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.
We have shown that the BuLi/LiDMAE reagent promotes the clean and regioselective C2 lithiation of 3- and 4-chloropyridines, while other reagents such as LDA or BuLi/TMEDA lead to classical ortho lithiation products or mixtures of regioisomers. The method was successfully applied to the preparation of various reactive 2,3- and 2,4-disubstituted pyridines.
A method for chemoselective reduction of 2-pyridyl ketones and related N-heteroaryl compounds catalyzed by cobalt stearate using DMF as a hydride source is developed. The ketone substrate is activated by chelation with cobalt, which makes the present method highly chemoselective. A possible reaction mechanism is proposed on the basis of control experiments.
TMSCH2Li–LiDMAE: a new nonnucleophilic reagent for C-2 lithiation of halopyridines
作者:Abdelatif Doudouh、Philippe C. Gros、Yves Fort、Christopher Woltermann
DOI:10.1016/j.tet.2006.04.059
日期:2006.6
A new superbasic reagent has been discovered by combining TMSCHL(2)i and LiDMAE in hexane. This reagent was found highly efficient for the C-2 lithiation of sensitive chloro- and fluoropyridines. The metallation occurred chemo- and regioselectively at 0 degrees C avoiding the nucleophilic addition or substrate degradation commonly obtained with other alkyllithiums even at lower temperatures. (c) 2006 Elsevier Ltd. All rights reserved.