Active-alkali metal promoted reductive desulfurization of dibenzothiophene and its hindered analogues

Article abstract of DOI:10.1016/j.tet.2012.10.044

Dibenzothiophene and some related organosulfur compounds are efficiently reductively desulfurized under mild reaction conditions, with Na and/or Li metal in the presence of a catalytic amount of tetraphenylethylene in THF at room temperature. This simple methodology was applied to the synthesis of several substituted biphenyls, thus realizing a connection between the directing properties of the sulfur atom of dibenzothiophene and the efficiency of 1,2-dianions of tetraphenylethane as homogenous electron transfer reagents.

Full text of DOI:10.1016/j.tet.2012.10.044

Tetrahedron 69 (2013) 207e211
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Tetrahedron
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Active-alkali metal promoted reductive desulfurization of
dibenzothiophene and its hindered analogues
*
Mario Pittalis , Ugo Azzena, Luisa Pisano
ꢀ
Dipartimento di Chimica, Universita di Sassari, via Vienna 2, I-07100 Sassari, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Dibenzothiophene and some related organosulfur compounds are efficiently reductively desulfurized
under mild reaction conditions, with Na and/or Li metal in the presence of a catalytic amount of tet-
raphenylethylene in THF at room temperature. This simple methodology was applied to the synthesis of
several substituted biphenyls, thus realizing a connection between the directing properties of the sulfur
atom of dibenzothiophene and the efficiency of 1,2-dianions of tetraphenylethane as homogenous
electron transfer reagents.
Received 14 August 2012
Received in revised form 24 September
2012
Accepted 15 October 2012
Available online 22 October 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Alkali metals
Reduction
Desulfurization
Dibenzothiophenes
Biphenyls
1. Introduction
Accordingly, there is an increasing interest in new methodolo-
gies to convert efficiently CeS bonds into CeH bonds including,
Reductive desulfurization of organic compounds is of impor-
tance both in organic synthesis and in industry. In the former case,
a synthetic transformation utilizes a sulfur-containing directing
group that can then be reductively removed in a successive step of
the whole synthetic procedure.1 Particularly, the sulfur atom in
benzo- and dibenzothiophene heterocycles exerts a strong direct-
ing effect towards metallations and electrophilic aromatic sub-
stitutions.2 Moreover, benzo- and dibenzothiophenes are between
the most abundant sulfur-containing impurities in crude oils, and
their desulfurization is a mandatory issue in the production of non
polluting fuels.3
inter alia, homogeneous catalytic hydrogenation,6 oxidative pro-
cesses,7 Grignard-mediated catalytic desulfurizations,8 reductive
desulfurizations with Ni-containing reagents.1a,9
Despite their ready availability and high reducing power, rela-
tively few reports concern the exhaustive reductive desulfurization
of dibenzothiophene with alkali metals. Indeed, most of its re-
ductions with alkali metals were projected to afford 2-
mercaptobiphenyl, in the form of the corresponding intermediate
diorganometal, as the main reaction product.10,11 Indeed, with
a noticeable exception involving the employment of alkali metals
absorbed into silica gel,12 low yields of biphenyl were obtained
unless high temperatures were used.13e16
Following our interest in the development of efficient alkali
metal-mediated synthetic procedures17 and alternative protocols
for the chemical transformation of widespread environmental
contaminants,18 we wish to report here on the effectiveness of
sodium and lithium metals, in the presence of catalytic amounts of
tetraphenylethylene (TPE), in promoting the reductive de-
sulfurization of dibenzothiophene, as well as of the corresponding
sulfone and sulfoxide. This methodology was additionally applied
to the synthesis of several substituted biphenyls, realizing a con-
nection between the directing properties of the sulfur atom of
dibenzothiophene and the efficiency of TPE to behave as a homo-
geneous electron shuttle, thus allowing the employment of alkali
metals in highly efficient single electron transfer (SET) reductions.19
At an industrial level, catalytic hydrogenation of fuels and coals
(hydrodesulfurization, HDS) is realized under relatively harsh re-
action conditions (high pressures and temperatures), in the pres-
ence of heterogeneous catalysts, such as
g-Al2O3-supported Co, Mo,
W and Ni compounds.4,5
The most refractory sulfur-containing compounds present in
fuels are known to be the alkyl-dibenzothiophenes, especially
those substituted at the
4 and 6 positions, such as 4,6-
dimethyldibenzothiophene (4,6-DMDBT, 1e), and their low re-
activity is generally attributed to the steric hindrance.5
* Corresponding author. Tel.: þ39 079229549; fax: þ39 079229559; e-mail ad-
dress: pittalis@uniss.it (M. Pittalis).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.10.044

