7,7-Dicyano-8,8-bis[4-(N,N-dimethylamino)phenyl]-meta-xylylenes: The first stable zwitterionic metaxylylene derivatives

Article abstract of DOI:10.1246/cl.2003.322

Treatment of 3-lithiophenyldicyanomethide ions with Michler's ketone afforded the title compounds as stable crystals upon weak-acidic work-up. The molecular structure and spectroscopic data reveal the zwitterionic nature of the compounds.

Full text of DOI:10.1246/cl.2003.322

322
Chemistry Letters Vol.32, No.4 (2003)
7,7-Dicyano-8,8-bis[4-(N,N-dimethylamino)phenyl]-meta-xylylenes:
The First Stable Zwitterionic Metaxylylene Derivatives
Takeshi Kawase,ꢀ Tohru Iwata, and Masaji Odaꢀ
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
(Received December 26, 2002; CL-021103)
Treatment of 3-lithiophenyldicyanomethide ions with Mi-
acetate from the weak acidic solutions. Metallic green plates of 6a
chler’s ketone afforded the title compounds as stable crystals
upon weak-acidic work-up. The molecular structure and spectro-
scopic data reveal the zwitterionic nature of the compounds.
precipitated from the extract upon standing for overnight. The
crystal is poorly soluble in common organic solvents. On the other
hand, the tert-butyl derivative 6b is relatively soluble in organic
solvents and readily purified by column chromatography on silica
gel. The reaction of 3a with 4,40-dimethoxybenzophenone
afforded a corresponding triarylmethanol 75 in good yield upon
similar work-up. Dehydrative treatments of 7 under several
conditions have failed to give the corresponding zwitterionic
species so far.
Metaxylylene 1 is potentially regarded as a resonance hybrid
between a diradical form A and a zwitterionic form B. Although
the diradical A has been received much attention mainly from the
viewpoints of high-spin organic compounds,1 few studies on the
zwitter ion B have been performed so far.2 Recently we have
reported the highly polarized character of 7,7-dicyano-8,8-bis(4-
dimethylaminophenyl)-p-quinodimethane (2).3 The results sug-
gest that appropriate substituents on each exo-methylene carbon
would stabilize the zwitterionic structure to a great extent. Several
nearly zwitterionic quinoid systems have been synthesized so
far.4 In these systems, steric crowding plays an important role in
increasing the dipolarity. Here we report the synthesis, properties
and molecular structure of the first zwitterionic metaxylylene
derivative.
Scheme 1. (i) a) 2.2 eq. n-BuLi/THF, ꢁ78 ꢂC, 1 h, b) Michler’s
ketone, rt, 1 h, c) sat. NH4Cl aq, 64% for 6a, 63% for 6b.
Appearance of NMR signals reveals little contribution of the
diradical structure A in 6.5 The chemical shifts of the dicyano-
methylene carbon (Cb: d 29.42) and the central quarternary
carbon (Ca: d 183.10) of 6b in CDCl3 are comparable to those of
phenyldicyanomethyl anion 8 (d 27.80 in CD3OD) and Malachite
Green 9 (d 187.4 in DMSO-d6),7 respectively. The difference, Ád
(Ca-Cb) = 153.7 ppm, is 38.3 ppm larger than that of 2 (Ád =
115.4), and almost comparable to that of the 13Cshifts
dependence on the p charge (167 ppm/p electron),8 clearly
suggesting the zwitterionic nature of 6b. Although the signals of
quarternary carbons of 6a was not observed because of the low
solubility, the similar IR frequencies of nitrile stretching as those
of 6b (n/cmꢁ1: 2125, 2168 for 6a and 2128, 2170 for 6b) also
indicate the zwitterionic nature of 6a.
