Polysulfane antitumor agents from o-benzyne. An odd-even alternation found in the stability of products o-C6H4Sx (x = 1-8)

Article abstract of DOI:10.1021/jo049418w

Benzyne is shown to add elemental sulfur and give rise to a series of polysulfane compounds. A computational and experimental study is presented. Odd-membered o-C₆H₄S_x rings (x = 1-8), except x = 1, which suffers from ring strain, have enhanced stability compared to even-membered rings. The acquisition of odd-even data may shed new light, revealing patterns on polysulfane stability and structure.

Full text of DOI:10.1021/jo049418w

SCHEME 1
P olysu lfa n e An titu m or Agen ts fr om
o-Ben zyn e. An Od d -Even Alter n a tion
F ou n d in th e Sta bility of P r od u cts o-C6H4Sx
(x ) 1-8)
Edyta M. Brzostowska and Alexander Greer*
Department of Chemistry, Graduate School and University
Center & The City University of New York (CUNY),
Brooklyn College, Brooklyn, New York 11210
agreer@brooklyn.cuny.edu
Received April 7, 2004
SCHEME 2
Abstr a ct: Benzyne is shown to add elemental sulfur and
give rise to a series of polysulfane compounds. A computa-
tional and experimental study is presented. Odd-membered
o-C6H4Sx rings (x ) 1-8), except x ) 1, which suffers from
ring strain, have enhanced stability compared to even-
membered rings. The acquisition of “odd-even” data may
shed new light, revealing patterns on polysulfane stability
and structure.
We present the study of a reaction that can make
ortho-fused polysulfur chemical bonds. o-Benzopoly-
sulfanes possess antitumor and antimicrobial activity.1
Previous research has demonstrated that elemental
sulfur can react with unsaturated chemical groups such
as benzyne2,3 and alkynes.4-6 A reaction where thian-
threne (1) and benzopentathiepin (2, o-C6H4S5) are
formed utilizing o-benzyne as a precursor has been
established previously by Nakayama et al. (Scheme 1).2,3
The report suggested an initial formation of benzothiirene
(3, o-C6H4S) or benzodithiete (4, o-C6H4S2) intermedi-
ates.2,3 A more detailed study of the o-benzyne-elemental
sulfur reaction has not yet appeared. The study of
o-benzyne with elemental sulfur is presented here that
provides evidence for the decomposition of an initial
o-C6H4S8 intermediate (5) to give thianthrene 1, penta-
thiepin 2, trithiole 6, and tetrathiocin 7 products (Scheme
2).
Various heterocycles may be expected to arise from the
reaction of o-benzyne with cyclic S8. B3LYP/6-31G(d)
calculations are used to predict the stability of possible
ortho-fused heterocycles, o-C6H4Sx (where x ) 1-8). Low-
energy conformers of the rings containing up to 8 sulfurs
were identified by a search of conformational space. An
oscillation pattern emerges in the relative stability of
these heterocycles (Figure 1). The odd-membered o-C6H4Sx
rings (except x ) 1, which suffers from ring strain) have
enhanced conformational stability compared to the even-
membered rings.
Figure 1 is a measure of the stability of o-C6H4Sx. We
compared the energy difference of o-C6H4S8 5 relative to
o-C6H4Sx plus a fraction of cyclic S8’s energy in an
isodesmic reaction. Compounds o-C6H4S7 and o-C6H4S5
possess staggered lone-pair electrons in the stable crown
and chair forms, respectively. The five-membered ring
molecule o-C6H4S3 adopts a stable half-chair conforma-
tion. Lone-pair interactions in polysulfur systems are
known.7 Sulfur lone pairs are in nonequivalent orbitals:
one pair in the 3p orbital (axial to the plane of the
(1) (a) Davidson, B. S.; Molinski, T. F.; Barrows, L. R.; Ireland, C.
