Palladium-catalyzed direct ortho alkoxylation of aromatic azo compounds with alcohols

Article abstract of DOI:10.1021/jo401623j

An efficient and highly regioselective synthesis of 2-alkoxy aromatic azo compounds via palladium(II)-catalyzed alkoxylation of azobenzene derivatives directed by the azo group using alcohols as the alkoxylation reagents and PhI(OAc)₂ as the ox

Full text of DOI:10.1021/jo401623j

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Note
Palladium-Catalyzed Direct ortho Alkoxylation
of Aromatic Azo Compounds with Alcohols
Zhangwei Yin, Xiaoqing Jiang, and Peipei Sun
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 05 Sep 2013
Downloaded from http://pubs.acs.org on September 5, 2013
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Palladium-Catalyzed Direct ortho Alkoxylation of Aromatic Azo
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Compounds with Alcohols
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Zhangwei Yina, Xiaoqing Jiang,a and Peipei Suna,b*
a Jiangsu Key Laboratory of Biofunctional Materials, College of Chemistry and Materials Science,
Nanjing Normal University, Nanjing 210097, China
b Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Nanjing 210023, China
sunpeipei@njnu.edu.cn
ABSTRACT: An efficient and highly regioselective synthesis of 2-alkoxyl aromatic azo compounds
was developed via the palladium(II)-catalyzed alkoxylation of azobenzene derivatives directed by azo
group using alcohols as the alkoxylation reagents and PhI(OAc)2 as the oxidant. The method was
applicable to both primary and secondary alcohols and the moderate to good yields were achieved.
In the past decade, transition-metal-catalyzed direct functionalization of unreactive C-H bonds
have achieved great success and been applied in the syntheses of many useful
polyfunctional compounds. A series of coordinating groups were employed as the directing groups in
this strategy.1 Compared to the formation of C–C bond, the reports of the formation of C–O bond are
less well established, maybe because of the electronegativity of the elements as well as the
metal–ligand bond strength.2 Introducing alkoxy group on the aromatic ring has great value in organic
synthesis because of the strong electron-donating conjugation effect of this group. And also, the aryl
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ether functionality is a common motif of pharmaceutical, functional materials and many other fine
chemicals.3 Therefore to develop new methodology of direct alkoxylation of C–H bond is becoming a
valuable and atom-economical strategy to construct this structural block.4 A few research groups,
including our group reported the directed ortho-alkoxylation of the C(sp2)−H bonds of arenes or
C(sp3)−H bonds catalyzed by palladium, in which oxime ether, N-methoxyamide, amide and cyano
were respectively used as the directing goups.5 These successful results push us to continue to carry
out our research in this area.
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Aromatic azo derivatives, a group of important conjugated compounds, are widely used not only
in the traditional chemical industry, such as dyes and pigments,6 but more noticeably, in many newly
rising areas of science and technology, such as photochemical molecular switch, super-molecular
chemistry of host-guest recognition, self-assembly liquid crystal material, analysis of biomedical
imaging and chemical, light driven molecular motor, energy conversion and so on.7 Therefore the
synthesis of aromatic azo derivatives, as well as the modification and functionalization to these
compounds in recent years have once again become the academic hot-spot. A few examples concerned
with the palladium-catalyzed functionalization to aromatic azo compounds.8 Very recently,
palladium-catalyzed ortho-acylation and halogenation of aromatic azo compounds were reported
respectively.9 A rhodium(III)-catalyzed C−H bond addition of azobenzenes to aldehydes was also
developed.10 Herein we want to describe a Pd−catalyzed ortho-alkoxylation of the C(sp2)−H bond at
aromatic ring directed by azo group using alcohols as alkoxylation reagents to synthesize 2-alkoxyl
aromatic azo compounds.
