Superior Effect of a π-Acceptor Ligand (Phosphine−Electron-Deficient Olefin Ligand) in the Negishi Coupling Involving Alkylzinc Reagents

Abstract

Palladium-catalyzed Negishi cross-coupling involving primary and secondary alkyls, even in the presence of beta-H, can be achieved at ambient temperature using chelating ligands containing a phosphine and an electron-deficient olefin. The superior effects of the ligands were shown not only in the desired cross-coupling product yields but also in the fast reaction at mild conditions. This reaction has been also scaled up to 50 g in 0.005 mol % catalyst (20,000 TONs) at room temperature.

Phosphine–alkene ligand-mediated alkyl–alkyl and alkyl–halide elimination processes from palladium(ii)

“…25 In complex 4, the Pd coordination geometry is distortedtetrahedral, with the Md-Pd-P planes of the two P-alkene 15 ligands forming an 89.1° dihedral angle. Although the  2 -coordinated C=C bonds of 4 are longer (1.401(2) and 1.406(2) Å) than that of 3, as would be expected, the difference is only moderate due to the comparatively poor π-donor character of d 10 palladium species. The rate of reductive elimination from complex 5 is likely to be much greater than that from tetracoordinate complex 3 since prior reports have clearly established rate enhancements in reductive elimination with increasing coordination 35 number.…”

“…The bond distances to palladium fall within the range observed for other complexes of the type cis-[PdCl 2 ( 2 -C=C)(PR 3 )]. 20,21,22,23,24 Consistent with moderate Pdπ*(C=C) retro-donation, the C(2)=C(3) bond in 3 10 (1.359(3) Å) is longer than the corresponding bond in an uncoordinated analogue of 2, N-BOC-aza-benzonorbornadiene oxadisilole (1.315(4) Å). 25 In complex 4, the Pd coordination geometry is distortedtetrahedral, with the Md-Pd-P planes of the two P-alkene 15 ligands forming an 89.1° dihedral angle.…”

“…For example, a phosphine bearing a pendant electron deficient alkene has been shown to favour reductive elimination over a competing β-hydride elimination 35 reaction pathway in Negishi cross-couplings. 10,11 The system's selectivity was attributed to the presence of the strongly π-accepting alkene moiety.…”

An Electron‐Deficient Diene as Ligand for Palladium‐Catalyzed Cross‐Coupling Reactions: An Efficient Alkylation of Aryl Iodides by Primary and Secondary Alkylzinc Reagents

“…[24,54] When the catalyst generated in situ from PdCl 2 A C H T U N G T R E N N U N G (MeCN) 2 and L 1 was tested in the reaction of 1a and 1.5 equiv. of isopropylzinc chloride, the ratio between ethyl 2-isopropylbenzoate 5a and ethyl 2-propylbenzoate 6a was determined by GC to be 5:1 and the total yield was 91%.…”

“…Afterwards, we designed a novel phosphine-olefin ligand and successfully applied it in Csp 2 À Csp 3 Negishi coupling. [54] But when a secondary alkylzinc chloride was used as nucleophile, isomerization to form VIII remained problematic. To further challenge the Csp 3 -related bond formations and probe the application of olefin-type ligands in Csp 3 -related cross-coupling reactions, we synthesized a bidentate olefin ligand L 1 ( Figure 1).…”