Visible Light-Induced [3+2] Cyclization Reactions of Hydrazones with Hypervalent Iodine Diazo Reagents for the Synthesis of 1-Amino-1,2,3-Triazoles

Article abstract of DOI:10.1002/adsc.202001436

In this study, visible-light-induced [3+2] cyclization reactions of hydrazones with hypervalent iodine diazo reagents as diazomethyl radical precursors are reported. Mild reaction conditions, a broad substrate scope, and excellent functional group compatibility were observed. Furthermore, the synthetic utility was demonstrated by gram-scale synthesis and elaboration to several value-added products. This protocol broadens the scope of diazo chemistry, and is applicable to the late-stage functionalization of natural products. (Figure presented.).

Full text of DOI:10.1002/adsc.202001436

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DOI: 10.1002/adsc.202001436
Visible Light-Induced [3+2] Cyclization Reactions of Hydrazones
with Hypervalent Iodine Diazo Reagents for the Synthesis of
1-Amino-1,2,3-Triazoles
Jun-Ying Dong,a He Wang,a,* Shukuan Mao,a Xin Wang,a Ming-Dong Zhou,a and
Lei Lia,*
a
School of Chemistry and Materials Science, Liaoning Shihua University, Dandong Road 1,
Fushun 113001, People’s Republic of China
E-mail: hewang@lnpu.edu.cn; lilei@lnpu.edu.cn
Manuscript received: November 17, 2020; Revised manuscript received: February 17, 2021;
Version of record online: Februar 25, 2021
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202001436
Abstract: In this study, visible-light-induced [3+2] cyclization reactions of hydrazones with hypervalent
iodine diazo reagents as diazomethyl radical precursors are reported. Mild reaction conditions, a broad
substrate scope, and excellent functional group compatibility were observed. Furthermore, the synthetic utility
was demonstrated by gram-scale synthesis and elaboration to several value-added products. This protocol
broadens the scope of diazo chemistry, and is applicable to the late-stage functionalization of natural products.
Keywords: [3+2] Cyclization reactions; Hydrazones; Hypervalent iodine diazo reagents; Diazomethyl radical;
Visible-light photoredox catalysis
Introduction
Owing to their high and versatile reactivity, diazo
compounds are highly valuable building blocks in
organic synthesis.[1] For example, diazo compounds
have wide-ranging applications as metal carbene
precursors,[2] 1,3-dipoles,[3] C nucleophiles,[4] and ter-
minal N electrophiles.[5] In contrast, the utility of the
diazo compounds as diazomethyl radical precursors
have rarely been studied. Recently, Suero and co-
workers have utilized hypervalent iodine(III)
reagents[6] by photoredox catalysis to generate
diazomethyl radicals (Scheme 1a), which provided an
efficient way to site-selective aromatic diazometh-
ylation in aromatic feedstocks and drug molecules. The
diazomethyl radical can act as carbyne equivalent to
construct a chiral carbon center via an assembly point Scheme 1. Representative reactions of the retention of the
dinitrogen functionality under light irradiation conditions.
