Monopyrrolo-Annelated Tetrathiafulvalenes

Full text of DOI:10.1021/jo962303a

J . Org. Chem. 1997, 62, 1903-1905
1903
Sch em e 1
Mon op yr r olo-An n ela ted
Tetr a th ia fu lva len es
Kyukwan Zong and Michael P. Cava*
Department of Chemistry, The University of Alabama,
Box 870336, Tuscaloosa, Alabama 35487-0336
Received December 10, 1996
In tr od u ction
The chemistry of tetrathiafulvalene (TTF) and its
derivatives has been intensely studied for more than two
decades because of the ability of these compounds to form
stable radical cation salts which have great potential as
cationic components of molecular conductors or even
superconductors.1
In a recent paper,2 we reported practical syntheses of
symmetrical bis[3,4-d]pyrrolo-annelated TTF derivatives
and their favorable properties as a new class of donor
molecules. These pyrrolo-annelated TTFs have appre-
ciably lower oxidation potentials as compared to other
annelated TTF derivatives and enjoy the great symmetry
advantage of not existing as cis and trans isomers. As a
further extension of this work, we now report the
synthesis of a number of monopyrrolo[3,4-d]-annelated
TTF derivatives by various procedures, as well as their
characterization and electrochemical properties.
Ta ble 1. P r ep a r a tion of Mixed P yr r olo[3,4-d ]-An n ela ted
TTF Der iva tives by Cr oss-Cou p lin g Rea ction sa
combination of
entry
two reagents
product (yield %)
1
2
3
4
5
6
1 + 4
1 + 5
2 + 4
2 + 5
3 + 4
3 + 5
6 (27)
7 (48)
8 (trace)
9 (38)
10 (trace)
11 (12)
a
Conditions: P(OEt)3, 111 °C, 12 h.
Resu lts a n d Discu ssion
is less reactive toward coupling than the other half unit.
However, other factors may well be involved which are
not evident at this time.
P h osp h ite-P r om oted Cr oss-Cou p lin gs. Phosphite
cross-couplings were first investigated because this method
is generally simple when yields are good and when
products are separable.3 Several half-unit precursors of
unsymmetrical TTF derivatives were prepared by known
procedures.1,4,5 The mixture of a trithiocarbonate and a
pyrrolo-annelated dithiocarbonate2 in neat triethyl phos-
phite was refluxed for 10 h, and the mixed TTF along
with the two symmetrical ones were separated by chro-
matography. Unlike in some other cases, the polarity of
the two precursors differed appreciably, and the resulting
mixture of coupled products was easily resolved. Com-
pounds 6, 7, 9, and 11 were synthesized in this way in
fair to good yields, but compounds 8 and 10 could be
detected only in trace amounts (Scheme 1, Table 1). In
all cases, the use of freshly distilled triethyl phosphite
is essential. Interestingly, the reverse combination of
half-unit precursors, that is, a dithiocarbonate for one
and the pyrrolo-annelated trithiocarbonate2 for the other,
failed to give mixed products, and only symmetrical TTF
derivatives were obtained. Also, neither thione-thione
nor carbonyl-carbonyl combinations provided mixed TTF
derivatives in significant yields. It was concluded from
the observations that the pyrrolo-annelated dithiole unit
Oth er Un sym m etr ica l Cou p lin g P r oced u r es. Sev-
eral unsymmetrical monopyrrolo-TTFs were also ob-
tained, making use of non-phosphite coupling methods.
Thus, the little used reaction of a phosphorus ylide with
an alkylselenodithiolium salt7 afforded a good alternative
synthesis of compound 6. S-Methylation of thione 4a 2
with methyl triflate gave salt 15, which was reacted with
sodium hydroselenide to give selone 16. Direct methy-
lation of crude 16 gave the Se-methylated triflate salt
17, which reacted with the known phosphonium salt 148
in the presence of triethylamine to give the mixed TTF
6 in quite satisfactory yield (43%) (Scheme 2).
A number of unsymmetrical TTFs have been prepared,
making use of trithioorthoformate anions as intermedi-
ates.9,10 We employed this methodology in an alternate
synthesis of TTF 7. Thus, the readily prepared trithio-
formate 13 was reacted sequentially with n-BuLi, thione
18, and MeI to give the hexathiooxalate 19 as an orange
oil. Thermolysis of 19 in refluxing trichloroethane af-
forded 7 in 48% yield (Scheme 3).
