Facile synthesis and characterization of naphthidines as a new class of highly nonplanar electron donors giving robust radical cations

Article abstract of DOI:10.1021/jo0517444

Naphthidines 2 were prepared by nickel-catalyzed amination of 1-chloronaphthalene followed by oxidative homocoupling of 1-naphthalene amines 1 using titanium(IV) tetrachloride. The electronic and magnetic properties of materials 2 were investigated by cyclic voltammetry and other electrochemical techniques, EPR and UV-visible spectroscopies, and magnetic susceptibility. It was demonstrated that compounds 2 could be easily and reversibly oxidized via a two-electron-transfer reaction into their bis(radical cation) 2^2.2+, which displays a substantial stability at room temperature (the half-life of 2^2.2+ estimated by EPR at 25 °C was 10 days). B3LYP/6-31G* optimized structures of N,N′-bis(4-methoxyphenyl)-(1,1′-binaphthyl)- 4,4′-diamine 2g shows significant differences in the torsion angle between the naphthalene moieties depending on its oxidation state. Twisted structures are preferred for neutral compounds, whereas more planar are favored for the oxidized forms 2g⁺ and 2g^2.2+ to realize spin and/or charge delocalizations over the whole π-system. Such conformation changes concerted with the electron transfers contribute to explain the unusual two-electron process observed in the electrochemical behavior of 2g instead of the two single-electron transfers that would have been expected in the case of two successive oxidations. It is finally shown that the oxidation of 2g in CH₂Cl₂ with thianthrenium perchlorate (ThClO4) generates the dication 2g^2.2+ with singlet spin-multiplicity.

Full text of DOI:10.1021/jo0517444

Facile Synthesis and Characterization of Naphthidines as a New
Class of Highly Nonplanar Electron Donors Giving Robust Radical
Cations
Christophe Desmarets,† Benoˆıt Champagne,§ Alain Walcarius,*,‡ Christine Bellouard,|
Rafik Omar-Amrani,† Abdelaziz Ahajji, Yves Fort,† and Raphae¨l Schneider*,†
Synthe`se Organome´tallique et Re´actiVite´, UMR 7565, Faculte´ des Sciences, BP 239, 54506 VandoeuVre les
Nancy, France, Laboratoire de Chimie Physique et Microbiologie pour l’EnVironnement, UMR 7564,
CNRS - UniVersite´ Henri Poincare´ Nancy I, 405 rue de VandoeuVre, 54600 Villers-les-Nancy, France,
Laboratoire de Chimie The´orique Applique´e, FUNDP, Rue de Bruxelles 61, 5000 Namur, Belgium,
Laboratoire de Physique des Mate´riaux, UMR 7556, Faculte´ des Sciences, BP 239, 54506 VandoeuVre les
Nancy, France, and Laboratoire d’Etudes et de Recherche sur le Mate´riau Bois, UMR 1093, Faculte´ des
Sciences, BP 239, 54506 VandoeuVre les Nancy, France
raphael.schneider@sor.uhp-nancy.fr
ReceiVed August 18, 2005
Naphthidines 2 were prepared by nickel-catalyzed amination of 1-chloronaphthalene followed by oxidative
homocoupling of 1-naphthalene amines 1 using titanium(IV) tetrachloride. The electronic and magnetic
properties of materials 2 were investigated by cyclic voltammetry and other electrochemical techniques,
EPR and UV-visible spectroscopies, and magnetic susceptibility. It was demonstrated that compounds
2 could be easily and reversibly oxidized via a two-electron-transfer reaction into their bis(radical cation)
2
2.2+, which displays a substantial stability at room temperature (the half-life of 22.2+ estimated by EPR
at 25 °C was 10 days). B3LYP/6-31G* optimized structures of N,N′-bis(4-methoxyphenyl)-(1,1′-
binaphthyl)-4,4′-diamine 2g shows significant differences in the torsion angle between the naphthalene
moieties depending on its oxidation state. Twisted structures are preferred for neutral compounds, whereas
more planar are favored for the oxidized forms 2g•+ and 2g2.2+ to realize spin and/or charge delocalizations
over the whole π-system. Such conformation changes concerted with the electron transfers contribute to
explain the unusual two-electron process observed in the electrochemical behavior of 2g instead of the
two single-electron transfers that would have been expected in the case of two successive oxidations. It
is finally shown that the oxidation of 2g in CH2Cl2 with thianthrenium perchlorate (ThClO4) generates
the dication 2g2.2+ with singlet spin-multiplicity.
Introduction
due to their potential applications in organic light-emitting
diodes (OLEDs), field effect transistors, charge storage devices,
photodiodes, sensors, etc.1 These organic electroactive and
photoactive materials are usually based on thiophene, pyrrole,
carbazole, or arylamine moieties. In the case of arylamines, a
great number of studies were devoted to 1,4-phenylenediamines2
and 4,4′-diaminobiphenyls3 because these unique families of
materials are highly electron-rich and can produce delocalized
Conjugated organic materials (both low molecular weight
compounds and polymers) have recently received much attention
† UMR 7565, Faculte´ des Sciences.
‡ UMR 7564, CNRS - Universite´ Henri Poincare´ Nancy I.
§ FUNDP.
| UMR 7556, Faculte´ des Sciences.
UMR 1093, Faculte´ des Sciences.
10.1021/jo0517444 CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/19/2006
J. Org. Chem. 2006, 71, 1351-1361
1351

Desmarets et al.
radical cations. Compared to 1,4-phenylenediamines and 4,4′-
diaminobiphenyls, which have found wide applications in
OLEDs due their high stability in the oxidized form, binaphthyl
conjugated aromatic amines have been much less studied,
although naphthalene, 1,4- or 1,5-ethylnylnaphthalene, or bi-
naphthyl-based oligomers were developed as spacers or con-
nectors4 in studies dealing with energy transfer.5
FIGURE 1. Structure of the SIPr ligand.
Among the physical properties sought in new materials,
especially for application in OLEDs, is their thermal and
morphological stability. Amorphous materials possessing high
glass transition temperatures (Tg) should have better opportunity
for retaining the film morphology during device operation. A
very simple concept for the formation of amorphous glass is
nonplanar molecular structure, because easy packing of mol-
ecules and hence ready crystallization can be retarded. Non-
planar configuration has been achieved with the use of star-
shaped molecules6 or incorporation of bulky moieties in the
molecules.7 As a result of the rotation barrier around the central
carbon-carbon bond and the torsion angle between the naphthyl
units, naphthidines may constitute attractive systems to study
switching phenomena in organic redox systems and therefore
for the development of efficient hole-transport materials.
