Interweaving Visible-Light and Iron Catalysis for Nitrene Formation and Transformation with Dioxazolones

Article abstract of DOI:10.1002/anie.202016234

Herein, visible-light-driven iron-catalyzed nitrene transfer reactions with dioxazolones for intermolecular C(sp³)-N, N=S, and N=P bond formation are described. These reactions occur with exogenous-ligand-free process and feature satisfactory to excellent yields (up to 99 %), an ample substrate scope (109 examples) under mild reaction conditions. In contrast to intramolecular C?H amidations strategies, an intermolecular regioselective C?H amidation via visible-light-induced nitrene transfer reactions is devised. Mechanistic studies indicate that the reaction proceeds via a radical pathway. Computational studies show that the decarboxylation of dioxazolone depends on the conversion of ground sextet state dioxazolone-bounding iron species to quartet spin state via visible-light irradiation.

Full text of DOI:10.1002/anie.202016234

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How to cite:
International Edition: doi.org/10.1002/anie.202016234
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Homogeneous Catalysis
Hot Paper
Interweaving Visible-Light and Iron Catalysis for Nitrene Formation
and Transformation with Dioxazolones
Jing-Jing Tang, Xiaoqiang Yu,* Yi Wang, Yoshinori Yamamoto, and Ming Bao*
Abstract: Herein, visible-light-driven iron-catalyzed nitrene
transfer reactions with dioxazolones for intermolecular C(sp3)-
=
=
N, N S, and N P bond formation are described. These
reactions occur with exogenous-ligand-free process and feature
satisfactory to excellent yields (up to 99%), an ample substrate
scope (109 examples) under mild reaction conditions. In
À
contrast to intramolecular C H amidations strategies, an
À
intermolecular regioselective C H amidation via visible-
light-induced nitrene transfer reactions is devised. Mechanistic
studies indicate that the reaction proceeds via a radical path-
way. Computational studies show that the decarboxylation of
dioxazolone depends on the conversion of ground sextet state
dioxazolone-bounding iron species to quartet spin state via
visible-light irradiation.
Introduction
Amides are widespread structural units in nature, since
the amide bond is the core linkage of natural proteins and
peptides.[1] Thus, much effort has been devoted to the
development of efficient synthetic approaches to introduce
amide groups into molecules.[2] The nitrene transformation
reaction assisted by transition metal has emerged as an
attractive approach for constructing amide compounds due to
its high activities.[3] However, traditional nitrene precursors,
such as organic azides[4] and iminoiodinanes,[5] are limited by
safety problems, high cost, and poor atom economy. In
contrast, dioxazolones[6] have been developed smoothly in
organic chemistry because they are easy to handle, readily
available, and have bench-top stable features. The seminal
work in catalytic nitrene transformation with dioxazolones for
C N and N S bond construction was reported by Bolm[7] and
Chang[8] groups. While precious transition metals such as
iridium,[9] rhodium,[10] and ruthenium[11] complexes showed
high catalytic activity in nitrene transfer reactions with
dioxazolones, earth-abundant first-row metals[12] are in their
infancy (Scheme 1a).
À
=
Scheme 1. Transition-metal-catalyzed nitrene formation and transfor-
mation with dioxazolones.
Among all the available transition metals, iron[13] is
generally regarded as one of the most abundant, inexpensive,
benign, and relatively nontoxic metals. Diverse nitrene trans-
fer reactions with organic azides[14] and iminoiodinanes[15]
have been effectively promoted by iron catalysts. Several
representative works were independently reported by Che,[16]
Arnold,[17] and Betley.[18] During the preparation of this
[*] J.-J. Tang, Prof. X. Yu, Y. Wang, Prof. Y. Yamamoto, Prof. M. Bao
State Key Laboratory of Fine Chemicals
Dalian University of Technology
manuscript, Chang and co-workers[19] reported (Phthalocya-
Dalian 116023 (China)
E-mail: yuxiaoqiang@dlut.edu.cn
mingbao@dlut.edu.cn
III
À
nine)Fe Cl-catalyzed intramolecular C H amidation toward
g-lactam synthesis with dioxazolones, which showed high
turnover numbers under aerobic conditions, and the spin
interconversion of the iron species was tuned by substrate
coordination. Undoubtedly, realizing efficient, general, and
simple iron-catalyzed nitrene transfer reactions with dioxa-
zolones under mild conditions would be highly appealing.
