Thiophene Oxidation and Reduction Chemistry - Thiophenes - page 270

H
H

H +

H -

H

R R R R 4-position

S S

R R

S

I

H +

H

H

H

R R

R R

S

S

II

2,5-dihydrothiophene
not observed.

H -

R R

S

Scheme 111 The general mechanism of ionic hydrogenation of thiophenes [ 126 , 127 ]

Table 2 Ionic hydrogenation of thiophenes with CF 3 CO 2 H · HSiEt 3
Thiophene Product Ratio (thiophene:silane:acid) Time Yield (%)

1:2:8 15 80

S S

R R

1:2:9 80 60

S S

CH 3 CH 3

1:3:13 20 75

S S

Ph Ph

1:2:8 25 80

S S

H 3 C CH 3 H 3 C CH 3

S S

1:2:20 50 0

Ph Ph Ph Ph

1:3:11 35 20 + 30

S S S

+

1:2:9 35 65

S S

COOMe COOMe

5 5

1:2:7 40 65

S S

COOH COOH

4 4

the hydrogenation; however, when these groups were positioned at the terminus of
2-substituted side-chains, no effect was observed and the ionic hydrogenation
reaction proceeded successfully (Table 2 ).
Benzo[ b ]thiophene itself reacts slowly under ionic hydrogenation conditions to
give 2,3-dihydrobenzo[ b ]thiophene, whereas 2-alkyl or 3-alkyl benzo[ b ]thiophenes
are readily reduced. As shown in Table 3 , the ionic hydrogenation of

3-methylbenzo[ b ]thiophene is a little faster than that of 2-methylbenzo[ b ]

thiophene.