α-(Aryithio)imidoyl Radicals: [3 + 2] Radical Annulation of Aryl Isothiocyanates with 2-Cyano-Substituted Aryl Radicals

Article abstract of DOI:10.1021/jo971128a

A novel cascade radical reaction is described involving aryl isothiocyanates and 2-cyanoaryl radicals. The mechanism entails the formation of an α-(arylthio)imidoyl radical, a 5-exo-dig cyclization onto a cyano group, and a final 6-membered ring closure o

Full text of DOI:10.1021/jo971128a

8394
J . Org. Chem. 1997, 62, 8394-8399
r-(Ar ylth io)im id oyl Ra d ica ls: [3 + 2] Ra d ica l An n u la tion of Ar yl
Isoth iocya n a tes w ith 2-Cya n o-Su bstitu ted Ar yl Ra d ica ls
Rino Leardini, Daniele Nanni, Patrizia Pareschi, Antonio Tundo, and Giuseppe Zanardi*
Dipartimento di Chimica Organica “A. Mangini”, Universita` di Bologna, viale Risorgimento 4,
I-40136 Bologna, Italy
Received J une 20, 1997X
A novel cascade radical reaction is described involving aryl isothiocyanates and 2-cyanoaryl radicals.
The mechanism entails the formation of an R-(arylthio)imidoyl radical, a 5-exo-dig cyclization onto
a cyano group, and a final 6-membered ring closure of an iminyl radical. The competitive
5-membered spiro-cyclization of the iminyl, leading to an isomeric product, was only observed in
the case of a disubstituted aryl isothiocyanate. The whole process involves a rare example of [3 +
2] radical annulation and allows the one-pot synthesis of tetracondensed nitrogen heterocycles in
good yields.
In tr od u ction
for cyclizations, annulations, and cascade reactions lead-
ing to heterocyclic compounds such as phenanthridine,3e
quinoline,2n,p,s,u,3d,f-g,4b benzotriazine,3h indole,2r pyrroline,2q
and quinoxaline2t,u derivatives.
Although radical chemistry was pioneered at the
beginning of this century, it has not received considerable
attention until the past decades, when free-radical reac-
tions were recognized as powerful tools in organic syn-
thesis. Over the past 20 years free radical methodology
has grown more popular and now the literature displays
an extraordinary number of multistep syntheses involv-
ing radicals as key intermediates.1 In recent years, our
group has drawn special attention to imidoyl radicals.
These species were first studied in the late 1960s and
have been generated by addition of alkyl, alkoxy, sulfa-
nyl, silyl, or stannyl radicals to isonitriles,2 hydrogen
abstraction from imines,3 and homolytic cleavage of
suitable imidoylic precursors.4 The reactivity of imidoyl
radicals has been widely explored through fragmentation
A further way to generate imidoyl radicals, particularly
R-thio-substituted imidoyl radicals, has involved the
addition of carbon,5 tin, or silicon6 radicals to the sulfur
atom of isothiocyanates. In principle, this procedure
could prove very useful in the synthesis of heterocyclic
compounds containing both a nitrogen and a sulfur atom
from readily accessible starting materials. Although it
has been known for a few decades, this strategy has found
little application in organic synthesis. It is worth men-
tioning two examples, both employing stannyl or silyl
radicals: the conversion of glycosyl isothiocyanates to
glycosyl isonitriles and/or alditols6f and the synthesis of
thiolactames6g or thiopyroglutammates6h by tributyltin
hydride-mediated cyclization of alkenyl isothiocyanates.
The latter is the only example reported in the literature
concerning the synthesis of heterocyclic compounds by
radical addition to isothiocyanates. Surprisingly, the
addition of carbon-centered radicals is even more uncom-
mon, the only instance being the isomerization of the
reactions,2a-e,g,j-l,3a,j
intra-2n,p-s,u,4a,b
and
inter-
molecular2m,3d,f-h,4b additions to unsaturated bonds, and
cyclizations onto aromatic rings,3e sulfur atoms,3i or cyano
groups.2t,u Their synthetic potential has been exploited
X Abstract published in Advance ACS Abstracts, November 1, 1997.
(1) For a general view on free radicals in organic synthesis, see:
Giese, B. Radicals in Organic Synthesis: Formation of Carbon-Carbon
Bonds; Pergamon: Oxford, 1986. Curran, D. P. Synthesis 1988, 417
and 489. J asperse, C. P.; Curran, D. P.; Fevig, T. L. Chem. Rev.
(Washington, D.C.) 1991, 91, 1237. Motherwell, W. B.; Crich, D. Free
Radical Chain Reactions in Organic Synthesis; Academic: London,
1992. Perkins, M. J . Radical Chemistry, Ellis Horwood: London, 1994.
Bunce, R. A. Tetrahedron 1995, 51, 13103. Malacria, M. Chem. Rev.
(Washington, D.C.) 1996, 96, 289. Curran, D. P.; Porter, N. A.; Giese,
B. Stereochemistry of Radical Reactions; VCH: Weinheim, 1996.
(2) (a) Shaw, D. H.; Pritchard, H. O. Can. J . Chem. 1967, 45, 2749.
(b) Saegusa, T.; Kobayashi, S.; Ito, Y.; Yasuda, N. J . Am. Chem. Soc.
1968, 90, 4182. (c) Saegusa, T.; Kobayashi, S.; Ito, Y. J . Org. Chem.
1970, 35, 2118. (d) Banks, R. E.; Haszeldine, R. N.; Stephens, C. W.
Tetrahedron Lett. 1972, 3699. (e) Singer, L. A.; Kim, S. S. Tetrahedron
Lett. 1974, 861. (f) Blum, P. M.; Roberts, B. P. J . Chem. Soc., Chem.
Commun. 1976, 535. (g) Kim, S. S. Tetrahedron Lett. 1977, 2741. (h)
Barton, D. H. R.; Bringman, G.; Motherwell, W. B. J . Chem. Soc.,
Perkin Trans. 1 1980, 2665. (i) Blum, P. M.; Roberts, B. P. J . Chem.
