One-pot synthesis of linearly fused n-heterocyles from their angular analogues and studies of their redox and electrochromic properties

Article abstract of DOI:10.1021/jo902432w

Chemical Equation Presented In an unusual chemical transformation, the angular heterocyclic compound [1]ClO₄, upon reaction with a primary aromatic amine, is transformed to a linear heterocyclic compound 3 via the angular intermediate [2]ClO₄ in one pot. Characterization of the compounds was primarily achieved by ¹H and ¹³C NMR and mass spectral data. Analyses of single-crystal X-ray structures of the representative compounds confirmed, their identities. These compounds exhibit notable spectral and redox properties. Their redox behavior is followed by spectral characterization of the electro-generated radicals. Electrochromic properties of the reference compounds are also explored.

Full text of DOI:10.1021/jo902432w

pubs.acs.org/joc
During our work on azo-based ligands in coordination
One-Pot Synthesis of Linearly Fused N-Heterocyles
from Their Angular Analogues and Studies of Their
Redox and Electrochromic Properties
chemistry, we recently discovered5 an unusual chemical
transformation on proton-catalyzed one-pot synthesis of
planar triazinium heterocycles from trans-2-(arylazo)pyri-
dines. The transformation involves trans f cis isomerization
and cyclization via a new C-N bond formation6 reaction.
The flat triazinium heterocyclic cations are found to be
interesting in the context of their easy reducible properties,
luminescent properties, and most importantly, their strong
DNA binding and photonuclease activities.5 Moreover, the
6-Cl-substituted triazinium compound, [1]ClO4, is of further
interest because it undergoes a variety of novel chemical
transformations associated with C-Cl bond activation gen-
erating scope of synthesis of new heteroaromatic systems.
For example, we recently have reported6a a redox-driven
dimerization reaction leading to the synthesis of a new
aromatic azo dye with versatile redox properties. Herein
we report yet another unusual chemical transformation that
resulted in the isolation of a linearly fused nitrogen-contain-
ing heterocycle 3 from the direct reaction of the planar and
angular triazinium salt [1]ClO4 with primary aromatic
amines. As far as we are aware, similar chemical transforma-
tion of angularly fused polyheterocycles directly to linearly
fused polyheterocyles under a mild laboratory condition is
not available in the literature. For comparison, flash vacuum
pyrolysis up to 525 °C led7 to valence bond isomerization of
the angularly fused starting compound to the corresponding
linearly fused derivatives [1,2,4]triazolo[4,3-b][1,2,4]benzo-
triazine. The chemical transformations concerning us here
are depicted in Scheme 1. Literature survey has revealed
that linearly fused tricyclic triazinium salts are usually pre-
pared8 from R-diaminoisoquinolinium salts and R-dioxo
compounds.
Mominul Sinan,† Kumaresh Ghosh,‡ and
Sreebrata Goswami*,†
†DepartmentofInorganic Chemistry, Indian Associationfor the
Cultivation of Science, Kolkata 700 032, India, and
‡Department of Chemistry, University of Kalyani, Nadia, India
icsg@iacs.res.in
Received November 14, 2009
In an unusual chemical transformation, the angular
heterocyclic compound [1]ClO4, upon reaction with a
primary aromatic amine, is transformed to a linear hetero-
cyclic compound 3 via the angular intermediate [2]ClO4
in one pot. Characterization of the compounds was
primarily achieved by 1H and 13C NMR and mass
spectral data. Analyses of single-crystal X-ray structures
of the representative compounds confirmed their identi-
ties. These compounds exhibit notable spectral and redox
properties. Their redox behavior is followed by spectral
characterization of the electro-generated radicals. Electro-
chromic properties of the reference compounds are also
explored.