Prof. Y. Yamamoto
WPI-Advanced Institute for Materials Research
Tohoku University, Sendai 980-8577 (Japan)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016234.
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Table 1: Optimization of the reaction conditions.[a]
However, theoretical calculation based on ground sextet state
FeCl3 and phenyl substituted dioxazolone 1a suggested that
a high activation barrier (DG = 29.9 kcalmolÀ1) is required in
decarboxylation by iron species 6TS1 (Scheme 1b), which was
used to be carried out under a high temperature. In general,
acylnitrene is easily decomposed and forms isocyanate via
Curtius-type rearrangement[20] under thermal conditions due
to its intrinsic instability. However, theoretical calculations
showed a lower barrier of CO2 extrusion in the quartet spin
state surface (4TS1) (DG = 22.0 kcalmolÀ1), although the
energy of dioxazolone-bounding iron intermediate 4INT1
Entry
Catalyst
Solvent
Wavelength [nm]
Yield [%][a]
1
2
3
4
5
6
7
8
FeCl3
FeBr3
FeF3
DCM
DCM
DCM
DCM
DCM
DCM
DCM
Toluene
DCE
450
450
450
450
450
450
450
450
450
450
450
450
365
400
500
450
–
61
17
32
ND
38
7
ND
18
31
ND
56
trace
40
42
trace
73
trace
trace
18
Fe(acac)3
FeCl2
FeSO4
FeIIPc
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
–
6
was much higher than that of INT1.
Inspired by the rapidly developed photochemistry,[21] we
hypothesized that if the ground state dioxazolone-bounding
iron complex could be activated by visible-light irradiation,
the formed higher active species could suffer an effective
interconversion of spin state, that is, dioxazolone-bounding
iron species 6INT1 would be easily converted to species 4INT1
by visible-light irradiation. Thus, the decarboxylation of
species 4TS1 would spontaneously occur to form Fe-acyl
nitrene intermediate 4INT3, and the following transformation
furnished various amidation and imidation products (Sche-
me 1c). Herein, we reported an efficient method that visible
9
10
11
12
13
14
15
16[b]
17[c]
18[c,d]
19
DMF
1,4-Dioxane
MeCN
DCM
DCM
DCM
DCM
DCM
DCM
DCM
–
450
light induced and simple iron catalyzed nitrene transforma-
3
=
tion with dioxazolones for intermolecular C(sp )-N, N S, and
[a] Reaction conditions: 1a (0.2 mmol), 2a (1.5 equiv), Cat. (10 mol%)
and Sol. (3.0 mL) at 408C under light irradiation for 16 h, yield of isolated
product. [b] Cat. (15 mol%). [c] Without light irradiation. [d] 1208C. See
Supporting Information for full details. Entry 16 as standard conditions.
=
N P bond formation.
Results and Discussion
15 mol% loading of FeCl3 to the reaction (Entry 16). Control
À
Considered the precedent works and guided by theoret-
ical calculation, we initially investigated the feasibility of our
reactions confirm that only a trace of C H amidation product
3aa was detected at 408C or 1208C in the absence visible
light. When the reaction was carried out in the absence of
FeCl3, 3aa was obtained in 18% yield (entry 19). Therefore,
À
hypothesis. Our studies commenced by probing the C H
amidation of 1,3-diphenylpropanedione 2a with 1a in the
presence of simple iron salt and visible light. The high
amidation efficiency of such 1,3-dicarbonyl substrates re-
mained challenging using the existing methods.[22] Then, 2a
(1.5 equiv) and iron catalyst (10 mol%) were added to
a solution of 1a in dichloromethane (DCM), and the mixture
was stirred at 408C for 16 h under blue LEDs (450 nm, 10 W)
irradiation. Several iron salts, including FeCl3, FeBr3, FeF3,
Fe(acac)3, FeCl2, and FeSO4 were examined (Table 1, En-
tries 1–6). FeCl3 exhibited a high activity to furnish amidation
product 3aa in 61% yield (Entry 1 vs. Entries 2–6). No
desired product was observed in the presence of FeIIPc
À
the intermolecular C H amidation reaction of 1,3-dicarbonyl
substrate 2 was performed by using FeCl3 as catalyst in DCM
at 408C for 16 h under blue LEDs (450 nm, 10 W) irradiation.