Soc., Perkin Trans. 2 1983, 209. (j) Meier, M.; Ru¨chardt, C. Tetrahedron
Lett. 1983, 24, 4671. (k) Stork, G.; Sher, P. M. J . Am. Chem. Soc. 1983,
105, 6765. (l) Wirth, T.; Ru¨chardt, C. Chimia 1988, 42, 230. (m) Barton,
D. H. R.; Ozbalik, N.; Vacher, B. Tetrahedron 1988, 44, 3501. (n)
Curran, D. P.; Liu, H. J . Am. Chem. Soc. 1991, 113, 2127. (o) Diart,
V.; Roberts, B. P. J . Chem. Soc., Perkin Trans. 2 1992, 1761. (p) Curran,
D. P.; Liu, H. J . Am. Chem. Soc. 1992, 114, 5863. (q) Bachi, M. D.;
Balanov, A.; Bar-Ner, N. J . Org. Chem. 1994, 59, 7752. (r) Fukuyama,
T.; Xiaoqi, C.; Peng, G. J . Am. Chem. Soc. 1994, 116, 3127. (s) Curran,
D. P.; J osien, H.; Ko, S.-B. Angew. Chem., Int. Ed. Engl. 1995, 34, 2683.
(t) Nanni, D.; Pareschi, P.; Rizzoli, C.; Sgarabotto, P.; Tundo, A.
Tetrahedron 1995, 51, 9045. (u) Curran, D. P.; Liu, H.; J osien, H.; Ko,
S.-B. Tetrahedron 1996, 52, 11385.
(3) (a) Ohta, H.; Tokumaru, K. J . Chem. Soc., Chem. Commun. 1970,
1601. (b) Danen, W. C.; West, C. T. J . Am. Chem. Soc. 1973, 95, 6872.
(c) Davies, A. G.; Nedelec, J .-Y.; Sutcliffe, R. J . Chem. Soc., Perkin
Trans. 2 1983, 209. (d) Leardini, R.; Pedulli, G. F.; Tundo, A.; Zanardi,
G. J . Chem. Soc., Chem. Commun. 1984, 1320. (e) Leardini, R.; Tundo,
A.; Zanardi, G.; Pedulli, G. F. Synthesis 1985, 107. (f) Leardini, R.;
Nanni, D.; Pedulli, G. F.; Tundo, A.; Zanardi, G. J . Chem. Soc., Perkin
Trans. 1 1986, 1591. (g) Leardini, R.; Nanni, D.; Tundo, A.; Zanardi,
G. Gazz. Chim. Ital. 1989, 119, 637. (h) Leardini, R.; Nanni, D.; Tundo,
A.; Zanardi, G. J . Chem. Soc., Chem. Commun. 1989, 757. (i) Leardini,
R.; Nanni, D.; Santori, M.; Zanardi, G. Tetrahedron 1992, 48, 3961. (j)
Guidotti, S.; Leardini, R.; Nanni, D.; Pareschi, P.; Zanardi, G.
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D. Tetrahedron 1995, 51, 12143. (l) Nanni, D.; Pareschi, P.; Tundo, A.
Tetrahedron Lett. 1996, 37, 9337.
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K. Bull. Chem. Soc. J pn. 1995, 68, 1497.
(5) Barton, D. H. R.; J aszberenyi, J . Cs.; Theodorakis, E. A.
Tetrahedron 1992, 48, 2613.
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Noltes, J . G.; J anssen, M. J . J . Organomet. Chem. 1964, 1, 365. (c)
J ohn, D. I.; Tyrrel, N. D. J . Chem. Soc., Chem. Commun. 1979, 345.
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Hay Motherwell, R. S.; Porter, A. E. A. J . Chem. Soc., Perkin Trans. 1
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ington, D.C.) 1989, 89, 1413 and references therein.
S0022-3263(97)01128-6 CCC: $14.00 © 1997 American Chemical Society

R-(Arylthio)imidoyl Radicals
Sch em e 1
J . Org. Chem., Vol. 62, No. 24, 1997 8395
Sch em e 2
Ta ble 2. Yield s of Ben zoth ien oqu in oxa lin es 12c,i,k ,l
Obta in ed in th e Rea ction of Isoth iocya n a tes 6, 9, a n d 10
w ith Tetr a flu or obor a tes 1 a n d 2 (Cr ow n -Eth er Meth od )
entry
X
Y
Z
12 (%)
c
i
k
l
Me
Br
O2N
O2N
H
H
H
H
H
H
H
Cl
56
77
57
69
Ta ble 1. Yield s of Ben zoth ien oqu in oxa lin es 12a -n
Obta in ed in th e Rea ction of Isoth iocya n a tes 5-11 w ith
Tetr a flu or obor a tes 1 a n d 2
acetate and [18-crown-6].8 Reactions c, i, k , and l were
carried out again under these conditions. The isothio-
cyanate (15 mmol) and the diazonium salt (5 mmol) were
dissolved in ethyl acetate (20 mL) together with potas-
sium acetate (10 mmol) and the crown ether (0.25 mmol).
After 4-5 h under stirring at room temperature, the
benzothienoquinoxalines 12c,i,k ,l were obtained in the
yields reported in Table 2. As one can see, the modified
procedure gave results comparable to (entries c and k )
or significantly better (entries i and l) than the ones
achieved by the pyridine method.
The reaction mechanism can be outlined as shown in
Scheme 2. The aryl radical (3, 4) attacks the sulfur atom
of the isothiocyanate (5-11) giving the imidoyl radical
13. Subsequent tandem 5-exo-dig cyclization of 13 onto
the carbon atom of the cyano group and 6-membered ring
closure of iminyl 14 onto the isothiocyanate ring afford
the benzothienoquinoxaline 12.
The intramolecular addition of a carbon radical to a
cyano group is well-documented9,2t-u and, in the case of
13, the 5-exo cyclization should be even more favored by
the nucleophilic character of imidoyl radicals.3g In
principle, we cannot rule out the possibility that imidoyl
13 may undergo a homolytic R-fragmentation leading to
an aryl isocyanide and involving loss of a stabilized
arylsulfanyl radical. Indeed, this process has been
observed with R-(tributyltin)thio-6d,f,g and R-(triphenyl-
methyl)imidoyl3l radicals. In our case, the facile 5-exo
cyclization seems to prevent the fragmentation of 13, so
that isonitriles and disulfides were detected in few cases
and only in trace amounts.
entry
X
Y
Z
12 (%)
a
b
c
d
e
f
g
h
i
j
k
l
m
n
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CN
CN
H
Cl
H
Cl
H
Cl
H
Cl
H
Cl
H
Cl
H
Cl
70
70
57
70
80
70
70
67
57
65
53
34
65
75
Me
Me
MeO
MeO
Cl
Cl
Br
Br
O2N
O2N
H
H
isothiocyanate moiety to thiocyanate group in the addi-
tion of alkyl radicals to sulfonyl isothiocyanates.5
Here we report a new cascade radical reaction of some
aryl isothiocyanates with 2-cyanoaryl radicals. The
whole sequence involves a [3 + 2] radical annulation, via
an intermediate R-(arylthio)imidoyl radical, followed by
cyclization of an iminyl radical. It allows the one-pot
synthesis of the polycondensed framework of substituted
benzothienoquinoxalines starting from commercially avail-
able or easily accessible compounds.