The C-Cl bond in the chloro-substituted triazinium com-
pound [1]ClO4 is activated6a and susceptible to nucleophilic
substitution reactions. Accordingly, the reaction of ArNH2
(Scheme 1, R = H, CH3, OCH3) with [1]ClO4 in CH3CN at
reflux for 1 h produced the amine-substituted compound
[2]ClO4 ([2a]ClO4 - [2c]ClO4) as the major product9 (yield,
ca. 70%) along with the molecular compound, 3 (3a-c) as
the minor product (yield, ca. 10%). However, the reaction of
[1]ClO4 with ArNH2 at room temperature produced the
compound [2]ClO4 as the only product (yield, >70%). It
may be relevant to note here that the reference transforma-
tions in [1]ClO4 were not possible with ammonia or with an
Nitrogen-containing heterocyles exhibit1 a diverse array
of favorable biological and pharmacological properties.
Also these are useful materials2-4 as fluorescent dyes,
DNA probes, NLO materials, ionic liquids, etc. Therefore,
there is an obvious need to obtain this class of new materials
preferably using easy synthesis in the laboratory.
(5) Sinan, M.; Panda, M.; Ghosh, A.; Dhara, K.; Fanwick, P. E.;
Chattopadhyay, D. J.; Goswami, S. J. Am. Chem. Soc. 2008, 130, 5185.
(6) (a) Sinan, M.; Panda, M.; Banerjee, P.; Shinisha, C. B.; Sunoj, R. B.;
Goswami, S. Org. Lett. 2009, 11, 3218. (b) Samanta, S.; Goswami, S. J. Am.
Chem. Soc. 2009, 131, 924. (c) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.;
Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805. (d) Surry, D. S.; Buchwald, S.
L. Angew. Chem., Int. Ed. 2008, 47, 6338. (e) Hartwig, J. F. Angew. Chem.,
Int. Ed. 1998, 37, 2046.
(1) (a) Gil, C.; Brase, S. J. Comb. Chem. 2009, 11, 175. (b) Butler, M. S. J.
Nat. Prod. 2004, 67, 2141.
(2) (a) Graves, D. E.; Velea, L. M. Curr. Org. Chem. 2000, 4, 915. (b)
ꢀ
Bischoff, G.; Hoffmann, S.; Martı
Chem. 2002, 9, 321. (c) Martınez, R.; Chacon-Garcı
2005, 12, 127.
´
nez, R.; Chacon-Garcı
´
a, L. Curr. Med.
a, L. Curr.Med. Chem.
ꢀ
´
´
(3) (a) Facchetti, A.; Abbotto, A.; Beverina, L.; van der Boom, M. E.;
Dutta, P.; Evmenenko, G.; Marks, T. J.; Pagani, G. A. Chem. Mater. 2002,
14, 4996. (b) Mata, J. A.; Uriel, S.; Llusar, R.; Peris, E. Organometallics 2000,
19, 3797. (c) Seddon, K. R. Nat. Mater. 2003, 2, 363. (d) Welton, T. Chem.
Rev. 1999, 99, 2071.
ꢀ
ꢀ
ꢀ
(7) Pelaez, W. J.; Yranzo, G. I.; Grof, C; Riedl, Z.; Hajos, G. Tetrahedron
2005, 61, 7489.
ꢀ
(8) Hajos, G.; Messmer, A.; Batori, S.; Riedl, Z. Bull. Soc. Chim. Belg.
1992, 101, 597.
(4) (a) Qiu, F.; Cao, Y.; Xu, H.; Jiang, Y.; Zhou, Y.; Liu, J. Dyes Pigments
2007, 75, 454. (b) Ooyama, Y.; Egawa, H.; Yoshida, K. Eur. J. Org. Chem.
2008, 31, 5239 and references therein.. (c) Ishow, E.; Bellaıche, C.; Bouteiller,
L.; Nakatani, K.; Delaire, J. A. J. Am. Chem. Soc. 2003, 125, 15744.
(9) The reaction of [1]ClO4 with an primary aromatic amine containing an
electrowithdrawing group like 4-nitroanline is sluggish. The compound,
[2]ClO4 (with R= -NO2, Scheme 1), was isolated in <15% yield even after
4 h reflux.
