An X-ray Study of the Effect of the Bite Angle of Chelating Ligands on the Geometry of Palladium(allyl) Complexes:  Implications for the Regioselectivity in the Allylic Alkylation

Abstract

X-ray crystal structures of a series of cationic (P-P)palladium(1,1-(CH(3))(2)C(3)H(3)) complexes (P-P = dppe (1,2-bis(diphenylphosphino)ethane), dppf (1,1'-bis(diphenylphosphino)ferrocene), and DPEphos (2,2'-bis(diphenylphosphino)diphenyl ether)) and the (Xantphos)Pd(C(3)H(5))BF(4) (Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) complex have been determined. In the solid state structure, the phenyl rings of the ligand are oriented in the direction of the nonsymmetrically bound [1,1-(CH(3))(2)C(3)H(3)] moiety. An increase of the bite angle of the chelating ligand results in an increase of the cone angle. In complexes containing ligands having a large cone angle, the distances between the phenyl rings and the allyl moiety become small, resulting in a distortion of the symmetry of the palladium-allyl bond. In solution, two types of dynamic exchange have been observed, the pi-sigma rearrangement and the apparent rotation of the allyl moiety. At the same time, the folded structure of the ligand changes from an endo to an exo orientation or vice versa. The regioselectivity in the palladium-catalyzed allylic alkylation of 3-methyl-but-2-enyl acetate is determined by the cone angle of the bidentate phosphine ligand. Nucleophilic attack by a malonate anion takes place preferentially at the allylic carbon atom having the largest distance to palladium. Ligands with a larger cone angle direct the regioselectivity to the formation of the branched product, from 8% for dppe (1) to 61% found for Xantphos (6). The influence of the cone angle on the regioselectivity has been assigned to a sterically induced electronic effect.

Palladium-catalyzed desulfinylative C–C allylation of Grignard reagents and enolates using allylsulfonyl chlorides and esters

“…In all cases the linear product 5a was favored (product of no allylic rearrangement). The regioselectivity (product ratio 5a/6a) was the best (96:4, yield 52%) using Pd 2 (dba) 3 and xantphos ligand [66][67][68][69][70][71] (entry 6). The best regioselectivity (95:5) and yield (75%) were realized using PdCl 2 (PhCN) 2 as catalyst, for a reaction in THF at room temperature (entry 16).…”

NMR Spectroscopic Studies of Palladium(II) Complexes of Bidentate Diphenylphosphane Ligands with Acetate and Tosylate Anions: Complex Formation and Structures

“…As can be seen in Entry 14 (L14, o-H vs. 94°). [35] This imposes a steric constraint on the adjacent coordination sites, [36,37] thus disfavouring bischelate complex formation.…”

Apparent Allyl Rotation and Pd−N Bond Rupture in Allylpalladium Complexes with N‐Donor Ligands − Evidence of an Associative Mechanism

“…Two main mechanisms have been proposed for the apparent allyl rotation: (i) associative mechanisms [20,28,29,32,35] that involve five-coordinate intermediates (coordination of the solvent, the anion or other molecules) that can undergo allyl pseudorotation, and (ii) dissociative mechanisms [18,19,21Ϫ23,25,26,31,33] with formation of T-shaped three-coordinate intermediates after dissociation of monodentate or didentate (partial dissociation) ligands. Consequently, one question that is open to debate is whether the apparent allyl rotation in derivatives with N-donor ligands involves PdϪN bond breaking or not.…”