Resu lts a n d Discu ssion
The aryl radicals 3 and 4 can be easily generated in
pyridine at low temperature from the corresponding
diazonium tetrafluoroborates 1 and 2.7 In a typical
experiment, the aryl isothiocyanate (5-11, 15 mmol) was
dissolved in pyridine; the tetrafluoroborate (5 mmol) was
then added portionwise under stirring at -10 to -20 °C.
After evaporation of the solvent, the residue was crystal-
lized or chromatographed to give the benzothienoqui-
noxalines 12 in generally good yields (Scheme 1 and
Table 1).
The reaction was sometimes affected by the low
solubility of the starting aryl isothiocyanates in pyridine
at low temperature. To overcome this problem, the aryl
radicals were generated by an alternative method. The
tetrafluoroborates 1 and 2 were decomposed in ethyl
acetate at room temperature in the presence of potassium
The homolytic aromatic substitution by the iminyl
radical is well-precedented as well.10 Under FVP (flash
vacuum pyrolysis) conditions, iminyls are known to give
rise to both 5- and 6-membered cyclizations onto aromatic
(8) Ru¨chardt, C.; Freudenberg, B. Tetrahedron Lett. 1964, 3623. See
also: Rosenberg, D. E.; Beadle, J . R.; Korzeniowski, S. H.; Gokel, G.
W. Tetrahedron Lett. 1980, 21, 4141.
(9) Clive, D. L. J .; Beaulieu, P. L.; Set, L. J . Org. Chem. 1984, 49,
1313. Chenera, B.; Chuang, C. P.; Hart, D. J .; Hsu, L. Y. J . Org. Chem.
1985, 50, 5409. Tsang, R.; Fraser-Reid, B. J . Am. Chem. Soc. 1986,
108, 2116. Beckwith, A. L. J .; O’Shea, D. M.; Sendaba, G.; Westwood,
S. W. J . Chem. Soc., Chem. Commun. 1987, 666. Yeung, B. W.;
Contelles, J . L. M.; Fraser-Reid, B. J . Chem. Soc., Chem. Commun.
1989, 1160. Dickson, J . K., J r.; Tsang, R.; Llera, J . M.; Fraser-Reid,
B. J . Org. Chem. 1989, 54, 5350. Knapp, S.; Gibson, F. S.; Choe, Y. H.
Tetrahedron Lett. 1990, 31, 5397. Snider, B. B.; Buckman, B. O. J .
Org. Chem. 1992, 57, 322. Yang, C.-C.; Chang, H.-T.; Fang, J .-M. J .
Org. Chem. 1993, 58, 3100.
(7) Sakakura, T.; Hara, M.; Tanaka, M. J . Chem. Soc., Chem.
Commun. 1985, 1545.

8396 J . Org. Chem., Vol. 62, No. 24, 1997
Leardini et al.
Sch em e 3
Sch em e 5
Sch em e 4
The substituents on the aromatic ring of the isothio-
cyanate clearly play an important role in the reaction
mechanism. Both experimental and computational stud-
ies are underway to get some more information about the
1,5-spirocyclization of such iminyl radicals as 14.12
rings,10d-g affording isomeric products through rear-
rangement of a spirocyclohexadienyl radical. In our
reaction this would result in the pathway shown in
Scheme 3.
Finally, it is worth pointing out that reactions m and
n exclusively afforded products derived from 5-membered
cyclization of the imidoyl radical on the cyano group of
the diazonium salt. An analogous ring closure on the
other nitrile moiety was not observed at all. This result
suggests that the 5-exo-dig cyclization leading to the
thiophene ring (Scheme 5, path a) should be highly
favored compared to the similar 5-exo-dig ring closure
that would afford a pyrrole ring and would be expected
to occur in competition with the other one (path b).
Actually, the reaction of isothiocyanate 11 with phenyl
On one hand, iminyl 14 can give 6-membered ring
closure, leading to quinoxaline 12 through the cyclohexa-
dienyl radical 16 (path b). On the other, it can undergo
a competitive 5-membered ipso-cyclization to the spiranic
intermediate 15; rearrangement of 15 and oxidation of
the cyclohexadienyl 17 eventually afford the isomeric
quinoxaline 18 (path a).11 Careful chromatographic and
spectral analyses of the reaction mixtures showed the
exclusive presence of quinoxaline 12 (see Experimental
Section). This compound could also arise from radical
15 (paths a and c). Nevertheless, in our opinion, if the
spirocyclohexadienyl radical is formed, it should rather
rearrange by migration of either C-N bondssgiving a
mixture of 12 and 18sthan afford 12 through translo-
cation of only one C-N bond (path c). This let us exclude,
for reactions a -n , the intervention of the intermediate
15 to a significant extent.
(12) All the attempts to perform X-ray crystallographic determina-
tions on the reaction products failed because of the unsuitable
crystalline properties. Structure 12 was assigned to the products on
the basis of the following arguments. It has been reported (ref 2t) that
an iminyl radical very similar to 14 gives an exclusive 1,6-ring
closure: in that case, the structure of the reaction product has been
confirmed by X-ray diffraction. An analogous, exclusive 6-membered
cyclization has been observed by Curran and co-workers (ref 2u) with
an iminyl radical generated by addition of an imidoyl radical to a cyano
group. Furthermore, to assign structure 18 instead of 12 to the reaction
The feasibility of the spirocyclization was however
established by the reaction of isothiocyanate 19 (Scheme
4). In this case, 19 afforded small amounts of the
isomeric product 18o, as well as major quantities of
quinoxaline 12o; 18o can be easily accounted for through
rearrangement of the spiranic radical 17o (X ) Y ) Cl).
product, we have to postulate
a mechanism involving exclusive
spirocyclization of iminyl 14 (Scheme 3, path a) and one-way-rear-
rangement of the resulting spirocyclohexadienyl radical 15 through
selective cleavage of the preexisting carbon-nitrogen bond (Scheme
3, path c). In our opinion, this is a very unlikely pathway. Finally, we
generated radical 13e by addition of (2-cyanophenyl)sulfanyl radical
to 4-methoxyphenyl isocyanide isonitrile. The reaction afforded major
amounts of quinoxaline 12e together with traces of a crystalline
byproduct derived from addition of another sulfanyl radical to 12e.
