styrene was employed
(Table 2, entry 3).11 It was found that the solvent could also influ-
ence the outcome of the metalloradical aziridination process by
[Co(TPP)]. While chlorobenzene was identified to be an optimal
R1
O
S
O
O
S
O
[Co(TPP)]
N2
+
O
N3
+
N
O
2R
room
1R
temperature
2R
ð1Þ
Results and discussion
After examining different organic azides, we turned our
attention to aryloxysulfonyl azides (ArOSO2N3), which have not
been previously explored as precursors of a-Co(III)-aminyl radicals
for Co(II)-based metalloradical catalysis.8 In comparison with aryl-
sulfonyl azides (ArSO2N3), which have been previously shown to be
ineffective for catalytic aziridination by [Co(TPP)],4a,9 ArOSO2N3
should be more electron-deficient due to the electron-withdrawing
effect of the aryloxy groups and might be easier to be activated by
[Co(TPP)]. With this assumption in mind, we synthesized a series
of aryloxysulfonyl azides from the corresponding phenols in a
Table 1
Preparation of aryloxysulfonyl azidesa
O
S
O
i) pyridine, 0 o
C
R
OH
R
O
N3
ii) SO2Cl2 in DCM, 5 h
iii) NaN3 in 0 oC to rt, 16 h
1
2
Entry
1
Product
Yieldb (%)
40
Entry
4
Product
Yieldb (%)
41
O
S
O
S
2d
N3
2a
N3
F
O
O
O
O
O
O
2b
2c
2e
N3
Cl
Br
O
O
S
O
O
S
N3
Ph
O
S
O
2
3
39
40
5
45
N3
O
a
Reaction conditions: phenols 1 (10 mmol), sulfuryl chloride (22 mmol), NaN3 (30 mmol).
Isolated yields.
b
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V. Subbarayan et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
Table 2
Table 3
Catalytic aziridination of styrene with 4-phenylphenoxysulfonyl azide by metallo-
Activation of aryloxysulfonyl azides for olefin aziridination by [Co(TPP)]a,b
porphyrin complexesa
O
N
S
O
O
Ph
O
S
O
[M(TPP)] (5 mol %)
4 A MS, rt, 20 h
+ N2
+
N3
Ph
O
2e
3a
4ea
Entry
Azide:styrene
[M(TPP)]
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
1:10
1:5
1:1.2
1:5
[Co(TPP)]
[Co(TPP)]
[Co(TPP)])
[Co(TPP)])
[Co(TPP)])
[Fe(TPP)Cl]
[Mn(TPP)Cl]
[Zn(TPP)]
[VO(TPP)]
[Cu(TPP)]
[Ni(TPP)]
[Cr(TPP)]
PhCl
PhCl
PhCl
CH3CN
THF
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
95
82
12
NR
NR
8
8
3
6
8
1:5
1:10
1:10
1:10
1:10
1:10
1:10
1:10
a
Reaction conditions: azide 2 (0.1 mmol), styrene 3a (0.5 mmol), 4 Å MS (100 mg),
[Co(TPP)] (5 mol %), PhCl (1 mL), 20 h, rt.
Isolated yields.
b
9
10
11
12
4
4
Table 4
a
[Co(TPP)]-catalyzed aziridination of various styrene derivatives with aryloxysulfonyl
Reaction conditions: azide 2e (0.1 mmol), styrene 3a (1 mmol) unless otherwise
azidesa,b
stated, 4 Å MS (100 mg), catalyst (5 mol %) unless otherwise stated, solvent (1 mL),
20 h, rt.
O
S
O
O
S
O
[Co(TPP)] (5 mol %)
PhCl, 4 A MS, rt, 20 h
N
O
R
O
b
1H NMR yields.
