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2005
Metallation
O 0279
16- 055
Unusual tBuLi Induced Ortholithiation versus Halogen—Lithium Exchange in
Bromopyridines: Two Alternative Strategies for Functionalization. — The reaction of 3-bromopyridine derivatives (I) and (V) with various lithiating agents is studied.
The reaction with nBuLi proceeds with the expected bromine—lithium exchange in
every case, while an unexpected lithiation at position 4 is observed with the use of
tBuLi. The lithiation—substitution at position 4 proceeds cleanly for the 2-chloro-5-bromopyridine (I) and with a wide range of electrophiles due to the enhanced acidity
of 4-position. In contrast, for derivatives (V) the balance between Br—Li exchange
and ortholithiation is strongly influenced by the nature of electrophile, the order of
addition of reagents and the temperature. Therefore, a different reaction mechanism is
proposed for these substrates. — (PIERRAT, P.; GROS*, P.; FORT, Y.; Synlett 2004,
13, 2319-2322; Synth. Organomet. React., CNRS, Univ. Henri Poincare,
F-54506 Vandoeuvre-les-Nancy, Fr.; Eng.) — Mischke
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