The structure of this compound was fully established by X-ray
diffraction, which confirmed the expected position of the substituent
on the quinoxaline ring (full details of this result will be reported
elsewhere). In light of this discussion, it seems quite reasonable to
assign structure 18o to the minor product of isothiocyanate 19. In fact,
there is no reason for supposing that, also in this case, iminyl 14o
cannot give mainly 1,6-ring closure affording 12o. The minor quantity
of 5-membered cyclization, leading to the isomer 18o, is probably the
result of both a statistical factor (one of the two ortho-positions is not
accessible for cyclization) and an increased stabilization of the spiro-
cyclohexadienyl by the two chlorine atoms (if we postulate a reversible
cyclization of 14).
(10) (a) Forrester, A. R.; Gill, M.; Thomson, R. H. J . Chem. Soc.,
Chem. Commun. 1976, 677. (b) Sakuragi, H.; Ishikawa, S.-I.; Nish-
imura, T.; Yoshida, M.; Inamoto, N.; Tokumaru, K. Bull. Chem. Soc.
J pn. 1976, 49, 1949. (c) Forrester, A. R.; Gill, M.; Sadd, J . S.; Thomson,
R. H. J . Chem. Soc., Perkin Trans. 1 1979, 612. (d) McNab, H. J . Chem.
Soc., Perkin Trans. 1 1984, 371. e) McNab, H. J . Chem. Soc., Perkin
Trans. 1 1984, 377. (f) McNab, H.; Smith, G. S. J . Chem. Soc., Perkin
Trans. 1 1984, 381. (g) Hickson, C. L.; McNab, H. J . Chem. Soc., Perkin
Trans. 1 1984, 1569.
(11) Formation and rearrangement of azaspirocyclohexadienyl radi-
cals have been reported in the ring closure of analogous vinyl
radicals: see refs 2n, 2t, 3f, and 4b.

R-(Arylthio)imidoyl Radicals
J . Org. Chem., Vol. 62, No. 24, 1997 8397
to -20 °C in a 50 mL round-bottomed flask under a very
efficient magnetic stirring. The tetrafluoroborate (5 mmol)
was then added portionwise (in ca. 1 h), keeping the temper-
ature in the range -10 to -20 °C. The reaction mixture was
warmed to room temperature, and the pyridine was evapo-
rated. The residue was suspended in methylene chloride and
filtered, and the solid was washed several times with meth-
ylene chloride. The solvent was evaporated and the residue
chromatographed according to the procedure described for each
compound. To exclude the presence of isomeric quinoxaline
derivatives, each fraction separated by chromatography was
carefully analyzed by mass spectrometry (the compounds are
not suitable for GC-MS analysis). 1H NMR analyses were
performed on all the fractions containing the required molec-
ular weight, and every sample (except the reaction of isothio-
cyanate 19) showed perfectly identical spectra, with no extra
peaks ascribable to the isomeric compounds. 1H NMR spectra
of the reaction crudes were also recorded. This strategy was
also applied to the following methods 2 and 3.
radicals gave no trace of compound 23, yielding unreacted
starting material, tars, and small amounts of diphenyl
disulfide. Even assuming a reversible addition step, the
1,6-cyclization of the intermediate iminyl 22 should be
sufficiently fast to drive the reaction toward the final
polycyclic compound. Therefore, this result seems to
indicate the complete inability of imidoyl radicals to give
a pyrrole ring through 1,5-cyclization onto a cyano
group.13
Work is in progress to obtain some quantitative data
on the intermediates and transition states involved in
the 5-exo-dig cyclization of imidoyl radicals to C-N triple
bonds.
The addition of 2-cyanoaryl radicals to aryl isothiocy-
anates is a novel example of cascade radical reaction that
allows the straightforward synthesis of tetracondensed
heteroaromatic products starting from very simple com-
pounds. Furthermore, reactions a -n are completely
regioselective, whereas the reported synthesis of ben-
zothienoquinoxalines starting from benzothiophene di-
oxide and substituted phenylenediamines always yields
mixtures of isomers.14 Our reaction is a new example of
rare [3 + 2] radical annulation via R-thio-substituted
imidoyl radicals. It is also the first instance of applica-
tion to the synthesis of heterocyclic compounds of a
radical addition, tandem cyclization strategy involving
isothiocyanates and carbon-centered radicals.
Meth od 2 (w ith P yr id in e). A pyridine solution of the aryl
isothiocyanate (15 mmol) was kept at -10 to -20 °C in a 50
mL round-bottomed flask under a very efficient magnetic
stirring. The tetrafluoroborate (5 mmol) was then added
portionwise (in ca. 1 h), keeping the temperature in the range
-10 to -20 °C. The reaction mixture was warmed to room
temperature, and the pyridine was evaporated. The residue
was treated with methylene chloride, which dissolved the
starting aryl isothiocyanate almost exclusively. The solid,
which contained most of the benzothienoquinoxaline, was
washed several times with methylene chloride; the filtrates
were combined, the solvent was evaporated, and the residue
was chromatographed according to the procedure described in
each reaction. The solid insoluble in methylene chloride was
poured into water, and the resulting suspension was refluxed
for a few minutes. After filtration, the residue was dissolved
in hot chloroform, the solution was dried over sodium sulfate
and filtered, and the solvent was evaporated. The residue was
combined with the benzothienoquinoxaline previously obtained
by column chromatography and crystallized.