Given the optimized reaction conditions, the scope and
À
limitation of this type of C H amidation reactions were
explored, and the results are summarized in Scheme 2.
Table 1, Entry 16 describes that the desired product 3aa was
obtained in 73% yield. The reactions of 1,3-diphenylpropa-
nediones (2b and 2c) bearing a MeO- or Br- substituents on
the para positions of the benzene rings provided the
corresponding products 3ab and 3ac in 86% and 65% yield,
respectively. Furthermore, phenyl b-keto esters could be
successfully involved under the standard conditions, affording
catalyst, which showed a good catalytic activity in intra-
[19]
À
molecular C H amidation toward g-lactam synthesis (En-
À
À
try 7). Intermolecular C H amidation reaction would be
the C H amidation products with moderate yields (3ad–3af
suppressed after adding the bidentate nitrogen ligands
(Table S1 Entries 27 and 28 in Supporting Information for
details). Then, the solvents were screened. Compared with
DCM, 1,4-dioxane had a slightly lower yield, toluene was
notably less effective, 1,2-dichloroethane (DCE) produced
a low yield, and N,N-dimethylformamide (DMF) and MeCN
showed nearly no reactivity (Entries 8–12). A series of
controls of wavelengths (365, 400, and 500 nm) was then
carried out, and lower yields or traces of the product were
and 3bd). Unexpectedly, ethyl 2-oxocyclohexanecarboxylate
(2g) bearing large steric hindrance group could also undergo
the reaction to construct a quaternary carbon center product
(3ag) with a lower yield (46%). Thiophene dioxazolone (1q)
or furan dioxazolone (1r) were also examined in the present
method, and corresponding products 3qd, 3qe, and 3rd were
obtained in 47%, 53%, and 33% yields, respectively. The
reactions of methyl substituted dioxazolone (1s) also pro-
À
ceeded smoothly to afford C H amidation products (3sb–3se,
À
formed (Entries 13–15). The yield of C H amidation product
and 3sh) with moderate yields.
3aa could be further improved to 73% with the addition of
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Scheme 3. Scope of visible-light-induced iron-catalyzed intermolecular
amidation of diphenyl methane substrates. [a] Reaction conditions:
1 (0.2 mmol), 4 (4.0 equiv), FeCl3 (15 mol%), AgNTf2 (15 mol%) and
DCM (5.0 mL) at 408C under 10 W blue LEDs irradiation
(lmax =450 nm) for 16 h, yield of isolated product. [b] 4 (2.0 equiv),
without additive.
yield. Mono- and di-substituted diphenyl methane (4b and
À
4d) underwent this C H amidation reaction to give 5ab and
5ad with moderate to high yields. Product 5ac carrying an
iodo functional group was formed in poor yield. Isochroman
could also be involved, providing 5ae in 74% yield without
any silver additive. The reaction of Me-substituted dioxazo-
lones (1s) resulted in no reaction and 1s was recovered in
90% yield (see Scheme S1 in Supporting Information).
Further, other coupling partners such as toluene, ethylben-
zene, and allylbenzene also give no desired product under the
photo-induced condition.
Scheme 2. Scope of visible-light-induced iron-catalyzed intermolecular
amidation of 1,3-dicarbonyl compounds. [a] Reaction conditions:
1 (0.2 mmol), 2 (1.5 equiv), FeCl3 (15 mol%) and DCM (3.0 mL) at
408C under 10 W blue LEDs irradiation (lmax =450 nm) for 16 h, yield
of isolated product. [b] 2 (2.0 equiv).
Sulfimide and sulfoximine structural motifs have fre-
quently been found in the frameworks of various pharma-
ceuticals.[23] In 2014, Bolm and co-workers elegantly reported
pioneering advances in nitrene transfer reaction for the
synthesis of N-acyl sulfimides and sulfoximines with dioxa-
zolones via a light-induced catalytic method, in which
a ruthenium N-acyl nitrene was proposed as a key interme-
diate.[7a] Considering mechanistic exploration and preliminary
results, we anticipated that nitrene transformation of sulfur
substrates with dioxazolones might be realized in the
presence of a simple iron catalyst. It was found that the
reaction of sulfoxides with dioxazolones proceeded smoothly
by using FeCl3 (10 mol%) as catalyst in DCM at 408C for 12 h
under blue LEDs (450 nm, 10 W) irradiation. Scheme 4
Inspired by the preliminary results, the scope of various
diphenylmethane derivatives was next examined (Scheme 3).