DOI: 10.1021/jo902432w
r
Published on Web 02/12/2010
J. Org. Chem. 2010, 75, 2065–2068 2065
2010 American Chemical Society

Sinan et al.
JOCNote
SCHEME 1. Synthetic Reactions
FIGURE 1. Molecular view of [2a]ClO4. Selected bond lengths:
C5-N2, 1.361(6); N2-N3, 1.310(5); C6-N3, 1.355(6); C6-C7,
1.403(6); C7-C8, 1.353(6); C8-C9, 1.420(6); C9-N4, 1.362(5);
C12-N4, 1.420(6).
alkylamine like benzylamine. In these cases, a mixture of
large number of unidentified products (overlapping bands of
different colors on TLC plate) was observed after prolonged
reflux.
Interestingly, the preformed compound [2]ClO4, in turn,
reacts freely with ArNH2 in acetonitrile at reflux to produce 3
in >80% yield in 4 h. The above results together indicate
that the compound [2]ClO4 is an intermediate for the trans-
formation, [1]ClO4 f 3. Most importantly, the product of
the conversion, [2]ClO4 f 3 does not depend on the nature of
ArNH2 indicating that the amine acts just as an initiator in
the above angular to linear heterocyclic conversion. For
example, [2a]ClO4 f 3a conversion occurs with similar
efficiency by both aniline as well as p-toluidine. Moreover,
the compound [2b]ClO4 is converted to 3b in acetonitrile
upon boiling in the presence of aniline. Furthermore, we wish
to note here that the afore-mentioned transformation ceases
in the presence of a radical scavenger like TEMPO (2,2,6,6-
tetramethylpiperidinoxyl radical) or BHT (2,6-di-tert-butyl-
4-methylphenol) when a 1.25 equiv quantity is used.
Microanalytical, positive-ion ESI-mass spectra, together
with NMR spectral data (Figures S1-S8, Supporting
Information) of the compounds, convincingly support the
formulation of the compounds as shown in the Scheme 1.
For example, the compounds [2b]ClO4 and 3b both showed
an intense peak at m/z 287 amu due to [2b]þ and [H3b]þ.
Finally, structure determination by X-ray diffraction of the
representative compounds [2a]ClO4 and 3a confirms their
identities as well as the geometries. Molecular views of the
compounds [2a]ClO4 and 3a are shown in Figures 1 and 2.
Their ORTEPs are available as Figures S9-S11 (Supporting
Information), and crystallographic data are collected in
Table S1 (Supporting Information). Comparison of the
bond lengths of [2a]ClO4 with those of 3a clearly reveals that
electronic structures of the tricyclo ring in the above two
compounds are different. The carbon-nitrogen bonds con-
necting the amine nitrogen and the tricyclo ring in these
two compounds are different. For example, the C8-N4
length (1.307(4) A) in 3a is noticeably shorter10 than the
FIGURE 2. Molecular view of 3a. Selected bond lengths: N1-N2,
1.385(3); C11-N3, 1.301(4); C10-C11, 1.450(4); C9-C10,
1.330(4); C8-C9, 1.473(4); C8-N4, 1.307(4); C12-N4, 1.411(4).
corresponding length C9-N4 (1.362(5) A) in [2a]ClO4. The
observed C-N bond contraction in [2a]ClO4 is attributed to
delocalization6a of the nonbonding electron on the amine
nitrogen to the cationic tricyclo ring. Moreover, d(N-N) in
the above two compounds are different. The bond is elon-
gated (1.385(3) A) in 3a, indicating6a that it is a single bond.
The linearly fused molecule 3a has a definite π-π interaction
(Figure S12, Supporting Information) with an intercentroid
distance of 3.526 A. The solutions of the compounds [2a]ClO4-
[2c]ClO4 in acetonitrile are 1:1 electrolytic [Λeqiv(CH3CN),
105-110 Ω-1 mol-1 cm2], while those of 3a-c are nonelec-
trolytic.