Exp er im en ta l Section
Gen er a l P r oced u r es. 1H and 13C NMR spectra were
recorded in deuteriochloroform using tetramethylsilane as an
internal standard. Mass spectra (MS) and high-resolution
mass spectra (HRMS) were performed by electron impact with
a beam energy of 70 eV: relative intensities are given in
parentheses. Column chromatography was carried out on 60-Å
silica gel using light petroleum ether (40-70 °C) and a light
petroleum ether/diethyl ether gradient (from 0 up to 100%
diethyl ether) as eluant; diethyl ether was sometimes replaced
by methylene chloride. Previously reported reaction products
were identified by spectral comparison and mixed melting
point determination with authentic specimens.
Sta r tin g Ma ter ia ls. Isothiocyanates 5-10, 19, 2-ami-
nobenzonitrile, 2-amino-4-chlorobenzonitrile, thiophosgene,
boron trifluoride etherate, tert-butyl nitrite (Aldrich), and [18-
crown-6] (Merck) were commercially available. Tetrafluorobo-
rates 1 and 215 and benzo[4,5]thieno[2,3-b]-quinoxaline (12a )16
were prepared according to the literature.
2-Isoth iocya n a toben zon itr ile (11).17 A methylene chlo-
ride (25 mL) solution of 2-aminobenzonitrile (100 mmol) was
added dropwise at 20 °C to a stirred solution of thiophosgene
(125 mmol) in a methylene chloride (15 mL)/water (35 mL)
mixture. After 3 h, methylene chloride (20 mL) was added
and the organic layer was separated and dried (sodium
sulfate). The solvent was evaporated and the residue was
crystallized to give the title compound (80%), mp ) 60-62 °C
(from light petroleum ether).
Gen er a l P r oced u r e for th e Rea ction s of th e Ar yl
Isoth iocya n a tes 5-11 a n d 19 w ith th e Tetr a flu or obo-
r a tes 1 a n d 2. Meth od 1 (w ith P yr id in e). A pyridine
solution of the aryl isothiocyanate (15 mmol) was kept at -10
Meth od 3 (w ith Cr ow n Eth er ). [18-Crown-6] (0.25
mmol), potassium acetate (10 mmol), and the tetrafluoroborate
(5 mmol) were added to an ethyl acetate (20 mL) solution of
aryl isothiocyanate (15 mmol) in a 20 mL Erlenmeyer flask.
The reaction mixture was kept at room temperature for 4 h
under very efficient magnetic stirring. The solvent was
evaporated, and the residue was suspended in methylene
chloride, which dissolved the starting aryl isothiocyanate
almost exclusively. The solid, which contained most of the
benzothienoquinoxaline, was washed several times with me-
thylene chloride; the filtrates were combined, the solvent was
evaporated, and the residue was chromatographed according
to the procedure described in each reaction. The solid insoluble
in methylene chloride was poured into water, and the resulting
suspension was refluxed for a few minutes. After filtration,
the residue was dissolved in hot chloroform, the solution was
dried over sodium sulfate and filtered, and the solvent was
evaporated. The residue was combined with the benzothieno-
quinoxaline previously obtained by column chromatography
and crystallized.
Ben zo[4,5]th ien o[2,3-b]qu in oxa lin e (12a ). According to
method 1, column chromatography (light petroleum ether)
gave starting phenyl isothiocyanate (5). Further elution with
light petroleum ether/diethyl ether 50:50 v/v afforded 12a (0.83
g, 70%): mp ) 166-167 °C (from ethanol); lit.16 mp ) 166-
167 °C.
(13) An analogous R-phenylimidoyl radical, generated in the reaction
of N-(phenylmethylene)-2-aminobenzonitrile with diisopropyl peroxy-
dicarbonate (see refs 3d-k), did not afford any cyclization product
either.
(14) Banerji, K. D.; Mazumdar, A. K. D.; Sen, K. K. J . Indian Chem.
Soc. 1973, 50, 268.
(15) Doyle, M. P.; Bryker, W. J . J . Org. Chem. 1979, 44, 1572.
(16) Bezdrik, A.; Friedla¨nder, P.; Koeniger, P. Chem. Ber. 1908, 41,
227.
(17) Pazdera, P.; Ondracek, D. Czech. Pat. CS 270,981, 1991; Chem.
Abstr. 1992, 117, 69590u.
3-Ch lor oben zo[4,5]th ien o[2,3-b]qu in oxa lin e (12b). Ac-
cording to method 2, column chromatography (light petroleum
ether) gave the starting isothiocyanate (5). Further elution
with light petroleum ether/methylene chloride 67:33 v/v af-
forded 12b (0.95 g, 70%): mp ) 244-245 °C (from benzene);
1H NMR (300 MHz) 7.06 (1H, dd, J 1 ) 8.1 Hz, J 2 ) 1.7 Hz),
7.10 (1H, d, J ) 1.7 Hz), 7.32-7.40 (2H, m), 8.11-8.18 (1H,

8398 J . Org. Chem., Vol. 62, No. 24, 1997
Leardini et al.
m), 8.21-8.30 (2H, m + d, J ) 8.1 Hz); MS18 m/z 272 (M+ + 2,
37), 270 (M+, 100), 235 (7), 135 (12); HRMS calcd for C14H7-
ClN2S 270.00185, found 270.0020. Anal. Calcd for C14H7-
ClN2S: C, 62.11; H, 2.61; N, 10.35; S, 11.84. Found: C, 62.23;
H, 2.61; N, 10.31; S, 11.90.
ether) gave starting 4-chlorophenyl isothiocyanate (8). Further
elution with light petroleum ether/methylene chloride 67:33
v/v afforded 12g (0.95 g, 70%): mp ) 224-226 °C (from
benzene); 1H NMR (300 MHz) δ 7.11-7.28 (3H, m), 7.31 (1H,
dd, J 1 ) 8.9 Hz, J 2 ) 2.2 Hz), 7.86 (1H, d, J ) 8.9 Hz), 8.29
(1H, d, J ) 2.2 Hz), 8.45-8.53 (1H, m); 13C NMR (75 MHz)
124.3, 125.3, 126.6, 129.1, 130.2, 131.3, 131.9, 132.0, 135.4,
140.6, 141.0, 141.5, 149.2, 158.2; MS m/z 272 (M+ + 2, 37),
270 (M+, 100), 235 (5), 135 (11); HRMS calcd for C14H7ClN2S
270.00185, found 270.0020. Anal. Calcd for C14H7ClN2S: C,
62.11; H, 2.61; N, 10.35; S, 11.84. Found: C, 62.03; H, 2.61;
N, 10.37; S, 11.85.
