Ru(bpy)2Cl2: A catalyst able to shift the course of the photorearrangement in the Boulton-Katritzky reaction

Article abstract of DOI:10.1016/j.tetlet.2015.10.030

The Boulton-Katritzky reaction represents one of the most popular and efficient strategies used to realize azole-into-azole conversions. For example, under different experimental conditions, it allows the rearrangement of Z-arylhydrazones of 3-benzoyl-5-p

Full text of DOI:10.1016/j.tetlet.2015.10.030

Tetrahedron Letters 56 (2015) 6598–6601
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Tetrahedron Letters
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Ru(bpy)2Cl2: a catalyst able to shift the course of the
photorearrangement in the Boulton–Katritzky reaction
b
c
b
a
Maurizio D’Auria a, , Vincenzo Frenna , Magda Monari , Antonio Palumbo-Piccionello , Rocco Racioppi ,
⇑
Domenico Spinelli c, Licia Viggiani a
a Dipartimento di Scienze, Università della Basilicata, Viale dell’Ateneo Lucano 10, 85100 Potenza, Italy
b Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche, Università di Palermo, Viale delle Scienze, Ed. 17, 90128 Palermo, Italy
c Dipartimento di Chimica ‘G. Ciamician’, Alma Mater Studiorum—Università di Bologna, Via F. Selmi 2, 40126 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2015
Revised 1 October 2015
Accepted 7 October 2015
Available online 8 October 2015
The Boulton–Katritzky reaction represents one of the most popular and efficient strategies used to
realize azole-into-azole conversions. For example, under different experimental conditions, it allows
the rearrangement of Z-arylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazoles (1) into 2-aryl-4-
benzoylamino-5-phenyl-2H-1,2,3-triazoles (2) in very high yields. Moreover, we have recently realized
this conversion also by UV-photostimulation. Now we have enlarged the scope of the reaction irradiating
with visible or UV light an acetonitrile solution of some Z-arylhydrazones (1a–e) in the presence of catalytic
amounts of Ru(bpy)2Cl2. We have observed the unexpected formation of the 1-aryl-5-benzoylamino-3-
phenyl-1,2,4-triazoles (3a–e) eventually together with the expected 2a–d in high yields.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
1,2,4-Oxadiazole
Photochemical rearrangement
Ruthenium catalyst
Regiochemistry
DFT
Introduction
In this Letter we like to report the effect of the presence of Ru
(bpy)2Cl2 on the course of the BKR: we shall offer a methodology
The use of Ruthenium complexes in organic synthesis has
received great attention in recent years.1 Ru(bpy)3Cl2, after excita-
tion, is able to give a single electron transfer (SET) process that can
be used for both oxidation and reduction reactions. Thus, it can
reduce a C–Br bond to give an electron deficient radical able to
add with high stereoselectivity to chiral enamines.2 The same com-
plex was used to catalyze [2+2] cycloaddition reactions with high
diastereoselectivity.3 Some reviews cover the recent acquisitions
in this field,4 but, on the whole, the use of Ru(bpy)2Cl2 in organic
synthesis has not been significantly investigated.
unexpectedly able to change regioselectivity in a photostimulated
reaction. We think that this result can exceed the interest in the
chemistry of azoles and could open the way to further applications
thus becoming of general interest and utility in organic syntheses.9
The regioselectivity that we have observed in the BKR of Z-aryl-
hydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole in different
solvents in the absence of photostimulation7a–d (via
a SNi
process)7c or by photostimulation in acetonitrile in the absence
of catalysts,8 has been related to the fact that the attack of the aryl-
hydrazonic NH on the N-2 atom of the 1,2,4-oxadiazole ring gave a
heterocyclic system (the 1,2,3-triazole), much more aromatic of
the starting 1,2,4-oxadiazole,10 while the eventual attack on the
N-4 atom of the above ring seems not able to furnish a stable hete-
rocyclic compound.
Ru(bpy)2Cl2 shows an absorption at kmax = 538 nm (e 9890 in
N-methylformamide)5 and is able to give photosubstitution reac-
tions with molecules possessing a lone pair,6 because of its ability
to give reactive adducts.
