AgNO3 as nitrogen source for rhodium(III)-catalyzed synthesis of 2-aryl-2H -benzotriazoles from azobenzenes

Article abstract of DOI:10.1039/c6cc04341k

A new approach has been established for Rh(iii)-catalyzed direct aza oxidative cyclization of non-prefunctionalized azobenzenes to provide 2-aryl-2H-benzotriazoles in good yields, in which AgNO₃ instead of conventional azide reagents for the first time functions as the nitrogen source for the nitrogenation reaction. Preliminary mechanistic studies suggest that the Rh(iii)-catalyst could account for the nitration reaction, and subsequently cationic silver species might both play a vital role in the fission of the nitrogen-oxygen bonds in nitro groups and promote aza oxidative cyclization.

Full text of DOI:10.1039/c6cc04341k

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AgNO3 as Nitrogen Source for Rhodium(III)-Catalyzed Synthesis of
2-Aryl-2H-Benzotriazoles from Azobenzenes
Jixing Li,ab Hui Zhou,ab Jinlong Zhang,a Huameng Yang,a and Gaoxi Jiang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new approach has been established for Rh(III)-catalyzed direct should be suffered from the extremely difficult cleavage of two
aza oxidative cyclization of nonprefunctionalized azobenzenes to intrinsic N=O double bonds. Herein, we report the first
provide 2-aryl-2H-benzotriazoles in good yields, in which AgNO3 example with inorganic nitrate as nitrogen source for
instead of conventional azide reagents for the first time functions nitrogenation reaction.
as the nitrogen source for nitrogenation reaction. Preliminary
Benzotriazole represents an important class of compounds
mechanistic studies suggest that Rh(III)-catalyst could be account bearing a unique fused five-membered ring with three vicinal
for nitration reaction and subsequently cationic silver species nitrogen atoms.[5] In principle, such five-membered ring
itself might play a vital role for the fission of nitrogen-oxygen presents two tautomeric forms, 1H (or 3H) and
bond of nitro group and promote aza oxidative cyclization.
thermodynamically unstable 2H isomers that make their
selective synthesis more intriguing.[6] Significantly, 2-aryl-2H-
benzotriazoles, i.e., 2H isomers are core motif extensively
existing in pharmaceuticals and organic photoelectronic
materials.[7] Therefore, considerable effort has been directed
toward the regioselective synthesis of these useful units in the
last few years. Typical well documented approaches are
cyclization of prefunctionalized azobenzenes with an intrinsic
azido, amino, or nitro group on ortho-position of the aryl ring
(Scheme 2, eq 1).[8] Additionally, the copper catalyzed cross
coupling cyclization of 2-haloaryltriazenes with NaN3 has also
been developed by Chen and co-workers (eq 2).[9] From the
viewpoint of atom/step economy, undoubtedly, the direct
Nitrogen-containing compounds are fundamental moieties in
biology, chemistry, and pharmaceutical molecules.[1] Over the
past decades, noteworthy progress for direct implantation
nitrogen into hydrocarbons by transition-metal catalyzed
strategy with organo nitrogen donors has been achieved
(Scheme 1).[2] Comprehensively, sodium azide undertakes the
[M]
N
N
N
R- 3, R-O O, R- H2, DMF, ...
N
Na
3
"N"
O
O
T his work
N
MO
R
N
(M- O3)
N
N
N
cyclization
(1)
N
N
Xn
Scheme 1. Typical nitrogen sources in organochemistry.
N
R
N
:
Xn NO2, NH2, N3
most primal nitrogen donor.[3] Since it’s dramatically explosive,
the excavation of new safe surrogates is still in strong demand.
Inorganic nitrate, thanks to its eminent merit of
inexpensiveness, stability, hypotoxicity, and ease of handling,
is widely used as an oxidant and nitro source in organic
synthesis,[4] but unprecedented as a nitrogen source that
R
N
N
N
[Cu] / NaN3
N
N
(2)
(3)
N
R
R
I, Br
R
N
N
N
aza oxidative cyclization
N
H
conditions: (i) Ts 3, [Rh]/AgNTf2, then PhI(OAc)2 (2.0 eq)[11]
N
a.State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research
Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of
Sciences, Lanzhou 730000, P. R. China
N
(ii) TMS 3 (2.0 eq), [Pd]/TBHP (2.0 eq) in DMSO[12]
N
Present work
)
(iii) Ag O3, [Rh]/AgNTf2, no extra oxidant (
E-mail: gxjiang@licp.cas.cn
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
Scheme 2. Typical approaches and diverse nitrogen sources for the
synthesis of 2-aryl-2H-benzotriazole.
b.