9-Meth ylben zo[4,5]th ien o[2,3-b]qu in oxa lin e (12c). Ac-
cording to method 1, column chromatography (light petroleum
ether) gave starting 4-methylphenyl isothiocyanate (6). Fur-
ther elution with light petroleum ether/diethyl ether 85:15 v/v
afforded 12c (0.71 g, 57%): mp ) 203-205 °C (from benzene);
1H NMR (200 MHz) 2.62 (3H, s, Me), 7.52-7.71 (3H, m), 7.84-
7.91 (1H, m), 8.01-8.08 (2H, m), 8.53-8.60 (1H, m); 13C NMR
(50 MHz) 22.5, 124.2, 124.9, 126.3, 128.4, 128.9, 131.4, 132.2,
132.8, 140.2, 140.6, 141.2, 148.3, 157.0; MS m/z 250 (M+, 100),
249 (27), 125 (14); HRMS calcd for C15H10N2S 250.0565, found
250.0563. Anal. Calcd for C15H10N2S: C, 71.97; H, 4.03; N,
11.19; S, 12.81. Found: C, 72.01; H, 4.02; N, 11.17; S, 12.80.
The reaction was also carried out according to a simplified
method 3: quinoxaline 12c is soluble in methylene chloride
and it can be easily obtained by extraction of the inorganic
residue followed by column chromatography. Elution with
light petroleum ether gave 6; further elution with light
petroleum ether/diethyl ether 85:15 v/v afforded 12c (0.70 g,
56%); mp ) 203-205 °C (from benzene).
3,9-Dich lor oben zo[4,5]th ien o[2,3-b]qu in oxa lin e (12h ).
According to method 2, column chromatography (light petro-
leum ether) gave the starting isothiocyanate 8. Further
elution with light petroleum ether/methylene chloride 60:40
v/v afforded 12h (1.02 g, 67%): mp ) 216-218 °C (from ethyl
acetate); 1H NMR (200 MHz) δ 7.56 (1H, dd, J 1 ) 8.5 Hz, J 2
)
1.7 Hz), 7.75 (1H, dd, J 1 ) 9.0 Hz, J 2 ) 2.4 Hz), 7.86 (1H, d,
J ) 1.7 Hz), 8.08 (1H, d, J ) 9.0 Hz), 8.24 (1H, d, J ) 2.4 Hz),
8.45 (1H, d, J ) 8.5 Hz); 13C NMR (75 MHz) 124.1, 126.1, 127.4,
129.0, 130.3, 130.35, 131.6, 135.8, 138.3, 140.5, 141.6, 142.2,
148.2, 157.8; MS m/z 308 (M+ + 4, 13), 306 (M+ + 2, 69), 304
(M+, 100), 269 (7), 234 (6), 152 (9); HRMS calcd for C14H6-
Cl2N2S 303.9629, found 303.9627. Anal. Calcd for C14H6-
Cl2N2S: C, 55.10; H, 1.98; N, 9.18; S, 10.51. Found: C, 55.00;
H, 1.98; N, 9.20; S, 10.52.
3-Ch lor o-9-m e t h ylb e n zo[4,5]t h ie n o[2,3-b]q u in oxa -
lin e (12d ). According to method 2, column chromatography
(light petroleum ether) gave the starting isothiocyanate 6.
Further elution with light petroleum ether/methylene chloride
67:33 v/v afforded 12d (1.0 g, 70%): mp ) 211-214 °C (from
9-Br om oben zo[4,5]th ien o[2,3-b]qu in oxa lin e (12i). Ac-
cording to a simplified method 2 (the quinoxaline is not
dissolved by methylene chloride during the first filtration),
crystallization of the residue gave 12i (0.89 g, 57%): mp )
1
ligroin); H NMR (200 MHz) 2.62 (3H, m, Me), 7.53 (1H, dd,
J 1 ) 7.9 Hz, J 2 ) 1.6 Hz), 7.61-7.68 (1H, m), 7.84 (1H, d, J )
1.6 Hz), 7.98-8.06 (2H, m), 8.45 (1H, d, J ) 7.9 Hz); 13C NMR
(75 MHz) 22.6, 124.0, 125.7, 127.1, 128.5, 128.9, 130.7, 133.1,
137.5, 140.6, 140.65, 141.3, 141.8, 147.4, 156.6; MS m/z 286
(M+ + 2, 35), 284 (M+, 100), 283 (23), 249 (4), 248 (4), 142
(10); HRMS calcd for C15H9ClN2S 284.0175, found 284.0175.
Anal. Calcd for C15H9ClN2S: C, 63.27; H, 3.19; N, 9.84; S,
11.26. Found: C, 63.27; H, 3.19; N, 9.85; S, 11.25.
1
237-239 °C (from benzene); H NMR (300 MHz) δ 7.59 (1H,
td, J t ) 7.7 Hz, J d ) 0.9 Hz), 7.68 (1H, td, J t ) 7.9 Hz, J d
)
1.0 Hz), 7.84-7.89 (2H, m), 8.02 (1H, d, J ) 8.8 Hz), 8.45 (1H,
d, J ) 1.9 Hz), 8.53-8.58 (1H, m); 13C NMR (75 MHz) 123.5,
124.3, 125.4, 126.6, 130.3, 131.9, 132.1, 132.5, 133.8, 140.8,
141.0, 141.8, 149.1, 158.5; MS m/z 316 (M+ + 2, 100), 314 (M+,
93), 235 (15), 158 (9); HRMS calcd for C14H7BrN2S 313.9513,
found 313.9513. Anal. Calcd for C14H7BrN2S: C, 53.35; H,
2.24; N, 8.89; S, 10.17. Found: C, 53.31; H, 2.24; N, 8.91; S,
10.19.
9-Meth oxyben zo[4,5]th ien o[2,3-b]qu in oxalin e (12e). Ac-
cording to method 1, column chromatography (light petroleum
ether) gave starting 4-methoxyphenyl isothiocyanate (7).
Further elution with light petroleum ether/diethyl ether 90:
10 v/v afforded 12e (1.06 g, 80%); mp ) 187-188 °C (from
ethanol/chloroform); 1H NMR (300 MHz) 3.40 (3H, s, OMe),
7.41 (1H, dd, J 1 ) 9.1 Hz, J 2 ) 2.6 Hz), 7.48 (1H, d, J ) 2.6
The reaction was also carried out according to a simplified
method 3: quinoxaline 12i is completely insoluble in methyl-
ene chloride and can be easily obtained by crystallization of
the residue of the first filtration. We obtained 1.21 g (77%) of
12i; mp ) 237-239 °C (from benzene).
