C(3) Functional Derivatives of the Blatter Radical

Article abstract of DOI:10.1021/acs.orglett.9b02563

A series of 3-substituted 1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yls 1 was prepared by addition of PhLi to 3-substituted benzo[e][1,2,4]triazines 2 followed by aerial oxidation. The scope of the C(3) substituents in the reaction was investigated, an

Full text of DOI:10.1021/acs.orglett.9b02563

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
C(3) Functional Derivatives of the Blatter Radical
,†,‡,§
Dominika Pomikło,† Agnieszka Bodzioch,† Anna Pietrzak,§,⊥ and Piotr Kaszynski*
́
†
́
́
Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Łodz, Poland
§Department of Chemistry, Middle Tennessee State University, Murfreesboro, Tennessee 37132, United States
⊥
́
́
́
́
Faculty of Chemistry, Łodz University of Technology, 90-924 Łodz, Poland
‡
́
́
́
́
Faculty of Chemistry, University of Łodz, 91-403 Łodz, Poland
S
* Supporting Information
ABSTRACT: A series of 3-substituted 1-phenyl-1,4-dihydrobenzo[e][1,2,4]-
triazin-4-yls 1 was prepared by addition of PhLi to 3-substituted benzo[e][1,2,4]-
triazines 2 followed by aerial oxidation. The scope of the C(3) substituents in the
reaction was investigated, and 10 structurally diverse radicals 1 were isolated, their
stability was assessed and properties were investigated with spectroscopic and
electrochemical methods. Two radicals were analyzed with single-crystal XRD
methods. Experimental data are compared to DFT results and correlated with
Hammett substituent parameters.
he exceptional stability, broad absorption in the visible
range, and favorable electrochemical properties make the
benzo[e][1,2,4]triazinyl, and they include CN,16,19 CHO,19
NO2,16 halogens,13,16 and OMe substituents.13,16 Also trans-
formations in the presence of the unpaired electron are
numerous,20,21 which have been extensively used in the
preparation of some functional materials. Efficient methods
have also been demonstrated for postcyclization ring
substitution13,14,20−22 and ring annulation.23−25
Functionalization of the C(3) atom is, however, much less
developed. Arguably, this is a particularly important position
since it is the only functionalizable position with a sizable
negative spin density in the benzo[e][1,2,4]triazinyl system,
and the substituent is expected26 to have a significant impact
on electronic properties of the radical. A literature search
demonstrates that only a handful of substituents have been
introduced into this position using mainly methods A and B,
and they include aromatic (Ph,11 substituted Ph,3−6,16 2-
pyridinyl,17 and 2-thienyl16,17), aliphatic (CH3,17 t-butyl,27,28
and adamantyl29), CF3,17 and NHAr (in method A′ after basic
hydrolysis of NH(CHO)Ar).15 Surprisingly, there is no report
on the C(3)-unsubstituted derivative (X = H).
T
1,4-dihydrobenzo[e][1,2,4]triazin-4-yl radical an attractive and
increasingly important structural element of functional
materials,1,2 such as photoconductive liquid crystals3−6 and
those for molecular electronics,7 energy storage,8 controlled
polymerization,9 and biophysical10 applications. Most of these
applications require the presence of appropriate functional
groups that enable the radical to be incorporated into more
complex molecular architectures. Therefore, further progress in
application of the 1,4-dihydrobenzo[e][1,2,4]triazin-4-yl
hinges upon accessibility of its multifunctional derivatives.
There are three major methods leading to 1,4-dihydrobenzo-
[e][1,2,4]triazin-4-yl radicals 1 (Figure 1): 6-π electrocycliza-
Here, we present the preparation of C(3)-substituted 1-
phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl radicals 1 by
addition of PhLi to the readily available benzo[e][1,2,4]-
triazines26 2 followed by oxidation of the resulting anions 3
(method C, Figure 1). The scope of the substituents X is
investigated, stability of radicals 1 is assessed, and the impact of
the C(3) substituent X on spectroscopic and electrochemical
properties in series 1 is analyzed.