The suggested mechanistic path for the formation of
compounds 3 from [2]ClO4 is outlined in Scheme 2. The
formation of 3 from [2]ClO4 is initiated by aromatic amines
under refluxing conditions. We envision that aromatic amine
radical (ArNH•), produced by one electron oxidation and
proton loss, attacks the carbon center adjacent to the pyri-
dinium nitrogen leading to the formation of an intermediate
A. Upon subsequent C-N bond cleavage, A is transformed
into B, which is resonance stabilized. Out of the several
possible resonating structures of B, the form C seems to
be the most stable one. Radical attack at the terminal
double bond through this resonating structure leads to the
monoradical species D that on removal of ArNH• and a
proton loss produces the final product 3. To have further
support, a similar reaction of [2]ClO4 was carried out
using thiophenol in place of an aromatic amine. The
reaction did occur producing the compound 3 in nearly
70% yield (for experimental details, see the Supporting
Information).
ꢀ
(10) (a) Chatterjee, S.; Singh, P.; Fiedler, J.; Bakova, R.; Zalis, S.; Kaim,
W.; Goswami, S. Dalton Trans. 2009, 7778. (b) Mitra, K. N.; Majumdar, P.;
ꢀ ꢁ
~
Peng, S. -M.; Castineiras, A.; Goswami, S. Chem. Commun. 1997, 1267. (c)
~
Mitra, K. N.; Choudhury, S.; Castineiras, A.; Goswami, S. J. Chem. Soc.,
Dalton Trans. 1998, 2901.
2066 J. Org. Chem. Vol. 75, No. 6, 2010

Sinan et al.
JOCNote
SCHEME 2. Plausible Mechanism of [2]ClO4 f 3 Conversion
FIGURE 3. Cyclic voltammograms of [2a]ClO4 (---) and 3a(—).
Absorption and emission spectra of the compounds
[2]ClO4 and 3 are submitted as Supporting Information
(Figures S13 and S14 and Table S2). The compound
[2]ClO4 in CH3CN exhibited emission at 580 nm associated
with a shoulder at 550 nm at room temperature (300 K) upon
excitation at 490 nm (quantum yield 0.025). The emission is
attributed to the intramolecular charge transfer between the
electron-rich anilino moiety and the electron-deficient pyr-
idinium ring.11 The lifetime of a representative [2a]ClO4 at
300 K was found to be 3.92 ns. In comparison, the linearly
fused heterocycle 3 showed emission in CH3CN at 428 nm
when excited at 270 nm (quantum yield 0.008), and the
lifetime of a representative 3a at 300 K was found to be
6.62 ns. A decrease in temperature resulted in considerable
increment in the fluorescence quantum yields (quantum
yields 0.136 and 0.032 for the compounds [2]ClO4 and 3,
respectively, at ethanol-methanol glass) because nonradia-
tive processes related to thermal agitation are less efficient at
a glassy state.
FIGURE 4. UV-vis spectra of [2a]ClO4 (red) and 2a• (black).
The compounds [2]ClO4 and 3 showed a reversible reduc-
tion wave near -0.4 and -0.3 V versus the Ag-AgCl,
respectively (Table S3, Supporting Information). Cyclic
voltammograms of the two compounds [2a]ClO4 and 3a
are displayed in Figure 3. The electrolytically reduced com-
pounds, 2a• and 3a•- showed a single line sharp EPR
spectrum at g, 2.001 and 2.003, respectively (Figure S15,
Supporting Information), characterizing the formation of
the free radical in each case. Interestingly, the colors of the
solutions of the electrogenerated reduced compounds are
totally different12 than those of the parent compounds. For
example, the red color of the compound [2a]ClO4 became
pale yellow upon reduction (Figure 4). The original com-
pounds are generated quantitatively by the oxidation of the
reduced complexes, suggesting that the redox processes are
reversible.
FIGURE 5. Change in absorbance for regeneration of the com-
pounds (a) [2a•] f [2a]ClO4 and (b) [3a•-] f 3a as a function of time.