Monopyrrolo[3,4-d]-annelated TTF derivatives 9 and
11 were conveniently deprotected by treatment with 30%
sodium methoxide. Alkylation of 20 and 21 by reaction
with sodium hydride followed by methyl iodide afforded
the N-methylated derivatives 22 and 23 in excellent
yields. However, deprotection of 7 failed to give more
(1) (a) Schukat, G.; Richter, A. M.; Fangha¨nel, E. Sulfur Rep. 1987,
7, 177. (b) Garin, J . Adv. Heterocycl. Chem. 1995, 62, 249. (c) Williams,
J . M.; Ferraro, J . R.; Thorn, R. J .; Carlson, D. D.; Geiser, U.; Wang, H.
H.; Kini, A. M.; Whangbo, M.-H. Organic Superconductors (Including
Fullerenes); Synthesis, Structure, Properties, and Theory; Grimes, R.
N., Ed.; Prentice Hall: Englewood Cliffs, NJ , 1992.
(2) Zong, K. K.; Chen, W.; Cava, M. P.; Rogers, R. D. J . Org. Chem.
1996, 61, 8117.
(7) Sudmale, I. V.; Tormos, G. V.; Khodorkovsky, V. Y.; Edzina, A.
S.; Neilands, O. J .; Cava, M. P. J . Org. Chem. 1993, 58, 1355.
(8) Hansen, T. K.; Lakshmikantham, M. V.; Cava, M. P.; Metzger,
R. M.; Becher, J . J . Org. Chem. 1991, 56, 2720.
(9) Brown, C. A.; Miller, R. D.; Lindsay, C. M.; Smith, K. Tetrahedron
Lett. 1984, 25, 991.
(3) Kini, A. M.; Tytko, S. F.; Hunt, J . E.; Williams, J . M. Tetrahedron
Lett. 1987, 28, 4153.
(4) Svenstrup, N. Becher, J . Synthesis 1995, 215.
(5) Narita, M.; Pittman, J r., C. V. Synthesis 1976, 489.
(6) Gonnella, N. C.; Cava, M. P. J . Org. Chem. 1978, 43, 369.
(10) Lindsay, C. M.; Smith, K.; Brown, C. A.; Betterton-Cruz, K.
Tetrahedron Lett. 1984, 25, 995.
S0022-3263(96)02303-1 CCC: $14.00 © 1997 American Chemical Society

1904 J . Org. Chem., Vol. 62, No. 6, 1997
Notes
Sch em e 2
Sch em e 4
4a
Ta ble 2. Cyclic Volta m m etr y of Com p ou n d s vs SCE
(Sca n Sp eed 100 m V/s)a
1
2
compd
E1/2
E1/2
6
7
9
11
20
21
22
23
TTF
ET
0.391
0.535
0.423
0.532
0.263
0.385
0.255
0.382
0.390
0.510
0.855
0.980
0.929
0.929
0.766
0.822
0.769
0.877
0.765
0.921
Sch em e 3
a
Cyclic voltammetry was carried out in a 0.1 M solution of
Bu4NPF6 in dichloromethane at rt.
useful donor ET. Compounds 6, 21, and 23 are compa-
rable to TTF as donors, while 20 and 22 are even better
donors than TTF because of the methyl substituents.
The new unsymmetrical TTF derivatives show good
solubility in most common organic solvents. This prop-
erty, coupled with their attractive redox values, make
them attractive donors for the synthesis of new conduct-
ing charge-transfer salts. Work in this direction is being
initiated.
Exp er im en ta l Section
than trace amounts of 24 using either sodium methoxide
or thermolysis (Scheme 4).11 The nature of the decom-
position products formed in these attempts was not
further investigated, but we suspect that rupture of the
ethano bridge of 7 under the reaction conditions is
involved.
Cyclic Volta m m etr y. Electrochemical studies indi-
cated that all of the new monopyrrolo[3,4-d]-annelated
TTFs have excellent reversible redox properties and quite
low oxidation potentials (Table 2). As expected, the
highest oxidation potentials of the series are shown by
the BOC-protected compounds 7, 9, and 11, but even
these values are low enough to be close to that of the
Gen er a l Meth od for Cr oss-Cou p lin g Rea ction s. A mix-
ture of 1.0 equiv of the appropriate thione and 1.0 equiv of the
oxo analog in freshly distilled P(OEt)3 (6.0 equiv) was immersed
under N2 in a preheated oil bath (110-120 °C). The mixture
was stirred vigorously for 10 h, cooled to rt, and diluted with
twice the volume of methanol for precipitation of the products.
