Photocatalytic Intramolecular C-H Amination Using N-Oxyureas as Nitrene Precursors

Article abstract of DOI:10.1021/acs.orglett.0c02200

Nitrenes are remarkable high-energy chemical species that enable direct C-N bond formation, typically via controlled reactions of metal-stabilized nitrenes. Here, in contrast, the combined use of photocatalysis with careful engineering of the precursor enabled C-H amination forming imidazolidinones and related nitrogen heterocycles from readily accessible hydroxylamine precursors. Preliminary mechanistic results are consistent with the formation of free carbamoyl triplet nitrenes as reactive intermediates.

Full text of DOI:10.1021/acs.orglett.0c02200

pubs.acs.org/OrgLett
Letter
Photocatalytic Intramolecular C−H Amination Using N‑Oxyureas as
Nitrene Precursors
Ryan A. Ivanovich, Dilan E. Polat, and Andre M. Beauchemin*
‡
‡
́
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ABSTRACT: Nitrenes are remarkable high-energy chemical species that
enable direct C−N bond formation, typically via controlled reactions of
metal-stabilized nitrenes. Here, in contrast, the combined use of
photocatalysis with careful engineering of the precursor enabled C−H
amination forming imidazolidinones and related nitrogen heterocycles
from readily accessible hydroxylamine precursors. Preliminary mecha-
nistic results are consistent with the formation of free carbamoyl triplet nitrenes as reactive intermediates.
Scheme 1. (A) Cyclic Urea Pharmaceutical Diversity; (B)
itrogen atoms impart useful physiochemical properties
and thus are common in the structures of important
Prior Art: Hydroxylamines as Nitrene Precursors; (C) New
Imidazolidinone Synthesis via Photocatalytic C−H
Amination
N
medicines, materials, and fine chemicals. Arguably, direct C−H
aminations are ideal synthetic transformations.1 Nitrenes and
N-centered radicals have historically been powerful inter-
mediates in this regard, enabling both inter- and intramolecular
amination reactions.2,3 The development of new synthetic
methods to generate these reactive N-centered intermediates
more selectively is inextricably linked to the identification of
new precursors. In this regard, acyloxy hydroxylamine
precursors were identified as N atom transfer precursors in
nature (Scheme 1A),4 and several emergent C−H amination
strategies accessing N-centered radicals5−14 or nitrenes15−22
have been reported. Despite sweeping advances, important
synthetic limitations remain. While nitrene C−H amination
reactions from hydroxylamine precursors have established
routes for the synthesis of cyclic carbamates15,23and more
recentlyγ-lactams,16−18,20,22,23 hydroxylamine precursors for
C−H amination reactions affording cyclic ureasimidazolidi-
noneshave not yet been reported (Scheme 1B). This is
surprising, given the diversity of imidazolidinone-based
pharmaceuticals (Scheme 1A).
The synthetic attention devoted to metal nitrenes has
emboldened the notion that the high reactivity of free nitrenes
renders them unselective and as such nearly halted their
comparative study in its infancy. Indeed, Lwowski originally
reported that UV irradiation of azidoformates led to poor yield
and selectivity for intermolecular C(sp3)−H amination, and
intramolecular reactions were unexpectedly unsuccessful.24
However, photocatalysis has prompted a resurgence in the
study of free nitrenes. Yet, reports are rare and have been
limited to triplet sensitization of azidoformates or acyl azides,
which undergo either aziridination25 or C(sp2)−H amination,
respectively.26,27 Arguably, azides may not be ideal nitrene
precursors for photocatalytic transformations, and the shortage
of more convenient, stable precursors could account for the
relative scarcity of reactions of triplet nitrenes. While preparing
this manuscript, Chang reported a new photocatalytic C(sp3)−
H amination strategy affording cyclic carbamates and γ-lactams
via triplet sensitization of N-benzoyloxy hydroxylamine
Received: July 2, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02200
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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derivatives,23and other related EnT processes have recently
emerged.28 In their system, a bis-trifluoromethyl benzoyl
activating group was necessary to lower the triplet energy (ET)
of the precursor for effective energy transfer (EnT). In stark
contrast, while developing triplet nitrene C−H amination
reactions using analogous urea derived precursors, we had
identified that precursors containing a bis-trifluoromethyl
benzoyl activating group were completely ineffective to form
the rare class of carbamoyl nitrenes (Scheme 1B). Herein, we
report a photocatalytic strategy to form such triplet nitrenes
from engineered O-acyl hydroxylamine precursors that enables
new photocatalytic C−H amination reactions forming
imidazolidinones and related heterocycles (Scheme 1C). Our
preliminary results clearly highlight the subtle yet significant
differences that hydroxylamine nitrene precursors impart on
photocatalytic C−H amination reactions.
a
Table 1. Optimization of Acyloxy Urea C−H Amination
entry
sm
solvent
temp (°C)
base
catalyst yield (%)
1
2
3
4
5
6
7
8
9
10
1a
1b
1a
1a
1a
1a
1a
1a
1b
1c
1d
1d
1d
1d
(CH2Cl)2
(CH2Cl)2
CH2Cl2
CH2Cl2
(CH2Cl)2
(CH2Cl)2
MeCN
MeCN
MeCN
MeCN
MeCN
100
120
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
TMG
TMG
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N