In the framework of our interest in the study of azole
reactivity,7a–d we have recently reported that the irradiation of
some arylhydrazones of 3-benzoyl-1,2,4-oxadiazole 1 gave the
corresponding triazoles 2 (Scheme 1)8 furnishing a photochemical
version of the Boulton–Katritzky reaction (BKR).7
Results and discussion
We have observed that the irradiation of an acetonitrile solution
of some Z-arylhydrazones (1a–e: see Scheme 2 and entries 1, 3, 5, 7
and 9 of Table 1) in the presence of catalytic amounts of Ru(bpy)2
Cl2Á3H2O with visible light gives in high yields (60–95%) unex-
pected reaction products, whose structures have been ascertained
by 1H, 13C NMR, and ESI-MS spectra and definitively supported by
⇑
Corresponding author. Tel.: +39 0971 205480; fax: +39 0971 205678.
E-mail address: maurizio.dauria@unibas.it (M. D’Auria).
http://dx.doi.org/10.1016/j.tetlet.2015.10.030
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

M. D’Auria et al. / Tetrahedron Letters 56 (2015) 6598–6601
6599
Table 1
Photochemical reaction of Z-arylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazoles
1a–c in the presence of Ru(bpy)2Cl2Á3H2O
1
R
N
NH
N
Entry Substrate Irradiation
condition
Irradiation
time (h)
Product 213
(Yield, %)
Product 313
(Yield, %)
N
NH
O
ν
h
1
N
N
R
O
N
1
2
3
4
5
6
7
8
9
10
1a
1a
1b
1b
1c
1c
1d
1d
1e
1e
Visible11
UV12
48
120
48
48
48
3a (95)
3a (60)
3b (93)
3b (45)
3c (75)
3c (44)
3d (60)
3d (34)
3e (85)
3e (60)
3
2a (30)
2b (45)
2c (25)
2d (25)
R
2
Visible11
UV12
R
3
R
Visible11
UV12
2
1
R
48
Visible11
UV12
216
216
48
2
Visible11
UV12
Scheme 1. Photochemical rearrangement of 1,2,4-oxadiazoles
1,2,3-triazoles 2.
1
to give
48
X-ray analysis (Fig. 1): they are the unknown 1-aryl-5-
benzoylamino-3-phenyl-1,2,4-triazoles (3a–e). Of course indepen-
dent experiments (blank tests) have shown that compounds 1a–e
in acetonitrile under visible-stimulation at 20 °C stay practically
unchanged in the absence of Ru(bpy)2Cl2Á3H2O. It is interesting
to know that the presence of a nitro group in meta position on
the arylhydrazonic aromatic group induced a marked decrease of
the reactivity of starting material. The E-isomer of 3d showed the
same behavior.
For the sake of comparison we have carried out in parallel the
reaction under UV-stimulation and we have observed the
formation of compound 3a–e (main products) and of the expected
products of BKR, that is, compounds 2a–d (see Scheme 2 and
entries 2, 4, 6, and 8 of Table 1). Only compound 1e gave only 3e
as the reaction product in both the reaction conditions tested.
Probably the formation of 2a–d occurs because of the
UV-stimulation (see above).8
The role of Ruthenium complex in this reaction can be under-
stood on the basis of some previous results6 and is supported by
calculations. We have optimized the structure of two possible
adducts between Ru(pby)2Cl2 and the Z-phenylhydrazone of
3-benzoyl-5-phenyl-1,2,4-oxadiazole in acetonitrile. On the basis
of the previous reported work,6 we can formulate the hypothesis
that a chloride is substituted by acetonitrile, while the other
chloride can be substituted by the 1,2,4-oxadiazole derivative.
Figure 1. ORTEP drawing of 3a showing the atom labeling scheme. Displacement
ellipsoids are drawn at the 30% probability level. CCDC-1029103 contains the
supplementary crystallographic data for this Letter. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
2+
NHPh
NHPh
N
O
2+
N
N
N
Ph
Ph
O
Ph
N
N
visible light
Ph
N
N
N
NH
N
N
N
irradiation
O
N
N
Ru
Ru
Ru(bpy) Cl
2
2
N
1
N
N
N
1
N
R
N
H
R
N
O
N
C
CH
N
C
CH
3
R
2
R
3
3
3
R
1
2
A
B
R
1
2
3
a: R = Ph; R = H; R = CH
3
Figure 2. Possible complexes between Ru(bpy)2Cl2 and an 1,2,4-oxadiazole
derivative in acetonitrile.