Reactions of a series of 14 3-substituted benzo[e][1,2,4]-
triazines26 2b−2o with PhLi were conducted under conditions
previously used to obtain the Blatter radical 1a from 2a in 70−
78% yield,18 and the results are shown in Table 1. Thus, 1.3
Figure 1. Selected methods for construction of the 1,4-dihydrobenzo-
[e][1,2,4]triazin-4-yl skeleton.
tion of azoimines (methods A,11−14 A′,15 and the aza-Wittig
modification16), reductive cyclocondensation of N-aryl hydra-
zides (method B),17 and azaphilic addition of ArLi to
benzo[e][1,2,4]triazines 2 followed by aerial oxidation of the
resulting anion 3 (method C).18 In methods A and B,
substituent X in radical 1 is derived from the carboxylic acid,
which, to large extent, defines its nature.
A review of the literature indicates that several functional
groups have been introduced to the benzene ring of the
Received: July 24, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02563
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
used in the reaction with 2n, the crude reaction mixture
contained minor quantities of the expected radical 1n along
with the 3-phenylbenzo[e][1,2,4]triazine (2a), in addition to
radical 1a. This is consistent with the observed facile
substitution of the CN group with OH, upon basic hydrolysis
of 2n,26 and similar substitution of the 3-PO(OEt)2 group in
2o.30
Table 1. Preparation of 1-Phenyl-1,4-
dihydrobenzo[e][1,2,4]triazin-4-yls 1 from
a
Benzo[e][1,2,4]triazines 2
b
b
X
yield (%) of 1
X
yield (%) of 1
c
a, Ph
70−78
46
29
59
81
31
13−19
70
i, OEt
j, NH2
k, morpholin-4-yl
l, NHPh
m, COO−[NMe4]+
n, CN
23
b, thien-2-yl
c, ferrocenyl
d, CCPh
e, CF3
f, C5H11
g, H
13−15
67
Overall, C(3) substituents in benzo[e][1,2,4]triazines 2 fall
into four categories: those that give high yields of radicals 1
upon reactions with PhLi (CF3, aryl (Ph), CCPh, SBu-t, and
morpholin-4-yl), those that give complex mixtures of products
and the corresponding radicals 1 can be isolated in low yields
(ferrocene, alkyl, OEt, NH2, and H), those for which no
desired products could be isolated (NHPh and COO−), and
those that undergo clean aromatic nucleophilic displacement
with PhLi (CN and PO(OEt)2). Most radicals are stable after
isolation except for those containing H, C5H11, and OEt at the
C(3) position, which slowly decompose upon standing.
Analysis of electrophilicity indices expressed as condensed
electronic Fukui functions,31,32 f+,30 indicates that the N(1)
position is typically preferred for the Nu− addition, and the
resulting anion is also most favored thermodynamically for
addition of Ph− to 2a and 2n.30 This regioselectivity is also
predicted for anion 2j‑, formed upon deprotonation of amine
2j, but in anion 2l−, the C(3) position appears to be most
electrophilic.
d
0
d
0
e
0, 59
e
o, PO(OEt)2
0, 58
h, SBu-t
a
According to Scheme 1. Typical procedure: (1) 1.3 equiv of PhLi in
Bu2O (0.8 mL) was added to 0.5 mmol of 2 in THF (1.5 mL) at −78
b
°C; (2) H2O (0.5 mL), air. Yield of isolated compounds after
c
d
chromatography. Reference 18. Complex mixture of products.
e
Yield of Blatter radical 1a.
equiv of PhLi was added to 3-thienylbenzo[e][1,2,4]triazine
(2b) in THF solution at −78 °C, the resulting 3b was
quenched with water and the formed leuco form was oxidized
to 1b by exposure to air (Scheme 1). The known16,17 radical
Scheme 1. Preparation of Radicals 1 from
Benzo[e][1,2,4]triazines 2
The structures of two radicals, 1h and 1k, were confirmed
with single-crystal XRD analysis (Figure 2). Dimensions of the
1b was isolated in 46% yield by column chromatography. A
similar reaction of 3-ferrocenyl (2c), 3-CCPh (2d), 3-CF3
(2e),17 and 3-C5H11 (2f) derivatives gave the corresponding
radicals in yields 31−81%. Radicals 1d and 1f exhibited limited
stability in solutions and to chromatography. Instability of the
latter radical is amplified by its liquid nature, while radical 1d
appears to be stable in the solid state.
