Rate and Mechanism of the Oxidative Addition of Vinyl Triflates and Halides to Palladium(0) Complexes in DMF

Abstract

In a coordinating solvent such as DMF, the fast oxidative addition of vinyl triflates to Pd0(PPh3)4 performed under stoichiometric conditions gives the cationic complexes [(η1-vinyl)Pd(PPh3)2(DMF)]+TfO-, which have been characterized by conductivity measurements, electrospray mass spectrometry, and NMR spectroscopy, before their decomposition to vinylphosphonium salts [vinyl-PPh3]+TfO- and Pd0 complexes. [(η1-vinyl)Pd(PPh3)2(DMF)]+TfO- complexes are less stable than [(aryl)Pd(PPh3)2(DMF)]+TfO- formed in the oxidative addition of aryl triflates to Pd0(PPh3)4. The rate constant of the oxidative addition of vinyl triflates and bromides to Pd0(PPh3)4 has been determined and compared to that of aryl triflates and halides. The following reactivity orders are established in DMF: vinyl-OTf ≫ vinyl-Br > PhBr and vinyl-OTf ≫ PhOTf.

The Mechanisms of the Stille Reaction

“…[97] A recent study showed that the oxidative addition of alkenyl triflates to [Pd(PPh 3 ) 4 ] in DMF gives [Pd(h 1 -alkenyl)(PPh 3 ) 2 (dmf)](TfO) under stoichiometric conditions (Tf = trifluoromethanesulfonyl). [98] The following reactivity order was established: alkenyl-OTf @ vinyl-Br > PhBr and vinyl-OTf @ PhOTf.…”

Mechanism of palladium‐catalyzed reactions: role of chloride ions

“…the oxidative addition of aryl triflates, 25 vinyl triflates, 29 and allylic acetates 35 to palladium(0) complexes (acceleration of the oxidative addition of aryl and vinyl triflates and irreversibility of the oxidative addition of allylic acetates).…”

“…Chloride ions modify the structure of the aryl-, vinylor allyl-palladium(II) complexes, generated in the oxidative addition, by formation of neutral aryl-, vinyl-or η 1 -allyl-palladium(II) chloride complexes respectively, 25,29,35,36 instead of cationic aryl-, vinyl-or (η 3 -allyl)palladium(II) complexes.…”

“…3, Table 2) shows that vinyl triflates are considerably more reactive than aryl triflates (compare Tables 1 and 2). 29 The oxidative additions were so fast that the kinetics were investigated in the presence of extra PPh 3 (10 equivalents) in order to decrease the rate of the reaction by decreasing the concentration of the reactive complex Pd 0 (PPh 3 ) 2 relative to that of the unreactive complex…”

“…29 Consequently, since the accelerating effect of the chloride ions on the oxidative addition is very low, one can assume that the beneficial role of chloride ions in Stille reactions involving vinyl triflates is not due to a faster oxidative Similarly, neutral complexes η 1 -allyl-PdCl(P,P) (7α) and (7γ ) were generated with bidentate P,P ligands (Scheme 6).…”