functionalization. Inspired by these results, and as part
of our ongoing research on hydrazones[7] and diazo
compounds,[5b–d,8] we proposed that 1-amino-1,2,3-
triazoles could be constructed by reacting hydrazones functional materials, ligands, agrochemicals and
with hypervalent iodine diazo reagents using an aminyl pharmaceuticals.[10] Among them, 1-amino-1,2,3-tria-
radical-polar crossover strategy.[9]
zoles are known to have interesting biological activ-
The 1,2,3-triazoles skeleton has been recognised as ities and used as α7 nicotinic acetylcholine receptor
one of the most privileged structural motifs present in agonists or dual inhibitors of carbonic anhydrases I/
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II.[11] Despite the considerable advances in the [3+2]
cycloaddition reaction of either diazo compounds or
azides with unsaturated molecules for the synthesis of
triazoles, there are just few methodologies to synthe-
size 1-amino-1,2,3-triazoles.[11] Therefore, the develop-
ment of general, mild, and efficient synthetic strategies
for the 1-amino-1,2,3-triazoles moiety is highly desir-
able. Until recently, Alcarazo and co-workers have
reported the reaction between the α-diazo sulfonium
triflates and N,N-dialkyl hydrazones to afford 1-
(dialkylamino)-1,2,3-triazoles under photoredox catal-
ysis, which further elaborated into mesoionic carbene
ligands.[12]
Table 1. Optimization of reaction conditions.[a,b]
Entry
Catalyst (2 mol%)
Visible light
Yield (%)
Visible-light-induced photoredox-catalyzed radical
cross-coupling reactions are well-established powerful
strategies in modern organic synthesis.[13] Recently,
with the growth of modern photochemistry, photo-
catalyzed reactions of diazo compounds have attracted
considerable attention. In general, diazo compounds
can act as radical acceptors,[14a] carbene[14b–g] and
radical precursors,[7a,14h] and nucleophiles[15a] in photo-
catalytic systems to develop new and valuable chem-
ical transformations. In such transformations, diazo
compounds often require N2 extrusion as a driving
force under light irradiation conditions,[16] with few
reported examples showing retention of the diazo
functionality.[7,15] For example, Zhou and coworkers
reported a photocatalytic cross-dehydrogenative cou-
pling reaction of tertiary amines and diazo compounds,
affording various β-amino-α-diazocarbonyl compounds
(Scheme 1b).[15a] Furthermore, the Yu group developed
a visible-light-induced decarboxylative radical cross-
coupling of alkyl N-hydroxyphthalimide ester with
diazoacetates for the synthesis of N-alkyl hydrazones
(Scheme 1c).[15b] To increase the efficiency of diazo
functionality utilization, developing practical methods
with high generality for retention of the dinitrogen
functionality under visible-light catalysis remains
appealing and challenging. Thus, we report the first
visible-light-induced [3+2] cyclization reactions of
hydrazones with hypervalent iodine diazo reagents as
1
2
3
4
5
6
7
8
[Ru(bpy)3]Cl2
[Ru(bpy)3]Cl2
[Ru(bpy)3]Cl2
[Ru(bpy)3]Cl2
[Ru(bpy)3](PF6)2
Ru(bpy)3Cl2·H2O
Rose Bengal
Eosin Y
1 W Blue
51
56
52
46
51
36
35
0
11
0
23
0
1 W White
1 W Green
2 W White
1 W White
1 W White
1 W White
1 W White
1 W White
1 W White
1 W White
1 W White
No light
9
Ir(ppy)3
10
11
12
13
14[c]
15[d]
16[e]
4CzIPN
Mes-Acr+
No
[Ru(bpy)3]Cl2
[Ru(bpy)3]Cl2
[Ru(bpy)3]Cl2
[Ru(bpy)3]Cl2
0
1 W White
1 W White
1 W White
63
57
38
[a] Reactions were conducted using 1a (0.2 mmol), 2a
(0.3 mmol), catalyst (2 mol%), and NaHCO3 (0.3 mmol) in
CH3CN (2 mL) at room temperature for 40 min under N2
atmosphere.
[b] Isolated yield.
[c] Under air.
[d] Ru(bpy)3Cl2 (1 mol%).
[e] Under O2. Mes-Acr+ =9-mesityl-10-methylacridinium per-
chlorate. 4CzIPN=1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicya-
nobenzene.
diazomethyl radical precursors to afford 1-amino- a slightly lower yield (entry 3). When the reaction was
1,2,3-triazoles under mild conditions (Scheme 1d).
