Pd and Ni complexes of a novel vinylidene β‐diketimine ligand: Their application as catalysts in Heck coupling and alkyne trimerization

Abstract

A β-diketimine ligand with vinylidene substitution at γ-carbon, CH 2 C(CH 3 CNAr) 2 (Ar = 2,6-diisopropylphenyl) (L 2 ), was synthesized by treating β-diketimine H 2 C(CH 3 CNAr) 2 with n-BuLi followed by paraformaldehyde. L 2 formed the homobimetallic ether-bridged β-diketiminate complex [O{(CH 2 -β-diketiminate) Pd(OAc)} 2 ] (1) with (PdOAc) 2 . It also gave complexes [L 2 PdCl 2 ] (2) and [L 2 NiBr 2 ] (3) when treated with PdCl 2 (CH 3 CN) 2 and NiBr 2 (dimethoxyethane), respectively. All the compounds were characterized using 1 H/ 13 C NMR spectroscopy and single-crystal X-ray diffraction studies. The catalytic activity of Pd and Ni complexes 1, 2 and 3 was explored in Heck coupling and alkyne trimerization reactions and it was found that they are very good catalysts. The results are reported in detail.

Enhanced Catalytic Activity of Nickel Complexes of an Adaptive Diphosphine–Benzophenone Ligand in Alkyne Cyclotrimerization

“…The preference for electron poor alkynes is commonly observed in Ni-catalyzed cyclotrimerization of terminal alkynes. 37m,37v−37y In comparison with the Ni-catalysts reported in literature, 37 complex p -tol 1 is a competitive catalyst in terms of activity and regioselectivity (from 90:10 to 97:3). A catalyst loading of p -tol 1 of 0.05 mol % mediated the cyclotrimerization of ethyl propiolate ( 11 ) into triethylbenzene-1,2,4-tricarboxylate ( 11a ) in 87% isolated yield, reaching a TON of 1740 after 16 h.…”

“…At 50 °C, 4-ethynylanisole ( 13 ) produces the desired 1,2,4-cyctrotrimerization product in a lower yield than the weakly electron-withdrawing analogue 1-ethynyl-4-fluorobenzene ( 12 ), supporting a preference for electron-withdrawing substrate (entries 2 and 3). The preference for electron poor alkynes is commonly observed in Ni-catalyzed cyclotrimerization of terminal alkynes. ,− In comparison with the Ni-catalysts reported in literature, complex p ‑tol 1 is a competitive catalyst in terms of activity and regioselectivity (from 90:10 to 97:3). A catalyst loading of p ‑tol 1 of 0.05 mol % mediated the cyclotrimerization of ethyl propiolate ( 11 ) into triethylbenzene-1,2,4-tricarboxylate ( 11a ) in 87% isolated yield, reaching a TON of 1740 after 16 h.…”

“…Metal catalyzed cyclotrimerization is an elegant method for the formation of cyclic frameworks, particularly convenient when the desired product is not accessible via traditional aromatic substitution reactions. Many transition metal systems have been developed (e.g., Ti, Fe, Co, Ni, Mo, Ru, Rh, Pd, Ir). Literature precedents for the nickel-catalyzed conversion of alkynes to substituted benzene regio-isomers are known, both from an intramolecular ,− ,,,, and intermolecular approach. ,,,− ,,,,− For the [2 + 2 + 2] intermolecular alkyne cyclotrimerization, it starts from the early example by Reppe et al to the combination of divalent nickel halide salts and reductants , or even activated Ni-particles .…”

“…33 Metal catalyzed cyclotrimerization is an elegant method for the formation of cyclic frameworks, particularly convenient when the desired product is not accessible via traditional aromatic substitution reactions. Many transition metal systems have been developed (e.g., Ti, 34 Fe, 35 Co, 36 Ni, 37 Mo, 38 Ru, 39 Rh, 40 Pd, 41 Ir 42 ). Literature precedents for the nickel-catalyzed conversion of alkynes to substituted benzene regio-isomers are known, both from an intramolecular 36c,37g−37l,37n,37p,37r,37s and intermolecular approach.…”

Ni-Catalyzed Larock Indenone Annulation with Aliphatic- and Silyl-Substituted Alkynes Supported by Mechanistic Analysis

“…A common byproduct in the reactions with esters 1c and 1d was the corresponding isocoumarin (lactone). In comparison, Pd catalysts typically supply isocoumarins as the primary products in similar annulation reactions with methyl 2-iodobenzoate. , Alkyne cyclotrimerization products were frequently observed, depending on the relative steric hindrance of the alkyne . The solvent played a critical role in successful catalysis.…”