Reductive Couplings of 2-Halopyridines without External Ligand: Phosphine-Free Nickel-Catalyzed Synthesis of Symmetrical and Unsymmetrical 2,2′-Bipyridines

Abstract

An unexpectedly facile synthetic approach for symmetrical and unsymmetrical 2,2'-bipyridines through the Ni-catalyzed reductive couplings of 2-halopyridines was developed. The couplings were efficiently catalyzed by 5 mol % of NiCl2·6H2O without the use of external ligands. A variety of 2,2'-bipyridines including caerulomycin F have been efficiently synthesized.

Nickel-Catalyzed Reductive Couplings

“…Although no details on the mechanism were addressed, a possible oxidative addition of Ar 0 X to Ar-Ni I is plausible according to previous stoichiometric studies [24,25]. Application of Ni-catalyzed reductive biaryl synthesis to bipyridine derivatives was recently revealed by Weix, Duan, and Iranpoor (Scheme 34) [91][92][93]. The ligand-free conditions allow the construction of 2,2 0 -bipyridine in high yields.…”

“…The ligand-free conditions allow the construction of 2,2 0 -bipyridine in high yields. Duan also demonstrated cross-coupling between two different pyridyl halides by setting one of electrophiles in excess [92]. Iranpoor indicated that 1,3,5-triazine could be utilized for activation of aryl-O bonds [93].…”

Enabling the Cross-Coupling of Tertiary Organoboron Nucleophiles through Radical-Mediated Alkyl Transfer

“…Representative ligands from the Ni cross-coupling literature ranging from phosphines, 18 pyridines, 19 terpyridines, 20 amino alcohols, 21 N -heterocyclic carbenes, 6 olefins, 22 to 1,3-diketones 23 were all introduced to both a Ni(0) and Ni(II) source in parallel. The reactions were then followed by UPLC to observe the desired coupling product (see Supporting Information for full details).…”

In Situ Solid‐State Generation of (BN)2‐Pyrenes and Electroluminescent Devices

“…[8] Molecules A2 a and A2 b were prepared via double lithiation of 3,3'-dimethyl-2,2'bipyridine, [10] followed by the addition of two equivalents of the appropriate BAr 2 Fr eagent. Theb ulky aryl groups on the boron center were chosen to enhance the chemical stability and reduce excimer emission of the BN-pyrenes,a sw ell as ensure high thermal stability of the precursors.C ompounds A2 a and A2 b were fully characterized by NMR spectroscopy and HRMS.T he crystal structure of A2 a was determined by X-ray diffraction analysis and is shown in Figure 2.…”

“…Interestingly,t he absorption and fluorescent peaks of B2 a and B2 b have very small Stokes shifts (ca. 10 nm) as shown in Figure 3, which is clearly due to their rigid structure and the symmetry-allowed S 0 !S 1 transitions in these BN-pyrenes.Incontrast to pyrene which is highly prone to excimer emission, [10] B2 a and B2 b do not display excimer emission, which is credited to the bulky aryl group on each boron atom. Compared to the green-emissive BN-phenanthrene B1 which has an emission peak at around 495 nm, [8a] the maxima of the absorption and fluorescent bands of B2 a and B2 b are red-shifted by ca.…”