I
86
69
16
9
10
0
60
99
65
0
II
III
IV
V
b
b
VI
VI
VI
VI
VI
VI

b
Building on our work on heteroatom-substituted isocyanate
cascade reactions,29 we initially envisioned a reaction in which
thermal substitution on an acyloxy O-isocyanate followed by α-
elimination to a singlet nitrene could lead to nitrogen-rich
heterocycles (Scheme 2). Although some success was achieved
b
b
11
12
13
14
91
0
c
MeCN
MeCN
MeCN
b

0
0
b
Et3N
a
Scheme 2. Initial Thermal Cascade Reaction Design and
Associated Challenges
Conditions: Starting material (sm) 1a−d (1.0 equiv), base (1.1
b
equiv), catalyst (2 mol %), solvent (0.2 M), 16 h under argon. 450
c
nm blue LEDs, rt, 16 h. No light. 1H NMR yields (1,3,5-
trimethoxybenzene (TMB) as an internal standard).
transfer (SET)-initiated N-centered radical pathway.31 Under
identical conditions, we achieved 60% yield (entry 7). Upon
screening a variety of conditions, Ru(bpy)3(PF6)2 emerged as a
uniquely proficient photocatalyst in the presence of triethyl-
amine as a base (entry 8).
At this point, we investigated the scope of the reaction (e.g.,
entry 9, see Figure S1), which revealed systematic issues. The
most problematic byproduct was identified as II (Scheme 2)
formed from reduction of the N−O bond (ca., 10−20% yield).
This prompted a return to substrate design and a systematic
probe of the acyl substituent. Reasoning that SET reduction
could occur from RuI (E1/2 RuII*/RuI = −1.33 V) in the
presence of triethylamine (E1/2 = −0.78 V) as a sacrificial
reductant,32 a variety of O-acyl hydroxylamines were screened.
Notably, a substrate possessing the same bis-trifluoromethyl
benzoate successful in Chang’s work failed to provide any
desired product (reagent 1c, entry 10).23 In stark contrast, a
substrate bearing an electron-rich p-methoxybenzoate proved
effective (reagent 1d, entry 11). Control experiments verified
the need for light, base, and catalyst (entries 12−14).
The scope of the reaction was again investigated, and
contrary to results obtained with benzoate derivatives, using
electron-rich p-methoxybenzoates as substrates resulted in
efficient amination of 3°, 2°, and aromatic C−H bonds (Figure
1). Tertiary C−H bonds were aminated efficiently (2a−2o).
C−H amination at α-hydroxy/α-ethereal positions led to the
corresponding aromatic imidazolones (2l−2n), highlighting
the versatility of this method beyond the synthesis of saturated
heterocycles. A hydantoin heterocycle (2o) was also obtained
by a targeted C−H amination at an acetal methine C−H.
Unfortunately, with these conditions, the p-methoxybenzoate
precursor was still unable to effect C−H amination using
analogous amide and carbamate precursors (2g−2i) in
acceptable yields; however, modification of the base allowed
extension to these classes of nitrene precursors.14,23 Pleasingly,
efficient C−H aminations on unactivated methylenes, which
in developing a cascade process (see SI), several competing
side-reactions, such as O-isocyanate trimerization, pre-emptive
N−O bond cleavage, and 1,2-rearrangement followed by N-
isocyanate dimerization,30 obstructed the development of this
cascade (Scheme 2, I−III). However, some side-reactions were
mitigated by a stepwise approach (Table 1, entry 1).
Unfortunately, these conditions were not broadly applicable
(entry 2, see SI for preliminary reaction scope) but clearly
establish where the metal-free background reaction lies. To
