Iron(iii) tetrakis(pentafluorophenyl)porpholactone catalyzes nitrogen atom transfer to CC and C-H bonds with organic azides

Article abstract of DOI:10.1039/c2dt11995a

We have demonstrated that iron porpholactones could be effective catalysts for nitrogen atom transfer reactions such as aziridination of alkenes and amidation of alkanes using organic azides.

Full text of DOI:10.1039/c2dt11995a

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Iron(III) tetrakis(pentafluorophenyl)porpholactone catalyzes nitrogen atom
transfer to C C and C–H bonds with organic azides†
Lei Liang, Hongbin Lv, Yi Yu, Peng Wang and Jun-Long Zhang*
Received 20th October 2011, Accepted 21st November 2011
DOI: 10.1039/c2dt11995a
We have demonstrated that iron porpholactones could be
effective catalysts for nitrogen atom transfer reactions such
as aziridination of alkenes and amidation of alkanes using
organic azides.
Nitrogen atom transfer reactions catalyzed by metallopor-
phyrins through metal nitrene intermediate(s) to C–H and C
C
bonds attract much attention in organic synthesis.4 For the choice
of nitrogen source, there has been a growing interest in the
use of organic azides, because the by-product is nitrogen gas
whereas the other nitrogen sources such as PhINTs, bromamine-
T or chloramine-T would lose PhI, NaCl and NaBr moieties.
Cenini,5 Che,6 and Zhang7 groups pioneered the development
of nitrene transfer reactions such as inter- and intramolecular
aziridination of alkenes, amidation of sulfides, and amidation of
alkanes with organic azides catalyzed by ruthenium, cobalt and
iron porphyrins. In this work, at the outset of the project, we aimed
to identify a suitable azide as the “nitrogen source” using styrene
and ethylbenzene as model substrates. Through optimization of
reaction conditions (Table S1, E◦SI†), typical reactions were carried
Porpholactones, as analogues of porphyrins, in which one pyrrole
is replaced by an oxazolone ring, are regarded as “intermediates”
to porphyrins and chlorins.1 With lactone moiety replacement,
partial saturation on porphyrin periphery results in restricted
p-conjugation and lowered molecular symmetry on energies of
the frontier molecular orbitals (MOs) predicted by an iterative
extended Hu¨ckel (IEH) calculation.1b Compared with porphyrins,
the energy of a porpholactone HOMO (the highest occupied
molecular orbital) is slightly lower, supported by the anodic
shift of the first oxidation potential of porpholactones rela-
tive to that of the corresponding porphyrins, indicating that
porpholactones are better at stabilizing the high valent metal
complexes than the corresponding porphyrins.2 However, the
effects of particular structures on catalysis remains unexplored,
although “porphyrin-like” properties had been recognized for
20 years.1b In contrast to various kinds of metalloporphyrin
catalyzed reactions, only two examples of oxidation of alkenes
and sulfides by metalloporpholactones have been reported.3
Thus, to expand the application of metalloporpholactones in
catalysis is of importance. Herein, we report the first example
of using iron porpholactone [Fe(F20-TPPL)Cl] (F20-TPPL = meso-
tetrakis(pentafluorophenyl)porpholactonato dianion) (Fig. 1) as
a catalyst for nitrogen atom transfer to C C and C–H bonds with
organic azides.
˚
out in 1,2-dichloroethane at 80 C in the presence of 4 A molecule
sieves for 12 h. We found that only p-toluenesulfonyl azide (TsN3)
and methylsulfonyl azide (MsN3) allowed for aziridination of
styrene and gave the corresponding aziridines in 87 and 61%
yield (Table 1, entries 1 and 2, respectively). For ethylbenzene,
only p-nitrophenyl azide is effective for amidation of a saturated
C–H bond (Table 1, entry 10). Using PhINTs as the nitrogen
source, [Fe(F20-TPPL)Cl] shows no reactivity towards styrene and
ethylbenzene (Table 1, entries 7 and 14). Control experiments in
the absence of [Fe(F20-TPPL)Cl] or only using FeCl3 as catalysts
were carried out.8 As shown in Table S2,† for the aziridination
of styrene, without metal salts or only using FeCl3, we isolated
styrene aziridine in yields of 30 and 52%, respectively, which are
lower than that obtained by [Fe(F20-TPPL)Cl]. For amidation
of ethylbenzene, control experiments could not afford the amide
product (Table S3, ESI†). In addition, we found sunlight to have
a negligible effect on these reactions.