However, because of the lack of suitable methods for the
synthesis of naphthidines, their hole-transport properties have
not been studied thoroughly even though good efficiencies
should be attained with these compounds. N,N,N′,N′-Tetra-
methylnaphthidine (TMN),8,9 N,N′-diphenylnaphthidine,10 and
the parent 5,5′-di(8-aminoquinoxalyl)11 are the sole compounds
containing a binaphthyl core or a heterocyclic analogue core
structure described in the literature. The synthesis of the former
compounds has mainly been achieved by oxidative dimerization
10a
9
using CrO3 or TiCl4 of the corresponding naphthylamines.
Although the reported methods are, in principle, applicable to
N-substituted naphthylamines, such procedures have not been
reported yet. Other methods involving anodic oxidation followed
by electrodimerization of N,N′-dimethyl-1-naphthylamine have
also been described, but the yields are poor (30-35%) because
the dimeric product is more readily oxidized than the starting
material.8 The use of a two-phase H2O-C6H5NO2 solvent
system for the electrochemical oxidation was reported to
overcome this difficulty, but no isolated yield is reported.12
In this paper, we first report the synthesis of a series of N,N′-
substituted naphthidines 2 using the nickel-catalyzed aryl
amination methodology we recently established.13,14 The elec-
tronic and magnetic properties of these materials were next
investigated by several electrochemical techniques (cyclic
voltammetry, chronoamperometry, coulometry), EPR and UV-
vis spectroscopies, and magnetic susceptibility, as well as
quantum chemical calculations within the density functional
theory (DFT) approach.
(1) (a) Leclerc, M.; Fa¨ıd, K. AdV. Mater. 1997, 9, 1087. (b) Handbook
of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L.,
Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998. (c) AdVances in
Synthetic Metals: Twenty Years of Progress in Science and Technology;
Bernier, P., Lefrant, S., Bidan, G., Eds.; Elsevier: Lausanne, 1999. (d)
Leclerc, M. J. Polym. Sci., Polym. Chem. 2001, 39, 2867.
(2) (a) Deuchert, K.; Hu¨nig, S. Angew. Chem., Int. Ed. Engl. 1978, 17,
875. (b) Louie, J.; Hartwig, J. F. J. Am. Chem. Soc. 1997, 119, 11695. (c)
Wienk, M. M.; Janssen, R. A. J. J. Am. Chem. Soc. 1997, 119, 4492. (d)
Nelsen, S. F.; Tran, H. Q.; Nagy, M. A. J. Am. Chem. Soc. 1998, 120, 298.
(e) Schumann, J.; Kanitz, A.; Hartmann, H. Synthesis 2002, 9, 1268. (f)
Yano, M.; Ishida, Y.; Aoyama, K.; Tatsumi, M.; Sato, K.; Shiomi, D.;
Ichimura, A.; Takui, T. Synth. Met. 2003, 137, 1275. (g) Kim, M.-J.; Seo,
E.-M.; Vak, D.; Kim, D.-Y. Chem. Mater. 2003, 15, 4021.
(3) (a) Suzuki, T.; Okubo, T.; Okada, A.; Yamashito, Y.; Miyashi, T.
Heterocycles 1993, 35, 395. (b) Lambert, C.; No¨ll, G. Angew. Chem., Int.
Ed. 1998, 37, 2107. (c) Lambert, C.; No¨ll, G. J. Am. Chem. Soc. 1999,
121, 8434. (d) Goodson, F. E.; Hauck, S. I.; Hartwig, J. F. J. Am. Chem.
Soc. 1999, 121, 7527. (e) Hreha, R. D.; George, C. P.; Haldi, A.; Domercq,
B.; Malagoli, M.; Barlow, S.; Bre´das, J.-L.; Kippelen, B.; Marder, S. R.
AdV. Funct. Mater. 2003, 13, 967. (f) Huang, Q.; Evmenenko, G.; Dutta,
P.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 14704. (g) Low, P. J.;
Paterson, M. A. J.; Puschmann, H.; Goeta, A. E.; Howard, J. A. K.; Lambert,
C.; Cherryman, J. C.; Tackley, D. R.; Leeming, S.; Brown, B. Chem. Eur.
J. 2004, 10, 83.
Results and Discussions
Synthesis. Preparation of naphthidines 2 was carried out by
the two-step synthetic procedure starting from 1-chloronaph-
thalene as shown in Scheme 1.
We have recently reported the use of a Ni(0) catalyst
associated with a strong electron-donating and sterically hin-
dered N-heterocyclic carbene15 (N,N′-bis(2,6-diisopropylphenyl)-
dihydroimidazol-2-ylidene, SIPr) (Figure 1) to allow mild
amination of aryl chlorides with several classes of amines.14
We first investigated the scope and limitations of this catalyst
system for the cross-coupling of 1-chloronaphthalene with
structurally and electronically diverse amines using the aryl
(9) Periasamy, M.; Jayakumar, K. N.; Bharati, P. J. Org. Chem. 2000,
65, 3548.
(4) (a) Chow, H.; Ng, M. Tetrahedron: Asymmetry 1996, 7, 2251. (b)
Chow, H.; Ng, M. Tetrahedron Lett. 1996, 37, 2979. (c) Rodriguez, J. G.;
Tejedor, J. L. J. Org. Chem. 2002, 67, 7631.
(5) (a) El-Ghayoury, A.; Harriman, A.; Khatyr, A.; Ziessel, R. Tetra-
hedron Lett. 1997, 38, 2471. (b) El-Ghayoury, A.; Harriman, A.; Khatyr,
A.; Ziessel, R. J. Phys. Chem. A 2000, 104, 1512. (c) El-Ghayoury, A.;
Harriman, A.; Khatyr, A.; Ziessel, R. Angew. Chem., Int. Ed. 2000, 39,
185.
(6) (a) Shirota, Y.; Kobata, T.; Noma, N. Chem. Lett. 1989, 1145. (b)
Higuchi, A.; Inada, H.; Kobata, T.; Shirota, Y. AdV. Mater. 1991, 3, 549.
(c) Inada, H.; Shirota, Y. J. Mater. Chem. 1993, 3, 319. (d) Kuwabara, Y.;
Ogawa, H.; Inada, H.; Noma, N.; Shirota, Y. AdV. Mater. 1994, 6, 677. (e)
Katsuma, K.; Shirota, Y. AdV. Mater. 1998, 10, 223. (f) Wu, I.-Y.; Lin, J.
T.; Tao, Y.-T.; E. Balasubramaniam, E. AdV. Mater. 2000, 12, 668.
(7) (a) Noda, T.; Ogawa, H.; Noma, N.; Shirota, Y. J. Mater. Chem.