minimize the decomposition pathways occurring from thermal
activation, extensive efforts using transition metal catalysts
were undertaken (entries 3−6, see SI for further expts).
Ultimately, catalysts reported to perform nitrenoid C−H
aminations typically catalyzed the 1,2-rearrangement side-
reaction, which was followed by rapid N-isocyanate dimeriza-
tion30 (see SI). We reasoned that selectively accessing the
triplet nitrene could provide a means to circumvent the 1,2-
rearrangement and enable productive C−H amination
reactions with free nitrenes. Toward this, we were drawn to
photoredox catalysis, intrigued by a report wherein acyloxy
ureas underwent intramolecular aziridination reactions, using
an Ir photoredox catalyst and a proposed single electron
B
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Figure 1. Reaction scope for photoredox catalyzed intramolecular C−H amination. aConditions: 1 (1.0 equiv), Et3N (1.1 equiv), Ru(bpy)3(PF6)2
(0.02 equiv), MeCN (0.2 M), 450 nm blue LEDs, rt, 16 h under argon. b1H NMR yield based on 1,3,5-trimethoxybenzene as an internal standard.
c1 (1.0 equiv), Bu4NOP(O)(OBu)2 (0.05 equiv), Ru(bpy)3(PF6)2 (0.02 equiv), CH2Cl2 (0.2 M), 450 nm blue LEDs, rt, 16 h under argon.
typically show reduced reactivity,21,23,33,34 were achieved (2p−
2u). Interestingly, formation of five- and six-membered rings
were observed in an 8:1 ratio when each position was 2° (2q);
however, when the 5-position was 2° and the 6-position was 3°
(2r), the ratio was 1.7:1.35,36 The reaction was not limited to
C(sp3)−H amination; a variety of sp2 centers were also
aminated (2y−2ac, 2af−2ag). Alkenes underwent aziridina-
tion, but the released p-methoxybenzoate performed a ring
opening reaction leading to a formal oxyamination of the
pendant olefin (2y−2aa).37−39 In contrast to the C(sp3)−H/
arene C(sp2)−H amination reactions, the aziridination
reaction delivered the corresponding urea, lactam, and
carbamate derivatives. Finally, the innate selectivity of the
C−H amination corroborated with the reported selectivity of
nitrenes, e.g., 3 °C−H > benzylic C−H ≈ ethereal > 2 °C−H
> 1 °C−H.40 Site competition experiments further demon-
strate that alkene aziridination, and aryl C(sp2)−H amination
seem to react more readily than 3 °C−H amination. Notably,
all precursors used for site competition experiments (Figure
Compelled by the intricacies of this reaction, we began a
study of the reaction mechanism. Although a pathway initiated
by an N-centered radical via SET-induced N−O bond cleavage
was unlikely given that electron-rich benzoates were required
for optimal reactivity (Figure 1), we hoped to further rule this
mechanism out. In the absence of Et3N as a sacrificial electron
donor, C−H amination was still observed (Scheme 3A). A
derivative possessing an N-methyl substituent was subjected to
the reaction, but after 72 h, there was no trace of C−H
amination. Instead, slow conversion to a mixture of urea
byproducts 3a and 3b was observed (Scheme 3B). The
isolation of 3b suggests that even in the presence of a
carbocation, C−H amination is not observed. Furthermore, the
C−H amination reaction with 1a′ was quantitative in the
presence of excess BHT (Scheme 3C); however, TEMPO
resulted in a complete starting material recovery (see SI).When
optically enriched 1an (92% ee) was subjected to the reaction
conditions, the resulting cyclic product 2k suffered total
racemization (Scheme 3D), thereby ruling out a singlet