Careful crystallization of 6a from an extract successfully
afforded good single crystals.9 The crystals contained both ethyl
acetate and THF as inclusion (6a : ethyl acetate : THF = 1 : 0.75 :
0.25 by NMR). Recrystallization of 6a in pure ethyl acetate or
THF have failed to afford good single crystals so far. Figure 1
shows molecular structures of 6a. Little pyramidization of both
Ca and Cb carbons and the low twisted angle between
dicyanomethyl group and A ring (>2ꢂ) indicate that 6a has a
fully conjugated system on the metaxylylene structure. The
molecule has a propeller structure similar to triphenylmethyl
cation 10 as expected. The average of twisted angles between aryl
groups (57.0ꢂ) is smaller than that of 10 (64.8ꢂ).10 The zwitterion
forms a dimeric structure in crystal: the dicyanomethylphenyl
group A is faced to the dimethylaminophenyl group B of an
adjacent molecule. The included solvent molecules are placed in
Treatment of 4-lithiophenyldicyanomethide lithium with
diarylketones effectively afforded the highly polarized para-
xylylenes.3 The results suggest that 3-lithiophenyldicyanome-
thide lithiums 3 would be promising synthetic tools for 1B. 3-
Bromophenyl- and 3-bromo-5-tert-butylphenylmalononitrile
(4a) and (4b)5 were prepared from m-dibromobenzene or 1,3-
dibromo-5-tert-butylbenzene and malononitrile by the Takaha-
shi’s method6 in 67% and 80% yields, respectively. Treatment of
4a with 2.4 equiv. of n-BuLi at ꢁ78 ꢂC for 1 h cleanly generated
the corresponding dianion 3a. The generation was confirmed by
quenching with methyliodide to give a dimethylated compound
55 in 88% yield.
Quenching of 3a and 3b with Michler’s ketone followed by
addition of aqueous ammonium chloride gave zwitter ions 6a and
6b as green solutions. The compounds are extractable with ethyl
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.4 (2003)
323
Scheme 2.
Research (No. 10146102 and 14340197) from Ministry of
Education, Science, Sports and Culture, Japan.
Figure 1. Left; ORTEP drawing of 6a: right; side view.
Included solvent molecules removed for clarity. Dihedral angle
between planes (ꢂ). plane A–plane B = 58.75, plane A–plane C
= 60.54, plane B–plane C= 51.75.
References and Notes
1
M. Baumgarten and K. Mullen, Top. Curr. Chem., 169, 1 (1994); A.
¨
Rajica, Chem. Rev., 94, 871 (1994); A. Rajca, Chem. —Eur. J., 8,
4835 (2002).
the interspace of 6a. The disorder is responsible for the low
accuracy of the analysis.
2
3
R. Gompper and H.-U. Wagner, Angew. Chem., Int. Ed., 27, 690
(1988).
T. Kawase, M. Wakabayashi, C. Takahashi, and M. Oda, Chem.
Lett., 1997, 1055; T. Kawase, M. Wakabayashi, and M. Oda, Chem.
Lett., 1997, 1057.
The absorption spectrum of 6a similar to that of 9 shows
small solvent effects (ꢀmax ¼ 607 nm/CH3OH; 608 nm/CH2Cl2).
In contrary to the quinodimethane 2, 6a exhibits no charge-
transfer band.11 The absorption change of 6a at various pH
(Figure 2) indicates that pKa of 11, the conjugate acid of 6a, is
estimated tobe 3.4 unit. Thus, 11 is thestronger acid than 8(pKa =
5.3) by 1.9 pK units. Whereas the color of 9 disappeared rapidly in
strong basic media (EtOH-NaOH aq. pH = 13.22), the long
wavelength absorption of 6a slowly decreased under the condi-
tions. After standing for a day, the solution of 6a discolored
completely. Regeneration of 6a by neutralizing the solution
ended in failure, which indicated that irreversible decomposition
4
5
K. Takahashi, J. Syn. Org. Chem. Jpn., 44, 806 (1986); A. Z.-Q.
Kahn and J. Sandstrom, J. Am. Chem. Soc., 110, 4843 (1988); S.
¨
Inoue, Y. Aso, and T. Otsubo, Chem. Commun., 1997, 1105.
4a: colorless needles (Bz/n-Hex); mp 99–101 ꢂC; 1H NMR (CDCl3,
270 MHz) d ppm = 5.04 (s, 1H), 7.39 (t, J ¼ 7:9 Hz, 1H), 7.47 (d,
J ¼ 7:9 Hz, 1H), 7.64 (d, J ¼ 7:9 Hz, 1H), 7.67 (s, 1H); 13CNMR
(CDCl3, 67.8 MHz) d ppm = 27.46, 111.29, 123.76, 125.81,
128.03, 130.20, 131.38, 133.57; IR (KBr) n/cmꢁ1 2260 m (CꢃN).