1999, 9, 2177. (b) Wang, S.; Oldham, W. J., Jr.; Hudack, R. A., Jr.; Bazan,
G. C. J. Am. Chem. Soc. 2000, 122, 5695.
(10) (a) Braid, M. U.S. Patent 3,759,996, 1973. (b) Hornback, J. M.;
Gossage, H. E. J. Org. Chem. 1985, 50, 541.
(11) Suzuki, T.; Saito, M.; Kawai, H.; Fujiwara, K.; Tsuji, T. Tetrahedron
Lett. 2004, 45, 329.
(12) Vettorazzi, N.; Fernandez, H.; Silber, J. J.; Sereno, L. Electrochim.
Acta 1990, 35, 1081.
(13) (a) Brenner, E.; Fort, Y. Tetrahedron Lett. 1998, 39, 5359. (b)
Brenner, E.; Schneider, R.; Fort, Y. Tetrahedron 1999, 55, 12829. (c)
Brenner, E.; Schneider, R.; Fort, Y. Tetrahedron Lett. 2000, 41, 2881. (d)
Desmarets, C.; Schneider, R.; Fort, Y. Tetrahedron Lett. 2000, 41, 2875.
(e) Desmarets, C.; Schneider, R.; Fort, Y. Tetrahedron Lett. 2001, 42, 247.
(f) Desmarets, C.; Schneider, R.; Fort, Y. Tetrahedron 2001, 57, 7657. (g)
Brenner, E.; Schneider, R.; Fort, Y. Tetrahedron 2002, 58, 6913. (h)
Desmarets, C.; Schneider, R.; Fort, Y.; Walcarius, A. J. Chem. Soc., Perkin
Trans. 2 2002, 1844.
(14) (a) Gradel, B.; Brenner, E.; Schneider, R.; Fort, Y. Tetrahedron
Lett. 2001, 42, 5689. (b) Desmarets, C.; Schneider, R.; Fort, Y. J. Org.
Chem. 2002, 67, 3029. (c) Omar-Amrani, R.; Thomas, A.; Brenner, E.;
Schneider, R.; Fort, Y. Org. Lett. 2003, 5, 2311. (d) Omar-Amrani, R.;
Schneider, R.; Fort, Y. Synthesis 2004, 2527.
(8) (a) Miras, M. C.; Silber, J. J.; Sereno, L. Electrochim. Acta 1998,
33, 851. (b) Zon, M. A.; Fernandez, H. J. Electroanal. Chem. 1990, 295,
41.
1352 J. Org. Chem., Vol. 71, No. 4, 2006

Synthesis and Characterization of Naphthidines
SCHEME 1. Synthesis of Naphthidines 2
SCHEME 2. Proposed Mechanism for the TiCl4-Catalyzed
Homocoupling of Compounds 1
amination conditions [Ni(0) (5 mol %), SIPr (5 or 10 mol %),
t-BuONa (150 mol %), dioxane, 100 °C] we developed.14b,c
As can be seen from Table 1, reactions performed with
alicyclic amines (entries a and b) proceeded smoothly, affording
the corresponding 1-naphthylamines 1 in good yields. For
example, under the reaction conditions, pyrrolidine gave
compound 1a in 85% yield. Anilines gave results similar to
those of alicyclic amines. Diarylation of the starting aniline could
not be detected even after prolonged reaction times in the
presence of an excess of 1-chloronaphthalene. The presence of
two methyl groups at the ortho position of the aniline (entries
d and e) has no deleterious effect. Using hindered amines such
as 2,6-dimethylaniline or mesitylamine as substrates led to
products 1d and 1e in, respectively, 82% and 81% yield. In
every reaction, very little dechlorinated byproduct was observed
(less than 5%).
We next turned our attention to the following step involving
oxidative coupling of 1-naphthylamines 1 mediated by TiCl4.
In oxidative coupling of N,N-dimethylnaphthylamine, optimized
reaction conditions have recently been reported.9 When the
homocoupling of compounds 1 was conducted with 1.5 equiv
of TiCl4 between 0 and 25 °C as described by Periasamy,9 the
corresponding naphthidines 2 were isolated in modest yields
(26-33%). Careful attention to reaction conditions proved
critical in order to obtain products 2 in good yields. Much better
results (see Table 1) were obtained when the oxidative coupling
and the hydrolysis of the reaction medium were both conducted
between -5 and 0 °C. For example, the isolated yield of
naphthidine 2a, disubstituted by the pyrrolidino group, which
among dialkylamino groups exhibits the most pronounced
resonance effect and the strongest donor character,16 could be
improved from 26% to 76% when the whole coupling was
performed between -5 and 0 °C. Except compound 1b, the
transformation was found to be general for all 1-naphthylamines
1 studied, and their isolation was easily achieved by column
chromatography. Finally, 1-naphthylmorpholine 1b failed to
react under the reaction conditions. No side product was
observed. This failure does probably not originate from the
chelating properties of the oxygen atom toward Ti4+ since
oxygen-containing substrates such as 1g afforded the expected
homocoupled product 2g in high yields. Due to the electron-
withdrawing effect of the oxygen atom,17 it seems more likely
that the nucleophilicity of the nitrogen atom is too weak to
generate the aryl titanium and radical cation species involved
in the oxidative coupling (Scheme 2).
Electrochemical Studies. Cyclic voltammetry (CV) was used
to characterize the electrochemical behavior of compounds
2a,c-h. The measurements were carried out in acetonitrile at
room temperature using tetrabutylammonium hexafluorophos-
phate (0.1 M) as the supporting electrolyte and a saturated
Calomel electrode as the reference. CV experiments were
performed at various scan rates extending from 10 mV s-1 to 2
V s-1; within this whole range, all recorded voltammograms
were characterized by peak currents directly proportional to the
square root of potential scan rate, indicating that the electron-
transfer processes involving compounds 2a,c-h were diffusion-
controlled.18
Typical CV curves are depicted in Figure 2 for compounds
2a, 2c, 2d, and 2g. All of the novel compounds 2a,c-h exhibit
a single, two-electron, reversible signal in the potential range
0.5-0.8 V (oxidation of the 1,1′-binaphthyl-4,4′-bispyrrolidine
2a at 0.49 V and at ca. 0.7-0.8 V for N,N′-diaryl-(1,1′-
binaphthyl)-4,4′-diamine 2c-h), similarly to results reported for
the related N,N,N′,N′-tetramethylnaphthidine (TMN) and N,N′-
diphenylnaphthidine.8-10 The electrochemical characteristics of
compounds 2a,c-h are summarized in Table 2. The lower
oxidation potential of 2a is due to the stronger donating
properties of the pyrrolidinyl group compared to those of
arylamines.16 The two-electron-transfer process was confirmed
by coulometry (Table 2), corresponding to the one-step forma-
(17) (a) Roberts, S. M.; Suschitzky, H. J. Chem. Soc., Chem. Commun.
