Isomerization of (.pi.-allyl)palladium complexes via nucleophilic displacement by palladium(0). A common mechanism in palladium(0)-catalyzed allylic substitution

Abstract

Treatment of (ir-allyl)palladium complexes such as 6 and 9 with Pd(PPh3)4 leads to rapid isomerization at -15 °C in tetrahydrofuran and other solvents. At 0 °C and in the presence of more than 2 equiv of triphenylphosphine per palladium, the phosphine attacks the ir-allyl group to give allylic phosphonium salts 7 with concomitant formation of a palladium(0)-phosphine complex, and isomerization of 6 is observed. Attack by PPh3 on 6 was shown to be stereospecific and to proceed with inversion. Studies of the Pd(0)-catalyzed substitution of le (X = OAc) with several different nucleophiles support the hypothesis that

Catalytic enantioselective synthesis of 2-aryl-chromenes

“…For a related system, the possibility for racemization of the π-allylpalladium complex (such as 5b ) by an intermolecular nucleophilic attack (or anti -addition) of a Pd(0) has been investigated. 15 We currently favor Path A due to the lack of observed correlation between enantioselectivity and catalyst concentration, which disfavors Path B. Additionally, the use of bis-benzoylated substrates ( vs. acetates) proceeds at the approximately the same rate with the similar levels of er and yield (see ESI † for details). More extensive investigations of this process are currently underway.…”

From mononuclear to multinuclear complexes of palladium containing unsaturated hydrocarbon ligands

“…During our continued effort to elucidate new reaction patterns of p-allylpalladium complexes relevant to mechanisms of catalytic transformations [16], we encountered the nice equilibrium system (Scheme 3) for the purpose of evaluating the electronic effect of the substituents in the allyl ligand on the relative stability of the dinuclear and mononuclear series. Scheme 3 involves a rare transfer of allyl groups between metal atoms in the different oxidation states (redox transmetallation), where the discovery of Scheme 3 was the result of extension of our earlier finding that the p-allyl group is transferred from Pd(II) to Pd(0) with inversion of configuration at the sp 3 allyl carbon (Scheme 5) [17,18].…”

Palladium-catalyzed 1,3-butadiene telomerization with methanol. Improved catalyst performance using bis-o-methoxy substituted triarylphosphines

“…Based on this earlier work which had shown that phosphine ligands can be alkylated to give phosphonium cations during the telomerization reaction through a mechanism that mirrors a step in the formation of the desired reaction product, 1, we sought to determine if this process was involved in the early loss of catalyst activity when using the more basic phosphines. It is well known that phosphines are competent nucleophiles and they have been shown to react with metal p-allyls [17]. Thus, we sought to confirm that phosphine ligands were being converted to phosphonium cations of the type [Ar 3 PCH 2 CH]CHCH 2 CH 2 CH 2 CH]CH 2 ] þ under telomerization con ditions, and determine if this process could be related to the loss of catalytic activity observed with the more basic ligands at higher methanol concentrations.…”