†
DOI: 10.1039/x0xx00000x
Electronic
Supplementary
Information
(ESI)
available:
See
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catalytic aza oxidative cyclization of easily accessible product 2a was isolated. Encouraged by the rudimentary result,
nonprefunctionalized azobenzenes[10] is much more attractive several reactions were conducted to improve the chemical
(eq 3). In 2014, Lee and co-workers reported an “one-pot” yield. Screening solvents reveals that 1,2-dichloroethane (DCE)
cascade reaction for the synthesis of 2-aryl-2H-benzotriazoles is the best choice and provided 2a in 73% yield (entries 1-7).
that is Rh(III)-catalyzed amidation of azobenzenes with N- Changing AgNO3 to other nitrate salts, such as KNO3, Mg(NO3)2,
sulfonyl azide (TsN3) followed by oxidation with PhI(OAc)2 (eq Zn(NO3)2, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, and even organic
3, i).[11] Recently, Patel and co-workers realized a Pd(II)- solvent CH3NO2 didn’t improved the reaction (entries 8-14).
catalysis with azidotrimethylsilane (TMSN3) as the nitrogen Examination of counteranion additives was proved to be less
source and (over)stoichiometric amounts of TBHP as the effective (entries 15-20). It is noteworthy that decreasing the
oxidant (eq 3, ii).[12] Despite these important advances, further loading of AgNO3 from 2.0 equiv to 1.0 equiv also readily led to
exploration of simple and easily handled methods to obviate 2a in 67% yield (entry 21). Lowering the reaction temperature
the use of dangerous azides and excessive amounts of extra resulted in an adverse effect in the reaction (entry 22). Other
oxidant is of interest and significance yet a great challenge. In transition-metal compounds such as Pd(OAc)2, Cp*Ru(PPh3)2Cl2,
the context, we realized a new process of Rh(III)-catalysis with and [Cp*IrCl2]2 have no catalytic activity (see ESI).
AgNO3 as the nitrogen source (eq 3, iii). Preliminary
With the optimized conditions in hand, we next
mechanistic studies suggest that Rh(III)-catalyst could be investigated the scope of azobenzenes to evaluate the
account for nitration reaction by directing group-assisted C-H generality of the transformation. First, besides 1a, a series of
bond activation and subsequently cationic silver species itself symmetrically substituted azoarenes 1b
might play a vital role for the cleavage of nitrogen-oxygen the reactions proceeded smoothly to furnish the
bond of nitro group and promote aza oxidative cyclization. corresponding products 2b in 46−83% yields (Scheme 3).