We started to optimize the reaction conditions by using azobenzene (1a) as the substrate and
methanol (1b) as both the alkoxylation reagent and solvent. In the absence of Pd(OAc)2, the reaction
could not proceed at all. The presences of Pd(OAc)2 (10 mol%) and an oxidant PhI(OAc)2 led to a
formation of the desired monomethoxylation product 3aa with very high selectivity. In order to
improve the yield, A series of oxidants were tested, in which no conversion was observed when
Cu(OAc)2, AgOAc, BQ or O2 was used as the oxidant (entries 12~15), and a very low yield was
obtained when K2S2O8 or (NH4)2S2O8 was used (entries 10, 11). PhI(OAc)2 was proved to be the best
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oxidant, and the appropriate amount of it was 2 eq (entries 1, 9). Some additives such as AcOH, TFA,
TsOH and CH3SO3H were also tested. In the presence of AcOH, the yield did not increase evidently
(entries 2~4), but the presence of TFA, TsOH and CH3SO3H was obviously not favorable for this
reaction (entries 5~7). So no additive was employed in our determined reaction conditions. Other
palladium catalysts such as PdCl2, PdCl2(CH3CN)2 and PdCl2(PPh3)2 showed low catalytic activity
(entries 16~18), while Ru and Rh catalysts [RuCl2(p-cymene)]2 and [RhCp*Cl2]2 demonstrated
entirely no efficiency to this reaction (entries 19, 20). Different reaction temperatures were also
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investigated. At 80 C, the best result was obtained. Decreasing the temperature to 60 C or room
temperature brought the reduction of the yield. And also, there was no obvious promotion while
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increase the temperature to 100 C (entry 1). Besides, the yield decreased sharply when 5 mol%
Pd(OAc)2 was used (entry 8).
Table 1. Optimization of the Reaction Conditionsa
entry
catalyst
oxidant (eq)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
additive (eq)
none
yield (%)b
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
75 (23, 62, 73)c
AcOH (1)
AcOH (5)
AcOH (10)
TFA (1)
73
70
71
<10
<10
<10
47d
TsOH (1)
CH3SO3H (1)
none
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Pd(OAc)2
PhI(OAc)2 (1)
K2S2O8 (2)
none
none
none
none
none
none
none
none
none
none
none
none
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13
23
0
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20
Pd(OAc)2
Pd(OAc)2
(NH4)2S2O8 (2)
Cu(OAc)2 (2)
AgOAc (2)
Pd(OAc)2
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Pd(OAc)2
0
Pd(OAc)2
BQ (2)
0
Pd(OAc)2
O2 (1atm)
0
PdCl2
PhI(OAc)2 (2)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
PhI(OAc)2 (2)
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0
PdCl2(CH3CN)2
PdCl2(PPh3)2
[RuCl2(p-cymene)]2
[RhCp*Cl2]2
0
aUnless otherwise specified, all the reactions were carried out in a sealed tube in the presence of azobenzene (1a, 0.50
mmol), methanol (2a, 2mL), catalyst (10 mol%) and oxidant (1 mmol) under air atmosphere at 80 oC for 24 h. bIsolated
yields. cAt room temperature, 60 and 100 oC, respectively. dPd(OAc)2 (5 mol%) was used.
Table 2. The ortho-Alkoxylation of Azobenzene Derivatives with Alcoholsa
O
N
N
N
N
N
N
N
N
O
O
O
3ab 63%
3ac 77%b
3aa 75%
3ad 55%b
N
O
N
N
N
N
N
N
N
O
O
O
3ae 62% (36 h)b
3af 65% (36 h)b
3ba 69%
3ag 52% (72 h)b
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O
N
O
N
O
N
O
N
N
N
N
N
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3bc 70%b
3bb 57%
3bf 67%b
3bd 51%b
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Cl
N
Cl
N
Cl
N
O
N
Cl
N
Cl
N
Cl
N
N
O
O
O
3bg 47%b
3cc 59% (100 oC, 48 h)b
3ca 62%
3cb 43%
Cl
Br
N
Br
N
Br
N
O
N
Br
N
Br
N
Br
N
N
O
O
O
Cl
3dc 65% (100 oC, 36 h)b
3cd 35% (100 oC, 72 h) c
3db 47%
3da 66%
Br
O
O
O
N
O
N
N
N
N
N
O
N
O
N
O
O
O
O
Br
3ec 51%b
3eb 55%
3fa 42%
3dd 40% (100 oC, 72 h)b
aUnless otherwise specified, all reactions were carried out with aromatic azo compound (1, 0.5 mmol), alcohol (2,
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2mL), Pd(OAc)2 (10 mol %), PhI(OAc)2 (1.0 mmol) under air atmosphere at 80 C for 24 h. All listed yields are
isolated one. b20 equiv AcOH was used as additive.
With suitable conditions for the methoxylation of azobenzene in hand, the reactivities of different
alcohols as the alkoxylation reagents were investigated. The results were revealed in Table 2. To our
delight, the alkoxylation with ethanol (2b) afforded product 3ab in a yield of 63%. Unfortunately,
propanol (2c) and isopropanol (2d) did not undergo the reaction smoothly to give corresponding
alkoxylation products in the same condition. Then we attempted to employ some additive. To our
surprise, the propoxylation and isopropoxylation products (3ac, 3ad) were obtained with the yield of
77% and 55% respectively by adding 20 equiv AcOH. Under this reaction condition, the alkoxylation
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products with some other primary or secondary alcohols such as butanol (2e), isobutanol (2f) and
cyclopentanol (2g) were achieved in moderate yield for the reaction time of 36-72 hours (3ae~3ag),
while more sterically hindered tert-butyl alcohol failed to provide the desired product, which was
similar to the previous reported alkoxylation reactions.5 It was interesting that the reaction gave the
monoalkoxylation products with very high selectivity for the alcohols we used.