3
1
2
3
b: R = Ph; R = H; R = Cl
1
2
3
c: R = Ph; R = OCH ; R = H
3
1
2
3
d: R = Ph; R = H; R = NO
2
1
2
3
e: R = Ph; R = Cl; R = H
The complex can derive from an interaction between
Ruthenium and the N-2 atom (A) or the N-4 one (B) (Fig. 2). The
structures have been optimized at the DFT/B3LYP/LanL2DZ level
of theory on Gaussian 09.14 The complex A appears much more
stable than B (see Supporting information). Of course, the forma-
tion of the complex between Ruthenium and the N-2 atom in the
oxadiazole ring inhibits its reactivity toward the normal BKR
allowing the possibility of attack on the other nitrogen atom (the
N-4 atom) of the ring.
UV irradiation
Ru(bpy) Cl
2
2
2
+
3
Scheme 2. Photochemical reaction of 1,2,4-oxadiazoles 1a–e in the presence of Ru
(bpy)2Cl2Á3H2O.

6600
M. D’Auria et al. / Tetrahedron Letters 56 (2015) 6598–6601
Cl
N
N
N
N
Ru
Cl
hν
CH3CN
CH3
C
2+
N
N
N
N
N
NH
Ru
O
N
N
N
N
N
N
H
N
O
N
C
C
CH3
hν
hν
NHPh
N
2+
Ph
N
O
N
NPh
N
N
Ru
N
N
C
A
CH3
Figure 3. Catalytic cycle of Ru complex in the conversion 1 ? 3.
Ph
HN
Ph
N
N N
Ph
a
O
N
Ph
Ph
N
HN
N
N
H
Ph
N
Ph
N
N
Ph
Ph
O
O
b
Ph
N
N
Ph
C N
HN
Ph
O
Scheme 3. Evolution of the radical anion of 1 (R1 = Ph, R2 = R3 = H).
Figure 4. Reaction paths of the mechanism leading to 3.
The catalytic cycle of the Ruthenium complex can be rational-
ized as described in Figure 3. Calculations at the DFT/B3LYP/
LanL2DZ level of theory of the complex of Ruthenium with the
corresponding compound 3 showed that it cannot be formed
(see Supporting information).
In a previous work on this type of compounds the absorption of
the complexes deriving from a photochemical substitution of the
chlorine atoms with acetonitrile (C) was identified as charge
transfer bands. We can suppose that also A gave a charge transfer
transition. The evolution of the radical anion of the compound 1
(R1 = Ph, R2 = R3 = H) has been studied on the isolated molecule at
the DFT/B3LYP/6-311G+(d,p) level of theory and the results are
reported in Scheme 3 and Figure 4.
Ph
N
O
N
N Ph
Ph
N
H
Figure 5. The transition state ST2.
the hydrazonic N atom on the C-3 of the oxadiazole ring induces
a transposition reaction able to give directly product 3. In the tran-
sition state ST2 the first transposition of the benzylic carbon atom
on the nitrogen radical has just occurred (Fig. 5). In pathway b, the
cleavage of the C–C bond gave a nitrile and PhN(À)-N@C(Å)-Ph
radical anion able to give a [3+2] cycloaddition reaction to give
the product. Path b showed to present a highly endothermic step
The transfer on a hydrogen atom from the hydrazonic nitrogen
atom to the N-4 in the oxadiazole ring in the radical anion of 1
induces a ring opening with breaking of the O–N bond. Two possi-
ble evolution pathways have been tested. In path a, the attack of

M. D’Auria et al. / Tetrahedron Letters 56 (2015) 6598–6601
6601
11. Typical experimental procedure for irradiation with visible light. Compound
1a16 (0.42 mmol, 150 mg) was dissolved in acetonitrile (60 ml) in the presence
of Ru(bpy)2Cl2Á3H2O (5 Â 10À4 mmol). The mixture was deoxygenated by
(Fig. 4). These pathways are in agreement with the lower reactivity
of meta nitro substituted oxadiazole derivative. In fact, the pres-
ence of the nitro substituent can increase the delocalization of
the negative charge on the nitrogen atom.
nitrogen bubbling and irradiated with
a solar simulator (Suntest CPS+,
Heraeus) equipped with a Xenon lamp (1.1 KW), protected by a quartz plate.