Initially, we treated azobenzene 1a (0.1 mmol) with AgNO3 Notably, this protocol could tolerate well range of
-k were employed and
−
k
a
(2.0 eq) in the presence of [Cp*RhCl2]2 (5.0 mol %), AgNTf2 (20 substituents on the phenyl ring although the electronic nature
mol %), and H2O (0.1 mmol) in CH2Cl2 (0.5 mL) under 150 °C for
12 hours (Table 1, entry 1). To our delight, 42% of desired
Scheme 3. Substrate Scope for Symmetrical Azoarenes to 2-Aryl-2H-
Benzotriazoles.a
Table 1. Optimization of the Reaction Conditions.a
R
[Cp*RhCl2]2 (5.0 mol %)
AgNTf2 (20 mol %)
N
N
N
N
R
R
N
+
Ag O3
R
N
DCE, H2O, 150 C, 12 h
H
1a-k
2a-k
N
N
N
N
N
N
Me
N
Et
entry
1
“N” source
AgNO3
[X-]
solvent
CH2Cl2
DCE
yield (%)b
52(42)
88(73)
n.r c
n.r
N
N
Me
Et
b 2a
2b
2c
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgOTf
AgSbF6
AgBF4
73%,
76%,
81%,
2
AgNO3
3
AgNO3
Toluene
THF
N
N
N
N
n-Bu
N
i-Pr
N
t-Bu
4
AgNO3
n-Bu
i-Pr
t-Bu
N
N
N
5
AgNO3
DMF
PhNO3
CH3NO2
DCE
n.r
76%, 2d
83%, 2e
81%, 2f
6
AgNO3
25
7
AgNO3
47
N
N
N
N
N
N
N
N
Br
Cl
CO2Et
8
KNO3
(39)
45
N
Br
Cl
EtO2C
9
Mg(NO3)2
Zn(NO3)2
Co(NO3)2
Ni(NO3)2
Cu(NO3)2
CH3NO2
AgNO3
DCE
c
2g
2i
46% (16%),
54% (17%) ,
2h
58% (14%),
10
11
12
13
14
15
16
17
18
19
20
21d
22e
DCE
72(58)
70
DCE
Me
Me
Me
Me
Me
DCE
35
N
N
N
N
N
N
Me
DCE
20
55%, 2j
72%, 2k
DCE
48
a Reaction conditions: 1a-k (0.1 mmol), AgNO3 (0.2 mmol), [Cp*RhCl2]2 (5.0
mol %), AgNTf2 (20 mol %), and H2O (0.1 mmol) in DCE (0.5 mL) under 150 °C for
12 hours. b Yield of isolated product. c Recovered azoarene in parentheses.
DCE
68
AgNO3
DCE
35
AgNO3
DCE
50
AgNO3
Ag2CO3
NaOTf
NaBARF
AgNTf2
AgNTf2
DCE
n.r
AgNO3
DCE
30
and steric hindrance of substrates had influence on the
reaction result. Basically, the relatively electron-rich azoarenes
always exhibited better reactivity than relatively electron-
AgNO3
DCE
50
AgNO3
DCE
(67)
(45)
AgNO3
DCE
deficient ones. For instance, azobenzenes 1b
group converted readily into the desired 2-aryl-2H-
Benzotriazoles 2b in more than 76% yields while ones 1g
having chloro-, bromo-, and ester substituent afforded
products 2g in mere yield of 46-58%. Gratifyingly, ortho-
methyl and 3,4-dimethyl substituted azoarenes 1j were also
-f bearing alkyl
a Unless otherwise noted, the reaction was performed with 1a (0.1 mmol), “N”
source (0.2 mmol), [Cp*RhCl2]2 (5.0 mol %), counteranion additive [X-] (20
b
-f
-i
mol %), and H2O (0.1 mmol) in 0.5 mL of solvent under 150 °C for 12 hours.
Yield was determined by GC-Mass using a standard, yield of isolated product is
given in parentheses. c n.r means no reaction upon 1a. d 1.0 eq of AgNO3 used. e
Reaction carried out at 140 °C.
-i
-
k
2 | J. Name., 2012, 00, 1-3
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amenable to the reaction condition, providing 2j and 2k in 55%
and 72% yields, respectively.
Scheme 5. Preliminary Mechanistic Study
Compared to symmetrical azoarenes, unsymmetrically
substituted ones are much more challenging due to the
difficult control of regioselectivity. To excavate the influence of
electronic and steric effect and realize high regioselectivity, a
series of substrates were treated to the standard reaction
conditions (Scheme 4). Phenyldiazenes containing a Me (1l), Cl
[Cp*RhCl2]2 (5.0 mol %)
N
N
AgNTf2 (20 mol %)
N
N
N15O3
(1)
(2)
(3)
+
K
N15
DCE, H2O, 150 C, 12 h
H
2.0 eq
1a
2a', 42% (isolated)
N
N
N
N
[Cp*RhCl2]2 (5.0 mol %)
N
+
AgNO3
DCE, H2O, 150 C, 12 h
H
2.0 eq
without AgNTf2 : no reaction
(
1m), and CN (1n) group at 4-position on another aromatic ring
1a
2a
N
N
N
N
AgNTf2 (20 mol %)
N
Scheme 4. Regioselectivity for Unsymmetrical Azoarenes to 2-Aryl-2H-
Benzotriazoles.