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After screening of different alcohols, we explored the scope of differently substituted azobenzenes.
Substituents on the aromatic moiety of aromatic azo compounds influence the efficiency of this
coupling reaction significantly. It was unfortunate that the method seems not very efficient to the
ortho- or para-substituted azobenzene derivatives in the present reaction conditions. For example, for
the methoxylation of 2,2'-dimethylazobenzene and 4,4'-dimethylazobenzene, only trace desired
products generated. However, a range of meta-substituted azobenzenes could be adopted in our
alkoxylation methodology. The reaction of azobenzene derivatives having an electron-donating group
such as methyl or methoxy on the meta position of phenyl ring proceeded smoothly to give the
corresponding products. By using primary alcohols (methanol, ethanol, propanol, isobutanol), the
satisfactory yields could be obtained in 51%~70% (3ba~3bc, 3bf, 3eb~3ec) while the secondary
alcohol (isopropanol, cyclopentanol) giving lower yield of 51% and 47% (3bd, 3bg), respectively.
Alkoxylation of aromatic azo compounds bearing a weak electron-withdrawing group (-Cl, -Br) group
at the meta position of the phenyl ring was less efficient apart from methoxylation reaction (3ca, 3da).
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For example, the propoxylation and isopropoxylation needed higher temperature (100 C) and longer
reaction time (36~72 h) to afford the products (3cc, 3cd, 3dc, 3dd). Other electron-withdrawing group,
such as acetyl, could also be functionalized to bring out the desired product, albeit with a relatively
low yield (3fa).
On the basis of previous related studies5,9 and our experiments, a possible mechanism of this
palladium-catalyzed alkoxylation of aromatic azo compounds is proposed as shown in Scheme 1. First,
the coordination of azobenzene (1) with Pd (II) catalyst and subsequent C-H activation to undergo
cyclopalladation formed the palladacycle A. Then the aryl palladium intermediate A was oxidized to
Pd (IV) species B by PhI(OAc)2 in the presence of alcohol. Next, the final product 3 was obtained via
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reductive elimination of B, followed by the regeneration of Pd (II) catalyst. In addition, Ritter11
proposed a Pd(II)/Pd(III) catalytic cycle through a bimetallic Pd(III) complex for a similar directed
C−O bond formation reactions. Recently, Sunoj5c suggested a Pd(II)/Pd(0) process for a direct
alkoxylation of N-methoxybenzamides. These possible mechanisms should not be excluded.
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Scheme 1. Plausible Reaction Mechanism
In summary, we have developed an efficient route for direct alkoxylation of aromatic azo
compounds via a Pd-catalyzed azo group directed sp2 C−H bond activation, in which PhI(OAc)2 was
found to be a particularly effective oxidant to these transformations and exhibited functional group
tolerance. A series of azobenzene derivatives with either electron-donating or electron-withdrawing
groups could be alkoxylated directly and efficiently. The method was applicable to both primary and
secondary alcohols. This work provided a convenient method for the syntheses of 2-alkoxyl aromatic
azo compounds from readily accessible starting materials under mild reaction conditions and therefore
is an important extension of the chemistry of azo compounds.
Experimental Section
General
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All reactions were run in a sealed tube with a Teflon lined cap under air atmosphere. Chemicals
were commercially available and were used without purification. Aromatic azo compounds substrates
were prepared according to the literature procedures.12 1H NMR and 13C NMR spectra were recorded
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in CDCl3 [using (CH3)4Si (for H, δ = 0.00; for 13C, δ = 77.00) as internal standard]. The following
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abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, m
= multiplet. Melting points are uncorrected. HRMS was obtained by ESI on TOF mass analyzer.
General Experimental Procedures and Characterizations
Azobenzene (0.5 mmol), alcohol (2 mL), Pd(OAc)2 (0.05 mmol) and PhI(OAc)2 (1.0 mmol) were
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added in a 25 mL sealed tube with a Teflon lined cap. The mixture was heated at 80 C (oil bath
temperature) for 24 h. After cooling to room temperature, the volatiles were removed under reduced
pressure, and the residue was purified by a flash column chromatography on silica gel using
hexane/ethyl acetate as eluent to give the corresponding product.
