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Scheme 5
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Scheme 6 Direct arylations of 2-(trimethylsilyl)-5-methylthiophene.
same starting compound. This reaction shows that the inhibited
desilylation is not limited to the use of TMS substituent.
The regiocontrol of direct arylations is an important issue. As
the desilylation of aromatics is relatively easy, the presence of
silyl substituents on such compounds might be useful to block
specific highly reactive carbons to provide alternative regioisomeric
arylated compounds. In order to demonstrate the potential of silyl
substituents as blocking groups, we examined the reactivity of
2-(trimethylsilyl)-5-methylthiophene (Scheme 6). Interestingly, in
the course of this reaction the arylation was found to be
regioselective in favour of the formation of the 4-arylated
compounds 23 and 24. After desilylation, this reaction gives a
simple access to 3-arylated thiophenes which cannot be obtained
by direct arylation of 2-substituted thiophenes.7e,9k As the
hindrance of silyl groups14 can be easily modulated, this
method has potential to inhibit the arylation at the positions
adjacent to the silyl substituents of aromatics, allowing highly
regioselective direct arylations.
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It should be noted that TMS groups on thiophene can be
easily transformed into other functional groups.5 For example,
in the treatment of 16 with N-bromosuccinimide, 2-bromo-
5-naphthalen-1-yl-thiophene 25 was obtained in 92% yield.5b
In summary, we have demonstrated that silyl-substituted
thiophenes can be directly arylated with aryl bromides without
desilylation, using the simple Pd(OAc)2 precatalyst but in the
presence of the profitable influence of a diphosphine ligand
(dppb) for both conversion and desilylation inhibition. It
should be noted that a wide range of functions on the aryl
bromide are tolerated. Such a functional group tolerance
should allow the easy modification of the structures and in
consequence of the physical properties of materials.
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L. C. is grateful to the Chinese Scholarship Council for a
PhD grant.
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This journal is The Royal Society of Chemistry 2011