First example of opening and hydrogenation of 2,3-dihydrobenzo[b]thiophene
to 2-ethylthiophenol assisted by a soluble metal complex
Claudio Bianchini,* Andrea Meli, Werner Oberhauser and Francesco Vizza
Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, ISSECC-CNR, 50132
Firenze, Italy. E-mail: bianchin@fi.cnr.it
Received (in Cambridge, UK) 4th February 1999, Accepted 8th March 1999
1
Displacement of THF by dihydrobenzo[b]thiophene
ligand than THF. Very few h -DHBT complexes have been
reported,7 but their existence has frequently been claimed in
homogeneous and heterogeneous catalytic hydrogenation cy-
cles of BT.1,2,3,7
1
(DHBT) in [(triphos)Ir(H)2(THF)]BPh4 yields the h -S-
1
DHBT adduct [(triphos)Ir(H)2(h -DHBT)]BPh4 which re-
acts in THF at room temperature with KOBut to give the
1
1
neutral 2-vinylthiophenolate derivative (triphos)Ir(H)2{h -
As shown in Scheme 2, the reaction of the h -S-DHBT
o-S(C6H4)CHNCH2} via C2–S bond cleavage; the latter
compound is hydrogenated under mild conditions (2 bar H2,
80 °C) to the 2-ethylthiophenolate derivative (triphos)-
Ir(H)2{o-S(C6H4)Et} and under harsh conditions (30 bar H2,
160 °C) to free 2-ethylthiophenol and (triphos)IrH3.
adduct 2 in THF with an equivalent amount of KOBut at room
temperature,8 followed by treatment with a high pressure of H2
at 160 °C yields the hydrogenolysis product ETP and converts
all the iridium precursor to the trihydride (triphos)IrH3 3.9 The
overall process was studied in situ by multinuclear NMR
spectroscopy in THF-d8 using a sapphire high-pressure NMR
tube, initially under nitrogen and then under H2.3 The strong
base has been found to transform the DHBT ligand in 2 into a
2-vinylthiophenolate group yielding the neutral complex (tri-
The hydrodesulfurization (HDS) of thiophenic molecules
occurring in the course of the hydrotreating catalysis of
petroleum feedstocks is a complex process whose mechanism is
still quite speculative.1 As is outlined in Scheme 1 for the model
substrate benzo[b]thiophene (BT), the hydrogenation of thio-
phenes over heterogeneous catalysts may lead to the formation
of hydrocarbons and H2S via two principal mechanisms.
Kinetic studies at elevated H2 pressures suggest that the
reactions go through hydrogenated intermediates (Scheme 1,
path a), especially with higher analogs of thiophene such as BT
in which the electron density is less localized on the sulfur
atom.2 The prehydrogenation path has never been confirmed by
the homogeneous modeling studies which are all consistent with
the alternative hydrogenolysis route (Scheme 1, path b).3
The present paper constitutes a preliminary account of the
first example of opening and hydrogenation of 2,3-dihy-
drobenzo[b]thiophene (DHBT) to 2-ethylthiophenol (ETP)
assisted by a transition metal complex. C–S bond cleavage of
tetrahydrothiophene (THT) has previously been found to occur
on polymetallic species.4,5 Notably, the C–S bond cleavage of
THT in Cl3W(m-THT)WCl3 was obtained by reaction with a
nucleophile that remains attached to the C2 carbon atom.4
1
phos)Ir(H)2{h -o-S(C6H4)CHNCH2} 4.† The tube was then
pressurized with 2 bar of H2. No reaction occurred at room
temperature. Only at 80 °C, the 1H NMR spectrum showed the
signals of the vinyl hydrogens of 4 to decrease in intensity and
the 31P NMR spectrum showed the appearance of a new AM2
spin system. Within 15 h at 80 °C, all 4 converted to the known
2-ethylthiophenolate derivative (triphos)Ir(H)2{o-S(C6H4)Et}
5.10 Both 4 and 5 were isolated in the solid state by simply
scaling up the NMR conditions. The hydrogen pressure in the
tube was increased to 30 bar and the temperature was raised to
160 °C to observe a further chemical transformation: free
2-ethylthiophenol was formed quantitatively (1H NMR,
GCMS)3d together with the known trihydride (triphos)IrH3 3.9
The opening of the DHBT ligand by KOBut most likely
occurs via an E2 elimination as shown in Scheme 3(a).11 Indeed,
an E2 elimination mechanism has been proposed to account for
the hydrodesulfurization (HDS) of DHBT over the surface of
real heterogeneous catalysts [Scheme 3(b)].12
The Ministero dell’Ambiente of Italy (contract PR1/C) is
gratefully acknowledged for financial support.
1
The iridium complex [(triphos)Ir(H)2(h -DHBT)]BPh4 2 has
been synthesized by adding a slight excess of DHBT to a
6
tetrahydrofuran (THF) solution of [(triphos)Ir(H)2(THF)]BPh4
1 [triphos = MeC(CH2PPh2)3].† At room temperature, the
reaction is quantitative even in THF as DHBT is a much better
+
SH
P
P
P
H
H
Ir
S
P
Ir
H
i
ii
+
P
P
H
H
S
30 bar H2
160 °C
2
H2
H2
20 °C
a
b
ButOK
–THF
ButOH
H2
+
+
H2S
P
P
Ir
S
P
P
H
P
H
H
S
SH
P
Ir
S
P
P
H
H
H
2 bar H2
80 °C
P
Ir
+
H2
H2
O
S
1
4
5
SH
Scheme 1 Principal mechanisms proposed for the HDS of benzo[b]thio-
Scheme 2 Reagents and conditions: i, ButOK, 20 °C; ii, 30 bar H2,
phene over heterogeneous catalysts.
160 °C.
Chem. Commun., 1999, 671–672
671
Bianchini, Claudio