(E)-1-(2-Methoxyphenyl)-2-phenyldiazene (3aa)13 Yield: 75% (79 mg). Red thick oil. 1H NMR
(CDCl3, 400 MHz): δ 7.95-7.93 (m, 2H), 7.70 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H), 7.55-7.45 (m, 4H),
7.12 (dd, J1 = 8.0 Hz, J2 = 0.6 Hz, 1H), 7.07-7.03 (m, 1H), 4.06 (s, 3H). 13C NMR (CDCl3, 100 MHz):
δ 157.0, 153.2, 142.3, 132.5, 130.8, 129.0, 123.0, 120.8, 117.0, 112.7, 56.4.
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(E)-1-(2-Ethoxyphenyl)-2-phenyldiazene (3ab) Yield: 63% (72 mg). Reddish orange oil. H NMR
(CDCl3, 400 MHz): δ 7.94 (d, J = 1.2 Hz, 2H), 7.69 (dd, J1 = 8.0 Hz, J2 = 2.0 Hz, 1H), 7.55-7.42 (m,
4H), 7.12 (dd, J1 = 8.0 Hz, J2 = 1.0 Hz, 1H), 7.06-7.02 (m, 1H), 4.29 (q, J = 6.8 Hz, 2H), 1.55 (t, J =
7.2 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ 156.5, 153.1, 142.6, 132.4, 130.7, 129.1, 123.0, 120.8,
117.0, 114.5, 65.3, 14.9. HRMS-ESI (m/z): calcd for C14H15N2O [M + H]+ 227.1179, found 227.1168.
(E)-1-Phenyl-2-(2-propoxyphenyl)diazene (3ac) Yield: 77% (93 mg). Red thick oil. 1H NMR (CDCl3,
400 MHz): δ 7.97-7.94 (m, 2H), 7.70 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H), 7.56-7.42 (m, 4H), 7.12 (dd,
J1 = 8.4 Hz, J2 = 1.2 Hz, 1H), 7.06-7.02 (m, 1H), 4.20 (t, J = 6.4 Hz, 2H), 1.99-1.92 (m, 2H), 1.13 (t, J
= 7.2 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ 156.7, 153.2, 142.7, 132.3, 130.7, 129.0, 123.0, 120.8,
117.0, 114.7, 71.3, 22.7, 10.6. HRMS-ESI (m/z): calcd for C15H17N2O [M + H]+ 241.1341, found
241.1346.
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(E)-1-(2-Isopropoxyphenyl)-2-phenyldiazene (3ad) Yield: 55% (66 mg). Red thick oil. H NMR
(CDCl3, 400 MHz): δ 7.94 (d, J = 1.2Hz, 2H), 7.69 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H), 7.56-7.48 (m,
3H), 7.44-7.40 (m, 1H), 7.14 (dd, J1 = 8.0 Hz, J2 = 1.0 Hz, 1H), 7.08-7.04 (m, 1H), 4.72 (m, 1H), 1.45
(d, J = 4.0 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 156.0, 153.1, 144.1, 132.1, 130.7, 129.0, 123.0,
121.5, 118.5, 117.1, 73.8, 22.3. HRMS-ESI (m/z): calcd for C15H17N2O [M + H]+ 241.1341, found
241.1338.
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(E)-1-(2-Butoxyphenyl)-2-phenyldiazene (3ae) Yield: 62% (79 mg). Red thick oil. H NMR (CDCl3,
400 MHz): δ 7.96-7.94 (m, 2H), 7.70 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H), 7.56-7.42 (m, 4H), 7.12 (dd,
J1 = 8.4 Hz, J2 = 0.8 Hz, 1H), 7.06-7.02 (m, 1H), 4.22 (t, J = 6.4 Hz, 2H), 1.96-1.89 (m, 2H),
1.64-1.55 (m, 2H), 1.04 (t, J = 7.2 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ 156.7, 153.2, 142.7, 132.3,
130.7, 129.0, 123.0, 120.8, 117.0, 114.7, 69.6, 31.4, 19.3, 13.9. HRMS-ESI (m/z): calcd for
C16H19N2O [M + H]+ 255.1497, found 255.1483.
(E)-1-(2-Isobutoxyphenyl)-2-phenyldiazene (3af) Yield: 65% (83 mg). Red thick oil. 1H NMR (CDCl3,
400 MHz): δ 7.94 (d, J = 2.8 Hz, 2H), 7.70 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H), 7.56-7.41 (m, 4H), 7.11
(dd, J1 = 8.4 Hz, J2 = 0.8 Hz, 1H), 7.06-7.01 (m, 1H), 3.97 (d, J = 6.4 Hz, 2H), 2.29-2.22 (m, 1H),
1.12 (d, J = 6.4 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 156.8, 153.2, 142.7, 132.3, 130.6, 129.0,
123.0, 120.7, 117.0, 114.7, 76.1, 28.5, 19.3. HRMS-ESI (m/z): calcd for C16H19N2O [M + H]+
255.1497, found 255.1501.