Reactions of PhLi with 3-SBu-t and 3-(morpholin-4-yl)
derivatives 2h and 2k, respectively, gave the corresponding
radicals 1h and 1k in about 70% yield. In contrast, reactions of
the 3-OEt (2i) and 3-NH2 (2j) derivatives and the parent
benzo[e][1,2,4]triazine (2g) gave complex mixtures of
products, from which the desired radicals could be isolated
in 13−23% yield. The 3-amino and 3-unsubstituted radicals, 1j
and 1g, appear to be moderately stable after isolation, while the
3-OEt 1i undergoes slow decomposition on standing, which
precluded accurate combustion analysis. The formation of the
Blatter radical 1a by substitution of the OEt group was not
observed.
No expected radicals were observed in reactions of PhLi
with C(3)-NHPh (2l) and C(3)-COO−[NMe4]+ (2m)
derivatives. In the former case, a species with an apparently
additional Ph group (addition of a total of two Ph groups) was
isolated in 35% yield, but it was not investigated further.30 This
is presumably due to the dominant imino tautomeric form of
2l26 and the highest calculated electrophilicity, f+, at the C(3)
position of the corresponding anion.30
Finally, treatment of the 3-CN and 3-PO(OEt)2 derivatives
2n and 2o, respectively, with PhLi led to the formation of the
Blatter radical 1a apparently by replacement of the C(3)
substituent with the Ph group. When 0.25 equiv of PhLi was
Figure 2. Molecular structures of 1h (left) and 1k (right). For
selected geometrical parameters see the text and the SI. Displacement
ellipsoids are drawn at the 50% probability level.
benzo[e][1,2,4]triazine heterocycle are typical with the ring
being puckered (10.1°) along the N(1)···N(4) line in 1h and
essentially planar in 1k. The C(3)−S distance in 1h is 1.765(2)
Å. In the two independent molecules of 1k the C(3)−N
distance is 1.372(2) Å and 1.386(2) Å with the nitrogen atoms
away from the plane defined by the three connected C atoms
by 0.171 and 0.302 Å, respectively.
Spectroscopic and electrochemical analyses of radicals in
series 1 revealed the effects of the C(3) substituent. Thus, all
radicals exhibit a broad, low intensity absorption in the visible
range with poorly defined absorption maxima. The most
pronounced effect on the absorption spectrum is exhibited by
the 3-(morpholin-4-yl) derivative 1k (Figure 3).
The observed trend in excitation energies is reproduced
computationally. TD DFT analysis suggests that the two
lowest energy excitations calculated at about 500 and 400 nm
involve mainly the α-HOMO → α-LUMO and β-HOMO →
β-LUMO transitions. Substitution of the C(3) position in the
parent radical 1g has stronger impact on the β-HOMO (Figure
4) energy (range in the series ΔE = 1.12 eV) than on the α-
B
DOI: 10.1021/acs.orglett.9b02563
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
Figure 5. Cyclic voltammograms for 3-phenyl (1a, black), 3-CF3 (1e,
red), and 3-(morpholin-4-yl) (1k, blue) 1-phenyl-1,4-dihydrobenzo-
[e][1,2,4]triazin-4-yls (∼0.5 mM) in CH2Cl2 [n-Bu4N]+ [PF6]− (50
mM), ca. 20 °C, 50 mV s−1, glassy carbon electrode.
Figure 3. UV−vis spectra for radicals 3-Ph 1a (black) and 3-
(morpholin-4-yl) 1k (red) in CH2Cl2.
Table 2. Electrochemical Data for 1-Phenyl-1,4-
a
dihydrobenzo[e][1,2,4]triazin-4-yls 1
b
c
c
−1/0
0/+1
E1/2
(V)
E1/2
(V)
Ecell
ELUMO
(eV)
EHOMO
(eV)
X
(V)
d
a, Ph
b, thien-2-yl
−0.92
−0.88
f
+0.28
+0.30
+0.38
+0.72
+0.40
+0.58
+0.24
+0.30
+0.15
+0.08
1.20
1.18
−3.81
−3.92
−4.81
−4.88
−4.62
e
c, ferrocenyl
g
d, CCPh
−0.78
−0.71
−0.97
−0.90
−0.96
−0.98
1.18
1.29
1.21
1.20
1.11
1.06
−3.95
−3.99
−3.80
−3.85
−3.83
−3.80
−4.94
−5.15
−4.78
−4.87
−4.65
−4.65
h
e, CF3
i
Figure 4. CAM-B3LYP/6-31++G(2d,p)//B3LYP/6-31G(2d,p)-de-
rived β-HOMO contours and energies for radicals 1d (left) and 1k
(right) in CH2Cl2 dielectric medium.