The reaction mixture was placed in a refrigerator overnight and
filtered. The collected solid was usually a mixture of three TTF
derivatives (two symmetric and one mixed), TLC of which
(CHCl3/hexane ) 1:1) usually showed three different spots, the
middle one of which usually represented the mixed TTF deriva-
tive. Purification by chromatography on silica using as eluent
chloroform/hexane ) 1:1 afforded the desired product as a yellow
to orange solid.
Com p ou n d 6: orange solid (27%); mp 195-198 °C dec; 1H
NMR (360 MHz, CDCl3) δ 7.50-7.40 (m, 3H), 7.25-7.1 (m, 2H),
3.29 (s, 4H), 1.96 (s, 6H); 13C NMR (90 MHz, CDCl3) δ 138.1,
(11) Rawal, V. H.; Cava, M. P. Tetrahedron Lett. 1985, 26, 6141.

Notes
J . Org. Chem., Vol. 62, No. 6, 1997 1905
129.3, 128.2, 122.8, 119.8, 117.9, 115.9, 113.5, 109.2, 30.2, 12.5.
Anal. Calcd for C18H15NS6: C, 49.39; H, 3.45; N, 3.20; S, 43.95.
Found: C, 49.61; H, 3.50; N, 2.93; S, 43.65.
Hexa th ioor th ooxa la te 19. To a solution of 13 (650 mg, 2.7
mmol) in dry THF (100 mL) was added n-BuLi (1.5 M, 1.8 mL,
2.7 mmol). After 30 min, a solution of 2,5-dimethyl-N-BOC-
pyrrolo[3,4-d]trithiocarbonate (815 mg, 2.7 mmol) in THF (100
mL) was added. After 1 h of stirring, excess MeI (1.0 mL) was
added. The reaction mixture was concentrated, and the crude
product was dissolved in dichloromethane (50 mL), washed with
water, and dried over sodium sulfate. Purification by chroma-
tography on silica using dichloromethane/hexane (1:1) as eluent
gave the product (820 mg, 55%) as a thick orange oil: 1H NMR
(360 MHz, CDCl3) δ 3.25 (s, 4H), 2.52 (s, 3H), 2.48 (s, 3H), 2.28
(s, 6H), 1.56 (s, 9H); 13C NMR (90 MHz, CDCl3) δ 149.4, 123.5,
120.7, 111.3, 83.8, 77.2, 30.0, 28.1, 18.7, 17.6, 16.3.
Com p ou n d 7: orange solid (48%); mp 195-197 °C dec; 1H
NMR (360 MHz, CDCl3) δ 3.29 (s, 4H), 2.32 (s, 6H), 1.58 (s, 9H);
13C NMR (90 MHz, CDCl3) δ 149.4, 121.3, 120.8, 113.4, 110.9,
84.0, 30.1, 28.1, 16.3; MS m/e 125 (55), 149 (57), 169 (100), 181
(14), 201 (12), 213 (50), 245 (10), 257 (12), 271 (14), 333 (50),
361 (73). Anal. Calcd for C17H19NO2S6: C, 44.22; H, 4.14; N,
3.03; S, 41.67. Found: C, 44.77; H, 4.31; N, 3.22; S, 41.97.
Com p ou n d 9: yellow cotton-like crystals (38%); mp 195-198
1
°C dec; H NMR (360 MHz, CDCl3) δ 2.31 (s, 6H), 1.94 (s, 6H),
1.57 (s, 9H); 13C NMR (90 MHz, CDCl3) δ 149.9, 130.0, 122.3,
122.2, 120.6, 115.7, 83.8, 28.1, 16.3, 13.7. Anal. Calcd for
Syn th esis of 7 via Clea va ge of Hexa th ioor th ooxa la te 19.
A solution of hexathioorthooxalate 19 (200 mg) in 1,1,2-trichlo-
roethane was refluxed for 10 h. After the solvent was removed,
the residue was purified by chromatography on silica using
dichloromethane/hexane (1:1) to provide the product (7, 80 mg,
48%) as an orange solid, identical to that obtained by phosphite
coupling.
C
17H21NO2S4: C, 51.09; H, 5.30; N, 3.50; S, 32.10. Found: C,
50.94; H, 5.34; N, 3.47; S, 32.19.
