Enantioselective Synthesis of Six-Membered Palladacycles Having Metal-Bound Stereogenic Carbons:  Isolation and Reactivity of Palladacycles Containing Readily Accessible β-Hydrogens

Abstract

Five enantiopure palladacycles containing palladium bonded to a stereogenic carbon and an N-coordinated oxindole were synthesized by the reaction of alkenyl aryl triflates 2 and 9 with Pd(0) bisphosphine complexes. Two palladacyclic complexes, 3beta and 10alpha, were characterized by single-crystal X-ray crystallography. The reactivity of neutral palladacycles 3beta and 10beta was studied in detail. These unusual palladium alkyls, which have three accessible beta-hydrogens, are thermally stable at temperatures as high as 120 degrees C. At higher temperature, or at room temperature in the presence of weak acids, these complexes epimerize at the stereogenic carbon bonded to palladium. The mechanism of the acid-promoted epimerization was studied in detail. During this epimerization, cationic palladium alkyls 13/14 and 33 and cationic palladium hydride alkene complexes 31 and 32 are in rapid equilibrium.

Syntheses and characterizations of methylpalladium complexes bearing a biphenyl-based bulky phosphine ligand: Weak interactions suggested by NBO and QTAIM analyses

“…4 [46]. The Pd(1)-C(17) distance (2.955(4) Å ), which is the shortest distance among those between Pd and carbon atoms of the terminal phenyl ring, is shorter than the sum of van der Waals radii (3.30 Å ) [47].…”

Asymmetric organic synthesis with stoichiometric transition metal complexes

“…In a subsequent study, the mechanism of epimerization of the palladium bonded stereocenter in the isolated palladacycles was investigated in more detail (Scheme 10). 17 Palladacycles bearing (R)-BINAP and (R,R)-BDPP ligands were characterized by NMR spectroscopic methods, and in two cases by X-ray crystallographic analyses. The epimerization took place under thermal conditions (above 120 o C), or at room temperature in the presence of weak acids.…”

Directed Mizoroki–Heck Reactions

“…It remains Scheme 12 Carbamate-directed, regioselective Mizoroki-Heck reaction by Tamaru [55] Directed Mizoroki-Heck Reactions 183 unclear whether it is the oxygen (shown) or the nitrogen (not shown) functioning as the donor under the basic reaction conditions. Overman isolated stable σ -alkylpalladium(II) complexes, which are stabilized by the nitrogen of a deprotonated N-monosubstituted amide functionality [57][58][59].…”