The complexes, [2a•] and [3a•-] were generated in situ by mixing
excess of aqueous sodium dithionite solution to the aqueous-
methanolic (1:1) solutions of [2a]ClO4 and 3a, respectively.
of sodium dithionite solution. The most notable observa-
tion of these experiments is the regeneration of the
original compounds (>95% regeneration in 0.5 h) with
time (Figure 5; for details of the experiment, see the
Supporting Information, Figure S16) generating the scope
of using these as catalysts in the radical-induced redox
transformations.
In summary, we have introduced one-pot synthesis of
linear pyridinyl-1,2,4-triazine from its angular analogue.
Primary aromatic amines mediate this transformation, and
such a facile but useful chemical transformation is not
available in the literature. The end product 3 as well as the
cationic intermediate [2]ClO4 undergo reduction at low
cathodic potentials producing stable polycyclic radical
species. The compounds show potential prospect of using
them as catalysts in radical induced chemical reactions. It
may be relevant to add here that the electrochromic property
Notably, these compounds can also be reduced chemically
by an aqueous solution of sodium dithionite. The lowest
energy transition in the original compounds, [2a]ClO4 and
3a, disappeared completely upon addition of excess (10 times)
(11) Koner, R. R.; Ray, M. Inorg. Chem. 2008, 47, 9122.
(12) (a) Suzuki, T.; Yoshino, T.; Nishida, J.; Ohkita, M.; Tsuji, T. J. Org.
Chem. 2000, 65, 5514. (b) Ito, S.; Kikuchi, S.; Okujima, T.; Morita, N.; Asao,
T. J. Org. Chem. 2001, 66, 2470.
J. Org. Chem. Vol. 75, No. 6, 2010 2067

Sinan et al.
JOCNote
in the linearly fused heterocyclic compound 3 resembles to
7.26 Hz), 2.19 (s, 3H); 13C NMR (75 MHz, CD3CN) δ 155.6 (C),
143.6 (C), 142.8 (C-H), 140.1 (C), 137.4 (C), 134.8 (C-H),
134.7 (C), 130.7 (C-H), 130.6 (C-H), 128.9 (C-H), 127.6 (C),
125.8 (C-H), 123.5 (C-H), 123.2 (C-H), 93.3 (C-H), 20.3
(CH3); IR (KBr) 1620, 1313, 1087 cm-1; ESI-MS m/z 287 [M]þ.
Anal. Calcd for C18H15N4ClO4: C, 55.89; H, 3.91; N, 14.49.
Found: C, 55.84; H, 3.92; N, 14.50.
that of a closely related biomolecule, riboflavin.13
Experimental Section
Synthesis of the Compounds [2a]ClO4 and 3a. Aniline
(0.14 mL, 1.5 mmol) was added to a 10 mL acetonitrile solution
of [1]ClO4 (0.317 g, 1 mmol), and the mixture was stirred at
300 K for 1 h. The initial yellowish brown solution became red
during this period. The crude mass obtained by evaporation of
the solution was purified on a preparative alumina TLC plate
using the solvent mixture chloroform/acetonitrile (4:1) to afford
1
[2c]ClO4: yield (0.286 g, 71%); mp 218-219 °C; H NMR
(300 MHz, CD3CN) δ 9.19 (d, 1H, J = 6.90 Hz), 8.95 (s, 1H),
8.66 (d, 1H, J = 8.55 Hz), 8.58 (t, 1H, J = 8.46 Hz), 8.45 (d, 1H,
J = 9.27 Hz), 8.07 (t, 1H, J = 6.97 Hz), 7.65 (d, 1H, J = 9.20
Hz), 7.56 (s, 1H), 7.34 (d, 2H, J = 8.94 Hz), 7.04 (d, 2H, J = 8.92
Hz), 3.82 (s, 3H); 13C NMR (75 MHz, CD3CN) δ 158.9 (C),
156.3 (C), 143.6 (C), 142.6 (C-H), 140.3 (C), 134.8 (C-H),
130.5 (C-H), 130.1 (C), 128.8 (C-H), 127.7 (C), 125.7 (C-H),
125.6 (C-H), 123.3 (C-H), 115.4 (C-H), 92.8 (C-H), 55.4
(CH3); IR (KBr) 1620, 1310, 1085 cm-1; ESI-MS m/z 303 [M]þ.