(E)-1-(2-(Cyclopentyloxy)phenyl)-2-phenyldiazene (3ag) Yield: 52% (69 mg). Red thick oil. 1H NMR
(CDCl3, 400 MHz): δ 7.96-7.94 (m, 2H), 7.69 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H), 7.57-7.40 (m, 4H),
7.14-7.01 (m, 2H), 5.00 (t, J = 2.4 Hz, 1H), 2.08-1.87 (m, 6H), 1.69-1.62 (m, 2H). 13C NMR (CDCl3,
100 MHz): δ 155.9, 153.2, 143.5, 132.1, 130.6, 129.1, 123.0, 120.8, 117.0, 116.8, 81.8, 33.0, 24.0.
HRMS-ESI (m/z): calcd for C17H19N2O [M + H]+ 267.1497, found 267.1492.
(E)-1-(2-Methoxy-5-methylphenyl)-2-m-tolyldiazene (3ba) Yield: 69% (83 mg). Reddish orange solid,
mp: 71-73 oC. 1H NMR (CDCl3, 400 MHz): δ 7.76-7.73 (m, 2H), 7.49 (d, J = 1.6 Hz, 1H), 7.41 (t, J
= 8.0 Hz, 1H), 7.30-7.26 (m, 2H), 7.01 (d, J = 8.0 Hz, 1H), 4.03 (s, 3H), 2.47 (s, 3H), 2.37 (s, 3H). 13
C
NMR (CDCl3, 100 MHz): δ 155.0, 153.3, 142.0, 138.9, 132.9, 131.5, 130.3, 128.8, 122.6, 121.0,
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117.2, 112.7, 56.5, 21.3, 20.5. HRMS-ESI (m/z): calcd for C15H17N2O [M + H]+ 241.1341, found
241.1352.
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(E)-1-(2-Ethoxy-5-methylphenyl)-2-m-tolyldiazene (3bb) Yield: 57% (72 mg). Orange solid, mp:
57-59 oC. 1H NMR (CDCl3, 400 MHz): δ 7.75 (d, J = 6.4 Hz, 2H), 7.48 (d, J = 1.6 Hz, 1H), 7.41 (t, J
= 8.0 Hz, 1H), 7.30 (s, 1H), 7.24 (dd, J1 = 8.4 Hz, J2 = 2.0 Hz, 1H), 7.01 (d, J = 8.4 Hz, 1H), 4.26 (q, J
= 6.8 Hz, 2H), 2.47 (s, 3H), 2.36 (s, 3H), 1.53 (t, J = 7.0 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ
154.5, 153.2, 142.4, 138.9, 132.8, 131.5, 130.4, 128.8, 123.1, 120.6, 117.2, 114.8, 65.6, 21.4, 20.5,
14.9. HRMS-ESI (m/z): calcd for C16H18N2ONa [M + Na]+ 277.1317, found 277.1333.
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(E)-1-(5-Methyl-2-propoxyphenyl)-2-m-tolyldiazene (3bc) Yield: 70% (94 mg). Red thick oil. H
NMR (CDCl3, 400 MHz): δ 7.76-7.75 (m, 2H), 7.49 (d, J = 1.6 Hz, 1H), 7.43-7.39 (m, 1H), 7.30-7.23
(m, 2H), 7.02 (d, J = 8.4 Hz, 1H), 4.15 (t, J = 6.4 Hz, 2H), 2.48 (s, 3H), 2.36 (s, 3H), 1.96-1.91 (m,
2H), 1.11 (t, J = 7.6 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ 154.7, 153.3, 142.5, 138.8, 132.8, 131.4,
130.4, 128.8, 123.2, 120.5, 117.2, 115.0, 71.7, 22.7, 21.4, 20.5, 10.5. HRMS-ESI (m/z): calcd for
C17H21N2O [M + H]+ 269.1648, found 269.1642.