performed with white (2 W) LEDs, it was less effective
(entry 4). The screening of organometallic photocata-
lysts, such as Ru(bpy)3Cl2 ·H2O, Ru(bpy)3(PF6)2, and
Ir(ppy)3, and other nonmetallic photocatalysts, such as
Results and Discussion
Initial investigations were conducted using N-morpho- rose bengal, eosin Y, 4CzIPN, and MesÀ Acr+ (entries
lino-1-phenylmethanimine (1a) and hypervalent iodine 6–11), found that Ru(bpy)3Cl2 exhibited the best
diazo reagent 2a as model substrates under blue LED catalytic activity regarding the overall reaction out-
irradiation (1 W) for 40 min. In the presence of come (entry 2). The solvent effect was also screened,
NaHCO3 as base, the reaction catalyzed by Ru- with common solvents, such as DMF, DCE, THF,
(bpy)3Cl2 (2 mol%) proceeded in acetonitrile at room DMA, DMSO, and 1,4-dioxane, giving disappointing
temperature, and desired 1-amino-1,2,3-triazole 3aa results (Table S1, entries 1–7). Various bases were also
was obtained in 51% yield (Table 1, entry 1). By tested, with Na2CO3 (54%), NaOH (45%), and AcONa
changing the light source from blue to white (1 W) (45%) found to be less effective than NaHCO3, while
LED, the yield of 3aa increased to 56% (entry 2). tBuOK, KHCO3, K2CO3, Na2HPO4, and DBU did not
Irradiating the reaction by green (1 W) LED resulted in afford the target product (Table S1, entries 8–16).
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Furthermore, increasing or decreasing base and solvent zones with halo, trifluoromethyl, and cyano substitu-
loadings did not improve the results (Table S1, entries ents were smoothly converted into corresponding
17–20). Control experiments, conducted by performing products 3ba–3fa in 54%–78% yields. The molecular
the reaction in the dark or in the absence of photo- structure of 3ca was unambiguously confirmed using
catalyst, failed to provide the desired product, further X-ray crystallography (CCDC 2033586).[17] Substrates
suggesting that the reaction was photocatalyzed (en- bearing electron-donating groups (such as methyl, tert-
tries 12 and 13). When the reaction was performed butyl, and methoxyl groups) on the aromatic ring were
under air, the yield of 3aa was improved to 63% all suitable for photoredox catalysis, affording slightly
(entry 14). Decreasing the Ru(bpy)3Cl2 loading resulted lower yields (3ga–3ia). From the point of view of the
in a slightly lower yield (entry 15). When the reaction reaction mechanism, hydrazones bearing electron-
proceeded in O2, under the optimal conditions, a donating groups at the phenyl ring may decrease the
significant decrease in yield (38%) was observed acidity of α-H, which may not facilitate radical
(entry 16), suggesting that the incremental of the yield addition or deprotonation. Ortho- and meta-substituted
is not related with the presence of O2. Finally, no or aryl aldehyde hydrazones were also compatible with
poor conversion was observed when 2a was replaced the reaction conditions, forming target products 3ka–
by other hypervalent iodine diazo reagents such as 2b 3ua in good yields. For disubstituted substrates, the
and 2c.
reactions afforded products 3va–3ya in 40%–65%
With optimal reaction conditions established, the yields. In addition to arene aldehydes, a heterocyclic
scope and generality of N,N-dialkylhydrazones in the aldehyde was shown to be compatible, affording
[3+2] cyclization reaction were explored (Table 2). corresponding product 3za in 31% yield. To demon-
Pleasingly, when the reaction was irradiated with strate the synthetic applicability of this catalytic
sunlight instead of a white LED (1 W), target product system, 3aa was synthesized on a 6-mmol scale,
3aa was also obtained in 61% yield. Furthermore, affording a 52% isolated yield.
various aldehyde hydrazones reacted with 2a under the
We next explored the scope of hydrazones and
optimized conditions to furnish the corresponding1- hypervalent iodine diazo reagents under the optimized
amino-1,2,3-triazoles (3aa–3za) in good yields. For reaction conditions (Table 3). The results showed that
example, various para-substituted aryl aldehyde hydra- hydrazones 1aa and 1ab also reacted with 2a to
produce desired products 3aaa and 3aba. Notably,
diverse substituents, such as benzyl, n-butyl, i-butyl,
Table 2. Scope of hydrazones.[a,b]
and but-3-en-1-yl, were all tolerated by this catalytic
system, leading to corresponding products 3ad–3ag in
acceptable yields. Furthermore, various hypervalent
iodine diazo reagents reacted with para-cyano-substi-
Table 3. Scope of hydrazones and hypervalent iodine diazo
reagents.[a,b]
[a] Reactions were conducted using 1 (0.2 mmol), 2a
(0.3 mmol), Ru(bpy)3Cl2 (2 mol%), and NaHCO3 (0.3 mmol)
in CH3CN (2 mL) at room temperature for 40 min under air.