Anal. Calcd for C18H15N4ClO5: C, 53.67; H, 3.75; N, 13.91.
Found: C, 53.69; H, 3.74; N, 13.89.
1
the compound [2a]ClO4 (0.268 g, 72%): mp 216-217 °C; H
NMR (300 MHz, CD3CN) δ 9.24 (d, 1H, J = 6.51 Hz), 8.98 (s,
1H), 8.73 (d, 1H, J = 8.67 Hz), 8.63 (t, 1H, J = 7.53 Hz), 8.54 (d,
1H, J = 9.09 Hz), 8.10 (t, 1H, J = 6.47 Hz), 7.75-7.71 (m, 2H),
7.54 (t, 2H, J = 7.83 Hz), 7.45 (d, 2H, J = 7.50 Hz), 7.38 (t, 1H,
J = 7.35 Hz); 13C NMR (125 MHz, CD3CN) δ 155.5 (C), 143.5
(C), 142.8 (C-H), 140.2 (C), 137.6 (C), 134.8 (C-H), 130.6
(C-H), 130.3 (C-H), 128.9 (C-H), 127.5 (C), 127.1 (C-H),
125.9 (C-H), 123.5 (C-H), 123.4 (C-H), 93.5(C-H); IR (KBr)
1618, 1315, 1085 cm-1; ESI-MS m/z 273 [M]þ. Anal. Calcd for
C17H13N4ClO4: C, 54.78; H, 3.52; N, 15.03. Found: C, 54.76; H,
3.49; N, 15.06.
3b: yield (0.24 g, 84%); mp 208-209 °C; 1H NMR (300 MHz,
CDCl3) δ 7.75 (d, 1H, J = 6.84 Hz), 7.41-7.31 (m, 2H), 7.14 (d,
2H, J = 8.04 Hz), 7.08 (d, 1H, J = 9.63 Hz), 6.94-6.89 (m, 3H),
6.56 (t, 1H, J = 6.69 Hz), 6.29 (s, 1H), 2.34 (s, 3H); 13C NMR (75
MHz, CD3CN) δ 160.5 (C), 148.1 (C), 147.7 (C), 143.5 (C-H),
139.5(C), 136.4 (C-H), 136.1 (C-H), 133.3 (C), 129.7 (C-H),
126.4 (C-H), 124.1 (C-H), 120.9 (C), 120.7 (C-H), 111.0
Conversion of [2a]ClO4 into 3a was achieved from the pre-
formed compound [2a]ClO4 (0.373 g, 1 mmol) by reacting it with
aniline (1.4 mL, 15 mmol) in 10 mL of acetonitrile under
refluxing conditions for 4 h. The color of the solution changed
from red to brownish pink during this period. Purification of the
reaction mixture on a preparative alumina TLC plate using
chloroform as eluent yielded 3a (0.23 g, 85%): mp 207-208 °C;
1H NMR (300 MHz, CDCl3) δ 7.69 (d, 1H, J = 7.20 Hz),
7.35-7.25 (m, 3H), 7.05-7.01 (m, 2H), 6.93 (d, 2H, J = 7.35
Hz), 6.85 (d, 2H, J = 9.04 Hz), 6.48 (t, 1H, J = 6.30 Hz), 6.17 (s,
1H); 13C NMR (CDCl3, 75 MHz) δ 160 (C), 149.4 (C), 147.4 (C),
147.1 (C), 142.3 (C-H), 140.4 (C), 136.4 (C-H), 136.3 (C-H),
129.2 (C-H), 126.9 (C-H), 124.3(C-H), 124.2 (C-H), 121.1
(C-H), 101.8 (C-H), 21 (CH3); IR (KBr) 1640, 1505 cm-1
;
ESI-MS m/z 287 [HM]þ. Anal. Calcd for C18H14N4: C, 75.5; H,
4.93; N, 19.57. Found: C, 75.51; H, 4.94; N, 19.55.