When the reaction of 1a with diphenylmethane 4a was
carried out under standard conditions (Table 1, Entry 16), the
À
desired C H amidation product 5aa was obtained in only
26% yield. The yield of 5aa was remarkably increased (71%
and 74%) when silver salts AgSbF6 (15 mol%) or AgNTf2
(15 mol%) were added to the model reaction individually
(Table S2, Entries 35 and 36, see Supporting Information for
details). The role of silver salts was converted the neutral iron
catalyst to the putative highly reactive iron cationic species.[8b]
À
Thus, the C H amidation reactions of 4a with various
dioxazolones were carried out in the presence of AgNTf2
(15 mol%) additive. The results show that aryl substituted
t
showed that substituted dioxazolones bearing Me, Bu, and
t
dioxazolones bearing Me, Bu, MeO, CO2Me, CF3, F, Cl, and
MeO on benzene rings at the para position resulted in
moderate yields of corresponding products 7ba–7da, while
substrate 1 f bearing an electron-withdrawing group (CF3) on
the phenyl ring at the para position afforded the desired
products 7 fa in 83% yield. These results indicate that the
Br groups on benzene rings provided the corresponding
products 5aa–5pa in moderate to satisfactory yields. Notably,
thiophen substituted dioxazolone 1q was also suitable for the
present method to afford the desired product 5qa in 64%
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Scheme 4. Scope of visible-light-induced iron-catalyzed imidization of
sulfoxides. [a] Reaction conditions: 1 (0.2 mmol), 6 (1.2 equiv), FeCl3
(10 mol%) and DCM (3.0 mL) at 408C, under 10 W blue LEDs
irradiation (lmax =450 nm) for 12 h, yield of isolated product.
Scheme 5. Scope of visible-light-induced iron-catalyzed imidization of
sulfides. [a] Reaction conditions: 1 (0.2 mmol), 8 (1.2 equiv), FeBr3
(5 mol%) and DCM (3.0 mL) at 408C under 10 W blue LEDs irradi-
ation (lmax =450 nm) for 12 h, yield of isolated product.
electronic property of the substituent linked to the phenyl
ring influenced the reactivity of aryl substituted dioxazolone.
Many versatile functional groups on phenyl moieties of
dioxazolones, such as halides (F, Cl, and Br), were all suitable
in this transformation, which afforded the corresponding
products (7ga–7ia, 7ma, and 7na) in satisfactory to good
yields. Thiophen substituted dioxazolone 1q was also suitable
for the present method to afford desired product 7qa in 79%
yield. Alkyl substituted dioxazolones were also examined,
and corresponding products 7sa and 7ta were obtained in
91% and 79% yields, respectively. Next, sulfoxide substrates
were also studied. Alkyl substituted sulfoxides 6d and 6e also
worked very well to furnish the corresponding sulfoximines
7ad and 7ae in satisfactory yields.
Having successfully demonstrated the versality of our
approach with respect to sulfoxides and its tolerance toward
various functional groups, the developed reaction protocol
was next applied to sulfide 8 (Scheme 5). A 73% yield of
desired sulfimide 9aa was obtained when the reaction of 1a
with methyl(phenyl)sulfane 8a was carried out with FeCl3
(5 mol%) catalyst under blue LEDs (450 nm, 10 W) irradi-
ation. Further screening of iron catalysts showed that FeBr3
was the most activated catalyst for the sulfide imidation and
afforded 9aa in 98% yield (Table S4, Entry 3, see Supporting
Information for details). Scheme 5 showed that substrates
containing electron-neutral, electron-donating and electron-
deficient groups (H, Me, MeO, tBu, F, Cl, Br, and CF3) at the
ortho, meta, and para positions of the aryl substituted
dioxazolones (1a–1o) successfully generated the desired
products 9aa–9oa in excellent to moderate yields (98–
60%). Moreover, dioxazolone bearing heterocycle, such as
thiophene moiety (1q) was suitable for furnishing the desired
product 9qa in 58% yield. Alkyl substituted dioxazolones 1s
and 1t were tested, and corresponding sulfimides 9sa and 9ta
were obtained in 84% and 83% yields, respectively. Next,
various sulfide substrates were examined using the present
method. Thioanisole bearing Me, MeO, NO2, and F groups on
benzene rings provided the corresponding products 9ab–9ae
in satisfactory yields. Phenyl thioether containing alkyl (Et,
Cyp, tBu, and Bn) groups proceed smoothly to afford 9af–9ai
in moderate yields. The reaction conditions were also suitable
for diphenyl sulfide, thus providing the corresponding prod-
uct 9aj in 62% yield.