1967, 893. (b) Perepichka, I. F.; Popov, A. F.; Kostenko, L. I.; Piskunova,
Zh. P. Org. React. 1986, 23, 317. (c) Hartwig, J. F.; Richards, S.; Baranavo,
D.; Paul, F. J. Am. Chem. Soc. 1996, 118, 3626. (d) Beletskaya, I. P.;
Bessmertnykh, A. G.; Guilard, R. Tetrahedron Lett. 1999, 40, 6393.
(18) Bard, A. J.; Faulkner, L. R., Electrochemical Methods, Fundamentals
and Applications, 2nd ed.; John Wiley & Sons: New York, 2001.
(15) (a) Herrmann, W. A.; Reisinger, C. P.; Spiegler, M. J. Organomet.
Chem. 1998, 557, 93. (b) Bo¨hm, V. P. W.; Gsto¨ttmayr, C. W. K.; Weskamp,
T.; Herrmann, W. A. J. Organomet. Chem. 2000, 595, 186. (c) Bourissou,
D.; Guerret, O.; Gabba¨ı, F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39. (d)
Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(16) Brown, H. C.; Okamoto, Y. J. Am. Chem. Soc. 1958, 80, 4979.
J. Org. Chem, Vol. 71, No. 4, 2006 1353

Desmarets et al.
TABLE 1. Synthesis of 1-Naphthylamines 1 and Naphthidines 2
1354 J. Org. Chem., Vol. 71, No. 4, 2006

Synthesis and Characterization of Naphthidines
Table 1 (Continued)
1
a All the yields refer to pure isolated products characterized by H and 13C NMR, IR and elemental analysis.
TABLE 2. Electrochemical Characteristics of Compounds 2a,c-h
nb
a
a
a
compound
Epa
Epc
ipa/ipc
2a
2c
2d
2e
2f
0.49
0.80
0.72
0.73
0.74
0.73
0.68
0.44
0.75
0.64
0.69
0.69
0.69
0.62
1.02
0.99
1.08
1.05
0.98
1.01
0.97
1.9
2.2
2.1
1.9
nm
2.0
nm
2g
2h
a Epa, anodic peak potential; Epc, cathodic peak potential; ipa/ipc, ratio of
anodic-to-cathodic peak currents. Other conditions: 0.1 M Bu4NPF6 in
CH3CN, potential vs SCE, Pt electrode, 25°C, scan rate 20 mV/s.
where both linear and spherical diffusion are expected to occur
successively and using the well-known ferrocene/ferricinium
redox couple as a single-electron-transfer reference (see Sup-
porting Information). They revealed a value of n ) 1.8 ( 0.2,
confirming the two-electron transfer reaction in the early times
of the electrochemical experiment. The chronoamperometric data
also led to the evaluation of the diffusion coefficient of
FIGURE 2. Cyclic voltammograms of compounds 2a, 2c, 2d, and 2g
(1 × 10-3 M in acetonitrile containing 0.1 M Bu4NPF6) recorded at a
derivative 2g, which was calculated as about 8 × 10-6 cm2 s-1
,
potential scan rate of 20 mV s-1
.
a value of the same order of magnitude as that reported for
TMN.19 Further evidence to support the reversible two-electron
transfer in CV is given by the variation of peak currents of 2g
as a function of square root of scan rate, which is 23/2 times
higher than that of ferrocene (at the same concentration),
according to the Randles-Sevcik equation,17 after correction
for the lower diffusion coefficient of 2g with respect to that of
ferrocene (2 × 10-5 cm2 s-1).20,21
tion of the dication 22.2+ (evidenced by several spectroscopic
techniques, see below). The anodic-to-cathodic peak separation
observed in CV, however, was in the range 40-60 mV,
suggesting indeed the transfer of more than one electron, but it
was not strictly equal to 30 mV (the theoretical value expected
in case of two-electron-transfer processes).18 This might be due
to nonideal reversible behavior (electron-transfer kinetic limita-
tions), which was more or less pronounced depending on the
compounds (compare, e.g., 2d and 2g in Figure 2).
The above results are consistent with a single reversible two-
electron oxidation of 2 into a stable di(radical cation), 22.2+
.
To further point out the two-electron process, even in the
small time scale of the CV experiments, additional chrono-
amperometric measurements have been performed on derivative
2g using an ultramicroelectrode (radius 50 µm) in conditions
(19) Miras, M. C.; Silber, J. J.; Sereno, L. J. Electroanal. Chem. 1986,
201, 367.
(20) Martin, R. D.; Unwin, P. R. J. Electroanal. Chem. 1997, 439, 123.
(21) Ikeuchi, H.; Kanakubo, M. Electrochemistry (Tokyo) 2001, 69, 34.
J. Org. Chem, Vol. 71, No. 4, 2006 1355

Desmarets et al.
FIGURE 3. Cyclic voltammograms of compound 2g recorded in two
potential windows, at a scan rate of 50 mV s-1 (other conditions as in
Figure 2).
This occurs probably via two successive electron transfers (EE
mechanism) involving first the formation of the radical cation
2•+, which is then oxidized to 22.2+ at a potential value lower
than that corresponding to the first electron transfer (E°2 < E°1),
as previously suggested for the electrochemical oxidation of
TMN.19 Such situations where the second electron transfer is
much easier than the first one can be indeed observed when,
e.g., conformation changes are concerted with the electron
transfer.22 This will be discussed below from results obtained
by theoretical calculations made on derivatives 2g, 2g•+, and
2g2.2+. The high stability of the di(radical cation) was also
pointed out by thorough electrolysis monitored by linear scan
voltammetry at rotating disk electrode at various completion
levels of the reaction (see Supporting Information).
FIGURE 4. UV-vis spectra of the stepwise oxidation of 2a (top)
and 2g (bottom) with ThClO4 in CHCl3 at 25 °C.
It should be noted finally that extending the potential scanning
range up to the anodic limit (2.5 V) led to the observation of a
second irreversible oxidation peak at about 1.7 V, only for
compound 2g (Figure 3), whereas all of the CV curves recorded
with the other naphthidine derivatives did not display any
additional well-defined signal.
Although no attempt was made to interpret this additional
oxidation step of compound 2g, it is likely that this could arise
from the oxidation of the p-methoxyphenyl moieties.23 This
second oxidation has only little influence on the reversibility
of the first oxidation peak at 0.73 V. These results also
demonstrate that, under the applied conditions, the di(radical
cation)s 22.2+ are stable and do not lead to, e.g., dimerization,
as was generally observed in the course of the decomposition
of arylamine radical cations.24
whereas with ThClO4 the oxidation process was accomplished
immediately. However, absorption spectra of the PIFA-oxidized
species do not differ from those of the ThClO4-oxidized ones.