+
AgNO3
DCE, H2O, 150 C, 12 h
H
2.0 eq
without [Cp*RhCl2]2 : no reaction
1a
2a
[Cp*RhCl2]2 (5.0 mol %)
AgNTf2 (20 mol %)
N
N
N
O-
N
N
N
N
(4)
(5)
+
+
DCE, H2O, 150 C, 12 h
N
NO2
1r
2a, 15% (by GC-MS)
2a-[O], 5%
N
N
N
DCE, H2O, 150 C, 12 h
N
N
NO2
1r
10 mol% of AgNTf2 : 30%
1.0 equiv of AgNTf2 : 79% (isolated)
2a
[Cp*RhCl2]2 is also necessary for the catalytic transformation
(eq 3). 1-(2-nitrophenyl)-2-phenyldiazene 1r partly converted
into 2a in the presence of catalytic amount of Rh(III) and
AgNTf2 with the formation of unstable triazole 1-oxide 2a-[O]
simultaneously (eq 4). Furthermore, 1r could turn into 2a
completely by increasing the amount of AgNTf2 from 10 mol %
to 1.0 eq (eq 5). Although the mechanism of this catalytic
transformation is not completely clear yet, on the basis of
previous reports[13] and the above-mentioned observations, a
possible reaction process is postulated in Scheme 6. The
counter anion exchange between [Cp*RhCl2]2 and AgNTf2 gives
Scheme 6. The Postulated Reaction Process.
gave a mixture of two inseparable constitutional isomers in 73-
58% yield but with a regularly increased ratio from 1:1 to 10:1.
The observation declares that the nitrogenation of Csp2-H bond
is preferred at the electron-rich aromatic rings. In the light of
the understanding, 1-(3,5-dimethylphenyl)-2-phenyldiazene 1o
was employed and as expected provided 2o exclusively in good
yield. Additionally, two azoarenes having two unsymmetrical
ortho-C-H reaction sites were used to investigate the steric
hindrance effect. For 1p, a ratio of 5:1 with total 66% yield of
2p and 2p’ was obtained that disclosed the capacious C-H
position is favourable. Thus increasing the steric hindrance
from methyl to isopropyl (1q) furnished 2q alone in acceptable
yield.
a cationic Rh(III) species, which facilitates the directing group-
assisted C−H acꢀvaꢀon of azoarene
I to provide a five-
membered rhodacycles II with release of HNTf2. Then II reacts
with AgNO3 to afford the nitrate ion-containing Rh(III) complex
III. Elimination of III in the presence of HNTf2 liberates 2-
nitroazoarene IV and another Rh(III) compound, the latter
might regenerate the catalytic Rh(III) complex by reaction with
AgNTf2. Oxidative addition of IV and cationic Ag(I) species
To explore the mechanism of this transformation, several
controlled reactions were performed (Scheme 5). First,
isotopic tracer experiments were executed. Treatment of 1a
with commercially available KN15O3 instead of AgNO3 to the
standard reaction conditions delivered the corresponding
isotope labeled product 2a’ in 42% yield (eq 1). The result
confirms undoubtedly that the inorganic nitrate functions as
the real nitrogen source. No reaction occurred in the absence
of AgNTf2 reveals the strong effect of counter anion (eq 2). And
might assemble the seven-membered Ag(III) intermediate
followed by aza cyclization to form triazole 1-oxide VI and a
cationic [Ag(III)O]+ species[14]
Subsequently, the second
V,
.
oxidation of VI with cationic Ag(I) species produces the desired
adduct.
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In conclusion, that inorganic AgNO3 instead of conventional
azide reagents for the first time functions as the nitrogen
source for nitrogenation reaction was realized. The Rh(III)-
catalyzed process without addition of extra oxidant
streamlines the synthesis of 2-aryl-2H-benzotriazoles from
nonprefunctionalized azobenzenes. Preliminary mechanistic
studies suggest that Rh(III)-catalyst could be account for
directing group-assisted nitration reaction and cationic Ag(I)
species itself might play a vital role for the fission of nitrogen-
oxygen bond of nitro group to promote subsequent aza
oxidative cyclization.
5
6
Financial support from Hundred Talent Program of Chinese
Academy of Sciences (CAS) is gratefully acknowledged.
7
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