32 Palladium in Heterocyclic Chemistry
in the presence of a base. The difference in requirements is such that highly selective couplings can be carried out, as is
clearly demonstrated by the use of a stannyl arylboronate (under neutral, non-polar conditions) to prepare a boronic
acid ester.
High-yielding Suzuki-Miyaura substitutions of halogen in substrates containing both a halogen and a boronic acid
are possible using N-methyliminodiacetic acid (MIDA) to form a protected boronate, which resists coupling under
anhydrous conditions. Following the cross-coupling, the boronic acid can be released under mild conditions, ready for
further reaction.
Negishi cross-couplings on 2-bromo-5- and -6-tri-n-butylstannyl pyridines are possible due to the relatively high reac-
tivity of -bromopyridines, which allows a relatively low temperature to be used.