To investigate the ligand effect on the reactivity of N atom trans-
fer to alkenes and alkanes, we chose [Fe(F20-TPP)Cl] and [Fe(F15-
TPC)] (F20-TPP = meso-tetrakis(pentafluorophenyl)porphyrino
dianion and F15-TPC = meso-tris(pentafluoro-phenyl)corrolato
trianion) as the catalysts and styrene, p-chlorostyrene and ethyl-
benzene as the substrates. As shown in Table S4,† [Fe(F20-
TPPL)Cl] exhibited higher reactivity (88, 80 and 88%, entries 1,
5 and 8, respectively) than [Fe(F20-TPP)Cl] and [Fe(F15-TPC)]
(entries 2, 3, 6, 7, 9 and 10). Replacing iron with manganese
resulted in a decrease of the reactivity of aziridination of styrene
to 53% yield (entry 4, Table S4, ESI†). A time course plot (Fig.
S1, ESI†) for the aziridination of styrene by [Fe(F20-TPPL)Cl] and
Fig. 1 Iron porphorinoid catalysts used in this work.
Beijing National Laboratory for Molecular Sciences, State Key Laboratory
of Rare Earth Materials Chemistry and Applications, College of Chemistry
and Molecular Engineering, Peking University, Beijing, 100871, P.R. China.
E-mail: zhangjunlong@pku.edu.cn; Fax: +86-10-62767034
† Electronic supplementary information (ESI) available: Experimental
procedures and characterization and spectral reproduction for new
compounds. See DOI: 10.1039/c2dt11995a
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Table 2 Aziridination of alkenes with TsN3 by [Fe(F20-TPPL)Cl]a
Table 1 Organic azides for catalytic aziridination of styrene and amida-
tion of ethylbenzene by [Fe(F20-TPPL)Cl]a
Entry
1
Alkene
Product
Yield (%)b
87
Entry Azide
1
Sub.
Yield (%)b Entry Sub.
Yield (%)b
styrene 87
styrene 61
styrene n.dc
8
ethylbenzene 0
2
3
81
87
2
9
ethylbenzene 0
3
4
10
11
ethylbenzene 88
ethylbenzene 0
styrene
0
4
5
6
7
88
5
6
styrene
styrene
0
0
12
13
ethylbenzene 0
ethylbenzene 0
80
89
7
PhINTs
styrene
0
14
ethylbenzene 0
a
˚
Performed under N2 in the presence of 4 A molecule sieves: [substrate] =
1 mmol mL-1; 1 mol% catalyst based on azide; styrene/azide = 3 : 1,
ethylbenzene/azide = 5 : 1 in 1,2-dichloroethane at 80 ◦C. b Isolated yields
based on the conversion of azides. c n.d. is not determined.
<20
[Fe(F20-TPP)Cl] shows a higher reaction rate (TOFs: 0.83 vs. 0.16
min-1) obtained by [Fe(F20-TPPL)Cl].
8
75
The scope of alkenes for a [Fe(F20-TPPL)Cl]/TsN3-based cat-
alytic system was investigated and the results are summarized in
Table 2. For p-substituted styrene, except for p-methoxystyrene,
the corresponding aziridines were obtained in isolated yields
of 75–89% (Table 2, entries 1–6). When p-methoxystyrene was
employed as the substrate, we could not isolate the aziridine
product but instead a tetrahydropyrrole product was isolated in
low yield (<20%, Table 2, entry 7), which was first observed in
metal catalyzed aziridination of alkenes using organic azides. We
attempted to use p-N,N-dimethyl styrene as the substrate, but
only observed a polymeric product. Using 3,5-dimethoxystyrene
and m-methoxystyrene as the substrate gave the corresponding
aziridines in moderate to low yield (75% to less than 20%, Table 2,
entries 8 and 9), indicating the electron donating group and steric
effect on styrene affects the formation of aziridines and consequent
reactions. The other terminal alkenes such as 2-vinylnaphthalene
and allylbenzene gave the aziridines in good yields (Table 2, entries
10 and 11), whereas a-methylstyrene give the allylic amidation
product and no aziridine was detected (82% yield, Table 2, entry
12), which is in accord with previously reported literature using
[Fe(F20-TPP)Cl] as the catalyst.6a In addition, dihydronaphthalene
with an intramolecular double bond was used and the aziridine
product was isolated in moderate yield (58%, Table 2, entry 13).
