Synthesis and reactivity of a chlorinated 1,8-bis(diarylmethylium) naphthalenediyl dication

Article abstract of DOI:10.1021/ol0477071

(Chemical Equation Presented) 1,8-Bis(bis(p-chlorophenyl)methylium) naphthalenediyl dication has been prepared by treatment of the corresponding diol with a mixture of [HBF₄]_aq and (CF ₃CO)₂O. The proximity of t

Full text of DOI:10.1021/ol0477071

ORGANIC
LETTERS
2005
Vol. 7, No. 2
283-285
Synthesis and Reactivity of a
Chlorinated
1,8-Bis(diarylmethylium)naphthalenediyl
Dication
Huadong Wang and Franc¸ois P. Gabba1ı*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843
francois@tamu.edu
Received November 8, 2004
ABSTRACT
1,8-Bis(bis(p-chlorophenyl)methylium)naphthalenediyl dication has been prepared by treatment of the corresponding diol with a mixture of
[HBF4]aq and (CF3CO)2O. The proximity of the methylium centers leads to strong electrostatic repulsions that are exacerbated by the electron-
withdrawing p-chloro substituents. As indicated by cyclic voltammetry, this dication is the strongest oxidant of the 1,8-bis(methylium)-
naphthalenediyl series. It undergoes a reductive chlorination with chloride and reacts with bromide or iodide to afford the corresponding
acenaphthenes.
In addition to the synthetic challenges associated with their
preparation and isolation, dications1 featuring two proximal
triarylmethylium moieties linked by a 2,2′-biphenylene2 or
a [1,1′-binaphthalene]-2,2′-diyl3 have attracted a great deal
of interest because of their unusual redox properties. They
undergo a two-electron reduction to afford the corresponding
acenaphthenes. Because reoxidation occurs at a much more
positive potential, such derivatives display redox bistability
and have often been considered as switches.2,3 In the oxidized
state, these derivatives feature an intercationic separation of
3.5-3.7 Å. Speculating that a shorter intercationic separation
could serve to increase the Coulombic repulsions and, there-
fore, electrophilic character of such compounds, we decided
to investigate the use of the naphthalenediyl backbone to
link two triarylmethylium moieties. As part of this effort,
we have described the synthesis of two dications of general
formula [1,8-(Ar2C)2C10H6]2+ with Ar ) p-MeOC6H4 (12+)
and C6H5 (22+).4,5 This series was complemented by Suzuki,
who reported a derivative in which the diarylmethylium
moieties have been replaced by the more stable methyl-
acridium cations.6 Structurally, these derivatives feature short
intercationic separations close to 3 Å. In the absence of
strongly electron-donating groups stabilizing the methylium
centers, such dications display unusual reactivities. For
example, 12+ reacts with hydride to afford the corresponding
acenaphthene derivative.5 A recent report by Ichikawa also
indicates that 22+ is a useful organic oxidant for the self-
coupling of N,N-dialkylanilines.7 In this communication, we
(1) (a) Prakash, G. K. S.; Rawdah, T. N.; Olah, G. A. Angew. Chem.,
Int. Ed. Engl. 1983, 22, 390. (b) Hart, H.; Sulzberg, T.; Rafos, R. R. J. Am.
Chem. Soc. 1963, 85, 1800.
(2) (a) Carey, K. A.; Clegg, W.; Elsegood, M. R. J.; Golding, B. T.;
Hill, M. N. S.; Maskill, H. J. Chem. Soc., Perkin Trans. 1 2002, 2673. (b)
Suzuki, T.; Nishida, J.; Tsuji, T. Chem. Commun. 1998, 2193. (c) Suzuki,
T.; Nishida, J.; Tsuji, T. Angew. Chem., Int. Ed. Engl. 1997, 36, 1329.
(3) (a) Higuchi, H.; Ohta, E.; Kawai, H.; Fujiwara, K.; Tsuji, T.; Suzuki,
T. J. Org. Chem. 2003, 68, 6605. (b) Suzuki, T.; Yamamoto, R.; Higuchi,
H.; Hirota, E.; Ohkita, M.; Tsuji, T. J. Chem. Soc., Perkin Trans. 2 2002,
1937. (c) Nishida, J.; Suzuki, T.; Ohkita, M.; Tsuji. T. Angew. Chem., Int.
Ed. 2001, 40, 3251.
(4) Wang, H.; Webster, C. E.; Perez, L. M.; Hall, M. B.; Gabba¨ı, F. P.
J. Am. Chem. Soc. 2004, 126, 8189.
(5) Wang, H.; Gabba¨ı, F. P. Angew. Chem., Int. Ed. 2004, 43, 184.
(6) Kawai, H.; Takeda, T.; Fujiwara, K.; Suzuki, T. Tetrahedron Lett.
2004, 45, 8289.
(7) Saitoh, T.; Yoshida, S.; Ichikawa, J. Org. Lett. 2004, 6, 4563.
10.1021/ol0477071 CCC: $30.25
© 2005 American Chemical Society
Published on Web 12/29/2004

