Long-chain substituted tetracyanoquinodimethanes: A convenient synthetic approach

Article abstract of DOI:10.1081/SCC-100103524

2-Alkylcyclohexane-1,4-diones, key precursors of TCNQ derivatives, have been synthesized by modified Birch reduction of 2,5-dimethoxybenzoic acid by Li and Et₂NH in an ultrasonic bath and subsequent alkylation.

Full text of DOI:10.1081/SCC-100103524

SYNTHETIC COMMUNICATIONS, 31(7), 971–974 (2001)
LONG-CHAIN SUBSTITUTED
TETRACYANOQUINODIMETHANES:
A CONVENIENT SYNTHETIC APPROACH
M. Utinans,1 V. Khodorkovsky,2 and O. Neilands1,
*
1Department of Organic Chemistry, Riga Technical
University, Riga, Latvia
2Department of Chemistry, Ben Gurion University of
Negev, 84105, Israel
ABSTRACT
2-Alkylcyclohexane-1,4-diones, key precursors of TCNQ
derivatives, have been synthesized by modified Birch reduc-
tion of 2,5-dimethoxybenzoic acid by Li and Et2N H in an
ultrasonic bath and subsequent alkylation.
Derivatives of tetracyanoquinodimethane (TCNQ) exhibit strong
electron-accepting properties and are widely employed as components for
charge transfer complexes and ion radical salts with metal-like conductivity.
Surface-active derivatives of TCNQ have been used for fabrication of
Langmuir-Blodget (LB) films with unusual photophysical behavior.1
The presence of a long alkyl chain in such compounds should increase the
solubility of their complexes with electron donors in nonpolar solvents, and
allows studies of electron transfer phenomena in solutions. However, to the
best of our knowledge, a systematic study of such complexes and LB films
has never been attempted, owing to the tedious and poorly reproducible
synthetic procedures published to date.
* Corresponding author.
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UTINANS, KHODORKOVSKY, AND NEILANDS
Indeed, the best yields of derivatives 5 have been obtained using
2-alkyl-cyclohexane-1,4-diones (4) as precursors. There are three synthetic
routes toward 2-alkylcyclohexane-1,4-diones (4) known: 1) Hydrogenation
of 2-alkylhydroquinones by hydrogen in the presence of Raney nickel as a
catalyst at high temperatures and at high pressures, giving rise to 2-alkyl-
1,-4-cyclohexanedioles, which can be further oxidized to corresponding
diones 4.2 The known multistep low-yield synthetic route to 2-alkylhydro-
quinones presents a severe limitation for the practical use of this method;
2) Birch reduction of poorly available 2-alkyl-1,4-dimethoxybenzenes
with sodium or lithium at low temperatures in liquid ammonia/THF
or Et2O mixture;2–5 and 3) Reduction of commercially available 2,5-
dimethoxybenzoic acid (1) by lithium in liquid ammonia with subsequent
alkylation.6,7
The last method is the most attractive; however, according to
our experience, it gives low yields in cases when long-chain alkylhalides
(practically insoluble in the reaction mixture) are used for alkylation
of salt 2.
We describe here a novel way for reduction of acid 1 in solvent system
HMPA–Et2NH–THF close to room temperature and alkylation, especially
with long-chain alkyl halides. The alkylation with hexadecylhalides already
1
has been described in our previous paper.8 The H NMR and IR spectra,
as well as melting points, in comparison with those described in the
literature, and satisfactory elemental analysis for unknown compounds
(4 and 5, R ¼ C13H27, C17H35), conform as to their structure and are of
sufficient purity (Tables 1, 2).