R
O
N3
Ar
+
N2
+
Ar
2
3
4
O
O
S
O
O
S
O
N
S
O
Ph
N
O
Ph
N
Ph
Ph
O
Ph
tBu
M = Co(II):
[Co(TPP)]
Me
4ec
entry 3: ; 87% yield
= Fe(III)Cl: [Fe(TPP)Cl]
= Mn(III)Cl: [Mn(TPP)Cl]
N
N
N
N
4ea
4eb
entry 1:
; 95% yield
entry 2:
; 89% yield
Ph
M
O
O
O
S
O
Ph
= Zn(II):
[Zn(TPP)]
N
S
O
Ph
N S O
N
O
Ph
= V(IV)(O): [VO(TPP)]
O
O
= Cu(II):
= Ni(II):
[Cu(TPP)]
[Ni(TPP)]
Cl
Br
F
= Cr(III)Cl: [Cr(TPP)Cl]
Ph
entry 6: 4ef; 94% yield
entry 5: 4ee; 94% yield
entry 4: 4ed; 90% yield
O
S
O
S
O
S
O
S
Figure 1. Structures of porphyrin complexes with metal ions.
N
O
Ph
N
O
Ph
N
O
N
O
O
O
O
O
Br
solvent, it was shown that coordinative and more polar solvents
such as acetonitrile and THF completely suppressed the catalytic
process, leading to no formation of the aziridine product (Table 2,
entries 4–5). In stark contrast to the catalytic capability of its Co(II)
metalloradical complex, other transition metal complexes of TPP,
such as Fe(III), Mn(III), Zn(II), V(IV), Cu(II), Ni(II), and Cr(III) com-
plexes (Fig. 1), were found to be unable to activate aryloxysulfonyl
azide 2e effectively for aziridination of styrene (Table 2, entries 6–
12). It is evident that the metalloradical character of [Co(TPP)] is
key to the success of the catalytic aziridination process with the
azide (Scheme 1).
F3
C
Me
entry 7: 4eg; 49% yield
entry 8: 4eh; 80% yield
entry 9: 4aa; 82% yield entry 10: 4ai; 72% yield
O
S
O
O
S
O
O
N
O
N
O
N
S
O
O
Me
F
Me
entry 12: 4aj; 31% yield
entry 13: 4ad; 71% yield
entry 11: 4ac; 54% yield
O
S
O
O
O
S
O
N
O
N
S
O
N
O
O
F3C
entry 16: 4ah; 78% yield
Cl
entry 14: 4ae; 73% yield
Br
entry 15: 4af; 51% yield
The commercially available metalloradical catalyst [Co(TPP)]
was demonstrated to be effective in activating different aryloxysul-
fonyl azides at room temperature for radical aziridination. As sum-
marized in Table 3, in addition to 4-phenylphenoxysulfonyl azide
2e, phenoxysulfonyl azide 2a could be activated by [Co(TPP)] to
aziridinate styrene to give the corresponding aziridine 4aa in a
high yield as well (Table 3, entry 2). Various halogenated phenoxy-
sulfonyl azides, including chloro-, bromo-, and fluoro-substituted
azides (2b–d), could also be applied for [Co(TPP)]-catalyzed
aziridination of styrene, affording the corresponding aziridine
products 4ba, 4ca, and 4da, respectively, in excellent yields
(Table 3, entries 3–5).
To investigate the substrate scope of the Co(II)-based catalytic
system, 4-phenylphenoxysulfonyl azide (2e) and phenoxysulfonyl
azide (2a) were selected as representative nitrogen sources for
aziridination reactions of different olefins. As shown in Table 4,
the [Co(TPP)]-catalyzed room-temperature aziridination was
demonstrated to be suitable for a wide range of aromatic olefins
a Reaction conditions: azide 2 (0.1 mmol), olefin 3 (0.5 mmol), 4 Å MS (100 mg),
[Co(TPP)] (5 mol %), PhCl (1 mL), 20 h, rt.
Isolated yields.
b
with various steric and electronic properties. For example, styrene
derivatives with electron-donating substituents, including methyl
and tert-butyl groups, could serve as effective substrates for the
metalloradical aziridination, yielding the desired aziridines in high
yields (Table 4, entries 1–3 and entries 9–12). As well, fluoro-,
chloro-, and bromo-substituted styrenes underwent the aziridina-
tion reactions smoothly by [Co(TPP)], affording the corresponding
aziridine products effectively (Table 4, entries 4–7 and entries
13–15). Moreover, the electron-deficient 4-trifluoromethylstyrene
could also be aziridinated by both azides 2e and 2a to produce
the aziridines 4eh and 4ah, respectively, in good yields (Table 4,
entries
8 and 16). It was noted that the reactivity of the
[Co(TPP)]-catalyzed aziridination could be significantly influenced
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4
V. Subbarayan et al. / Tetrahedron Letters xxx (2015) xxx–xxx
2775–2823; (d) Jiang, H. L.; Zhang, X. P. Oxidation C–N Bond Formation by
Oxidation (Aziridines) In Comprehensive Chirality; Carreira, E. M., Yamamoto, H.,
Eds.; Elsevier: Amsterdam, 2012; Vol. 5, pp 168–182; e Cui, X.; Zhang, X. P.