208
M. Pittalis et al. / Tetrahedron 69 (2013) 207e211
2. Results and discussion
2.2. Reductive desulfurization reactions
2.1. Starting materials
Na and/or Li metal were activated by stirring a THF suspension
of the freshly cut metal with TPE under a N2 atmosphere for 1 h at
rt. The reductions were run by the dropwise addition of a solution
of the appropriate substrate to the resulting deep red mixture over
Chart 1 shows the various dibenzothiophenes examined in this
work, together with the corresponding desulfurization products.
S
S
O
2a
2a
1a, DBT
1b, DBTO
S
S
CH3
CH3
CH3
O
O
2a
1c, DBTO2
1d, 4-MeDBT
2b
CH3
H3C
S
S
CH3
H3C
CH3 H3C
H3C
2c
2c
1e, 4,6-Me2DBT
1f, 2,8-Me2DBT
Ph
Ph
Ph
Ph
S
2d
1g, 2,8-Ph2DBT
Chart 1. Formulae and acronyms of the various benzothiophenes investigated and the corresponding desulfurization products.
Dibenzothiophene (DBT, 1a) and dibenzothiophene sulfone
30 min. Reaction mixtures were stirred at the same temperature for
the appropriate time (see Table 1), and quenched with H2O. Work-
up with ethyl acetate or diethyl ether led to a mass recovery typ-
icallyꢀ90%, and the crude mixtures were analyzed by 1H and 13C
NMR, IR spectroscopy and GC/MS analysis.
It is worth noting that we found no evidence for the
formation of significant amounts of the corresponding 2-
mercaptobiphenyls. Reaction conditions and yields are re-
ported in Table 1.
(DBTO2, 1c) are commercially available, whilst dibenzothiophene
sulfoxide (DBTO, 1b), was synthesized according to a literature
procedure.20
Dibenzothiophenes 1d,2a 1e,2a 1f21 and 1g2b were synthesized
according to literature procedures as depicted in Scheme 1, taking
advantage of the directing properties of the sulfur atom of diben-
zothiophene to direct metallation (for 1d and 1e) and electrophilic
aromatic substitution (for 1f and 1g) reactions.
Ph
Ph
Br
Br
ref. 2b
ref. 2b
cross
EAS
S
S
S
coupling
1g
1a
, 68%
70%
ref. 2a
ref. 21
CH3
H3C
S
S
R2
R1
R1 = H, R2 = CH3, 1d, 88%
1e
1f, 72%
R1 = R2 = CH3, , 49%
Scheme 1. Synthesis of dibenzothiophenes 1deg, according to literature procedures: Ref. 2b, EAS: Br2, CHCl3, 0 ꢁC to rt; Ref. 2b, cross coupling: PhB(OH)2, Pd(PPh3)4, K2CO3, DME/
H2O, 85 ꢁC; Ref. 2a, R1¼H, R2¼CH3: n-BuLi, hexane/THF, ꢂ78 ꢁC, then CH3I; Ref. 2a, R1¼R2¼CH3: n-BuLi, hexane, TMEDA, ꢂ78 ꢁC, then CH3I; Ref. 21: n-BuLi, hexane/Et2O, 0 ꢁC, then
CH3I. EAS¼Electrophilic aromatic substitution.