M. J . Am. Chem. Soc. 1991, 113, 4709. (b) Searle, P. A.; Molinski, T.
F. J . Org. Chem. 1994, 59, 6600. (c) Compagnone, R. S.; Faulkner, D.
J .; Carte, B. K.; Chan, G.; Hemling, M. A.; Hofmann, G. A.; Mattern,
M. R. Tetrahedron 1994, 50, 12785. (d) Ford, P. W.; Narbut, M. R.;
Belli, J .; Davidson, B. S. J . Org. Chem. 1994, 59, 5955. (e) Litaudon,
M.; Trigalo, F.; Martin, M.-T.; Frappier, F.; Guyot, M. Tetrahedron
1994, 50, 5323. (f) Makarieva, T. N.; Stonik, V. A.; Dmitrenok, A. S.;
Grebnev, B. B.; Iskov, V. V.; Rebachyk, N. M. J . Nat. Prod. 1995, 58,
254. (g) Sato, R.; Ohyama, T.; Ogawa, S. Heterocycles 1995, 41, 893.
(h) Sato, R.; Ohyama, T.; Kawagoe, T.; Baba, I.; Nakajo, S.; Kimura,
T.; Ogawa, S. Heterocycles 2001, 55, 145. (i) Lee, A. H. F.; Chen, J .;
Liu, D.; Leung, T. Y. C.; Chan, A. S. C.; Li, T. J . Am. Chem. Soc. 2002,
124, 13972. (j) Tonika, C.; Gates, K. S. Bioorg. Med. Chem. Lett. 2003,
13, 1349. (k) Brzostowska, E. M.; Greer, A. J . Am. Chem. Soc. 2003,
125, 396. (l) Konstantinova, L. S.; Rakitin, O. A.; Rees, C. W. Chem.
Rev. 2004, 104, 2617.
(2) Nakayama, J .; Kashiwagi, M.; Yomoda, R.; Hoshino, M. Nippon
Kagaku Kaishi 1987, 7, 1424.
(3) Nakayama, J .; Akimoto, K. Sulfur Reports 1994, 16, 61.
(4) Choi, K. S.; Akiyama, I.; Hoshino, M.; Nakayama, J . Bull. Chem.
Soc. J pn. 1993, 66, 623.
(5) Nakayama, J .; Ishii, A. Adv. Heterocycl. Chem. 2000, 77, 221.
(6) For a review on 1,2-dithietes, see: Nielsen, S. B.; Senning, A.
Sulfur Reports 1995, 16, 371.
10.1021/jo049418w CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/10/2004
J . Org. Chem. 2004, 69, 5483-5485
5483

overlap in the p-orbitals for the odd- vs even-membered
rings. With o-C6H4Sx (x ) 3, 5, 7), an even number of out-
of-phase overlaps is achieved, whereas with o-C6H4Sx
(x ) 2, 4, 6, 8), an odd number of out-of-phase overlaps
is achieved (e.g., compare structures A and B). Even vs
odd out-of-phase overlap in o-C6H4Sx (x ) 1-8) is
reminiscent of Hu¨ckel and Mo¨bius π electron systems.
Similar “odd-even” trends may apply to allotropes of
elemental sulfur but are not described in the literature.
F IGURE 1. Oscillations in the stability of o-benzopolysulfanes
o-C6H4Sx gauged by a fraction of cyclic S8 subtracted from
o-C6H4S8 in an isodesmic reaction, i.e., “relative energy” is the
energy of the reaction o-C6H4S8 f o-C6H4Sx + (8 - x)/8 cyclic
S8. Data shown were obtained with B3LYP/6-31G(d) gas-phase
calculations. The temperature sensitivity of the stability of
polysulfur compounds appears to be small (see Supporting
Information). The perfect linear correlation of energy for
(8 - x)/8 cyclic S8, where x ) 1 to 8, allowed for the study of
o-C6H4Sx heterocycle stability plotted here.
Described below is an experimental study of the
o-benzyne-Sx reaction. The identification of o-C6H4S5 2
and o-C6H4S3 6 as products is based on an NMR and GC/
MS analysis. Formation of o-C6H4S2 4 is suggested on
the basis of a secondary reaction that traps o-benzyne to
give thianthrene 1 as well as a o-C6H4S2 4 dimerization
process to give tetrathiocin 7 (Scheme 2B).