Phosphinimidic amides have been widely applied in
biochemistry and coordination of transition metals.[24] We
anticipated that the present method could also be suitable for
the construction of iminophosphoranes (Scheme 6). The
desired product 11aa was obtained practically in a quantita-
tive yield with FeCl3 (5 mol%) as catalyst in DCM for only
15 minutes (Table S5, entry 1). Reactions of dioxazolones
t
(1a–1n) bearing electron-donating groups (Me, Bu, and
MeO), electron-withdrawing (CF3) and halides (F, Cl, and Br)
with 10a efficiently produced the desired products (11aa–
11na) in good to excellent yields (98–80%). Heteroaromatic
substrates 1q and 1r also showed a high reactivity to afford
11qa and 11ra in 90% and 97% yields, respectively.
Remarkably, this transformation was not only limited to
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Scheme 6. Scope of visible-light-induced iron-catalyzed imidization of
phosphines. [a] Reaction conditions: 1 (0.2 mmol), 10 (1.0 equiv),
FeCl3 (5 mol%) and DCM (1.0 mL) at room temperature under 10 W
blue LEDs irradiation (lmax =450 nm) for 15 min, yield of isolated
product. [b] 30 min. [c] 1 (0.4 mmol), 10 (0.2 mmol), 30 min.
dioxazolones bearing arenes, as alkyl substituted dioxazo-
lones 1s and 1t were also tolerated in this reaction, generating
the desired products 11sa and 11ta in excellent yields. Next,
various phosphorus substrates were examined in the nitrene
transformation. Reactions of 1a with triphenylphosphine
derivatives (10b–10h), possessing Me, MeO, NMe2, F, and Cl
functional groups also exhibited high effectiveness (98–78%).
Notably, 1,2-bis(diphenylphosphino)ethane (DPPE) and 1,3-
bis(diphenyphosphino)propane (DPPP) containing two phos-
phorus atoms were successfully converted into phosphinimi-
dic amides 11ai and 11aj in 89% and 92% yield, respectively.
X-Ray crystal structure analysis of 11ai enabled the deter-
mination of the product. Reactions of trimethylphosphite
(10k), triethylphosphite (10l), and triisopropyl phosphite
(10m) with 1a were also tolerated and afforded the corre-
sponding desired products 11ak–11am in moderate to good
yields (60–80%).
Scheme 7. Mechanistic studies.
ment. Moderate KIE values were observed from intra- and
À
À
intermolecular competition of C H and C D amidation (kH/
kD = 1.1 and 1.4, respectively, Scheme 7a). This result dem-
onstrated that the HAA step was not the rate-limiting step,
which was consistent with the result by the DFT calculation.
Then, intermolecular amidations of p-methyl benzamide 12
À
with 2d was performed to measure whether C N bond
formation occurs through an amide intermediate (Sche-
me 7b). In this manner, 3bd was not obtained based on
HPLC analysis. This result indicates that amide species is
À
about not a potential intermediate for C N bond formation.
Inter and intramolecular competitive experiments were
performed to shed light on the nature of metallonitrene
species (Scheme 7c). When the reaction was carried out
under the standard reaction conditions, phenyl-dioxazolone
1a was reacted with 1,3-dicarbonyl 2a preferentially. Fur-
After the successful development of transformations, we
then turned our attention to characterizing the reaction
mechanism (Scheme 7, see the Supporting Information for
full details). Firstly, we performed an isotope labeling experi-
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thermore, the radical nature of this nitrene transformation
sponding quartet state. Based on our findings, a dioxazolone-
was confirmed by the employment of radical-trapping re-
agent. When 3.0 equiv of butylated hydroxytoluene (BHT)
was added as an external radical scavenger, the target
reaction was completely inhibited, and a BHT-trapped acyl
nitrene adduct A was obtained in 9% yield (Scheme 7d). The
reaction was also conducted by adding 3.0 equiv of 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) under the optimized
reaction, and the TEMPO-trapped acyl nitrene adducts were
detected by LC-HRMS with cluster peaks at 277.1905 and
432.3218 m/z. LC-HRMS analysis also showed a TEMPO-
trapped 1,3-dicarbonyl substrate adduct with a cluster peak at
348.2166 m/z. This result suggested that the visible-light
bounding iron intermediate was postulated to be formed. The
density functional theory (DFT) displayed that the coordina-
tion of dioxazolone to the unsaturated iron metal center was
exothermic by 5.9 kcalmolÀ1, and the Fe/N species spin state
was kept simultaneously. In the sextet spin state surface, the
activation energy was calculated to generate the key acylni-
6
trene iron intermediate via a transition state TS1, in which
the bond distances of the N-O and C-O to be cleaved were
lengthened to 1.87 ꢀ and 2.13 ꢀ, respectively. The predicted
energy barrier of dissociation of CO2 from sextet spin state
6INT1 was very high (29.9 kcalmolÀ1) via a concerted (single
step) decarboxylation (Figure 2).