M. Pittalis et al. / Tetrahedron 69 (2013) 207e211
209
Table 1
Desulfurization of compounds 1 with alkali metals and catalytic amounts of TPEa
Entry Substrate Reductant Cat (% mol) Metal/1 M ratio Product Yieldb
+M2S
S
SM M
M
M
1
1a
1a
1b
1c
1c
1d
1d
1e
1e
1e
1f
Na/TPE
Na/TPE
Li/TPE
20
10
10
20
20
20
20
20
10
10
8
2a
2a
2a
2a
2a
2b
2b
2c
2c
2c
2c
2d
>90
>90
90
2
3
4
5
6
7
8
9
10
11
12
Na/TPE
Na/TPE
Na/TPE
Na/TPE
Na/TPE
Na/Li/TPE 20
Na/Li/TPE 20
Na/TPE
Li/TPE
10
13
10
15
15
20f
12g
10
8
72c
>90d
60e
M
M
>90
<5
>90
>90
>90
90
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph2C CPh2
M
M
20
15
1g
a
All reactions were run in dry THF, under N2, for 14 h at rt, unless otherwise
indicated.
b
c
d
e
f
As determined by 1H NMR analyses of the crude reaction mixtures.
28% of 1a was also recovered.
2M
Scheme 2. A simplified mechanism for the TPE-catalyzed reductive desulfurization of
DBT, 1a; M¼Li or Na.
Reaction run for 26 h at rt.
40% of 1a was also recovered.
Reaction run with 10 equiv of Na and 10 equiv of Li.
Reaction run with 5 equiv of Na and 7 equiv of Li.
g
We next investigated the reductive cleavage of several
substituted dibenzothiophenes to check the feasibility to apply our
procedure to the regioselective synthesis of substituted biphenyls.
The reaction of 4-methyldibenzothiophene (1d, 4-MeDBT) with
metallic Na in a 1:10 M ratio and in the presence of 20% mol of TPE,
afforded the desired 3-methylbiphenyl, 2b, in 60% yield, along with
some unreacted starting material (Table 1, entry 6). Complete
conversion of 4-MeDBT, 1d, into biphenyl 2b was realized by simply
increasing the relative amount of metal (Table 1, entry 7).
More complex results were obtained in the reductive de-
sulfurization of 4,6-dimethyldibenzothiophene (1e, 4,6-Me2DBT),
a substrate, which is known for its particularly low reactivity under
HDS reaction conditions;5 in agreement with these antecedents,
4,6-Me2DBT, 1e, proved completely unreactive under the reaction
conditions employed to desulfurize 4-MeDBT, 1d (Table 1, entry 8).
However, taking advantage of the observation of Farmer et al. on
the synergistic effect of Na used in conjunction with Li in the re-
ductive desulfurization of dibenzothiophenes 1aec,13 we success-
fully achieved an almost quantitative conversion of 4,6-Me2DBT, 1e,
into the corresponding biphenyl derivative 2c, by reacting the
starting material with an excess of a Na/Li/TPE reducing system
(Table 1, entries 9 and 10).
We first investigated in some details the reductive de-
sulfurization of dibenzothiophene (1a, DBT). After several attempts,
we found that the reaction of 1a with an excess of Na metal and
a catalytic amount (20% mol) of TPE resulted, after 14 h at rt, in
complete conversion of the starting material, with almost quanti-
tative formation of biphenyl 2a. A minor amount of 1,1,2,2-
tetraphenylethane, i.e., the reduction product of TPE, was also re-
covered. Interestingly, a comparable result was obtained in the
presence of a reduced amount (10% mol) of TPE (Table 1, entries 1
and 2).
A similar reaction run in the absence of TPE led to complete
consumption of the starting material, but afforded a somewhat
more complex reaction mixture containing, besides the desired
biphenyl, 2a, minor amounts of products of reduction of the aro-
matic ring(s), most probably formed by further reaction of the main
reaction product with the excess of the metal (not reported in Table
1). Additionally, it is worth noting that previously reported Na
mediated exhaustive conversions of 1a into 2a were carried out in
the presence of a large excess of Na metal in tetradecane at 150 ꢁC
during 24 h,14 or employing an excess of an almost 1:1 mixture of
Na and Li as reducing agent in refluxing dioxane.13
As a comparison, it is worth noting that the isomeric 2,8-
dimethyldibenzothiophene (1f, 2,8-Me2DBT) was readily desulfur-
ized with the Na/TPE reducing system (Table 1, entry 11), an ob-
servation further highlighting the correlation between low
reactivity and steric hindrance generated by the alkyl groups near
the sulfur atom.5
On the other side, the presence of phenyl substituents greatly
enhanced the reactivity of 2,8-diphenyldibenzothiophene (1g, 2,8-
Ph2DBT) towards our reductive desulfurization procedure. Indeed,
reduction of 1g with the Na/TPE reducing system under different
conditions led to complete conversion of the starting material as
well as to the formation, besides the expected m-quaterphenyl, 2d,
of relevant amounts of m-terphenyl as well as of products of re-
duction of the aromatic ring(s), as identified by GC/MS analyses of
the crude mixtures (not reported in Table 1). Better results were
obtained by performing the reduction with an excess of Li metal
and a catalytic amount of TPE (Table 1, entry 12).
From a mechanistic point of view, these results indicate that TPE
acts as an electron shuttle through the formation of a vic-dio-
rganometallic derivative,19 able to promote an effective electron
transfer from the metal to the substrate under homogeneous and
mild conditions, as schematically outlined in Scheme 2 where, for
simplicity, only dianionic intermediates are reported. Indeed, the
exhaustive desulfurization most probably proceeds via the in-
termediate formation of several different intermediates, including
radical anions, monoanions and neutral species, the latest species
probably forming via hydrogen atom abstraction or protonation
reactions.22,23
The above reported procedure was further extended to the re-
ductive cleavage of the oxidized dibenzothiophenes 1b and 1c. In
contrast with what was observed with 1a, we were unable to get
clean desulfurization of dibenzothiophene-5-oxide (1b, DBTO) by
reacting it with Na and TPE under different reaction conditions
(experiments not reported in Table 1). Better results were obtained
employing a Li/TPE reducing system, leading to complete conver-
sion of the starting material into biphenyl 2a (Table 1, entry 3). It is
however worth noting that the Na/TPE reducing system proved
effective in the desulfurization of dibenzothiophene sulfone (1c,
DBTO2), although complete conversion of the starting material re-
quired a quite long reaction time and the presence of an excess of
the metal (Table 1, entries 4 and 5).
Finally, we investigated the reductive desulfurization of ben-
zothiophene (1h, BT) under the above reported reaction conditions.
Indeed BT is a known contaminant of many fuels and several papers
report its desulfurization to afford styrene14 or ethylbenzene,1a
depending upon the reaction conditions.
Although the Na/TPE reducing system proved unreactive, we
were able to achieve complete desulfurization of 1h by employing