[a] Reactions were conducted using 1 (0.2 mmol), 2 (0.3 mmol),
Ru(bpy)3Cl2 (2 mol%), and NaHCO3 (0.3 mmol) in CH3CN
(2 mL) at room temperature for 40 min under air.
[b] Isolated yield.
[b] Isolated yield.
[d]
[c] Reaction performed with 6 mmol of 1a.
Under sunlight
irradiation.
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tuted aryl aldehyde hydrazones with a highly reactive
To further demonstrate the synthetic utility of this
profile, affording the target products in 56%–77% protocol and the target products, we conducted a one-
yields (3fd–3fg). As for ketone-derived hydrazone pot reaction of 4-cyanobenzaldehyde and morpholin-4-
1ac, the reaction did not take place under the standard amine in CH3CN for 48 h, which led to in situ
conditions.
generation of aldehyde hydrazone 1f in the presence of
MgSO4. Subsequent addition of Ru(bpy)3Cl2, 2a, and
NaHCO3 allowed the reaction to proceed smoothly
under the optimized conditions, with product 3fa
isolated in 60% yield (Scheme 2, eq 1). Furthermore,
product 3ca was readily reduced to alcohol 4ca by
LiAlH4 in THF under mild conditions (Scheme 2,
eq 2). The hydrolysis of 3ca provided convenient
access to corresponding acid 5ca in excellent yield by
simple treatment with aqueous LiOH and acid
(Scheme 2, eq 3). Notably, this methodology can be
utilized for the late-stage functionalization of natural
product derivatives. For example, aldehyde hydrazones
of estrone, diacetone-D-glucose, and epiandrosterone
(1ad, 1ae, and 1af) were synthesized and applied to
this transformation under the optimized reaction
conditions. Desired 1-amino-1,2,3-triazoles 3ada,
3aea, and 3afa were obtained in 41%, 43%, and 62%
yield, respectively (Scheme 2, eq 4).
To gain insight into the reaction mechanism,
various control experiments were performed
(Scheme 3, eqs. 5–10). As reported by Alcarazo’s
group, the reaction mechanism suggests that α-diazo
hydrazone ultimately tautomerize to the corresponding
1,2,3-triazole.[12] First, we investigated whether the α-
diazo hydrazone was a key intermediate in this visible-
light-induced [3+2] cyclization reaction.[18] Therefore,
aldehyde hydrazone 1c was reacted with diazo
compound 2a in the absence of base under the
optimized conditions, with α-diazo hydrazone 6c
isolated in 19% yield (Scheme 3, eq 5). Furthermore,
when α-diazo hydrazone 6c was used as a starting
material under the standard visible-light-induced [3+
2] cyclization reaction conditions or in the presence of
base, desired product 3ca was not obtained (Scheme 3,
eq 6). These results indicated that the aminyl radical-
polar mechanism was critical for visible-light-induced
photoredox catalysis of a single-electron transfer
(SET) process. Furthermore, o-iodobenzoate 7 was
isolated in 61% yield under the standard reaction
conditions (Scheme 3, eq 7). When a stoichiometric
amount of radical scavengers 2,2,6,6-tetramethyl-1-
piperidinoxyl (TEMPO), 1,1-diphenylethylene, or 2,6-
di-tert-butyl-4-methylphenol (BHT) was added under
the optimized conditions, the reaction gave dramati-
cally lower yields (Scheme 3, eqs. 8–10). Correspond-
ing radical trapping adducts 8 and 9 were also isolated
in 14% and 30% yields, respectively. The molecular
structure of 9 was unambiguously confirmed using X-
ray crystallography (CCDC 2055775).[17] Stern-Volmer
fluorescence quenching experiments indicated that 2a
efficiently quenched the *[Ru(bpy)3]2+ excited state
(Figure 1).