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(E)-1-(2-Isopropoxy-5-methylphenyl)-2-m-tolyldiazene (3bd) Yield: 51% (68 mg). Red thick oil. H
NMR (CDCl3, 400 MHz): δ 7.78-7.76 (m, 2H), 7.50 (d, J = 1.6 Hz, 1H), 7.45-7.40 (m, 1H), 7.31-7.29
(m, 1H), 7.25-7.22 (m, 1H), 7.05 (d, J = 8.0 Hz, 1H), 4.65 (m, 1H), 2.49 (s, 3H), 2.38 (s, 3H), 1.44 (d,
J = 6.0 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 154.0, 153.2, 144.0, 138.9, 132.7, 131.5, 131.4,
128.8, 123.5, 120.3, 119.2, 117.2, 74.4, 22.3, 21.4, 20.6. HRMS-ESI (m/z): calcd for C17H21N2O [M +
H]+ 269.1648, found 269.1653.
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(E)-1-(2-Isobutoxy-5-methylphenyl)-2-m-tolyldiazene (3bf) Yield: 67% (95 mg). Red thick oil. H
NMR (CDCl3, 400 MHz): δ 7.78-7.76 (m, 2H), 7.49 (d, J = 1.6 Hz, 1H), 7.44-7.40 (m, 1H), 7.30-7.29
(m, 1H), 7.25-7.23 (m, 1H), 7.01 (d, J = 8.4 Hz, 1H), 3.95 (d, J = 6.4 Hz, 2H), 2.48 (s, 3H), 2.37 (s,
3H), 2.29-2.19 (m, 1H), 1.11 (d, J = 6.8 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 154.9, 153.3, 142.5,
138.8, 132.8, 131.4, 130.3, 128.8, 123.4, 120.3, 117.1, 115.1, 76.6, 28.5, 21.4, 20.5, 19.3. HRMS-ESI
(m/z): calcd for C18H23N2O [M + H]+ 283.1805, found 283.1803.
(E)-1-(2-(Cyclopentyloxy)-5-methylphenyl)-2-m-tolyldiazene (3bg) Yield: 47% (69 mg). Red thick oil.
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1H NMR (CDCl3, 400 MHz): δ 7.76-7.74 (m, 2H), 7.48-7.47 (m, 1H), 7.41 (t, J = 8.2 Hz, 1H), 7.30 (t,
J = 0.4 Hz, 1H), 7.24-7.21 (m, 1H), 7.02 (d, J = 8.4 Hz, 1H), 4.95 (m, 1H), 2.48 (s, 3H), 2.36 (s, 3H),
2.04-2.02 (m, 2H), 1.95-1.89 (m, 4H), 1.66 (m, 2H). 13C NMR (CDCl3, 100 MHz): δ 153.8, 153.2,
143.4, 138.8, 132.7, 131.4, 130.5, 128.8, 123.4, 120.3, 117.3, 117.2, 82.3, 32.9, 23.9, 21.4, 20.5.
HRMS-ESI (m/z): calcd for C19H23N2O [M + H]+ 295.1810, found 295.1823.
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(E)-1-(5-Chloro-2-methoxyphenyl)-2-(3-chlorophenyl)diazene (3ca) Yield: 62% (87 mg). Orange
solid, mp:77-79 oC. 1H NMR (CDCl3, 400 MHz): δ 7.91-7.89 (m, 1H), 7.86-7.81 (m, 1H), 7.69 (d, J =
2.8 Hz, 1H), 7.49-7.42 (m, 3H), 7.05 (d, J = 8.8 Hz, 1H), 4.05 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ
156.0, 153.6, 142.1, 135.2, 132.3, 130.9, 130.1, 126.5, 122.8, 121.8, 117.0, 114.1, 56.7. HRMS-ESI
(m/z): calcd for C13H11Cl2N2O [M + H]+ 281.0248, found 281.0272.
(E)-1-(5-Chloro-2-ethoxyphenyl)-2-(3-chlorophenyl)diazene (3cb) Yield: 43% (63 mg). Orange solid,
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mp: 54-55 oC. H NMR (CDCl3, 400 MHz): δ 7.91-7.90 (m, 1H), 7.87-7.84 (m, 1H), 7.69 (d, J = 2.8
Hz, 1H), 7.48-7.45 (m, 2H), 7.40 (dd, J1 = 8.8 Hz, J2 = 2.4 Hz, 1H), 7.05 (d, J = 9.2 Hz, 1H), 4.30 (q,
J = 7.2 Hz, 2H), 1.55 (t, J = 6.8 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ 155.5, 153.6, 142.4, 135.1,
132.2, 130.9, 130.1, 126.5, 122.6, 122.0, 117.0, 115.8, 65.7, 14.8. HRMS-ESI (m/z): calcd for
C14H13Cl2N2O [M + H]+ 295.0405, found 295.0418.
(E)-1-(5-Chloro-2-propoxyphenyl)-2-(3-chlorophenyl)diazene (3cc) Yield: 59% (91 mg). Red thick oil.
