Modular Palladium Bipyrazoles for the Isomerization of Allylbenzenes – Mechanistic Considerations and Insights into Catalyst Design and Activity, Role of Solvent, and Additive Effects

Abstract

The catalytic activity of novel bidentate N,N-chelated palladium complexes derived from electron excessive, backbone fused 3,3′-bipyrazoles in the selective isomerization of terminal arylpropenoids and 1-alkenes is described. The catalysts are easily modified by appropriate wing tip substitution, while maintaining the same bulky, rigid unreactive aliphatic backbone. Eleven novel palladium complexes with different electronic and steric properties were investigated. Their performance in the palladium(II)-catalyzed isomerization of a series of substituted allylbenzenes was evaluated in terms of electronic as well as steric effects. Besides the clear finding of a general trend towards higher catalyst activity with more electron-donating properties of the coordinated N,N-bidentate ligands, we found that the catalytic process strongly depends on the choice of solvents and additives. Extensive solvent screening revealed that reactions run best in a 2:1 toluene-methanol mixture, with the alcohol employed being a crucial factor in terms of electronic and steric factors. A reaction mechanism involving a hydride addition–elimination mechanism starting with a palladium hydride species generated in situ in alcoholic solutions, as corroborated by experiments using deuterium labeled allylbenzene, seems to be most likely. The proposed mechanism is also supported by the observed reaction rate orders of κ_obs[cat.]≈1 (0.94), κ_obs [substrate]=0.20→1.0 (t→∞) and κobs [methanol]=−0.51 for the isomerization of allylbenzene. Furthermore, the influence of acid and base, as well as the role of the halide coordinated to the catalyst, are discussed. The system catalyzes the isomerization of allylbenzenes very efficiently yielding high E:Z selectivities under very mild conditions (room temperature) and at low catalyst loadings of 1 mol% palladium even in unpurified solvents. The integrity and stability of the catalyst system were confirmed by multiple addition reaction cycles, successive filtration and isolation experiments, and the lack of palladium black formation

Pd‐Catalyzed Hydrogen‐Transfer Reactions from Alcohols to C=C, C=O, and C=N Bonds

“…According to deuterium‐labeling‐based mechanistic investigations, the isomerization of allylbenzenes under Trapp's experimental conditions [Equation (26)] occurs, as in the previous section, through the addition of a palladium hydride to the C=C bond of the substrate 48. A simplified mechanism is depicted in Scheme .…”

Platinum–(phosphinito–phosphinous acid) complexes as bi-talented catalysts for oxidative fragmentation of piperidinols: an entry to primary amines

“…The terminology "metal-catalyzed hydrogen transfer methodology" refers to any chemical process relying on the generation of a metal hydride from a hydrogen source and its transfer to an unsaturated hydrogen acceptor. 1,2 A substantial number of consistent versions of this concept have been reported and implemented with palladium catalysts for alcohols oxidation, [3][4][5] hydrogenation, 6,7 C-C, 8,9 C-N, 10 C-halogen bond formation, 11 double bond isomerization, 12 cross aldolisation 13 or else amination 14 with a view to increasing the molecular complexity. The critical feature of metal-catalyzed hydrogen transfer methodology lays in the design of suitable ligands, able to induce the formation of a reactive metal hydride intermediate.…”

Synthesis and Crystal Structure of C1-Symmetric 3,3′-Bi(1,1′-dinaphthyl-camphopyrazole)

“…Optical rotations were measured on a Perkin-Elmer 341 polarimeter. (1,1 -dinaphthyl-camphopyrazole) [Systematic Name: (1,7 ,8,8,8 ,4 ,5,5 ,6,6 ,7,…”

“…However, chiral pyrazole ligands derived from camphor coordinated to transition metals and the catalytic applications of the latter have been less explored [4]. Recently, chiral 3,3 -bipyrazole compounds have been proposed as important ligands that exert electronic and steric stronger influence to the metal center [5,6]. These ligands were prepared by condensation reaction between a bis-1.3-diketone previously reported [7,8] and hydrazine hydrate, followed by aryl-or alkylation.…”