Com p ou n d 11: orange crystals (12%); mp 64-65 °C; 1H NMR
(360 MHz, CDCl3) δ 2.82 (t, J ) 7.3 Hz, 4H), 2.32 (s, 6H), 1.61
(m, 4H), 1.59 (s, 9H), 1.49-1.20 (m, 52H), 0.89 (t, J ) 7.0 Hz,
6H); 13C NMR (90 MHz, CDCl3) δ 149.4, 127.4, 121.6, 120.8,
116.6, 113.3, 84.0, 36.3, 31.9, 29.7, 29.6, 29.5, 29.4, 29.1, 28.5,
28.1, 22.7, 16.3, 14.1; MS m/e 111 (100), 126 (92), 159 (28), 196
(17), 224 (49), 250 (17), 283 (10), 314 (24), 629 (10), 784 (5), 885
(4). Anal. Calcd for C47H81NO2S6: C, 63.82; H, 9.23; N, 1.58;
S, 21.75. Found: C, 63.53; H, 9.22; N, 1.56; S, 22.00.
Gen er a l P r oced u r e for Dep r otection of th e BOC Gr ou p .
A solution of the BOC-TTF derivative (1.0 equiv) in dry THF
was degassed by a nitrogen stream for 20 min. To this solution
was added 30% NaOCH3 in methanol (2.5 equiv) at rt, and the
mixture was stirred for 7 h under nitrogen. Water was added,
and the resulting precipitate was collected by filtration, washed
with water and methanol, and purified by chromatography on
silica using chloroform/hexane (2:1) to afford the deprotected
TTF.
Tr ip h en ylp h osp h on iu m sa lt 14: white crystals (90% from
13); mp 195 °C dec (ref 8, 198 °C); all spectroscopic data
corresponded to those from ref 8.
Tr ifla te Sa lt 15. To a solution of thione 4a (6.0 g, 21.63
mmol) in dichloromethane (25 mL) was added methyl triflate
(3.55 g, 21.63 mmol) [CAUTION: methyl triflate is a powerful
alkylating agent; it should be handled with rubber gloves and
in the hood] via a syringe at rt. The reaction mixture was stirred
for 4 h, and about triple the volume of dry ether was added for
precipitation. The resulting red solid was collected by filtration,
dried, and used for the next reaction without further purifica-
tion: yield quantitative; 1H NMR (360 MHz, CDCl3) δ 7.60-
7.20 (m, 5H), 3.25 (s, 3H), 2.22 (s, 6H).
Com p ou n d 20: 88%, a light yellow solid; mp 228-230 °C dec;
1H NMR (360 MHz, CDCl3) δ 7.75 (s, 1H), 2.12 (s, 6H), 1.99 (s,
6H); 13C NMR (90 MHz, CDCl3) δ 122.3, 118.8, 114.1, 113.1,
105.2, 20.3, 12.6, 11.4. Anal. Calcd for C12H13NS4: C, 48.12;
H, 4.37; N, 4.67; S, 42.82. Found: C, 48.24; H, 4.36; N, 4.63; S,
42.72.
1
Com p ou n d 21: 85%, a dark orange solid; mp 83-85 °C; H
NMR (360 MHz, CDCl3) δ 7.55 (s, 1H), 2.82 (t, J ) 7.3 Hz, 4H),
2.16 (s, 6H), 1.70-1.10 (m, 56H), 0.88 (t, J ) 7.3 Hz, 6H); 13C
NMR (90 MHz, CDCl3) δ 127.4, 120.1, 117.2, 116.0, 111.4, 36.2,
31.9, 30.0-28.6 (11C), 28.5, 22.7, 14.1, 12.5. Anal. Calcd for
2,5-Dim et h yl-N-p h en ylp yr r olo[3,4-d ]d it h ioselon e (16).
To a suspension of 15 (8.0 g, 18.12 mmol) in dry EtOH (80 mL)
was added a sodium hydroselenide solution prepared from
selenium (2.86 g, 36.23 mmol) and sodium borohydride (1.37 g,
36.23 mmol) in dry EtOH (20 mL) at rt. Upon addition of the
hydroselenide, the deep red color faded immediately to pale
yellow. After 1 h of stirring, the reaction mixture was diluted
with water (150 mL) and extracted with CHCl3 (2 × 200 mL).