9-Br om o-3-ch lor ob en zo[4,5]t h ien o[2,3-b]q u in oxa lin e
(12j). According to method 2, column chromatography (light
petroleum ether) gave the starting isothiocyanate 9. Further
elution with light petroleum ether/methylene chloride 40:60
v/v afforded 12j (1.13 g, 65%): mp ) 218-220 °C (from ethyl
Hz), 7.53 (1H, td, J t ) 7.5 Hz, J d ) 1.0 Hz), 7.60 (1H, td, J t
)
7.7 Hz, J d ) 1.2 Hz), 7.80-7.84 (1H, m), 7.98 (1H, d, J ) 9.2
Hz), 8.46-8.51 (1H, m); 13C NMR (50 MHz) 56.2, 107.1, 123.9,
124.1, 124.7, 126.1, 129.8, 131.2, 132.1, 138.3, 140.7, 142.7,
148.1, 155.2, 160.8; MS m/z 266 (M+, 100), 251 (13), 223 (41),
133 (8); HRMS calcd for C15H10N2OS 266.0514, found 266.0512.
Anal. Calcd for C15H10N2OS C, 67.65; H, 3.78; N, 10.52, S,
12.04. Found: C, 67.70; H, 3.77; N, 10.50; S, 12.02.
acetate); 1H NMR (300 MHz) δ 7.56 (1H, dd, J 1 ) 8.3 Hz, J 2
)
1.7 Hz), 7.84-7.91 (2H, dd + d, J 1dd ) 8.9 Hz, J 2dd ) 2.0 Hz,
J d ) 1.6 Hz), 8.01 (1H, d, J ) 8.9 Hz), 8.41-8.47 (2H, d + d,
3-Ch lor o-9-m et h oxyb en zo[4,5]t h ien o[2,3-b]q u in oxa -
lin e (12f). According to a modified method 2, column chro-
matography (light petroleum ether) of the combined organic
phases (methylene chloride and hot chloroform) gave the
starting isothiocyanate 7. Further elution with light petro-
leum ether/methylene chloride 50:50 v/v afforded 12f (1.05 g,
J ) 2.0 Hz, J ) 8.3 Hz); MS m/z 352 (M+ + 4, 28), 350 (M+
+
2, 100), 348 (M+, 80), 269 (10), 234 (7), 175 (9), 174 (6); HRMS
calcd for C14H6BrClN2S 347.9124, found 347.9122. Anal.
Calcd for C14H6BrClN2S: C, 48.09; H, 1.73; N, 8.01; S, 9.17.
Found: C, 47.99; H, 1.73; N, 7.99; S, 9.19.
1
70%): mp ) 234-236 °C (from ethyl acetate); H NMR (200
MHz) 4.00 (3H, s, OMe), 7.44-7.58 (3H, m), 7.85 (1H, d, J )
1.8 Hz), 8.03 (1H, dd, J 1 ) 8.8 Hz, J 2 ) 0.7 Hz), 8.44 (1H, d,
J ) 8.6 Hz); 13C NMR (50 MHz) 56.6, 107.1, 124.0, 124.3, 125.5,
127.0, 129.9, 130.6, 137.4, 138.4, 141.9, 142.9, 147.3, 154.8,
161.1; MS m/z 302 (M+ + 2, 37), 300 (M+, 100), 285 (10), 257
(39), 150 (8); HRMS calcd for C15H9ClN2OS 300.0124, found
300.0125. Anal. Calcd for C15H9ClN2OS: C, 59.90; H, 3.02;
N, 9.31; S, 10.66. Found: C, 60.01; H, 3.03; N, 9.29; S, 10.66.
9-Ch lor oben zo[4,5]th ien o[2,3-b]qu in oxa lin e (12 g). Ac-
cording to method 2, column chromatography (light petroleum
9-Nitr oben zo[4,5]th ien o[2,3-b]qu in oxa lin e (12k ). Ac-
cording to method 2, column chromatography (light petroleum
ether/methylene chloride 70:30 v/v) gave starting 4-nitrophenyl
isothiocyanate (10). Further elution with light petroleum
ether/methylene chloride 50:50 v/v afforded 12k (0.74 g,
53%): mp ) 267-269 °C (from dioxane/benzene); 1H NMR (200
MHz) δ 7.65 (1H, td, J t ) 7.6 Hz, J d ) 0.9 Hz), 7.75 (1H, td, J t
) 7.6 Hz, J d ) 1.3 Hz), 7.88-7.95 (1H, m), 8.29 (1H, d, J )
9.3 Hz), 8.57 (1H, dd, J 1 ) 9.3 Hz, J 2 ) 2.5 Hz), 8.59-8.66
(1H, m), 9.20 (1H, d, J ) 2.5 Hz); MS m/z 281 (M+, 100), 251
(5), 235 (46), 223 (16), 140 (2); HRMS calcd for C14H7N3O2S
281.0259, found 281.0259. Anal. Calcd for C14H7N3O2S: C,
59.78; H, 2.51; N, 14.94; S, 11.40. Found: C, 59.85; H, 2.51;
N, 14.97; S, 11.38.
(18) All of the mass spectra of the benzothienoquinoxalines with
even M+ show peaks corresponding to m/z ) M+/2, whereas the
benzothienoquinoxalines with odd M+ are characterized by peaks at
m/z ) M+/2 ( 0.5. This is probably due to double-charged molecular
ions. See also: Pring, B. G.; Stjernstro¨m, N. E. Acta Chem. Scand.
1968, 22, 549.