would like to report the synthesis and unusual properties of
[1,8-(p-ClC6H4)2C)2C10H6]2+ (32+) which is the most oxidiz-
ing dication thus far prepared in the 1,8-bis(methylium)-
naphthalenediyl series.
The reaction of 1,8-dilithionaphthalene‚TMEDA8 with 2
molar equiv of 4,4-dichlorobenzophenone affords the cor-
responding diol (3(OH)2), [1,8-(p-ClC6H4)2(HO)C)2C10H6]
(Scheme 1). Upon treatment with a mixture of [HBF4]aq and
1-diarylmethylium-8-diarylmethylnaphthalenediyl intermedi-
ate ([2‚H]+ or [3‚H]+), which then undergoes rapid depro-
tonation.5 However, in light of a recent report concerning
the lack of reactivity of a 1-diarylmethylium-8-diarylmeth-
ylnaphthalenediyl with hydrides,10 an outer sphere electron-
transfer reaction can also be invoked to rationalize this
hydride-induced reduction. This reaction afforded isolable
quantities of 2 and 3 that have been fully characterized and
whose crystal structures have been determined (Figure 1).
Scheme 1. Synthesis of 1,8-Bis(methylium)naphthalenediyl
Dication 32+a
a Reagents and conditions: (a) (p-ClC6H4)2CO, THF, -78 °C;
(b) [HBF4]aq/(CF3CO)2O, 25 °C.
(CF3CO)2O, this diol undergoes a double dehydroxylation
reaction to afford the corresponding dication (32+) as the
BF4- salt (Scheme 1). The 1H NMR spectrum of 32+ features
the expected signals for a symmetrically peri-substituted
naphthalene derivative with the hydrogen atoms at the 2-
and 7-positions shifted downfield by 1.04 ppm with respect
to the diol 3(OH)2. The detection of four signals for the
p-chlorophenyl hydrogen atoms indicates restricted motion
of the aryl substituent.
Figure 1. Ortep view of 2 showing the long C01-C02 bond of
1.701(3) Å (50% ellipsoids). H-atoms omitted for clarity.
The bonds of 1.701(3) Å (2) and 1.633(3) Å (3) linking the
former methylium carbon centers are notably longer than
typical C(sp3)-C(sp3) bonds.6,11
To compare the redox properties of these derivatives, the
2/22+ and 3/32+ redox couples were studied by cyclic
voltammetry and compared to that previously reported for
1/12+ (Figure 2). The cyclic voltammogram of 2 and 3 shows
Unlike triarylmethylium cations, which add hydrides to
afford the neutral triaryl methane derivatives,9 treatment of
2
2+ and 32+ with LiHBEt3, KH, or (p-Me2NC6H4)3CH leads
to reductive coupling and formation of the corresponding
1,1,2,2-tetra(aryl)acenaphthenes as observed for 12+ (Scheme
2). Since no deuterium incorporation is observed when this
Scheme 2. Reduction Reactions Observed of 12+, 22+, and
32+a
Figure 2. Cyclic voltammogram of 2 and 3 in CH2Cl2 with a
Pt working electrode; scan rate 100 mV/sec, 0.1 M NBu4PF6 (vs
Fc/Fc+).
a Reagents and conditions: (a) LiHBEt3, MeCN, 25 °C; (b) for
Br- and I-, nBu4NX (X ) Br, I), MeCN, 25 °C.
a two-electron oxidation peak at 1.10 V for 2 and 1.11 V
for 3 (vs Fc/Fc+), which corresponds to the formation of
the dication 22+ and 32+, respectively. This oxidation does
not meet the electrochemical criteria of reversibility since
reaction is carried out with LiDBEt3, it is reasonable to
assume that this reaction proceeds through formation of a
(8) Neugebauer, W.; Clark, T.; von Rague´ Schleyer, P. Chem. Ber. 1983,
116, 3283.
(9) Cheng, T.-Y.; Bullock, R. M. J. Am. Chem. Soc., 1999, 121, 3150.
(10) Kawai, H.; Nagasu, T.; Takeda, T.; Fujiwara, K.; Tsuji, T.; Ohkita,
M.; Nishida, J.; Suzuki, T. Tetrahedron Lett. 2004, 45, 4553.
284
Org. Lett., Vol. 7, No. 2, 2005

