Pd-Catalyzed regioselective synthesis of 2,6-disubstituted pyridines through denitrogenation of pyridotriazoles and 3,8-diarylation of imidazo[1,2-<i>a</i>]pyridines
6-disubstituted pyridines from pyridotriazoles throughpalladium-catalyzedaerobic oxidative denitrogenative reactions has been described. Denitrogenation of arylated pyridotriazoles generates metal-carbene intermediates in situ and provides selectively 6-aryl-2-benzoylpyridines. The same conditions have been extended to regioselective C-3 and C-8 diarylation of several imidazo[1,2-a]pyridines.
已经描述了通过钯催化的需氧氧化脱氮反应由吡啶三唑合成2,6-二取代的吡啶。芳基化吡啶三唑的脱氮原位生成金属-卡宾中间体,并选择性地提供6-芳基-2-苯甲酰基吡啶。相同的条件已经扩展到几种咪唑并[1,2- a ]吡啶的区域选择性C-3和C-8二芳基化。
Palladium-N-heterocyclic carbene an efficient catalytic system for the carbonylative cross-coupling of pyridine halides with boronic acids
Carbonylative cross-coupling of different pyridyl halides with various boronic acids was studied using catalytic systems constituted of N-heterocyclic carbene type ligands and palladium. These systems easily obtained in situ from the corresponding imidazolium salt and palladium acetate appear more efficient toward bromopyridines than catalysts based on hindered and basic alkylphosphines such as tricyclohexylphosphine
The use of N-heterocyclic carbene-type ligands with palladium catalysts allows the activation of chloropyridines and chloroquinoline towards carbonylative cross-coupling with phenylboronic acid for the synthesis of unsymmetrical biaryl ketones.
The main side-products arise from direct, non-carbonylative cross-coupling. Under optimized conditions, benzoylpyridines are recovered in high yields (80–95%). The order of reactivity decreases from iodo- to bromopyridines and from 2-, 4- to 3-substituted halopyridines. The reactivity of dihalopyridines has been investigated; 2,6-dibromopyridine (7) and 3,5-dibromopyridine (11) are selectively transformed
The proper choice of solvent, catalyst precursor and CO pressure enables the easy and selective transformation of mono- and dihalopyridines into phenyl pyridyl ketones in 81–95% yields.