The oxidation of compounds 2a and 2g with ThClO4 was
monitored by UV-vis spectroscopy. A 0.1 mM solution of 2a
or 2g in chloroform was treated repeatedly with 0.5 equiv of
ThClO4 in chloroform. Neutral 2 compounds were colorless in
solution. With the addition of the oxidant, the solution turned
immediately deep blue. Upon stepwise addition of ThClO4, the
absorption of neutral amine at λmax ) 341 and 345 nm,
respectively, for 2a and 2g decreases and new bands appear at
λmax ) 520 nm for 2a and at 490, 537, and 577 nm for 2g
(Figure 4). The UV-vis absorption characteristics of the
ThClO4-oxidized species do not differ from those obtained when
performing spectroscopic measurements in the course of the
electrolytic transformation of 2a and 2g at various reaction times
(Figure 5), indicating that the products generated by chemical
oxidation were the same as those produced electrochemically.
Note that the spectra of 2a and 2g after chemical oxidation
showed a single additional band at ca. 255 nm owing to the
presence of reduced ThClO4. The appearing bands were ascribed
to formation of 22.2+ and reached a maximum intensity after
addition of exactly 2 equiv of ThClO4, demonstrating the near
quantitative formation of the di(radical cation) 22.2+ in which
each naphthylamine unit carries an unpaired electron. This
quantitative oxidation was also pointed out in the electrochemi-
UV-Visible and EPR Spectra of Oxidized Species. Quan-
titative conversion of naphthidines 2 to their di(radical cation)
could be achieved either by chemical oxidation with thianthre-
nium perchlorate (ThClO4),25 or phenyliodine(III)bis(trifluoro-
acetate) (PIFA).26 PIFA is a weak oxidant for compounds 2,
(22) (a) Bellec, N.; Boubekeur, K.; Carlier, R.; Hapiot, P.; Lorcy, D.;
Tallec, A. J. Phys. Chem. A 2000, 104, 9750. (b) Carlier, R.; Hapiot, P.;
Lorcy, D.; Robert, A.; Tallec, A. Electrochim. Acta 2001, 46, 3269.
(23) Zweig, A.; Hodgson, W. G.; Jura, W. H. J. Am. Chem. Soc. 1964,
86, 157.
(24) Seto, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy,
D. W.; Adams, R. N. J. Am. Chem. Soc. 1966, 88, 3498.
(25) Murata, Y.; Shine, H. J. J. Org. Chem. 1969, 34, 3368. Caution!
Thianthrenium perchlorate is a shock-sensitive solid that should be handled
only on a small scale and with due care.
(26) Eberson, L.; Hartshorn, M. P.; Persson, O. Acta Chem. Scand. 1995,
49, 640.
1356 J. Org. Chem., Vol. 71, No. 4, 2006

Synthesis and Characterization of Naphthidines
FIGURE 6. EPR spectra recorded at 100 K of 2g oxidized by 2 equiv
ThClO4 in a frozen CH2Cl2 matrix.
FIGURE 5. UV-vis spectra of the electrochemical oxidation of 2a
(top) and 2g (bottom) in CH3CN at 25 °C.
cal reactions, as 2 mol of electrons were necessary to complete
the electrolyses. Upon further addition of ThClO4 or by stirring
the di(radical cation) at room temperature for 1 day, no further
evolution in the absorption spectra was observed. In accordance
with CV studies, this result illustrates the high stability of the
generated di(radical cation) at room temperature and shows that
no follow-up reaction such as dimerization occurs.
The corroborating evidence was obtained from the EPR
spectrum of the oxidized solutions. Oxidation of 2g by less than
1 or 2 molar equiv of ThClO4 yields EPR-active solutions. When
less than 1 equiv of ThClO4 is added to a solution of 2g in
dichloromethane, the EPR spectrum of the deep blue solution
at 25 °C reveals only a single broad line with a spectral width
of ca. 6.6 G (data not shown). Apart from an increasing intensity,
the same spectrum was observed using 2 equiv of ThClO4. The
spectra obtained suggest that ThClO4 immediately oxidizes 2g
into 2g2.2+ and that the mono(radical cation) 2g.+ cannot be
obtained with this oxidant. Figure 6 displays the EPR spectrum
of 2g2.2+ at 100 K in a frozen CH2Cl2 matrix, which shows a
similar pattern as at 25 °C but slightly wider (ca. 10.0 G). More
noteworthy is that, in both samples, the triplet-allowed ∆ms )
(2 resonance27 was not detected in a half-field region of the
allowed ∆ms ) (1, clearly indicating that the dication behaves
like a pair of independent radicals in doublet spin state.
FIGURE 7. EPR spectra recorded at 100 K of (A) 2g oxidized with
1 equiv of PIFA in CH2Cl2 and (B) 2g oxidized with 2 equiv of PIFA
in CH2Cl2.
When the oxidation was performed with 1 equiv of PIFA, a
milder oxidant than ThClO4, the oxidation of compound 2g is
slower. The EPR spectrum recorded at 100 K reveals five poorly
resolved lines having a splitting constant of ca. 8.2 G (Figure
7A). Such a small hyperfine coupling constant is indicative of
electron delocalization into the π-systems linked to the amino
group.28 The observed quintet spectrum is typical for benzidine
cation radical with an unpaired electron equally coupled to two
nitrogen atoms29 and was attributed to the mono(radical cation)
(27) (a) Selby, T. D.; Blackstock, S. C. J. Am. Chem. Soc. 1999, 121,
7152. (b) Domingo, V. M.; Aleman, C.; Brillas, E.; Julia, L. J. Org. Chem.
2001, 66, 4058. (c) Ito, A.; Nakano, Y.; Kato, T.; Tanaka, K. Chem.
Commun. 2005, 403.
J. Org. Chem, Vol. 71, No. 4, 2006 1357

Desmarets et al.
at a relatively low computational cost and the B3LYP exchange-
correlation functional, which includes 20% Hartree-Fock
exchange, is known to provide accurate equilibrium geom-
etries.31 The B3LYP XC functional has also been adopted in a
number of recent investigations on radicals.32 Nevertheless, two
other well-known XC functionals (PBE0 and BHandHLYP)
were also used in selected cases to test whether the B3LYP
trends are robust. These other results summarized in Supporting
Information lead to the same conclusions about the structural
and electronic effects associated with the successive oxidations.