Cobalt-Mediated Carbene Transfer Reactions. In Contemporary Carbene
Chemistry; Moss, R. A., Doyle, M. P., Eds.; John Wiley & Sons, 2013; pp 491–
525. Chapter 15.
by the steric hindrance of the aromatic olefins. While the aziridina-
tion reactions of para- and meta-substituted styrenes typically
gave good to excellent yields of the corresponding aziridines, more
sterically hindered ortho-substituted styrenes, such as 2-bromo-
and 2-methylstyrenes, could be only aziridinated by [Co(TPP)] in
moderate yields (Table 4).
2. For studies on the radical mechanism of [Co(Por)]-catalyzed olefin
aziridination, see: (a) Olivos Suarez, A. I.; Jiang, H. J.; Zhang, X. P.; de Bruin,
B. Dalton Trans. 2011, 40, 5697–5705; (b) Hopmann, K. H.; Ghosh, A. ACS Catal.
2011, 1, 597–600; For related studies on the radical mechanism of [Co(Por)]-
Conclusions
catalyzed C–H amination, including EPR observation of a-Co(III)-aminyl radical
(also known as Co(III)-nitrene radical) intermediates, see: (c) Lyaskovskyy, V.;
Suarez, A. I. O.; Lu, H. J.; Jiang, H. L.; Zhang, X. P.; de Bruin, B. J. Am. Chem. Soc.
2011, 133, 12264–12273; For detailed studies on the similar radical
In summary, we have identified aryloxysulfonyl azides
(ArOSO2N3) as a new type of nitrogen sources for radical aziridina-
tion by Co(II)-based metalloradical catalysis. Due to their high
reactivity, we have shown that aryloxysulfonyl azides can be even
activated by simple metalloradical catalyst [Co(TPP)] even at room
temperature to affect productive aziridination reactions of differ-
ent aromatic olefins. The [Co(TPP)]/ArOSO2N3-based aziridination
system has several practical attributes associated with metallo-
radical catalysis. In addition to the neutral and mild reaction con-
ditions, environmentally benign dinitrogen is the only byproduct
from the catalytic process. The commercial availability of [Co(TPP)]
should further enhance the practicality of this room-temperature
aziridination method. The resulting N-aryloxysulfonyl aziridines,
which can be readily converted to unprotected N–H aziridine
derivatives through facile removal of the aryloxysulfonyl groups,12
may find applications for the synthesis of nitrogen-containing
compounds of pharmaceutical importance.12b,12c,13 More broadly,
the demonstrated high reactivity of aryloxysulfonyl azides may sti-
mulate the exploration of their applications for the development of
mechanism involving
a-Co(III)-alkyl radical (also known as Co(III)-carbene
radical) intermediates, see: (d) Dzik, W. I.; Xu, X.; Zhang, X. P.; Reek, J. N. H.; de
Bruin, B. J. Am. Chem. Soc. 2010, 132, 10891–10902; (e) Belof, J. L.; Cioce, C. R.;
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8. For an example of the use of trichloroethyloxysulfonyl azide for metalloradical
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other Co(II)-based metalloradical processes where the
a-Co(III)-
aminyl radicals serve as the key intermediates (Scheme 1).
Acknowledgments
We are grateful for financial support by NSF (CHE-1152767)
and NIH (R01-GM098777).
Supplementary data
11. No significant improvements were observed when the reaction was conducted
in higher catalyst loadings, at elevated temperatures or using excess azides,
due to the limited solubility of [Co(TPP)] and decomposition of the azide.
12. (a) Cohen, S. B.; Halcomb, R. L. J. Am. Chem. Soc. 2002, 124, 2534–2543; (b)
Nicolaou, K. C.; Huang, X.; Snyder, S. A.; Bheema Rao, P.; Bella, M.; Reddy, M. V.
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.01.
186.
References and notes
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Please cite this article in press as: Subbarayan, V.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.01.186