The reaction was also carried out according to method 3:
column chromatography (light petroleum ether/methylene

R-(Arylthio)imidoyl Radicals
J . Org. Chem., Vol. 62, No. 24, 1997 8399
chloride 50:50 v/v) gave 0.80 g (57%) of quinoxaline 12k ; mp
267-269 °C (from dioxane/benzene).
starting isothiocyanate 11. Further elution with light petro-
leum ether/methylene chloride 60:40 v/v afforded 12n (1.11 g,
75%): mp ) 302-304 °C (from benzene); 1H NMR (300 MHz)
3-Ch lor o-9-n it r ob en zo[4,5]t h ien o[2,3-b]q u in oxa lin e
(12l). According to method 2, column chromatography (light
petroleum ether/methylene chloride 70:30 v/v) gave the start-
ing isothiocyanate 10. Further elution with light petroleum
ether/methylene chloride 50:50 v/v afforded 12l (0.53 g, 34%):
mp ) 250-252 °C (from benzene); 1H NMR (200 MHz) δ 7.61
(1H, dd, J 1 ) 8.1 Hz, J 2 ) 1.5 Hz), 7.90 (1H, d, J ) 1.5 Hz),
8.28 (1H, d, J ) 8.9 Hz), 8.51 (1H, d, J ) 8.1 Hz), 8.58 (1H,
dd, J 1 ) 8.9 Hz, J 2 ) 2.3 Hz), 9.16 (1H, d, J ) 2.3 Hz); MS m/z
317 (M+ + 2, 33), 315 (M+, 100), 285 (3), 271 (16), 269 (44),
259 (6), 257 (14), 234 (12), 157 (7); HRMS calcd for C14H6-
ClN3O2S 314.9869, found 314.9870. Anal. Calcd for
C14H6ClN3O2S: C, 53.26; H, 1.92; N, 13.31; S, 10.16. Found:
C, 53.17; H, 1.92; N, 13.33; S, 10.17.
δ 7.61 (1H, dd, J 1 ) 8.4 Hz, J 2 ) 1.8 Hz), 7.88 (1H, dd, J 1
)
8.4 Hz, J 2 ) 7.1 Hz), 7.93 (1H, d, J ) 1.8 Hz), 8.23 (1H, dd, J 1
) 7.1 Hz, J 2 ) 1.2 Hz), 8.49-8.54 (2H, m); MS m/z 297 (M+
+
2, 42), 295 (M+, 100), 260 (3), 147 (8); HRMS calcd for C15H6-
ClN3S 294.9971, found 294.9972. Anal. Calcd for C15H6-
ClN3S: C, 60.92; H, 2.04; N, 14.21; S, 10.84. Found: C, 61.00;
H, 2.04; N, 14.18; S, 10.84.
7,9-Dich lor oben zo[4,5]th ien o[2,3-b]qu in oxa lin e (12o)
a n d 8,10-d ich lor ob en zo[4,5]t h ien o-[2,3-b]q u in oxa lin e
(18o). According to method 2, column chromatography (light
petroleum ether/diethyl ether 90:10 v/v) gave starting 2,4-
dichlorophenyl isothiocyanate (19). Further elution with light
petroleum ether/diethyl ether 60:40 v/v afforded 12o (0.76 g,
50%), mp ) 231-233 °C (from benzene) [1H NMR (200 MHz)
δ 7.00-7.19 (3H, m), 7.44 (1H, d, J ) 2.0 Hz), 8.04 (1H, d, J
) 2.0 Hz), 8.34-8.43 (1H, m); MS m/z 308 (M+ + 4, 12), 306
(M+ + 2, 65), 304 (M+, 100), 269 (3), 234 (5), 152 (10), 108 (7);
HRMS calcd for C14H6Cl2N2S 303.9629, found 303.9621. Anal.
Calcd for C14H6Cl2N2S: C, 55.10; H, 1.98; N, 9.18; S, 10.51.
Found: C, 55.30; H, 1.99; N, 9.22; S, 10.46], and 18o (0.12 g,
8%), mp ) 255-257 °C (from benzene) [1H NMR (200 MHz) δ
7.15-7.30 (1H, m), 7.50-7.95 (3H, m + d, J ) 1.9 Hz), 8.08
(1H, d, J ) 1.9 Hz), 8.55-8.70 (1H, m); MS m/z 308 (M+ + 4,
13), 306 (M+ + 2, 67), 304 (M+, 100), 269 (3), 234 (7), 152 (11),
108 (10); HRMS calcd for C14H6Cl2N2S 303.9629, found
303.9623. Anal. Calcd for C14H6Cl2N2S: C, 55.10; H, 1.98;
N, 9.18; S, 10.51. Found: C, 55.28; H, 1.99; N, 9.23; S, 10.47].
The reaction was also carried out according to method 3:
column chromatography (light petroleum ether/methylene
chloride 50:50 v/v) gave 1.09 g (69%) of quinoxaline 12l.
Be n zo[4,5]t h ie n o[2,3-b]q u in oxa lin e -7-ca r b on it r ile
(12m ). According to a modified method 2, column chroma-
tography (light petroleum ether/methylene chloride 80:20 v/v)
of the combined organic phases (methylene chloride and hot
chloroform) gave starting 2-isothiocyanatobenzonitrile (11).
Further elution with light petroleum ether/methylene chloride
60:40 v/v afforded 12m (0.85 g, 65%): mp ) 264-265 °C (from
1
ethyl acetate); H NMR (300 MHz) δ 7.63 (1H, ddd, J 1 ) 7.8
Hz, J 2 ) 7.3 Hz, J 3 ) 0.9 Hz), 7.74 (1H, ddd, J 1 ) 8.0 Hz, J 2
) 7.3 Hz, J 3 ) 1.2 Hz), 7.86 (1H, dd, J 1 ) 8.5 Hz, J 2 ) 7.3
Hz), 7.91-7.95 (1H, m), 8.22 (1H, dd, J 1 ) 7.3 Hz, J 2 ) 1.2
Hz), 8.52 (1H, dd, J 1 ) 8.5 Hz, J 2 ) 1.2 Hz), 8.57-8.62 (1H,
m); MS m/z 261 (M+, 100), 234 (3), 131 (12); HRMS calcd for
C15H7N3S 261.0361, found 261.0360. Anal. Calcd for
C15H7N3S: C, 68.95; H, 2.70; N, 16.08; S, 12.27. Found: C,
69.10; H, 2.70; N, 16.01; S, 12.28.
Ack n ow led gm en t. The authors gratefully acknowl-
edge financial support from MURST, CNR (Rome), and
Universita` di Bologna (Progetto di Finanziamento Tri-
ennale del Dipartimento di Chimica Organica “A.
Mangini”).
3-Ch lor ob en zo[4,5]t h ien o[2,3-b]q u in oxa lin e-7-ca r b o-
n itr ile (12n ). According to method 2, column chromatography
(light petroleum ether/methylene chloride 80:20 v/v) gave the
J O971128A