1H NMR (CDCl3, 400 MHz): δ 7.90 (d, J = 1.2 Hz, 1H), 7.86-7.83 (m, 1H), 7.68 (d, J = 2.4 Hz, 1H),
7.48-7.46 (m, 2H), 7.40 (dd, J1 = 8.8 Hz, J2 = 2.4 Hz, 1H), 7.05 (d, J = 8.8 Hz, 1H), 4.16 (t, d = 6.4 Hz,
2H), 1.94 (m, 2H), 1.12 (t, J = 7.6 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ 155.7, 153.6, 142.5, 135.1,
132.0, 130.9, 130.1, 126.5, 122.5, 122.1, 117.0, 115.9, 71.6, 22.6, 10.5. HRMS-ESI (m/z): calcd for
C15H15Cl2N2O [M + H]+ 309.0561, found 309.0567.
(E)-1-(5-Chloro-2-isopropoxyphenyl)-2-(3-chlorophenyl)diazene (3cd) Yield: 35% (54 mg). Orange
solid, mp: 50-52 oC. 1H NMR (CDCl3, 400 MHz): δ 7.91-7.90 (m, 1H), 7.86-7.83 (m, 1H), 7.68 (d, J =
2.4 Hz, 1H), 7.50-7.45 (m, 2H), 7.39 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1H), 7.06 (d, J = 5.6 Hz, 1H), 4.70
(m, 1H), 1.45 (d, J = 6.0 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 154.9, 153.6, 143.8, 135.1, 132.1,
130.9, 130.1, 127.1, 122.6, 122.2, 119.4, 117.1, 74.1, 22.2. HRMS-ESI (m/z): calcd for C15H15Cl2N2O
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[M + H]+ 309.0561, found 309.0566.
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(E)-1-(5-Bromo-2-methoxyphenyl)-2-(3-bromophenyl)diazene (3da) Yield: 66% (122 mg). Orange
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solid, mp: 140-142 oC. H NMR (CDCl3, 400 MHz): δ 8.05 (t, J = 2.0 Hz, 1H), 7.91-7.88 (m, 1H),
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7.81 (d, J = 2.4 Hz, 1H), 7.63-7.56 (m, 2H), 7.42 (t, J = 8.0 Hz, 1H), 7.01 (d, J = 8.8 Hz, 1H), 4.05 (s,
3H). 13C NMR (CDCl3, 100 MHz): δ 156.4, 153.7, 142.4, 135.2, 133.9, 130.5, 124.6, 123.4, 123.2,
120.0, 114.6, 113.8, 56.6. HRMS-ESI (m/z): calcd for C13H11Br2N2O [M + H]+ 370.9218, found
370.9240.
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(E)-1-(5-Bromo-2-ethoxyphenyl)-2-(3-bromophenyl)diazene (3db) Yield: 47% (90 mg). Orange solid,
mp: 77-79 oC. 1H NMR (CDCl3, 400 MHz): δ 8.06 (t, J = 2.0 Hz, 1H), 7.91-7.88 (m, 1H), 7.81 (d, J =
2.4 Hz, 1H), 7.63-7.60 (m, 1H), 7.56-7.53 (m, 1H), 7.42 (t, J = 8.0 Hz, 1H), 7.00 (d, J = 8.8 Hz, 1H),
4.27 (q, J = 6.8 Hz, 2H), 1.55 (t, J = 7.2 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ 155.9, 153.7, 142.7,
135.1, 133.8, 130.4, 125.0, 123.2, 123.1, 120.0, 116.2, 113.8, 65.6, 14.8. HRMS-ESI (m/z): calcd for
C14H13Br2N2O [M + H]+ 384.9374, found 384.9377.
(E)-1-(5-Bromo-2-propoxyphenyl)-2-(3-bromophenyl)diazene (3dc) Yield: 65% (129 mg). Orange
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solid, mp: 55-57 oC. H NMR (CDCl3, 400 MHz): δ 8.06 (t, J = 2.0 Hz, 1H), 7.90-7.88 (m, 1H), 7.81
(d, J = 2.4 Hz, 1H), 7.63-7.60 (m, 1H), 7.53 (dd, J1 = 8.8 Hz, J2 = 2.8 Hz, 1H), 7.41 (t, J = 8.0 Hz, 1H),
7.00 (d, J = 8.8 Hz, 1H), 4.15 (t, J = 6.4 Hz, 2H), 1.94 (m, 2H), 1.12 (t, J = 6.8 Hz, 3H). 13C NMR
(CDCl3, 100 MHz): δ 156.1, 153.7, 142.8, 135.1, 133.8, 130.4, 125.1, 123.1, 123.0, 120.0, 116.3,
113.7, 71.6, 22.6, 10.4. HRMS-ESI (m/z): calcd for C15H15Br2N2O [M + H]+ 398.9531, found
398.9515.