Benzyne was generated by low-temperature (-60 °C
reaction of n-BuLi with o-dihalobenzene in diethyl ether)8
and elevated-temperature methods (83 °C decomposition
of benzenediazonium-2-carboxylate in 1,2-dichloroet-
hane).9 The production of o-benzyne was demonstrated
with furan trapping where 1,4-dihydronaphthalene-1,4-
endoxide was detected in 80% yields. Elemental sulfur
was added to trap the in situ-generated o-benzyne and
gave thianthrene 1, pentathiepin 2, trithiole 6, and
tetrathiocin 7 in low overall yields [1 (17.2%), 2 (11.9%),
6 (1.3%), 7 (2.6%)]. Acyclic polysulfanes are probably
formed concomitantly. One of the byproducts obtained,
o-C6H4ClSH (1%), suggests that the o-benzyne precursor,
o-C6H4ClLi, reacts with elemental sulfur and generates
thiolates, which can equilibrate polysulfides. Concentra-
tions of 1, 2, 6, and 7 are identical to within experimental
error with the o-benzyne precursors o-C6H4ClLi or o-C6H4-
CO2-N2+, which provide evidence for thermodynamic
product ratios. Pentathiepin 2 displayed a weak molec-
ular ion peak m/z 236 [M+] and a base peak at m/z 172
[M+ - 2S] representing the loss of two sulfur atoms. The
molecular ion of trithiole 6 is abundant and represents
the base peak m/z 172 [M+]. Mass spectrometry was also
F IGURE 2. Odd-even alternation arising from eclipsing
strain measured in o-C6H4Sx heterocycles based on B3LYP/
6-31G(d) calculations. The dihedral angle of pentathiepin
o-C6H4S5 (73.0°) is used as a reference point from which the
segment S1-S2-S3-S4 in o-C6H4Sx (x ) 4-8), C-S1-S2-
S3 in o-C6H4S3, and C-C-S1-S2 in o-C6H4S2 are compared.
The difference gives rise to the |∆θ| values.
molecule) and the other in an sp (equatorial) orbital.
Sulfur also possesses a vacant σ* orbital.
The conformations in o-C6H4Sx (x ) 3, 5, 7) can better
adopt gauche interactions, whereas eclipsing interactions
occur in the even-numbered cases. The extent of eclipsing
is measured by comparison of the polysulfur torsion angle
(θ ) S1-S2-S3-S4) in o-C6H4Sx as positive or negative
to the pentathiepin θ angle of 73.0° as a reference point
described as |∆θ|. Figure 2 reveals that the conformations
for the even-membered rings exhibit enhanced eclipsing
strain with higher |∆θ| values. The origin of the oscil-
lating feature in Figure 1 is also likely the result of the
1
used to confirm the presence of thianthrene 1. H NMR
data on the o-benzyne sulfuration reaction yielded con-
clusive evidence for the formation of thianthrene 1,
pentathiepin 2, and tetrathiocin 7.10 The expected upfield
chemical shifts for trithiole 6 are obscured by the reagent
and byproduct peaks. The assignment of 2 is based on
two sets of dds: a downfield set at δ 7.85 (2H) and an
(8) Gilman, H.; Gorsich, R. D. J . Am. Chem. Soc. 1957, 79, 2625.
(9) Logullo, M.; Seitz, A. H.; Friedman, L. Org. Synth. 1973, 5, 54.
(10) (a) Field, L.; Stephens, W. D.; Lippert, E. L. J . Org. Chem. 1961,
26, 4782. (b) Sato, M.; Lakshmikantham, M. V.; Cava, M. P.; Garito,
A. F. J . Org. Chem. 1978, 43, 2084. (c) Shimizu, T.; Kamigata, N. J .
Organomet. Chem. 2000, 611, 106.
(7) (a) Steudel, R. Chem. Rev. 2002, 102, 3905. (b) Kustos, M.;
Steudel, R. J . Org. Chem. 1995, 60, 8056. (c) Davidson, B. S.; Ford, P.
W.; Wahlman, M. Tetrahedron Lett. 1994, 35, 7185. (d) Conformational
Analysis of Medium-Sized Heterocycles; Glass, R. S., Ed.; VCH Publish-
ers: New York, 1988.
5484 J . Org. Chem., Vol. 69, No. 16, 2004

TABLE 1. P r od u ct Distr ibu tion of o-Ben zosu lfa n es th a t
Ar ise fr om th e Cyclic S8 Su lfu r a tion of o-Ben zyn ea -c
upfield set at δ 7.34 (2H). Similar chemical shifts are
found for 1 [δ 7.58 (4H); δ 7.32 (4H)] and 7 [δ 8.66 (4H)].