The representation of the Kohn-Sham orbitals and spin densities
has been performed using the MOLEKEL program.33 Excitation
energies for the neutral species have been evaluated using the
MOSF code34 at the ZINDO35 level using the Mataga-
Nishimoto-Weiss parametrization.
FIGURE 8. Normalized magnetization obtained at 2 K and 3 K as a
function of H/T fitted by a S ) 2/2 Brillouin function.
Neutral Compound. In the minimum energy conformation
of the electronic ground state, the naphthyl groups define a
torsion angle of 75.3°. This is slightly less than for the
binaphthyl species where the dihedral angle amounts to 78.2°.
On the other hand, this angle is more than twice larger than in
the parent N,N,N′,N′-tetraphenylbenzidine (33.9°), which shows
that the quasi perpendicular arrangement of the naphthyl units
is induced by their extended nature. The bond lengths are
essentially similar to those of the binaphthyl and, to a lower
extent, of the naphthalene compound, the only difference being
associated with the part of the naphthyl units that are influenced
by the electron-donating NH-Ph groups (Figure 9). Moreover,
the structure is not perfectly symmetric, and the bond lengths
of the two naphthyl units can differ by as much as 0.001-2 Å.
For simplicity, the values reported in Figure 9 are the averaged
length of the equivalent bonds. As for the binaphthyl compound,
there exists another equilibrium conformation. It has a similar
2g•+. The spectrum suggests therefore a fast intramolecular
charge transfer between the neutral and singly oxidized naph-
thalene amine moieties of 2g•+ or a delocalization of the
unpaired electron over the whole system (see Theoretical
Calculations), even at low temperature. After only 2 equiv of
PIFA is added, the EPR spectrum gradually changes and the
quintet signal is superimposed with the broad transition previ-
ously observed with ThClO4 in dichloromethane (Figure 7B).
No half-field ∆ms ) (2 absorption was observed, which verifies
the doublet nature of the dication. Finally, the mono- and di-
(radical cation)s generated from compound 2g are stable at room
temperature under aerobic conditions, displaying a qualitative
half-life of, respectively, ca. 8 and 10 days. Actually, both 2g•+
and 2g2.2+ were isolated from the CH2Cl2 solution by adding
diethyl ether. However, we have not hitherto succeeded in
obtaining single crystals suitable for the X-ray diffraction study.
Magnetization Measurements. Magnetization (M) of 2g2.2+
was measured using a PPMS/Quantum Design magnetometer.
Raw data obtained were corrected from the sample holder. To
take into account the diamagnetism of the solution, the same
linear slope was also subtracted from the magnetizations
measured at 2 K and 3 K so that magnetizations of 2g2.2+ are
superimposed when they are plotted as a function of H/T. The
normalized magnetizations (M/Ms) are depicted in Figure 8. The
H/T dependence is very close to the Brillouin curve for S )
2/2, indicating the presence of triplet contribution to diradical
2g2.2+. The fraction of diradical in the triplet state is, however,
very small. Indeed, the saturation magnetization amounts to
roughly 2% of the signal expected when the whole solution is
in the S ) 2/2 state.
(30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. GAUSSIAN 03,
Revision B.04; Gaussian, Inc.: Pittsburgh, PA, 2003.
(31) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density
Functional Theory; Wiley-VCH: Weinheim, Germany, 2000.
(32) (a) Brown, E. C.; Borden, W. T. J. Phys. Chem. A 2002, 106, 2963.
(b) Zhang, D. Y.; Hrovat, D. A.; Abe, M.; Thatcher Borden, W. J. Am.
Chem. Soc. 2003, 125, 12823. (c) Bendikov, M.; Duong, H. M.; Starkey,
K.; Houk, K. N.; Carter, E. A.; Wudl, F. J. Am. Chem. Soc. 2004, 126,
7416. (d) Serwinski, P. R.; Esat, B.; Lahti, P. M.; Liao, Y.; Walton, R.;
Lan, J. J. Org. Chem. 2004, 69, 5247. (e) Kubo, T.; Sakamoto, M.; Akabane,
M.; Fujiwara, Y.; Yamamoto, K.; Akita, M.; Inoue, K.; Takui, T.; Nakasuji,
K. Angew. Chem., Int. Ed. 2004, 43, 6474. (f) Kwon, O.; Barlow, S.; Odom,
S. A.; Beverina, L.; Thompson, N. J.; Zojer, E.; Bredas, J.-L.; Marder, S.
R. J. Phys. Chem. A 2005, 109, 9346.
Theoretical Calculations
To gain a deeper understanding of the experimental trends,
the molecular structures and electronic properties of the neutral
and oxidized states of 2g were theoretically investigated. Using
the GAUSSIAN03 package,30 ground state characterizations
were performed within the density functional theory (DFT)
approach using the gradient-corrected B3LYP hybrid exchange-
correlation functional and the 6-31G* basis set (B3LYP/
6-31G*). DFT calculations include electron correlation effects
(33) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. MOLEKEL 4.3;
Swiss Center for Scientific Computing: Manno, Switzerland, 2000-2002.
Portmann, S.; Lu¨thi, H. P. Chimia 2000, 54, 766.
(28) Gerson, F.; Huber, W. Electron Spin Resonance Spectroscopy of
Organic Radicals; Wiley-VCH: Weinheim, 2003; p 361.
(29) (a) Stamires, D. N.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 2557.
(b) Dollish, F. R.; Hall, W. K. J. Phys. Chem. 1965, 69, 2127. (c) Seo, E.
T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy, D. W.; Adams, R.
N. J. Am. Chem. Soc. 1966, 88, 3498.
(34) MOS-F (semiempirical Molecular Orbital package for Spectroscopy,
Fujitsu), V4, 1999.
(35) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111. Martin,
C. H.; Zerner, M. C. In Inorganic Electronic Structure and Spectroscopy;
Solomon, E. I., Lever, A. B. P., Eds.; Wiley: New York, 1999; Vol. 1, p
555.