reduction of the dication occurs at a much more cathodic
potential (Ered(22+) ) 0.20 V, Ered(32+) ) 0.35 V vs Fc/Fc+).
The reduction peak potentials measured for this series of
compounds (Ered(32+) ) 0.35 V > Ered(22+) ) 0.20 V >
Ered(12+) ) -0.17 V) falls in the order that can be expected
on the basis of the corresponding Hammett parameters for
Cl (σ+ ) 0.11), H (σ+ ) 0), and MeO (σ+ ) -0.78).12
Following the observation that compound 22+ reacts with
fluoride to afford a cation that features an unusual C-F f
C bridge, we decided to investigate the reactions of 12+, 22+,
and 32+ with heavier halides. Although 12+ fails to react with
chloride, 22+ and 32+ undergo a reductive chlorination
reaction that affords the acenaphthenes 2Cl and 3Cl (Scheme
3). Both of these compounds have been fully characterized,
naphthalene 4-position followed by deprotonation at the same
position. Presumably, the chloride anion is too large to enter
the narrow pocket available between the methylium centers.
A similar argument has been used to rationalize the anion
binding selectivity of 1,8-diborylnaphthalenes, which com-
plex fluoride but not chloride, bromide, or iodide.13 Remark-
ably, treatment of 22+ and 32+ with bromide leads to reductive
coupling and formation of the corresponding acenaphthenes
2 and 3 (Scheme 2). In the case of 12+, the reaction with
bromide is very sluggish and only affords low yields of 1.
Finally, 12+, 22+, and 32+ are spontaneously reduced in the
presence of iodide (Scheme 2). The coupling reactions
observed in the presence of bromide anion are especially
surprising and substantiate the unusual oxidant properties of
22+ and 32+.
The reactivity of 12+, 22+, and 32+ results from the unique
structural and electronic properties of the dications. The
proximity of the methylium centers leads to increased elec-
trostatic repulsive interactions and favors any reactions that
will allow for their removal. These effects are further
exacerbated by the presence of the electron-withdrawing
p-chloro substituents in 32+, which on the basis of its elec-
trochemistry is the strongest oxidant of the series. Attempts
to introduce more electron-withdrawing substituents at the
para-position of the phenyl rings have thus far been un-
successful.
Scheme 3. Reductive Chlorination Reactions Observed for
22+ and 32+ in the Presence of Chloridea
Acknowledgment. Support from the Robert A. Welch
Foundation (Grant A-1423) and the National Science Foun-
dation (CHE-0094264) is gratefully acknowledged.
a Reagents and conditions: (a) KCl/18-crown-6, MeCN, 25 °C.
Supporting Information Available: Experimental pro-
cedures, analytical data, and X-ray structure data in CIF
format. This material is available free of charge via the
Internet at http://pubs.acs.org.
and the crystal structure of 3Cl has been determined. This
reduction most likely proceeds by nucleophilic attack at the
(11) (a) Baldridge, K. K.; Kasahara, Y.; Ogawa, K.; Siegel, J. S.; Tanaka,
K.; Toda, F. J. Am. Chem. Soc. 1998, 120, 6167 and references therein. (b)
Bettinger, H. F.; von Rague´ Schleyer, P.; Schaefer, H. F. J. Chem. Soc.,
Chem. Commun. 1998, 769. (c) Toda, F.; Tanaka, K.; Watanabe, M.;
Tamura, K.; Miyahara, I.; Nakai, T.; Hirotsu, K. J. Org. Chem. 1999, 64,
3102. (d) Kahr, B.; Engen, D. V.; Mislow, K. J. Am. Chem. Soc. 1986,
108, 8305. (e) Suzuki, T.; Ono, K.; Kawai, H.; Tsuji, T. J. Chem. Soc.,
Perkin Trans. 2 2001, 1798.
OL0477071
(12) Arentt, E. M.; Flowers, R. A., II; Ludwig, R. T.; Meekhof, A. E.;
Walek, S. A. J. Phys. Org. Chem. 1997, 10, 499.
(13) (a) Katz, H. E. J. Org. Chem. 1985, 50, 5027. (b) Sole´, S.; Gabba¨ı
F. P. Chem. Commun. 2004, 1284.
Org. Lett., Vol. 7, No. 2, 2005
285