nitrene. This contrasts to the results of Chang, where an
1
1E) isomerized readily as indicated by H NMR.
C
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Scheme 3. Preliminary Mechanistic Studies for the
Photocatalytic C−H Amination Using N-Oxyureas
Table S1), (thermal stepwise, Tables S2−S4), (metal
catalyzed, Table S5), (early photoredox, Tables S6 and
S7), other mechanistic experiments (Figures S2 ad S3),
HPLC (Figures S4−S7), cyclic voltammetry (Figures
S8−S10), and NMR spectra (PDF)
FAIR data, including the primary NMR FID files, for
compounds 7a; 6a−6e; 5a; 5b; 5d; 5e; 5f; 5h; 4a−4d;
4.5, 3a; 2a; 2c′; 2d; 2e; 2h; 2k; 2l; 2m; 2n; 2o; 2s; 2x;
2aq and 2aq′; 2ap and 2ap′; 2am′; 2am; 2ak; 2aj; 2af;
2ae; 2ad; 2ab; 2aa; 1a−1z; 1a′; 1b′; 1d′; 1aa; 1af; 1ae;
1ad; 1ac; 1al; 1ak; 1aj; 1ah; 1ap; 1ao; 1an; 1am; 1as;
1ar and 1aq (ZIP)
AUTHOR INFORMATION
Corresponding Author
■
́
Andre M. Beauchemin − Centre for Catalysis Research and
Innovation, Department of Chemistry and Biomolecular
Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5,
Canada; orcid.org/0000-0003-0667-6215;
Email: abeauche@uottawa.ca
Authors
Ryan A. Ivanovich − Centre for Catalysis Research and
Innovation, Department of Chemistry and Biomolecular
Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5,
Canada
Dilan E. Polat − Centre for Catalysis Research and Innovation,
Department of Chemistry and Biomolecular Sciences, University
of Ottawa, Ottawa, Ontario K1N 6N5, Canada
enantioretentive C−H amination via triplet nitrene was
proposed.23 Perhaps the more stabilized benzylic radical in
our system allowed racemization to occur before radical
recombination occurred. Lastly, as a probe of the possibility for
an EnT reaction pathway, 1a′ was photochemically irradiated
with UVA light, which led to a modest 25% yield. Upon the
addition of 20 mol % tetramethylguanidine (TMG) as a base,
the yield was increased to 85% (Scheme 3E). The ability for
the reaction to proceed in the presence of high energy light
alone is usually an indicator of an EnT pathway.41−43 However,
density functional theory (DFT) studies predicted that EnT to
the neutral precursor was exceptionally endergonic for the
excited photocatalyst. While EnT to the anionic species was
predicted to be only minimally endergonic, the base cannot
deprotonate the precursor. Taken together, experimental
results seem consistent with a triplet nitrene pathway, but at
this stage, more studies are needed given the significant
substrate differences from Chang’s study and use of a milder
organic base.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02200
Author Contributions
‡R.A.I. and D.E.P. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the University of Ottawa and NSERC for the
generous financial support. R.A.I. thanks NSERC for a CGS-D
scholarship. We thank Prof. Derek Pratt (uOttawa) for
insightful discussions. We thank Katie Harriman (uOttawa)
for assistance with CV experiments. We thank Prof. Tom Woo
(uOttawa) for access to computational resources.
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development and mechanistic investigations are underway and
will be reported in due course.
■
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02200.
Experimental procedures, characterization data, compu-
tational results, optimization tables (thermal cascade,
D
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