f, C5H11
h, SBu-t
j, NH2
k, morpholin-4-yl
HOMO energy (range in the series ΔE = 0.44 eV), and the
observed trend approximately follows the Hammett σp
parameter.30 In general, π substituents, such as Ph, thienyl,
and phenylethynyl, significantly stabilize the α-LUMO (by up
to 0.93 eV in 1d), while all substituents, with the exception of
the CF3, destabilize the β-HOMO orbital with the strongest
effect observed in the 3-(morpholin-4-yl) derivative 1k (by
0.89 eV). Consequently, the lowest energy excitation has the
α-HOMO → α-LUMO character for 3-(het)aryl, 3-ethynyl
and 3-CF3 derivatives, while for those with substituents
containing an electron pair (alkoxy and amines) it has the β-
HOMO→ β-LUMO character.
a
Measured in CH2Cl2 [n-Bu4N]+ [PF6]− (50 mM), ca. 20 °C, 50 mV
s−1, glassy carbon electrode. Potentials referenced to Fc/Fc+ (0.46 V
b
c
vs SCE; ref 34). Ecell = E1/20/+1 − E1/2−1/0. Calculated from the onset
of oxidation or reduction: EHOMO/LUMO = −(E(onset ox/red vrs Fc+/Fc)
+
5.1) (eV); ref 35. Reference 18. Lit.16 −0.82 and +0.32 V.
d
e
f
g
Ambiguous assignments. The second oxidation potential is ascribed
to the ferrocenyl group. Lit.17 −0.78 and +0.36 V vs SCE. Lit.17 for
h
i
X = Me: −0.95 and +0.37 V vs SCE.
The C(3) substituent also impacts the hfcc and consequently
the shape of EPR spectra of the radicals (e.g., Figure 6).30 In all
Electrochemical analysis of series 1 gave more clear evidence
of the C(3) substituent effect on the electronic structure of the
benzo[e][1,2,4]triazinyl heterocycle.
A comparison of redox potentials in series 1 shows that
replacement of the Ph substituent at the C(3) position in 1a
with CF3, the most electron accepting substituent in the series,
0/+1
increases the E1/2
by 0.30 V in 1e, while replacement with
the most electron releasing morpholin-4-yl group lowers the
oxidation potential by 0.20 V in 1k (Figure 5, Table 2).30 The
−1/0
reduction potentials E1/2
follow the same trend, which is
consistent with other electrochemical studies17 and correlates
well with the Hammett σp parameters.30,33 The observed
30
0/+1
nearly twice larger slope for the E1/2
potential vs σp plot
than for the E1/2−1/0 vs σp plot suggests a greater impact of the
substituent on the HOMO than on the LUMO in series 1.
Interestingly, the ferrocenyl derivative 1c exhibits two quasi-
reversible oxidation processes due to the oxidation of the
heterocycle (+0.38 V) and the ferrocenyl group (+0.72 V).
Electrochemical analysis of 1g and 1i gave poor results due to
instability of the sample.
Figure 6. Experimental (black) and simulated (red) EPR spectra for
radical 1d recorded in CH2Cl2. Inset shows an assignment of the
resulting hfcc.
C
DOI: 10.1021/acs.orglett.9b02563
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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(3) Jasinski, M.; Kapuscinski, S.; Kaszynski, P. Stability of a
columnar liquid crystalline phase in isomeric derivatives of the 1,4-
dihydrobenzo[e][1,2,4]triazin-4-yl: Conformational effects in the
core. J. Mol. Liq. 2019, 277, 1054−1059.
radicals 1 the aN(1), aN(2), and aN(4) hfcc values fall into typical
ranges of 7.4−8.1, 4.1−4.9, and 4.9−5.8 G, respectively.
Correlation analysis of the data demonstrates that the aN1 hfcc
are essentially independent of the C(3) substituent (slope = 0
within the error), the aN2 values show a weak increasing trend
(when the CF3 derivative is omitted), and a clear decreasing
trend for aN4 hfcc is observed with increasing values of the
substituent parameter σp.30,33
In summary, we have demonstrated access to benzo[e]-
[1,2,4]triazin-4-yl with a significantly expanded range and
diversity of substituents at the C(3) position. The newly
available derivatives include the electroactive C(3)-ferrocenyl
derivative 1c, C(3)-acetylene derivative 1d, and derivative 1j
containing a particularly important and versatile NH2
functionality.36 We have also established limitation of the
azaphilic addition method and found its incompatibility with
C(3) substituents such as COO−, NHPh, CN, and PO(OR)2.