1358 J. Org. Chem., Vol. 71, No. 4, 2006

Synthesis and Characterization of Naphthidines
FIGURE 9. B3LYP/6-31G*-optimized bond lengths (Å) calculated for 2g (neutral, closed-shell), 2g•+ (radical cation, open-shell, doublet), 2g2.2+
(diradical dication, open-shell, singlet), 2g2.2+ (diradical dication, open-shell, triplet), binaphthyl, and naphthalene. Only the central part of the
naphthidines is depicted.
energy (∆E ) 0.2 kcal/mol) and a dihedral angle between the
naphthyl units of 111.0°. The two conformations are character-
ized by quasi identical geometrical parameters. The large
dihedral angles (75.3° or 111.0°) prevent electron delocalization
between the two naphthylamine moieties. This is evidenced by
the similarity between the geometrical parameters of 1g and
2g, as well as between their frontier orbitals. Figure 10 sketches
the two highest occupied molecular orbitals (HOMOs) and two
lowest unoccupied molecular orbitals (LUMOs) of 2g. The
HOMO and HOMO-1 of 2g mimic the HOMO of 1g, which is
mostly localized on the MeO-Ph-NH part (see Supporting
Information), with little delocalization between the two naph-
thylamine moieties. The LUMO of 1g (see Supporting Informa-
tion) is localized on the naphthyl unit so that in the course of
the TiCl4-mediated homocoupling, in- and out-of-phase com-
FIGURE 10. B3LYP/6-31G* Kohn-Sham orbitals (isocontour of 0.04
au) for 2g: (a) HOMO-1, (b) HOMO, (c) LUMO, (d) LUMO+1.
binations of the MOs of 1g result in different MOs, at least in
what concerns the junction between the naphthyl units.
The ZINDO calculations predict that the lowest-energy
absorption band observed for 2g at 345 nm (3.59 eV) is due to
the second electronic excited state, which is estimated to lie
3.81 eV (325 nm, f ) 0.841) above the ground state and
corresponds mainly to a HOMO-LUMO transition (ꢀHOMO
)
-4.77 eV, ꢀLUMO ) -0.98 eV). For 1g, the corresponding
J. Org. Chem, Vol. 71, No. 4, 2006 1359

Desmarets et al.
FIGURE 12. B3LYP/6-31G* spin densities (isocontour of 0.0015
FIGURE 11. B3LYP/6-31G* Kohn-Sham orbitals (isocontour of 0.04
au) of 2g•+: (a) HOMO(R), (b) HOMO(â), (c) LUMO(â).
au): (a) 2g•+, (b) triplet 2g2.2+, (c) singlet 2g2.2+
.
second electron from a MO presenting an antibonding inter-
ring interaction. Then, two types of diradical species can be
formed as a function of the spin pairing of the electrons: a
singlet and a triplet state. The B3LYP/6-31G* calculations do
not provide a definitive answer concerning the relative stability
of the singlet and triplet diradical dication states because the
energy difference for the corresponding optimized geometry is
only 0.4 kcal/mol in favor of the singlet. This difference reaches
0.8 kcal/mol when correcting the singlet energy for spin
contamination using the expression of Yamaguchi et al.36 Then,
keeping the geometries optimized for the isolated species while
including solvent effects using the IEFPCM scheme37 increases
this difference to 1.3 kcal/mol, whereas performing a geometry
optimization with the solvent gives a difference of 0.8 kcal/
mol. The other XC functionals also predict that the singlet
diradical is more stable than the triplet (PBE0, 3.56 kcal/mol;
BHandHLYP, 1.87 kcal/mol), although the BHandHLYP value
presents a larger spin contamination. On the other hand, the
energy difference and the changes in geometrical parameters,
as discussed later, being small, the unpaired electrons appear
weakly coupled. Although the energy differences are very small,
the most stable conformer for both the singlet and triplet
diradicals presents a torsion angle larger than 90°: 122.0° for
the singlet and 111.6° for the triplet. The other stable conformers
have torsion angles of 67.7° and 75.1°, for the singlet and triplet,
respectively. In addition to the torsion angle between the
naphthyl units, the triplet diradical structure with an inter-ring
CC bond length of 1.496 Å and NC distances of 1.407 Å
resembles much that of the neutral compound (Figure 9). The
quinono¨ıd character is stronger in the singlet diradical but still
much weaker than in 2g•+ (Figure 9).
quantities are 3.91 eV and 317 nm, showing again the weak
electron delocalization between the two naphthyl units.
Radical Cation Compound. The minimum energy confor-
mations of the electronic ground state are characterized by
dihedral angles of 55.6° and 136.3°, with the former being 1.4
kcal/mol higher in energy. Following this reduction of the
dihedral torsion angle by about 20°, the oxidation of 2g is
accompanied by an increase in quinono¨ıd character, which also
results in a shortening of the inter-ring CC bond by 0.031 Å
and of the CN bonds by 0.036 Å, as well as in the typical
reversal of the bond lengths in the two rings of the naphthyl
moieties that participate directly in the electron conjugation
(Figure 9). In the parent N,N,N′,N′-tetraphenylbenzidine, removal
of one electron leads also to a reduction of the torsion angle. It
amounts to 14.6° to reach a torsion angle of 19.3°. Although in
2g the MOs are localized on either side of the molecule, in
2g•+, they are delocalized over both units because of the increase
in planarity. This is illustrated in the case of the HOMO(R),
HOMO(â), and LUMO(â) (Figure 11a-c), which present the
same pattern as their parent HOMO and HOMO-1 of 2g and as
their parent HOMO of 1g. The spin density is distributed over
the whole system (Figure 12a), whereas within the Mulliken
population analysis scheme it presents maximum values of 0.14
|e| on the N atoms. The excess positive charge is also borne by
all atoms of the system. The decrease in planarity when going
from 2g•+ to 2g can thus be related to the addition of an electron
to the LUMO(â), which exhibits an antibonding inter-ring
interaction.
Dication Compound. In the second oxidation step, the
removed electron can originate either from the same MO as at
the first oxidation step or from another MO. In the former case,
a singlet closed-shell dication is formed, whereas the second
situation corresponds to creating a diradical dication. B3LYP/
6-31G* calculations predict that the singlet closed-shell system,
which is characterized by an increased quinono¨ıd character
[dCC ) 1.437 versus 1.495 Å and dCN ) 1.354 versus 1.409 Å
for the neutral species] and a torsion angle of 41.5°, is less stable
by about 4-5 kcal/mol than the diradical structures. This
reduction of the torsion angle is related to the removal of the
Starting from the radical cation, the triplet is obtained by
removing the electron from the â HOMO, which exhibits a
bonding interaction between the naphthyl moieties (Figure 11b).
This explains the increase in the torsion and the reduction in
(36) Yamaguchi, K.; Jensen, F.; Dorigo, A.; Houk, K. N. Chem. Phys.
Lett. 1988, 149, 537.
(37) See, for instance: Tomasi, J.; Cammi, R.; Mennucci, B.; Cappelli,
C.; Corni, S. Phys. Chem. Chem. Phys. 2002, 4, 5697 and references therein.