Scheme 2. Demonstration of synthetic utility.
Scheme 3. Mechanistic investigations.
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products underwent further transformations, demon-
strating that this method is applicable to the late-stage
functionalization of natural products. Preliminary
mechanistic studies were conducted and a plausible
catalytic cycle was proposed.
Experimental Section
Figure 1. Stern-Volmer experiments on reagent 2a.
General Information
Experimental All chemicals were obtained from commercial
1
sources and were used as received unless otherwise noted. H,
13C and 19F NMR spectra were recorded using CDCl3 or DMSO
as a solvent on a 400 MHz spectrometer at 298 K. The chemical
shift is given in dimensionless δ values and is frequency
Based on the above results and literature
reports,[7,10] a reaction mechanism was proposed, as
shown in Scheme 4. Initially, photoredox catalyst [Ru-
(bpy)3]2+ is photoexcited to the *[Ru(bpy)3]2+ excited
state (E1/2(RuIII/*RuII)=À 0.81 V vs. SCE in
CH3CN).[13] Single-electron reduction of reagent 2a
(reduction potential, Ered =0.95 V vs. SCE in CH3CN)
gives diazomethyl radical A[7] and strong oxidant
[Ru(bpy)3]3+. The SET process was confirmed by
1
referenced relative to TMS in H and 13C NMR spectroscopy.
High-resolution mass spectra (HRMS) were obtained on an
Agilent Q-TOF 6224 spectrometer. The intensity data were
recorded on a Bruker D8 QUEST with Mo-Kα radiation (λ=
0.71073 Å). The crystal structure was solved by means of direct
methods and refined by employing full-matrix least squares on
F2 (SHELXTL-2014). The photocatalytic reactions were per-
formed on WATTCAS Parallel Light Reactor (WP-TEC-
1020HSL) with 1 W LED. Column chromatography was
performed on silica gel (300–400 mesh) using ethyl acetate
(EA)/petroleum ether (PE).
fluorescence-quenching
experiments
(Figure 1).
Diazomethyl radical A then directly attacks the C=N
bond to deliver aminyl radical intermediate B, which
undergoes single-electron oxidation by [Ru(bpy)3]3+ to
form amino-cation intermediate C and regenerates
[Ru(bpy)3]2+, completing the catalytic cycle. Finally,
intramolecular cyclization affords the corresponding 1-
amino-1,2,3-triazoles in the presence of base.
General Procedure for the Preparation of 3 (3aa as
Example)
A solution of 1a (0.2 mmol), 2a (0.3 mmol), Ru(bpy)3Cl2
(2 mol%, 2.6 mg), and NaHCO3 (0.3 mmol, 25.2 mg) in CH3CN
(2.0 mL) was stirred at room temperature with 1 W white LED
for 40 min under air atmosphere. After the reaction was
Conclusion
The synthesis of 1-amino-1,2,3-triazoles using hyper-
valent iodine diazo reagents as diazomethyl radical complete (monitored by TLC), the solvent was removed under
reduced pressure. The crude residue was purified by silica gel
column chromatography (EtOAc/petroleum ether=1:5, V/V) to
afford pure product 3aa.
precursors under photoredox catalysis has been re-
ported for the first time. This [3+2] cyclization
reaction is distinguished by mild reaction conditions, a
broad substrate scope, and excellent functional-group
compatibility. Furthermore, the 1-amino-1,2,3-triazoles
Acknowledgements
We acknowledge financial support from the National Natural
Science Foundation of China (21702087, and 21801105), the
LiaoNing Revitalization Talents Program (XLYC1907010 and
XLYC1902085), the Research Project Fund of Liaoning Provin-
cial Department of Education (L2020033), and talent Scientific
Research Fund of Liaoning Shihua University (2016XJJ-078
and 2016XJJ-079).
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