4b: colorless needles (Bz/n-Hex); mp 72–74 ꢂC; 1H NMR (CDCl3,
270 MHz) d ppm = 1.33 (s, 9H), 5.02 (s, 1H), 7.41 (t, J ¼ 1:7 Hz,
1H), 7.48 (t, J ¼ 1:7 Hz, 1H), 7.63 (d, J ¼ 1:7 Hz, 1H); 13CNMR
(CDCl3, 67.8 MHz) d ppm = 27.75, 31.00, 35.30, 111.38, 123.06,
123.79, 127.37, 127.64, 131.00, 155.80; IR (KBr) n/cmꢁ1 2251 m
(CꢃN). 5: colorless oil; 1H NMR (CDCl3, 270 MHz) d ppm = 2.10
(s, 3H), 2.42 (s, 3H), 7.25–7.28 (m, 1H), 7.36–7.38 (m, 3H), IR
(KBr) n/cmꢁ1 2250 m (CꢃN). 6a: green plates; mp >220 ꢂC; Mass
(FAB): m=z ¼ 392 (M)þ; 1H NMR (CD3OD, 270 MHz) d ppm =
3.27 (s, 12H), 6.59 (d, J ¼ 6:6 Hz, 1H), 6.82 (s, 1H), 7.00 (d,
J ¼ 9:2 Hz, 4H), 7.17–7.28 (m, 2H), 7.44 (d, J ¼ 9:2 Hz, 4H). 6b:
green plates; m.p. 204–206 ꢂC; Mass (FAB): m=z ¼ 449 (M+H)þ;
1H NMR (CDCl3, 600 MHz) d ppm = 1.25 (s, 9H), 3.28 (s, 6H),
6.47 (t, J ¼ 1:8 Hz, 1H), 6.78 (d, J ¼ 9:3 Hz, 4H), 6.79 (t,
J ¼ 1:8 Hz, 1H), 7.39 (d, J ¼ 9:3 Hz, 4H), 7.45 (t, J ¼ 1:8 Hz,
1H); 13C NMR (CDCl3, 150 MHz) d ppm = 29.42, 31.27, 34.74,
40.71, 112.76, 122.72, 123.67, 123.93, 126.92, 127.59, 139.26,
141.13, 142.74, 150.77, 156.70, 183.10. 7: colorless needles (Bz/n-
Hex); mp 96–98 ꢂC; 1H NMR (CDCl3, 400 MHz) d ppm = 3.77 (s,
6H), 4.99 (s, 1H), 6.82 (AA’BB’, J ¼ 2:6, 8.9 Hz, 4H), 7.12
(AA0BB0, J ¼ 2:6, 8.9 Hz, 4H), 7.33–7.37 (m, 1H), 7.39–7.41 (m,
2H), 7.53 (s, 1H); IR (KBr) n/cmꢁ1 2255 w (CꢃN), 3489 br (OH).
M. Uno, K. Sato, M. Masuda, W. Ueda, and S. Takahashi,
Tetrahedron Lett., 26, 1553 (1985).
þ
occurred in the basic media (Scheme 2). The accurate pKR value
was not determinable; however, the cationic character of 6a is
apparently higher than that of 9. The presence of both charges in
the molecule would play an important role in increasing the
stability.
6
7
8
9
C. Avendano, C. de Diego, and J. Elguero, Magn. Reson. Chem., 28,
1011 (1990).
D. H. O’Brein, A. J. Hart, and C. R. Russel, J. Am. Chem. Soc., 97,
4410 (1975).
Figure 2. Change of electronic spectra of 6a at various pH in
EtOH–buffer solutions (CH3COOH-CH3COONa).
Crystal data for 6aÁ (CH3COOC2H5)0:75(THF)0:25 (The composi-
tion was determined by NMR.): monoclinic; space group P21=c
In conclusion, we report the first synthesis of zwitterionic
metaxylylene derivatives 6 as stable crystalline materials. With
keeping their planarity, 6 retain the individual electronic proper-
ties of each component probably owing to the novel metaxylylene
conjugation.
ꢀ
ꢀ
ꢀ
(#14), a ¼ 10:147ð1Þ A, b ¼ 22:508ð3Þ A, c ¼ 10:837ð1Þ A,
ꢁ ¼ 99:723ð4Þꢂ, V ¼ 2439:6ð5Þ3, Z ¼ 4, Dcalcd ¼ 1:069 gÁcm3,
R1 ¼ 9:0%. The data were deposited in Cambridge Crystallo-
graphic Data Center (CCDC 202240).
10 A. H. G. de Mesquita, C. H. Macgillavry, and K. Eriks, Acta
Crystallogr., 18, 437 (1965).
11 W. Rettig, Angew. Chem., Int. Ed., 25, 971 (1986).
This work was supported by Grant-in-Aids for Scientific