1360 J. Org. Chem., Vol. 71, No. 4, 2006

Synthesis and Characterization of Naphthidines
and the mixture was heated to reflux. A solution of t-BuOH (15
mmol) in 3 mL of dioxane was then added dropwise followed by
the amine (15 mmol), and the mixture was further stirred for 0.5
h. A solution of 1-chloronaphthalene (10 mmol) in 3 mL of dioxane
was then added, and the reaction was monitored by GC. After
complete consumption of the aryl chloride, the mixture was cooled
to room temperature and adsorbed onto silica gel. The crude reaction
mixture was purified by silica gel chromatography to furnish
the quinono¨ıd character. On the other hand, the structure of the
singlet diradical is essentially explained starting from the neutral
species where both the HOMO and HOMO-1 are doubled, one
of each spin type on each moiety of the naphthidine and where
the electrons on the R HOMO-1 and â HOMO-1 are removed
(Figure 10a). The spin density topology substantiates this
explanation because the larger amplitudes are now located on
the NH-Ph-OMe parts of the system (Figure 13c), whereas the
corresponding HOMOs and LUMOs present related topologies
(see Supporting Information).
Properties. When accounting for solvent effects within the
IEFPCM scheme,37 the first ionization energy, which corre-
sponds to the transition between the neutral and radical cation
compounds, amounts to 4.44 eV. The second ionization energy
to form the di(radical cation) is slightly larger and attains 4.93
eV. The small difference between the two ionization energies
explains why the CV exhibits a single, two-electron signal. More
details can be found in Supporting Information.
1
compound 1a as a pale yellow oil (85%). H NMR (400 MHz,
CDCl3) δ 7.76-7.74 (m, 2H), 7.67-7.65 (m, 1H), 7.53-7.49 (m,
1H), 7.44-7.38 (m, 2H), 6.90 (d, J ) 7,20 Hz, 1H), 3.30-3.28
(m, 4H), 1.94-1.92 (m, 4H); 13C NMR (100 MHz, CDCl3) δ 143.8,
130.2, 128.1, 127.1, 126.5, 125.6, 124.1, 121.2, 111.3, 52.5, 24.5.
MS m/z ) 197.
General Procedure 2. TiCl4-Mediated Oxidative Coupling of
Naphthylamines 1. The average yields for each reaction are shown
in Table 1. A typical procedure is given for entry 1.
1,1′-Binaphthyl-4,4′-bis-pyrrolidine (2a). A solution of 1-(1-
naphthyl)pyrrolidine 1a (5 mmol) in anhydrous CH2Cl2 (5 mL) was
chilled to -5 °C under nitrogen. TiCl4 (1.7 mL of 1:1 solution of
TiCl4/CH2Cl2, 7.7 mmol) was added dropwise for 5 min. The
reaction mixture was stirred at -5 °C for 1 h and stirred further at
0 °C for 8 h. A saturated K2CO3 solution (10 mL) was then added,
and the mixture was stirred for 0.5 h at 0 °C. The layers were
separated, and the aqueous layer was extracted with CH2Cl2 (2 ×
15 mL). The organic layers were combined, washed with brine
solution (5 mL), dried over MgSO4, and concentrated. The crude
reaction mixture was purified by silica gel chromatography to
Conclusions
In conclusion, naphthidines 2, easily available by a nickel-
catalyzed aryl amination reaction between 1-chloronaphthalene
and amines followed by TiCl4-mediated oxidative coupling,
represent a new class of easily oxidizable compounds. Cyclic
voltammetry, coulometric measurements, and EPR reveal that
naphtidines 2 exhibit good donor properties, showing a single
two-electron oxidation wave to form the di(radical cation)
species 22.2+ that display remarkable stability at room temper-
ature in solution and in the solid state. Such stability is an
important factor when considering potential applications of these
compounds, especially in OLEDs.
1
furnish 2a as a yellow solid (76%). Mp ) 177 °C. H NMR (400
MHz, CDCl3) δ 8.30 (d, J ) 8.3 Hz, 2H), 7.42-7.33 (m, 6H),
7.24-7.20 (m, 2H), 7.06 (d, J ) 7.6 Hz, 2H), 3.42 (m, 4H), 2.05
(s, 4H); 13C NMR (100 MHz, CDCl3) δ 147.3, 134.4, 132.1, 128.2,
128.1, 127.1, 125.4, 124.7, 124.1, 111.2, 52.8, 24.8. Anal. Calcd.
for C28H28N2: C, 85.67, H, 7.19, N, 7.14. Found: C, 85.59, H, 6.95,
N, 7.46.
The oxidation process has been theoretically studied by
optimizing the structures of the monocation and dication species
arising from naphthidine 2g. The important conformational
changes of 2g upon oxidation account for its electrochemical
properties. DFT theoretical calculations predict that the first
oxidation of compound 2g into 2g•+ is accompanied by an
important decrease of the torsion angle between the two naphthyl
moieties, which results in an increase of the quinoid character,
while the second oxidation into 2g2.2+ partially restores the large
torsion angle and the aromatic character. The coalescence of
the two oxidation peaks is therefore attributed to the structural
rearrangement that accompanies the redox process.
Acknowledgment. The authors would like to thank Pr.
Andre´ MERLIN (LERMAB, Universite´ Henri Poncare´, Nancy
1) for EPR spectra. B.C. thanks the Belgian National Fund for
Scientific Research for his Research Director position. The
calculations were performed on the Interuniversity Scientific
Computing Facility (ISCF), installed at the Faculte´s Universi-
taires Notre-Dame de la Paix (Namur, Belgium), for which the
authors gratefully acknowledge the financial support of the
FNRS-FRFC and the “Loterie Nationale” for the convention
no. 2.4578.02, and of the FUNDP.
Supporting Information Available: General experimental
information, experimental procedures, characterization data of
compounds 1a-h and 2a-h, chronoamperometry of naphthidines,
dependence of anodic-to-cathodic peak potentials on scan rate at
different concentrations of naphthidine 2g, voltammetric in situ
monitoring of the electrolysis of compound 2g, UV-visible spectra
of the stepwise chemical oxidation of compounds 2c-f and 2h,
and computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
Through magnetization experiments, we have finally shown
that the di(radical cations) 22.2+ have an energetic preference
for spin pairing yielding a singlet diradical ground state.
Experimental Section
General Procedure 1. Coupling of 1-Chloronaphthalene with
Secondary Cyclic Amines. The average yields for each reaction
are shown in Table 1. A typical procedure is given for entry 1.
1-(1-Naphthyl)pyrrolidine38 (1a). A 50 mL Schlenk tube was
loaded with degreased NaH (16 mmol), Ni(acac)2 (0.5 mmol,
5 mol %), SIPr‚HCl (0.5 mmol, 5 mol %), and 6 mL of dioxane,
JO0517444
(38) Srinivas, G.; Periasamy, M. Tetrahedron Lett. 2002, 43, 2785.
J. Org. Chem, Vol. 71, No. 4, 2006 1361