Unconventional Site Selectivity in Palladium-Catalyzed Cross-Couplings of Dichloroheteroarenes Under Ligand-Controlled and Ligand-Free Systems

Overview

Journal J Org Chem

Publisher American Chemical Society

Specialty Chemistry

Date 2022 May 18

PMID 35584051

Authors

Jacob P Norman

Nathaniel G Larson

Emily D Entz

Sharon R Neufeldt

Affiliations

Soon will be listed here.

Abstract

Halides adjacent to nitrogen are conventionally more reactive in Pd-catalyzed cross-couplings of dihalogenated -heteroarenes. However, a very sterically hindered -heterocyclic carbene ligand is shown to promote room-temperature cross-coupling at C4 of 2,4-dichloropyridines with high selectivity (∼10:1). This work represents the first highly selective method with a broad scope for C4-coupling of these substrates where selectivity is clearly under ligand control. Under the optimized conditions, diverse substituted 2,4-dichloropyridines and related compounds undergo cross-coupling to form C4-C and C4-C bonds using organoboron, -zinc, and -magnesium reagents. The synthetic utility of this method is highlighted in multistep syntheses that combine C4-selective cross-coupling with subsequent nucleophilic aromatic substitution reactions. The majority of the products herein (71%) have not been previously reported, emphasizing the ability of this methodology to open up underexplored chemical space. Remarkably, we find that ligand-free "Jeffery" conditions enhance the C4 selectivity of Suzuki coupling by an order of magnitude (>99:1). These ligand-free conditions enable the first C5-selective cross-couplings of 2,5-dichloropyridine and 2,5-dichloropyrimidine.

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