À
induced iron-catalyzed C H amidation may involve a radical
Subsequently, the absorption properties of the iron
species INT1 was studied. The calculation of Fe complex
6
pathway. Taking the results together, we speculated that the
nature of the iron nitrene species was more likely with
a radical character, although the formation of such eletro-
philic metallonitrene species cannot be ruled out.
showed an absorption at l = 449 nm (oscillator strength of
0.007). These results agreed with the wavelength tolerance of
the actual operating process. Based on our findings, the iron
species 6INT1 was further studied for the photoredox nitrene
transformation. Visible-light excitation of the iron species
6INT1 would generate the excited-state 6INT1* complex.
Afterward, the electrons of the excited-state 6INT1* complex
underwent spin reversal to obtain a quartet state iron species
4INT1 with an energy of 32.8 kcalmolÀ1 compared with the
ground state FeCl3. Furthermore, a lower barrier of CO2
The optical absorption for dioxazolone and FeCl3 was
tested by UV/Vis spectroscopy to further confirm our
suspicion (Figure 1, left). Based on 1a light-absorption
feature (lmax < 300 nm), utilizing visible light to drive photo-
catalytic nitrene transfer reaction will likely be a key chal-
lenge. The UV/Vis absorbance spectra showed that compared
with pristine FeCl3 (lmax ꢀ 430 nm), the gradual red-shift trend
was observed in the visible-light region when FeCl3 and 1a
were combined. This change was proposed to result from the
absorption of the dioxazolone-bounding iron species.
4
extrusion (22.0 kcalmolÀ1) in the quartet spin state via TS1
was observed. Unexpectedly, unlike sextet state iron species
6TS1, decarboxylation of iron species 4TS1 proceeded via
À
Although we failed to isolate the Fe/N complex, optical
absorption studies strongly supported the initial formation of
a photochemically competent complex generated by FeCl3
and dioxazolone. Next, the influence of light irradiation for
the reaction was explored. Then, the light on/off experiment
(Figure 1, right) based on 1,3-diphenylpropanedione 2a
a stepwise pathway. First, the N O bond of high active species
4TS1 was cleaved to afford INT2, and this process was
À1
À
calculated to be exergonic by 10.2 kcalmol . Next, C O
bond cleavage of the active INT2 occurred through a tran-
sition state 4TS2, which was exothermic by 2.0 kcalmolÀ1
.
Then, the CO2 escaped from 4TS2 to furnish acylnitrene iron
intermediate 4INT3, which was thermodynamically permitted
(DG = À40.1 kcalmolÀ1). Therefore, the CO2 extrusion in the
quartet spin state is considered favorable over the sextet spin
state surface.
À
revealed that the C N bond construction efficiently took
place under light irradiation and suppressed in the absence of
light, which suggests the importance of visible-light irradi-
ation. Moreover, we obtained the quantum yield of F = 0.83.
These findings support a mechanism that does not involve
a light-initiated radical chain mechanism.