3c: yield (0.247 g, 81%); mp 207-208 °C; 1H NMR
(300 MHz, CDCl3) δ 7.64 (d, 1H, J = 6.60 Hz), 7.28-7.21
(m, 2H), 6.99-6.89 (m, 3H), 6.84-6.80 (m, 3H), 6.45 (t, 1H, J =
6.84 Hz), 6.25 (s, 1H), 3.74 (s, 3H); 13C NMR (CDCl3, 75 MHz)
δ 160.3 (C), 156.7 (C), 148.1 (C), 147.8 (C), 143.3 (C-H), 139.9
(C), 136.5 (C-H), 136.3 (C-H), 126.5 (C-H), 124.2 (C-H),
122.4 (C-H), 114.5 (C-H), 111.4 (C-H), 101.6 (C-H), 55.7
(CH3), (one carbon in the aromatic region is unresolved); IR
(KBr) 1642, 1502 cm-1; ESI-MS m/z 303 [HM]þ. Anal. Calcd
for C18H14N4O: C, 71.51; H, 4.67; N, 18.53. Found: C, 71.48; H,
4.66; N, 18.55.
(C-H), 112.1 (C-H), 101.1(C-H); IR (KBr) 1643, 1502 cm-1
;
ESI-MS m/z 273 [HM]þ. Anal. Calcd for C17H12N4: C, 74.98; H,
4.44; N, 20.58. Found: C, 74.95; H, 4.43; N, 20.60.
The substituted compounds [2b]ClO4, [2c]ClO4, and 3b, 3c
were synthesized using the appropriate ArNH2 according to the
methods as noted for the synthesis of [2a]ClO4 and 3a. Their
yields and characterization data are as follows.
Acknowledgment. The research was supported by the
Department of Science and Technology (DST), India
funded Project, SR/S1/IC-24/2006. We are thankful to the
Editor and the reviewers for their suggestions at the revision
stage. Crystallography was performed at the DST-funded
National Single Crystal Diffractometer Facility at the
Department of Inorganic Chemistry, IACS. M.S. is thankful
to the Council of Scientific and Industrial Research for
fellowship support.
[2b]ClO4: yield (0.29 g, 75%); mp 215-217 °C; 1H NMR
(300 MHz, CD3CN) δ 9.24 (d, 1H, J = 6.69 Hz), 9.05 (s, 1H),
8.73 (d, 1H, J = 8.54 Hz), 8.63 (t, 1H, J = 7.47 Hz), 8.53 (d, 1H,
J = 9 Hz), 8.09 (t, 1H, J = 6.60 Hz), 7.76-7.72 (m, 2H), 7.54 (t,
2H, J = 7.77 Hz), 7.44 (d, 1H, J = 7.53 Hz), 7.38 (t, 1H, J =
(13) (a) Yang, K. -Y.; Swenson, R. P. Biochemistry 2007, 46, 2289. (b)
Roth, J. P.; Wincek, R.; Nodet, G.; Edmondson, D. E.; McIntire, W. S.;
Klinman, J. P. J. Am. Chem. Soc. 2004, 126, 15120. (c) Carvalhal, R. F.;
Mendes, R. K.; Kuboto, L. T. Int. J. Electrochem. Sci. 2007, 2, 979. (d)
Gropper, S. S.; Smith, J. L.; Groff, J. L. Riboflavin. In Advanced Nutrition and
Human Metabolism, 5th ed.; Wadsworth CENGAG Learning: Mason, OH, 2009;
Chapter 9, pp 329-333.
Supporting Information Available: X-ray crystallographic
table of [2a]ClO4, 3a, and 3b and relevant figures for character-
ization. This material is available free of charge via the Internet
at http://pubs.acs.org.
2068 J. Org. Chem. Vol. 75, No. 6, 2010