Homoleptic zincate-promoted room-temperature halogen-metal exchange of bromopyridines.

ABSTRACT

Homoleptic lithium tri- and tetraalkyl zincates were reacted with a set of bromopyridines. Efficient and chemoselective bromine-metal exchanges were realized at room temperature with a substoichiometric amount of nBu(4)ZnLi(2)·TMEDA reagent (1/3 equiv; TMEDA=N,N,N',N'-tetramethylethylenediamine). This reactivity contrasted with that of tBu(4)ZnLi(2)·TMEDA, which was inefficient below one equivalent. DFT calculations allowed us to rationalize the formation of N···Li stabilized polypyridyl zincates in the reaction. The one-pot difunctionalization of dibromopyridines was also realized using the reagent stoichiometrically. The direct creation of C-Zn bonds in bromopyridines enabled us to perform efficient Negishi-type cross-couplings.