Chemistry Reference
In-Depth Information
complex
H
2
S
S
Scheme 133
Model hydrogenation reaction of benzo[
b
]thiophene by a transition-metal complex
[
147
-
151
]
Study of the homogeneous catalytic hydrogenation of BT to DHBT in the
presence of [Rh(Cp*)(MeCN)
3
]
2
utilized deuteration experiments and detailed
NMR studies to elucidate a catalytic cycle [
147
]. The main cycle starts with the
2
-
coordination of BT to a rhodium polyhydrido species, followed by hydride transfer,
hydrogenation, finally generating DHBT.
Further study of the homogeneous hydrogenation of BT to DHBT using the
precatalysts [M(COD)(PPh
3
)
2
]PF
6
,M
¼
Rh, Ir] was conducted [
148
]. In the postu-
lated catalytic cycle, BT initially binds
η
1
-S to a M(III) dihydride (M
η
¼
Ir, Rh). Then
1
-S-DHBT)(PPh
3
)
2
]PF
6
, which is
likely in equilibrium with the isomeric dihydrido-
dissociation of one BT molecule yields Ir(H)
2
(
η
2
-BT complex. Next, selective
hydrogenation of the C-2-C-3 double bond occurs through a hydrido-2-benzo[
b
]
thienyl species, yielding intermediates containing the hydrogenated product [M(
η
1
-
S-DHBT)(PPh
3
)
2
]
+
. Displacement of DHBT by a new molecule of BT produces
[M(
η
1
-S-BT)(PPh
3
)
2
]
+
, which reacts with hydrogen to restart the cycle.
Regioselective hydrogenation of BT to DHBT with ruthenium(II) tris-
acetonitrile complex [(triphos)Ru(NCMe)
3
](BPh
4
)
2
as catalyst in homogeneous
phase takes place under mild reaction conditions [
149
].
It was found that RuHCl(TPPMS)
2
(THQ)
2
and RuHCl(TPPMS)
2
(An)
2
can be
used as catalyst in a liquid-biphase hydrogenation of BT. The initial catalyst
conditions were
T
η
136
C, P (H
2
)
35 atm, 1:1 mixture of water/decalin. Under
these experimental conditions, the only product detected was DHBT with no
evidence of C-S bond cleavage [
150
].
The complex OsH(CO)(
¼
¼
3
-OCOCH
3
)(PPh
3
)
2
also catalyzes reduction of BT to
DHBT under mild conditions. The proposed catalytic cycle, which is based on
kinetic experiments, is shown in Scheme
134
[
151
]. Among the several elementary
reactions, oxidative addition of hydrogen to
127
forms the dihydrido species Os
(H)
2
(CO)(
κ
1
-C
8
H
7
S)(PPh
3
)
2
, which is considered to be the rate-
determining step of the catalytic cycle. The fast reductive elimination of the
2,3-DHBT product regenerates the catalyst and restarts the cycle.
Optically active 2,3-dihydrobenzo[
b
]thiophene and tetrahydrothiophene deri-
vatives play a crucial role in organic, biological, and medicinal chemistry and are
widely distributed. The first homogeneous asymmetric hydrogenation of substituted
thiophenes and benzo[
b
]thiophenes [
152
] utilized a Ru-NHC complex generated in
situ from [Ru(cod)(2-methylallyl)
2
] and SINpEt · HBF
4
as the catalyst, the
2-substituted benzo[
b
]thiophene being converted into the corresponding
2,3-dihydrobenzo[
b
]thiophene smoothly with highly optical activity and in mode-
rate to excellent yields (Scheme
135
).
Inspired by the successful hydrogenation of substituted benzo[
b
]thiophenes, this
catalytic system was applied to the reduction of mono- or disubstituted thiophenes
1
-OCOCH
3
)(
κ
η