TETRACYANOQUINODIMETHANES
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Table 1. Characterization of Synthesized 2-Alkylcyclohexane-1,4-diones 4
4
Alkyl halide Yield M.p.
ꢀC
1H NMR, CDCl3
RX
%
ꢀ, ppm
C12H25I
C13H27I
C15H31Br
C16H33I
C17H35Br
C18H37I
45
46
43
41
43
36
702,6
66–67 0.86, t, 3H; 1.23, m, 24H; 2.43, m, 1H; 2.67, m, 6H.
0.86, t, 3H; 1.25, m, 22H; 2.45, m, 1H; 2.69, m, 6H.
782
0.87, t, 3H; 1.27, m, 28H; 2.41, m, 1H; 2.7, m, 6H.
80–8118 0.87, t, 3H; 1.26, m, 30H; 2.45, m, 1H; 2.68, m, 6H.
81–82 0.85, t, 3H; 1.23, m, 32H; 2.41, m, 1H; 2.69, m, 6H.
872–4 0.84, t, 3H; 1.24, m, 34H; 2.44, m, 1H; 2.67, m, 6H.
Table 2. Characterization of Alkyltetracyanoquinodimethanes 5
5
Yield
%
M.p.
ꢀC
1H NMR, CDCl3
R
ꢀ, ppm
C12H25
64
58
47
38
41
44
119–1202,6
119–120
1202
0.89, t, 3H; 1.29, s, 18H; 1.52, m, 2H; 3.0, t, 2H;
7.32, s, 1H; 7.54, d, 1H; 7.61, d, 1H.
0.87, t, 3H; 1.27, s, 20H; 1.52, m, 2H; 2.99, t, 2H;
7.32, s, 1H; 7.49, d, 1H; 7.59, d, 1H.
0.88, t, 3H; 1.28, s, 24H; 1.53, m, 2H; 2.97, t, 2H;
7.31, s, 1H; 7.54, d, 1H; 7.59, d, 1H.
0.89, t, 3H; 1.26, s, 26H; 1.51, m, 2H; 2.97, t, 2H;
7.32, s, 1H; 7.53, d, 1H; 7.62, d, 1H.
0.87, t, 3H; 1.28, s, 28H; 1.55, m, 2H; 3.01, t, 2H;
7.30, s, 1H; 7.48, d, 1H; 7.56, d, 1H.
0.87, t, 3H; 1.25, s, 30H; 1.53, m, 2H; 2.98, t, 2H;
7.34, s, 1H; 7.51, d, 1H; 7.57, d, 1H.
C13H27
C15H31
C16H33
C17H35
C18H37
120–1218
122–123
1242–4
EXPERIMENTAL
A General Procedure for Preparation of
2-Alkylcyclohexane-1,4-diones (4)
To a solution of 1.82 g (0.01 mol) 2,5-dimethoxybenzoic acid in 20 mL
THF and 12 mL HMPA (fresh-distilled solvents) under argon, 4 mL
diethylamine was added. After the formation of diethylammonium salt was
complete, 0.15 g (0.021 mol) lithium was added. The reduction was carried
out in an ultrasonic bath at 10ꢀ–15ꢀC and took about 20–30 min. A solution

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UTINANS, KHODORKOVSKY, AND NEILANDS
of alkyl halide (0.01 mol) in 15 mL THF was added dropwise until the red
color disappeared. After 1 h, pale yellow reaction mixture was diluted with
150 mL of water, acidified with hydrochloric acid to pHꢁ1, THF was distilled
off, and the mixture was refluxed for 2.5 h, maintaining pHꢁ1 (if necessary,
adjusted by hydrochloric acid). After cooling, the precipitated solid was col-
lected and recrystallized from ethanol. Resulting alkylcyclohexanediones 4
can be used for synthesis of TCNQ without additional purification.
A General Procedure for Preparation of
2-Alkyl Substituted TCNQ (5)
Cyclohexanedione 4 (0.002 mol), malonodinitrile (0.004 mol) and 0.05 g
ꢁ-alanine in a mixture of 5 mL of THF and 1 mL H2O were refluxed for 7 h.
After cooling to room temperature, 40 mL of water was added and the pro-
duct was extracted with 50 mL CHCl3. Extract was dried over MgSO4 and
evaporated in vacuo. The residue was dissolved in 15 mL of MeCNand 0.6 g
bromosuccinimide in 5 mL of MeCNwas added. Pyridine (0.24 mL) was
added to the reaction mixture after 30 min. After an additional 10 min,
H2O (50 mL) was added. The product was collected by filtration and dried
at room temperature. Purification of TCNQ derivatives can be carried out by
column chromatography using any appropriate solvent systems.
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Accepted May 31, 2000