The catalytic amidation of saturated C–H bonds was also
investigated. As described above, p-nitrophenyl azide was used
as the nitrogen source reagent and p-nitro-N-(1-phenylthyl) ben-
zenamine was obtained in 88% yield with complete azide con-
sumption. Using 4-nitrophenyl azide, the benzylic C–H bonds of
tetralin, diphenylmethane and fluorene were successfully aminated
in good yields (85–91%, Table 3, entries 2–4). Chemoselective
allylic amidation of 1-phenyl cyclohexene was observed (the ratio
of 3- to 6-amines is 1.7 : 1, Table 3, entry 5) and [Fe(F20-TPPL)Cl]
9
<20c
10
81
11
12
84
82
13
58
a Performed under N2 in the presence of
4
molecule sieves:
˚
A
alkenes/azide = 3 : 1, 0.5 mol% catalyst (based on azide). b Isolated
yields based on the conversion of azides. c Tetrahydropyrrole(<10%) was
detected.
prefers to catalyze the amidation of the 3-position of cyclohexene.6a
Furthermore, amidation of the unfunctionalized saturated C–H
bonds such as cycloheptane, cyclooctane, adamantane and decalin
were accomplished in moderate to good yields (71–80%, Table 3,
entries 6–9). For adamatane and decalin with both of 2◦ and 3◦ C–
H bonds, only amidation of 3◦ C–H bond products were obtained,
which is similar to the previous observation in amidation of
alkanes using metalloporphyrins.5,6,7a It is noteworthy that [Fe(F20-
TPPL)Cl] is also competent in the amidation of thioanisole with
high yield (Table 3, entry 10).
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Table 3 Amidation of saturated C–H bonds by [Fe(F20-TPPL)Cl]a
Entry
1
Alkane
Product
Yield (%)b
88
2
3
91
85
Fig. 2 [Fe(F20-TPPL)Cl] catalyzed aziridination of C60.
Fig. 3 are proposed: 1) ring-opening of the aziridine catalyzed by
[Fe(F20-TPPL)Cl] followed by nucleophilic attack with activated
sp2 C–H bond, which is known for Lewis acids such as Ag or Cu
complexes;11 2) generating the “NR¢–CH2–CHR∑” radical before
formation of the aziridine, which then reacts with another styrene
and then ring closure occurs to form tetrahydropyrrole. For route
1, direct reaction of aziridine and styrene in the presence of
iron porpholactone results in the tetrahydropyrrole in trace yield
(<5%), indicating route 1 might not be the mechanism. According
to our experiments, only p- and m-methoxystyrene as substrates
form tetrahydropyrrole and p- and m-methoxy substituents might
stabilize the possible radicals such as the (NR¢–CH2–CHR∑). In
addition, TEMPO was added as a radical quencher and no product
is detected, indicating the formation of radicals for this reaction is
important.
4
5
90
28
51
6
7
71
78
8
9
80
72
10
85
a
˚
Performed under N2 in the presence of 4 A molecule sieves: substrate =
0.5 mmol mL-1; substrate/azide = 5 : 1, 0.5 mol% catalyst (based on azide).
b Isolated yields based on the conversion of azides.
Fig. 3 Possible mechanisms for tetrahydropyrrole formation in the
presence of [Fe(F20-TPPL)Cl].
Thus, we propose route 2 might be the possible mechanism. Fur-
ther investigations for the catalytic tetrahydropyrrole formations
and mechanistic study are undergoing.
This project was supported by NKBRSFC (grant no.
2010CB912302) and NSFC (grant no.20971007). Thanks to Prof.
Z. Shi for fullerene supply.
To demonstrate the potential application of [Fe(F20-TPPL)Cl] in
organic synthesis, we used fullerene as the substrate to examine the
ability to produce C60 aziridine, which is an important intermediate
for further functionalization of fullerenes as useful optical and
electromaterials. Under the catalytic reaction conditions described
in Fig. 2, we isolated C60 aziridine in 43% yield, which was
comparable to that obtained with CuCl as catalyst (10 mol%
loading) and using PhINTs as the nitrogen source.9
In general, metalloporphyrins which catalyze N atom transfer
to C C and C–H bonds are associated with a reactive metal-
imido/nitrene key intermediate.4 The lower energy of the porpho-
lactone HOMO and the better stability of high valent Fe than
porphyrin,10 which imposes a radical-type reactivity to the “ni-
trene” moiety, led to the different reactivity for iron porpholactone
and porphyrin. For the formation of tetrahydropyrrole products
(Table 2, entries 7 and 9), two possible mechanisms shown in
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