The irradiation chamber was maintained at 20 °C by both circulating water
from a thermostatic bath and a conditioned airflow. After 2 days the solvent
was evaporated and the crude mixture was chromatographed on silica gel (9:1
hexanes/ethyl acetate).
Conclusion
12. Typical experimental procedure for irradiation with UV light. Compound 1a16
(0.42 mmol, 150 mg) was dissolved in acetonitrile (60 ml) in the presence of Ru
(bpy)2Cl2Á3H2O (5 Â 10À4 mmol). The mixture was deoxygenated by nitrogen
In the framework of our general interest in the study of the
rearrangement of azoles7a–d,8 and extending the application of
UV–visible irradiation as promoter of the reactivity in organic
systems we have examined the photorearrangement of some
Z-arylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazoles (1a–e)
in the presence of Ru(bpy)2Cl2, thus obtaining only the
unexpected 1-aryl-5-benzoyamino-3-pheny-1,2,4-triazoles (3a–e)
under visible stimulation and in contrast a mixture of 3a–e
(main products) and 2-aryl-4-benzoylamino-5-phenyl-2H-1,2,3-
triazoles (2a–d) under UV-stimulation.
The observed reactivity allowed us to report a new strategy to
obtain 1,2,4-triazole derivatives known for their biological proper-
ties as anti-inflammatory, CNS stimulants, sedatives, anti-anxiety,
antimicrobial, antimycotic, antitumor, and antiviral properties.15
Furthermore, the observed result (the shift of the course of the
reaction) represents the first instance of this kind of effect exerted
by Ru(bpy)2Cl2 in an organic reaction and opens the way to new
uses and applications of Ruthenium complexes. We think that this
observation on the ability of Ru(bpy)2Cl2 to modify the organic
reactivity represents a new and interesting result, which can open
the way to further applications of this catalyst in organic
syntheses.
bubbling and irradiated with
a Photochemical Multirays Reactor (Helios-
Italquartz, Milan, Italy) equipped with ten 15 W lamps whose output was
centered at 366 nm. After 5 days the solvent was evaporated and the crude
mixture was chromatographed on silica gel (9:1 hexanes/ethyl acetate).
13. Compound 2a: mp 213 °C; 1H NMR (CDCl3, 500 MHz) d: 7.95–7.90 (m, 5H),
7.81 (d, J = 6.5 Hz, 2H), 7.59 (t, J = 7.5 Hz, 1H), 7.50 (t, J = 8 Hz, 2H), 7.42–7.38
(m, 4H,), 7.17 (d, J = 8 Hz, 1H), and 2.42 ppm (s, 3H); 13C NMR, CDCl3, d: 142.4,
139.5, 133.2, 132.6, 129.0, 128.7, 127.5, 127.1, 127.0, 119.2, 115.8; MS (EI) m/z:
354, 105, 77. Compound 2b: mp 205 °C; 1H NMR (CDCl3, 500 MHz) d: 8.16 (s,
1H), 8.09 (t, J = 2 Hz, 1H), 7.96 (d, J = 8.5 Hz, 1H), 7.91 (d, J = 7.5 Hz, 2H), 7.79 (d,
J = 7.5 Hz, 2H), 7.59 (t, J = 7.5 Hz, 1H), 7.44–7.38 (m, 6H), and 7.32 ppm (d,
J = 10 Hz, 1H); 13C NMR, CDCl3, d:140.6, 140.3, 135.1, 133.0, 132.6, 130.3, 129.3,
129.1, 128.9, 127.6, 127.4, 127.0, 118.8, and 116.5 ppm. Compound 2c: mp
178 °C; 1H NMR (CDCl3, 400 MHz) d: 8.04 (s, 1H), 7.96 (d, J = 9.2 Hz, 2H), 7.89
(d, J = 7.2 Hz, 2H), 7.77 (d, J = 7.2 Hz, 2H), 7.56 (t, J = 7.6 Hz, 1H), 7.46 (t,
J = 7.6 Hz, 2H), 7.41–7.34 (m, 3H), 6.96 (d, J = 9.2 Hz, 2H), and 3.84 ppm (s, 3H);
13C NMR, CDCl3, d: 159.3, 142.3, 139.9, 133.6, 133.5, 132.7, 129.9, 129.1, 129.1,
127.7, 127.2, 120.3, 114.5, and 55.8 ppm. MS (ESI) m/z: 371 (M++H). Compound
2d: 1H NMR (CDCl3, 500 MHz) d: 8.30 (s, 1H), 8.25 (d, J = 1.5 Hz, 1H), 7.96 (d,
J = 8.5 Hz, 1H), 7.60 (t, J = 2 Hz, 1H,), 7.78 (dd, J1 = 7.5 Hz, J2 = 1.0 Hz, 2H), and
7.66–7.43 ppm (m, 9H); 13C NMR, CDCl3, d: 135.3, 134.2, 133.9, 132.1, 129.6,
129.0, 127.3, 127.0, 126.4, 122.5, 121.9, and 120.5 ppm. MS (EI) m/z: 385.