The behavior of experiment supports that mechanism
involves radical intermediates. Computational studies also
showed that acylnitrene iron intermediate INT3 abstracted
4
À
Computational studies were performed by C H amida-
tion to gain an in-depth understanding on the reaction
mechanism (Figure 2, see the Supporting Information for
details). 1a and FeCl3 were selected as a model substrate and
catalyst for this theoretical study. Theoretical calculation
showed that the sextet state of FeCl3 was the ground state,
which was 38.5 kcalmolÀ1 lower in energy than the corre-
an H-atom via one electron pathway. The calculated energy
barrier for the hydrgen-atom abstraction (HAA) pathway
was 14.0 kcalmolÀ1 via a transition state TS3. Next, a more
4
stable sextet state iron complex 6INT4 generated from erratic
4TS3 is found to be the ground state, which is 18.0 kcalmolÀ1
lower in energy than the corresponding quartet spin state
(6INT4), and this process was calculated to be exergonic by
31.7 kcalmolÀ1. As the last stage of the catalysis process,
6
a radical recombination pathway to the presupposed INT4
occurred (DG = 8.2 kcalmolÀ1), thereby generating product-
6
4
catalyst adduct INT5 via TS4, which was very exothermic
(DG = À70.3 kcalmolÀ1).
DFT calculations revealed that the energy barrier for
HAA process was lower than the corresponding decarbox-
ylation, suggesting that the decarboxylation is more challeng-
ing. The calculated results also show that the decarboxylation
and HAA pathway took place in the quartet spin state,
Figure 1. UV/Vis absorption spectra (left) and light on/off experiment
(right).
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Figure 2. Energy profiles (in kcalmolÀ1) for Fe-catalyzed nitrene transformation. Bond lengths are shown in ꢀ. The excited state is calculated via
TD-DFT/M06/6–311+G(d,p)/SDD, CPCM(DCM) for INT1(sextet).
whereas radical recombination pathway occurred in the sextet
spin state surface.
Based on these control experiments and calculations, the
following mechanism of iron-catalyzed amidation with diox-
azolones is proposed (Scheme 8). The first step is the initial
6
formation of the sextet state precatalyst iron species INT1
from reaction of FeCl3 with dioxazolone, which is excited by
visible light to form excited species 6INT1*. Then, the
6
activated INT1* undergoes spin reversal to afford a quartet
4
state iron species INT1. Subsequently, the putative highly
reactive iron acyl nitrene intermediate 4INT3 could easily be
generated with releasing CO2 by a stepwise pathway via
4INT2. Next, iron-nitrene/imido complex 4INT3 abstracts
À
a hydrogen atom from C H bonds to generate the more
stable sextet state iron complex INT4, followed by a rapid
6
radical recombination to deliver the product-catalyst adduct
6INT5. Concomitantly, photocatalytically active iron species
6INT1 is regenerated with dioxazolone by ligand exchange,
starting a new catalytic cycle.
Conclusion
We demonstrated an efficient, practical Fe-catalyzed
nitrene transfer reaction to realize intermolecular amidation
and imidation with dioxazolones under visible-light irradia-
tion without an exogenous photosensitizer. A wide range of
Scheme 8. Proposed mechanism.
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facile batch conditions. Mechanistic studies by experiment
and computation showed that the spin state of dioxazolone-
bounding iron species changed under visible-light irradiation
to realize decarboxylation. Further extensive mechanistic
studies and synthetic applications of this transformation are
currently underway in our laboratory.
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Acknowledgements
We thank Dr. Jialin Wen (SUSTech) for helpful discussion
and paper revisions. We are grateful to the National Natural
Science Foundation of China (Nos. 21572028 and 21372035)
for their financial support. This work was supported by a grant
from the National Joint Fund for Regional Innovation and
Development (Grant No. U20A20143). This work was also
supported by the Natural Science Foundation of Liaoning,
China (No. 2019JH3/30100001 and 201602181) and LiaoNing
Revitalization Talents Program (XLYC1802030).
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Conflict of interest
The authors declare no conflict of interest.
Keywords: dioxazolones · iron catalysis · nitrene transfer ·
spin state interconverting · visible light
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Manuscript received: December 7, 2020
Revised manuscript received: February 2, 2021
Accepted manuscript online: April 11, 2021
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Version of record online: && &&
,
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Research Articles
Homogeneous Catalysis
J.-J. Tang, X. Yu,* Y. Wang, Y. Yamamoto,
M. Bao*
&&&& — &&&&
Interweaving Visible-Light and Iron
Catalysis for Nitrene Formation and
Transformation with Dioxazolones
Visible-light-driven iron-catalyzed nitrene
conversion of the ground sextet state of
transfer reactions with dioxazolones are
dioxazolone-bounding iron species to
quartet spin state under visible-light irra-
diation.
3
=
used for intermolecular C(sp )-N, N S
=
and N P bond formation. The key acyl
nitrene iron intermediate is formed by
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