The expanded series of derivatives permitted analysis of C(3)
substituent effects on electronic properties of the benzo[e]-
[1,2,4]triazin-4-yl system, which, in turn, provides a tool for
designing of radicals with greater functional flexibility and
structural variety for modern materials applications.
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ASSOCIATED CONTENT
* Supporting Information
■
S
(12) Koutentis, P. A.; Lo Re, D. Catalytic Oxidation of N-
Phenylamidrazones to 1,3-Diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-
yls: An Improved Synthesis of Blatter’s Radical. Synthesis 2010, 2010,
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(13) Constantinides, C. P.; Koutentis, P. A.; Loizou, G. Synthesis of
7-aryl/heteraryl-1,3-diphenyl-1,2,4-benzotriazinyls via palladium cata-
lyzed Stille and Suzuki-Miyaura reactions. Org. Biomol. Chem. 2011, 9,
3122−3125.
(14) Constantinides, C. P.; Koutentis, P. A.; Rawson, J. M.
Antiferromagnetic Interactions in 1D Heisenberg Linear Chains of
7-(4-Fluorophenyl) and 7-Phenyl-Substituted 1,3-Diphenyl-1,4-dihy-
dro- 1,2,4-benzotriazin-4-yl Radicals. Chem. - Eur. J. 2012, 18, 15433−
15438.
(15) Grant, J. A.; Lu, Z.; Tucker, D. E.; Hockin, B. M.; Yufit, D. S.;
Fox, M. A.; Kataky, R.; Chechik, V.; O’Donoghue, A. C. New Blatter-
type radicals from a bench-stable carbene. Nature Commun. 2017, 8,
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02563.
Synthetic and analytical details, UV−vis and EPR
spectra, electrochemical data, details of XRD analysis,
and computational results (PDF)
Accession Codes
CCDC 1915133 and 1915134 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
(16) Savva, A. C.; Mirallai, S. I.; Zissimou, G. A.; Berezin, A. A.;
Demetriades, M.; Kourtellaris, A.; Constantinides, C. P.; Nicolaides,
C.; Trypiniotis, T.; Koutentis, P. A. Preparation of Blatter Radicals via
Aza-Wittig Chemistry: The Reaction of N-Aryliminophosphoranes
with 1-(Het)aroyl-2-aryldiazenes. J. Org. Chem. 2017, 82, 7564−7575.
(17) Berezin, A. A.; Zissimou, G.; Constantinides, C. P.; Beldjoudi,
Y.; Rawson, J. M.; Koutentis, P. A. Route to Benzo- and Pyrido-Fused
1,2,4-Triazinyl Radicals via N’-(Het)aryl-N’-[2-nitro(het)aryl]-
hydrazides. J. Org. Chem. 2014, 79, 314−327.
■
Corresponding Author
*E-mail: piotrk@cbmm.lodz.pl.
ORCID
Anna Pietrzak: 0000-0003-3415-8650
́
Piotr Kaszynski: 0000-0002-2325-8560
Notes
́
(18) Constantinides, C. P.; Obijalska, E.; Kaszynski, P. Access to 1,4-
The authors declare no competing financial interest.
Dihydrobenzo[e][1,2,4]triazin-4-yl Derivtives. Org. Lett. 2016, 18,
916−919.
ACKNOWLEDGMENTS
(19) Gallagher, N.; Zhang, H.; Junghoefer, T.; Giangrisostomi, E.;
Ovsyannikov, R.; Pink, M.; Rajca, S.; Casu, M. B.; Rajca, A. Thermally
and Magnetically Robust Triplet Ground State Diradical. J. Am. Chem.
Soc. 2019, 141, 4764−4774.
■
This work was supported by the Foundation for Polish Science
Grant (TEAM/2016-3/24) and National Science Foundation
(XRD facility MRI-1626549).
́
(20) Bodzioch, A.; Zheng, M.; Kaszynski, P.; Utecht, G. Functional
Group Transformations in Derivatives of 1,4-Dihydrobenzo[1,2,4]-
triazinyl Radical. J. Org. Chem. 2014, 79, 7294−7310.
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DOI: 10.1021/acs.orglett.9b02563
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