210
M. Pittalis et al. / Tetrahedron 69 (2013) 207e211
a more effective Na/Li/TPE reducing system (6/10/0.2 M ratio) with
an overall metal/1h¼16:1 M ratio (Scheme 3).
13C NMR spectra were recorded at 75 MHz in CDCl3 on a Varian VXR
300 spectrometerusing TMS as a reference. IR spectrawere recorded
on a FTIR Jacso 680 P. TLC analyses were performed on Macher-
eyeNagel silica gel pre-coated plastic sheets (0.20 mm).
4.2. Starting materials
Na/Li/TPE
THF, rt
S
Dibenzothiophene, 1a, dibenzothiophene sulfone, 1c, and benzo-
thiophene, 1h, are commercially available and were used without
2e, 80%
1h, BT
further
purification;
dibenzothiophene-5-oxide,
1b,20
4-
methyldibenzothiophene, 1d,2a 4,6-dimethyldibenzothiophene,
1e,2a 2,8-dimethyldibenzothiophene, 1f,21 and 2,8-diphenyldibenzo-
thiophene,1g,2b were synthesized according to literature procedures.
Scheme 3. Reductive desulphurization of benzothiophene, 1h.
Under these conditions, we observed formation of 1,4-
diphenylbutane 2e as the main reaction product, as well as for-
mation of minor amounts of 1,1,2,2-tetraphenylethane (see above)
and diphenylmethane. From a mechanistic point of view, 1,4-
diphenylbutane should derive by the reductive dimerization of
styrene, formed after the sulfur extrusion, and favoured by the
excess of the alkali metal present in the reaction mixture. More-
over, the formation of diphenylmethane, also occasionally detected
in very low amounts in some of the previously described reaction
mixtures, can be rationalized taking into account the already de-
scribed reaction of tetraphenylethane with an alkali metal.24
4.3. Reductive desulfurization procedure
Deep red suspensions of Na, Li or Na and Li metals in the presence
of a catalytic amount of TPE (Na/TPE, Li/TPE or Na/Li/TPE; for the
relative molar ratios, see Table 1) were prepared by vigorously stir-
ring the freshly cut metal in dry THF (10 mL) during 1 h at rt. To this
mixture, a solution of theappropriatedibenzothiophene,1, (2 mmol)
dissolved in dry THF (5 mL) was added dropwise within 30 min. The
reaction mixture was vigorously stirred at rt during 14 h, after which
time it was quenched by slow dropwise addition of H2O (15 mL). The
organic solvent was evaporated in vacuo and the resulting mixture
was extracted with Et2O or AcOEt (3ꢃ10 mL) and the organic phases
were collected, washed with H2O (10 mL), brine (10 mL), and dried
(Na2SO4). After evaporation of the solvent, the resulting mixtures
were analyzed by GC/MS, and the reaction products 2a,26 2b,26 2c,27
2d28 and 2e29 were characterized by 1H, 13C NMR and IR spectros-
copies, and by comparison with literature data.
3. Conclusions
The results reported above represent a significant improvement
with respect to previously reported reductions of dibenzothio-
phenes mediated by bulk alkali metals. Indeed, the employment of
TPE as an electron shuttle,25 i.e., the set up of homogeneous reaction
conditions, provides the means to realize the exhaustive de-
sulfurization reaction under particularly mild reaction conditions.
The effectiveness of our protocol is highlighted by the results ob-
tained in the reductive cleavage of substituted dibenzothiophenes,
thus allowing their employment as readily available starting mate-
rials in the regioselective synthesis of substituted biphenyls.
Finally, and in agreement with the findings obtained by Farmer
et al.13 under heterogeneous reaction conditions, it is worth noting
that Na, used in conjunction with Li, led to the generation of a par-
ticularly reactive reducing system even under homogeneous con-
ditions, as evidenced bythe results obtained in the desulfurization of
4,6-Me2DBT 1e a substrate usually considered highly unreactive
under HDS reaction conditions, as well as of BT 1h.
Acknowledgements
ꢀ
Financial support from the Universita di Sassari (Fondo di Ate-
neo per la Ricerca) is gratefully acknowledged. M.P. acknowledge
financial support from the Regione Autonoma della Sardegna
(Italy), trough the project ‘Promozione della Ricerca Scientifica e
dell’Innovazione Tecnologica in Sardegna’.
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