(E)-1-(5-Bromo-2-isopropoxyphenyl)-2-(3-bromophenyl)diazene (3dd) Yield: 40% (79 mg). Orange
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solid, mp: 62-64 oC. H NMR (CDCl3, 400 MHz): δ 8.06 (t, J = 2.0 Hz, 1H), 7.90-7.88 (m, 1H), 7.81
(d, J = 2.4 Hz, 1H), 7.63-7.60 (m, 1H), 7.53 (dd, J1 = 8.8 Hz, J2 = 2.8 Hz, 1H), 7.42 (t, J = 8.0 Hz, 1H),
7.03 (d, J = 8.8 Hz, 1H), 4.69 (m, 1H), 1.45 (d, J = 6.0 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 155.4,
153.7, 144.0, 135.0, 133.8, 130.4, 125.1, 123.1, 123.0, 120.1, 119.7, 114.3, 74.0, 22.6, 22.2.
HRMS-ESI (m/z): calcd for C15H15Br2N2O [M + H]+ 398.9531, found 398.9542.
(E)-1-(2-Ethoxy-5-methoxyphenyl)-2-(3-methoxyphenyl)diazene (3eb) Yield: 55% (79 mg). Red thick
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oil. H NMR (CDCl3, 400 MHz): δ 7.61-7.58 (m, 1H), 7.49 (t, J = 2.4 Hz, 1H), 7.44 (t, J = 8.0 Hz,
1H), 7.29 (t, J = 2.8 Hz, 1H), 7.08-7.03 (m, 3H), 4.25 (q, J = 6.8 Hz, 2H), 3.92 (s, 3H), 3.86 (s, 3H),
1.51 (t, J = 7.2 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ 160.3, 154.3, 151.5, 142.9, 129.7, 119.6,
117.6, 117.3, 117.1, 106.2, 100.4, 66.9, 55.8, 55.4, 15.1. HRMS-ESI (m/z): calcd for C16H19N2O3 [M
+ H]+ 287.1396, found 287.1401.
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(E)-1-(5-Methoxy-2-propoxyphenyl)-2-(3-methoxyphenyl)diazene (3ec) Yield: 51% (76 mg). Red
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thick oil. H NMR (CDCl3, 400 MHz): δ 7.61-7.58 (m, 1H), 7.50 (t, J = 2.4 Hz, 1H), 7.47-7.42 (m,
1H), 7.29 (t, J = 2.8 Hz, 1H), 7.09-7.00 (m, 3H), 4.14 (t, J = 6.4 Hz, 2H), 3.91 (s, 3H), 3.86 (s, 3H),
1.91-1.90 (m, 2H), 1.11 (t, J = 7.2 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ 160.3, 154.3, 154.2, 151.8,
142.9, 129.7, 119.6, 117.7, 117.4, 117.3, 105.7, 100.3, 72.9, 55.8, 55.4, 22.9, 10.6. HRMS-ESI (m/z):
calcd for C17H21N2O3 [M + H]+ 301.1552, found 301.1556.
(E)-1-(5-Acetyl-2-methoxyphenyl)-2-(3-acetylphenyl)diazene (3fa) Yield: 42% (62 mg). Orange solid,
mp: 129-131 oC. H NMR (CDCl3, 400 MHz): δ 8.50 (t, J = 2.0 Hz, 1H), 8.27 (d, J = 2.0 Hz, 1H),
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8.17-8.09 (m, 3H), 7.64 (t, J = 8.0 Hz, 1H), 7.19 (d, J = 8.8 Hz, 1H), 4.13 (s, 3H), 2.72 (s, 3H), 2.65 (s,
3H). 13C NMR (CDCl3, 100 MHz): δ 197.5, 196.7, 160.5, 153.0, 141.4, 138.2, 132.8, 130.5, 130.1,
129.5, 126.8, 123.6, 117.6, 112.5, 56.6, 26.8, 26.5. HRMS-ESI (m/z): calcd for C17H17N2O3 [M + H]+
297.1239, found 297.1243.
ASSOCIATED CONTENT
Supporting Information
Copies of 1H NMR and 13C NMR spectra for all products. This material is available free of charge via
the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: sunpeipei@njnu.edu.cn.
Notes
The authors declare no competing financial interest.
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■ ACKNOWLEDGMENTS
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This work was supported by the National Natural Science Foundation of China (Project 21272117 and
20972068) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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