Suppression of alkene isomerisation in products from intramolecular heck reactions by addition of Tl(1) salts.

Reversible Formation of a Cationic Palladium(II) Hydride [HPd(PPh 3 ) 2 ] + in the Oxidative Addition of Acetic or Formic Acid to Palladium(0) in DMF

“…The rate constant of the overall forward reaction [k f in Equation (13)] has been determined in the presence of 100 equivalents of HX, so that the reverse reaction is negligible (irreversible reaction as in the uppermost curve in Figure 1).…”

“…When starting from Pd(PPh 3 ) 4 alone, the concentration of PPh 3 is not constant [Equation (11)] and the kinetic law becomes: [22] The value of the rate constant of the overall reverse reactions [k b in Equation (13)] was then deduced from the equilibrium constant K ϭ k f /k b (Table 1).…”

“…HPdXL 2 complexes are key intermediates in Heck reactions since re-addition of these complexes to the double bond after arylation is the origin of double bond isomerization. [9,10] In the presence of Ag ϩ [11,12] or Tl ϩ [13] cations able to capture the halide, a cationic complex HPdL 2 ϩ may also be formed when aryl triflates are used [14,15] due to the triflate anion's low affinity for palladium(II) complexes. [15,16] In such cases, re-addition of HPdL 2 ϩ to the olefin after arylation may not take place.…”

“…[15] This is one explanation, among others, for the better regioselectivity generally observed when Heck reactions are performed under ionic conditions. [12,13] In our attempts to define the mechanism of the palladium-catalyzed electro-hydrocarbonylation of aryl iodides to aryl aldehydes, performed in the presence of formic acid, [4] we established that the catalytic cycle was initiated by the reversible oxidative addition of formic acid to a palladium(0) complex to afford a cationic hydride complex HPdL 2 ϩ together with formate anions. [4] We now report some more details of this reaction and on its extension to acetic acid.…”

Synthetic Studies on Cyathin Terpenoids: Enantioselective Synthesis of the Tricyclic Core of Cyathin through Intramolecular Heck Cyclisation

“…In the event, treatment of triflate (+)-27 under standard Heck conditions [Pd(OAc) 2 cat., K 2 CO 3 ] was disappointing, giving rise to an inseparable mixture of trienes 28a-c in low yield (Ͻ 30 %), along with a minute amount of allylic acetate 29 (Ͻ 10 %). Although the overall yield increased to 70 % when the reaction was conducted with Pd(PPh 3 ) 4 in toluene, we failed to avoid the double migration by the use of various additives such as silver [26] or thallium [27] salts. Since acetate 29 was formed by addition of acetate ions from Pd(OAc) 2 to the intermediate η 3 -palladium complex, we anticipated that adding acetate ions might drive the reaction course toward acetate 29.…”