The combined organic layers were washed with water and brine
and dried over sodium sulfate. Purification by chromatography
on silica (CHCl3/hexane ) 1:1) afforded selone 16 (75%) as an
orange solid: mp 135-145 °C; 1H NMR (360 MHz, CDCl3) δ
7.60-7.40 (m, 3H), 7.30-7.20 (m, 2H), 2.03 (s, 6H). The NMR
spectrum showed that the product contained some starting
thione which could not be readily separated. The crude product
was used directly in the next step.
Tr ifla te Sa lt 17. To a solution of the crude selone 16 (5.0 g,
15.42 mmol) in dichloromethane (25 mL) was added methyl
triflate (2.53 g, 15.42 mmol) via a syringe at rt. The reaction
mixture was stirred for 4 h, and about a triple volume of dry
ether was added for precipitation. The resulting red solid was
collected by filtration and used directly for the next reaction after
being vacuum-dried: yield quantitative; 1H NMR (360 MHz,
CDCl3) δ 7.60-7.20 (m, 5H), 3.30 (s, 3H), 2.20 (s, 6H).
Cr oss-Cou p lin g of Tr ip h en ylp h osp h on iu m Sa lt 14 a n d
Meth ylselen otr ifla te Sa lt 17. To a solution of triphenylphos-
phonium salt 14 (517 mg, 1.06 mmol) and triflate salt 17 (640
mg, 1.06 mmol) in acetonitrile (5 mL) was added triethylamine
(1.5 mL) at rt. The deep-red color of the solution faded
immediately. After 1 h of stirring, the reaction mixture was
placed in a refrigerator for 2 h. The solid was filtered and
dissolved in dichloromethane (30 mL). The organic phase was
washed with water, dried over sodium sulfate, and concentrated.
Purification by chromatography on silica, using CHCl3/hexane
(1:1) as eluent, afforded the mixed TTF derivative 6 (200 mg,
43%) as an orange solid. This compound was identical to the
one obtained by phosphite cross-coupling by all spectroscopic
criteria.
C
42H73NS6: C, 64.31; H, 9.38; N, 1.78; S, 24.52. Found: C, 64.38;
H, 9.40; N, 1.70; S, 24.43.
Gen er a l P r oced u r e for N-Alk yla tion of Dep r otected
TTF Der iva tive. To a solution of deprotected TTF 20 or 21
(1.0 equiv) in dry DMF was added sodium hydride (2.0 equiv,
hexane-washed and dried) under nitrogen at rt. The reaction
mixture was stirred for 30 min, excess iodomethane (4.0 equiv)
was added, and the mixture was stirred for a further 2 h. Water
was introduced carefully, and the precipitate was purified by
chromatography on silica using chloroform/hexane (1:1).
Com p ou n d 22: 92%, a yellow solid; mp 213-215 °C; 1H NMR
(360 MHz, CDCl3) δ 3.34 (s, 3H), 2.11 (s, 6H), 1.99 (s, 6H); 13C
NMR (90 MHz, CDCl3) δ 128.0, 124.4, 122.3, 114.9, 109.4, 20.3,
12.0, 10.6. Anal. Calcd for C13H15NS4: C, 49.80; H, 4.86; N,
4.47; S, 40.91. Found: C, 49.84; H, 4.86; N, 4.42; S, 40.80.
Com p ou n d 23: 87%, dark orange solid; mp 65-67 °C; 1H
NMR (360 MHz, CDCl3) δ 3.47 (2, 3H), 2.80 (t, J ) 7.3 Hz, 4H),
2.13 (s, 6H), 1.70-1.10 (m, 56H), 0.88 (t, J ) 7.3 Hz, 6H); 13C
NMR (90 MHz, CDCl3) δ 127.4, 119.7, 118.9, 114.4, 110.8, 36.2,
31.9, 31.0, 29.7-29.0 (11C), 28.5, 22.7, 14.1, 12.0. Anal. Calcd
for C43H75NS6: C, 64.68; H, 9.46; N, 1.75; S, 24.09. Found: C,
64.63; H, 9.39; N, 1.80; S, 24.16.
1
Com p ou n d 24: <5%, red crystals, mp 210 °C dec; H NMR
(360 MHz, CDCl3) δ 7.40 (s, 1H), 3.28 (s, 4H), 2.15 (s, 6H); MS
m/e 105 (96), 112 (53), 126 (34), 149 (100), 169 (35), 361 (29);
HRMS calcd 360.9215, found 360.9214.
Ack n ow led gm en t. We thank the National Science
Foundation (Grants CHE-9224899 and CHE-9612350)
for support of this work.
J O962303A