Compound 3a: 1H NMR (CDCl3, 400 MHz) d: 10.00 (br s, 1H), 8.03 (s, 2H), 7.90
(s, 2H), 7.60–7.10 (m, 10H), and 2.35 ppm (s, 3H); 13C NMR and DEPT, CDCl3, d:
144.0 (C), 141.9 (C), 137.6 (C), 137.1 (CH), 134.4 (CH), 133.7 (CH), 133.6 (CH),
133.2 (CH), 133.2 (CH), 132.6 (CH), 130.9 (CH), 128.4 (CH), 124.6 (CH), and
25.5 ppm (CH3). MS (EI) m/z: 354, 277, 207, 105, 77. Compound 3b: 1H NMR
(CDCl3, 400 MHz) d: 9.2 (br s, 1H), 8.10–7.90 (m, 6H), and 7.50–7.27 ppm (m,
8H); 13C NMR and DEPT, CDCl3, d: 138.3 (C), 135.1 (C), 132.9 (CH), 130.6 (C),
130.6 (CH), 130.4 (C), 129.1 (CH), 128.8 (CH), 127.7 (CH), 127.3 (CH), 126.5
(CH), 123.1 (C), 120.7 (C), 119.0 (C), and 116.8 ppm (C). MS (EI) m/z: 374, 207,
105, 77, 51. Compound 3c: 1H NMR (CDCl3, 400 MHz) d: 8.90 (br s, 1H), 8.06 (br
s, 2H), 8.01–7.90 (m, 2H), 7.69–7.60 (m, 2H), 7.59–7.50 (m, 2H), 7.49 (br s, 4H),
6.99 (d, 2H, J = 8 Hz), and 3.83 ppm (s, 3H); 13C NMR, CDCl3, d: 159.4, 132.6,
130.4, 129.9, 128.7, 128.7, 128.1, 127.5, 126.2, 124.8, 114.4, 114.3, and
55.5 ppm. MS (EI) m/z: 370, 342, 293, 247, 207. Compound 3d: 1H NMR (CDCl3,
500 MHz) d: 9.55 (br s, 1H), 8.91 (t, J = 2.5 Hz, 1H), 8.40 (dd J1 = 1.0 Hz,
J2 = 7.5 Hz, 1H), 8.18 (dd, J1 = 8.5 Hz, J2 = 2. Hz, 1H), 7.93 (d, J = 7.5 Hz, 2H,), 7.80
(d, J = 7.5 Hz, 2H), 7.64 (dd, J1 = J2 = 8.5 Hz, 1H), 7.57 (dd, J1 = J2 = 7.0 Hz, 1H),
7.52 (dd, J1 = J2 = 7.5 Hz, 2H), 7.48–7.38 (m, 3H); 13C NMR, CDCl3, d: 166.3,
148.9, 143.5, 141.4, 130.9, 130.4, 130.2, 129.1, 129.0, 128.9, 127.6, 127.5, 127.2,
127.0, and 123.9 ppm. MS (EI) m/z: 385. Compound 3e: 1H NMR (CDCl3,
500 MHz) d: 8.8 (br s, 1H), 8.2–7.6 (m, 4H), 7.60 (s, 2H), and 7.50 ppm (br s,
8H). 13C NMR, CDCl3, d: 136.1, 132.6, 130.4, 129.3, 128.9, 128.6, 126.2, and
124.0 ppm. MS (EI) m/z: 374.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.10.
030.
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