The reaction shown in Scheme 2 stems from the
proposal that benzooctathiecin 5 forms. There was no
direct detection of 5 in the benzyne reaction. However,
experimental evidence for the decomposition of an oc-
tathiecin was obtained by synthesizing 5 and 5′ from the
method of Steudel using ZnS6(TMEDA).11 In the presence
of heat (40 °C) or an added nucleophile n-PrSH in
dicholormethane, gas chromatographic analyses of the
reaction revealed the formation of 2, 6, and 7 and 2′, 6′,
and 7′ from 5 and 5′, respectively (see Supporting
Information). These results provide evidence for the
formation of 5 in the benzyne-Sx reaction and are
consistent with the idea that heat or nucleophiles can
influence the equilibrium between polysulfane com-
pounds,7a,11-13 which offers a reason for the decomposition
of o-C6H4S8 5 under the conditions. Formation of stable
arene annulated polysulfur products from precursor 5
likely occurs by reversible exchange reactions involving
complex mixtures of polysulfanes.
a
GC/MS and NMR detection of products formed in diethyl ether
b
at -60 °C after addition of cyclic S8 to o-benzyne. o-Benzyne
concentration in the reaction is approximately 200 mM. Each entry
is an average of four runs. c Reactions were taken to ∼20%
completion.
form from o-benzyne is enhanced with higher elemental
sulfur concentrations. We find that elemental sulfur is
soluble to the extent of 0.0078 g in 10 g of diethyl ether
at -60 °C, which leads to the conclusion that amounts
of elemental sulfur above ∼10 mM. This result indicates
that a heterogeneous process amplifies the production of
polysulfanes 2, 6, and 7 but not sulfane 1.
In conclusion, this study describes a reaction that can
make ortho-fused polysulfur bonds. We find that the odd-
membered o-C6H4Sx rings (x ) 3, 5, 7) have enhanced
stability compared to the even-membered rings. The
acquisition of odd-even data can reveal patterns on the
stability and the structure of cyclic polysulfanes. The
mechanism of polysulfane product formation appears to
be governed by reversible, equilibrium, exchange pro-
cesses.
The product distribution also bears on the solubility
of elemental Sx in the benzyne system. The yield of
thianthrene 1 decreased by 17% when the initial cyclic
S8 concentration is varied from 0.8 to 85 mM (Table 1).
Such an effect between o-benzyne and cyclic S8 concen-
tration and the suppression of 1 is consistent with a
reduced o-benzyne trapping of the dithiete intermediate
4 (Scheme 2B). It follows that the ability of 2, 6, and 7 to
Ack n ow led gm en t. The City University of New
York and PSC-CUNY provided financial support for this
work. Kym Faull and Ken Conklin (Pasarow UCLA
Mass Spectrometry Laboratory) and Cliff Soll (Hunter
College Mass Spectrometry Facility) were responsible
for making several mass measurements. We thank the
reviewers for their comments and P. Haberfield for
several enlightening discussions.
(11) Steudel, R.; Hassenberg, K.; Munchow, V.; Schumann, O.;
Pickardt, J . Eur. J . Inorg. Chem. 2000, 921.
(12) (a) Sato, R. Heteroatom Chem. 2002, 13, 419. (b) Lee, W. K.;
Liu, B.; Park, C. W.; Shine, H. J .; Guzman-J imenez, I. Y.; Whitmire,
K. H. J . Org. Chem. 1999, 64, 9207.
(13) Dimethyl disulfide was recently shown to add to benzyne to
give o-C6H4(SCH3)2. (a) Thoen, K. K.; Smith, R. L.; Nousiainen, J . J .;
Nelson, E. D.; Kentta¨maa, H. I. J . Am. Chem. Soc. 1996, 118, 8669.
(b) Stirk, K. M.; Orlowski, J . C.; Leeck, D. T.; Kentta¨maa, H. I. J . Am.
Chem. Soc. 1992, 114, 8604. (c) Thoen, K. K.; Kentta¨maa, H. I. J . Am.
Chem. Soc. 1999, 121, 800.
Su p p or tin g In for m a tion Ava ila ble: Experimental sec-
tion on reactions and product characterizations and descrip-
tions of the geometries of the stationary points and absolute
energies. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O049418W
J . Org. Chem, Vol. 69, No. 16, 2004 5485